Transcript star

Chapter 26
The Universe
A star is a large, glowing ball of gas in space,
which generates energy through nuclear fusion
in its core.
The closest star to Earth is the sun, which is
considered to be a fairly average star.
Stars are so far away that astronomers cannot
measure their distances directly.
Astronomers are able to observe stars from two
different positions–opposite sides of Earth’s
Nearby stars appear to move against the moredistant background stars.
• The apparent change in position of an object
with respect to a distant background is called
• Astronomers measure the parallax of nearby
stars to determine their distance from Earth.
With the invention of the telescope, astronomers could
measure the positions of stars with much greater
• The closer a star is to Earth, the greater is its parallax.
• Astronomers have measured the parallax of many nearby
stars and determined their distances from Earth.
The Light-Year
Because stars are so far apart, it’s not practical to
measure their distances in units that might be used on
Earth, such as kilometers.
• A light-year is the distance that light travels in a vacuum in
a year, which is about 9.5 trillion kilometers.
• Proxima Centauri, the closest star to the sun, is about 4.3
light-years away.
Astronomers classify stars by their color, size,
and brightness. Other important properties
of stars include their chemical composition
and mass.
Most stars have a chemical makeup that is
similar to the sun, with hydrogen and helium
together making up 96 to 99.9 percent of the
star’s mass.
Color and Temperature
A star’s color indicates the temperature of its surface.
• The hottest stars, with surface temperatures above 30,000
K, appear blue.
• The surfaces (photospheres) of relatively cool red stars are
still a toasty 3000 K or so.
• Stars with surface temperatures between 5000 and 6000 K
appear yellow, like the sun.
Astronomers have discovered that the
brightness of stars can vary by a factor of more
than a billion.
Stars that look bright may actually be farther
away than stars that appear dim.
The sun appears very bright to us because it is
much closer than other stars.
The brightness of a star as it appears from Earth
is called its apparent brightness.
The apparent brightness of a star decreases as
its distance from you increases.
Absolute brightness is how bright a star really is.
A star’s absolute brightness is a characteristic of
the star and does not depend on how far it is
from Earth.
You can calculate a star’s absolute brightness if
you know its distance from Earth and its
apparent brightness.
Size and Mass
Once astronomers know a star’s temperature
and absolute brightness, they can estimate its
diameter and then calculate its volume.
The masses of many stars can be determined by
observing the gravitational interaction of stars
that occur in pairs.
For most stars, there is a relationship between
mass and absolute brightness.
A spectrograph is an instrument that spreads
light from a hot glowing object into a spectrum.
Astronomers can use spectrographs to identify
the various elements in a star’s atmosphere.
This is the spectrum of a star. The dark
absorption lines indicate the presence of
various elements in the star.
H-R diagrams are used to estimate the sizes
of stars and their distances, and to infer how
stars change over time.
Stars can be classified by locating them on a
graph showing two easily determined
Such a graph is called a Hertzsprung-Russell
diagram, or H-R diagram.
An H-R diagram is a graph of the surface
temperature, or color, and absolute brightness
of a sample of stars.
The horizontal axis shows the surface
temperatures of stars. A star’s color is directly
related to its surface temperature.
The hottest blue stars are on the left and the
coolest red stars are on the right.
Surface temperatures of stars range from less
than 3000 K to more than 30,000 K.
The vertical axis of the H-R diagram shows
absolute brightness, with the brightest stars at
the top and the faintest at the bottom.
The absolute brightnesses of stars vary even
more than temperature, ranging from about one
ten-thousandth to a million times that of the
• A star’s
on an H-R
indicates its
and surface
e (or color).
Main-Sequence Stars
Stars occur only in certain places on the H-R
Most stars are found along a diagonal band running
from the bright hot stars on the upper left to the
dim cool stars on the lower right. Astronomers call
this diagonal band on the H-R diagram the main
About 90% of all stars are found on the main
sequence. The sun lies near the middle of this band.
Giants and Dwarfs
In general, two factors determine a star’s absolute
brightness: its size and its surface temperature.
