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When it comes to the size of stars, there are dwarves, 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 bang: Supernovae. And now we know that before they leave they also suffer from coughing fits – epic eruptions of dust clouds that howl at high speed, causing a rapid and profound change in the star’s brightness.
If this sounds familiar to you, keep Betelgeuse in mind. We will come back to that.
But in this case, we’re talking about the VY star 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 Big Dog (one of Orion’s hunting dogs). In this case, the constellation is appropriate: VY CMa is a huge star, well above 2 billion kilometers wide.
For comparison, the Sun is 1.4 million km in diameter. VY CMa is over a thousand times larger. A thousand. Replace the Sun with VY CMa and it would extend almost to Saturn’s orbit.
It would be a shame for the 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 aspect. Stars like this don’t last long, only a few million years, and as they get older they generate so much light that they blow off their own surface, matter is thrown into them by the intensity of the sun. 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 it throws far too much infrared light for a star of its kind, 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 cool word). It is warmed by starlight and therefore glows in the infrared, causing the observed excess.
Very high resolution observations of VY CMa show this dust, and they also show that it is quite complex. There are nodes, clusters, arcs and diffuse clouds around the star. New observations using Hubble, however, have allowed astronomers to measure how fast all that dust is moving – much of it has been thrown out at tens of thousands of kilometers per hour. Quickly. VY CMa made it big.
The beauty of this is that they then measured the star’s distance to these different clusters and used it in conjunction with speed to trace the clusters back in time, to see when they were ejected. What they found is really interesting… the age of the different tufts and other characteristics indicate that they were torn from the star around 70, 120, 200 and 250 years ago.
Looking at the star’s historical observations, these periods coincide with periods of great variability in the star’s brightness, darkening and brightening by an important factor.
In other words, a physical mechanism in the star caused it to eject these huge clouds of dust, and those clouds then passed between us and the star, dimming it. The last major eruption dates back to the late 1800s, when the star faded a lot. It was visible to the naked eye (barely), but after this rash it has subsided and hasn’t really cleared up since.
And it’s so interesting because everyone’s favorite star Betelgeuse, which has yet to explode, just suffered a huge gradation event at the end of 2019. For several months, the star shone. at half its usual reddish hue, and astronomers are still arguing over what caused it. The two main contenders are a cooling effect that caused its brightness to drop, and the other is – you guessed it – eruptions of dust that blocked the star. I actually prefer the latter explanation; there is a lot of dust around Betelgeuse, and we know that it sometimes blows this material in big clouds. But a drop in temperature cannot yet be ruled out.
Yet Betelgeuse is a red supergiant. Weaker, smaller, and less luminous mass than 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, it makes sense for the same to happen with Big B.
There are other differences, some of which are important. Betelgeuse is a regular variable star, undergoing cyclical changes in brightness of the order of a year due to physics taking place deep in its lower atmosphere. VY CMa is an irregular variable, and changes in its brightness take many years to complete, are more likely due to things happening in its very upper atmosphere. We must therefore be careful when extrapolating from one star to another. But still, it’s a provocative idea.
Stars like this fascinate and terrify me. It’s hard to understand how incredibly huge they are, how powerful they are, and how they live their lives. But they are crucial for galactic evolution; they create heavy elements like iron in their cores which are distributed in space when they explode. This material then goes into the making of new stars, new planets … and we. Literally me and you.
The iron in your blood pumped through your body was once the core of an explosive star like VY CMa, which first pumped it into the galaxy. If that alone isn’t reason enough to study stars like this, nothing is.
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