Download Stars change over their life cycles.

KEY CONCEPT
Stars change over their
life cycles.
Sunshine State
STANDARDS
SC.E.1.3.3: The student
understands that our
Sun is one of many
stars in our galaxy.
SC.E.1.3.4: The student
knows that stars
appear to be made of
similar chemical elements, although they
differ in age, size,
temperature, and
distance.
BEFORE, you learned
NOW, you will learn
• The Sun is our local star
• The other stars are outside our
solar system
• There are huge distances
between objects in the universe
• How stars are classified
• How stars form and change
EXPLORE Characteristics of Stars
How does distance affect brightness?
PROCEDURE
1
VOCABULARY
light-year p. 786
parallax p. 787
nebula p. 789
main sequence p. 790
neutron star p. 790
black hole p. 790
In a darkened room, shine a flashlight
onto a dark surface from 30 cm
away while your partner shines a
flashlight onto the surface from
the same distance. Observe the two
spots of light.
MATERIALS
• 2 flashlights
• meter stick
• dark surface
2 Move one of the flashlights back 15 cm
and then another 15 cm. Compare the
two spots of light each time you move
the flashlight.
WHAT DO YOU THINK?
• How did distance affect the brightness of the light on the dark surface?
• How does the distance of a star from Earth affect our view of it?
MAIN IDEA WEB
A main idea web would
be a good choice for
taking notes about the
characteristics of stars.
We classify stars by their characteristics.
Like our Sun, all stars are huge balls of glowing gas that produce or have
produced energy by fusion. However, stars differ in size, brightness, and
temperature. Some stars are smaller, fainter, and cooler than the Sun.
Others are much bigger, brighter, and hotter.
Stars look like small points of light because they are very far away.
At most, only a few thousand can be seen without a telescope.
To describe the distances between stars, astronomers often use a unit
called the light-year. A light-year is the distance light travels in one
year, which is about 9.5 trillion kilometers (6 trillion mi). Outside the
solar system, the star closest to Earth is about 4 light-years away.
786 Unit 6: Space Science
Brightness and Distance
If you look at stars, you will probably notice that some appear to be
brighter than others. The amount of light a star gives off and its
distance from Earth determine how bright it appears to an observer.
A star that gives off a huge amount of light can appear faint if it is far
away. On the other hand, a star that gives off much less light can
appear bright if it is closer to Earth. Therefore, to determine the true
brightness of a star, astronomers must measure its distance from Earth.
One way astronomers measure distance is by using parallax, which
is the apparent shift in the position of an object when viewed from
different locations. Look at an object with your right eye closed. Now
quickly open it and close your left eye. The object will seem to move
slightly because you are viewing it from a different angle. The same kind
of shift occurs when astronomers view stars from different locations.
To measure the parallax of a star, astronomers plot the star’s position
in the sky from opposite sides of Earth’s orbit around the Sun. They then
use the apparent shift in position and the diameter of Earth’s orbit to
calculate the star’s distance.
Check Your Reading
What factors affect how bright a star appears from Earth?
Parallax
SKILL FOCUS
How does the distance of an object
affect parallax?
Measuring
PROCEDURE
1
MATERIALS
Stand 1 m away from a classmate. Have the classmate hold up a meter stick
at eye level.
2 With your left eye closed, hold a capped pen up close to your face. Look at
• meter stick
• capped pen
TIME
10 minutes
the pen with your right eye, and line it up with the zero mark on the meter
stick. Then open your left eye and quickly close your right eye. Observe how
many centimeters the pen seems to move. Record your observation.
3 Repeat step 2 with the pen held at arm’s length and then with the pen
held at half your arm’s length. Record your observation each time.
WHAT DO YOU THINK?
• How many centimeters did the pen appear to move each time you observed it?
• How is parallax affected when you change the distance of the pen from you?
CHALLENGE How could you use this method to estimate distances that
you cannot measure directly?
Chapter 22: Stars, Galaxies, and the Universe 787
Size
It is hard to get a sense of how large stars are from viewing them in
the sky. Even the Sun, which is much closer than any other star, is far
larger than its appearance suggests. The diameter of the Sun is about
100 times greater than that of Earth. A jet plane flying 800 kilometers per
hour (500 mi/h) would travel around Earth’s equator in about two days.
If you could travel around the Sun’s equator at the same speed, the
trip would take more than seven months.
