Download Chapter 25 Beyond Our Solar System

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Chapter 25 Beyond Our Solar System
Summary
25.1 Properties of Stars
Color is a clue to a star’s temperature.
Binary stars are used to determine the star property most difficult to
calculate—its mass.
• The word constellation is used to designate an area of the sky that
contains a specific pattern of stars.
• Two stars that orbit each other, pulled toward each other by gravity,
are called binary stars.
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The nearest stars have the largest parallax angles, while those of
distant stars are too small to measure.
• Parallax is the slight shifting in the apparent position of a nearby star
due to the orbital motion of Earth. Parallax is the most basic way to
measure star distance.
• The light-year is the distance light travels in one year—about
9.5 trillion kilometers.
Three factors control the apparent brightness of a star as seen from
Earth: how big it is, how hot it is, and how far away it is.
• A star’s brightness as it appears form Earth is called its apparent
magnitude.
• Astronomers are also interested in how bright a star actually is, or its
absolute magnitude.
A Hertzsprung-Russell diagram shows the relationship between the
absolute magnitude and temperature of stars.
• A main sequence star is a star that falls into the main sequence
category on the Hertzsprung-Russel diagram. This category contains
90 percent of stars.
• Red giants are very bright stars that lie above and to the right of the
main sequence in the H-R diagram.
• Supergiants are very large, very bright red giants.
• Cepheid variables are stars that get brighter and fainter in a regular
pattern.
• A nova is a sudden brightening of a star.
• Nebulae are clouds of dust and gases found in “the vacuum of space.”
25.2 Stellar Evolution
• A medium-mass star like the sun goes through several stages of
development. It starts as a nebula, which contracts into a protostar—a
developing star not yet hot enough for nuclear fusion to occur.
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Chapter 25
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Beyond Our Solar System
When the core of a protostar has reached about 10 million K, pressure
within is so great that nuclear fusion of hydrogen begins, and a star is
born.
• At some point after fusion begins, a star becomes a balanced, mainsequence star. For an average star, this stage lasts 90 percent of the
star’s life.
• Once all of the hydrogen in a star’s core is consumed, the star expands
and cools, becoming a red giant.
All stars, regardless of their size, eventually run out of fuel and
collapse due to gravity.
• The final stage of a star’s life cycle depends on the star’s mass. Lowmass stars go from being a main-sequence star to becoming a white
dwarf. Medium-mass stars become planetary nebulae. Massive stars
end in a supernova.
• A supernova is a brilliant explosion that causes a star to become
millions of times brighter than its prenova stage.
• The process that produces chemical elements inside stars is called
nucleosynthesis.
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The sun began as a nebula, will spend much of its life as a mainsequence star, and then will become a red giant, planetary nebula, white
dwarf, and finally, a black dwarf.
• White dwarfs are the remains of low-mass and medium-mass stars.
• Neutron stars, which are smaller and more massive than white
dwarfs, are thought to be the remnants of supernova events.
• A spinning neutron star that appears to give off pulses of radio waves
is called a pulsar.
• Dense objects with gravity so strong that not even light can escape
their surface are called black holes.
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Chapter 25
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Beyond Our Solar System
25.3 The Universe
The Milky Way is a large spiral galaxy whose disk is about 100,000
light-years wide and about 10,000 light-years thick at the nucleus.
• Galaxies are large groups of stars, dust, and gases held together by
gravity.
• There are three types of galaxies. Spiral galaxies are disk shaped with
arms extending from the center. Most galaxies are elliptical galaxies,
which range in shape from round to oval. A small percent of galaxies
have irregular shapes, and are called irregular galaxies.
In addition to shape and size, one of the major differences among
different types of galaxies is the age of their stars.
• A galaxy cluster is a group of galaxies.
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The red shifts of distant galaxies indicate that the universe is
expanding.
• Hubble’s law states that galaxies are retreating from us at a speed that
is proportional to their distance.
The big bang theory states that at one time, the entire universe was
confined to a dense, hot, supermassive ball. Then, about 13.7 billion years
ago, a violent explosion occurred, hurling this material in all directions.
• According to the big bang theory, the universe began as a violent
explosion from which the universe continues to expand, evolve,
and cool.
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