What happens when Betelgeuse explodes?

If Betelgeuse explodes right now, could we see it with naked eye? It is over 400 light years away, so you might think that people would see it long after it actually happens?

Originally posted @ Medium!

Betelgeuse is already one of the brightest stars in the night sky, sitting somewhere around the 8th or 9th brightest star in the night sky. (These lists don’t include the Sun, which is somewhat obviously always the brightest object in the sky.) It sits in the constellation Orion, along with a number of other bright stars, and makes up the left hand shoulder of the warrior. It looks visibly orange in the night sky, and is classified as a red supergiant star, in the later stages of its life. It’s also one of the few stars that’s close enough for us to resolve in more detail than a point source, and the pictures are pretty fun.

If Betelgeuse were to go supernova right now — as in, if you could break physics and travel to the star instantaneously to check on it — you’re absolutely correct to think that it would take us quite a while to notice. Betelgeuse is about 600 light years away from our solar system, so the light traveling from Betelgeuse has about 600 years of travel before it will reach us. If the star had physically exploded in 2015, we wouldn’t spot the light from that explosion until 2615. We’re constantly observing this star (and pretty much everything in the Universe) as it was, a significant period of time ago. This is also why astronomers say that in studying the night sky, we study the past. The more distant the object, the further in the past we observe. 600 light years, in the grand scheme of things, is pretty close; we’re still dealing with our local neighborhood inside our own galaxy.

Supernovae are incredibly bright phenomena. At the brightest point of the explosion, a supernova can outshine the whole galaxy it lives in. A single star has managed to, for a short time, be a brighter source of light than the several billion other stars in its galaxy combined. This is tremendously bright. Supernovae do have a “rising time” of about a week, when the star is increasing in brightness — it stays at its peak brightness for a few days, and then slowly declines into obscurity over a period of a couple of weeks.


But how bright would Betelgeuse specifically be? We can do some math to work this out, making the assumption that Betelgeuse explodes as a Type II supernova. The exact style of supernova is still up for a bit of debate, depending on the exact rotation speed and mass loss of the star over the next hundred thousand years. Regardless of the exact method of its explosion, all the supernovae options for this star have a peak brightness of approximately the same value, so for a quick calculation that’s good enough to determine what we’d see with the naked eye.

There are two ways of measuring brightness in the astronomy world; the first is absolute magnitude, which is the brightness of the star, as it would be measured from a fixed distance. (It’s arbitrary, but the fixed distance chosen is 10 parsecs, or about 33 light years.) This is trying to get to a measure of intrinsic brightness — as though we could line up everything in the sky at equal distance from us, and compare them to each other that way. We can’t actually measure the brightness of a star this way, but we can apply some corrections based on the distance to the star to get to it. The absolute magnitude of a Type II supernova is around -17. Because astronomers have the worst conventions in the world (for largely “historical reasons”), negative numbers mean brighter objects. The sun has an absolute magnitude of 4.83, which, once we translate out of “magnitudes”, means that the sun is 500 million times fainter than the supernova, when measured at the same distance. This huge difference in relative brightness is why a supernova can outshine an entire galaxy.


The other method of measuring brightness is a bit more straightforward. It’s the apparent brightness — i.e., how bright does it appear to us as viewed from the Earth. In this frame of reference, more distant objects will always appear fainter, regardless of how intrinsically bright they are. Because Betelgeuse is still fairly distant from us, the apparent brightness would be significantly less than the absolute magnitude. Based on the distance to Betelgeuse, we can work out that the apparent magnitude of the peak of the explosion would be -10. The sun, in apparent magnitude, is the brightest thing in our sky, and is checking in at an apparent magnitude of -26.74. Once again translated out of magnitudes, this means that the Sun as seen from the Earth is a whopping ~5 million times brighter than Betelgeuse’s explosion, so our supernova certainly won’t be anywhere near as bright as our sun in the daytime. That’s not to say you wouldn’t be able to see it — it would definitely be bright enough to see during the daytime, as long as you were looking in the right direction. (After all, you can still see Venus in the daytime, if you know where to look!)

