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LESSON 2 CHAPTER16
Transcript of LESSON 2 CHAPTER16
Lesson 2 What is a star? How the Sun Stacks Up as a Star
The Sun is a star. Stars, including the Sun,
are gigantic balls of very hot gases that give off
electromagnetic radiation. As stars go, the Sun
is not unusual at all. It is of medium size. Stars
known as giants may be 8 to 100 times as large
as the Sun. Supergiants are even larger. They may
be up to 300 times as large as the Sun. Other stars
are much smaller—only about the size of Earth.
Compared to the Earth, though, the Sun is huge.
If you think of the Sun as a gumball machine and
the Earth as a gumball, it would take a million
Earth gumballs to fill the Sun gumball machine.
The Sun gives off enormous amounts of thermal
energy and light energy. These energies come from
powerful reactions involving the Sun’s two main
components, hydrogen and helium gas. Deep
inside the core of the Sun, the nuclei of hydrogen
atoms have such a high temperature and kinetic
energy that when they collide, they fuse together.
The nuclei combine to form a new nucleus and
a new element, helium. Huge amounts of energy
are released as this happens, which is what makes
the Sun shine.
Brightness, Color, and
Temperature of Stars
The Sun is the closest star to Earth. It is by
far the brightest star in the sky. It’s easy to
think that the stars that look the brightest
are the closest. However, Barnard’s Star is
the third closest to Earth, but it can’t be seen
without a telescope.
So just what is it that makes some stars
look bright? The brightest stars are the
stars that give off the most energy. But a
star’s size, temperature, and distance
from Earth all play a part in how bright a
star looks to us. The dazzling white star
Sirius, for example, is the brightest star
in the night sky, but it is only the ninth
closest star to Earth. It is larger, hotter,
and more than 20 times as bright as
the Sun. It doesn’t look brighter to us
because it is much farther from us than
the Sun. If we could line up all the stars
at the same distance from Earth, we could
see which stars are really the brightest.
A star’s color tells you how hot it is. Red
stars, like Barnard’s star are the coolest.
Somewhat hotter are orange and yellow
stars, like the Sun with a temperatures of
about 5500°C. The hottest stars are white or
blue-white. Even though red stars are said
to be “cool,” they are still extremely hot.
Barnard’s Star is about 2250° C (4000° F). At
that temperature a piece of iron would melt
instantly and boil away into a gas.
Sunspots may be
the size of Earth
Prominences may rise at a speed
of 1000 kilometers per second.
Some may reach a height of over
one million kilometers.
Solar flares give off more
light than other parts of the
Sun. They also give off
more X rays and other electromagnetic radiation.
The Life of Stars
Thanks to powerful telescopes, scientists have glimpsed new stars being born and ancient
stars dying. New stars form in a cloud of gas and dust called a nebula. As particles of gas
and dust churn around, gravity begins to pull together a clump of particles
into a ball. Gravity increases in the ball, and more and more
particles are pulled in. At the same time, the temperature
rises. If it gets hot enough, hydrogen will begin changing
into helium and releasing tremendous amounts of energy.
The massive and dense clump of particles will officially have become a star.
Stars live an extremely long time, but they don't live forever. In billions of years, the Sun will use up all of the hydrogen (its "fuel") in its core. It will become several thousand times brighter and expand to about 170 times its current size—about out to where Mars orbits now.
In this photograph,
stars are seen
forming and dying
As the Sun expands, its temperature will be slightly cooler, so it will be red, rather than yellow. It will be known as a red giant star. At this point, it will be using helium as fuel. When the helium is used up, the core will shrink to about the size of Earth and the remaining layers of gas will float off into space. The core will become a white dwarf star. A white dwarf has no fuel to convert to radiant energy, only the leftover thermal energy from its energy-producing days keeps the star hot for a long time. Over a period of several million years, a white dwarf cools down and becomes a cold object called a black dwarf.
Massive stars go out in a blaze of glory. When a massive star's core runs out of fuel, it starts shrinking in on itself until it can shrink no farther. Powerful shock waves from this sudden stop fan outward, and particles of matter spin off into space carrying huge amounts of energy with them. A gigantic explosion occurs that is millions or billions of times as bright as the star ever was. This explosion is known as a supernova. It hurls matter and energy far out into space. Usually, all that will be left behind is a ball of neutrons that is about 20 km (12 mi) across. This city-sized object is called a neutron star.
