# Hottest Possible Hydrogen-Fusing Stars

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I guess this is more a question about stellar models than anything else. I was wondering what is predicted to be the hottest possible stars that would still be hydrogen-burning.

What complicates this more is that the most massive stars will probably only sit close to the main sequence (as very early O-type) for several thousand years before gaining a WNh spectrum despite still being relatively early in the lifetime. So I'll be somewhat more precise and ask what is the hottest possible star predicted at ZAMS that won't just blow itself apart by radiation pressure.

Also tangential to the topic but have there been any stars found with an O1 spectrum, and is this even a spectral type that has models made for it?

An answer to your question is contained within What is the largest hydrogen-burning star? The hottest observed main sequence stars are of type O3V, with photospheric temperatures of about 50,000 K.

However, it is indeed possible that hotter main sequence stars may exist in the present-day universe, but have simply evolved into Wolf-Rayet stars (and lost a lot of mass). Indeed, Crowther et al. (2010) claim to see evidence for such objects in the cluster R136 in the Large Magellanic Cloud.

How hot could such objects be? A theoretical study by Bromm et al. (2001) suggests that very massive, metal-rich present-day stars might reach 65,000 K at a few hundred solar masses (see their Fig.1).

The hottest zero age main sequence stars ever would probably be very massive ($$sim 1000M_{odot}$$) ultra-metal poor, or even metal-free population III stars. These actually begin life as He burning stars until they produce enough carbon to commence the CNO H-burning cycle. Equation 6 and Fig.1 in Bromm et al. (2001), who provide a theoretical study of such objects, suggests effective temperatures of about $$1.1 imes 10^{5}$$ K for such stars, with very little detailed dependence on mass and metallicity.

I guess such a star might be classified as O1, but according to Bromm et al. they are basically blackbodies once the mass exceeds about 300 solar masses.

Another key point to bear in mind is that if one is asking for the "hottest" star, then presumably it is "effective temperature" that is referenced. Effective temperature is related to the luminosity L and the radius R from which the light emerges (not the static surface of the star, R can be well out in a dense stellar wind) by the Stefan-Boltzmann law, which says T_eff ~ L^(1/4) / R^(1/2). Normally for main-sequence stars, whose R rises much more slowly than L, that means the T_eff is higher at higher L. But really high L stars have dense radiatively-driven winds, and eventually the wind could get so dense that the mass-loss rate from the star might be proportional to L. If so, the radius R from which the light emerges (where the optical depth is near unity) is also proportional to L, so then T_eff ~ L^(1/4) / L^(1/2) ~ L^(-1/4).

This says we should expect T_eff to reach a peak and then drop if L is further increased. The peak T_eff should be when the winds are getting so dense that the mass-loss rate becomes proportional to L (when a significant amount of L is being used to lift the mass), and that is more or less also where the WNh spectrum replaces the O spectrum (something like what is trying to be the O1 spectrum). So the kinds of T_eff already referred to, above 50,000 K or so, is going to be the highest T_eff possible for a core hydrogen burning star. After that, even higher L will simply mean that you see a much larger star with a lower T_eff.

Incidentally, this probably points to a flaw in the Bromm et al. paper cited above. That paper is really about Pop III stars, so zero metallicity and not much in the way of stellar winds also. So when they talk about T_eff, they appear to use an expression that involves L and the static radius, but that's not the T_eff that matters for observing stars, or for their UV flux. The latter should use the R from which the light emerges, which can be in the wind. The distinction is not important for the pop III conclusions of their paper, but it does matter for the Pop I results shown in Figure 1. That is probably not the correct T_eff, so I doubt T_eff really can get up to 65,000 K for Pop I stars.

## Bellatrix is Orion’s 3rd-brightest star

View at EarthSky Community Photos. | Victor C. Rogus in Sedona, Arizona, captured this telescopic view of the star Bellatrix on November 15, 2020. Many eyes were turned toward this star then because Comet C/2020 M3 (Atlas) was near it and just past its closest to Earth. Thank you, Victor.

Bluish-white Bellatrix – aka Gamma Orionis – is the third-brightest star in the easy-to-recognize constellation Orion the Hunter. This star marks the left shoulder of Orion a brighter star, reddish Betelgeuse, marks the Hunter’s right shoulder. Another brighter star, Rigel, marks the Hunter’s left foot.

In the month of November, Orion is just coming back to a convenient viewing time in the evening sky. You’ll find it in your sky between your local sunset and your local midnight. Try Stellarium to find out exactly when Orion rises over your eastern horizon.

The constellation Orion is easy to spot. Three medium-bright stars in a short, straight row – known as Orion’s Belt – marks its mid-section. Betelgeuse and Rigel are Orion’s brightest stars, and Bellatrix is 3rd-brightest. A print of the copperplate engraving for Johann Bayer’s Uranometria (1661) showing the constellation Orion. Image via U.S. Naval Observatory Library/ Wikimedia Commons.

Bellatrix in lore and culture

In popular culture today, many know the name Bellatrix as J.K. Rowling’s dark witch, Bellatrix Lestrange, in the Harry Potter fantasy book and film series. Rowling frequently employed names from astronomy for important characters in her series: Sirius, Andromeda, Cygnus, Draco and Luna, among others.

