Astronomy

Are stars expected to become dimmer before a supernova?

Are stars expected to become dimmer before a supernova?


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With the recent news about the "fainting" of Betelgeuse and the speculation that this might be a precursor to a supernova, I'm wondering if there is any theoretical/observational basis for this interpretation, or whether this is a case of reading too much into a greater-than-average change in an intrinsically variable star. Why would a star become dimmer in the run-up to a supernova?


The connection between the dimming and a putative supernova relies on the interpretation that the decrease in luminosity may be due to circumstellar material, ejected in the years/decades/centuries immediately preceding a supernova. There are several mechanisms that could lead to this sort of mass loss (see slides 24-25), including

  • gravity-wave driven envelope loss (in red supergiants, during neon/oxygen core burning)
  • pulsations due to pair-instability in the days to decades prior to the supernova
  • turbulent eruptions à la luminous blue variables

These then lead to Type IIn supernovae, with narrow lines arising from interactions with the previously ejected circumstellar material. If the dimming is due to extinction by circumstellar dust from eruptions due to any of these mechanisms, it could mean that a supernova is imminent on a timescale of days to years.


The Mystery of Betelgeuse's Dimming Has Finally Been Solved

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Betelgeuse began to dim in late 2019, decreasing in brightness by as much as 35 percent before brightening again in April 2020. Photograph: Galaxy Picture Library/Alamy

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In December 2019, astronomers noticed a strange, dramatic dimming in the light from Betelgeuse, a bright red star in the Orion constellation. They puzzled over the phenomenon and wondered whether it was a sign that the star was about to go supernova. Several months later, they had narrowed the most likely explanations to two: a short-lived cold patch on the star's southern surface (akin to a sun spot), or a clump of dust making the star seem dimmer to observers on Earth. We now have our answer, according to a new paper published in the journal Nature. Dust is the primary culprit, but it is linked to the brief emergence of a cold spot.

This story originally appeared on Ars Technica, a trusted source for technology news, tech policy analysis, reviews, and more. Ars is owned by WIRED's parent company, Condé Nast.

As Ars' John Timmer reported last year, Betelgeuse is one of the closest massive stars to Earth, about 700 light years away. It's an old star that has reached the stage where it glows a dull red and expands, with the hot core only having a tenuous gravitational grip on its outer layers. The star has something akin to a heartbeat, albeit an extremely slow and irregular one. Over time, the star cycles through periods when its surface expands and then contracts.

One of these cycles is fairly regular, taking a bit over five years to complete. Layered on that is a shorter, more irregular cycle that takes anywhere from under a year to 1.5 years to complete. While they're easy to track with ground-based telescopes, these shifts don't cause the sort of radical changes in the star's light that would account for the changes seen during the dimming event.

In late 2019, Betelgeuse dimmed so much that the difference was visible to the naked eye. The dimming persisted, decreasing in brightness by 35 percent in mid-February, before brightening again in April 2020.

Telescopes pointed at the giant were able to determine that—rather than a tidy, uniform drop in luminance—Betelgeuse's dimming was unevenly distributed, giving the star an odd, squished shape when viewed from Earth. That raised lots of questions about what was going on with the giant, with some experts speculating that because of Betelgeuse's size and advanced age, the strange behavior was a sign of a supernova in the making.

By mid-2020, astronomers had changed their tune. An international team of observers happened to have the Hubble Space Telescope pointed at Betelgeuse before, during, and after the dimming event. Combined with some timely ground observations, this UV data indicated that a big burp that formed a cloud of dust near the star may have caused the star to get darker.

"With Hubble, we could see the material as it left the star's surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim," said Andrea Dupree, an astronomer at the Harvard-Smithsonian Center for Astrophysics who made those observations. She is also a co-author on the new paper.

The findings last year showed that an outer layer of the star, called the photosphere, had begun unevenly accelerating outward right before Betelgeuse began to dim. At its peak, the photosphere was moving at around 7 kilometers per second, reversing the outward push as the dimming of the star became more dramatic.

Dupree and her colleagues suggested that as the star expanded in one of its usual cycles, a portion of the surface accelerated much more rapidly, thanks to a convection cell that had traveled from the interior of the star to its surface. Those two events combined pushed out sufficient material far enough from the star that it cooled down, forming stardust. That dust could account for the dimming.

The new Nature paper expands on those earlier observations due to images captured by the European Southern Observatory's (ESO) Very Large Telescope (VLT) in January and March 2020. "For once, we were seeing the appearance of a star changing in real time on a scale of weeks," said coauthor Miguel Montargès, from the Observatoire de Paris, France, and KU Leuven, Belgium.

