From what distance could the Sun's planetary nebula be seen entirely with free eye?

From what distance could the Sun's planetary nebula be seen entirely with free eye?

When collapsing to a white dwarf, a red giant Sun would emit a planetary nebula. How far from the Sun would that nebula stretch at most?

Also: At about what distance could you see the planetary nebula completely in all its beauty, I mean so that the entire nebula would match into your eye's field of view? The Earth and Mars would be deep within the nebula I guess, as well as the main asteroid belt. If you watched the Sun from Eris' aphelion or Sedna's perihelion, I guess that would be best to observe the entire nebula or ain't I correct? And what would the nebula look like from Proxima Centauri b?

In other words, how close to the Sun's planetary nebula can you be and still see it entirely in its beauty?

Planetary nebula

The term "planetary nebula" is a misnomer because they are unrelated to planets. The term originates from the planet-like round shape of these nebulae observed by astronomers through early telescopes. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets however, as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "very dim but perfectly outlined it is as large as Jupiter and resembles a fading planet". [3] [4] [5] Though the modern interpretation is different, the old term is still used.

All planetary nebulae form at the end of the life of a star of intermediate mass, about 1-8 solar masses. It is expected that the Sun will form a planetary nebula at the end of its life cycle. [6] They are relatively short-lived phenomena, lasting perhaps a few tens of millennia, compared to considerably longer phases of stellar evolution. [7] Once all of the red giant's atmosphere has been dissipated, energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus (P.N.N.), ionizes the ejected material. [2] Absorbed ultraviolet light then energizes the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.

Planetary nebulae probably play a crucial role in the chemical evolution of the Milky Way by expelling elements into the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances.

Starting from the 1990s, Hubble Space Telescope images revealed that many planetary nebulae have extremely complex and varied morphologies. About one-fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms that produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may play a role.


Discovery Edit

The first planetary nebula discovered (though not yet termed as such) was the Dumbbell Nebula in the constellation of Vulpecula. It was observed by Charles Messier on July 12, 1764 and listed as M27 in his catalogue of nebulous objects. [8] To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like Uranus. As early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "a very dull nebula, but perfectly outlined as large as Jupiter and looks like a fading planet". [3] [4] [5]

The nature of these objects remained unclear. In 1782, William Herschel, discoverer of Uranus, found the Saturn Nebula (NGC 7009) and described it as "A curious nebula, or what else to call it I do not know". He later described these objects as seeming to be planets "of the starry kind". [9] As noted by Darquier before him, Herschel found that the disk resembled a planet but it was too faint to be one. In 1785, Herschel wrote to Jérôme Lalande:

These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like a planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude. [10]

He assigned these to Class IV of his catalogue of "nebulae", eventually listing 78 "planetary nebulae", most of which are in fact galaxies. [11]

Herschel used the term "planetary nebulae" for these objects. The origin of this term not known. [8] [12] The label "planetary nebula" became ingrained in the terminology used by astronomers to categorize these types of nebulae, and is still in use by astronomers today. [13] [14]

Spectra Edit

The nature of planetary nebulae remained unknown until the first spectroscopic observations were made in the mid-19th century. Using a prism to disperse their light, William Huggins was one of the earliest astronomers to study the optical spectra of astronomical objects. [12]

On August 29, 1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed Cat's Eye Nebula. [8] His observations of stars had shown that their spectra consisted of a continuum of radiation with many dark lines superimposed. He found that many nebulous objects such as the Andromeda Nebula (as it was then known) had spectra that were quite similar. However, when Huggins looked at the Cat's Eye Nebula, he found a very different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of emission lines. [12] Brightest of these was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known element. [15]

At first, it was hypothesized that the line might be due to an unknown element, which was named nebulium. A similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868. [8] While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, "nebulium" was not. In the early 20th century, Henry Norris Russell proposed that, rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions. [8]

Physicists showed in the 1920s that in gas at extremely low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities. [16] Electron transitions from these levels in nitrogen and oxygen ions ( O + , O 2+ (a.k.a. O iii ), and N + ) give rise to the 500.7 nm emission line and others. [8] These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas. [17]

Central stars Edit

The central stars of planetary nebulae are very hot. [2] Only when a star has exhausted most of its nuclear fuel can it collapse to a small size. Planetary nebulae are understood as a final stage of stellar evolution. Spectroscopic observations show that all planetary nebulae are expanding. This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life. [8]

Modern observations Edit

Towards the end of the 20th century, technological improvements helped to further the study of planetary nebulae. [19] Space telescopes allowed astronomers to study light wavelengths outside those that the Earth's atmosphere transmits. Infrared and ultraviolet studies of planetary nebulae allowed much more accurate determinations of nebular temperatures, densities and elemental abundances. [20] [21] Charge-coupled device technology allowed much fainter spectral lines to be measured accurately than had previously been possible. The Hubble Space Telescope also showed that while many nebulae appear to have simple and regular structures when observed from the ground, the very high optical resolution achievable by telescopes above the Earth's atmosphere reveals extremely complex structures. [22] [23]

Under the Morgan-Keenan spectral classification scheme, planetary nebulae are classified as Type-P, although this notation is seldom used in practice. [24]

Stars greater than 8 solar masses (M) will probably end their lives in dramatic supernovae explosions, while planetary nebulae seemingly only occur at the end of the lives of intermediate and low mass stars between 0.8 M to 8.0 M. [25] Progenitor stars that form planetary nebulae will spend most of their lifetimes converting their hydrogen into helium in the star's core by nuclear fusion at about 15 million K. This generated energy creates outward pressure from fusion reactions in the core, balancing the crushing inward pressures of the star's gravity. [26] This state of equilibrium is known as the main sequence, which can last for tens of millions to billions of years, depending on the mass.

When the hydrogen source in the core starts to diminish, gravity starts compressing the core, causing a rise in temperature to about 100 million K. [27] Such higher core temperatures then make the star's cooler outer layers expand to create much larger red giant stars. This end phase causes a dramatic rise in stellar luminosity, where the released energy is distributed over a much larger surface area, which in fact causes the average surface temperature to be lower. In stellar evolution terms, stars undergoing such increases in luminosity are known as asymptotic giant branch stars (AGB). [27] During this phase, the star can lose 50 to 70% of its total mass from its stellar wind. [28]

For the more massive asymptotic giant branch stars that form planetary nebulae, whose progenitors exceed about 3M, their cores will continue to contract. When temperatures reach about 100 million K, the available helium nuclei fuse into carbon and oxygen, so that the star again resumes radiating energy, temporarily stopping the core's contraction. This new helium burning phase (fusion of helium nuclei) forms a growing inner core of inert carbon and oxygen. Above it is a thin helium-burning shell, surrounded in turn by a hydrogen-burning shell. However, this new phase lasts only 20,000 years or so, a very short period compared to the entire lifetime of the star.

The venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures exceeding about 30,000 K, there are enough emitted ultraviolet photons to ionize the ejected atmosphere, causing the gas to shine as a planetary nebula. [27]

After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins [19] as gases blow away from the central star at speeds of a few kilometers per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB. [19] As the gases expand, the central star undergoes a two-stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling when the hydrogen shell is exhausted through fusion and mass loss. [19] In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. [8] [19] During the first phase, the central star maintains constant luminosity, [19] while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a white dwarf, and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution. [19] For a typical planetary nebula, about 10,000 years [19] passes between its formation and recombination of the resulting plasma. [8]

Planetary nebulae may play a very important role in galactic evolution. Newly born stars consist almost entirely of hydrogen and helium, [31] but as stars evolve through the asymptotic giant branch phase, [32] they create heavier elements via nuclear fusion which are eventually expelled by strong stellar winds. [33] Planetary nebulae usually contain larger proportions of elements such as carbon, nitrogen and oxygen, and these are recycled into the interstellar medium via these powerful winds. In this way, planetary nebulae greatly enrich the Milky Way and their nebulae with these heavier elements – collectively known by astronomers as metals and specifically referred to by the metallicity parameter Z. [34]

Subsequent generations of stars formed from such nebulae also tend to have higher metallicities. Although these metals are present in stars in relatively tiny amounts, they have marked effects on stellar evolution and fusion reactions. When stars formed earlier in the universe they theoretically contained smaller quantities of heavier elements. [35] Known examples are the metal poor Population II stars. (See Stellar population.) [36] [37] Identification of stellar metallicity content is found by spectroscopy.

Physical characteristics Edit

A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles per cm 3 . [38] (The Earth's atmosphere, by comparison, contains 2.5 × 10 19 particles per cm 3 .) Young planetary nebulae have the highest densities, sometimes as high as 10 6 particles per cm 3 . As nebulae age, their expansion causes their density to decrease. The masses of planetary nebulae range from 0.1 to 1 solar masses. [38]

Radiation from the central star heats the gases to temperatures of about 10,000 K. [39] The gas temperature in central regions is usually much higher than at the periphery reaching 16,000–25,000 K. [40] The volume in the vicinity of the central star is often filled with a very hot (coronal) gas having the temperature of about 1,000,000 K. This gas originates from the surface of the central star in the form of the fast stellar wind. [41]

Nebulae may be described as matter bounded or radiation bounded. In the former case, there is not enough matter in the nebula to absorb all the UV photons emitted by the star, and the visible nebula is fully ionized. In the latter case, there are not enough UV photons being emitted by the central star to ionize all the surrounding gas, and an ionization front propagates outward into the circumstellar envelope of neutral atoms. [42]

Numbers and distribution Edit

About 3000 planetary nebulae are now known to exist in our galaxy, [43] out of 200 billion stars. Their very short lifetime compared to total stellar lifetime accounts for their rarity. They are found mostly near the plane of the Milky Way, with the greatest concentration near the galactic center. [44]

Morphology Edit

Only about 20% of planetary nebulae are spherically symmetric (for example, see Abell 39). [45] A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar, quadrupolar, [46] and other types, [47] although the majority of them belong to just three types: spherical, elliptical and bipolar. Bipolar nebulae are concentrated in the galactic plane, probably produced by relatively young massive progenitor stars and bipolars in the galactic bulge appear to prefer orienting their orbital axes parallel to the galactic plane. [48] On the other hand, spherical nebulae are probably produced by old stars similar to the Sun. [41]

The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different. [49] Nevertheless, the reason for the huge variety of physical shapes is not fully understood. [47] Gravitational interactions with companion stars if the central stars are binary stars may be one cause. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae. [50] In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes. [51] [52]

Planetary nebulae have been detected as members in four Galactic globular clusters: Messier 15, Messier 22, NGC 6441 and Palomar 6. Evidence also points to the potential discovery of planetary nebulae in globular clusters in the galaxy M31. [53] However, there is currently only one case of a planetary nebula discovered in an open cluster that is agreed upon by independent researchers. [54] [55] [56] That case pertains to the planetary nebula PHR 1315-6555 and the open cluster Andrews-Lindsay 1. Indeed, through cluster membership, PHR 1315-6555 possesses among the most precise distances established for a planetary nebula (i.e., a 4% distance solution). The cases of NGC 2818 and NGC 2348 in Messier 46, exhibit mismatched velocities between the planetary nebulae and the clusters, which indicates they are line-of-sight coincidences. [44] [57] [58] A subsample of tentative cases that may potentially be cluster/PN pairs includes Abell 8 and Bica 6, [59] [60] and He 2-86 and NGC 4463. [61]

Theoretical models predict that planetary nebulae can form from main-sequence stars of between one and eight solar masses, which puts the progenitor star's age at greater than 40 million years. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a planetary nebula within. [44] For one reason, the planetary nebula phase for more massive stars is on the order of millennia, which is a blink of the eye in astronomic terms. Also, partly because of their small total mass, open clusters have relatively poor gravitational cohesion and tend to disperse after a relatively short time, typically from 100 to 600 million years. [62]

The distances to planetary nebulae are generally poorly determined. [63] It is possible to determine distances to the nearest planetary nebula by measuring their expansion rates. High resolution observations taken several years apart will show the expansion of the nebula perpendicular to the line of sight, while spectroscopic observations of the Doppler shift will reveal the velocity of expansion in the line of sight. Comparing the angular expansion with the derived velocity of expansion will reveal the distance to the nebula. [22]

The issue of how such a diverse range of nebular shapes can be produced is a debatable topic. It is theorised that interactions between material moving away from the star at different speeds gives rise to most observed shapes. [47] However, some astronomers postulate that close binary central stars might be responsible for the more complex and extreme planetary nebulae. [64] Several have been shown to exhibit strong magnetic fields, [65] and their interactions with ionized gas could explain some planetary nebulae shapes. [52]

There are two main methods of determining metal abundances in nebulae. These rely on recombination lines and collisionally excited lines. Large discrepancies are sometimes seen between the results derived from the two methods. This may be explained by the presence of small temperature fluctuations within planetary nebulae. The discrepancies may be too large to be caused by temperature effects, and some hypothesize the existence of cold knots containing very little hydrogen to explain the observations. However, such knots have yet to be observed. [66]

A Dying Star Fades Away Before Hubble’s Very Eyes

This image compares two drastically different portraits of the Stingray nebula captured by NASA’s . [+] Hubble Space Telescope 20 years apart. The image on the left, taken with the Wide Field and Planetary Camera 2 in March 1996, shows the nebula’s central star in the final stages of its life. The gas being puffed off by the dying star is much brighter when compared to the image of the nebula at the right, captured in January 2016 using the Wide Field Camera 3.

NASA, ESA, B. Balick (University of Washington), M. Guerrero (Instituto de Astrofísica de Andalucía), and G. Ramos-Larios (Universidad de Guadalajara)

All stars, even our Sun, will someday eventually die.

After burning on the main sequence for billions of years, the Sun will expand into a red giant, . [+] switch to helium burning, move to the asymptotic branch, and then eject its outer layers. As the core contracts, it heats up, illuminating the gas in a planetary nebula. Over about 20,000 years, that nebula will fade away, eventually becoming invisible.

Wikimedia Commons user Szczureq

Upon exhausting their core’s nuclear fuel, Sun-like stars die in a predictable fashion.

Near the end of a Sun-like star's life, it begins to blow off its outer layers into the depths of . [+] space, forming a protoplanetary nebula like the Egg Nebula, seen here. Its outer layers have not yet been heated to sufficient temperatures by the central, contracting star to create a true planetary nebula just yet.

NASA and the Hubble Heritage Team (STScI / AURA), Hubble Space Telescope / ACS

The core contracts, forming white dwarfs, which heats and illuminates the blown-off outer layers, creating planetary nebulae.