In the history of the sky, Bellatrix comes from Latin and means female warrior. Bellatrix is also sometimes called the Amazon Star.

According to Jim Kaler, before Orion’s bluish-white shoulder star carried the name Bellatrix, that name belonged to a different star. The golden star Capella carried the name Bellatrix in medieval times.

Star chart showing the constellation Orion the Hunter and its brightest stars, via IAU/ Roger Sinnott & Rick Fienberg, Sky & Telescope/ Wikimedia Commons.

The brightness of Bellatrix, and surrounding nebulosity

Rigel and Betelgeuse are generally considered the sky’s eighth and 11th brightest stars, with magnitudes (brightnesses) of 0.13 and 0.50 respectively. Bellatrix is the sky’s 26th brightest star, approximately (estimates vary depending on the source), and its magnitude vacillates between 1.59 and 1.64.

While Bellatrix has an apparent magnitude of 1.64, it has an absolute magnitude of -3.3. Apparent magnitude is a measure of how bright things look to an observer, or how bright it “appears.” Absolute magnitude is a star’s intrinsic brightness. All stars’ absolute magnitudes are measurements of how bright they would be if they were 10 parsecs (32 light-years) away from Earth. Absolute magnitude is not a term observers often use it’s more used in the scientific community when making a comparison between stars. Consider the star Saiph, representing the knee opposite Bellatrix’s shoulder star in Orion. Saiph “appears” dimmer than Bellatrix, and its apparent magnitude is 2.05. But Saiph’s absolute magnitude is -6.8! You can probably guess that this means Saiph is much farther away from us than Bellatrix, and you’d be right. Bellatrix is 303 light-years from Earth while Saiph is 1,826 light-years away!

Bellatrix appears to have a white or bluish-white hue. When viewed through binoculars or a telescope, you might pick up a gaseous glow surrounding it.

The glow does not originate with Bellatrix itself but with its location within the great Orion Molecular Cloud complex, a star-forming region in the same direction in space as the constellation Orion. In Burnham’s Celestial Handbook, Volume Two, Robert Burnham wrote:

A faint diffuse nebulosity surrounding [Bellatrix], shown on the Skalnate Pleso Atlas, is merely an illuminated portion of the general nebulous haze which envelops much of Orion, and which naturally becomes visible in the vicinity of any highly luminous star.

Similar masses of nebulous haze are scattered across the entire region to the south of Bellatrix, culminating in the splendor of the Great Nebula itself.

A long-exposure telescope image of the Orion Molecular Cloud complex, a star-forming region. This region permeates the sky in the direction of Orion. See Orion’s 3 Belt stars, and Betelgeuse, Rigel and Bellatrix? This photo – taken by Rogelio Bernal Andreo in October 2010 – appeared as the Astronomy Picture of the Day on October 23, 2010. Image via Wikimedia Commons.

Science of Bellatrix

Bellatrix is in spectral class B2 III. B stars are on the brighter, hotter and whiter end of the spectrum, and the number 2 means it is an early type compared to a larger number. The III denotes that it is a giant star, although Jim Kaler of the University of Illinois has said that Bellatrix in not an actual giant, but just has the spectral signature of a giant. He asserts that Bellatrix is probably a hydrogen-fusing dwarf, still on its way to the true giant stage.

At nine times the mass of our sun, Bellatrix could be a supernova candidate, but – if that doesn’t occur – it will become a massive white dwarf. Bellatrix may have a companion in Gamma Orionis B, a red dwarf that for the past 100-plus years has maintained a constant separation of 178 arcseconds. But Bellatrix’s possible companion star is not close enough for its matter to flow into Bellatrix and trigger a supernova.

Bellatrix is one of the hottest stars that you can see without optical aid. It has a temperature of 21,750 Kelvin (38,690 F). Compare that to our sun, which is a mere 5,778 Kelvin (about 10,000 F).

Bellatrix may be six times the size of our sun.

Skywatchers know Orion as a hallmark of the winter sky, at least in the Northern Hemisphere. The celestial equator cuts across Orion’s waist, with Bellatrix and the Hunter’s upper body on one side and his knees/legs/feet on the other. Orion can therefore be seen in both the Northern and Southern Hemispheres from around October through April. The image of Orion that Northern Hemisphere inhabitants are familiar with would look upside down to them if they viewed the constellation from south of the equator.

In the Southern Hemisphere, Orion is standing on his head. Use Stellarium to learn how you see Orion.

Orion hovers above ESO’s Paranal Observatory in Chile in this photo from March 2020. Orion is upside down in the Southern Hemisphere. The bright orangish star is Betelgeuse, with the other shoulder star to the lower left being Bellatrix. Image via ESO/ Y. Beletsky.

Bottom line: Bellatrix is the third-brightest star in Orion. The hot bluish-white shoulder star has a name that means “female warrior.”

