Download Stars

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
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Is the largest object in our solar system.
All of the other objects in the solar system
move around it.
Our Sun
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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 star
b. It gives off energy
c. 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.
b.
c.
d.
yellow star
supergiant
red star
blue star