Those images, combined with earlier observations in January and December 2019, allowed astronomers to directly witness the stardust formation, matching the observations of Dupree and her colleagues last year. The ESO team concluded that a gas bubble was ejected and pushed further out by the star's outward pulsation. When a convection-driven cold patch appeared on the surface, the local temperature decrease was sufficient to condense the heavier elements (like silicon) into solid dust, forming a dusty veil that obscured the star's brightness in its southern hemisphere. The astronomers speculate that a similar expelling of dust from cool stars could end up becoming building blocks of planets.

The ESO team found no evidence to support the impending supernova hypothesis. "The lack of an explosive conclusion might seem disappointing, but [these] results go beyond explaining one brief wink of a nearby star," University of Washington astronomer Emily Levesque (who is not a co-author) wrote in an accompanying Nature commentary. She raises the prospect of other red supergiants also showing signs of dimming. "Next-generation facilities focused on monitoring stellar brightness over time, or on studying the signature of dust in the infrared spectra of stars, could prove invaluable for expanding the lessons learned here."

One of those next-generation facilities is ESO's Extremely Large Telescope (ELT), slated to achieve first light in 2026. "With the ability to reach unparalleled spatial resolutions, the ELT will enable us to directly image Betelgeuse in remarkable detail," said co-author Emily Cannon of KU Leuven. "It will also significantly expand the sample of red supergiants for which we can resolve the surface through direct imaging, further helping us to unravel the mysteries behind the winds of these massive stars."


A Star That Would Not Die

Supernovae, the explosions of stars, have been observed by the thousands. And in all cases, the transient astronomical events signaled the death of those stars.

Now, astrophysicists at UC Santa Barbara and astronomers at Las Cumbres Observatory (LCO) have reported a remarkable exception: a star that exploded multiple times over a period of more than 50 years. Their observations, published in the journal Nature, are challenging existing theories on these cosmic catastrophes.

"This supernova breaks everything we thought we knew about how they work," said lead author Iair Arcavi, a NASA Einstein postdoctoral fellow in UC Santa Barbara's Department of Physics and at LCO. "It's the biggest puzzle I've encountered in almost a decade of studying stellar explosions."

When iPTF14hls was discovered in September 2014 by the Caltech-led Palomar Transient Factory, it looked like an ordinary supernova. But several months later, the scientific team noticed that the supernova, once faded, was growing brighter. It was a phenomenon they had never seen before.

A normal supernova rises to peak brightness and fades over 100 days. Supernova iPTF14hls, on the other hand, grew brighter and dimmer at least five times over three years.

When the scientists examined archival data, they were astonished to find evidence of an explosion in 1954 at the same location. Somehow this star survived that explosion and then exploded again in 2014. In the study, the authors calculated that the exploding star was at least 50 times more massive than the sun and probably much larger.

"Supernova iPTF14hls may be the most massive stellar explosion ever seen," explained co-author Lars Bildsten, director of UCSB's Kavli Institute for Theoretical Physics. "For me, the most remarkable aspect of this supernova was its long duration, something we have never seen before. It certainly puzzled all of us as it just continued shining." As part of this effort, Bildsten worked with UC Berkeley astrophysicist Dan Kasen, exploring many possible explanations.

The earlier explosion in 1954 provided an important clue, suggesting that iPTF14hls could be the first example of a pulsational pair-instability supernova. Theory holds that the cores of massive stars become so hot that energy is converted into matter and antimatter. This causes an explosion that blows off the star's outer layers and leaves the core intact. Such a process can repeat over decades before the final explosion and subsequent collapse to a black hole.

"These explosions were only expected to be seen in the early universe and should be extinct today," said co-author Andy Howell, a UCSB adjunct faculty member who leads the supernova group at LCO. "This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur."

The pulsational pair-instability theory may not fully explain all the data obtained for this event because the energy released by the supernova is more than the theory predicts. This means iPTF14hls may be a completely new kind of supernova.

LCO's supernova group continues to monitor iPTF14hls, which remains bright three years after it was discovered. Their global telescope network is uniquely designed for this type of sustained observation, which has allowed researchers to observe iPTF14hls every few days for several years. Such long-term consistent monitoring is essential for the study of this very unusual event.

"We could not have kept tabs on iPTF14hls for this long and collected data that challenges all existing supernova theories if it weren't for the global telescope network," Arcavi said. "I can't wait to see what we'll find by continuing to look at the sky in the new ways that such a setup allows."


