Many stars have names. Here are a couple, both in Orion. Orion rises in the east shortly after sunset this time of year.
Betelgeuse is the bright orange star at the upper left of the big quadrilateral of Orion; it’s considered Orion’s shoulder. Rigel is the bright one at the lower right, generally considered to be Orion’s foot.
Orion. The seven major stars (the three in the belt, and the four in the quadrilateral) are all big, bright stars and with one exception they’re all a similar distance away from Earth. The upper right is Bellatrix, the lower left is Saiph, the belt stars are Alnitak, Alnilam, and Mintaka. Bellatrix is considerably closer to us than the others, “only” 250 light years away, the others are 650 to 2000 light years away.
Rigel
That’s the bright star at the lower right in Orion (if you’re in Australia, the upper left). That is one very, very bright star!! It doesn’t LOOK as bright as Sirius (the very bright star to the east and a bit south of Orion), but Sirius is about 8 light years away, and Rigel is 860 light years away… a hundred times as far. Yet it isn’t that much dimmer than Sirius. Sirius is the brightest star in the night sky, and Rigel is #7.
Rigel is 120,000 times as bright as the Sun, intrinsically. If the Earth were orbiting Rigel… well, it wouldn’t be. Earth would be getting 120,000 times as much energy and would be melted, then boiled away. (Imagine if the Sun in our sky gave off over 100,000 times as much heat as it does!) The Earth would have to be 364 times further away, which is to say 364 Astronomical Units (AUs), to get down to the same amount of electromagnetic (EM) radiation that it actually gets orbiting our Sun at 1 AU.
That’s ten times the distance to Pluto for our hypothetical planet orbiting Rigel, to get the SAME amount of energy the Earth does, orbiting our much dimmer Sun.
Rigel is bluish in color, like most of the visible stars in the sky. That is because its surface temperature is some 12,100 degrees Kelvin, which is over 21,000 F.
Have you ever seen anything that hot here on Earth? No. So you might be surprised to find that when things get that hot, they glow bluish. As things get hotter, they go from invisible (but you can feel the heat), to dull red, to orange (embers), to yellow (a tungsten bulb), to white (the Sun)… to blue. Each with more and more heat, and towards the end, with more and more ultraviolet (UV). In fact the only place to see things that are blue hot is in the night sky. (No the flames on your gas stove aren’t that hot—they are blue for a very different reason.)
Something that is blue hot is giving off more ultraviolet than visible light. So that 120,000-times-brighter-than-the-Sun light from Rigel, is mostly ultraviolet. Our hypothetical planet at 364 AUs distance is getting most of its sunlight as UV. Proportionately less of it is coming as visible light. If you were outside on a clear day there, there’d be the feel of an overcast day, but you’d be getting sunburned in a hurry. And the “sun” would be this tiny point of painfully bright blue light.
A couple more things to say about Rigel: It is 21 times as massive as the Sun, and maybe 80 times as wide.
Betelgeuse
Our other star is Betelgeuse. (“Beetle Juice,” if you must.)
Orion’s upper left shoulder should look a bit redder than the other stars (which will look bluish white). That is Betelgeuse.
Betelgeuse is a “red giant.” It is about 700 light years away (not quite as far as Rigel), and is anywhere between 90,000 and 150,000 times as bright as the Sun, intrinsically (it actually varies in brightness).
It’s redder because the surface is a lot cooler than our Sun, 3590K, not that far off from an incandescent light bulb. Yes, it’s at a different part of the red/orange/white/blue-hot continuum.
It’s 11 times as massive as the Sun, but it is anywhere between 700 and 1,000 times as wide as the Sun! If the Earth were orbiting Betelgeuse it would be inside the star. A bit toasty!
Betelgeuse is so big that, as far away as it is, we were actually able to measure its diameter from here on Earth in 1920! Most stars look like points in even a powerful telescope, but not Betelgeuse.
Earth would be comfortable at a similar distance from Betelgeuse, about 350 AUs, but because the star drastically varies in brightness, much more so than our Sun, climate change would be far, far worse.
They’re Not Much Alike.
Now, it turns out a star is actually a very simple thing. It’s a lot of gas, trying to contract under its own gravity. The main difference between stars should be in how massive they are.
Rigel and Betelgeuse are of similar masses (closer to each other, proportionately, than either is to the Sun), yet Rigel, the bigger of the two, seems more like the Sun than Betelgeuse. Hotter and bigger, but not swollen to a ridiculous size. In fact, astronomers place stars like Rigel and the Sun in a category called the “Main Sequence,” a progression from small, red stars up to giant blue ones. Some Main Sequence stars are the exact mass of Betelgeuse, and they’re not red and swollen.
Betelgeuse is not on the Main Sequence. It’s a different sort of animal.
What gives? Why are they so different?
To start finding out the answer, let’s look at their masses once again.
Did you notice how these stars are thousands of times brighter… but only 10 or 20 times as massive?
