When it comes to star sizes, there are dwarves, there are giants and there are supergiants.
And then there are hypergiants.
These are a lot massive stars that live quickly, die young and come out with a huge explosion: supernovae. And now we know that before they leave they also suffer from coughing attacks: epic eruptions of dust clouds screaming at high speed, causing the star to change rapidly and profoundly in its brightness.
If this sounds familiar to you, yes, keep in mind at Betelgeuse. We will be back.
But in this case we are talking about the star VY Canis Majoris (or VY CMa in short). This ridiculously inflated red hypergiant is about 4,000 light-years away in the constellation of Canis Major, the big dog (one of Orion’s hunting dogs). In this case, the constellation is appropriate: VY CMa is an immense, well-finished star 2 billion miles wide.
In comparison, the Sun is 1.4 million km in diameter. VY CMa is more than a thousand times larger. A thousands. Replace the Sun with VY CMa and it would extend almost to the orbit of Saturn.
That would be very bad for Earth. We would be inside her. And given that the star generates hundreds of thousands of times the energy of the Sun, our planet would not last long.
So yes, this star is crushing in every way. These stars do not last long, only a few million years, and as they age, they generate so much light that they exploit their own surfaces, matter there escaping by the intensity of radiation from below. VY CMa probably started with up to 40 times the mass of the Sun, but has already lost about half of it. And this is where our story really begins.
Observations of the star show that too much infrared light bursts for such a star, which is a telltale 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 circumstellar, which is just a funny word). The light from the stars heats it up and therefore shines in the infrared, causing the observed excess.
High-resolution observations of VY CMa show this powder and also show that it is quite complex. There are knots, groups, arcs and fuzzy clouds around the star. The new observations used by Hubble, however, allowed astronomers to measure the speed at which all this dust moves, much of which was ejected at tens of thousands of miles per hour. Fast. VY CMa makes things big.
The beauty of this is that they then measured the distance of the star to these various groups and used it along with the speed to track the groups back in time, to see when they were expelled. What they found is interesting, in fact … the ages of the various groups and other characteristics indicate that they were flown from the star about 70, 120, 200 and 250 years ago.
Observing historical observations of the star, these periods coincide with periods of great variability in the brightness of the star, fading and shining by a large factor.
In other words, some physical mechanism of the star caused these huge clouds of dust to come out, and these clouds passed between us and the star, attenuating 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 darkened and hasn’t looked brighter since.
And this is very interesting because everyone’s favorite star, although exploded, Betelgeuse has just suffered a huge darkening in late 2019. For several months, the star shone in the middle of its usual red hue, and astronomers are still arguing about what caused it. The two main contenders are a cooling effect that decreased its brightness 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 this material rips in big clouds. But a drop in temperature cannot yet be ruled out.
Still, Betelgeuse is a red supergiant. Lower mass, smaller and not as bright as VY CMa (which, after all, is one of the brightest stars in the entire galaxy), but very similar. If VY CMa blows dust and darkens, then it makes sense that the same could happen with Big B.
There are other differences, some of which are important. Betelgeuse is a regular variable star, which undergoes cyclical changes in brightness on the order of one year due to physics happening in its lower atmosphere. VY CMa is an irregular variable and changes in its brightness take many years to complete, things are more likely to go through its very high atmosphere. Therefore, care must be taken extrapolating from one star to another. But still, it’s a provocative idea.
Stars like this fascinate and terrify me. It is difficult to grasp the enormous immensity they have, how powerful they are and how they live their lives. But they are crucial to galactic evolution; they create heavy elements such as iron in the nuclei that are distributed throughout space when they explode. This material is aimed at creating new stars, new planets … and We. Literally, me and you.
The blood iron pumped by the body was once in the core of an exploding star like VY CMa, which first pumped it into the galaxy. If that just isn’t reason enough to study stars like that, nothing is.