An H-R diagram shows a star’s absolute brightness and
surface temperature.
• If you compare two stars at the same temperature, the
brighter one must be larger.
• Hotter stars are brighter than cooler stars of the same size.
The very bright stars at the upper right of the HR diagram are called supergiants.
Supergiants are much brighter than mainsequence stars of the same temperature, so
they must be very large compared with mainsequence stars.
Supergiants range in size from 100 to 1000 times
the diameter of the sun.
Just below the supergiants on the H-R diagram
are the giants—large, bright stars that are
smaller and fainter than supergiants
Below the main sequence in the lower part of the H-R
diagram are white dwarfs.
• A white dwarf is the small, dense remains of a low- or
medium-mass star.
• White dwarfs are hot but dimmer than main-sequence
stars of the same temperature.
• The diameter of a red giant is typically 10–100
times that of the sun and more than 1000
times that of a white dwarf.
Life Cycle of Stars
A star is formed when a contracting cloud of
gas and dust becomes so dense and hot that
nuclear fusion begins.
Life Cycle of Stars
A nebula is a large cloud of gas and dust spread out
over a large volume of space.
• Some nebulas are glowing clouds lit from within by bright
• Other nebulas are cold, dark clouds that block the light
from more-distant stars beyond the nebulas.
Stars form in the densest regions of nebulae.
Gravity pulls a nebula’s dust and gas into a
denser cloud. As the nebula contracts, it heats
Life Cycle of Stars
A contracting cloud of gas and dust with enough
mass to form a star is called a protostar.
As a protostar contracts, its internal pressure
and temperature continue to rise.
Pressure from fusion supports the star against
the tremendous inward pull of gravity.
Life Cycle of Stars
A star’s mass determines the star’s place on
the main sequence and how long it will stay
Life Cycle of Stars
Stars spend about 90 percent of their lives on the
main sequence.
In all main-sequence stars, nuclear fusion converts
hydrogen into helium at a stable rate. There is an
equilibrium between the outward thermal pressure
from fusion and gravity’s inward pull.
The amount of gas and dust available when a star
forms determines the mass of each young star.
Life Cycle of Stars
The most massive stars have large cores and
therefore produce the most energy. High-mass
stars become the bluest and brightest mainsequence stars.
These blue stars are about 300,000 times
brighter than the sun.
Because blue stars burn so brightly, they use up
their fuel relatively quickly and last only a few
million years.
Life Cycle of Stars
Stars similar to the sun occupy the middle of the
main sequence.
A yellow star like the sun has a surface
temperature of about 6000 K and will remain
stable on the main sequence for about 10 billion
Life Cycle of Stars
Small nebulas produce small, cool stars that are
long-lived. A star can have a mass as low as a tenth
of the sun’s mass.
The gravitational force in such low-mass stars is just
strong enough to create a small core where nuclear
fusion takes place. This lower energy production
results in red stars, the coolest of all visible stars.
A red star, with a surface temperature of about
3500 K, may stay on the main sequence for more
than 100 billion years.
Life Cycle of Stars
The dwindling supply of fuel in a star’s core
ultimately leads to the star’s death as a white
dwarf, neutron star, or black hole.
Life Cycle of Stars
When a star’s core begins to run out of hydrogen,
gravity gains the upper hand over pressure, and the
core starts to shrink.
• The core temperature rises enough to cause the hydrogen
in a shell outside the core to begin fusion.
• The energy flowing outward increases, causing the outer
regions of the star to expand. The expanding atmosphere
moves farther from the hot core and cools to red.
• The star becomes a red giant.
Life Cycle of Stars
• The collapsing core grows hot enough for
helium fusion to occur, producing carbon,
oxygen, and heavier elements.
• In helium fusion, the star stabilizes and its
outer layers shrink and warm up.
• The final stages of a star’s life depend on its
Life Cycle of Stars
Low- and Medium-Mass Stars
Low-mass and medium-mass stars, which can be as
much as eight times as massive as the sun, eventually
turn into white dwarfs.
