Download Stars, Galaxies, and the Universe Section 1

Stars, Galaxies, and the Universe
Section 1
Analyzing Starlight
• star a large celestial body that is composed of gas and
that emits light.
• Nuclear fusion is the combination of light atomic nuclei to
form heavier atomic nuclei
• Astronomers learn about stars primarily by analyzing the
light that the stars emit.
• Starlight passing through a spectrograph produces a
display of colors and lines called a spectrum.
Stars, Galaxies, and the Universe
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Analyzing Starlight, continued
• All stars have dark-line spectra, which are bands of color
crossed by dark lines where the color is diminished.
• A star’s dark-line spectrum reveals the star’s composition
and temperature.
• Stars are made up of different elements in the form of
gases.
• Because different elements absorb different wavelengths
of light, scientists can determine the elements that make
up a star by studying its spectrum.
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Analyzing Starlight, continued
The Compositions of Stars
• Scientists have learned that stars are made up of the
same elements that compose Earth.
• The most common element in stars is hydrogen.
• Helium is the second most common element in star.
• Small quantities of carbon, oxygen, and nitrogen are also
found in stars.
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Analyzing Starlight, continued
The Temperatures of Stars
• The surface temperature of a star is indicated by its color.
• Most star temperatures range from 2,800 ˚C to 24,000 ˚C.
• Blue stars have average surface temperatures of 35,000 ˚C.
• Yellow stars, such as the sun, have surface temperatures of about
5,500 ˚C.
• Red stars have average surface temperatures of 3,000 ˚C.
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Analyzing Starlight, continued
The Sizes and Masses of stars
• Stars vary in size and mass.
• Stars such as the sun are considered medium-sized
stars. The sun has a diameter of 1,390,000 km.
• Most of the stars you can see in the night sky are
medium-sized stars.
• Many stars also have about the same mass as the sun,
however some stars may be more or less massive.
Stars, Galaxies, and the Universe
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Stellar Motion
Apparent Motion
• The apparent motion of stars, or motion as it appears
from Earth, is caused by the movement of Earth.
• The stars seem as though they are moving counterclockwise around a central star called Polaris, the North
Star. Polaris is almost directly above the North Pole, and
thus the star does not appear to move much.
• Earth’s revolution around the sun causes the stars to
appear to shift slightly to the west at a given time every
night.
Stars, Galaxies, and the Universe
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Stellar Motion, continued
Circumpolar Stars
• Some stars are always visible in the night sky. These
stars never pass below the horizon.
• In the Northern Hemisphere, the movement of these
stars makes them appear to circle the North Star.
• These circling stars are called circumpolar stars.
Stars, Galaxies, and the Universe
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Stellar Motion, continued
Actual Motion of Stars
• Most stars have several types of actual motion.
• Stars move across the sky (seen only for close stars).
• Some stars may revolve around another star.
• Stars either move away from or toward our solar system.
Stars, Galaxies, and the Universe
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Stellar Motion, continued
Actual Motion of Stars
• Doppler effect an observed change in the frequency of
a wave when the source or observer is moving
• The spectrum of a star that is moving toward or away
from Earth appears to shift, due to the Doppler effect.
• Stars moving toward Earth are shifted slightly toward
blue, which is called blue shift.
• Stars moving away from Earth are shifted slightly toward
red, which is called red shift.
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Stellar Motion, continued
The spectrum of a star that is moving toward or away from
Earth appears to shift, as shown in the diagram below.
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Distances to Stars
• Distances between the stars and Earth are measured in
light-years.
• light-year the distance that light travels in one year.
• Because the speed of light is 300,000 km/s, light travels
about 9.46 trillion km in one year.
• For relatively close stars, scientists determine a star’s
distance by measuring parallax.
• parallax an apparent shift in the position of an object
when viewed from different locations.
Stars, Galaxies, and the Universe
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Stellar Brightness
• apparent magnitude the brightness of a star as seen
from the Earth
• The apparent magnitude of a star depends on both how
much light the star emits and how far the star is from
Earth.
