Stars are all born in the same way, but they live and die in different ways. A star's mass rules how hot it will be, how long it will shine, and how it will end.
Stars are born from great cosmic clouds called nebulae.
You need a lot of hydrogen for a star. Fortunately, nebulae supply the hydrogen, but can't squeeze it together to make a star. Collapsing the cloud is a job for gravity. As the cloud collapses, it heats up. At 10 million °C (18 million °F) nuclear reactions start, and a star is born. (You can find out more about starbirth by clicking on the link at the end of this article for How the Sun Was Born.)
The length of a star's life depends on its mass.
Mass is the amount of matter in an object. So you might think that the most massive stars live the longest, because they have the most fuel. Yet that isn't what happens. A little red dwarf could live for a trillion years. Our medium-sized Sun formed five billion years ago and should last for another five billion. It will live a thousand times longer than some of the most massive stars.
Everything is made of atoms
Before we continue the story we need to remember a few things about atoms.
- The center of an atom is called the nucleus [plural: nuclei].
- A nucleus is made of protons and neutrons.
- There are tiny electrons orbiting the nucleus.
- The electrons' orbits are big compared to the size of the nucleus. Most of an atom is empty space. The atom is only solid because the electrons move at extremely high speeds.
Stars are nuclear reactors.
A young star uses hydrogen as its fuel, but it doesn't burn hydrogen in the way a furnace burns gas. In the center of a star its core hydrogen nuclei join together to make helium nuclei. We call this nuclear fusion. It gives out the energy that makes a star shine. And the energy from fusion does more than produce light and heat. Gravity pulled the hydrogen together to make a star, but gravity doesn't go away when that job is done. It keeps the star from flying apart, but something still has to stop the star from collapsing. The outward force from the energy of nuclear fusion is what balances gravity.
When the hydrogen is used up, the core is made of helium.
When the hydrogen is gone, nuclear fusion stops. There's no force to balance gravity, so the core starts to shrink. Helium is also a nuclear fuel, but the star may be too small to start helium fusion. If so, it just collapses into a white dwarf. A white dwarf is about the size of the Earth, but with as much mass as the Sun. (See the article How the Sun Will Die to find out more.)
However if there's enough mass, nuclear fusion starts up again using helium. Helium fuses to make carbon. There may even be enough mass for carbon fusion, but at some point a medium-sized star can't go on. It collapses into a white dwarf just as smaller stars do.
A supergiant destroys itself in a massive explosion.
A really massive star doesn't end up as a white dwarf. It can keep fusing heavier and heavier elements until it has an iron core. But it's bad news when its core is iron. Iron doesn't fuse, iron is the end. Fusion stops. Gravity wins, and it wins instantly. The core collapses completely in a dramatic way. It's so violent that the electrons in the atoms are pushed into the nucleus. There they react with the protons to make neutrons plus teensy energetic particles called neutrinos.
There's an atomic force that finally stops the collapse. But the empty space between the electrons and the nucleus is gone, and what's left is a dense star made of neutrons. A neutron star is about 20 km (12 mi) across, but with a mass greater than that of the Sun.
It's not just the core that collapses. The outer layers also fall in and hit the collapsed core at high speed. The in-falling material bounces off the core and takes the tremendous energy of the neutrinos away with it. This is the supernova an epic explosion that for a time shines as brightly as an entire galaxy.
What's left after the supernova?
Usually there's a neutron star left, but a star can be so massive that its story doesn't end with a neutron star. The more mass there is, the stronger its gravity is. In the most massive stars even atomic forces can't stop gravity from collapsing the neutron star. All that's left is a black hole. This is a very small region of space where the mass bends the space around it so tightly that nothing can escape from it not even light. We can't see a black hole, but can sometimes see the effects of its gravity on another body.
The material that the supernova throws out from the star forms a supernova remnant. Almost all of the chemical elements are made only in stars, including the chemicals of life. If the high-speed remnant meets a nebula, the shock wave triggers star formation. It also enriches the nebula with the new elements. Billions of years ago something like this was the beginning of our Solar System. We are recycled stars.