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astronomy

Cranky Stars Go “Boom” Eventually

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Everybody’s been in a lather about one of the most famous stars in the sky. That would be Betelgeuse and its “mysterious” dimming or “fainting” that is catching astronomers’ attention. I’ve seen a lot of headlines about how scientists are perplexed or confused or whatever by this star losing some brightness. Even worse are the headlines about how it’s acting strangely, or that its behavior is very weird or that it’s about to explode.

So, I started digging around about it because whenever I see clickbait headlines, I know that somebody didn’t understand the astronomy. Turns out that Betelgeuse is pretty much acting as it has throughout its later life. That’s offering astronomers more insight into its late-stage behavior, which gives a peek at how old, massive stars age and eventually die.

Betelgeuse, which is the bright left “shoulder” of the constellation of Orion, is a variable star. That means it pulsates in brightness over time. It goes from being the eleventh-brightest star in the sky to being not even in the top twenty brightest stars. Right how, it is, in fact, the faintest it’s been more than a hundred years. Sounds pretty interesting, no? It certainly is if you’re a variable star researcher.

chart of Orion showing location of Betelgeuse
A chart view of the stars of Orion, showing Betelgeuse in the shoulder of the giant. The three belt stars run through the middle, and just below them is the Orion Nebula starbirth region. Courtesy Zwergelstern on Wikimedia Commons.

It turns out that Betelgeuse goes through several cycles of dimming and brightening over time. These are due to changes in size and temperature. It’s also possible that the current dimming is being caused by an outburst of material from the star that’s partially obscuring our view. A lot of stars “huff off” material as they age and that stuff creates a shell around the star. That would cause it to appear dimmer to us because some of the light is absorbed or blocked by the material in the shell.

artist concept of Betelgeuse showing its size and possible "look".
An artist’s conception of what Betelgeuse looks like, and its size compared to other objects. It’s one of many supermassive red giants that will eventually collapse in part of supernova events. Credit: ESO/L Calcada

Behavior of Cranky Old Stars

Apparently all of this is completely normal for Betelgeuse. It’s an aging supermassive star and objects like that are cranky as they get older. They oscillate and spew. And, eventually, they blow up. So, does all this dimming and mass loss mean that Betelgeuse is about to explode as a supernova? Probably not. Although, to be fair, going supernova IS the end state for this star. But, it’s not likely to happen for a while.

It might be as little as around a hundred thousand years before this aging red supergiant collapses in on itself and then bursts out as a supernova. Or it could take a bit longer. But, it’s not likely to be tomorrow. Or the next day. Or anytime soon. Kind of disappointing to those of us who’d like to witness a relatively nearby supernova in our lifetimes, but that’s the breaks.

What WILL happen when Betelgeuse gets to its bursting point? Its core will collapse when it runs out of fuel to sustain fusion reactions. The weight of all the layers of gas above it will cause them to crash in on the core. There will be a huge bounce (a rebound) and a lot of Betelgeuse’s material will burst out in a catastrophic explosion. That’ll spread “star stuff” throughout its neighborhood. It won’t all be gone, though. The heavily smashed core will remain, most likely as a neutron star. There won’t be enough mass for it to form a black hole, unfortunately. But, the fireworks show will be a great one, for our great-great-great^n grandkids. They’ll see it from Earth at a relatively safe distance of about 642 light-years.

Another Cranky Type of Star

While Betelgeuse grabs all the attention, another type of star IS going to explode by century’s end. It’s called binary star V Sagittae (V Sge), and it’s not just one star. It’s in a class of object called a “cataclysmic variable binary star”. V Sge has a rather ordinary star orbiting in a binary dance with a white dwarf star. The regular star is losing mass, which is falling onto the white dwarf. That action is powering a hugely powerful stellar wind, as well. Scientists have recently announced that this star is going to give us quite a light show by century’s end. At that point, it will become the most luminous star in the galaxy. It’ll be at least as bright as Sirius, which is currently the brightest star in our night-time sky. It’ll be quite the spectacle.

artist's concept of binary variable star.
Artist’s concept of a binary star; in the case of V Sge, the normal star is much more massive than its white dwarf neighbor, and has lots of mass to “dump” on the dwarf. There are many binary stars in our galaxy and beyond. Courtesy NASA.

The action will start long before V Sge is set to explode. It won’t just flare up suddenly. We’ll have plenty of warning. Over the next decades, It will brighten up quite a bit. About the year 2083, the accretion of material from the normal star to the white dwarf will speed up. That will dump a LOT of material onto the white dwarf.

