How Stars Live and Change

By the end of this lesson, you will be able to describe how a star is born, lives, and dies, and use a special graph to compare and classify stars.

Every star begins inside a nebula, which is a giant cloud of gas and dust floating in space. Gravity pulls that material together until the center gets so hot and squeezed that hydrogen atoms begin joining together, releasing enormous energy. That is when a star switches on. Most of a star's life is spent in a stable stage called the main sequence, the long period when a star steadily burns hydrogen in its core. Our Sun is a main sequence star and is about halfway through this stage.

What happens next depends on mass. A star about the size of the Sun swells into a red giant, puffs away its outer layers, and leaves behind a small, hot core called a white dwarf that slowly cools. A star many times more massive burns through its fuel much faster, explodes as a supernova, and leaves behind either a dense neutron star or a black hole.

Here is an everyday way to think about star color. On a gas stove in your kitchen, the blue flame is hotter than the orange glow of charcoal in a backyard grill. Stars work the same way: blue stars are the hottest, white and yellow stars are in the middle, and red stars are the coolest.

Scientists organize stars on the Hertzsprung-Russell diagram, a graph that plots a star's temperature across the bottom against its brightness up the side. Most stars fall along a diagonal band, the main sequence. Red giants sit in the upper right because they are cool but very bright, and white dwarfs sit in the lower left because they are hot but dim.

Try it: If a star is very hot but very dim, which group does it belong to?