Doesn’t that mean the star will burn itself out that much sooner? If Rigel has got 20 times the gas, but it’s 120,000 times as bright as the Sun, that means it’s burning through its fuel supply 120,000 times as fast, and it should last only 1/6,000th the time. (To be sure a star only burns what’s at its center, not the surface layers, so that’s not quite the right comparison to make.)
The Sun is expected to last 10,000 million years (and we’re 4,500 million years into that). Rigel’s lifespan, total, can’t be much more than 10 million years; it’s estimated to already be 8 million years old.
Rigel lives boldly, but very, very briefly. A cosmic butterfly.
Betelgeuse is of comparable brightness to Rigel, but half the mass. It’s ripping through its available fuel even faster, proportionately speaking. And yet its big and cool on its surface. It makes sense to be big, if it’s cool, or cool, if it’s big. It has a MUCH higher surface area than Rigel, so each square meter of it has to radiate a lot less, for the total output to be the same. And the way to do that is to be cooler.
But that doesn’t explain why it’s so different from Rigel, and our Sun, as to not be on the Main Sequence.
There’s more to the story. Lots more.
The Life Of A Star–Youth
As I said earlier, a star is a simple thing, really. It’s a big ball of gas that wants to contract under its own gravity. As it does so, it heats up, just like compressing the gas in a bicycle pump makes it get hotter. Heating it up increases the pressure; the pressure resists the tendency to contract. Eventually a balance is reached.
But that heat eventually radiates off, the pressure drops, and the star contracts. What one would see is stars glowing as they contract, shrinking as fast as the bleed-off of heat (and pressure) lets it.
Unless the star can find another source of energy, something that it can internally generate, to stave off the collapse.
And it does.
Any ball of gas sufficiently large (considerably larger than Jupiter) will eventually reach a point where the core is at a temperature of millions of degrees, and then the hydrogen starts fusing into helium. Four hydrogen atoms go through a series of reactions (exactly which series of reactions depends on the temperature, which depends on the mass of the star), to ultimately make one atom of helium. In the process, 0.7 percent of the mass of the hydrogen disappears—it becomes energy. It works out to 26.73 million electron volts of energy. (An MeV is a tiny amount of energy to us, but this is from 4 atoms of hydrogen, and there are about 600,000,000,000,000,000,000,000 atoms of hydrogen in 1 gram of the stuff… so, really, it’s quite a bit of energy!)
In fact, I’m going to point out that 12 atoms of hydrogen, fusing to 3 atoms of helium, works out to almost exactly 80 MeVs of energy. That will be important, later on.
So nuclear fusion gives off energy, lots of it. That energy heats the star up, and the contraction stops. The star finds a balance, and the star will stay pretty much the same size as long as it has hydrogen in its core to fuse to helium. (It actually gets a bit hotter as time goes on, but this is a very slow process.) Once it runs out of hydrogen, well, life will get interesting again.
The Sun is at just the right temperature and pressure, inside, that it’s going to take 10 billion years to burn all of its fuel (even though it’s burning 650 million tons of it a second), but that’s the right rate to keep it from either expanding too much and cooling off (which would allow it to contract again, heating it up), or contracting too much and heating up, which would cause it to expand again and cool off. It’s in balance, and it will stay in balance for another 5.5 billion years, when it runs out of hydrogen.
Back to Rigel.
Rigel, like the Sun, is burning hydrogen, to make helium. It’s doing so at a rate far faster than our Sun; it has to to maintain the very high temperature to keep 20 times the mass of the Sun from continuing to collapse. You see, the bigger the star, the hotter it has to get to keep from collapsing, but the hotter it gets, the faster the heat radiates away, and that means, the faster it burns through its fuel. And the shorter it will live.
So a star like Rigel is heavy, very hot, and blowing through its fuel FAST.
So now we understand why big stars are so much hotter and brighter.
But that just makes Betelgeuse more puzzling. It’s half as massive as Rigel, but it’s about as bright (it shouldn’t be), and it’s much cooler than our Sun (again, it shouldn’t be). The fact that it is so swollen, comparatively, means it should be much, much hotter at the very center, but wouldn’t that just make it even brighter?
What makes it so large, yet cooler than the Sun at the surface?
The Life of A Star–Middle and Old Age
Well, it turns out, Betelgeuse is a star that has already run out of hydrogen!
As I said, when a star runs out of hydrogen, life gets interesting. A very small star, lighter and smaller than our Sun, basically is done at this point. It will just contract until the atoms are touching each other, cooling off over billions of years. But this doesn’t happen until it’s tens of billions of years old. In fact, it would have to be older than the universe for this to have happened to it before now, so there shouldn’t be any of these out there.
A star our Sun’s size, or larger, will shrink too when it runs out of hydrogen, but the core will get hotter and hotter. Again, this will only be temporary heat up, unless another source of energy is found.
That source does exist. If the star is massive enough (and the Sun is, therefore so are Rigel and Betelgeuse), eventually the core gets much hotter than it was before, with higher pressure, and helium fuses to become carbon. It takes 3 helium nuclei to make a carbon nucleus, which means it took 12 of the original hydrogen atoms to make the 1 carbon nucleus.