## 10 Most Extreme Stars in the Universe

Image Credit: Comparison of UY Scuti to other stars (Sun, Sirius, Pollux, Arcturus) by S. Clester

Our Sun is often described as an “average yellow dwarf star”, but the truth of the matter is that there are no average stars. Stellar bodies come in a bewildering variety of masses, luminosities, radii, and temperatures within any given classification or type, from barely making it into any given classification to representing the most extreme example of their classification. While the items on this list represent these extremities, it must be remembered that complex issues exist around determining the “vital statistics” of stars.

For instance, to calculate a stars’ true diameter, its luminosity must be known to a high degree of accuracy. This is often very difficult to determine because many massive stars are frequently encased in dusty envelopes that absorb and/or scatter much of the star’s light. Similarly, determining a star’s mass depends largely on knowing (among many other things, such as its metallicity), its distance, which in some cases, is very difficult to determine or even impossible due to factors that are not clearly understood, as is the case with some stars in Orion’s Belt.

For these and other reasons, the stars on this list are merely the leading contenders for the title of “Most Extreme Star in My Category”, and issues like stellar variability over long periods, improved observational techniques, or other factors may unseat some of the stars on this list in the future. Nonetheless, the stars on this list represent current knowledge, so let us start with..

The Biggest Star – UY Scuti

Image: Rutherfurd Observatory/ Columbia University

The image above shows UY Scuti, a pulsating variable red supergiant star (the red spot to the right and above centre), which is located about 9,500 light years away in the constellation Scutum. In terms of sheer bulk, UY Scuti is the leading contender for the biggest star title, having a radius that is estimated to be 1,708 times that of the Sun, equivalent to 750 million miles, or 7.94 astronomical units, giving it a volume 5 billion times that of the Sun. If UY Scuti were placed over the Sun, its photosphere would extend past the orbit of Jupiter.

Although there is a margin of error of about 190 solar radii in determining the true diameter of UY Scuti, it would still take a hypothetical space ship moving at the speed of light (c) eight hours to travel around the star at its maximum diameter. By way of comparison, a space ship moving at the speed of light needs only 14.5 seconds to travel once around the Sun.

The Most Massive Star – RMC 136a1

Image: ESO/VLT

The bright light at the center of this near infrared image is the star RMC 136a1, situated in R163, the innermost region of the large open cluster NGC 2070 (in the Tarantula Nebula) in the Large Magellanic Cloud. It is located about 163,000 light years away in the constellation of Dorado.

RMC 136a1 weighs in at a whopping 315 solar masses, but at only about 800,000 years old it is still on the main sequence fusing hydrogen into helium. However, the star is undergoing extreme mass loss through an energetic solar wind that is blowing material away at a speed of about 2,600 km/sec, which is stronger than the gravitational forces that holds the star together.

At its current stage of evolution, R136a1 is losing mass at the rate of 5.1 × 10 −5 M (3.21 × 10 18 kg/s) per year, which translates into about one billion times higher than the Sun is losing its mass. At this rate, R136a1 would have lost about 50 solar masses worth of material since its formation.

The Brightest Star – R136a1

During 2010, investigators discovered that R136a1 is also the most luminous star known. While previous estimates put the stars’ brightness at about 1.5 million solar luminosities, R136a1 is now known to shine with at least 8.7 million solar luminosities, which means that R136a1 emits as much energy in five seconds, than the Sun emits in a full year.

To put this in perspective- if R136a1 were to replace the Sun in the solar system, we would see it at magnitude -39, which is at least 94,000 times brighter than the Sun it would have replaced. In addition, from a distance of 10 parsecs away, its absolute magnitude would be -7.6, which is more than three times brighter than Venus ever gets when viewed from Earth.

The Smallest True Star – EBLM J0555-57Ab

EBLM J0555-57Ab s the smallest star in a triple-star system that is located about 600 light years away in the constellation Pictor. As the smallest star in the system, EBLM J0555-57Ab has a mass of about 85 times that of Jupiter (about 0.018% of that of the Sun), and a radius that compares to that of Saturn. With these numbers, EBLM J0555-57Ab represents the lower mass limit at which true stars can initiate and sustain a process of hydrogen fusion, according to current models of star formation.

The other two stars in the system are EBLM J0555-57Aa, a F-type star, and EBLM J0555-57B, a magnitude 10.76 star about which not much is known. Although no orbital motion has been detected in the system, all there stars share a common proper motion, which suggests that they are gravitationally bound.

The Hottest Star – WR 102

Image: NASA/ESA/Hubble

With a surface temperature of 210,000K compared to 6,000K for the Sun, WR 102 (shown here illuminating the nebulosity that surrounds it) situated 9,800 light years away in the constellation Sagittarius is the hottest known star. It is also an oxygen-sequence Wolf-Rayet star, which shed their surplus mass extremely quickly, and are extremely rare, with only a few found per galaxy and four known in our Milky Way.

Its luminosity is estimated to be about 500,000 times that of the Sun, and while it has a diameter of less than half of that of the Sun, it is at least 20 times more massive. Based on its spectrum, WR 102 has reached, or is close to the end of its helium burning phase, and it is expected to self-destruct in a supernova explosion sometime in the next 1,500 years or so.