• absolute magnitude the brightness that a star would
have at a distance of 32.6 light-years from Earth
• The brighter a star is, the lower the number of its
absolute magnitude.
Stars, Galaxies, and the Universe
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Stellar Brightness
The lower the number of the star on the scale shown on the
diagram below, the brighter the star appears to observers.
Stars, Galaxies, and the Universe
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Classifying Stars
• One way scientists classify stars is by plotting the surface
temperatures of stars against their luminosity, or the total
amount of energy they give off each second.
• The Hertzsprung-Russell diagram, or H-R diagram, is a
simplified version of the graph that illustrates the resulting
pattern.
• Most stars fall within a band that runs diagonally through the
middle of the H-R diagram.
• main sequence the location on the H-R diagram where most
stars lie; it has a diagonal pattern from the lower right to the
upper left.
Stars, Galaxies, and the Universe
Classifying Stars, continued
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Stars, Galaxies, and the Universe
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Star Formation
• nebula a large cloud of gas and dust in interstellar
space; a region in space where stars are born
• A star beings in a nebula. When the nebula is
compressed, some of the particles move close to each
other and are pulled together by gravity.
• As described in Newton’s law of universal gravitation, as
gravity pulls particles of the nebula closer together, the
attraction on each other increases.
• This pulls more nearby particles toward an area of
increasing mass, and regions of dense matter begin to
build up within the nebula.
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Star Formation, continued
Protostars
• As gravity makes dense regions within a nebula more
compact, these regions spin and shrink and begin to
form a flattened disk. The disk has a central
concentration of matter called a protostar.
• The protostar continues to contract and increase in
temperature for several million years. Eventually the gas
in the region becomes so hot that its electrons are
stripped from their parent atoms.
• The nuclei and free electrons move independently, and
the gas is then considered a separate state of matter
called plasma.
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Star Formation, continued
The Birth of a Star
• A protostar’s temperature continually increases until it
reaches about 10,000,000 °C.
• At this temperature, nuclear fusion begins. Nuclear
fusion is a process in which less-massive atomic nuclei
combine to form more-massive nuclei. The process
releases enormous amounts of energy.
• The onset of nuclear fusion marks the birth of a star.
Once this process begins, it can continue for billions of
years.
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Star Formation, continued
A Delicate Balancing Act
• As gravity increases the pressure on the matter within
the star, the rate of fusion increase.
• In turn, the energy radiated from fusion reactions heats
the gas inside the star.
• The outward pressures of the radiation and the hot gas
resist the inward pull of gravity.
• This equilibrium makes the star stable in size.
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The Main-Sequence Stage
• The second and longest stage in the life of a star is the
main-sequence stage. During this stage, energy
continues to be generated in the core of the star as
hydrogen fuses into helium.
• A star that has a mass about the same as the sun’s
mass stays on the main sequence for about 10 billion
years.
• Scientists estimate that over a period of almost 5 billion
years, the sun has converted only 5% of its original
hydrogen nuclei into helium nuclei.
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Leaving the Main Sequence
• A star enters its third stage when about 20% of the
hydrogen atoms within its core have fused into
helium atoms.
Giant Stars
• A star’s shell of gases grows cooler as it expands. As
the gases in the outer shell become cooler, they
begin to glow with a reddish color. These stars are
known as giants.
• giant a very large and bright star whose hot core has
used most of its hydrogen.
Stars, Galaxies, and the Universe
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Leaving the Main Sequence, continued
Supergiants
• Main-sequence stars that are more massive than the sun
will become larger than giants in their third stage.
• These highly luminous stars are called supergiants.
These stars appear along the top of the H-R diagram.
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The Final Stages of a Sunlike Star
• In the evolution of a medium-sized star, fusion in the
core will stop after the helium atoms have fused into
carbon and oxygen.
Planetary Nebulas
• As the star’s outer gases drift away, the remaining core
heats these expanding gases.
• The gases appear as a planetary nebula, a cloud of gas
that forms around a sunlike star that is dying.
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The Final Stages of a Sunlike Star,
continued
White Dwarfs
• As a planetary nebula disperses, gravity causes the
remaining matter in the star to collapse inward until it
cannot be pressed further together.
