When it comes to star sizes, there are dwarfs, there are giants and there are supergiants.
And then there are hypergiants.
This is very massive stars that live fast, die young and go out with a huge bang: Supernovae. And now we know that before they leave, they also suffer from coughing fits: epic eruptions of dust clouds screaming away at a high speed, causing the star to change rapidly and deeply in brightness.
If this sounds familiar, then keep Betelgeuse in mind. We’ll get back to that.
But in this case we are talking about the star VY Canis Majoris (or VY CMa for short). This ridiculously inflated red hypergiant is 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 big star, past 2 billion kilometers wide.
By comparison, the sun is 1.4 million km wide. VY CMa is more than a thousand times larger. A thousand. Replace the sun with VY CMa and it will extend to the orbit of Saturn.
It would be too bad for the earth. We will be in it. And since the star generates the sun’s energy several hundred thousand times, our planet will not stay there long.
So yes, this star is crushing in every way. Stars like these do not last long, only a few million years, and as they age, they generate so much light that they blow from their own surfaces, and the matter is thrown away by the intensity of the radiation from below. VY CMa probably started with as much as 40 times the sun’s mass, but lost about half of it. And this is where our story really begins.
Observations of the star show that it emits too much infrared light for 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 (soot) material around the star (so we call it circular, which is just a cool word). It is heated by the starlight and thus glows in the infrared, causing the observed excess.
Extremely high resolution observations of VY CMa show this substance, and it also shows that it is quite complex. There are knots, tufts, arcs and diffuse clouds around the star. With new observations with Hubble, however, astronomers have been able to measure the speed at which all this matter moves – many of which are ejected at tens of thousands of kilometers per hour. Quickly. VY CMa do things great.
The beauty of this is that they then measured the distance from the star to these different clusters and used it along with the velocity to trace the clusters back in time to see when they were ejected. What they found is indeed interesting … the ages of the different clusters and other characteristics indicate that they were blown off the star about 70, 120, 200 and 250 years ago.
If we look at historical observations of the star, these periods coincide with times of large luminosity changes in the star, which are dimmed and illuminated by a large factor.
In other words, some physical mechanism in the star caused these enormous clouds of dust to bulge out, and these clouds then passed between us and the star and dimmed. The last major eruption was in the late 1800s, when the star faded a lot. It was previously (barely) visible to the naked eye, but after the eruption it faded and has not really brightened since.
And this is very interesting, because Betelgeuse’s favorite not yet completely exploding star Betelgeuse underwent a huge eclipse event at the end of 2019. For several months, the star has shown half of its usual red color, and astronomers are still debating what caused it. The two main competitors are a cooling effect that has dropped the brightness, and the other is – you can rest assured – dust bursts that have blocked the star. I actually prefer the latter explanation; there is abundant dust around Betelgeuse, and we know that it sometimes blows this material down in great clouds. But a drop in temperature can not yet be ruled out.
Yet Betelgeuse is a red supergiant. Lower mass, smaller and not as bright as VY CMa (which is, after all, one of the brightest stars in the entire galaxy), but much the same. If VY CMa blows dust and fades, it makes sense that the same could go on with Big B.
There are other differences, some of which are important. Betelgeuse is a regular variable star, which undergoes cyclical changes in brightness in the order of a year due to physics taking place deep in its lower atmosphere. VY CMa is an irregular variable, and the changes in the brightness of it take many years to complete are more likely due to the things going on in the upper atmosphere. So you have to be careful about extrapolating from one star to another. But still, it’s a challenging idea.
Stars like this fascinate and scare me. It is difficult to comprehend how soul-shifting they are, how powerful and how they live their lives. But it is crucial for galactic evolution; they create heavy elements such as iron in their nuclei that are scattered through space when they explode. This material then goes into making new stars, new planets … and we. Literally, you and me.
The iron in your blood that was pumped through your body was once in the core of an exploding star like VY CMa, which first pumped it into the galaxy. If that alone is not reason enough to study such stars, there is nothing.