The Fastest Moving Star – HE 0437-5439

Located about 200,000 light years away in the constellation Dorado, HE 0437-5439 is a massive, B-type main sequence star with an estimated age of about 30 million years. HE 0437-5439 appears to be receding from Earth at the break-neck speed of 723 km/sec (449 miles/s), or 2.6 million km/hour. At this extremely high-speed, the star is moving beyond the escape velocity of the Milky Way galaxy, and will eventually move into intergalactic space. By way of comparison, most other stars travel at a more sedate pace of about 100 km/sec or so.

Note, though, that there exists another contender for the title of fastest moving star. The star designated RX J0822-4300 (now moving away from the Puppis A supernova remnant) has been measured to move at 1,500 km/s, or 5,400,000 km/h (3 million miles/hour). However, since no clear mechanism has been found to explain the high recessional speed of RX J0822-4300, its velocity has been revised downward to more modest 672±115 km/s, which is not quite so difficult to explain theoretically. In both cases, it is possible for stars to attain such high speeds when they are ejected from binary systems, or when they pass around massive black holes that fail to capture them.

The Fastest Rotating True Star – VFTS 102

While most neutron stars are known to have very high spin rates, with PSR J1748-2446ad known to spin at 716 times a second, or almost one-quarter the speed of light, neutron stars are not normal stars and therefore do not qualify for a place on this list.

The fastest spinning star in which fusion processes are taking place is VFTS 102, a 25-solar mass star located about 160,000 light years away in the Tarantula Nebula in the Large Magellanic Cloud in the constellation Dorado.. This star is spinning at roughly 2 million km/sec at its equator, which is about 300 times faster than the Sun, with this rate of rotation thought to represent the highest theoretical limit at which gaseous stars can spin before they break apart.

The Reddest Naked-eye Stars – Mu Cephei / CE Tauri

Image: Greg Parker

The composite image above shows Herschel’s Garnet Star (Mu Cephei) in the constellation Cepheus , and the Ruby Star (CE Tauri) in the constellation Taurus in the top and bottom frames of the panel, respectively. While there are brighter red stars than these two, such as Betelgeuse, Arcturus and Aldebaran, there are no other naked-eye stars known that are redder.

It’s all in the numbers Mu Cephei has a B-V index of +2.35, and an apparent magnitude of 4.08, which makes it both brighter and redder than CE Tauri. CE Tauri on the other hand, has a B-V index of +2.07, which makes it redder than Betelgeuse, Arcturus, and Aldebaran, since their B-V indexes are 1.85, 1.34, and 1.78, respectively. Note that the higher the positive value of the B-V index, the redder the star is.

The Oldest Star in the Milky Way – HE 1523-0901

The image above is an artists’ impression of “Methuselah”, the oldest known star in the Milky Way, located about 7,500 light years away in the constellation Libra. Methuselah is visible in small telescopes, but note that while it is best seen from the southern hemisphere, it can be seen up to mid-European latitudes, too.

Methuselah is an extremely metal-poor, 0.8-solar mass star that was found among other metal-poor stars in the galaxy’s extended halo, and is thought to have formed directly from the remains of a previous generation of stars, or Population I stars. Using the ESO’s Very Large Telescope, the star’s age was determined to be about 13.2 billion years, which is nearly as old as the Universe itself.

However, while the margin of error in the method used to determine its age (mainly through measuring the rate of decay of the elements uranium and thorium it the star) is between 0.7 and 2.7 billion years, depending upon the assumptions at play in determining the uncertainty, this figure reduces to about 800,000 years when the same method is used to determine Methuselah’s age relative to similarly ancient stars.

Note that since the uncertainties regarding the relative age of the Universe and Methuselah are both very large, the star is not older than the Universe that contains it. It is just nearly as ancient as the Universe.

The Most Distant Star – SDSS J122952.66+112227.8

Image: NASA

The image above shows the galaxy IC 3418 that contains the most distant star known called SDSS J122952.66+112227.8, located about 55 million light years away in the Virgo constellation. The galaxy is well-known for its long tidal tail that represents the tug-of-war between the galaxy and the Virgo Super Cluster of Galaxies, which IC 3418 had run headlong into about 55 million years ago.

The star SDSS J122952.66+112227.8 is an O-type, blue supergiant that is illuminating a clump of gas being stripped from the main galaxy, and as such, it is the furthest known star to be definitively resolved by means other than its effects on its surroundings. While many stars are known at greater distances from Earth, these stars can only be identified through the events they cause, such as supernova events and gamma ray bursts.

## Stellar Evolution

A star is born, lives, and dies, much like everything else in nature. Using observations of stars in all phases of their lives, astronomers have constructed a lifecycle that all stars appear to go through. The fate and life of a star depends primarily on it's mass.

Hubble image of the Eagle Nebula, a stellar nursery. (Credit: NASA/ESA/Hubble Heritage Team)

All stars begin their lives from the collapse of material in a giant molecular cloud. These clouds are clouds that form between the stars and consist primarily of molecular gas and dust. Turbulence within the cloud causes knots to form which can then collapse under it's own gravitational attraction. As the knot collapses, the material at the center begins to heat up. That hot core is called a protostar and will eventually become a star.