Astronomy Midterm

-ISM is also important to understand the dynamics, chemistry and evolution of galaxies.

-Trumpler found that stars in more distant clusters are dimmer than expected form a simple inverse-square law

-interstellar medium makes stars look redder than they are: Reddening NOT redshifts!!

-If electrons gain sufficient energy from the light, they will be
stripped off the atom, leaving free electrons and hydrogen nuclei (protons). This is the ionized hydrogen gas (HII).

-This "warm" ionized ISM,so-called the HII region, has density and temperature of

-To produce an HII region, electrons need to be completely stripped off hydrogen atoms, which requires UV photons

-Usually associated with hot O, B, stars

-HII regions can be detected in visible light

-The hot gas makes up a small mass fraction of ISM, but occupies a large interstellar volume

-Started with nuclear fusion at sometime in the past, and will run out of fuel in the future

-starts in the stage of the star formation from a dense ISM through the gravitational contraction and heating (no nuclear fusion)

-stars are often rotating and in a binary system, as well as accenting surrounding gas, which makes them highly variable typically in visible and X-ray lights

-more massive stars spend shorter times in the pre-main sequence

-Once the core temperature reaches

-beings as the stable hydrogen fusion is ignited at the core of the star: the main sequence is defined as the stage in which the star maintains the hydrostatic equilibrium by hydrogen fusion at its center

- the detailed evolutionary path and the eventual "fate" of stars are determined by their MASS

-proto-stars continue to collapse due to its own gravity gravitational energy loss and collisional heating among other falling gas particles produce radiation

-Although they are "cold" , proto-stars are visible in infared

-more massive: stay in main sequence for a shorter period

-less massive: stay in main sequence for longer time period

-As helium (not fusing) piles up, the gas pressure in the core would be overwhelmed by the gravity. Thus the core contracts and is heated. (Helium still does not fuse)

- This helium-core contracts to be heated, and releases gravitational energy.

-The hydrogen in the outer-layer of helium core becomes hot enough to fuse. (Helium core is still not fusing)

-Because of effective convection (or mixing), the
star does not form the hydrogen fusion shell. Hydrogen runs out to form a helium core.

- As the star becomes larger, it cools down.
* The size of a sun like star increases to

170R, and the surface temp decreases to

-the star is now cooler than it was a main sequence star, but is much brighter than it was in main sequence because of the large size. Which means that the star "moves" to upper-right hand side off the main sequence in the HR diagram ---> which is now a (red) giant

-when the gas temperature reaches 100 mill K in the helium core (to convert helium to carbon and oxygen) to create normal gas pressure

-electron degeneracy pressure is NOT dependent on the gas temperature

-now both hydrogen and helium shells are fusing, and the star expands and cools to become a red giant again (or red supergiant)

-the core is not made up with carbon, oxygen, neon or magnesium, depending on the mass

-with a mass of <1M, the core cannot reach a temperature that is hot enough to fuse these heavy elements

-As the hot core is exposed, it moves to the LEFT of the hr diagram

-hot, stellar core, but not hot enough to fuse carbon or oxygen

-high density state with no supply of thermal energy by nuclear fusion, electron degeneracy pressure becomes important

-electron degeneracy pressure provides the internal outward pressure to balance the inward gravity to keep the entire star in hydrostatic equilibrium

-the core continues to contract, and the temperature rises above 1 bill K.

-Neon fusion produces oxygen and magnesium--->-Oxygen fuses to magnesium, silicon, and sulfur---> & at 3 bill K, silicon fuses to iron.

-most stable nucleus among all elements with the largest binding energy between protons and neutrons

-in the iron-core, there is no fusion reaction, and the degenerate electron pressure keeps it from collapsing by gravity while its mass continues to grow

-when the iron-core mass reaches

1.4M, the core collapses to form a neutron star or black hole, the outer layers of the star explodes as a supernova

-as nuclear fusion progresses at higher temperatures beyond hydrogen, the fusion rate increases. Meanwhile, the number of atoms to be fused decreases.

