When it comes to star sizes, there are dwarves, there are giants, and there are supergiants.
And then there are hypergiants.
These are much massive stars that live fast, die young and go out with a huge bang: Supernovae. And now we know that before they go, they also suffer from coughing fits: epic eruptions of clouds of dust screaming away at high speed, causing the star to change rapidly and deeply in brightness.
If this sounds familiar, remember Betelgeuse. We return to it.
But in this case we are talking about the star VY Canis Majoris (or in short VY CMa). This ridiculously inflated red hypergiant is about 4,000 light-years away in the constellation Canis Major, the big dog (one of Orion̵
By comparison, the sun is 1.4 million km across. VY CMa is over a thousand times larger. ONE thousand. Replace the sun with VY CMa, and it extends almost to the orbit of Saturn.
That would be too bad for Earth. We would be inside it. And given that the star generates hundreds of thousands of times the Sun’s energy, our planet would not last long.
So yes, this star is crushing in every aspect. Stars like this do not last long, only a few million years, and as they get older, they generate so much light that they blow off their own surfaces, the matter being thrown away by the intensity of the radiation from below. VY CMa probably started with as much as 40 times the mass of the sun, but has already lost approx. half of it. And this is where our story really begins.
Observations of the star show that it blows too much infrared light next to a star of its kind, which is a sign that it is surrounded by dust. These are usually microscopic grains of rocky (silicate-laden) or carbonaceous (sooty) material around the star (so we call it circumference, which is just a cool word). It is heated by the starlight, and then it glows in the infrared, causing the observed excess.
Extremely high resolution observations of VY CMa show this dust and they also show that it is quite complex. There are nodes, lumps, arcs and diffuse clouds around the star. However, new observations using Hubble allowed astronomers to measure the speed at which all this dust moves – much of it was pushed out at tens of thousands of kilometers per hour. Quick. VY CMa makes things great.
The beauty of this is that they then measured the distance from the star to these various lumps and used it along with the speed to track the lumps backwards in time to see when they were pushed out. What they found is interesting … the age of the various lumps and other features indicates that they were blown up by the star about 70, 120, 200 and 250 years ago.
When looking at historical observations of the star, these periods coincide with times of great brightness variation in the star, dimmed and the light by a large factor.
In other words, some physical mechanism in the star caused it to bend these huge clouds of dust, and these clouds then passed between us and the star and attenuated it. The last major eruption was in the late 1800s, when the star faded a lot. It used to be visible to the naked eye (hardly), but after this eruption it dimmed and has not really lit up since.
And it’s so very interesting because everyone’s favorite, not quite explosive star Betelgeuse just underwent a huge attenuation event in late 2019. For months, the star shone to half its usual red hue, and astronomers are still arguing about what caused it. The two main competitors are a cooling effect that dropped its brightness, and the other is – you guessed it – eruptions of dust that blocked the star. I actually prefer the latter explanation; there is plenty of dust around Betelgeuse, and we know that it sometimes blows this material off in large clouds. But a temperature drop can not yet be ruled out.
Betelgeuse is still a red super giant. Lower mass, smaller and not as luminous as VY CMa (which after all is one of the brightest stars in the entire galaxy), but very similar. If VY CMa blows dust off and dampens, it makes sense that the same can happen with Big B.
There are other differences, some of which, however, are important. Betelgeuse is a regular variable star that undergoes cyclical changes in brightness on the order of one year due to physics taking place deep in its lower atmosphere. VY CMa is an irregular variable, and the changes in its brightness taking many years to implement are more likely due to things going on in its very upper atmosphere. So you have to be careful about extrapolating from one star to another. But still, it’s a provocative idea.
Stars like this fascinate and scare me. It’s hard to understand how soul-crushing huge they are, how powerful and how they live their lives. But they are essential for galactic evolution; they create heavy elements like iron in their nuclei, which are distributed throughout space when they explode. This material then goes into creating new stars, new planets … and U.S. Literally me and you.
The iron in your blood pumped through your body was once at the core of an exploding star like VY CMa, which first pumped it into the galaxy. If that alone is not reason to study stars like that, there is nothing.