The cloud doesn't collapse into just one large star, but different knots of material will each become it's own protostar. This is why these clouds of material are often called stellar nuseries – they are places where many stars form.

As the protostar gains mass, its core gets hotter and more dense. At some point, it will be hot enough and dense enough for hydrogen to start fusing into helium. It needs to be 15 million Kelvin in the core for fusion to begin. When the protostar starts fusing hydrogen, it enters the "main sequence" phase of its life.

Stars on the main sequence are those that are fusing hydrogen into helium in their cores. The radiation and heat from this reaction keep the force of gravity from collapsing the star during this phase of the star's life. This is also the longest phase of a star's life. Our sun will spend about 10 billion years on the main sequence. However, a more massive star uses its fuel faster, and may only be on the main sequence for millions of years.

Eventually the core of the star runs out of hydrogen. When that happens, the star can no longer hold up against gravity. Its inner layers start to collapse, which squishes the core, increasing the pressure and temperature in the core of the star. While the core collapses, the outer layers of material in the star to expand outward. The star expands to larger than it has ever been – a few hundred times bigger! At this point the star is called a red giant.

What happens next depends on how the mass of the star.

### The Fate of Medium-Sized Stars

Hubble image of planetary nebula IC 418, also known as the Spirograph Nebula. (Credit: NASA/Hubble Heritage Team)

When a medium-sized star (up to about 7 times the mass of the Sun) reaches the red giant phase of its life, the core will have enough heat and pressure to cause helium to fuse into carbon, giving the core a brief reprieve from its collapse.

Once the helium in the core is gone, the star will shed most of its mass, forming a cloud of material called a planetary nebula. The core of the star will cool and shrink, leaving behind a small, hot ball called a white dwarf. A white dwarf doesn't collapse against gravity because of the pressure of electrons repelling each other in its core.

### The Fate of Massive Stars

Chandra X-ray image of supernova remnant Cassiopeia A. The colors show different wavelengths of X-rays being emitted by the matter that has been ejected from the central star. In the center is a neutron star. (Credit: NASA/CSC/SAO)

A red giant star with more than 7 times the mass of the Sun is fated for a more spectacular ending.

These high-mass stars go through some of the same steps as the medium-mass stars. First, the outer layers swell out into a giant star, but even bigger, forming a red supergiant. Next, the core starts to shrink, becoming very hot and dense. Then, fusion of helium into carbon begins in the core. When the supply of helium runs out, the core will contract again, but since the core has more mass, it will become hot and dense enough to fuse carbon into neon. In fact, when the supply of carbon is used up, other fusion reactions occur, until the core is filled with iron atoms.

Up to this point, the fusion reactions put out energy, allowing the star to fight gravity. However, fusing iron requires an input of energy, rather than producing excess energy. With a core full of iron, the star will lose the fight against gravity.

The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive force between the positively-charged nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in an explosive shock wave. In one of the most spectacular events in the Universe, the shock propels the material away from the star in a tremendous explosion called a supernova. The material spews off into interstellar space.

About 75% of the mass of the star is ejected into space in the supernova. The fate of the left-over core depends on its mass. If the left-over core is about 1.4 to 5 times the mass of our Sun, it will collapse into a neutron star. If the core is larger, it will collapse into a black hole. To turn into a neutron star, a star must start with about 7 to 20 times the mass of the Sun before the supernova. Only stars with more than 20 times the mass of the Sun will become black holes.

## Hottest Possible Hydrogen-Fusing Stars - Astronomy

In any case, have a look at http:/ /ast.star.rl.ac.uk/hr.html. This page illustrates the "Hertzsprung-Russell Diagram," a graphical plot on which astronomers plot the characteristics of stars. In the HR diagram, total brightness (known as "Luminosity" or "Absolute Magnitude") is plotted against the surface temperature of the star. As the star goes through its life cycle, it moves along the HR diagram from one place to another. The page I sent you shows tracks for stars and "failed stars" like Jupiter.

The star begins as a cool cloud of gas, well to the right (cold side) of the diagram. Under its own gravity, the cloud begins to collapse. As it collapses, it releases its gravitational energy as radiation. So a young star is cool but bright. We usually don't see these proto-stars, however, because they are generally shrouded in dust and only give off light in the far infrared part of the spectrum.

The proto-star continues to collapse, and as it does, it gets hotter and hotter, moving to the left in the HR diagram. "Stars" less than a percent of the Sun's mass eventually halt their collapse due to gas pressure. These stars are "brown dwarfs" or "giant planets," like Jupiter. They never ignite their hydrogen, and gradually dim away. These are the downward tracks you see in the diagram.

Stars larger than about 0.05 to 0.07 solar masses contract, getting hotter and hotter, until their cores are hot enough to begin burning hydrogen. When a star "turns on" its hydrogen-burning phase, we say that it has reached the main sequence, and is a true star, not a protostar, brown dwarf, or planet. The main sequence is a line of stars plotted on the HR-diagram, which represent stars which are burning hydrogen, fusing it into helium. The fusion process in the core releases heat and light, supporting the star against further gravitational collapse, and making it shine. Stars spend most of their life in one spot on the main sequence. Very massive stars live at the top of the main sequence, shining very blue and very bright, while low-mass stars are dim and red. The sun is between these two extremes.