This Hubble Space Telescope image of the Helix Nebula shows a typical planetary nebula/white dwarf . [+] combination: the result of a Sun-like star reaching the end of its life. The central white dwarf is much fainter than a standard star, but is very hot and emits ionizing radiation. The illuminated nebula is made of ejecta from the star's outer layers, and is illuminated by the central stellar remnant.

NASA, ESA, and C.R. O'Dell (Vanderbilt University)

These nebulous remnants persist for

20,000 years, experiencing slow, gradual changes.

After 20 years of Hubble observations, however, the Stingray Nebula appears doubly special.

This animation shows how significant the fading of the Stingray Nebula has been since 1996. Note the . [+] background star, just to the upper left of the central, fading white dwarf, which remains constant over time, which confirms that the nebula itself is dimming significantly.

NASA, ESA, B. Balick (University of Washington), M. Guerrero (Instituto de Astrofísica de Andalucía), and G. Ramos-Larios (Universidad de Guadalajara)

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First, it’s faded away tremendously, becoming far less luminous.

Normally, a planetary nebula will appear similar to the Cat's Eye Nebula, shown here. A central core . [+] of expanding gas is lit up brightly by the central white dwarf, while the diffuse outer regions continue to expand, illuminated far more faintly. This is in contrast to the Stingray Nebula, which appears to be contracting.

Nordic Optical Telescope and Romano Corradi / Wikimedia Commons / CC BY-SA 3.0

Second, the shells of gas are contracting and diffusing, appearing less crisp.

The Dumbbell Nebula, as imaged here through an 8" amateur telescope, was the first planetary nebula . [+] ever discovered: by Charles Messier in 1764. The shells of gas are slowly expanding and their definition remains constant over time, typical for a planetary nebula. The Stingray Nebula, somehow, is different.

Mike Durkin madmiked/flickr

These changes are unprecedented, but different elemental signatures reveal clues.

This image from NASA’s Chandra X-ray Observatory shows the location of different elements in the . [+] Cassiopeia A supernova remnant including silicon (red), sulfur (yellow), calcium (green) and iron (purple). Each element reveals its own particular spectral signature and set of photometric emissions, enabling us to map out the location of various elements in all sorts of stellar remnants and nebulae.

Nitrogen and hydrogen emissions substantially decreased, but oxygen emissions plummeted almost a thousandfold.

This 2016 image from the Hubble Space Telescope, of the Stingray Nebula, brings out all the details . [+] in the nebula to the best of the image's ability, revealing a much fainter and less sharply defined nebula than earlier images. The central star has cooled significantly from its peak of 60,000 K, which it rose to from the 1970s until about

2000. The temperature has been dropping ever since.

This is driven by the central star’s temperature changes: rising from

60,000 K previously, and now dropping rapidly.

This image from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 . [+] surrounding a dim and dying star located about 3300 light-years away. The green color arises from emission line transitions in the ionized gas surrounding the dim, dying star. Typically, green colors only appear from doubly ionized oxygen, requiring temperatures of

At 50,000 K, oxygen loses two electrons, getting doubly ionized, emitting a brilliant green glow.

The Asymptotic Giant Branch star, LL Pegasi, is shown with its ejecta, along with a cutaway of its . [+] core. Surrounding the carbon-oxygen core is a shell of helium, which can fuse at the interface of the carbon-oxygen core. In the remnant powering the Stingray nebula, even though the outer hydrogen and helium has been mostly ejected, a transient helium-burning shell likely heated this remnant extremely recently, which now fades away.

ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al. (main) NOAO (inset)

This hints at a recent burst of fusion: where helium in a shell around the core ignited, illuminating the surroundings.

Initially, the Stingray Nebula, Hen 3-1357, exhibited bright blue shells near its center, as this . [+] 1996 image shows. It was touted as perhaps the youngest planetary nebula on record. Given its recent fading and dimming, that conclusion may be wildly incorrect.

NASA, ESA, B. Balick (University of Washington), M. Guerrero (Instituto de Astrofísica de Andalucía), and G. Ramos-Larios (Universidad de Guadalajara)

With that burst over, the nebula fades as the central engine cools.

The Stingray Nebula has faded dramatically, as this 2016 image shows as compared to earlier ones. It . [+] has dimmed in brightness and changed in shape, with the decreased oxygen emissions comprising the most notable change. The nebula no longer 'pops' against the bright background of empty space.

NASA, ESA, B. Balick (University of Washington), M. Guerrero (Instituto de Astrofísica de Andalucía), and G. Ramos-Larios (Universidad de Guadalajara)

Additionally, the gas contracts instead of expanding: something never previously observed.

The Medusa Nebula, shown here, is faint, diffuse, and shows a complex structure indicative of its . [+] old age. Planetary Nebulae only persist for about 10,000 to 20,000 years, and this one is apparently nearing the end of its life. As the gas becomes neutral or too diffuse to shine and the central white dwarf cools, the nebula fades away entirely.

Jschulman555 / Wikimedia Commons / Mt. Lemmon Skycenter

This planetary nebula could disappear entirely — a first — perhaps in merely 20-30 years.

From a wide-field view, it isn't clear where the Stingray Nebula is, but close observations reveal . [+] its location in the central, very blue star. In as little as 20-30 years, if the current fading trend continues unabated, the nebula will disappear entirely.

ESA/Hubble, Digitized Sky Survey 2. Acknowledgement: Davide De Martin

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Astronomers Discover Water-Building Molecule in Planetary Nebulas

This image presents the Helix Nebula first at optical wavelengths, as seen by the Hubble Space Telescope, then by Herschel’s SPIRE instrument at wavelengths around 250 micrometers. A spectrum is shown for the region identified on the image, showing the clear signature of CO and OH+ emission in the clumpy outer regions of the planetary nebula.

Using the Hubble Space Telescope and ESA’s Herschel Space Observatory, astronomers have discovered a water-building molecule in three planetary nebulas.

Using ESA’s Herschel Space Oobservatory, astronomers have discovered that a molecule vital for creating water exists in the burning embers of dying Sun-like stars.

When low- to middleweight stars like our Sun approach the end of their lives, they eventually become dense, white dwarf stars. In doing so, they cast off their outer layers of dust and gas into space, creating a kaleidoscope of intricate patterns known as planetary nebulas.

These actually have nothing to do with planets, but were named in the late 18th century by astronomer William Herschel, because they appeared as fuzzy circular objects through his telescope, somewhat like the planets in our Solar System.

Over two centuries later, planetary nebulas studied with William Herschel’s namesake, the Herschel space observatory, have yielded a surprising discovery.

Like the dramatic supernova explosions of weightier stars, the death cries of the stars responsible for planetary nebulas also enrich the local interstellar environment with elements from which the next generations of stars are born.

While supernovas are capable of forging the heaviest elements, planetary nebulas contain a large proportion of the lighter ‘elements of life’ such as carbon, nitrogen, and oxygen, made by nuclear fusion in the parent star.

A star like the Sun steadily burns hydrogen in its core for billions of years. But once the fuel begins to run out, the central star swells into a red giant, becoming unstable and shedding its outer layers to form a planetary nebula.

The Ring Nebula at optical wavelengths as seen by the Hubble Space Telescope, with Herschel data acquired with SPIRE and PACS over a wavelength range of 51–672 micrometers for the region identified. The spectra have been cropped and the scales stretched in order to show the OH+ emission, a molecular ion important for the formation of water. ESA’s Herschel space observatory is the first to detect this molecule in planetary nebulas – the product of dying Sun-like stars.