-with such a high luminosity (>10000) radiation pressure is huge to push out stellar material from the outer layers of the star
**These supergiant stars eject a large amount of their surface
material in the form of stellar winds

-supergiants are surrounded by dense circumstellar medium of this ejected gas

-evolution of massive stars is complicated involving several different stages, but primarily depending on their mass

-classified by their explosion mechanisms or by their observational characteristics

-theoretical classifications by the explosion mechanism are core-collapse and thermonuclear supernova

-all massive stars eventually explodes as a core-collapse supernova

-the iron core will stop nuclear fusion, and the mass grows to reach the Chandrasekhar limit

-The iron core collapses in

0.1 sec, and protons and electrons merge together to form neutrons

-the falling material of the outer core layers are bounced back within a millisec

10^57( billion x trillion x trillion x trillion x trillion)

-these neutrinos carry out

99% of the explosion energy

-thermonuclear explosion completely destroys the white dwarf

-hydrogen fusion may occur only at the surface of the white dwarf, and the surface layer of the white dwarf explodes, which is called a NOVA. ---> since only the outer layers explode. the white dwarf is not destroyed.

-the light curve reaches a peak luminosity and then smoothly fades away in several months

-type 1 supernova is divided into subtypes of 1a, 1b and 1c depending on their detailed spectral features

-type 1b & 1c supernovas are fainter than type 1a

-their light curve shows a peak (similar to that of types 1b & 1c)

-many type II supernova light curves show a plateau at

-main source of iron to chemically enrich interstellar medium

-no hydrogen lines are seen in type Ia supernova bc there is no hydrogen rich layer in the white dwarf progenitor

-can occur in all types of galaxies

-progenitors of type la supernova are all the "same" type of stars (white dwarfs) with the "same" mass
*their explosion energy, peak luminosity, and the light curve are identical*

-type la supernova is useful as standard candles to measure distances to galaxies

-they are the brightest candles with which we can measure farthest distances up to


So, is that actually about to occur?

This is the big mystery, and it’s one of the reasons why the star’s current behavior is intriguing. Scientists suspect that a precipitous drop in brightness could portend a star’s demise.

“As massive stars near the end of their lives, they experience insane and violent mass loss,” Nance says. In theory, all that ejected dust could shroud and darken the nearly dead star, causing it to dim from our perspective right before it goes supernova. In practice, though, it’s not yet certain whether stars are darkest before they blow—no one has yet been able to closely study a doomed star before, during, and after its demise.


Will Betelgeuse Explode? After ‘Unprecedented’ Dimming The Giant Star Is Now Changing Shape

The red supergiant star Betelgeuse, in the constellation of Orion, has been undergoing unprecedented . [+] dimming. This stunning image of the star’s surface, taken with the SPHERE instrument on ESO’s Very Large Telescope late last year, is among the first observations to come out of an observing campaign aimed at understanding why the star is becoming fainter. When compared with the image taken in January 2019, it shows how much the star has faded and how its apparent shape has changed.

Spectacular new images taken using the European Southern Observatory’s Very Large Telescope (VLT) in Cerro Paranal in Chile, published today, reveal that red supergiant star Betelgeuse isn’t just dimming, but could also be changing shape.

The star in the constellation of Orion has been visibly dimming since late 2019, and now stands at just 36% of its normal brightness. Astronomers and experienced stargazers can easily see the difference, and it’s got them talking . about the chance of the star becoming a supernova.

Is the dimming associated with a change in Betelgeuse that could lead to the star “going supernova?” In that scenario, Betelgeuse’s explosion could mean it shines as bright as a full moon for a few months.

So why has it got dimmer? A team led by Miguel Montargès, an astronomer at KU Leuven in Belgium, has been observing the star with the ESO’s Very Large Telescope since December. Among the team’s first observations is this stunning new image (above, main image) of Betelgeuse’s surface in visible light. It was taken late last year with the telescope’s SPHERE instrument.

By lucky chance the same team had photographed Betelgeuse in January 2019 prior to its dimming—in visible light and using the same telescope—giving them the invaluable before-and-after comparison in this video:

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The video shows how much the star has faded, but also how its apparent shape has changed. So what’s going on? “The two scenarios we are working on are a cooling of the surface due to exceptional stellar activity or dust ejection towards us,” says Montargès. “Of course, our knowledge of red supergiants remains incomplete, and this is still a work in progress, so a surprise can still happen.”

It’s thought that Betelgeuse is between 650 and 700 light years away, and that the star is around 15-20 times the mass of the sun. The mass makes a huge difference in calculating at what stage Betelgeuse is in its development.

Infrared light being emitted by the dust surrounding Betelgeuse in December 2019, obtained with the . [+] VISIR instrument on ESO’s Very Large Telescope.

ESO/P. Kervella/M. Montargès et al., Acknowledgement: Eric Pantin

Basically, Betelgeuse’s dimming—and its “new” apparent shape—is all down to dust.

