When it comes to star size, there are dwarfs, there are giants, and there are super giants.
And then there are hypergiants.
These are very massive stars that live fast, die young and go out with a bang: Supernovae. And now we know that before they leave, they also suffer from coughing fits: epic bursts of dust clouds that scream away at high speed, causing the star to change brightness quickly and deeply.
If that sounds familiar, keep Betelgeuse in mind. We will come back to it later.
But in this case we are talking about the star VY Canis Majoris (or VY CMa for short). This ridiculously bloated 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 star, far beyond 2 billion kilometers wide.
In comparison, the sun is 1.4 million km wide. VY CMa is more than a thousand times larger. A thousandReplace the sun with VY CMa and it would stretch almost to Saturn’s orbit.
That would be a shame for the Earth. We would be in it. And since the star generates hundreds of thousands of times the energy of the sun, our planet wouldn’t last long there.
So yes, this star is crushing in every aspect. Stars like these don’t last long, just a few million years, and as they age, they generate so much light that they blow off their own surface, the matter thrown there by the intensity of the radiation from below. VY CMa probably started with as much as 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 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 (sooty) material around the star (that’s what we call circumstellar, which is just a cool word). It is heated by the starlight and thus glows in the infrared, creating the perceived excess.
Extremely high resolution observations of VY CMa reveal this dust, and they also show that it is quite complex. There are knots, clumps, arcs and diffuse clouds around the star. New observations using Hubble allowed astronomers to measure the speed at which all this dust moves – much of it being ejected at tens of thousands of kilometers per hour. FastVY CMa does things big.
The great thing about this is that they then measured the distance from the star to these different clumps and used that in combination with the speed to track the clumps back in time to see when they were ejected. What they found is interesting indeed… the age of the various clumps and other features indicate that they were blown off the star around 70, 120, 200 and 250 years ago.
If we look at historical observations of the star, these periods coincide with times of great variability in the star’s brightness, dimming and lighting by a large factor.
In other words, some physical mechanism in the star caused these huge dust clouds to erupt, and these clouds then passed between us and the star, dimming it. The last major eruption was in the late 18th century, when the star faded to a great extent. It used to be visible to the naked eye (barely), but after that eruption, it dimmed and hasn’t really gotten any brighter since then.
And that’s so interesting because everyone’s favorite, not quite exploding star Betelgeuse just underwent a massive dimming event in late 2019. For several months, the star shone at half its usual ruddy hue, and astronomers are still arguing about the cause. The two main contenders are one cooling effect that reduced its brightness, and the other is – you guessed it – bursts of dust that blocked the star. I actually prefer the latter statement; there is an abundance of dust around Betelgeuse, and we know that it sometimes blows this material away 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 blows away and dims dust, it makes sense that the same could be going on with Big B.
There are other differences, some of which are important, however. Betelgeuse is a regular variable star that undergoes cyclical changes in brightness of the order of a year due to physics taking place deep in the lower atmosphere. VY CMa is an irregular variable, and it takes many years for the brightness changes to complete, and is more likely due to the things that are happening to it in the very highest atmosphere. So you have to be careful about extrapolating from one star to another. Still, it’s a provocative idea.
Stars like this fascinate and terrify me. It’s hard to grasp how overwhelmingly crushing they are, how powerful, 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 scattered throughout space when they explode. This material is then used to create new stars, new planets… and U.SLiterally, 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 pumps into the galaxy. If that alone isn’t reason enough to study stars like that, then there is nothing.