Download Astronomy Ch 20 The Universe

Astronomy Ch 20 The
Universe
What is the universe?
Sum of all space, matter, and energy
past, present, future
 http://hubblesite.org/explore_astrono
my/hubbles_universe_unfiltered/
Where do we live in the universe:
Our Solar System, Milky Way Galaxy,
Local Group Cluster, Virgo Super
Cluster
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Astronomy Ch 20 The
Universe
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Section 1: The Life and Death of Stars
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What Are Stars? Stars are huge spheres
of very hot gas that emit light and other
radiation. Stars are formed from clouds of
dust and gas, or nebulas, and go through
different stages as they age. (p. 693)
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Stars have influenced cultures for
thousands of years by helping people
mark the passage of time and by
providing markers for navigation in the
night sky.
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Stars are born in a nebula (left). After
billions of years, most stars become
old and lose their outer layers of gas.
As a star begins to die, it may become
a planetary nebula (right).
light-year
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Stars are located at various distances
from Earth. We use the light-year (ly)
to describe the distance from Earth to
far-away objects such as stars and
galaxies. A light-year is the distance
that light travels in one year, or about
9.5 × 1012 km.
Stars are powered by nuclear
fusion reactions
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A star is a huge sphere of very hot
hydrogen and helium gas that emits light. A
star is held together by the enormous
gravitational forces that result from its own
mass. Inside the core of a star, the pressure
is more than a billion times the atmospheric
pressure on Earth. The temperature in this
incredibly dense core is hotter than 15
million kelvins.
Energy from a nuclear fusion reaction in the core may
take tens of thousands of years to reach a star’s surface.
When the energy reaches the surface, the energy is
released into space as electromagnetic radiation
Studying Stars
The telescope allowed astronomers to study
stars in more detail for the first time.
Greeks noticed that stars had color and divided stars
by their apparent brightness
Astronomers did not learn about the nature of stars
until the optical telescope was invented.
As science and technology have improved, telescopes
have become more powerful and have thus allowed
us to see more.
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Some stars appear brighter than others.
The brightness of a star depends on the star’s
temperature, size, and distance from Earth.
The brightest star in the night sky is Sirius in the
constellation Canis Major. Sirius appears so
bright because it is close to Earth, only about
8.7 light-years away. The surface temperature
of Sirius is about 10,000 K.
The sun’s surface is only 6,000 K, but the sun is
so close to Earth that it prevents us from seeing
other stars during the day.
We learn about stars by studying energy.
Stars produce various types of electromagnetic
radiation: from visible light to X rays to radio
waves.
Scientists use optical telescopes to study visible
light and radio telescopes to study radio waves
emitted from astronomical objects.
Earth’s atmosphere blocks other wavelengths,
so telescopes in space are used to study a
wider range of the electromagnetic spectrum
than telescopes on the ground can detect.
Section 1: The Life and Death of
Stars
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The Life Cycle of Stars In a way
that is similar to other natural cycles,
stars are born, go through stages of
development, and eventually die. (p.
698)
This graph shows the intensity of
light at different wavelengths for
the sun and for two other stars.
The color spectra of most stars contain dark lines. Dark lines are
caused by gases in the outer layers of the stars that absorb the light
at these wavelengths.
Astronomers match the dark lines in starlight to the known lines of
elements found on Earth to determine what elements make up a
star..
Stars change in many ways over their life
cycle. Small and medium stars are born in
giant nebulas of dust and gas and often end
up as white dwarfs billions of years later. The
images in this life cycle are not shown to
scale.
Massive stars
 supernova -If the core that remains after such a supernova has
occurred has enough mass, the remnant can become a
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neutron star. Neutron stars are small in diameter, but they are very
massive. Just a teaspoon of matter from a neutron star would weigh
more than 100 million tons on Earth. Neutron stars can be detected
as
pulsars, or spinning neutron stars that pulsate radio waves.If the
leftover core is great enough, it will collapse to form something
else—a
black hole, which consists of matter so massive and compressed
that nothing, not even light, can escape its gravitational pull.
Because no light can escape a black hole, black holes cannot be
seen directly. Black holes can, however, be detected indirectly by
observing the radiation of light and X rays from objects that revolve
rapidly around them.
Section 2: The Milky Way and
Other Galaxies
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Galaxies
While the nearest stars are a few light-years away
from Earth, the nearest galaxy to our own is
millions of light-years from Earth. A galaxy is a
collection of millions to billions of stars. The
deeper scientists look into space, the more
galaxies they find. There may be more than 100
billion galaxies. If you counted 1,000 galaxies per
night, it would take 275,000 years to count all of
them.
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Gravity holds galaxies together.
Without gravity, everything in space might be a
veil of gas spread out through space. But because
of gravity, clouds of gas come together and
collapse to form stars. As the first stars in a galaxy
age, they throw off gas and dust or become
supernovas. New stars then form. The gas, dust,
and stars form galaxies because of gravity.