Here’s another dramatic new image (above)—this time at a wavelength of light similar to that detected by heat cameras—also taken in December 2019. It was taken using the VISIR instrument on the Very Large Telescope and it shows the infrared light being emitted by the dust surrounding Betelgeuse. It was taken by a team led by Pierre Kervella from the Observatory of Paris in France. The clouds of dust are formed when the star sheds its material back into space, something that astronomers know that Betelgeuse is prone to do. It’s why Betelgeuse is known to dim now and again—though it’s never got as dim as it is right now.

In this video, published today, you can also see the surface of Betelgeuse—that tiny black dot in the middle of the image.

“Over their lifetimes, red supergiants like Betelgeuse create and eject vast amounts of material even before they explode as supernovae,” said Emily Cannon, a PhD student at KU Leuven working with SPHERE images of red supergiants. “Modern technology has enabled us to study these objects, hundreds of light-years away, in unprecedented detail giving us the opportunity to unravel the mystery of what triggers their mass loss.”

Betelgeuse's distance from us means that we're seeing it as it was in the 14th century if it's already gone supernova then we're still waiting for the light to travel to us. This is true of everything in the night sky everything is "in the past." So if Betelgeuse literally did explode during our "today" (itself an Earth-bound concept) then it would be the people of Earth of the 27th century that would see it.

Betelgeuse is typically the eleventh-brightest star in the night sky, but lately it’s lost that claim. Will it go supernova? Yes, absolutely it will. When? Sometime in the next 100,000 years. In cosmic terms, that’s any second now .


Touring Orion's belt and bright stars

Many cultures have assigned significance to the distinctive row of three equally spaced stars that mark Orion's belt. Scandinavian countries have seen a distaff, a scythe and a sword. Predominantly Catholic countries referred to it as The Three Marys (from the New Testament). In the Middle East, they saw the three kings, or Magi. In China, it was known as The Weighing Beam, and the Three Stars. In fact, the top portion of the corresponding Chinese character 參 (shēn) has three identical symbols representing the three stars. In North America, the Lakota called it the Bison's Spine, with the surrounding stars and nearby constellations forming the rest of a great bison in their winter sky.

If you are willing to face the winter chill, bring your phone or tablet and astronomy app outside on a clear evening, and use it to find the stars and objects mentioned below. A backyard telescope will show you most of the objects. Or, you can tour the constellation indoors with an app such as SkySafari 5. Simply search for the objects by name, center them in the app and zoom in to see how they look in full color. Use the Information option to bring up historical and scientific details, as well as additional images taken by amateur astronomers and large observatories.

From east to west (left to right looking at the sky in the Northern Hemisphere), Orion's three belt stars are named Alnitak, Mintaka and Alnilam. In a telescope, Alnitak (Arabic for "the Girdle") is revealed to be double, with the larger star a blue supergiant about 820 light-years away and shining strongly in ultraviolet light. Its surface temperature is a scorching 31,000 kelvins &mdash that's 55,340 degrees Fahrenheit! (For comparison, our gentle yellow sun is a mere 6,200 K &mdash 10,700 degrees F.) Astrophotographers love the area around Alnitak. It's loaded with gorgeous gas clouds and nebulas, including the famous Horsehead Nebula and another called the Flame Nebula. A very large telescope is needed to see the spectacular objects with your own eyes, but your astronomy app will allow you to search for them and display full-color images. [Best Telescopes for the Money - 2017 Reviews and Guide]

The belt's middle star is called Alnilam, which means "string of pearls". It's another large, and very hot, blue-white star, about 1.5 times farther away than the other two belt stars. Aging rapidly and nearing the end of its hydrogen supply, this star is expected to become a red supergiant &mdash the precursor to a supernova, at any time. In fact, considering the star sits more than 1,300 light-years from Earth (with a corresponding delay in our seeing it), it might have happened already!

The third and westernmost star, called Mintaka ("belt"), is also a double star when viewed through a telescope. In fact, there are at least four stars making up what we see as Mintaka, although the others are only evident through spectroscopy. The brightest one has a partner that orbits it every 5.73 days in an eclipsing binary configuration that makes the star vary in brightness. These stars are also hot, blue giants, sitting about 900 light-years away. If you look carefully, Mintaka is actually somewhat dimmer than Alnitak and Alnilam.