Nighttime will be a different story. The brightness of Betelgeuse’s supernova is about the same as the quarter moon. It would also be about 16 times brighter than the brightest supernova known to have been seen from earth, which occurred in 1006, and was recorded by a number of early civilizations. (An image of what remains of that supernova is shown below.)


It was said that the supernova in 1006 was bright enough to cast a shadow at night. Betelgeuse, being significantly brighter, would likely also cast shadows — which, if you think about the brightness of a quarter moon, would make sense!


All that said, Betelgeuse isn’t expected to explode for another 100,000 years or so. We do expect a few supernova in our galaxy every few hundred years, so there are a number of stars that are nearing the ends of their lifetimes within our galaxy. It’s hard to predict exactly when a star will transition from “close to the end of its life” to “exploding in the next week”, so while we expect that none of these will be exploding in the next little while, it’s difficult to predict which one of the stars will be the first to go. In the mean time, we can take wonderful pictures of the more nearby stars, like the one below taken by Hubble, and watch them cast off their outer layers at an incredible rate.


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What's the Hertzsprung Gap?

What is the Hertzsprung Gap, and what happens to stars that would otherwise be in it?

The Hertzsprung Gap is a feature within the Hertzsprung-Russell diagram (usually abbreviated the HR Diagram), both of which have the same name attached to them because the same person who helped to construct the HR diagram, noted the gap within it, and it began to take his name as a descriptor.

I’ve talked about the HR diagram before, and what it tells us about stars in general, but as a brief summary, the HR diagram is a plot of the brightness of a star versus its colour, and the position of the star within this diagram can tell you about the age of the star, and more specifically, how far along in its lifetime the star has progressed.

The biggest single feature in the HR diagram is called the Main Sequence (we are not good at naming things in astronomy) which goes in a diagonal from the top left to the bottom right. This is where most stars stay for the majority of their lifetimes - any star which is burning hydrogen in its core, as our sun currently is, will sit somewhere along this line, with very large stars at the top left, and very small stars in the bottom right. Our sun sits somewhere in the middle.

The Red Giant Branch, which is the other major feature on the HR diagram we need to be familiar with in order to understand the Hertzsprung Gap, is just to the right of the Main Sequence. This is a narrow collection of stars which are very bright and very red. These are stars which are no longer burning hydrogen in their cores- they’ve exhausted all the hydrogen that exists in their cores and are now able to burn hydrogen in a shell around a core made primarily of helium, which for a star which is still burning hydrogen, acts like ash to a flame. These are all stars on their way to a death that will be more or less spectacular, depending on their mass.

Now, the Hertzsprung Gap is a region of the HR diagram just between the Main Sequence and the Red Giant Branch, and is notable for having almost no stars - hence the term “gap”. In the picture above, it’s represented by the narrowing of the background colour just below the “giants” grouping of stars. What causes the gap is linked to what the stars are doing. For most natural objects (stars count), gaps in a distribution usually mean one of two things. Either there’s some barrier to the object going in that area, or they move through it very quickly, and it’s just unlikely that you’ll spot something. In the case of the Hertzsprung Gap, it’s the latter. Stars are going through a phase just between finishing their hydrogen burning, and before starting to burn hydrogen in a shell around that core. Once they start up the hydrogen shell burning, they’ll lie along the Red Giant Branch, and if they’re still burning hydrogen in their core, they’ll still be on the Main Sequence.

The reason for the gap is that there’s a very short period of time between these two stages of a star’s life. The time gap is about 1000 years. A thousand years is basically the length of a blink, in astronomical terms. For comparison, our star will stay on the main sequence for about 8 billion years. Even high mass stars, which have relatively short lifetimes, stay on the main sequence for about 100 million years. 1000 years is really short. Because stars pass through the Hertzsprung Gap so quickly, it’s hard to spot them in that region of the diagram, and the stars that would otherwise be in the gap, if they didn’t move so quickly through it, are either still on the Main Sequence, or already on the Red Giant Branch.