The remaining core of the star eventually becomes a hot white dwarf pouring out ultraviolet radiation into its surroundings.

This intense radiation may destroy molecules that had previously been ejected by the star and that are bound up in the clumps or rings of material seen in the periphery of planetary nebulas.

The harsh radiation was also assumed to restrict the formation of new molecules in those regions.

But in two separate studies using Herschel astronomers have discovered that a molecule vital to the formation of water seems to rather like this harsh environment, and perhaps even depends upon it to form. The molecule, known as OH+, is a positively charged combination of single oxygen and hydrogen atoms.

In one study, led by Dr Isabel Aleman of the University of Leiden, the Netherlands, 11 planetary nebulas were analyzed and the molecule was found in just three.

What links the three is that they host the hottest stars, with temperatures exceeding 100,000ºC.

“We think that a critical clue is in the presence of the dense clumps of gas and dust, which are illuminated by UV and X-ray radiation emitted by the hot central star,” says Dr Aleman.

“This high-energy radiation interacts with the clumps to trigger chemical reactions that leads to the formation of the molecules.”

Meanwhile, another study, led by Dr Mireya Etxaluze of the Instituto de Ciencia de los Materiales de Madrid, Spain, focused on the Helix Nebula, one of the nearest planetary nebulas to our Solar System, at a distance of 700 light years.

The central star is about half the mass of our Sun, but has a far higher temperature of about 120,000ºC. The expelled shells of the star, which in optical images appear reminiscent of a human eye, are known to contain a rich variety of molecules.

Herschel mapped the presence of the crucial molecule across the Helix Nebula, and found it to be most abundant in locations where carbon monoxide molecules, previously ejected by the star, are most likely to be destroyed by the strong UV radiation.

Once oxygen atoms have been liberated from the carbon monoxide, they are available to make the oxygen–hydrogen molecules, further bolstering the hypothesis that the UV radiation may be promoting their creation.

The two studies are the first to identify in planetary nebulas this critical molecule needed for the formation of water, although it remains to be seen if the conditions would actually allow water formation to proceed.

“The proximity of the Helix Nebula means we have a natural laboratory on our cosmic doorstep to study in more detail the chemistry of these objects and their role in recycling molecules through the interstellar medium,” says Dr Etxaluze.

“Herschel has traced water across the Universe, from star-forming clouds to the asteroid belt in our own Solar System,” says Göran Pilbratt, ESA’s Herschel project scientist.

“Now we have even found that stars like our Sun could contribute to the formation of water in the Universe, even as they are in their death throes.”

From what distance could the Sun's planetary nebula be seen entirely with free eye? - Astronomy

When an intermediate mass star, such as the Sun (or any star with between about 0.7 and 8 solar masses)
nears the end of its life it becomes a helium shell-burning supergiant AGB star . These stars swell to around 100
times their original size, and the tenuous outer layers cool and redden (indeed they become so cool that dust
may form, obscuring and apparently dimming the central star). These stars are burning helium, which means
that they are hotter than hydrogen-burning Main Sequence stars and helium burning emits massive emits of
radiation in a shell just beneath the outer tenuous envelope and above the core. This intense radiation
pressure generates a superwind, which is much more intense than the normal stellar wind of a main sequence
star like the Sun. This wind meets the tenuous outer layers which are so far from the core as to be only loosely
bound by the star's gravity and as a result massive amounts of material are blown outwards from the star and
the outer envelope is shed in one or more expanding shells. These shells typically expand outwards at great
speeds, typically 30-60 kilometres per second. Over thousands of years these shells may be hundreds or
thousands of times the diameter of the Solar System. Relieved of the weight (pressure) of these overlying
layers, the core expands slightly, causing what is left of the envelope around the core to contract and heat up
considerably. When this core plus envelope remnant reach about 30 000 degrees K, they emit enough
ultraviolet radiation to cause the massively expanded outer shells to fluoresce, and a planetary nebula is born!

The planetary nebula appears as a ring when seen through a telescope (light passing through the edges has
passed through more gas creating the illusion that the spherical nebula is thickest here). This ring-like
appearance gives rise to the 'planetary' part of the name, but planetary nebulae actually have nothing to do
with planets (although at first they were mistaken for planets, debris and dust around the central star).
Sometimes, the central star is hard to see (as in the one above) in visible light, especially in a young nebula
which may contain a bubble of hot gas around the core. The core will eventually free itself of the expanding
nebula and cool to become a C-O white dwarf , after about 20 000 years. The Sun will (if left to its own devices)
become a planetary nebula in about 6 billion years time.

Above: the hot central core is visible in the centre of this nebula. Often a planetary nebulae have the appearance
of 'cosmic eyes' such as the Cat's Eye Nebula. Sometimes (largely for unexplained reasons) the core of planetary
nebulae appears off-centre.

Below: some (probably most) planetary nebulae are bilobed as the expanding material is preferentially shed
along a particular axis. Examples include the Hourglass Nebula (which also has a central 'eye' with a stellar core
that is off-centre) and the Egg Nebula. Others more complicated structures may also occur, including radiating
spokes of cooler gas, knots of gas at shock fronts and bipolar jets. Sometimes spherical shells and jets or a pair
of lobes occur together. Some of these more complex patterns probably arise when the star is part of a binary
star and so orbits a nearby companion star that may perturb the nebula with its gravity. It maybe that a companion
star will start to accrete (draw onto itself) some of the material from the nebula, some of which may be jetted off
from the companion star's poles. In short planetary nebulae occur in an extraordinary array of forms and are one
of the most beautiful classes of object in space.

Example: Cat's Eye Nebula (NGC 6543)

Credits: NASA , ESA , HEIC, and The Hubble Heritage Team (STScI /AURA ) Acknowledgment: R. Corradi (Isaac Newton Group of Telescopes, Spain) and Z. Tsvetanov (NASA)

The Cat's Eye Nebula is one of the most complicated planetary nebulae. Multiple rings (at least 9 to 11) surround the nebula, suggesting either the periodic expulsion of gas from the central star (at 1500 year intervals) or the transit of a pressure wave through the material, compacting it at periodic intervals. In addition a series of bubbles surround the central elliptical cloud and knots of material, suggestive of bipolar jets can also be seen. Perhaps the best explanation for this phenomenon is that of an accreting binary star - one of the star's may be dying, ejecting material which is perhaps being accreted by its partner with the central Cat's eye Nebula itself forming within the spheres of ejected gas about 1000 thousand years ago. An accretion disc around one of the stars could give rise to collimated stellar winds coming from the poles and such winds impacting on spherical shells of matter ejected by the red giant progenitor might go some way to explaining the structure. Estimates for the mass of the central star vary from about one solar mass to more than 5 solar masses. The rate of mass loss is estimated to be 3.2 x 10 -7 solar masses / year, but may have been much higher during the red giant phase (estimated at 1.5 x 10 -5 solar masses /year).

Balick, B. and preston, H.L. 1987. A wind-blown model for NGC 6543. 1987. The Astronomical J. 94(4): 958-963.

balick, B., Wilson, J. and Hajian, A.R. 2001. NGC 6543: The Rings Around the Cat's Eye. The Astronomical Journal 121(1):

Bianchi, L., Cerrato, S., & Grewing, M. 1986. Mass loss from central stars of planetary nebulae - The nucleus of NGC 6543. Astronomy and Astrophysics (ISSN 0004-6361), vol. 169, no. 1-2, Nov. 1986, p. 227-236.