But this is actually desperation.
Turning three helium nuclei into one carbon nucleus only releases 7.25 MeVs of energy. In other words, less than a tenth of the energy that the star got making the helium in the first place (80 MeV). Reburning the ash, releases less energy. So to put out the same energy every second as it did before, it has to go through its fuel over 10 times faster, by weight.
Yet the star must sustain a HIGHER core temperature than it was doing before. So going through the helium ash 10 times faster than it went through the hydrogen… isn’t enough!!
So the star gets hotter, and hotter. The good news is all this heat will cause hydrogen outside the core to fuse too, which means the star gets a bit of a boost.
But that higher core temperature causes the star’s upper layers to expand more—that’s what makes it big—and the much larger sphere has to radiate less energy per given area—which is what makes it cool.
Because the helium-to-carbon fusion is so much less productive, and the star needs more energy, it can only stay in the helium burning phase for a very short time.
Our Sun will have a helium burning phase. It will turn into a red giant, like Betelgeuse, but much smaller. We’re not sure whether it will swallow the Earth, but even if not, life will be toast here. But that’s 5 billion years from now, so you still have to do your taxes next year.
Once the helium starts to run low… the star, if it’s massive enough, moves on. (The Sun is not massive enough. Helium-to-carbon will be the end of the road for it.)
The star contracts, heats up, and starts fusing helium + carbon to make oxygen, helium plus oxygen to make neon, neon plus helium to make magnesium, or maybe even go directly: carbon plus carbon to make silicon. Each of these reactions requires more heat, and produces less energy per unit mass than the one before it.
So these phases are each shorter than the one before it. But the core being hot enough to (say) make oxygen makes the layer right outside the core hot enough to make carbon, and the layer outside of that is making helium. The star starts to resemble an onion with all these layers, with the one in the center going at a furious rate, desperately trying to get more and more energy out of a less and less energetic reaction, since it is ultimately holding the star up from collapse.
A red supergiant, burning heavier and heavier ash, trying to stave off collapse.
The Death of a Massive Star
Eventually the star has a core of iron. This core—much, much bigger than the Earth and made entirely iron—probably is built in 1 day; that’s how fast the star must rip through its fuel to produce the iron, and not collapse.
We’re well into the region of diminishing returns. But now we move into the realm of negative returns. Moving beyond iron actually consumes energy.
The star is done. It collapses. The heat of collapse actually does cause further reactions, but they just suck more energy out of the system, making the collapse even faster. But make no mistake—the star’s core is getting hotter and hotter, and more and more reactions are happening in it. Elements much heavier than iron are made, instantly. Huge amounts of neutrinos are generated—in fact they carry off most of the energy.
But what’s left over is still titanic. The star explodes. The mass of the outer layers is consumed instantly, and now, for just a few weeks or so, the star outshines a billion or more normal stars. It can be brighter than everything else in its galaxy.
This is a supernova.
And what it leaves behind is a neutron star, or maybe even a black hole. Not ordinary matter at all. It’s done generating energy, it’s done being a star as we know it.
R. I. P.
This is the Crab Nebula, also known as M-1. It’s a supernova remnant; the supernova blew up in 1054 and was seen by Chinese astronomers. It is 6,500 light years away, much further away than Betelgeuse. The star that blew up probably was not visible to the naked eye, yet people on Earth could see the supernova without difficulty.
In the center of this, is a rapidly rotating neutron star, known as a pulsar.
Death Watch for Betelgeuse
Remember when I said Betelgeuse was out of hydrogen in its core?
We know it’s done with hydrogen in its core, but don’t know exactly what it’s doing right now, for certain. Many of those higher reactions could be taking place simultaneously in different layers deep inside the star, but we have no real way of knowing. How many layers of different fusion are inside Betelgeuse?
If, today, it is working on making iron—tomorrow, it goes KABOOM. You will be able to see it in the day time, from 800 light years away. At night time, well, many nocturnal creatures will have their routines disrupted—it will probably be brighter than the Moon. Perhaps for their sake, we should hope it goes kablooey! during northern hemisphere summer time when Betelgeuse is not in the night sky.
It will happen. We don’t know when, but sometime in the next 100,000 years or so, Betelgeuse will light its own funeral pyre. It could be tonight. It could be 10,000 years from now. And once the supernova cools off—which will take a few years—Orion will lose his shoulder.
And Rigel isn’t all that far behind, in cosmic terms. Give it a few million years; it will run out of hydrogen, swell into a red giant bigger than Betelgeuse, and begin to die.
Whoever is watching at that time, will hopefully be consoled by the thought that all that stuff flung out into space is what will eventually make new stars, new planets, and, maybe, life.
For after all, that’s how we came to be. Everything in this universe that isn’t hydrogen or helium… all the carbon, oxygen, nitrogen, sodium, aluminum, silicon, titanium, iron, silver, gold, lead, and uranium, either came out of the exploded cores of big stars, or perhaps from collisions between their dead remnant neutron stars. Which means almost everything on Earth, and everything in
Stars died, so that you may live.
