Download Lesson 4, Stars

Chapter 8, Astronomy
LESSON 4, STARS
Objectives
 Define some of the properties of stars.
 Compare the evolutionary paths of star
types.
Main Idea
 Stars vary in their size, their brightness, and
their distance from Earth.
Vocabulary
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star
constellation
parallax
light-year
nebula
supernova
black hole
What are stars?
 A star is a large, hot ball of gases, held
together by gravity, that gives off its
own light.
 A constellation is a group of stars that
appear to form a pattern.
 As earth revolves, different
constellations can be seen, like Orion,
which is a winter constellation in the
Northern Hemisphere.
 Constellations are classified by the
seasons they appear in.
 Finding the Big Dipper in Ursa Major, the Great
Bear, can help you find Polaris, the North Star.
 If you are unsure of directions, the North
Star can help you.
 Because of our perspective, the stars in the sky
form pictures, as we look at them from Earth.
 If we looked at our stars far away from our
solar system, the pictures would not look the
same.
Finding the Distance
to a Star
 Viewed from different points in Earth’s orbit,
some stars seem to change position slightly
compared to stars farther away.
 The apparent shift in an objects position when
viewed from two locations is called parallax.
 Astronomers use parallax to find the distance
of a star from Earth.
 When astronomers measure the distance from
Earth to a star, even a kilometer far is too small.
 They use a measurement of distance called a
light year, the distance that light travels in one
year.
 One light year is more than 9 trillion kilometers
(6 trillion miles).
 The nearest star, Alpha Centauri, is about 4.3
light years away from Earth.
 The light we see when we look at Alpha
Centauri left that star about 4.3 years ago.
Quick Check
Compare and Contrast
How are the constellations alike?
How are they different?
The constellations all suggest patterns and
consists of clusters of stars.
The constellations have different numbers of
stars and are positioned at varying distances
from Earth.
Critical Thinking
It takes sunlight about 8 minutes to reach
Earth.
Is the Sun more than or less than a light
year away? Explain.
The Sun is far less than a light year away.
The reason is that a light year is the distance
that light travels in a year.
Light from the Sun takes 8 minutes to reach
Earth, which is much less than a year.
What are some properties
of stars?
 Some stars are brighter than others.
 Stars appear less bright the farther they
are from Earth.
 Sirius seems brighter than Rigel.
 However, Rigel is brighter than
Sirius.
 Color is one of a star’s properties, it tells
you the stars temperature.
 Red and orange colors indicate
cooler stars.
 Yellow indicates hotter stars.
 Blue – white indicates the hottest
stars.
 Stars come in different sizes.
 Red supergiants are the biggest.
 Our sun is average.
 A white dwarf is the smallest.
Brightness and Temperature
 Astronomers, Ejnar Hertzsprung and
Henry Norris Russell looked for
similarities between a star’s brightness
and temperature.
 They found that stars appear in groups.
 To read an H-R diagram start near the
lower left corner.
 The stars near here are very dim but
very hot.
 In the lower right corner are dim, very cool
stars.
 Diagonally up and to the left from the
lower right corner is a long band.
 Most stars here are main–sequence
stars.
 On the top of the scale of absolute
magnitude are supergiants.
 Giants are dimmer than supergiants.
 The giants are just below supergiants in the
chart.
Quick Check
 Compare and Contrast
 How are absolute magnitude and
apparent magnitude similar?
 Both types of magnitude are a measurement
of brightness.
 Absolute magnitude describes the actual
brightness of a star, not just how it appears
from Earth.
 Apparent magnitude describes how a star
looks to us on Earth.
 Critical Thinking
 What are three properties that all
stars have and how do they relate to
one another?
 All stars have magnitude, temperature,
and size.
 Size and temperature both affect
absolute magnitude.
How do stars develop?
 Stars are born, they mature, grow older,
and finally they die.
 The main factor that affects how a star
goes through this cycle of development
is the star’s mass.
Protostars
 Every star begins as a nebula, a huge
cloud of gas and dust in space.
 As a cloud shrinks, it heats up and
becomes a protostar or a young star.
 Protostars continue to gain mass
because of its gravitational pull.
Main-Sequence Stars
 Eventually the center of a protostar reaches
a temperature of millions of degrees Celsius
and a nuclear reaction starts.
 Hydrogen atoms fuse and forms helium
atoms releasing large amounts of energy.
 Stars spend most of their time as main-
sequence stars.
Red Giants and Supergiants
 As the star expands, its surface becomes
cooler and its color becomes redder.
 The star becomes a red giant or a
supergiant, depending on its mass.
Final Stages
 The final stages of a star’s life depends on its
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mass.
Stars up to ten times the mass of the Sun
become red giants.
Nuclear reactions give off energy causing the
star to release huge clouds of gas.
A layer of gases, called a planetary nebula,
forms around the star.
Its surface becomes white-hot, forming a white
dwarf.
 A white dwarf is so hot that it gives off enough
radiation to make its surrounding shell of gas
and dust glow.
 When the shell glows brightly enough, we see
it as a planetary nebula.
 A white dwarf may take billions of years to
cool off.
 When it cools off, it will become a black dwarf.
 Stars with masses greater than ten times
the mass of the Sun follow a different path.
 These large stars become supergiants.
 They use up energy at a fantastic rate,
giving off very large amounts of energy.
 In a relatively short amount of time, the star
can no longer fuse atoms and give off
energy at the same rate.
 When a supergiant can no longer
produce enough energy, it collapses and
then explodes.
 This is called a supernova.
 The next stage depends on the star’s mass.
 What remains of a supernova becomes a
neutron star, made of neutrons.
 As they rotate they blink like a lighthouse
beacon.
 This is a pulsar.
 If a star is very massive, the supernova
does not become a neutron star.
 Instead, the core collapses and it
becomes a tiny but very massive object
called a black hole.
 Black holes are detected by the effect
they have on other objects.
 Often, gasses form a nearby companion
that is pulled in by the intense gravity of
the black hole.
 When the gasses approach they emit x-
rays.
Quick Check
 Compare and Contrast
 Compare the development of a less-massive
star with that of a more-massive star.
 A less-massive star: begins as a nebula,
becomes a protostar, a main-sequence star, a
red giant, and finally, a white dwarf.
 A more-massive star: begins as a nebula,
becomes a protostar, a main-sequence star, a
very massive star, a supergiant, a supernova,
and finally, either a neutron star (pulsar) or a
black hole.
 Critical Thinking
 Why will the Sun not become a black
hole someday?
 The Sun does not have enough mass to
become a black hole.
What kind of star
is the Sun?
 The Sun is a main-sequence star in the
middle of the H-R diagram.
 The Sun has been shining for 5 billion
years and will continue for another 5
billion, until it becomes a red giant.
 The Sun contains 99.9% of the solar
system’s mass.
 It is 92% hydrogen.
 Its hydrogen is being changed into
helium by nuclear reactions.
 You should never look at the Sun directly
because its brightness can harm your eyes.
Quick Check
 Compare and Contrast
 How is the Sun like other stars?
 Like other stars the Sun is very hot and
gaseous. Nuclear reactions occur in its
core and large amounts of energy are
released.
 Critical Thinking
 Do you think the Sun is less massive
or more massive than other stars?
Why?
 Because of its position on the main
sequence, it is probably average.
 The Sun is probably neither less massive
nor more massive and is most likely an
average-size star.