Ultra-close stars discovered inside a planetary nebula

An image obtained with the Hubble Space Telescope of the planetary nebula M3-1, the central star of which is actually a binary system with one of the shortest orbital periods known. Credit: David Jones / Daniel López – IAC

An international team of astronomers have discovered two stars in a binary pair that complete an orbit around each other in a little over three hours, residing in the planetary nebula M3-1. Remarkably, the stars could drive a nova explosion, an entirely unexpected event based on our current understanding of binary star evolution. The team, led by David Jones of the Instituto Astrofisica de Canarias and the Universidad de La Laguna, report their findings in Monthly Notices of the Royal Astronomical Society: Letters.

Planetary nebulae are the glowing shells of gas and dust formed from the outer layers of stars like our own Sun, which they throw off during the final stages of their evolution. In many cases, interaction with a nearby companion star plays an important role in the ejection of this material and the formation of the elaborate structures seen in the resulting planetary nebulae.

The planetary nebula M3-1 is located in the constellation of Canis Major, at a distance of roughly 14,000 light years. M3-1 was a firm candidate to host a binary central star, as its structure with prominent jets and filaments is typical of these binary star interactions.

Using the telescopes of the European Southern Observatory (ESO) in Chile, Jones's team looked at M3-1 over a period of several years. In the process they discovered and studied the binary stars in the centre of the nebula.

"We knew M3-1 had to host a binary star, so we set about acquiring the observations required to prove this and to relate the properties of the nebula with the evolution of the star or stars that formed it" says Brent Miszalski, researcher at the Southern African Large Telescope, and co-author of the study.

The two stars are so close together that they cannot be resolved from the ground, so instead the presence of the second star is inferred from the variation of their observed combined brightness—most obviously by periodic eclipses of one star by the other which produce marked drops in the brightness.

"When we began the observations, it was immediately clear that the system was a binary" explains Henri Boffin, researcher at the European Southern Observatory in Germany. "We saw that the apparently single star at the centre of the nebula was rapidly changing in brightness, and we knew that this must be due to the presence of a companion star."

The team discovered that the central star of the planetary nebula M3-1 has one of the shortest orbital period binary central stars known to date, at just over three hours. The ESO observations also show that the two stars—most likely a white dwarf with a low-mass main sequence companion—are almost touching.

As a result, the pair are likely to undergo a so-called nova eruption, the result of the transfer of material from one star to the other. When this reaches a critical mass, a violent thermonuclear explosion takes place and the system temporarily increases in brightness by up to a million times.

"After the various observing campaigns in Chile, we had enough data to begin to understand the properties of the two stars—their masses, temperatures and radii" says Paulina Sowicka, a Ph.D. student at the Nicolas Copernicus Astronomical Center in Poland. "It was a real surprise that the two stars were so close together and so large that they were almost touching one another. A nova explosion could take place in just a few thousand years from now."

Theory suggests that binary stars should be well separated after the formation of a planetary nebula. It should then take a long time before they begin to interact again and events such as novae become possible.

In 2007, astronomers observed a different nova explosion, known as Nova Vul 2007, inside another planetary nebula.

Jones comments: "The 2007 event was particularly difficult to explain. By the time the two stars are close enough for a nova, the material in the planetary nebula should have expanded and dissipated so much that it's no longer visible."

The new event adds to the conundrum, adds Jones: "In the central stars of M3-1, we've found another candidate for a similar nova eruption in the relatively near future."

The team now hope to carry out further study of the nebula and others like it, helping to shed light on the physical processes and origins of novae and supernovae, some of the most spectacular and violent phenomena in the universe.


As seen from the northern hemisphere, the constellation's brighter stars form an easily recognizable asterism known as "the Teapot". [1] [2] The stars δ Sgr (Kaus Media), ε Sgr (Kaus Australis), ζ Sgr (Ascella), and φ Sgr form the body of the pot λ Sgr (Kaus Borealis) is the point of the lid γ 2 Sgr (Alnasl) is the tip of the spout and σ Sgr (Nunki) and τ Sgr the handle. These same stars originally formed the bow and arrow of Sagittarius. [3]

Marking the bottom of the teapot's "handle" (or the shoulder area of the archer), is the bright star (2.59 magnitude) Zeta Sagittarii (ζ Sgr), named Ascella, and the fainter Tau Sagittarii (τ Sgr).

To complete the teapot metaphor, under good conditions, a particularly dense area of the Milky Way can be seen rising in a north-westerly arc above the spout, like a puff of steam rising from a boiling kettle. [4]

The constellation as a whole is often depicted as having the rough appearance of a stick-figure archer drawing its bow, with the fainter stars providing the outline of the horse's body. Sagittarius famously points its arrow at the heart of Scorpius, represented by the reddish star Antares, as the two constellations race around the sky. Following the direct line formed by Delta Sagittarii (δ Sgr) and Gamma2 Sagittarii (γ 2 Sgr) leads nearly directly to Antares. Fittingly, Gamma2 Sagittarii is Alnasl, the Arabic word for "arrowhead", and Delta Sagittarii is called Kaus Media, the "center of the bow," from which the arrow protrudes. Kaus Media bisects Lambda Sagittarii (λ Sgr) and Epsilon Sagittarii (ε Sgr), whose names Kaus Borealis and Kaus Australis refer to the northern and southern portions of the bow, respectively. [5]

Sagittarius is one of the prominent features of the summer skies in the northern hemisphere although in Europe north of the Pyrenees it drags very low along the horizon and can be difficult to see clearly. In Scotland and Scandinavia it cannot be seen at all. In southern Brazil, South Africa, and central Australia (30° south), Sagittarius passes directly overhead. It is hidden behind the Sun's glare from mid-November to mid-January and is the location of the Sun at the winter solstice (December 21). By March, Sagittarius is rising at midnight. In June, it achieves opposition and can be seen all night. The June full moon appears in Sagittarius.

In classical antiquity, Capricorn was the location of the Sun at the winter solstice, but due to the precession of the equinoxes, this had shifted to Sagittarius by the time of the Roman Empire. By approximately 2700 AD, the Sun will be in Scorpius at the winter solstice.

Stars Edit

α Sgr (Rukbat, meaning "the archer's knee" [6] ) despite having the "alpha" designation, is not the brightest star of the constellation, having a magnitude of only 3.96. It is towards the bottom center of the map as shown. Instead, the brightest star is Epsilon Sagittarii (ε Sgr) ("Kaus Australis," or "southern part of the bow"), at magnitude 1.85. [7]

Sigma Sagittarii (σ Sgr) ("Nunki") is the constellation's second-brightest star at magnitude 2.08. Nunki is a B2V star approximately 260 light-years away. [6] "Nunki" is a Babylonian name of uncertain origin, but thought to represent the sacred Babylonian city of Eridu on the Euphrates, which would make Nunki the oldest star name currently in use. [5]

Zeta Sagittarii (ζ Sgr) ("Ascella"), with apparent magnitude 2.61 of A2 spectra, is actually a double star whose two components have magnitudes 3.3 and 3.5. [8]

Delta Sagittarii (δ Sgr) ("Kaus Meridionalis"), is a K2 spectra star with magnitude 2.71 about 350 light years from Earth. [8]

Eta Sagittarii (η Sgr) is a double star with component magnitudes of 3.18 and 10, while Pi Sagittarii (π Sgr) ("Albaldah") [9] is actually a triple system whose components have magnitudes 3.7, 3.8, and 6.0. [8]

The Bayer designation Beta Sagittarii (Beta Sgr, β Sagittarii, β Sgr) is shared by two star systems, β¹ Sagittarii, with apparent magnitude 3.96, and β² Sagittarii, magnitude 7.4. The two stars are separated by 0.36° in the sky and are 378 light-years from earth. Beta Sagittarii, located at a position associated with the forelegs of the centaur, has the traditional name "Arkab", meaning "Achilles tendon".