If the core was quite massive—more massive than three Suns—the core's own gravity will keep causing it to shrink in on itself until it becomes a black hole. A black hole is a point in space that has such a strong force of gravity that nothing within a certain distance of it can escape getting pulled into the black hole—not even lig
At the center of this nebula, two stars orbit each other. One of the stars is dying and has thrown off most of its gas layers, creating this butterfly-shaped cloud of gas and dust.
Ancient people divided the sky up into groups of stars. This made studying the
stars easier. Today we know that the stars—including the Sun—are part of even
larger groupings of stars that are all bound to each other by gravity.
The Sun, the Earth, and the other planets in the solar system are part of the
galaxy known as the Milky Way. A galaxy is a huge system of stars, dust, and
gas held together by gravity. There are billions of galaxies in the universe.
A few can be seen without a telescope, but they are so far away they look like single points of light. Using powerful telescopes, astronomers have learned that galaxies come in different shapes and sizes.
About three-fourths of the galaxies that have
been discovered are spiral galaxies. They
look like pinwheels. They have bright, bulging
middles and wispy arms that fan out from the
center. The stars in the arms of the galaxy are
circling the center bulge of the galaxy, much
as the Earth moves around the Sun.
Elliptical galaxies can be almost round or more
oval like a football. The largest galaxies we know
of are elliptical. There are also elliptical galaxies
that are many times smaller than our galaxy.
Some galaxies are neither spiral nor elliptical.
Galaxies that have no real shape are called
irregular galaxies. Irregular galaxies are probably
young galaxies in which stars are still forming.
This side view of the Milky Way shows the bright
bulging middle and wispy arms of this spiral galaxy.
Our solar system is toward the end of one of the
Milky Way’s arms, about 25,000 to 30,000 lightyears
from the center.
In the past, people looked up at the night sky and "connected the dots" formed by the stars. They saw patterns that reminded them of bears, dogs, a swan, a lion, and even a sea monster! Today, scientists divide the night sky into 88 constellations. A constellation is a group of stars that forms a pattern. A map of constellations looks much like a map of the United States with all the states in the country outlined. Some are rectangles, others have odd shapes. Many of the constellation names are the names of the star patterns people used long ago.
Dividing the sky into sections makes studying stars easier. A constellation is a little bit like a star's address. For example, if you tell someone which state you live in, it lets that person know which part of the country you live in. Knowing which constellation a star is in lets you know which part of the sky to look at to find that star.
Two stars that look close together in the same constellation are not necessarily very close in reality. One star may be billions of kilometers farther from Earth than the other. They appear to be close together because they are in the same direction from Earth.
People who live in different parts of the world see different sections of the sky and different constellations. The Earth can be divided into two halves along the equator. The half to the north of the equator is called the Northern Hemisphere. The half to the south is called the Southern Hemisphere. The United States is in the Northern Hemisphere. Ursa Major can be seen in the Northern Hemisphere. But it is not visible to people in the Southern Hemisphere.
Stars on the Move
Stars do not always appear in the same place in the sky.
They move in logical and predictable ways. Suppose you
looked at the sky early one evening and found the Big
Dipper. Two hours later you went back out and found
that the Big Dipper moved toward the eastern horizon.
Actually, the Big Dipper did not move, but you moved.
Every 24 hours, the Earth makes one complete rotation.
This rotation of the Earth is why the Sun seems to travel
across the sky every day—rising in the east and setting
in the west. It is also why the stars appear to move across
the sky in the same direction.
Ursa Major, which contains the Big Dipper, is visible all
year. But other constellations can be seen only at certain
times of the year. In the United States, you can see the
constellation Canis Major, or the Great Dog, only in winter.
Constellations change with the seasons because Earth is
traveling around the Sun. It takes Earth one year to travel
around the Sun. As the Earth makes its journey, different
parts of the sky come into view at night for people on Earth.
In a way, it’s not so different from riding a merry-go-round.
As you look outward during a ride, your view of the
Nothing in the universe stands still. Stars are moving
through space in various directions and at various
speeds. We can not see this is happening because
the stars are so very far away. But over very long periods
of time, the patterns of stars will change as some stars
move closer to or farther away from each other.
1. Which star shown has a diameter that is almost twice
the Sun’s diameter?
A. Beta Pegasi
C. Sirius A
D. Hadar B
2. Which star shown is almost 100 times as large as
F. Sirius A
G. Hadar B
I. Beta Pegasi
3. The planet Jupiter has a diameter of about 140,000 km.
It is about the size of which star?
A. the Sun
B. Beta Pegasi
C. Hadar B
D. Sirius A
4. List the 5 stars in the chart in order from
smallest to largest.