When it comes to star sizes, there are dwarfs, giants, and supergiants.
And then there are the hypergiants.
These are very massive stars that live fast, die young, and come out with a huge explosion: supernovae. And now we know that before they leave they also suffer coughing fits: epic eruptions of dust clouds that scream at high speed, causing the star to change rapidly and profoundly in brightness.
If that sounds familiar, yes, consider Betelgeuse. We will come back to that.
But in this case we are talking about the star VY Canis Majoris (or VY CMa for short). This ridiculously bloated red hypergiant is located about 4,000 light-years away in the constellation Canis Major, the Big Dog (one of Orion’s hunting dogs). In this case, the constellation is appropriate: VY CMa is a huge star, well above 2 trillion kilometers wide.
For comparison, the Sun is 1.4 million kilometers in diameter. VY CMa is a thousand times bigger. TO one thousand. Replace the Sun with VY CMa and it would stretch almost to the orbit of Saturn.
That would be a shame for Earth. We would be inside him. And since the star generates several hundred thousand times the energy of the Sun, our planet would not last long there.
So yeah, this star is crushing in every way. Stars like this don’t last long, only a few million years, and as they age they generate so much light that they blow onto their own surfaces, the matter thrown off by the intensity of radiation from below. VY CMa probably started out with up to 40 times the mass of the Sun, but has already lost about half. And this is where our story really begins.
Observations of the star show that it is emitting too much infrared light for such a star, which is a tell-tale sign that it is surrounded by dust. They are usually microscopic grains of rocky material (loaded with silicates) or carbonaceous (soot) around the star (so we call it circumstellar, which is a cool word). It is heated by starlight and therefore shines in the infrared, causing the excess observed.
Extremely high resolution observations of VY CMa show this dust, and also show that it is quite complex. There are knots, clusters, arcs, and fuzzy clouds around the star. However, the new observations using Hubble allowed astronomers to measure the speed at which all this dust is moving; much of it was ejected at tens of thousands of kilometers per hour. Fast. VY CMa does great things.
The beauty of this is that they then measured the distance from the star to these various groups and used that in conjunction with speed to track the groups back in time to see when they were ejected. What they found is really interesting … the ages of the various groups and other characteristics indicate that they were ripped from the star about 70, 120, 200 and 250 years ago.
When looking at historical observations of the star, these periods coincide with moments of great variability of brightness in the star, darkening and brightening by an important factor.
In other words, some physical mechanism in the star caused it to eject these huge clouds of dust, and these clouds then passed between us and the star, obscuring 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 (barely), but after that eruption it dimmed and hasn’t glowed since.
And that’s very interesting because everyone’s favorite star, Betelgeuse, which has yet to explode, underwent a tremendous dimming event at the end of 2019. For several months, the star glowed half its usual reddish hue, and astronomers are still arguing about what caused it. The two main contenders are a cooling effect that lowered its luminosity, and the other is, you guessed it, dust eruptions that blocked the star. In fact, I prefer the latter explanation; There is a lot of dust around Betelgeuse, and we know that sometimes it carries this material in large clouds. But a drop in temperature cannot yet be ruled out.
Still, Betelgeuse is a red supergiant. Lower mass, smaller, and not as luminous as VY CMa (which is, after all, one of the most luminous stars in the entire galaxy), but very similar. If VY CMa is dusting and dimming, then it makes sense that the same could be happening with Big B.
There are other differences, some of which are important. Betelgeuse is a regular variable star, undergoing cyclical brightness changes of the order of one year due to physics that take place deep in its lower atmosphere. VY CMa is an irregular variable, and changes in its brightness take many years to complete, more likely due to things happening in its upper atmosphere. Therefore, you must be careful when extrapolating from one star to the other. But still, it is a provocative idea.
Stars like this fascinate and terrify me. It is difficult to understand how overwhelmingly huge they are, how powerful and how they live their lives. But they are crucial for galactic evolution; they create heavy elements like iron in their cores that are distributed through space when they explode. This material is then dedicated to the creation of new stars, new planets … and We. Literally you and me.
The iron in your blood pumped through your body was once in the core of an exploding star like VY CMa, which pumped it first into the galaxy. If that alone isn’t reason enough to study stars like this, nothing is.