Nova Sagittarii 2015 No. 2 was discovered on 15 March 2015, [10] by John Seach of Chatsworth Island, NSW, Australia. It lies near the center of the constellation. It reached a peak magnitude of 4.3 before steadily fading.

Deep-sky objects Edit

The Milky Way is at its densest near Sagittarius, as this is where the galactic center lies. As a result, Sagittarius contains many star clusters and nebulae.

Star clouds Edit

Sagittarius contains two well-known star clouds, both considered fine binocular objects.

  • The Large Sagittarius Star Cloud is the brightest visible region of the Milky Way. It is a portion of the central bulge of the galaxy seen around the thick dust of the Great Rift, and is the innermost galactic structure that can be observed in visible wavelengths. It has several embedded clusters and superimposed dark nebulae. [11]
  • The Small Sagittarius Star Cloud, also known as Messier 24, has an apparent magnitude of 2.5. The cloud fills a space of significant volume to a depth of 10,000 to 16,000 light-years. Embedded in M24 is NGC 6603, a small star cluster that is very dense. NGC 6567, a dim planetary nebula, and Barnard 92, a Bok globule, are also nearby. [12]

Nebulae Edit

Sagittarius contains several well-known nebulae, including the Lagoon Nebula (Messier 8), near λ Sagittarii the Omega Nebula (Messier 17), near the border with Scutum and the Trifid Nebula (Messier 20), a large nebula containing some very young, hot stars.

  • The Lagoon Nebula (M8) is an emission nebula that is located 5,000 light-years from Earth and measures 140 light-years by 60 light-years (1.5°). Though it appears grey in telescopes to the unaided eye, long-exposure photographs reveal its pink hue, common to emission nebulae. [13] It is fairly bright, with an integrated magnitude of 3.0. [14] The Lagoon Nebula was discovered independently by John Flamsteed in 1680, [15]Guillaume Le Gentil in 1747, [13] and Charles Messier in 1764. [15] The central area of the Lagoon Nebula is also known as the Hourglass Nebula, so named for its distinctive shape. The Hourglass Nebula has its shape because of matter propelled by Herschel 36. The Lagoon Nebula also features three dark nebulae catalogued in Barnard's Catalog. [13] The Lagoon Nebula was instrumental in the discovery of Bok globules, as Bart Bok studied prints of the nebula intensively in 1947. Approximately 17,000 Bok globules were discovered in the nebula nine years later as a part of the Palomar Sky Survey studies later showed that Bok's hypothesis that the globules held protostars was correct. [16]
  • The Omega Nebula is a fairly bright nebula, sometimes called the Horseshoe Nebula or Swan Nebula. It has an integrated magnitude of 6.0 and is 4890 light-years from Earth. It was discovered in 1746 by Philippe Loys de Chésaux observers since him have differed greatly in how they view the nebula, hence its myriad of names. Most often viewed as a checkmark, it was seen as a swan by George F. Chambers in 1889, a loon by Roy Bishop, and as a curl of smoke by Camille Flammarion. [17]
  • The Trifid Nebula (M20, NGC 6514) is an emission nebula in Sagittarius that lies less than two degrees from the Lagoon Nebula. Discovered by French comet-hunter Charles Messier, it is located between 2,000 and 9,000 light-years from Earth and has a diameter of approximately 50 light-years. The outside of the Trifid Nebula is a bluish reflection nebula the interior is pink with two dark bands that divide it into three areas, sometimes called "lobes". Hydrogen in the nebula is ionized, creating its characteristic color, by a central triple star, which formed in the intersection of the two dark bands. [13] M20 is associated with a cluster that has a magnitude of 6.3. [18]
  • The Red Spider Nebula (NGC 6537) is a planetary nebula located at a distance of about 4000 light-years from Earth. is a star-forming region located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius, showing both emission (red) and reflection (blue) regions.

In addition, several other nebulae have been located within Sagittarius and are of interest to astronomy.

    is a planetary nebula with an approximate magnitude of 11. A large nebula at over one arcminute in diameter, it appears very close to the globular clusterNGC 6440. [19] is a dimmer globular at magnitude 9.2, though it is more distant than M71 at a distance of 26,000 light-years. It is a Shapley class VI cluster the classification means that it has an intermediate concentration at its core. It is approximately a degree away from the brighter globulars M22 and M28 NGC 6638 is southeast and southwest of the clusters respectively. [20]

Other deep sky objects Edit

In 1999 a violent outburst at V4641 Sgr was thought to have revealed the location of the closest known black hole to Earth, [22] but later investigation increased its estimated distance by a factor of 15. [23] The complex radio source Sagittarius A is also in Sagittarius, near its western boundary with Ophiuchus. Astronomers believe that one of its components, known as Sagittarius A*, is associated with a supermassive black hole at the center of the galaxy, with a mass of 2.6 million solar masses. [24] Although not visible to the eye, Sagittarius A* is located off the top of the spout of the Teapot asterism. [1] The Sagittarius Dwarf Elliptical Galaxy is located just outside the Milky Way.

Baade's Window is an area with very little obscuring dust that shows objects closer to the Milky Way's center than would normally be visible. NGC 6522, magnitude 8.6, and NGC 6528, magnitude 9.5, are both globular clusters visible through Baade's Window. 20,000 and 24,000 light-years from Earth, with Shapley classes of VI and V respectively, both are moderately concentrated at their cores. NGC 6528 is closer to the galactic core at an approximate distance of 2,000 light-years. [25]

2MASS-GC02, also known as Hurt 2, is a globular cluster at a distance of about 16 thousand light-years from Earth. It was discovered in 2000 by Joselino Vasquez, and confirmed by a team of astronomers under the leadership of R. J. Hurt at 2MASS. [26]

The space probe New Horizons is moving on a trajectory out of the Solar System as of 2016 that places the probe in front of Sagittarius as seen from the Earth. [27] New Horizons will exhaust its radioisotope thermoelectric generator long before it reaches any other stars.

The Wow! signal was a strong narrowband radio signal that appeared to have come from the direction of Sagittarius.

The Babylonians identified Sagittarius as the god Nergal, a centaur-like creature firing an arrow from a bow. [28] It is generally depicted with wings, with two heads, one panther head and one human head, as well as a scorpion's stinger raised above its more conventional horse's tail. The Sumerian name Pabilsag is composed of two elements – Pabil, meaning 'elder paternal kinsman' and Sag, meaning 'chief, head'. The name may thus be translated as the 'Forefather' or 'Chief Ancestor'. [29] The figure is reminiscent of modern depictions of Sagittarius.