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What happens when one star in a binary turns into a red giant?

When you have a binary star system with the stars close together, what happens when one of the stars starts to turn into a red giant?

Binary stars are actually pretty common (about a third of the stars in our galaxy are in some kind of binary), although the very close pairs are a little less common.

For the majority of the lifetime of the stars in a binary, both stars just spin around each other, burning their own hydrogen and orbiting tranquilly. However, unless the two stars are exactly the same size when they’re formed (not generally the case), then one star will run out of hydrogen before the other, and will transform into a red giant star.

Red giants are pretty fuzzy stars, only loosely held together by their own gravity. For some scale, our own Sun will become a red giant in its future. It will expand from its current (quite large) radius of 695,500 kilometers to something 200 times that large. But it won’t have gained any material to grow that much bigger - it just spreads out what’s already there.

If the star is on its own, this newfound wispiness doesn’t change much. But if there’s another star nearby, the other star can begin to tug on the outer layers of the red giant, pulling some of the outer layers towards itself. (This is the same thing that the moon does to our oceans.) Because the red giant is so big, the outer layers are only weakly tied to the centre of the star. If the companion star in the binary is close enough or massive enough, it can begin to tear the outer layers of the red giant away, and pull them towards itself. This will change the shape of the red giant from a diffuse sphere into a diffuse teardrop, with the point of the teardrop facing the companion star.

What the companion star does with that surface material depends on what kind of star the companion is. By the time one star has aged its way to being a red giant, its companion is in one of two states, which will be dictated by how massive the companion was when it began its life.

If the companion started out with a lower mass than its red giant partner, it will still be burning hydrogen in its core when the red giant begins to form. As it siphons gas off of the red giant, this companion star will grow in mass. Depending on how rapidly the red giant is growing, and how quickly the companion is siphoning material, the lower mass star can actually grow so large that it becomes more massive than the red giant. After having so much of its material drawn away from itself and onto its neighbor, the red giant will slow its donation of mass. This can result in a fairly stable configuration - the red giant, having lost a good amount of its atmosphere to its neighbor, will continue to slowly bleed gas into its neighbor’s gravitational well, and the neighbor will continue burning hydrogen until it has exhausted its own resources.

The other option for these binaries is if the red giant was the smaller of the two stars when they started their lives. This means that the star now turning into a red giant took much longer to reach the end of its life than its neighbor. (The bigger your mass, the shorter your life, if you’re a star.) So the companion star has already gone through its death throes, and can be one of a number of interesting stellar remnants.

The main options for your stellar companion in this case are: a white dwarf, a neutron star, or a black hole.

If your red giant is pouring gas down onto a white dwarf, you will eventually trigger some kind of explosion: either a nova or a supernova. A nova is a thermonuclear detonation on the surface of a white dwarf, and can recur multiple times, as it’s just a surface explosion. This kind of behavior makes these binaries fairly noticeable, because the brightness of the star will flare to many times its original brightness. A supernova, on the other hand, will detonate the entire white dwarf, blasting itself apart, and leaving nothing behind (also quite noticeable). This kind of supernova occurs when the white dwarf gains too much material to be stable (these stars are balancing gravity against an electron’s unwillingness to be pushed too close to another electron), and some trigger in the core sparks a runaway burning of material.

If the other object is a black hole or a neutron star, you’ll wind up with what’s called an X-ray binary, for the somewhat boring reason that it produces a lot of X-rays. For these objects, as the gas from the red giant is pulled off of the red giant star, it gets pulled into a very thin disk, surrounding the black hole or neutron star. The disk forms because it’s very hard for gas to lose a lot of momentum all at once and plunge straight down onto the black hole, but as a result, the gas winds up heating up to an incredible temperature before it makes it all the way to the neutron star or black hole. This heat causes the X-ray glow, and keeps the disk itself almost invisible in optical light.

So there you have it! A binary system of stars with one red giant will result in the companion tearing the outer layers of the red giant star away. From there, you wind up growing the object nearby, or causing a nova, a supernova, or the creation of a lot of X-rays.

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