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stars

A star is a huge glowing ball of hot gas. Deep inside its core, hydrogen atoms smash together, forming helium and releasing huge amounts of energy that heats the gas. This is called nuclear fusion, and it's why a star shines. As the hot gas pushes outward, it opposes the inward pull of gravity. This balance of forces is what makes a star, a star. It holds the star together and keeps it at a steady temperature for most of its life.

Stats

Description: a huge glowing ball of hot gas
Significance: As stars live and die, they produce almost all of the elements in the universe.
Observed: Astronomers use telescopes on Earth and in space to study all wavelengths of starlight.
Number in Milky Way Galaxy: More than 200 billion stars (200,000,000,000). Every star we can see with the naked eye is in the Milky Way.
Number in Universe: More than 40 sextillion stars (40,000,000,000,000,000,000,000)!

Story

It's All About Mass
All stars are born, mature, and eventually die. A star's mass determines how it will live and die (mass is the amount of matter contained in something).

Throughout the universe, stars are born in dense clouds of gas and dust. Gravity pulls the gas and dust into clumps. If the clump is massive enough, a star can form! The growing temperature and pressure causes its core to ignite and nuclear fusion begins.

After millions to trillions of years, a star will begin to run out of fuel. What happens next depends on its mass. The least massive stars will swell into red giants. They will eject their outer layers and their cores will shrink into small white dwarfs. Massive stars will swell into red or yellow supergiants before ejecting their outer layers. Then they will collapse in on themselves and explode as supernovas. The remaining cores shrink into small neutron stars. The most massive stars form black holes after they explode in super-novas. Black holes are so dense that not even light can escape their gravity!

Stumper

Stars vary in brightness, temperature, and color, which all depend on their mass. The most massive stars are bright and:

blue

red

yellow

Correct!

Bright, blue stars are the hottest. The least massive stars are cool, red, and dim. Yellow stars, like our Sun, are in between.

Stumper

How does the Sun's mass compare to other stars?

it is more massive than average

it is close to average

it is less massive than average

Correct!

The Sun is not your average star. Nearly 90% of stars are less massive than the Sun. Since they're less massive, they are also cooler, redder, and dimmer.

Stumper

Very massive stars explode when they die. They run out of fuel quickly and explode. This explosion is called a:

black hole

red giant

supernova

Correct!

Supernovas are very rare—only one percent (1 in 100) of stars are massive enough to die this way. Most stars are less massive. When they die, they first swell into a red giant. Then the outer layers blow off in a wind, while the core shrinks to become a white dwarf.

Fact or Fiction

All stars have about the same brightness. They just look brighter the closer they are.

Fact

Fiction

A star does look brighter the closer it is. But its brightness (what we see) also depends on its luminosity (how much light it emits). The more massive it is, the more luminous it shines.

Fact or Fiction

Without stars, our planet and everything on it would not exist.

Fact

Fiction

Fact

Stars are factories for elements. As they live and die, they form nearly all of the elements on the periodic table, like oxygen and carbon. These elements make up Earth—and us.

Quote

“

The most massive stars can be almost a million times as luminous as the Sun!

„

Mordecai-Mark Mac Low, astrophysicist

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