Greek mythology Edit

In Greek mythology, Sagittarius is usually identified as a centaur: half human, half horse. However, perhaps due to the Greeks' adoption of the Sumerian constellation, some confusion surrounds the identity of the archer. [5] Some identify Sagittarius as the centaur Chiron, the son of Philyra and Cronus, who was said to have changed himself into a horse to escape his jealous wife, Rhea, and tutor to Jason. As there are two centaurs in the sky, some identify Chiron with the other constellation, known as Centaurus. [5] Or, as an alternative tradition holds, that Chiron devised the constellations Sagittarius and Centaurus to help guide the Argonauts in their quest for the Golden Fleece. [30]

A competing mythological tradition, as espoused by Eratosthenes, identified the Archer not as a centaur but as the satyr Crotus, son of Pan, who Greeks credited with the invention of archery. [5] [31] According to myth, Crotus often went hunting on horseback and lived among the Muses, who requested that Zeus place him in the sky, where he is seen demonstrating archery. [5]

The arrow of this constellation points towards the star Antares, the "heart of the scorpion", and Sagittarius stands poised to attack should Scorpius ever attack the nearby Hercules, or to avenge Scorpius's slaying of Orion. [32]

Terebellum Edit

On the west side of the constellation, Ptolemy also described the asterism Terebellum consisting of four 4th magnitude stars, including the closest and fastest moving member, omega Sagittarii. [33]

Astrology Edit

As of 2002 [update] , the Sun appears in the constellation Sagittarius from 18 December to 18 January. In tropical astrology, the Sun is considered to be in the sign Sagittarius from 22 November to 21 December, and in sidereal astrology, from 16 December to 14 January. [ citation needed ]

Research Box Title


The object NGC 7027 is a star entering the final stages of life. It is going through spectacular death throes as it evolves into what astronomers call a "planetary nebula." A nebula is a visibly diffuse region composed of gas and dust. The term planetary nebula came about not because of any real association with planets, but because in early telescopes objects of this type often appeared planet-like to the astronomer – even glowing with a bright green color. We now know that green color to be from very hot oxygen atoms in the gas surrounding the central star.

After a star has depleted the majority of its nuclear fuel, profound changes occur as it enters a poorly understood phase of evolution. First, a combination of stellar pulsations and radiation pressure drives the atmosphere outward, forming an extended envelope around the star. The envelope can be so large that, if such a star were our Sun, the gas and dust expelled from the star might extend many times farther out than the average distance of the planet Pluto to the Sun (this will in fact be the Sun's fate). During this period the star loses material at very high rates a star several times the mass of the Sun might shed an amount equal to the total mass of the Sun in less than 10,000 years. The wind that propels the envelope has speeds in excess of 43,000 miles per hour.

The gas in the circumstellar envelope is mostly made up of simple molecules such as molecular hydrogen and carbon monoxide, combined with several other gases such as cyanide, sodium chloride, and possibly water vapor. Complex hydrocarbon molecules are also known to be present in circumstellar envelopes. Most importantly, the material cast off during this phase of stellar evolution includes a large abundance of the key elements to the origin of terrestrial life – carbon, nitrogen, and oxygen. These elements are created through nuclear fusion of hydrogen and helium in the stellar core.

While the envelope is being formed and ejected into the interstellar medium, the central star of the young planetary nebula continues to evolve. Its surface heats to temperatures in excess of 360,000 degrees F. The increase in ultraviolet radiation as the star heats first dissociates the molecules in the envelope and then ionizes the constituent atoms. This transition phase is very short – perhaps less than 1000 years. We have caught NGC 7027 at a very important time for study – in the middle of this transformation. It has a hot central star surrounded by an ionized region of gas external to which is the remnant stellar envelope made up of molecules and microscopic dust particles. The molecules in the envelope of NGC 7027 are being destroyed into their constituent atoms and ions. This is an object that will survive in its present state for only the blink of an eye (in cosmological time).


In February 1997, following a spectacular nighttime launch, astronauts aboard STS-82, the 22nd mission of the Space Shuttle Discovery, installed a new set of instruments in the Hubble Space Telescope. One of those instruments, the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) is designed to observe at wavelengths outside our normal view – the infrared – giving HST new eyes on the Universe.

The composite color image of NGC 7027 is among the first data of a planetary nebula taken with NICMOS. This image is new and unique, because it probes wavelengths of light we cannot see directly with our eyes with clarity never before achieved. The image is not exactly what it might at first appear – a color photograph. It is in fact a "pseudo" color image of how the object appears at wavelengths outside our normal view. This picture is actually three separate images taken at different wavelengths (1.10, 2.12, and 2.15 microns). The shorter wavelength is coded to blue and the longest to red and green. The red image is most sensitive to emission from one type of molecule, the most abundant one in the Universe – molecular hydrogen. Until NICMOS was installed, HST could not see emission from this important molecule. When combined with earlier HST/WFPC2 data of NGC 7027, a much more complete understanding can be obtained.

These NICMOS data reveal a wealth of new information. The often difficult to see central star of this object is clearly seen here in the near infrared. Surrounding it is an elongated apparently ring-shaped region of gas and dust cast off by the star. This region is highly ionized – atoms ripped apart into nuclei and electrons by radiation from the hot central star. This gas (appearing as white, and off white) has a temperature of several tens of thousands of degrees. We find that the object has two "cones" or "wings" of emission from molecular hydrogen (the red material) stimulated to glow in the infrared by ultraviolet light from the star – a process known as fluorescence. Molecular hydrogen glows in this way where it is being violently split into separate hydrogen atoms by the stellar ultraviolet radiation. It appears as it does in this image, because the cones are highly inclined to our line of sight. These "cones" end with rings of bright molecular hydrogen emission. In images taken at wavelengths where molecular hydrogen does not emit, the closest ring to the Earth can be seen as a dark band across the center of the nebula. This is caused by a large abundance of dust in this region, resulting in the light from behind to be attenuated along our line of sight. Outside of the of bright regions seen in these data is not just empty space, but is actually where most of the material shed by the star remains. It is invisible in this image because it is still shielded from the radiation of the star by material interior to it – obscured like trees in a forest.

Another interesting feature of these data is the appearance of a disturbance that might be caused by an as yet unseen jet of material that is inclined from the main axis of the object. If correct, the jet appears to extend symmetrically from the upper left to lower right of the image. In other planetary nebulae, similar types of jets can have speeds in excess of 150 miles per second.

From even the best ground-based telescopes, NGC 7027 looks fuzzy. This is in part because of the distorting effects of the Earth's atmosphere. It is also very small in appearance from the Earth. Spanning only about 15 seconds of arc on the sky, it is one of the smallest objects of its kind to be imaged by HST. Seeing the structures clearly as we do in this image is like seeing the face of Roosevelt on a dime at a distance of twenty miles. The actual size of the nebula is approximately 14,000 times the average distance between the Earth and Sun (or 14,000 times 93 million miles). HST and NICMOS have made it possible for us to see clearly for the first time the true nature of NGC 7027.

NICMOS has allowed astronomers to see clearly the interface from hot, glowing atomic gas to cold molecular gas. The origin of the newly seen filamentary structures is not well understood. The interface region is the pink and red colored cool molecular hydrogen gas. While WFPC2 is best used to study the hot, glowing gas, which is the bright, oval-shaped region surrounding the central star, NICMOS allows us to see deeper into the nebula and view material that cannot be seen at visible wavelengths. The material beyond this core is illuminated by light from the central star reflecting off dust in the cold gas surrounding the nebula. Combining exposures from the two cameras allows astronomers to clearly see the way the nebula is being shaped by winds and radiation.