To the upper left of his belt, the very bright, orange star Betelgeuse marks Orion's eastern shoulder. The ninth-brightest star in all the night sky, Betelgeuse is a red supergiant located about 500 light-years away. For comparison, if this star were in our solar system, all of the inner planets from Mercury to Mars would be inside the star! Even though Betelgeuse is much younger than our sun, it is a type of star that matures dramatically faster, and thus astronomers think it is approaching the end of its life and is massive enough to explode as a Type II supernova. And considering the light we see now left the star 500 years ago, it might have exploded already!

To the lower right of Orion's belt sits the hot, blue star Rigel. From our perspective on Earth, it appears about as bright as Betelgeuse, but it's much farther away &mdash meaning it emits considerably more light. Rigel, too, is a supergiant star burning with a surface temperature of 12,000 kelvins (21,140 degrees F, or 11,727 degrees C)! In a good telescope, a small companion star can be spotted very close to Rigel. In Arabic, Rigel means "the foot of the great one." In China, Rigel is known as 参宿七 (Sānsù Qī, "The Seventh of the Three Stars").

Orion's western shoulder is marked by the bright star Bellatrix, which translates to "Amazon Star," named after the warrior women of legend. Bellatrix is about 240 light-years away, and burns at a blistering-hot 21,500 kelvins (38,240 degrees F, or 37,967 degrees C). It, too, is well along in its life cycle and is soon expected to enter its next phase of evolution, and become an orange giant. (As the last stages of their lives begin, stars first swell and become orange giants. Later in the process, they redden and swell even larger.) Above and between Orion's shoulders is an open cluster of stars, 1,305 light-years away, that marks his head. The brightest star is named Meissa ("the Shining One"). You can use binoculars or a telescope to enjoy them better, and see how many you can count. [Awesome Binocular Astronomy with the help of Mobile Apps]

Completing our circuit of his body, the western foot, or knee, of Orion is the misnamed star Saiph ("Sword of the Giant"). Another hot, blue-white star blazing at 26,500 kelvins (47,240 degrees Fahrenheit, or 26,227 degrees Celsius), it's also nearing the transition to creaky-old red supergiant.

Orion's cloak, lion's pelt or shield is composed of a crooked line of about nine stars that run up and down off to the constellation's western side. The brightest star, in the middle of the string, is named Tabit ("the Endurer"). On the opposite side of the constellation, the upraised club dips into the Milky Way. As you look higher, you'll see that pairs of stars widen the club. By using binoculars, you'll be able to see the rich star fields there.

Orion's stars serve as pointers to other celestial signposts. Extending the belt stars to the west leads to the bright-orange star Aldebaran in Taurus. With your arm extended, measure two diameters of your fist in the opposite direction to spot Sirius, the brightest star in the entire night sky, in the constellation Canis Major. If you imagine drawing a line drawn from Bellatrix to Betelgeuse, it will point to Sirius' bright puppy, the star Procyon. And a line upward from Rigel through Betelgeuse leads to the two matched stars Castor and Pollux, the heads of Gemini, the twins.


What’s Happening at Betelgeuse?

Astronomers are puzzled by the dimming of one of the brightest stars in the sky, Betelgeuse in Orion.

Hertzsprung-Russell (H-R) Diagram plots luminosity vs temperature.

Amateur astronomers quickly learn to identify Betelgeuse, the right shoulder in the constellation Orion. After getting over the initial chuckle that a star could be named beetle-juice (more accurately, betel-jooz, an Arabic name also amusing, meaning ‘armpit of Orion’), the new astronomer learns it is a red giant star 650 light-years away. Astrophysics students learn that red giants are fated to blow up as supernovas, according to stellar evolution theory. They learn the Hertzsprung-Russell diagram that astronomers use to connect the dots between star types, showing how one type evolves into another over billions of years. The diagram was not made for stellar evolution theory, though it was merely a diagram to conveniently graph luminosity to temperature. Stellar evolution theory followed later.

However, astronomers are not quite sure about the recent observations of the famous red giant. Evan Gough at Universe Today has been following the news about unexpected dimming out there:

Betelgeuse is Continuing to Dim! It’s Down to 1.506 Magnitude (Universe Today, Jan 22).

Betelgeuse keeps getting dimmer and everyone is wondering what exactly that means. The star will go supernova at the end of its life, but that’s not projected to happen for tens of thousands of years or so. So what’s causing the dimming?

Betelgeuse Just Keeps Getting Dimmer, And We Have No Idea Why (Universe Today , Jan 23). Theory tells us that red giants will swell outward, then collapse.

Or could it be something else? We know a lot about stars, but we don’t know everything. We’ve also never been able to observe any other red super-giants the way we can with Betelgeuse.

