Welcome to Three Alpha! Since last time: In the Solar System, the longstanding mystery of the high temperature of the Sun’s corona may have been solved; in the Galaxy, the LOFAR radio array has discovered a new type of radio source with unexpectedly polarised emissions; and in the Universe, a new study has found that the “little red dots” are consuming matter below the Eddington limit.

Meanwhile, in this edition of the newsletter we’re focusing on how stars end up in binaries. Read on for more…

Born from the same cloud

One of the most well-known stars in the sky, the red giant Betelgeuse on the shoulder of Orion, seems to have a tiny stellar companion, according to research published last month. People have claimed to have found evidence for a companion in the past, which has never held up to scrutiny, but recently evidence has been growing that there really is a companion in a close orbit with Betelgeuse. The latest is a photograph of the companion showing it at its furthest point from Betelgeuse, in 2024. If this object really exists, it would potentially explain Betelgeuse’s long period (almost six year) variability. There is still some doubt, due to the extreme difficulty of detecting it, and the complicated process used to produce the image, but it is looking good.

So how surprising is this? How common is it for a star like Betelgeuse to have a companion, and how did the companion get there? Stars with companions like this are called binaries, and it is actually as common for stars to have companions than to not have them. Roughly half of all stars have binary companions, with perhaps as many as 70% of the most massive stars having them. The most massive stars are the ones, like Betelgeuse, which go on to become red giants in their old age. Substantially fewer red giants are in binaries than younger stars (less than about 40%), since close companions tend to be swallowed up when the star expands to become a red giant. So it is not that surprising for a star like Betelgeuse to have a binary companion, but it also would not be surprising if it was solo instead.

So how did all these stars end up with companions? The answer has to do with how stars form in the first place.

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All stars form in roughly the same way: a giant cloud of cold, dense gas gets disturbed, small bits of the cloud start to collapse due to gravity, and eventually those collapsing regions get dense enough for nuclear fusion to start, and a star is born. Shortly afterwards, planets, moons, asteroids, and everything else form from the material that is left.

So why does the cloud being disturbed make parts of it collapse? And what determines which parts of the cloud collapse and which don’t? It’s all down to something called the Jeans instability (I could make a bad joke about denim here, before revealing that it is named after the astronomer James Jeans). Before star formation starts, the cloud is in equilibrium: pressure (from the cloud’s heat) is in balance with the cloud’s gravity, stopping it from collapsing. The pressure depends on the cloud’s temperature (and in turn the density of the gas). The strength of the gravitational attraction depends on the density and the mass of the gas in a given volume. If something disturbs the gas, such as a shock wave from a nearby supernova, that can change the density of parts of the cloud (i.e. as the shockwave sweeps through) and can move material around, changing the mass of small regions of the cloud. The overall effect of this is that parts of the cloud can become unstable, as their gravity suddenly outweighs their internal pressure. Those regions begin to collapse. Other regions, less dense or warmer to begin with, will not collapse.

Once the collapse starts, the only thing powerful enough to stop it is the ignition of nuclear fusion in the newly forming protostar. Once the Jeans instability is triggered, there is no going back to the old equilibrium. Star formation is inevitable

This scenario also explains why so many stars end up in binaries. I mentioned that the gas clouds are huge. They contain many, many times the mass of a single star. And when they are disturbed, more than one region can pass the Jeans threshold and collapse into a new star. Regions that are close enough together can create stars that are gravitationally bound to each other. Those new stars can end up orbiting each other if they’re really close, and otherwise they might stay together in a star cluster without actually being in a binary.

Binary stars will live their whole lives together, potentially interacting and affecting each other. Often, stars in binaries can cause variations in each other’s brightness. Studying this stellar variability is a key way that we can use to learn what is happening to the stars. There are many types of stellar variability, and while many are not caused by binary companions, some are. One example is ellipsoidal variations, where the shape of a star is distorted by its companion’s gravity, making it appear brighter from certain angles, which we see as the stars orbit each other.

This is why astronomers have long suspected that Betelgeuse has an unseen companion: it has long-period variations (almost six years) which would be explained by a companion periodically getting closer during its orbit. This new detection of a possible companion fits the observed variations. It is still early, and the companion is not quite confirmed yet, but it is starting to look likely that Betelgeuse is really in a binary system.

Finally

Have you ever seen a planet disappear? Check out this video fading between observations of Saturn taken in two different wavelengths:

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].