It is data of this type that will aid astronomers to understand the complexities of stellar evolution – both early and late stages. An understanding of the physical and chemical processes that are taking place in the important transition zone viewed by NICMOS has importance to many other areas of astronomy, including a better understanding of the processes that take place in the regions surrounding new born stars.

These data were acquired as part of a HST/NICMOS study of compact planetary nebulae and proto-planetary nebulae (Objects with central stars still too cool for it to ionize the core region.). NGC 7027 is located about 3,000 light-years from the Sun in the direction of the constellation Cygnus the Swan.


Stars do not live forever. The Sun, for example, was born approximately four and half billion years ago, and is expected to have another 5 billion years left in its life. What is the eventual fate of the Sun, and stars like it? It is a popular myth that stars explode when they die, and violent events such as supernova explosions were thought to be the norm. Dr. Sun Kwok of the University of Calgary in Canada has promoted the idea that most stars do not explode, but undergo a quiescent death. The majority of stars, in fact over 95%, will go quietly – in other words "not with a bang but a whimper". This theory has been confirmed by observations from space, including observations from the Hubble Space Telescope.

During the last ten thousand years of a star's life, it goes through a glorious stage called "planetary nebula" phase, during which the star gives off a magnificent display of light (see recent releases of HST images of planetary nebulae). In order to better understand the cause of death of stars, we have to look back a few hundred years before the planetary nebulae phase, and examine the state of stars before they undergo this transformation. Over the past decade, Dr. Sun Kwok and Dr. Hrivnak have been searching for such stars, the very old stars which are just beginning to get "sick". Now two such "patients" have been imaged with the HST. The high quality HST pictures have provided much needed information for the diagnosis of the cause of death of stars.

The accompanying pictures are HST WFPC2 images of two "proto-planetary nebulae": the "Cotton Candy Nebula" (IRAS 17150-3224) and the "Silkworm Nebula" (IRAS 17441-2411). In both cases, we can see a series of concentric rings, representing the "puffs" given off by the star in the last few thousands years of their lives. These "puffs" provide evidence that the star has been "ill" for some time. After a number of these "puffs", the star wraps itself inside a cocoon. In these two HST images, we are seeing the first indication that the nebulae are emerging from their cocoons, like butterflies undergoing metamorphosis.

These observations were made with the HST WFPC2 camera in cycle 6 under GO program 6565 (Kwok et al.: Imaging of Proto-Planetary Nebulae". We have also made use of data in the public archive obtained under the snapshot program 6364 (Bobrowsky et al.: Snapshot survey of protoplanetary nebulae and AGB stars). These results will be published in an upcoming issue of Astrophysical Journal Letters.

Dr. Sun Kwok has published extensively in the field of planetary nebulae and is best known for his theory of planetary nebulae formation. He is the current chairman of the International Astronomical Union Working Group on Planetary Nebulae.


NGC 6818 is in the constellation Sagittarius at a distance of about 6000 light-years. It has a diameter of about 0.5 light-year. The Hubble telescope observation was taken March 10, 1997 by the Wide Field and Planetary Camera 2.

This is a composite of images taken in 3 filters: Halpha is red, Hbeta is blue, and [O III] (doubly ionized oxygen) 5007 is green. As a result of the longer exposure time at Hbeta, the central star of the planetary nebula appears blue. We see a roughly spherical outer envelope as well as a brighter vase-shaped interior "bubble". Astronomers believe that a fast wind from the hot central star is creating the elongated shape and in fact has caused a "blowout" at the two ends of the major axis (lower right and upper left).

This nebula looks like a twin of NGC 3918, another planetary nebula (PN) that has been imaged by the Hubble telescope. The structure of NGC 3918 is remarkably similar to that of NGC 6818 – it has an outer spherical envelope, an inner brighter elongated bubble, and it also shows a blowout orifice at one end of the major axis in the bottom right-hand corner. By finding and studying such similar objects, it is hoped that crucial details can be learned about the evolutionary life history of planetary nebulae. One could call this "comparative PNology."

Credits:W. Latter (SIRTF Science Center/IPAC/Caltech), S. Kwok (U. Calgary), R. Rubin (NASA/Ames), H. Bond (STScI)

New method to estimate more accurate distances between planetary nebulae and the Earth

A way of estimating more accurate distances to the thousands of so-called "planetary nebulae" dispersed across our Galaxy has just been announced by a team of three astronomers based at the University of Hong Kong (HKU). Dr David Frew, Professor Quentin Parker and Dr Ivan Bojicic, based on a culmination of ten years of research work, developed a new method for measuring more accurate distances between "planetary nebulae" and the Earth. With this technique, "planetary nebulae" finally get a more meaningful physical presence. The scientists have published their results in the Monthly Notices of the Royal Astronomical Society on November 18.

Ghostly and beautiful planetary nebulae" have nothing to do with planets but acquired this name because these glowing spheres of ionized gas resembled planets to early observers. They are the colourful, ejected shrouds of dying stars, which offer a brief window into the history of many stars' lives, including that of our Sun. Their stunning shapes make them popular with the wider public, but a full exploitation of their scientific potential has been blunted by poor distance determinations for them.

The solution, presented by these authors, is both simple and elegant. More accurate distances between the most common type of "planetary nebulae" and the Earth can be estimated simply with three sets of data: firstly, the size of the object on the sky taken from the latest high resolution surveys secondly, an accurate measurement of how bright the object is in the red hydrogen-alpha emission line and thirdly, an estimate of the dimming toward the nebula caused by so called interstellar-reddening. The resulting so-called "surface brightness -- radius relation' has been robustly calibrated using more than 300 planetary nebulae whose accurate distances have been determined via independent and reliable means (e.g. trigonometric parallax measurements of their central stars).

The first author of this research Dr David Frew, Research Assistant Professor, Department of Physics said: 'measuring distances to Galactic "planetary nebulae" has been an intractable problem for many decades, because of the extremely diverse nature of both the nebulae themselves and their central stars. However, understanding their true nature and physical characteristics depends crucially on knowing their distance. With our significantly improved distance estimates we can finally provide more meaningful values for many key parameters of scientific interest.'

The second author of this research Professor Parker, who is also the Head of the Physics Department in HKU, explained: 'the basic technique is not new but what marks out this work from what has gone before is the use of the most up-to-date and reliable measurements of all of those crucial properties'. This is combined with the use of the authors' own robust techniques to effectively remove "doppelgangers" and mimics that have seriously contaminated previous planetary nebulae catalogues, which added considerable scatter to previous statistical distance scales.

Incredibly, the new distance scale works over a factor of more than six powers of ten in surface brightness. The technique can provide distances accurate to 20 percent, which a major advance on previous estimators that can have errors of a factor of two or more. "In the past, the old distance scales worked fairly well for small planetary nebulae but got systematically worse for the larger nebulae. Ours is the first scale to be able to estimate distances for all planetary nebulae. As big planetary nebulae are the most common, we will use our new scale in making an unbiased census of planetary nebulae in the Milky Way, which will then help answer some important research questions." Dr Frew added.

This latest research by HKU astronomers promises a new era in our ability to study and understand this fascinating if brief period in the final stages of the lives of low- and mid-mass stars.

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