Scientists enjoy a good surprise, because it usually means more discoveries are coming. It also relieves boredom of thinking everything has already been figured out. Some are wondering if we are about to see a close-by supernova explosion that could rival the moon in brightness. Others are considering more mundane explanations, that maybe interstellar clouds are interfering and causing the dimming. Some stars are known to undergo cycles of brightening and dimming, but usually not red giants like Betelgeuse unless they are near the end of their lives.

Our sun is a speck compared to the red supergiants.

Betelgeuse: star’s weird dimming sparks rumours that its death is imminent (Daniel Brown, astronomer, at The Conversation). It could explode now or any time in the next 100,000 years, Brown says.

But this current substantial dimming is not necessarily a sign of its imminent death. That’s because, at this stage, we do not know enough about how a star’s brightness develops before such an event. That said, this makes Betelgeuse rather interesting for astronomers.

If it did occur, it would become the brightest supernova ever observed. In a matter of days, it would become as bright as the full moon, be visible during day time and be bright enough at night to cast shadows on Earth.

Nobody knows what will happen, if anything. If Betelgeuse were to explode, it would raise questions about why we are around at this time to witness such a rare phenomenon. But it would also be an opportunity to learn more about supernovas, and compare the observations with models.

Betelgeuse, the “armpit of Orion” is a familiar sight in the winter sky. Photo by David Coppedge

Update 2/26/2020: Nature says that Betelgeuse has started to brighten up again. Many stars are cyclical. Perhaps Betelgeuse has several modes of oscillation.

Stellar Evolution Theory Evolves

Astronomers divide supernovas into various types and subtypes (e.g., Type 1a, thought to represent material from a binary system falling onto a white dwarf). Type 1a supernovas have been important “standard candles” for measuring vast distances in space, but occasionally corrections need to be made to fit observations with theory. These articles show that theories are never finished:

Modeling a superluminous supernova (Science Magazine). Keith Smith says, “Superluminous supernovae can be up to 100 times brighter than normal supernovae, but there is no consensus on how such bright transients are produced.” He presents a recent model correction to explain this type.
Mysteriously bright supernova may have smashed up a huge gas cloud (New Scientist). Leah Crane shows how theories need to evolve to fit “weird” situations and “strange variants” of supernova types that astronomers thought they understood.

The sort of supernova that creates enough iron to match this one is called a type Ia, but those are usually 100 times dimmer than SN 2006gy. The best way the researchers found to make a type Ia supernova 100 times brighter is for it to slam into a cloud of material as it explodes, converting the kinetic energy of the blast into light.

The scenario that Jerkstrand and his team found that best matches SN 2006gy starts with a pair of stars orbiting one another in a shared cloud of gas. As the two spiralled towards one another, the gas was blown off, creating a cloud around the stars. When they collided, they blew up and the blast crashed through that cloud in an explosion of light.

“Scenarios” are stories trying to compare theory to observations, but if the observations cannot be made, the scenario is little more than an idle tale. As one astronomer quipped, ‘No observation should be considered valid until it has been confirmed by theory.” (That’s backward, folks.)

Theories of stars can be very sophisticated, with detailed mathematical equations describing their structure and expected behavior. Still, a model is only a simulation of reality – not reality itself. In astrophysics, unlike in biological evolution “scenarios,” scientists can apply known physical laws to the observations. They can try to refine their models, but never reach absolute certainty. “Strange variants” continue to arise. When enough anomalies arise within a paradigm, a scientific revolution may follow.

Nobody knows what will happen to Betelgeuse, but it’s intriguing to observe it and try to understand it. That’s a legitimate human enterprise. Invoking unseen occult forces is not.


The Stars

What are Stars?

A star is a massive, luminous sphere of plasma held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on the planet. Some other stars are visible from Earth during the night when they are not obscured by clouds or other atmospheric phenomena, appearing as a multitude of fixed luminous points because of their immense distance. Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

Billions of Stars in the Universe!

The observable universe consists of the billions of galaxies (and hence billions of stars) and other matter that can, in principle, be observed from Earth in the present day—because light (or other signals) from those objects has had time to reach the Earth since the beginning of the cosmological expansion. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical volume (a ball) centered on the observer, regardless of the shape of the universe as a whole. Every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.

Based on current estimates, there are between 200 – 400 billion stars in our galaxy (The Milky Way). There are possibly 100 billion galaxies in the Universe. So taking the average of our galaxy, gives approximately 3 x 1024 stars. So about 3 septillion. This has been equated to the same number of grains of sand that are on Earth!