Eventually, the hydrogen in the core whose fusion supports the star begins to run out. The core becomes mostly helium (the product of hydrogen fusion), and hydrogen burning moves out away from the core, forming a burning shell around the core. When this happens, the core begins to collapse again, but the outer regions of the star are pushed outwards. The star becomes brighter and cooler. This is the Red Giant stage. When the sun reaches the Red Giant stage, 5 billion years from now, it will likely grow to engulf Mercury, Venus, and the Earth.

If the star has little mass, it may end its life here, throwing off its outer layers, creating a planetary nebula out of its atmosphere, and a hot, dense "white dwarf" out of its core. The white dwarf shines only by residual left-over heat, and will eventually fade into a mere cinder. Cores of stars at least half as massive as the sun, however, will eventually collapse and heat enough to start burning helium in their cores. Once helium-burining starts, the star decends the giant branch again, reverses its swelling a bit, and lives happily fusing helium in its core, and hydrogen in a shell around the core.

Then the giant dance begins again. The core helium runs out, and the star once again becomes a red giant. Stars like the sun get off the bus here, and become planetary nebulae and white dwarfs. Heavier stars begin burning carbon. This happens over and over again with heavier and heavier elements. The star ascends and descends the giant branch many times. Lighter stars that don't get hot enough in their cores to burn the next element become planetary nebulae and white dwarfs. Stars that are heavy enough eventually heat their cores enough to begin the next stage of burning and descend the giant branch for a while. For each element, the process is quicker and quicker. A star might burn Hydrogen on the main sequence for billions of years, but once this process gets to, say, silicon, the star might burn silicon for only a few days.

A very massive star, more than 5-10 times solar mass, will ascend and descend the giant branch many times, until the star is ready to burn iron. But iron fusion doesn't release energy it sucks it up. So what happens is that the star is ascending the giant branch, its iron core is collapsing and heating, until iron is ready to fuse. As soon as it does, though, it absorbs all of the heat around it, chilling the core. All fusion abrubtly stops, and the star implodes. The rebound of this implosion is the greatest explosion known in the cosmos: a supernova. A single supernova can be brighter than an entire galaxy for a few days. After the supernova, depending on the mass of the original star, the core might be left over as a white dwarf, neutron star, or black hole.

So depending on its mass, a star ends its life either in a planetary nebula or supernova, leaving its core behind as a compact object: white dwarf, neutron star, or black hole.

#### Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

## Wolf Rayet stars

In stars in which fusion is present, the fusion happens in a core which is surrounded by great deal of non-fusing matter. One slight exception to this are a subset of Wolf-Rayed stars which "have now completely lost their outer hydrogen and are fusing helium or heavier elements in the core" Wikipedia).

Is it possible for a star to be composed of nothing but the fusing-core that is, is it possible for a star to be a fusing-body only with no separate core and separate surrounding non-fusing body? Could a star be massive enough, and could this star be fusing a specific type of "fuel" (i.e. hydrogen, deuterium, helium, etc.) which would allow for the force of the star's gravity to balance and contain the pressure of the fusing process, resulting in a sort of photosphere/chromosphere representing the balance of these two forces (i.e. gravity in and explosive fusion pressure out)?

I suspect that the answer is "no" this is not possible. I would like to read the language explanations (using simple math if necessary) of why such is not possible.

If possible. I think it would be fascinating to know one is looking directly at a steady fusion reaction directly without the necessity of seeing it only for an instant (i.e. nuclear explosion) and without the necessity of seeing the fusion reactions by products which have wormed their way through the outlying non-fusing layers of a star.

Edited by Otto Piechowski, 22 August 2015 - 12:18 PM.

### #2 graffias79

I think something like this can happen when a companion strips mass from it. Are you speaking of a star like EF Eridani? https://en.wikipedia. wiki/EF_Eridani

### #3 Otto Piechowski

No, I was not thinking of that in particular. It is a theoretical musing on my part, only.

### #4 graffias79

As long as it falls within the laws of physics, there's probably a star system that's an example of any scenario somewhere out there in the vast reaches of space!

### #5 llanitedave

In a nova explosion, if I remember right, a runaway fusion event is occurring on the surface of an accreting white dwarf. In a type Ia supernova, fusion is occuring on the surface and throughout the volume of the star. Those are the only two examples I can think of with "naked fusion" occurring, and obviously both are unconstrained.

### #6 Xshovelfighter

My first instinct would be no, for two general reasons that stick out to me:

1) As a single fuel source is fused into a heavier element (say H into He, etc), the heavier element will preferentially settle out. Basically, this "ash" from the fusion process will settle out and accumulate in the core due to gravity. This scenario would lead to a non-fusing core surrounded by a fusing shell. This doesn't follow your model.

2) Considering beyond the differential "settling" of this non-fusing ash - I think the temperature dependency of the progressive fusion process would limit a single fusing core. I know that the Helium fusion process is VERY temperature sensitive - it is a function of T^4. This means that it dependent upon a narrow temperature range. Extrapolating this to the fusion process of other progressively heavier elements, I doubt there would ever be a point where the primary fuel source and its resulting product could sustain fusion simultaneously.

