Supernova remnant, the Veil Nebula NGC 6992 [Photo: Antonio Ferretti & Albert Martνnez Castillo]

All stars are born in much the same way, but they don't die in the same way.

Stars are made from the gas in cosmic clouds called nebulae.
In a nebula there's plenty of gas – mostly hydrogen – spread very thinly. It's gravity's job to collapse the cloud, pulling the gas particles closer and closer together. As the cloud collapses, it gets hotter and hotter until it reaches 10 million degrees Celsius. Then nuclear reactions begin, and a star is born.

The length of a star's lifetime depends on its mass.
Mass is how much matter an object contains. We might think that the stars with the most matter live the longest because they have the most fuel. But that's not how it works. A little red dwarf may live for a trillion years. Our medium-sized Sun is halfway through the 10 billion years of its life. It will last a thousand times longer than really massive stars. The more massive the star, the hotter it burns and faster it uses up its nuclear fuel.

Note: Everything is made of atoms.
The center of an atom is its nucleus [plural: nuclei].
The nucleus is made of protons and neutrons.
Electrons zoom around outside the nucleus.
Compared with the size of the nucleus, he orbits of electrons are big – most of an atom seems to be empty space.

Stars are giant nuclear reactors.
Hydrogen is the fuel for a young star, but it doesn't burn the way fire burns wood. In the center of a star – its core – hydrogen nuclei are squeezed together to make helium nuclei. This is called nuclear fusion, and it releases the energy that makes a star shine.

Nuclear fusion is gravity's partner.
Gravity holds the star together, but on its own, it would collapse the star like it did the nebula. However, the energy from fusion is an outward force that balances gravity's pull.

A very massive star destroys itself in a very massive explosion.
When the hydrogen is used up, some stars can burn helium. If a star has at least eight times the mass of the Sun, it can even keep fusing heavier and heavier elements until it has an iron core.

But iron doesn't fuse, so nuclear fusion stops. Gravity wins instantly. The core collapses so violently that the electrons in the atoms smash into the protons in the nucleus to make neutrons. The empty spaces in the atoms are gone. What's left is a super dense neutron star. It's about 20 km (12 mi) across, with a mass around twice that of the Sun.

When the core collapses, the outer layers of the star fall in to the center. At high speed, the infalling material slams into the core and bounces off with tremendous energy. This is a supernova, an epic explosion that for a time shines as brightly as an entire galaxy.

What's left after the supernova?
After the supernova, what's left is a neutron star. But a few stars are so massive that the neutron star also collapses. All that's left is a black hole. This is a very small region of space where gravity is so strong that even light can't escape from it.

Cosmic recycling
The material that a supernova throws out forms a supernova remnant. It spreads out at millions of kilometers per hour and is rich in chemical elements. The star had made elements, and the energy of the supernova had created even heavier elements. When the remnant meets a nebula, it causes a shock wave that triggers star formation. It also enriches the nebula with the new elements. Long ago, the elements of the Solar System were made by stars older than the Sun. We're all made out of recycled stars.