What are Stars and How Long do they Live?

a star cluster with massive stars.
The star cluster Pismis 24, located in the heart of a nebula in the constellation Scorpius, is home to a number of very massive stars, including Pismis 24-1 (the brightest star in the center of this image). ESO/IDA/Danish 1.5/ R. Gendler, U.G. Jørgensen, J. Skottfelt, K. Harpsøe

When we think of stars, we can visualize our Sun as a good example. It's a superheated sphere of gas called a plasma, and it functions the same way that other stars do: by nuclear fusion at its core. The simple fact is that the universe is made up of many different types of stars. They may not look different from each other when we're looking into the heavens and simply see points of light.  However, each star in the galaxy goes through a lifespan that makes a human's life look like a flash in the dark by comparison. Each one has a specific age, an evolutionary path that differs depending on its mass and other factors. Here's a quick primer about stars — how they are born and live and what happens when they grow old.

Edited and updated by Carolyn Collins Petersen.

Alpha Centauri
Alpha Centauri (left) and its surrounding stars. This is a main sequence star, just as the Sun is. Ronald Royer / Getty Images

When is a  star born? When it begins to form from a cloud of gas and dust?  When it begins to shine? The answer lies in a region of a star that we cannot see: the core.

Astronomers consider that a star begins its life as a star when nuclear fusion commences in its core. At this point it is, regardless of mass, considered a main sequence star. This is a "life track" where the majority of a star's life is lived. 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.  More »

Red Giant Star
A red giant star is one step in a star's long lifetime. Günay Mutlu / Getty Images

The main sequence doesn't cover the star's entire life. It's just one segment of stellar existence. 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".  More »

White Dwarf
Some stars lose mass to their companions, as this one is doing. This accelerates the star's dying process. NASA/JPL-Caltech

When low-mass stars like our Sun reach the end of their lives, they enter the red giant phase. But the outward radiation pressure from the core eventually overwhelms the gravitational pressure of material wanting to fall inward. This lets the star expand farther and farther out into space.

Eventually, the outer envelope of the star begins to merge with interstellar space and all that is left behind is the remnant of the star's core. This core is 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 neutron star. Eventually it is this type of object that will be the sole remains of our Sun billions of years from now. More »

Neutron star
NASA / Goddard Space Flight Center

A neutron star, like a white dwarf or black hole, is actually not a star but a stellar remnant. When a massive star reaches the end of its life it undergoes a supernova explosion, leaving behind its incredibly dense core. A soup-can full of neutron star material would have about the same mass as our Moon. There only objects known to exist in the Universe that have greater density are black holes. More »

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Black hole
This black hole, in the center of the galaxy M87, is ejecting a stream of material out from itself. Such supermassive black holes are many times the mass of the Sun. A stellar mass black hole would be much smaller than this, and much less massive, since it's made from the mass of only one star. NASA

Black holes are the result of very massive stars collapsing in on themselves due to the massive gravity they create. When the star reaches the end of its main sequence life cycle, the ensuing supernova drives the outer part of the star outward, leaving only the core behind. The core will have become so dense that not even light can escape its grasp. These objects are so exotic that the laws of physics break down. More »

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Brown Dwarfs

Brown dwarf
Brown dwarfs are failed stars, that is -- objects that didn't have enough mass to become fully fledged stars. NASA/JPL-Caltech/Gemini Observatory/AURA/NSF

Brown dwarfs are not actually stars, but rather "failed" stars. They form in the same manner as normal stars, however they never quite accumulate enough mass to ignite nuclear fusion in their cores. Therefore they are noticeably smaller than main sequence stars. In fact those that have been detected are more similar to the planet Jupiter in size, though much more massive (and hence much denser).

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Variable Stars

star cluster
Variable stars exist throughout the galaxy, and even in globular clusters like this one. They vary in brightness on a regular period. NASA / Goddard Space Flight Center

Most stars we see in the night sky maintain a constant brightness (the twinkling we sometimes see is actually created by the motions of our own atmosphere), but some stars actually do vary in their brightness. Many stars owe their variation to their rotation (like rotating neutron stars, called pulsars) most variable stars change brightness because of their continual expansion and contraction. The period of pulsation observed is directly proportional to its intrinsic brightness. For this reason, variable stars are used to measure distances since their period and apparent brightness (how bright they appear to us on Earth) can be sued to calculate how far away they are from us.