• Stars remain in the giant stage until their hydrogen and
helium supplies dwindle and there are no other elements
to fuse.
• The energy coming from the star’s interior decreases.
• With less outward pressure, the star collapses.
Life Cycle of Stars
• The dying star is surrounded by a glowing
cloud of gas, called a planetary nebula.
• As the dying star blows off much of its mass,
only its hot core remains.
• This dense core is a white dwarf. A white dwarf
is about the same size as Earth but has about
the same mass as the sun.
• White dwarfs don’t undergo fusion, but glow
faintly from leftover thermal energy.
Life Cycle of Stars
High-Mass Stars
The life cycle of high-mass stars is very different from
the life cycle of lower-mass stars.
• As high-mass stars evolve from hydrogen fusion to the
fusion of other elements, they grow into brilliant
supergiants, which create new elements, the heaviest
being iron.
• A high-mass star dies quickly because it consumes fuel
very rapidly.
Life Cycle of Stars
• As fusion slows in a high-mass star, pressure
• Gravity eventually overcomes the lower
pressure, leading to a dramatic collapse of the
star’s outer layers.
• This collapse produces a supernova, an
explosion so violent that the dying star
becomes more brilliant than an entire galaxy.
Life Cycle of Stars
Supernovas produce enough energy to create elements
heavier than iron.
• These elements, and lighter ones such as carbon and
oxygen, are ejected into space by the explosion.
• As a supernova spews material into space, its core
continues to collapse.
Life Cycle of Stars
If the remaining core of a supernova has a mass less
than about three times the sun’s mass, it will become a
neutron star, the dense remnant of a high-mass star
that has exploded as a supernova.
• In a neutron star, electrons and protons are crushed
together by the star’s enormous gravity to form neutrons.
• Neutron stars are much smaller and denser than white
Life Cycle of Stars
A neutron star spins more and more rapidly as it
contracts. Some neutron stars spin hundreds of turns
per second!
• Neutron stars emit steady beams of radiation in narrow
• A spinning neutron star that appears to give off strong
pulses of radio waves is called a pulsar.
Life Cycle of Stars
Pulsars emit steady beams of radiation that
appear to pulse when the spinning beam
sweeps across Earth.
Life Cycle of Stars
If a star’s core after a supernova explosion is
more than about three times the sun’s mass, its
gravitational pull is very strong.
The core collapses beyond the neutron-star
stage to become a black hole.
A black hole is an object whose surface gravity is
so great that even electromagnetic waves,
traveling at the speed of light, cannot escape
from it.
Groups of Stars
A group of stars that appear to form a pattern as
seen from Earth is called a constellation.
The stars in a constellation are generally not
close to one another. They just happen to lie in
the same general direction of the sky as seen
from Earth.
Groups of Stars
Astronomers have determined that more
than half of all stars are members of star
Most stars occur in groups of two or more.
• A star system is a group of two or more stars that are held
together by gravity.
• A star system with two stars is called a binary star. The two
stars orbit each other.
Groups of Stars
Sometimes the smaller star in a binary star is too
dim to be seen easily from Earth but can still be
detected from the motion of the other star.
If one star passes in front of the other, blocking
some of the light from reaching Earth, the star
system is called an eclipsing binary.
The brightness of an eclipsing binary varies over
time in a regular pattern.
Groups of Stars
There are three basic kinds of star clusters:
open clusters, associations, and globular
Studying star clusters is useful because all the
stars formed together in the same nebula, so
they are about the same age and the same
distance from Earth.
Astronomers plot the stars of a cluster on an H-R
diagram to estimate the cluster’s age.
Groups of Stars
An open cluster has a disorganized or loose
appearance and contains no more than a few
thousand stars that are well spread out.
Open clusters often contain bright supergiants and
gas and dust clouds.
Associations are temporary groupings of bright,
young stars. In time, gravity from nearby stars
breaks these groups apart.
Associations are typically larger than open clusters.