Utilizing this same point, even if two consecutive fusion parent/daughter products could sustain fusion simultaneously, then there would just be a resulting 3rd, 4th, 5th, 6th, nth order fusion end product. In order for your model to work, all of these resulting products would have to contain the same temperature dependency - this is know to be false. Progressively heavier elements require progressively higher temperatures.

## What causes stars to switch from Hydrogen fusion to using heavier elements?

When talking about the life cycle of stars, people often say something along the lines of “when a star runs out of hydrogen to fuse into helium, it has to start performing fusion with heavier elements to keep producing energy.” But this glides over a lot of details and also frames the star as having some sort of agency, which is obviously not the case.

Why does a star “have to” switch to using heavier elements? (I.e. why doesn’t it just die when it runs out of hydrogen?)

Whatever mechanism causes the star to fuse heavier elements, why isn’t that process active during the entire life cycle of the star, including when there’s still hydrogen?

In short, gravity compresses the core as fuel is exhausted, which raises the temperature, which eventually gets it hot enough to make the ash from the previous fusion burning stage to itself start fusing.

During the main sequence hydrogen slowly burns to helium, so helium is accumulating in the core. The star is in an equilibrium, where the thermal pressure is sufficient to drive H to He fusion (but not much He to C/O fusion) which balances gravity. But that's just a story we tell to children so they can sleep at time.

Over the course of the H-burning main sequence life, you can see that the H is slowly getting exhausted. In order to keep up the rate of energy production to balance gravity, the core does have to get a bit hotter and denser because it has less H. So in reality, over the main sequence life of the star the core density and pressure are slowly increasing in order to maintain the balance- the core slowly contracts, increasing the density and pressure, and the equilibrium output moves slowly to a greater luminosity. As a result, the star finds a new balance- it gets more luminous and it grows, so its surface temperature decreases. This is very very gradual, and is not the transition to a giant phase yet. For most purposes, you can pretend that the luminosity during the main sequence part of the star's life is effectively constant. But if we're being honest, our sun is halfway through it's life and it is actually a few percent more luminous (and redder) than when it formed.

If you repeat this process until the point where H fusion is not sufficient to balance the star, you'll find that the core contracts due to gravity. This drives it to greater luminosity, coincidentally, causing the star to slowly grow in a giant phase. Most importantly, the contraction causes the core to reach higher temperatures, and this is the key to getting your next stage of fusion burning. Due to the greater Coulomb repulsion between larger nuclei, they need greater kinetic energies (and thus temperatures) in order to burn efficiently. It is ultimately the core contraction (inside a growing star!) which heats the core enough to 'unlock' the next stage in nuclear burning.

If you want to know more, the 'helium flash' is a really interesting feature of low mass stars as they make the transition to a giant phase and helium burning.

## 6. Death

When low-mass stars die, they collapse under their own weight until their centers act in some ways like a solid. A star like the Sun will collapse at the end to be about twice the size of Earth, a tremendous reduction in size. These white-hot dying stars are called white dwarfs by astronomers. A white dwarf is so compressed that&mdash if you could survive standing on its hot surface&mdashyour weight would be roughly a million times your Earth weight. Perhaps the best known white dwarf orbits the brightest star in the sky, Sirius. Since Sirius is known as the dog star, the companion white dwarf is sometimes nicknamed "the pup" it requires a significant telescope to see, but is fun to imagine when you see Sirius on winter evenings.

Massive stars have a very different ending in store. When the core of a massive star collapses, its powerful gravity takes it right through the white dwarf stage to produce one of two extremely bizarre objects&mdasheither a neutron star or a black hole (see below). In most cases, the rest of the star then blows up, in a gargantuan explosion called a supernova. These explosions produce so much energy that the star can briefly become brighter than the entire galaxy in which it is located (brighter than 100 billion Suns). Extreme versions of a supernova explosion, sometimes called "hypernovae," produce gamma-ray bursts like the one discovered by Michael Koppelman, as described in our film.

The most famous supernova in history was seen in July 1054. Records of it still survive, from China all the way to North American Indian lore. Seen today, roughly a thousand years later, its remnant, called the Crab Nebula, is one of the most interesting objects in the sky. It can be found with good binoculars or small telescopes in the constellation of Taurus, the Bull.

## Spectral Classes L, T, and Y

The scheme devised by Cannon worked well until 1988, when astronomers began to discover objects even cooler than M9-type stars. We use the word object because many of the new discoveries are not true stars. A star is defined as an object that during some part of its lifetime derives 100% of its energy from the same process that makes the Sun shine—the fusion of hydrogen nuclei (protons) into helium. Objects with masses less than about 7.5% of the mass of our Sun (about 0.075 MSun) do not become hot enough for hydrogen fusion to take place. Even before the first such “failed star” was found, this class of objects, with masses intermediate between stars and planets, was given the name brown dwarfs.

