STAR

Go outdoors at night and look up at the sky. There are twinkling points of light everywhere. You are seeing thousands of stars that are millions of miles away. The stars look tiny because they are so distant. But if you could see those stars up close, you would see huge balls of fire.

The closest star to you on Earth is the Sun. The Sun is a star at the center of our solar system. Our Sun is about 4.6 billion years old. There are stars that are older or younger than our Sun. There are stars that are much bigger. There are stars that have exploded and stars that are just being born.

WHAT IS A STAR?

A star is a big ball of hot, glowing gas. The gas is mostly hydrogen and helium. Stars give off heat, light, and other kinds of energy.

A star has several layers. The part at the center of a star is called its core. A star shines because of its core. The core is so hot and tightly packed that atoms crunch together. Atoms are tiny bits of matter much too small to see. Hydrogen atoms crunch together and become helium atoms. This is called nuclear fusion. Nuclear fusion gives off enough energy to make the stars shine.

GROUPS OF STARS

Stars are part of groups called galaxies. Our Sun is in the Milky Way Galaxy.

People in ancient times grouped stars by patterns they thought they saw in the sky. The patterns are called constellations. They thought the patterns looked like people, animals, or objects. The Big Dipper is a constellation of seven stars in the shape of a dipping ladle. Astronomers map where the stars are in the sky using the constellations.

WHAT COLOR ARE STARS?

Stars come in different colors. They can be deep red, orange, yellow, white, or even blue. The color of a star depends on how hot the star is. The coolest stars are reddish and the hottest stars are bluish.

It is hard to imagine how hot a star can be. The temperature at the surface of red stars is about 5400° Fahrenheit (about 3000° Celsius). Yellow stars have surface temperatures about 11,000° Fahrenheit (about 6000° Celsius). Our Sun is a yellow star. White stars are about 18,000° Fahrenheit (about 10,000° Celsius)!

A star looks as if it is just one color. Starlight, however, is made up of many colors. Light from our Sun has all the colors of the rainbow. Astronomers study the light of other stars. Patterns in the light can tell astronomers what the stars are made of and how hot they are.

HOW BRIGHT IS A STAR?

Some stars in the sky look brighter than others. Some stars really are brighter. Other stars just look brighter because they are closer.

Some stars are not nearly as bright as the Sun. Other stars are as much as 500,000 times brighter.

HOW BIG IS A STAR?

The Sun is huge compared to Earth. If the Sun were hollow, a million Earths could fit inside it!

Astronomers compare the size of other stars to the size of the Sun. For example, a star called Betelgeuse is about 1,000 times bigger than the Sun.

WHERE DO STARS COME FROM?

Stars are born from swirling clouds of gas and dust. Gravity pulls the gas and dust together. The gas and dust form a spinning ball. As it spins, it gets hotter. The gas and dust get tightly packed. Finally, nuclear fusion begins and the star starts to shine.

A STAR’S LIFE

There are different stages in a star’s life, just as there are different stages in the lives of people. Right after a star is born it starts to get smaller. After a million years of shrinking, the star enters the main sequence of its life.

After about 10 billion years, the star’s core runs out of fuel. The star grows many times larger than it was during the main sequence. At this stage the star is called a red giant. What happens next depends on the size of the star.

GIANTS AND DWARFS

Medium-sized stars like our Sun become white dwarfs. White dwarfs can explode. The outside gas layers blow off and make clouds called nebulas. The core keeps shrinking. A spoonful of white dwarf core could weigh more than a dump truck. After several billion years, the star loses all its energy and becomes a cold black dwarf.

Really big stars become supergiants. Supergiants become supernovas, which are big exploding stars. The explosion sends gas and dust into space to make new stars. The core gets packed tighter and tighter. Some cores then turn to iron and become neutron stars. Some supernova cores turn into black holes, which swallow everything around them in space. Not even light can escape from a black hole.

COULD ASTRONAUTS VISIT A STAR?

Stars beyond our solar system are too far away for a spacecraft to reach. The closest star is Proxima Centauri. It is more than 4 light-years from Earth. A light-year is the distance light travels in a year, about 6 trillion miles (about 9 trillion kilometers) Most stars are much farther away than Proxima Centauri. No spaceship can travel fast enough to reach even the nearest star during an astronaut’s lifetime. It takes billions of years for even light to reach the most distant stars.

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