Sizing up Stars

The sun is the star at the center of our solar system, and the one we know the most about.

It is close enough to us that we can measure its size precisely.

Astronomers use the sun as the benchmark for comparing and describing all other stars.

Star sizes are measured in solar radii star brightness is measured in solar luminosities. Our sun is the only star in our solar system. • Other stars seem tiny compared with the sun

Energy from Stars A star is a huge ball of hot gasses that give off its

own light. The light energy from a star travels through space. You see stars because this energy reaches you on

Earth.

The sun is only one of many stars you can see. There are many billions of stars, more than you can

count. You can see many stars with your eyes alone, but

you need a telescope to see most stars. They are very far from Earth and from each other.

When compared to other stars in the galaxy, our sun is actually fairly large.

It is more massive and brighter than about 85% of the stars.

That’s because the most common stars are red dwarfs, which are small dim stars.

Most of the stars we can see in the night sky, however, are brighter and larger that the sun.

Because they are so far away, they must be large and bright to be visible from Earth.

Color Stars come in different colors. A star’s color depends on how hot its surface is. You may think of the color red as hot and blue as cool.

When it comes to stars, the opposite is true. The hottest stars are blue and the coolest stars are red. In between are white, yellow, and orange stars. Our sun is a yellow star.

Sizes Stars also have different sizes. A star’s size is the distance from its

surface to its center. Our sun is a medium sized star. The largest stars are up to 1,000 times

larger than our sun. The largest stars are called giants and

supergiants. The smallest starts are called white

dwarfs.

Brightness

A star’s brightness depends on its size and how hot it is.

Larger and hotter stars are brighter than smaller and cooler stars.

Brighter stars send out more light energy.

How bright a star looks to us also depends on how far away it is.

A dim star that is close to earth can appear brighter than a more distant star that is actually very bright.

A few thousand stars look bright enough for us to see without a telescope.

These bright stars may give off more light than other stars, or they may just be closer to Earth.

Our Sun• Is the largest object in our solar

system.• All of the other objects in the solar

system move around it.

Our Sun Gives Earth light and

heat. The energy of the sun’s

light and heat is needed for life on Earth.

Is a yellow, average-sized star.

Many stars are much larger and hotter than our sun.

So why does our sun look larger and brighter to us than all other stars? Why do all other stars look tiny like

points of light? Imagine that you and a friend are standing at

one end of a playing field. You hold a baseball. Your friend holds a basketball. You know that a basketball is much larger than a baseball. But what seems to happen if you friend carries the basketball to the other end of the field? The basketball looks smaller than the baseball to

you. The basketball does not really get smaller. It looks smaller because it is much farther away.

•The same is true of the sun and the other stars.

•Stars are much larger and brighter than the sun

•look like tiny points of light to us because they are much farther away from Earth than the sun is.

Our sun is by far the closest star to Earth. Because it is so much closer to us, the sun looks much larger and brighter than any other star.

Lesson Review

1. How is our sun different from all other stars?

a. It is a yellow starb. It gives off energyc. It is a medium-size star. d. It is much closer to Earth.

2. Why do giant stars look small from Earth?

a. They are red.b. They are much bigger than the sun. c. They are very far away. d. They are smaller than supergiant stars

3. Which kind of star is the hottest?

a. yellow starb. supergiantc. red star d. blue star