Brown dwarfs are very difficult to observe because they are extremely faint and cool, and they put out most of their light in the infrared part of the spectrum. It was only after the construction of very large telescopes, like the Keck telescopes in Hawaii, and the development of very sensitive infrared detectors, that the search for brown dwarfs succeeded. The first brown dwarf was discovered in 1988, and, as of the summer of 2015, there are more than 2200 known brown dwarfs.

Initially, brown dwarfs were given spectral classes like M10 + or “much cooler than M9,” but so many are now known that it is possible to begin assigning spectral types. The hottest brown dwarfs are given types L0–L9 (temperatures in the range 2400–1300 K), whereas still cooler (1300–700 K) objects are given types T0–T9 (see [link]). In class L brown dwarfs, the lines of titanium oxide, which are strong in M stars, have disappeared. This is because the L dwarfs are so cool that atoms and molecules can gather together into dust particles in their atmospheres the titanium is locked up in the dust grains rather than being available to form molecules of titanium oxide. Lines of steam (hot water vapor) are present, along with lines of carbon monoxide and neutral sodium, potassium, cesium, and rubidium. Methane (CH4) lines are strong in class-T brown dwarfs, as methane exists in the atmosphere of the giant planets in our own solar system.

In 2009, astronomers discovered ultra-cool brown dwarfs with temperatures of 500–600 K. These objects exhibited absorption lines due to ammonia (NH3), which are not seen in T dwarfs. A new spectral class, Y, was created for these objects. As of 2015, over two dozen brown dwarfs belonging to spectral class Y have been discovered, some with temperatures comparable to that of the human body (about 300 K).

Brown Dwarfs. This illustration shows the sizes and surface temperatures of brown dwarfs Teide 1, Gliese 229B, and WISE1828 in relation to the Sun, a red dwarf star (Gliese 229A), and Jupiter. (credit: modification of work by MPIA/V. Joergens)

Most brown dwarfs start out with atmospheric temperatures and spectra like those of true stars with spectral classes of M6.5 and later, even though the brown dwarfs are not hot and dense enough in their interiors to fuse hydrogen. In fact, the spectra of brown dwarfs and true stars are so similar from spectral types late M through L that it is not possible to distinguish the two types of objects based on spectra alone. An independent measure of mass is required to determine whether a specific object is a brown dwarf or a very low mass star. Since brown dwarfs cool steadily throughout their lifetimes, the spectral type of a given brown dwarf changes with time over a billion years or more from late M through L, T, and Y spectral types.

## New Class of Pulsating Stars Discovered

This image shows HIP 52181 (blue star in the center), a pulsating subdwarf O-star. Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD / SDSS.

“Many stars pulsate, even our Sun does on a very small scale,” said Dr. Thomas Kupfer, a postdoctoral researcher in the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara.

“A true pulsator can vary in brightness by some 10% due to a periodic change in its temperature, radius or both. Those with the largest brightness changes are usually radial pulsators, ‘breathing’ in and out as the entire star changes size.”

Initially, Dr. Kupfer and colleagues were searching for binary stars with periods less than an hour in observations from the Zwicky Transient Facility, a sky survey at the Palomar Observatory.

Four stars — ZTF J071329.02-152125.2, ZTF J184521.40-254437.5, ZTF J191306.79-120544.6, and ZTF J182815.88+122530.5 — stood out due to large changes in their brightness over just a few minutes.

Follow-up data quickly confirmed that they were indeed pulsators, not binary pairs.

According to the team, these stars are hot subdwarf pulsators (hot subdwarfs are stars about one-tenth the diameter of our Sun with masses between 20 and 50% that of the Sun they’re incredibly hot — up to 90,000 degrees Fahrenheit (50,000 degrees Celsius).

“These stars have certainly completed fusing all of the hydrogen in their core into helium, explaining why they are so small and can oscillate so rapidly,” said Dr. Lars Bildsten, also from the Kavli Institute for Theoretical Physics.

“Scientists hadn’t previously predicted the existence of these stars, but in retrospect they fit well into the leading models of stellar evolution.”

Because of the stars’ low masses, they started life as typical Sun-like stars fusing hydrogen to helium in their cores. After exhausting the hydrogen in their cores, the stars expanded into the red giant stage.

Usually, a star will reach its largest radius and begin fusing helium deep in the core. However, the scientists think these newly-discovered pulsators had their outer material stolen by a companion before the helium became hot and dense enough to fuse.

In the past, hot subdwarfs were almost always related to stars which became red giants, started fusing helium in their cores, and then got stripped by a companion.

The new findings indicate that this group includes different types of stars.

“Some do helium fusion and some don’t,” Dr. Kupfer noted.

The stars’ pulsations allow researchers to probe their masses and radii and compare these measurements to stellar models, something that was not otherwise possible previously.

“We were able to understand the rapid pulsations by matching them to theoretical models with low mass cores made of relatively cold helium,” said Evan Bauer, a doctoral student at the University of California, Santa Barbara.

The results were published in the June 20, 2019 issue of the Astrophysical Journal Letters (arXiv.org preprint).

Thomas Kupfer et al. 2019. A New Class of Large-amplitude Radial-mode Hot Subdwarf Pulsators. ApJL 878, L35 doi: 10.3847/2041-8213/ab263c