What depends on how long a star lives
Our Sun has been on the main sequence for about 5 billion years, and will persist for another 5 billion years or so before it transitions to become a red giant star.
The main sequence doesn't cover the star's entire life. It's just one segment of stellar existence, and in some cases, it's a comparatively short part of the lifetime.
Once a star has used up all of its hydrogen fuel in the core, it transitions off the main sequence and becomes a red giant. Depending on the mass of the star, it can oscillate between various states before ultimately becoming either a white dwarf, a neutron star or collapse in on itself to become a black hole.
One of our nearest neighbors galactically speaking , Betelgeuse is currently in its red giant phase and is expected to go supernova at any time between now and the next million years. In cosmic time, that's practically "tomorrow". When low-mass stars like our Sun reach the end of their lives, they enter the red giant phase.
This is a bit of an unstable phase. When the two are balanced, the star is in what's called "hydrostatic equilibrium. In an aging star, the battle gets tougher. This lets the star expand farther and farther out to space. Eventually, after all the expansion and dissipation of the outer atmosphere of the star, all that is left is the remnant of the star's core. It's a smoldering ball of carbon and other various elements that glows as it cools.
While often referred to as a star, a white dwarf is not technically a star as it does not undergo nuclear fusion. Rather it is a stellar remnant , like a black hole or a neutron star. Eventually, it is this type of object that will be the sole remains of our Sun billions of years from now. A cloud of dust and gas, also known as a nebula , becomes a protostar, which goes on to become a main sequence star. Following this, stars develop in different ways depending on their size.
Stars that are a similar size to the Sun follow the left hand path:. Stars that are far greater in mass than the Sun follow the right hand path:. A star forms from massive clouds of dust and gas in space, also known as a nebula. Nebulae are mostly composed of hydrogen. Gravity begins to pull the dust and gas together. As they run out of hydrogen to fuse in their cores, they swell into red giant stars before shedding their outer layers.
The remnant left behind in these planetary nebulae is a white dwarf star. Like neutron stars, white dwarfs no longer fuse hydrogen into helium, instead depending on degeneracy pressure for support — this time, the electrons are degenerate, packed together and forced into higher energy states, rather than the neutrons.
Left to their own devices, white dwarfs will eventually fade into black dwarfs. No black dwarfs have been observed yet because a white dwarf takes longer than the current age of the universe to fade away.
And if the white dwarf is part of a binary system, it may avoid that fate altogether. By accreting matter from its companion star, the white dwarf can explode in a Type Ia supernova , leaving no remnant behind. The smallest stars in the universe have exceedingly long lives — in fact, none have faced their end yet.
Red dwarfs, stars with less than 0. The luminosity of the star is the energy released per unit time. For main sequence stars, the energy comes from hydrogen fusion and we have:. The mass converted into energy through burning will be a fraction f of the total mass of the star.