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Just as Earth revolves around the sun because of
gravity, the solar system revolves around the
center of the galaxy because of gravity. It takes
our solar system about 226 million years to
complete one orbit of our galaxy.
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Galaxies are often found together in clusters.
Galaxies are not spread out evenly through space. Galaxies
are grouped together and bound by gravity in clusters like the
cluster shown in Figure 3. The Milky Way galaxy and the
Andromeda galaxy are two of the largest members of the
Local Group, a cluster of more than 30 galaxies. New
members of the Local Group, are being discovered as more
telescopes, such as the Hubble Space Telescope that is
shown in Figure 2, become available.
Clusters of galaxies can form even larger groups called
superclusters. A typical supercluster contains thousands of
galaxies that contain trillions of stars in individual clusters.
Superclusters can be as large as 100 million light-years
across. They are the largest known structures in the universe.
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Section
2: The Milky Way and Other Galaxies
Types of Galaxies
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Galaxies can be divided into three major types: spiral, elliptical, and
irregular. The three types of galaxies have many stars, but differ
in structure.
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We live in the Milky Way galaxy.
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The Milky Way is a spiral galaxy.
Our galaxy is a huge spiraling disk of stars,
gas, and dust. This gas and dust is called
interstellar matter
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The Milky Way has a huge bulge in the
center that contains primarily old red stars
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The spiral arms contain young blue stars
Section 2: The Milky Way and
Other Galaxies
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How Galaxies Change Over Time By
studying closer galaxies that might be
similar to ancient ones, scientists can
slowly piece together the puzzle of how
galaxies evolve. (p. 706)
Section 3: Origin of the
Universe
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What Is the Universe? The universe
consists of all space, matter, and energy
that exists—now, in the past, or in the
future. (p. 708)
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We see the universe now as it was in the past.
Astronomers need large units of measure to express
distances. A light-year, which is approximately 9.5 ×
1012 km, Remember that while a year is a unit of time, a
light-year is a unit of distance.
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It takes time for light to travel in space. When we say
the sun is 8 light-minutes away, we are expressing not
only its distance from Earth, but also that we see the
sun as it was 8 min ago. We never see it as it is in the
present.
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When we see distant objects, we see them as they were
when they were younger.
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Astronomers can compare how galaxies age by looking
at many galaxies at various distances and thus at
different ages.
Section 3: Origin of the
Universe
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What Happened at the Beginning?
Scientists theorize that the universe
formed during a cataclysmic event known
as the big bang. (p. 710)
scientists estimate that the
universe is about 13.7 billion
years old.
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In 1965, scientists Arno Penzias and
Robert Wilson were making adjustments
to a new radio antenna that they had
built.
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They could not explain a steady but very
dim signal from all over the sky in the
form of radiation at microwave
wavelengths. They finally realized that the
signal was the cosmic background
radiation predicted by the big bang
theory.
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Radiation dominated the early universe.
According to the big bang theory, immediately
after the big bang, the universe was extremely
hot and made up of pure energy. There was a
period of rapid expansion that caused the
energy to cool and allowed sub-atomic particles,
such as protons, electrons, and neutrons, to
form. The first stars were born about 400 million
years after the big bang.
Expansion implies that the universe
was once smaller.
 Most galaxies are moving away from each
other.
 Long ago, the entire universe might have
been contained in an extremely small
space.
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During the past 100 years, other theories
for the origin of the universe have been
proposed. A few are still being studied,
but the big bang theory is the one that is
best supported by the current evidence
including the cosmic background radiation
and observations of the movement of
distant galaxies
Section 3: Origin of the
Universe
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Predicting the Future of the Universe
Scientists use their increasing knowledge
of the universe to hypothesize what might
happen to the universe in the future. (p.
715)
They depend on a mixture of theories and
precise observations of very faint and distant
objects.
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as telescopes
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New space telescopes that collect infrared
radiation and X rays are being built and
launched.
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Data in these regions of the electromagnetic
spectrum may provide important clues about the
beginning and future of the universe.
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One example of new, more sensitive
technology is the Chandra X-Ray
Observatory, which was launched into
orbit around Earth in 1999.
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This observatory can take photographs in
the X-ray part of the electromagnetic
spectrum. The presence of X rays indicates
matter at temperatures of more than one
million degrees.
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According to Einstein’s theory of relativity, mass
curves space, much in the same way that your
body curves a mattress when you sit on it.
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In 1919, observations of a total solar eclipse
showed that Einstein was correct. In the
direction of the sun, stars that could be seen
only during the eclipse were in slightly different
positions than expected.
The future of the universe is uncertain.
The universe is still expanding, but it may not do
so forever. The combined gravity of all of the
mass in the universe is also pulling the universe
inward, in the direction opposite to the
expansion. The competition between these two
forces leaves three possible outcomes for the
universe:
1.The universe will keep expanding forever.
2.The expansion of the universe will gradually
slow down, and the universe will approach a
limit in size.
3.The universe will stop expanding and start to fall
back on itself.
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