“Everyone must leave something behind when he dies, my grandfather said. A child or a book or a painting or a house or a wall built or a pair of shoes made. Or a garden planted. Something your hand touched some way so your soul has somewhere to go when you die, and when people look at that tree or that flower you planted, you’re there.” -Ray Bradbury
Today is Memorial Day here in the United States, where we honor all the soldiers who have fought and fallen for our country. The peace and prosperity that I have enjoyed my entire life is because of a price paid, many times over, mostly by people I’ve never met. So it goes with the Universe, too.
Over here at Starts With A Bang, I can think of no better way to celebrate it than by telling the story of what gets left behind by the stars that live, die, and give life to the next generation of stars and planets in the Universe. Because they didn’t start as stars, of course. They started as diffuse clouds of cold gas, long ago, that collapsed under their own gravity.
When those clouds collapse, and reach a certain density, star formation occurs. Out of this gas comes a whole variety of stars, dominated by hot, blue, massive stars, but full of the whole gamut of different young star types.
Take a look at the core of this cluster, and look past the brightest, hot blue stars here in cluster NGC 3603, and you’ll find something typical of all newly formed star clusters.
Yes, there are the hot, massive, ultra-luminous blue stars, but there are far more of the less massive, Sun-like stars among them, and an even greater number of dim, red stars in the mix. This cluster, a mere 20,000 light years away (in our own galaxy), is a typical example of a star-forming region in the Universe.
And every star in that image, just like every star ever formed in the Universe, will someday run out of fuel and die. But what will each star leave behind? Turns out, that’s entirely dependent on how much mass your star has.
The lowest mass stars, the Red Dwarfs (or the M-stars, above), with 40% the mass of the Sun (or less), burn their fuel the most slowly. While our Sun will live for billions of years, M-stars can live for many trillions of years, burning coolly and slowly through their fuel, eventually turning all their hydrogen into helium, and then simply contracting down in their entirety to form a degenerate ball of atoms: a white dwarf star. More than 1,000,000 times denser than water and over 1,000 times denser than the center of our Sun, a white dwarf packs the mass of maybe a hundred thousand Earths into the volume of less than one.
That degenerate dwarf is all a Red Dwarf will leave behind. While M-stars are most stars — about 75% by number — they’re also the least massive and arguably the least interesting: not a single red dwarf has been around long enough in our Universe to burn through all of its fuel. But the other types — from K-class stars all the way up through the lower-mass B-stars — will die in the same fashion our Sun will.
Unlike M-stars, these stars burn through their fuel more rapidly, so the hydrogen in the outer layers never gets a chance to burn. What’s more, is that the helium in the core can fuse further into carbon, nitrogen, oxygen, and sometimes even heavier elements: all the way up to iron for a few of these stars. When they reach the end of their lives, the result is simply spectacular.
A planetary nebula, like the Cat’s Eye Nebula shown here, consists of the outer layers of a star of one of these types, blown off in the violent death-throes of such a star, spanning only a few thousand years. The outer layers — half the mass of a star, on average — are made up of some 97% hydrogen, ideal for providing the fuel for future generations of stars, while the inner layers, made up of mostly Carbon and Oxygen, contract down to form a degenerate white dwarf.
These white dwarfs — the eventual fate of maybe 799 out of every 800 stars in the Universe — will someday be so common that they will outnumber all the living stars in the Universe. But not every star that lives will wind up as a white dwarf. These rarities, the one-in-800 stars that are massive enough, will die in the most spectacular of explosion of all: a type II supernova!
All stars born with more than about 4-5 times the mass of our Sun have enough fuel in them that they cannot form white dwarfs at their center; the white dwarf itself would be too massive, and must continue towards an even denser state! Instead, most commonly, the atoms themselves, normally made of protons, neutrons, and electrons, wind up collapsing almost entirely into neutrons, forming a tiny, ultra-dense ball known as a neutron star.
Because stars rotate, these neutron stars wind up spinning incredibly rapidly, and hence with incredible magnetic fields trillions of times what we find at the surface of our Sun. As these stars rotate, up to nearly 1,000 times per second, they send out electromagnetic radiation along the star’s north and south poles. The stars that point one of their poles at us appear to pulse, anywhere between about 1 and 1,000 times per second, and hence we call them pulsars.
The oldest, fastest pulsars are some of the best natural clocks in the Universe; you can look away for over a year and then look back, and you’ll know whether the pulse you’re looking at is a billion pulses into the future or a billion-and-one. Only recently have atomic clocks passed pulsars as the best clocks in the Universe. What’s more, is that it isn’t just the hydrogen-rich outer layers of a supernova that get blown off in a stellar death like this; it’s many of the heavier elements, too. In fact, type II supernovae are where practically all of the elements found on Earth originated!
But neutron stars aren’t the fate of all type II supernovae, just most of them. The rarest of all star types — the most massive O-stars — can actually have three different fates, depending on their masses. If your star is too massive to produce a neutron star, because even neutron stars have a mass limit, you will get a black hole to go with your supernova instead!
And this is true, unless your star — like maybe only one out of a billion stars — is more massive than 130 times our Sun is.
Because if you get more massive than that, your star can die in a very special type of explosion, known as a Pair-Instability Supernova, where a pressure drop at the core of a star causes runaway thermonuclear reactions, destroying the entire star and leaving absolutely nothing behind!
But there is one more possible fate, for the star types so massive that it’s thought we don’t even have one like it in our galaxy! If a star is more than 250 times as massive as our Sun, the star undergoes tremendous amounts of photodisintegration, where the entire core of the star collapses into a black hole, and except for a couple of highly collimated jets, there isn’t even a hint of an explosion — much less a supernova — at all.
And so, in memory of all the stars that have ever lived, now you, too, know what it is that they’ve left behind. For those of you who enjoyed the children’s version, consider this the one for adults: this is the beauty of the Universe inherent in the death and life of every star. Without all of this, we never would have gotten here, and billions of years in the future, the matter that makes us up will spread out among the cosmos, where it will create future generations of stars, planets, and possibly, once again, life.