90% of all stars are main sequence. The coolest / brightest stars are giants & supergiants.

The hottest / least bright star is a white dwarf. The brightest main seq. stars are blue.

The least bright main sequence stars are red.

Nebula – a cloud of dust and gas in space. Star formation begins with a Nebula – a cloud of dust and gas in space.

Stars are born in clouds of gas and dust — the ashes from earlier generations of dead stars. Gradually the cloud collapses under its own gravity, and its rotation makes it flatten out like a pizza dough spun in the air. This picture shows what you might see if you could peer through the protostar's birth cloud at the newly forming sun inside.

This is an example of what a brown dwarf might look like This is an example of what a brown dwarf might look like. Brown dwarfs are objects that form like stars, but do not have enough mass to ignite or reach the flash point for main-sequence nuclear reactions. They are thus not true stars, and may in fact be closer to planets such as Jupiter in appearance.

The cloud begins to contract, heat up and behave like a star. Protostar – collapsed cloud that begins to radiate heat (infrared) but has not fully ignited.

nebula protostar After ignition the star fuses hydrogen into helium in the core. The outward pressure from heat is in balance with the inward pressure of gravity and the star becomes a main sequence star. Main sequence – main stage of star’s life. Gravity/pressure forces in balance.

Because the interiors of stars are not mixed, helium ashes accumulate in the core, and fuel in the outer part never gets used. Energy production in the core falls, the weight of the outer layers forces the core to collapse and heat up. The unused shell of hydrogen ignites, expands and the star swells to become a Red giant (supergiant)– late stage – star grows larger, brighter and cooler.

About 6 or 7 billion years from now, the Sun will exhaust its hydrogen supply and swell to become a red giant. Its luminosity will balloon enormously, along with its radius. Mercury and Venus, the innermost two planets, might become completely engulfed. This image shows the Sun as it might appear from Earth. Even from there, the swollen star still fills the sky of the planet, whose atmosphere has been totally stripped away.

During the giant phase, helium fuses in the core leaving behind a carbon-oxygen core which grows hotter. When this C-O “bomb” reaches 600,000,000K (carbon detonation) it can become a supernova. If it survives it will continue to fuse heavier elements until it reaches iron, remember iron is the greatest element which can be formed by fusion, or the absolute final collapse into a white dwarf. White dwarf – dying star that has collapsed to the size of Earth and is slowly cooling off. Black dwarf – the end state of a white dwarf that has cooled to low temperature.

It is the far future, perhaps 8 billion years from now, and the Sun is facing a quiet death. All that remains is a tiny glowing ember called a white dwarf. At first the star will be fairly luminous by virtue of its temperature. But as it cools down it will grow steadily dimmer. This is a view from the Earth (if it survives) when the white dwarf has cooled to the extent that it provides barely more light than a few full moons.

nebula protostar If a star is massive dies it will become a Supernova – explosion caused by the collapse of the iron core of a massive star.

Crab Nebula – supernova remnant seen by the Chinese in 1054

nebula protostar High mass stars will then become a Neutron star – the core of a star that has collapsed – it is so dense only neutrons can exist.

Magnetars are neutron stars, left over from the death of a giant star Magnetars are neutron stars, left over from the death of a giant star. Their density and rapid rotation (several times per second) enormously increases the power of their magnetism. If you placed a magnetar where the Moon is, 400,000 kilometres away, it could still wipe your credit cards and rip spoons out of your pockets.

nebula protostar Super massive stars will become a Black hole – a mass that has collapsed to such a small size that its gravity prevents the escape of all radiation.