The fate of the normal star isn’t a good one. It will spiral in toward its white dwarf neighbor, losing more and more mass, which will cause them to brighten up. Eventually, the two stars will merge, and the final explosion will take place. What’s left will be a very massive white dwarf with a tightly packed core, and a vast envelope of burning hydrogen around it. The whole thing will be quite a show, and our great-grandkids will watch it from Earth at a very safe distance of about 8,000 light-years.

astronomy, dark matter, women and astronomy

Vera C. Rubin and a Dark Matter Observatory

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Vera Rubin
Dr Vera Cooper Rubin, who (along with a team of observers) confirmed the existence of dark matter through continued observations of galaxy motions.

One of the stories coming from the American Astronomical Society meeting involves a topic that we’ve discussed here before: dark matter. Actually, it focuses directly on one of the astronomers closely associated with determining the existence of this mysterious “stuff”: Dr. Vera C. Rubin.

We’ve all heard of dark matter. It’s a weird, “invisible” material that makes up about a quarter of the mass in the universe. Nobody is quite sure what it is, but astronomers are sure it’s out there. The fact that we know even that much is due largely to Dr. Rubin and her efforts at finding it.

The dark matter story begins with a question: why don’t galaxies rotate at the velocity we expect them to? Over many years, Dr. Rubin and her team observed galaxy rotations. They compiled their data into what are called “rotation curves” and noticed that galaxies don’t always rotate the way they were expected to. Why?

Ultimately, the answer was “dark matter”. This is a type of cosmic “stuff” first suggested by Swiss astronomer Dr. Fritz Zwicky as “dunkel materie”. It could constrain the motions of objects in galaxies. It was largely unknown and theoretical then, but its effects are often observed today.

To honor Dr. Rubin’s work, the Large Scale Synoptic Telescope (LSST), currently under construction in Chile, will be renamed the NSF Vera C. Rubin Observatory (VRO). It’s the first national observatory named after a woman. VRO will be heavily involved in the search for dark matter and the even-more-mysterious dark energy.

The Vera C. Rubin Observatory on Cerro Pachón, in Chile, as seen on Dec. 18, 2019. Credit: LSST/Vera Rubin Observatory.
The newly renamed Vera C. Rubin Observatory after sunset in December 2019. Credit: LSST/Vera Rubin Observatory.

Tracking Galaxy Motions Leads to Dark Matter

Vera Cooper Rubin began her astronomy career at Vassar college at a time when women weren’t expected to do science. She went to Cornell University and Georgetown Universities, gaining her Ph.D. in 1954. Her thesis suggested that galaxies clumped together in clusters. Today, that’s accepted observational fact. Back then, galaxy clusters were still a theoretical and not-too-popular idea. Still, throughout her career, Dr. Rubin studied galaxies both individually and in clusters, and charted the motions of their stars.

In the 1960s, Rubin began working at the Carnegie Institution of Washington’s Department of Terrestrial Magnetism. Rubin’s work focused directly on galactic and extragalactic dynamics. Those subjects deal with the motions of galaxies both singularly and in clusters. In particular, Dr. Rubin studied the rotation rates of galaxies and the material in them. As I mentioned above, the team promptly discovered a puzzle: the predicted motion of a galaxy’s rotation didn’t match the observed rotation.

To understand why that might seem strange, it’s important to understand that galaxies do rotate. Astronomers expected all the material in a galaxy to rotate at rates dependent on their distance from the center. However, if they do it fast enough, they could fly apart, IF the combined gravitational effect of all their stars was the only thing holding them together. In her team’s observations, the rotation of some galaxies didn’t perform according to expectations. It implied that the mass of their stars wasn’t enough to keep them “together”. Why why didn’t they come apart? Something else had to be holding them together. The difference between the predicted and observed galaxy rotation rates was dubbed the “galaxy rotation problem”.

Dark Matter as a Vera C. Rubin’s Solution

Rubin and others decided that there was some kind of unseen mass in or around the galaxy. It was holding the galactic pieces and parts together. Based on many observations made by Rubin and her colleague Kent Ford, the mystery began to unravel. It turned out that galaxies must have at least ten times as much “invisible” mass as they do visible mass in their stars and nebulae.

Calculations showed that this invisible mass really existed. And, it might be that “dark matter” that Zwicky first suggested in the 1930s. Zwicky himself was scoffed at when he came up with the idea and later on, Rubin and her colleagues faced a lot of the same skepticism. Yet, it made sense when they invoked dark matter as an explanation for the odd rotation curves they calculated.

Dark Matter Focus from Chile

Dr. Vera C. Rubin (who died in 2016) spent much of her later life working on the dark matter problem. For that reason, the renaming of LSST in her honor is appropriate. The Vera Rubin Observatory will begin official operations in 2022. Its telescope will be mated to a state-of-the-art 3200-megapixel camera. Together with other instruments, astronomers will use it to study the universe in search of dark matter. It will also look for evidence of dark energy, study the bodies of the solar system, explore the transient optical sky, and map our home galaxy, the Milky Way. It’s a perfectly fitting tribute to a woman who persisted on research that others felt wasn’t important; not only WAS it important, it led to a new area of astrophysical research.