Groups of Stars
A globular cluster is a large group of older stars.
Globular clusters usually lack sufficient amounts of gas
and dust to form new stars. They are spherical and
have a dense concentration of stars in the center.
Globular clusters can contain more than a million stars.
Globular clusters usually do not have short-lived blue
stars because these stars have already died out.
Astronomers estimate that the oldest globular clusters
are about 12 billion years old. Thus, the universe must
be at least that old.
Groups of Stars
Astronomers classify galaxies into four main
types: spiral, barred-spiral, elliptical, and
A galaxy is a huge group of individual stars, star
systems, star clusters, dust, and gas bound together by
• There are billions of galaxies in the universe.
• The largest galaxies consist of more than a trillion stars.
Galaxies vary widely in size and shape.
Groups of Stars
Spiral and Barred-Spiral Galaxies
Spiral galaxies have a bulge of stars at the center, with
arms extending outward like a pinwheel.
• These spiral arms contain gas, dust, and many bright young
• The Milky Way is a spiral galaxy.
Groups of Stars
Some spiral galaxies have a bar through the
center with the arms extending outward from
the bar on either side. These are called barredspiral galaxies.
Groups of Stars
Elliptical Galaxies
Elliptical galaxies are spherical or oval, with no trace of
spiral arms.
• Elliptical galaxies come in a wide range of sizes.
• Elliptical galaxies have very little gas or dust between stars.
They contain only old stars.
Groups of Stars
Irregular Galaxies
A small fraction of all galaxies are known as
irregular galaxies.
Irregular galaxies have a disorganized
appearance. They have many young stars and
large amounts of gas and dust.
Irregular galaxies come in many shapes, are
typically smaller than other types of galaxies,
and are often located near larger galaxies.
Groups of Stars
The Milky Way Galaxy
The Milky Way galaxy has an estimated 200 to
400 billion stars and a diameter of more than
100,000 light years.
Every individual star that you can see with the
unaided eye is in our galaxy.
The solar system lies in the Milky Way’s disk
within a spiral arm, about two thirds of the way
from the center.
Groups of Stars
In a side view, the Milky Way appears as a flat
disk with a central bulge. An overhead view of
the Milky Way shows its spiral shape.
Location of
solar system
Central bulge
Overhead View of Our Galaxy
Disk of spiral arms
containing mainly
young stars
Halo containing
oldest stars
Central bulge
containing mainly
older stars
Side View of Our Galaxy
Groups of Stars
The Milky Way’s flattened disk shape is caused
by its rotation.
The sun takes about 220 million years to
complete one orbit around the galaxy’s center.
Recent evidence suggests that there is a massive
black hole at our galaxy’s center.
Stars are forming in the galaxy's spiral arms.
Groups of Stars
By studying their spectra, astronomers have
determined that quasars are the enormously bright
centers of distant, young galaxies.
Quasars produce more light than hundreds of
galaxies the size of the Milky Way.
What makes a quasar so bright? The most likely
explanation involves matter spiraling into a supermassive black hole with the mass of a billion suns.
The Expanding Universe
Absorption lines
of a galaxy shift
toward the blue
end of the
spectrum when
it moves toward
Earth. The lines
shift to the red
end of the
spectrum when
a galaxy moves
away from Earth.
The Expanding Universe
When you observe a star that is ten light-years
away, you are seeing the star as it was ten years
ago, because light took ten years to travel from
the star to Earth.
Images of galaxies that are billions of light-years
away show how those galaxies looked billions of
years ago.
The Expanding Universe
The observed red shift in the spectra of galaxies
shows that the universe is expanding.
The Doppler effect can be used to determine how fast stars
or galaxies are approaching or moving away from Earth.
• When a star or galaxy is approaching Earth, the lines in its
spectrum are shifted toward the shorter (bluer) wavelengths.
• When the star or galaxy is moving away, the lines in its spectrum
shift toward the longer (redder) wavelengths.
The larger the observed shift, the greater is the speed.
