When it comes to star size, there are dwarfs, there are giants and there are supergiants.
And then there are hypergysts.
These are very massive stars that live fast, die young and come out with a huge explosion: Supernove. And now we know that before leaving, he also suffers from coughing fits: epic eruptions of dust clouds screaming at high speed, causing the star to change quickly and profoundly in brightness.
If it sounds familiar, yes, consider Betelgeuse. We will return to him.
But in this case we are talking about the star VY Canis Majoris (or VY CMa for short). This ridiculously swollen red hypergiant is about 4,000 light-years away in the constellation Canis Major, the Great Dog (one of Orion’s hunting dogs). In this case, the constellation is right: VY CMa is a huge star, far above 2 billion kilometers wide.
For comparison, the Sun is 1.4 million km long. VY CMa is over a thousand times higher. A thousand. Replace the Sun with VY CMa and it would extend almost into Saturn’s orbit.
It would be too bad for Earth. We would be inside. And given that the star generates several hundred thousand times the energy of the Sun, our planet would not last long there.
So, yes, this star is overwhelming in every way. Stars of this kind 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, the matter there was thrown by the intensity of the radiation below. The VY CMa probably started 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.
The star’s observations show that it emits too much infrared light for such a star, which is a telltale sign that it is surrounded by dust. This is usually microscopic grains of rocky material (loaded with silicate) or carbonic (soot) around the star (so we call it circumstellar, which is just a cool word). It is heated by starlight and thus glows infrared, causing the observed excess.
The extremely high observations of VY CMa show this dust and also show that it is quite complex. There are knots, clusters, arcs and clouds scattered around the star. New observations using Hubble, however, have allowed astronomers to measure the speed with which all this dust moves – much of it has been evacuated by tens of thousands of kilometers per hour. Fast. VY CMa does great things.
The beauty is that they then measured the distance from the star to these different agglomerations and used it along with the speed to track the agglomerations back in time, to see when they were expelled. What they found is really interesting … the ages of the different agglomerations and other characteristics indicate that they were blown by the star 70, 120, 200 and 250 years ago.
Looking at the historical observations of the star, these periods coincide with periods of great variability in the brightness of the star, diminishing and shining with a large factor.
In other words, a physical mechanism in the star caused it to burst on these huge clouds of dust, and these clouds then passed between us and the star, diminishing it. The last major eruption occurred in the late 1800s, when the star faded greatly. Before, it was visible to the naked eye (barely), but after that eruption it faded and has not lit up since.
And this is so interesting, because everyone’s favorite star, not yet exploded, Betelgeuse just suffered an extraordinary fading event in late 2019. For several months, the star shone in the middle of its usual red hue, and astronomers still she argues about what caused her. The two main competitors are a cooling effect that has reduced its brightness, and the other is – you guessed it – dust eruptions that have blocked the star. In fact, I prefer the latter explanation; there is abundant dust around Betelgeuse and we know that sometimes this material blows in big clouds. But a drop in temperature cannot yet be ruled out.
However, Betelgeuse is a red supergiant. Smaller 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 the dust and fades, then it makes sense the same thing could happen with Big B.
There are other differences, some of which are important. Betelgeuse is a common variable star, which undergoes cyclic changes in brightness of the order of one year due to the 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, most likely due to the fact that things take place in its upper atmosphere. So you have to be careful when extrapolating from one star to another. But still, it’s a challenging idea.
Stars like this fascinate and terrify me. It is difficult to understand how huge they are of crushing the soul, how strong and how they live their lives. But they are crucial to galactic evolution; they create heavy elements such as iron in their cores that are distributed in space when they explode. This material then goes into creating new stars, new planets … and us. 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 first pumped it into the galaxy. If that alone is not a sufficient reason to study such stars, nothing is.