Lives of Stars

Stars are born, live out their lives, and die. Their appearance changes dramatically along the way. The Sun will age to become a red giant star like Arcturus. (A red giant star is the form that most stars take at the end of their lives, after they use up their fuel and their outer layers swell.) The star Betelgeuse will eventually resemble the Crab Nebula (see image below), exploding as a supernova and leaving behind a neutron star remnant. The fate of the supergiant star Rigel is that of a black hole—an object so dense that nothing can escape its gravity, not even light. The wide variety of stars we see represents the different stages in their lives.

The evolution of a star is governed primarily by its mass. A star’s mass determines its nuclear fusion rate, and thus its luminosity (energy output) and its life expectancy. We define four broad mass categories, based on a star’s lifetime, how it dies, and the stellar remnant it leaves. The “life” of a star is the stable stage when it is fusing hydrogen to helium in its core.

Low-mass stars are the longest lived of the energy-producing objects in the universe. Though they far outnumber all other stars, they are the faintest ones, and thus are hard to detect. Some low-mass stars will live for trillions of years.

Stars of intermediate mass have lifetimes that range between 50 million and 20 billion years.  Nuclear reactions in these stars make most of the carbon and nitrogen in the universe.  When intermediate-mass stars die, they blow off their atmospheres, dispersing such elements across space.

Old age: red giant
This simulation shows how, after depleting the hydrogen in its core, an intermediate-mass star contracts and heats up until it sets off the fusion of helium into heavier elements. Helium fusion provides a second stable phase, covering the last 10 percent of the star’s life. The increase in energy production puffs out the star’s atmosphere, resulting in a highly luminous red giant star. When the Sun reaches this stage, it will engulf the orbit of Venus and evaporate Earth’s oceans.

High-mass stars are very luminous and short lived. They forge heavy elements in their cores, explode as supernovas, and expel these elements into space. Apart from hydrogen and helium, most of the elements in the universe, including those comprising Earth and everything on it, came from these stars.

Death: supernova
High-mass stars die in spectacular explosions called supernovas, as shown in this simulation. A supernova spews more than 90 percent of the star’s mass, including the newly formed heavy elements, out into the galaxy.

In any batch of newly formed stars, the most massive ones are the rarest and shortest lived. Only one in about five hundred thousand stars has more than twenty times the mass of the Sun. In spite of their rarity, these stars are so luminous that they are easily seen at great distances.

Remnant: black hole
In this simulation, a very high-mass star collapses past the density of white dwarfs and past the density of neutron stars. As the star gets smaller, the gravity on its shrinking surface grows, severely warping the fabric of space. Eventually, space curves back upon itself, cutting off all contact with the rest of the universe. We cannot see, and do not know, what happens inside because nothing can escape the abyss, not even light. We call such objects black holes.