A star the size
of the Sun
Diameter = 1.4
million kilometers
(900,000 mi)
Some stars are much larger than the Sun. Giant and supergiant
stars range from ten to hundreds of times larger. A supergiant called
Betelgeuse (BEET-uhl-JOOZ) is more than 600 times greater in diameter
than the Sun. If Betelgeuse replaced the Sun, it would fill space in our
solar system well beyond Earth’s orbit. Because giant and supergiant
stars have such huge surface areas to give off light, they are very bright.
Betelgeuse is one of the brightest stars in the sky, even though it is
522 light-years away.
There are also stars much smaller than the Sun. Stars called white
dwarfs are about 100 times smaller in diameter than the Sun, or roughly
the size of Earth. White dwarfs cannot be seen without a telescope.
Color and Temperature
If you observe stars closely, you may notice that they vary slightly in
color. Most stars look white. However, a few appear slightly blue or
red. The differences in color are due to differences in temperature.
White dwarf
1/100 the Sun’s
diameter
Giant star
10–100 times the
Sun’s diameter
Supergiant star
100–1000 times
the Sun’s diameter
You can see how temperature affects color by heating up metal.
For example, if you turn on a toaster, the metal coils inside will start to
glow a dull red. As they get hotter, the coils will turn a brighter orange.
The illustration on page 789 shows changes in the color of a metal bar
as it heats up.
Like the color of heated metal, the color of a star indicates its
temperature. Astronomers group stars into classes by color and surface
temperature. The chart on page 789 lists the color and temperature
range of each class of star. The coolest stars are red. The hottest stars are
blue-white. Our Sun—a yellow, G-class star—has a surface temperature
of about 6000°C.
Stars of every class give off light that is made up of a range of colors.
Astronomers can spread a star’s light into a spectrum to learn about the
star’s composition. The colors and lines in a spectrum reveal which
gases are present in the star’s outer layers.
Check Your Reading
788 Unit 6: Space Science
How does a star’s temperature affect its appearance?
Color and Temperature
Objects that radiate light change
color as they heat up.
When heated to about 1500°C,
a steel bar gives off white light.
Classification of Stars
Class
Color
Surface Temperature (°C)
O
blue-white
above 25,000
B
blue-white
10,000–25,000
A
white
7500–10,000
F
yellow-white
6000–7500
G
yellow
5000–6000
K
orange
3500–5000
M
red
below 3500
Stars are classified according to their colors
and temperatures. The Sun is a G-class star.
At about 1200°C the metal
gives off yellow light.
A steel bar glows red when
heated to about 600°C.
Stars have life cycles.
Although stars last for very long periods, they are not permanent.
Like living organisms, stars go through cycles of birth, maturity, and
death. The life cycle of a star varies, depending on the mass of the
star. Higher-mass stars develop more quickly than lower-mass stars.
Toward the end of their life cycles, higher-mass stars also behave
differently from lower-mass stars.
Colors have been added
to this photograph of the
Omega Nebula in order to
bring out details.
Stars form inside a cloud of gas and
dust called a nebula (NEHB-yuh-luh).
Gravity pulls gas and dust closer
together in some regions of a nebula.
As the matter contracts, it forms a hot,
dense sphere. The sphere becomes a
star if its center grows hot and dense
enough for fusion to occur.
When a star dies, its matter does
not disappear. Some of it may form a
nebula or move into an existing one.
There, the matter may eventually
become part of new stars.
Check Your Reading
How is gravity involved in the
formation of stars?
Chapter 22: Stars, Galaxies, and the Universe 789
Stages in the Life Cycles of Stars
The diagram on page 791 shows the stages that stars go through in
their life cycles. Notice that the length of a cycle and the way a star
changes depend on the mass of the star at its formation.
RESOURCE CENTER
CLASSZONE.COM
Learn more about life
cycles of stars.
FLORIDA
Content Review
reminder
Notice that in the lives of
stars, as useful energy of
the star system decreases,
disorder in the system
increases.
The stage in which stars produce energy through
the fusion of hydrogen into helium is called the main sequence.
Because they use their fuel slowly, lower-mass stars can remain in the
main-sequence stage for billions of years. The Sun has been a mainsequence star for 4.6 billion years and will remain one for about another
5 billion years. When a lower-mass star runs out of hydrogen, it expands
into a giant star, in which helium fuses into carbon. Over time a giant
star sheds its outer layers and becomes a white dwarf. A white dwarf is
simply the dead core of a giant star. Although no fusion occurs in white
dwarfs, they remain hot for billions of years.
