By Robert Hazen, Ph.D., George Mason University
If we examine different sizes of stars, and see their ultimate fates, we will see that they are all different. We know that a main sequence star, like the Sun, will eventually end as a white dwarf, slowly cooling down. But what about stars much smaller or much larger than the Sun? Let’s check out their life cycles.
Brown Dwarfs
The smallest stars are about a tenth the size of our Sun. These are called brown dwarfs, because they neither get very hot nor very bright.
These small stars burn their hydrogen very, very slowly, and thus they don’t radiate much energy to the surface; they just appear as dull, red-brown stars in the heavens. They are in the extreme lower-right-hand part of the Hertzsprung-Russell diagram; cool, low-energy stars that last for hundreds of billions of years.
Because brown dwarfs are so faint, they’re very difficult to detect with ground-based telescopes. Heat-seeking telescopes have the best chance of locating these objects.
Learn more about how a star forms.
Massive Stars
Stars that are more than a few times more massive than the Sun experience a very different fate.
Larger stars have several concentric layers of fusion reactions. Going deeper, the temperatures and pressures get higher and higher; one may find hydrogen burning to produce helium-4, but then layers of helium burning to produce carbon-12.
Stars that are ten times more massive than the Sun will burn up all their hydrogen fuel in perhaps a hundred million years, or less, and then they have to resort to other kinds of fusion reactions.
After you get to carbon, you go to oxygen-16; oxygen-16 burns to neon-20, to magnesium-24, and so forth; all the way up to iron-56. The last of these steps of fusion can take place in a matter of seconds in a star.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Supernova Metals
Iron-56 is the ultimate nuclear ash. You can get no more nuclear energy out of this material, and all that’s left is gravity, collapsing the star. In an instant, the star implodes and then rebounds out into space, in what’s called a supernova explosion, an explosion of unimaginable power.
In the process, nuclei of all different sizes are formed. Atoms start sticking together, and all of the remaining elements of the periodic table are formed in an instant in this explosion. Fusion only takes you up to iron, element 26, But then, in an instant, when that star explodes, everything else, all the heavier elements, are formed.
All the elements that we can think of—silver and gold; lead, bismuth, vanadium; uranium and plutonium—come from exploding stars.
Neutron Stars and Pulsars
Often, when a star explodes, it leaves behind a smaller, but nevertheless quite massive object that marks the place of the former star, and these remnants are among the strangest objects that we know of in the universe.
It collapses down into an incredibly dense object, in which electrons and protons fuse together to form neutrons. This object, that may be ten miles in diameter and have several times the mass of the Sun, is composed entirely of neutrons; in effect, it is a giant nucleus.
Neutron stars can rotate very rapidly, thousands of times per second. When they do this, they can create pulsing radio signals in space as they have very strong magnetic fields. These radio signals sweep by the Earth, and one can get very regular, pulsing radio signals from deep space. These objects are called pulsars.
Learn more about the nebular hypothesis.
Black Holes and More
If you have a residual mass after a supernova, more than about 30 times the mass of the Sun, something very different takes place.
The gravitational force is so immense that all that mass disappears into a point. The point is called a black hole, an object so massive that nothing, not even light, can escape from it. This represents the ultimate victory of gravity. Black holes themselves are invisible. They absorb light, so one can’t possibly see them.
However, one can see the effects of black holes in the surrounding space, and therefore we have ways of detecting black holes.
One of the ways that gravitational black holes may be observed is by the fact that they often rotate around an adjacent star, a binary star. If the black hole is massive, the other star may have a stream of matter that radiates from it to the black hole, which is sucking it in. These can actually be observed in space, with telescopes. Also, black holes have such massive gravity, they actually bend light that passes near them, creating gravitational lensing. So, if a black hole passes in front of a more distant visible object, then you actually may see that object suddenly brighten and dim, or you’ll see a ring of material, where before there was just a single dot.
So we see that it’s not just the Sun that is vital to our life. The stars are the factories of the chemical elements that make everything we can see. Start with hydrogen, and stars make the entire periodic table; and from that periodic table, all living things are brought forth. That’s why Carl Sagan said, “We are all made of star stuff.”
Common Questions about Brown Dwarfs, Supernovas, and Black Holes
Small stars burn their hydrogen very, very slowly, and don’t radiate very much energy to the surface; they just appear as dull, red-brown stars in the heavens. These can last for hundreds of billions of years.
When fusion stops, gravity collapses the star. In an instant, the star implodes and then rebounds out into space, in what’s called a supernova explosion, an explosion of unimaginable power. This explosion creates all the other elements which are familiar to us—silver and gold; lead, bismuth, vanadium; uranium and plutonium and others.
If you have a residual mass after a supernova, which is more than about 30 times the mass of the Sun, the gravitational force is so immense that all that mass disappears into a point. This is a black hole, an object so massive that not even light can escape from it.