• A hot, extremely dense core of matter—a white dwarf—is
left. White dwarfs shine for billions of years before they
cool completely.
• white dwarf a small, hot, dim star that is the leftover
center of an old sunlike star
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The Final Stages of a Sunlike Star,
continued
Novas and Supernovas
• If a white dwarf star revolves around a red giant, the
gravity of the white dwarf may capture gases from the
red giant.
• As these gases accumulate on the surface of the white
dwarf, pressure begins to build up.
• This pressure may cause large explosions, called a
nova.
• nova a star that suddenly becomes brighter
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The Final Stages of a Sunlike Star,
continued
Novas and Supernovas, continued
• A white dwarf may also become a supernova, which is a
star that has such a tremendous explosion that it blows
itself apart.
• Supernovas are a thousand times more violent than
novas.
• The explosions of supernovas completely destroy the
white dwarf star and may destroy much of the red giant.
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The Final Stages of Massive Stars
Supernovas in Massive Stars
• Massive stars may produce supernovas as part of their
life cycle.
• After the supergiant stage, the star collapses, producing
such high temperatures that nuclear fusion begins again.
This time, carbon atoms in the core fuse into heavier
elements until the core is almost entirely made of iron.
• When nuclear fusion stops, the star’s core begins to
collapse under its own gravity. This causes the outer
layers to explode outward with tremendous force.
Stars, Galaxies, and the Universe
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Stars, Galaxies, and the Universe
Supernova as seen in a galaxy
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The Final Stages of Massive Stars,
continued
Neutron Stars
• Stars more massive than the sun do not become white
dwarfs.
• After a star explodes as a supernova, the core may
contract into a neutron star.
• neutron star a star that has collapsed under gravity to
the point that the electrons and protons have smashed
together to form neutrons
Stars, Galaxies, and the Universe
Neutron Star
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Stars, Galaxies, and the Universe
Neutron Star Cross section
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Stars, Galaxies, and the Universe
The Final Stages of Massive Stars,
continued
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Stars, Galaxies, and the Universe
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The Final Stages of Massive Stars,
continued
Pulsars
• Some neutron stars emit a beam of radio waves that
sweeps across space and are detectable here on Earth.
• pulsar a rapidly spinning neutron star that emits pulses
of radio and optical energy
• These stars are called pulsars. For each pulse detected
on Earth, we know that the star has rotated within that
period.
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The Final Stages of Massive Stars,
continued
Black Holes
• Some massive stars produce leftovers too massive to
become a stable neutron star.
• These stars contract, and the force of the contraction
leaves a black hole.
• black hole an object so massive and dense that even
light cannot escape its gravity
Stars, Galaxies, and the Universe
Black Hole
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Stars, Galaxies, and the Universe
Black Hole
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Stars, Galaxies, and the Universe
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Constellations
Dividing Up the Sky
• constellation one of 88 regions into which the skay has
been divided in order to describe the locations of
celestial objects; a group of stars organized in a
recognizable pattern
• In 1930, astronomers around the world agreed upon a
standard set of 88 constellations.
• You can use a map of the constellations to locate a
particular star.
Stars, Galaxies, and the Universe
Constellations, continued
The Constellation Orion
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Stars, Galaxies, and the Universe
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Multiple-Star Systems
• When two or more stars are closely associated, they form
multiple-star systems.
• Binary stars are pairs of stars that revolve around each other
and are held together by gravity. The center of mass, or
barycenter, is somewhere between the two stars.
• In star systems that have more than two stars, two stars may
revolve rapidly around a common barycenter, while a third
star revolves more slowly at a greater distance from the pair.
• Astronomers estimate that more than half of all sunlike stars
are part of multiple-star systems.
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Stars, Galaxies, and the Universe
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Star Clusters
• Sometimes, nebulas collapse to form groups of
hundreds or thousands of stars called clusters.
• Globular clusters have a spherical shape and can
contain up to one million stars.
• An open cluster is loosely shaped and rarely contains
more than a few hundred stars.