Lower-Mass Stars
Stars more than eight times as massive as our Sun
spend much less time in the main-sequence stage because they use
their fuel rapidly. After millions of years, a higher-mass star expands to
become a supergiant star. In the core of a supergiant, fusion produces
heavier and heavier elements. When an iron core forms, fusion stops
and gravity causes the core to collapse. Then part of the core bounces
outward, and the star erupts in an explosion called a supernova.
Higher-Mass Stars
For a brief period, a supernova can give off as much light as a galaxy.
The outer layers of the exploded star shoot out into space, carrying
with them heavy elements that formed inside the star. Eventually this
matter may become part of new stars and planets.
Neutron Stars and Black Holes
The collapsed core of a supergiant star may form an extremely dense
body called a neutron star. Neutron stars measure only about 20 kilometers (12 mi) in diameter, but their masses are one to three times
that of the Sun.
Neutron stars emit little visible light. However, they strongly emit
other forms of radiation, such as x-rays. Some neutron stars emit beams
of radio waves as they spin. These stars are called pulsars because they
seem to pulse as the beams rotate.
A pulsar emits beams of
radio waves as it spins
rapidly. The pulsar seems
to pulse as the beams
rotate toward and away
from Earth.
790 Unit 6: Space Science
Sometimes a supernova leaves behind a core with a mass more than
three times that of the Sun. In such a case, the core does not end up as
a neutron star. Instead, it collapses even further, forming an invisible
object called a black hole. The gravity of a black hole is so strong that
no form of radiation can escape from it.
Check Your Reading
How do lower-mass stars differ from higher-mass stars after the
main-sequence stage?
Life Cycles of Stars
A star forms inside a cloud of gas and dust called a nebula.
The life cycle of a star depends on its mass.
Lower-Mass Stars
A lower-mass star
can fuse hydrogen
into helium for billions of years. This
stage is called the
main sequence.
Higher-Mass Stars
A higher-mass star
remains in the
main-sequence
stage for millions
of years.
After the
main-sequence
stage, the star
expands into a
supergiant.
After the
main-sequence
stage, the star
expands into a
giant star.
When a giant star
sheds its outer
layers, it leaves
behind a dead
core called a
white dwarf.
When fusion
can no longer
occur in the
supergiant, it
undergoes an
explosion called
a supernova.
A high-mass star leaves
behind a densely
packed core called a
neutron star.
A star with an extremely
high mass leaves behind an
invisible black hole.
Astronomers can sometimes detect matter and
energy around a black hole.
How do the stars shown in this illustration differ in the
main-sequence stage of their life cycles?
Chapter 22: Stars, Galaxies, and the Universe 791
Star Systems
Unlike our Sun, most stars do not exist alone. Instead, they are
grouped with one or more companion stars. The stars are held together
by the force of gravity between them. A binary star system consists of
two stars that orbit each other. A multiple star system consists of more
than two stars.
In many star systems, the stars are
too close together to be seen individually. However, astronomers have
developed ways of detecting such systems. For example, in a binary star
system, one of the stars may orbit in
front of the other when viewed from
Earth. The star that orbits in front
will briefly block some of the other
star’s light, providing a clue that
more than one star is present. The
illustration at right shows a binary
star system that can be detected this
way. Sometimes astronomers can also
figure out whether a star is really a
star system by studying its spectrum.
Binary Star System
Some binary star systems appear
to dim briefly when one star
orbits in front of the other and
blocks some of its light.
When neither star is in front
of the other, the star system
appears to give off more light.
Star systems are an important
source of information about star
masses. Astronomers cannot measure the mass of a star directly.
However, they can figure out a star’s mass by observing the effect
of the star’s gravity on a companion star.
Check Your Reading
Why are star systems important to astronomers?
KEY CONCEPTS
CRITICAL THINKING
1. Why must astronomers figure
out a star’s distance to calculate
its actual brightness?
4. Analyze Some of the brightest stars are red supergiants.
How can stars with cooler red
surfaces be so bright?
2. How are color and temperature related in stars?
3. How does a star’s mass affect
its life cycle?
792 Unit 6: Space Science
5. Infer Will the Sun eventually
become a black hole? Why or
why not?
CHALLENGE
6. Infer At what stage in the life
cycle of the Sun will it be
impossible for life to exist on
Earth? Explain.