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The Lifetime of a Normal Star
After the main sequence star is formed, a normal-sized star (like Earth's sun) burns brightly for approximately 10 billion years until it depletes its hydrogen core. The core collapses without a heat source to support it against gravity, and the pull of gravity increases the density of its core until it is at a point high enough to convert helium to carbon.
Once the helium is exhausted, the outer layer swells into a red giant and burns for approximately 100 million years. The red giant stage ends when the star sheds its layers, becoming a planetary nebula that lasts about 100,000 years. The core of the star remains in its final form; a white dwarf and then, after it cools, a black dwarf.
The Lifetime of a Large Star
After the main sequence is formed, a larger star burns very brightly for approximately 50 million years until it depletes the hydrogen at its core. The star collapses into its own gravity, and the density converts helium to carbon, just as a normal star evolves. However, the carbon core of a massive star continues to contract, reaching temperatures that burn carbon to oxygen, neon, silicon, sulfur and then iron while it's in the red supergiant phase. The star remains in this form for approximately one million years.
Iron is the most stable form of nuclear matter. Once the core has evolved to iron, further collapsing will bounce off, causing an explosion called a supernova.
After the supernova, some stars become small nebulas, while others become a compact neutron star (seen as a radio pulsar) or a black hole.
The Neutron Star and Black Hole
Neutron stars are what sometimes remains of the core of massive stars. The core of the star collapses during the supernova, turning each electron-proton pair into a neutron, which stops the collapse of the star's core and evolves to a neutron star. Neutron stars are very dense: One teaspoonful of its matter on Earth would weigh about 100 million tons. Smaller neutron stars are heavier than larger ones.
The neutron star stops the collapsing of the star after the supernova; however, the gravitational pull of a massive star is too strong. It overwhelms the nuclear force and collapses the atom to a point where its density is infinite. The gravitational wells of black holes are so strong that even light cannot escape. A black hole can be seen only by observing its effect on matter that surrounds it.
Earth's Sun
The sun is the largest object in the Milky Way galaxy. It's a normal star (not massive-sized), and its core is approximately 70 percent hydrogen and 28 percent helium. These percentages will change as it depletes its hydrogen core.
When the sun evolves to a red giant, it will become the size of Earth's orbit, demolishing its orbiting planets. It will eventually shed its layers, becoming a planetary nebula, then a white dwarf and, finally, a black dwarf.