The newest estimates gained by the Hubble space telescope places the estimate of 500 billion Galaxies each with about 300 billion stars for each galaxy.

A star-forming region in the Large Magellanic Cloud. NASA/ESA image

Basic Facts about Stars

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star’s interior and then radiates into outer space. Once a star’s hydrogen is nearly exhausted, almost all naturally occurring elements heavier than helium are created, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when very massive stars explode. Near the end of its life, a star can also contain a proportion of degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, luminosity, and spectrum respectively. The total mass of a star is the principal determinant of its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

Betelgeuse is a red supergiant star approaching the end of its life cycle. Image courtesy of NASA

How Stars are Born

A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes. The star’s internal pressure prevents it from collapsing further under its own gravity.

Star Birth: Carina Nebula, a sprawling and complex Escher-like region of gas and dust about 7500 light years away. It’s the scene of chaotic star birth and death, slammed and reslammed by winds from stars being born and others busy blowing up. Image courtesy of NASA

Death of Stars

Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of its matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or (if it is sufficiently massive) a black hole.

The Star Explodes!

A supernova is a stellar explosion. Supernova are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval a supernova can radiate as much energy as the Sun is expected to emit over its entire life span. The explosion expels much or all of a star’s material at a velocity of up to 30,000 km/s (10% of the speed of light), driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

The Star Explodes! The Crab Nebula, remnants of a supernova that was first observed around 1050 AD. Image courtesy of NASA

The Star becomes a White Dwarf!

A white dwarf, also called a degenerate dwarf, is a stellar remnant composed mostly of electron-degenerate matter. They are very dense a white dwarf’s mass is comparable to that of the Sun, and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy.

A white dwarf star in orbit around Sirius (artist’s impression). Image courtesy of NASA

The Star becomes a Neutron Star

A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without net electrical charge and with slightly larger mass than protons. Neutron stars are very hot and are supported against further collapse by quantum degeneracy pressure due to the phenomenon described by the Pauli exclusion principle. This principle states that no two neutrons (or any other fermionic particles) can occupy the same place and quantum state simultaneously.

A very small dense star that is composed mostly of tightly-packed neutrons (neutronium). Image courtesy of NASA

The Star becomes a Black Hole

A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return. The hole is called “black” because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics. Quantum field theory in curved spacetime predicts that event horizons emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Gravitational collapse occurs when an star’s internal pressure is insufficient to resist the object’s own gravity. For stars this usually occurs either because a star has too little “fuel” left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature. In either case the star’s temperature is no longer high enough to prevent it from collapsing under its own weight. The collapse may be stopped by the degeneracy pressure of the star’s constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. The type of compact star formed depends on the mass of the remnant—the matter left over after the outer layers have been blown away, such from a supernova explosion or by pulsations leading to a planetary nebula. Note that this mass can be substantially less than the original star—remnants exceeding 5 solar masses are produced by stars that were over 20 solar masses before the collapse.

If the mass of the remnant exceeds about 3–4 solar masses — either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. No known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the implosion and the object will inevitably collapse to form a black hole.

The gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the early universe may have resulted in very massive stars, which upon their collapse would have produced black holes of up to 103 solar masses. These black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.

Simulated view of a black hole (center) in front of the Large Magellanic Cloud. Note the gravitational lensing effect, which produces two enlarged but highly distorted views of the Cloud. Across the top, the Milky Way disk appears distorted into an arc. Image courtesy of Wikipedia

Stars of all Sizes!

A size comparison between known planets, our sun, and other stars. Image courtesy of Wikipedia

Stellar Evolution: The Life Cycle of Stars

The cycle of a star – from birth to death to rebirth. Image courtesy of Wikipedia.


Key Concepts and Summary

A supernova occurs on average once every 25 to 100 years in the Milky Way Galaxy. Despite the odds, no supernova in our Galaxy has been observed from Earth since the invention of the telescope. However, one nearby supernova (SN 1987A) has been observed in a neighboring galaxy, the Large Magellanic Cloud. The star that evolved to become SN 1987A began its life as a blue supergiant, evolved to become a red supergiant, and returned to being a blue supergiant at the time it exploded. Studies of SN 1987A have detected neutrinos from the core collapse and confirmed theoretical calculations of what happens during such explosions, including the formation of elements beyond iron. Supernovae are a main source of high-energy cosmic rays and can be dangerous for any living organisms in nearby star systems.