The Expanding Universe
In the mid-1920s, Edwin Hubble discovered that
the light from most galaxies undergoes a red
shift—that is, their light is shifted toward the
red wavelengths.
This red shift showed that nearly all galaxies are
getting farther away from Earth.
The Expanding Universe
Hubble also found that more-distant galaxies
have greater red shifts.
This relationship, called Hubble’s Law, says that
the speed at which a galaxy is moving away is
proportional to its distance from us.
The most distant observed galaxies are moving
away at more than 90 percent of the speed of
The Expanding Universe
The space between the galaxies is expanding in all
directions. The universe as a whole is becoming
Hubble’s law expresses the relationship between
the velocity that a galaxy is moving away from Earth
and its distance from us. The ratio of these variables
is a constant called Hubble’s constant. Hubble’s
constant can be estimated by finding the slope of a
graph of velocity versus distance for a set of
The Expanding Universe
Hubble’s constant is one of the most important
and debated numbers in astronomy. It expresses
how fast the universe is expanding, and can be
used to estimate the age of the universe.
The Expanding Universe
Astronomers theorize that the universe came
into being at a single moment, in an event
called the big bang.
The existence of cosmic microwave
background radiation and the red shift in the
spectra of distant galaxies strongly support
the big bang theory.
The Expanding Universe
The motion of galaxies indicates that the
universe is expanding uniformly.
The big bang theory states that the universe
began in an instant, billions of years ago, in an
enormous explosion.
The Expanding Universe
After the Big Bang
The universe expanded and cooled down after the big
• After a few hundred thousand years of expansion, the
universe was cool enough for atoms to form.
• Gravity pulled atoms together into gas clouds that
eventually evolved into stars in young galaxies.
• The sun and solar system formed about 4.6 billion years
ago, when the universe was about two thirds of its present
The Expanding Universe
The universe began with the
big bang 13.7 billion years
The first stars and galaxies
formed 200 million years
The solar system, and Earth,
formed about 9 billion years
after the big bang.
Big bang occurred
13.7 billion years
First stars and
galaxies formed
200 million years
after big bang.
Solar system
formed 4.6 billion
years ago.
Earth today
The Expanding Universe
Evidence for the Theory
In 1965, Arno Penzias and Robert Wilson, using a radio
telescope, noticed a faint distant glow in every
• Today this glow is called the cosmic microwave background
• This glow is energy produced during the big bang, still
traveling throughout the universe.
The Expanding Universe
The big bang theory describes how the
expansion and cooling of the universe over time
could have led to the present universe of stars
and galaxies.
It offers the best current scientific explanation of
the expansion of the observable universe.
Variations of the theory continue to be
proposed and are being tested with new
The Expanding Universe
Age of the Universe
Since astronomers know how fast the universe is
expanding now, they can infer how long it has been
If you traveled backward in time, all of the matter in
the universe would be at its starting point 13 to 14
billion years ago.
Recent measurements of the microwave
background radiation have led to a more precise
age. Astronomers now estimate that the universe is
13.7 billion years old.
The Expanding Universe
Dark matter cannot be seen directly, but its
presence can be detected by observing its
gravitational effects on visible matter.
To have a gravitational force strong enough to
reverse the expansion, there must be sufficient
mass in the universe.
If there is less than this amount of mass, the
universe will continue to expand.
The Expanding Universe
Much of the matter in the universe can’t be
seen by astronomers.
Dark matter is matter that does not give off
Galaxies like ours may contain as much as ten
times more dark matter than visible matter.
The Expanding Universe
There are many unanswered questions about
dark matter. Astronomers don’t know what it is
made of or how it is distributed through the
Much of the mass of the universe may be
composed of dark matter.
The Expanding Universe
In the past few years, astronomers have discovered
that the rate of expansion of the universe may be
Galaxies appear to be moving apart faster now than
expected. The reason for this is uncertain.
A mysterious force called dark energy is theorized
to be causing the rate of expansion to increase.
If the expansion is accelerating, it’s likely that the
universe will expand forever.