Yesterday, I wrote about a star called W Hydrae that was once very much like the Sun, but is much older. It has expanded into a red giant, and is close enough (about 300 light-years away) that, using sophisticated techniques, we can actually see its disk.
I also wrote about how huge it is, about 600 million kilometers across. That's big.
But some stars make that look positively dinky. Like, say, V766 Centauri. How wide is this star?
Over two billion kilometers. That's 1,500 times bigger than the Sun!
Those three images of V766 Cen are actual images, and the disk you see in it is the actual disk of the star! That's stunning to me, in no small part (haha) because the star is over 11,000 light-years away (a huge distance!). The only reason we don't see it as a single dot is that it is so ridiculously huge.
So why is it so big? The star I wrote about yesterday has a mass very similar to that of the Sun. Stars like that expand when they run out of hydrogen fuel in their cores. The core fills with helium "ash," heats up, that energy gets dumped into the outer layers, and the gas there reacts by expanding and cooling (I explain all this in my episode of Crash Course Astronomy: Low Mass Stars). For some stars this is the end of the road. For others, the helium can build up enough to start fusing into carbon, but in general that's about it. Once carbon builds up in the core and the helium runs out, the star is, for all intents and purposes, dead. It doesn't have the pressure needed to fuse that carbon, and the star dies (to be fair, a lot of really interesting stuff still happens, but the star's life as a star is pretty much done).
But stars with more mass have a different fate. If the star itself has more than about eight times the mass of the Sun, the core gets so hot and the pressure there so intense that the carbon that builds up can fuse into heavier elements like neon and magnesium. Neon builds up, fuses into oxygen, and off we go again. Oxygen begets silicon, and silicon begets iron, and then, well, the star explodes. You can find out more about this in — you guessed it — Crash Course Astronomy: High Mass Stars.
Each time the star fuses heavier elements, the temperature in the core goes up. Way up, like over a billion degrees up. That generates absurd amounts of energy, dwarfing the energy inside a regular low-mass star when it becomes a giant. High-mass stars get so big we call them supergiants. Though the definition is hazy, the ones that really push the envelope are called hypergiants, even bigger than supergiants. Only a handful are known in the galaxy.
V766 Cen is kinda maybe sorta a hypergiant. It turns out making this classification is hard to do; it can be difficult to pin down their size, mass, and other properties. They are so luminous that they can literally barely hold themselves together; they exist on the thin hairy edge of exploding. V766 Cen is over 600,000 times as luminous as the Sun! Replace the Sun with it and the Earth would be fried to a crisp. Heck, we'd be inside it.
Stars like this also undergo violent episodes of mass loss, blasting out huge winds of material from their surfaces. As I wrote about W Hydrae, too, the "surface" is a hard thing to define; the material just gets less and less dense as you move up from the interior until it just blends into space (much like Earth's atmosphere, which doesn't have a hard-and-fast edge to it).
The images above were taken using a detector called PIONIER, for Precision Integrated-Optics Near-infrared Imaging ExpeRiment. It's used with the Interferometer of the Very Large Telescope (VLTI), which combines the power of four 1.8 meter telescopes separated by up to 100 meters, essentially creating a virtual telescope 100 meters across. The resolution (ability to see small details) of such a beast is phenomenal, many dozens of times better than Hubble, for example.
From left to right the images were taken in 2014, 2016, and 2017 (note that the date of the earliest observation in the image is a typo; it was indeed 2014). In the earliest one the star's shape is distorted. It's thought that this is due to convection inside the star: Warmer material from deep inside the star welling up and rising to the surface.
The images from 2016 and 2017 also show an odd hot spot, but this can't be from the same cause. The temperature of that spot, and the amount of light it gives off, is far too high for it to be due to something on the star itself. It has been previously shown that V766 Cen has a companion star close by, so close that they actually have formed a peanut-shaped object called a contact binary*. It's possible that's what's being seen in the later images; in the 2014 image the smaller companion was behind the bigger primary star.
Heh. "Smaller." It's likely to be a supergiant star itself, one with 2-20 times the mass of the Sun, and also swollen up near the end of its life, enough to dwarf the Sun in stature! In fact it may be about the same size of W Hydrae … but next to the immense primary, it looks downright diminutive.
These newer observations don't pin down much about the second star. The orbit may be something like 1300 days long, but the size of the orbit isn't constrained much by these images, and the size of the orbit is key to knowing the masses of the two stars. They could combine to have as little as 30 times the Sun's mass, or as much as over 100! Either way, these are monster objects.
But a hypergiant? Maybe not; the 2014 image shows the primary star of V766 Cen to have the characteristics more like a red supergiant … but a huge one, at the top of the scale. Some red supergiants can evolve into hypergiants as the next step in fusion happens in the core, and that may be what V766 Cen is doing right now.
Either way, this star is not much longer for this Universe. At this stage in its evolution it's already well along the path of fusing heavier and heavier elements. Soon, maybe in the next million years or so (and probably a lot less), it'll build up iron in its core, and that leads to one inevitable conclusion: Boom. Supernova.
At its distance of 11,000 light-years or so, that explosion will be bright as seen from Earth, probably a few times brighter than Venus. That's incredible! It's 10% of the way across the galaxy from us, but when it explodes it'll be bright enough to cast shadows at night! We'll be safe from any dangers from it, but that doesn't lessen just how ridiculously overwrought everything about this star is.
Sometimes we take our Sun for granted. It's just there, every day (well, literally every day), just shining away pretty steadily, safe and boring.
You know what? That's perfectly OK by me. Things could be a lot worse.
* Even more fun, there's a third star orbiting farther out, a B0 supergiant star itself that dwarfs the Sun in mass and size. I'm pretty glad this system is so far away. It's terrifying.