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Galaxies
• galaxy a collection of stars, dust, and gas bound
together by gravity
• Galaxies are the major building blocks of the universe.
• A typical galaxy, such as the Milky Way, has a diameter
of about 100,000 light-years and may contain more than
200 billion stars.
• Astronomers estimate that the universe contains
hundreds of billions of galaxies.
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Galaxies, continued
Distances to Galaxies
• Giant stars called Cepheid variables brighten and
fade in a regular pattern. Most Cepheids have regular
cycles. The longer the cycle, the brighter the star’s
absolute magnitude.
• Scientists compare the Cepheid’s absolute
magnitude and the Cepheid’s apparent magnitude to
calculate the distance to the Cepheid variable.
• This distance tells scientists the distance to the
galaxy in which the Cepheid is located.
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Galaxies, continued
Types of Galaxies
• Galaxies are classified by shape into three main types.
• A spiral galaxy has a nucleus of bright stars and
flattened arms that spiral around the nucleus.
• Elliptical galaxies vary in shape, from nearly spherical to
very elongated, are extremely bright in the center, and
do not have spiral arms.
• An irregular galaxy has no particular shape, and is fairly
rich in dust and gas.
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Galaxies, continued
The Milky Way
• The Milky Way galaxy is a spiral galaxy in which the sun is
one of hundreds of billions of stars.
• Each star orbits around the center of the Milky Way galaxy. It
takes the sun about 225 million years to complete one orbit
around the galaxy.
• Two irregular galaxies, the Large Magellanic Cloud and Small
Magellanic Cloud, are our closest neighbors.
• Within 5 million light-years of the Milky Way are about 30
other galaxies. These galaxies and the Milky Way galaxy are
collectively called the Local Group.
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Quasars
• quasar quasi-stellar radio source; a very luminous object
that produces energy at a high rate
• Quasars appear as points of light, similar to stars. Some
quasars project a jet of gas.
• Quasars are located in the centers of galaxies that are
distant from Earth. Galaxies that have quasars in them
differ from other galaxies in that the quasars in their
centers are very bright.
• Quasars are among the most distant objects that have
been observed from Earth.
Stars, Galaxies, and the Universe
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Hubble’s Observations
• cosmology the study of the origin, properties,
processes, and evolution of the universe
• Cosmologists and astronomers can use the light given
off by an entire galaxy to create the spectrum for that
galaxy.
• Edwin Hubble used galactic spectra to uncover new
information about our universe.
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Hubble’s Observations, continued
Measuring Red Shifts
• Hubble found that the spectra of galaxies, except for the
few closest to Earth, were shifted toward the red end of
the spectrum.
• Hubble determined the speed at which the galaxies were
moving away from Earth.
• Hubble found that the most distant galaxies showed the
greatest red shift and thus were moving away from Earth
the fastest.
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Hubble’s Observations, continued
The Expanding Universe
• Using Hubble’s observations, astronomers have been
able to determine that the universe is expanding.
• The expanding universe can be thought of as a raisin
cake rising in the oven. If you were able to sit on one
raisin, you would see all the other raisins moving away
from you.
• Similarly, galaxies in the universe are moving farther
away from each other due to the expansion of the
universe.
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A Theory Emerges
• Although cosmologists have proposed several different
theories to explain the expansion of the universe, the
current and most widely accepted is the big bang theory.
• big bang theory the theory that all matter and energy in
the universe was compressed into an extremely small
volume that 3 to 15 billion years ago exploded and
began expanding in all directions
• By the mid-20th century, almost all astronomers and
cosmologists accepted the big bang theory.
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A Theory Emerges, continued
Cosmic Background Radiation
cosmic background radiation radiation uniformly
detected from every direction in space; considered a
remnant of the big bang
• Astronomers believe that cosmic background radiation
formed shortly after the big bang.
• The background radiation has cooled after the big bang,
and is now about 270 °C below zero.
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A Theory Emerges, continued
Ripples in Space
• Maps of cosmic background radiation over the whole sky
look very smooth. But on satellite maps that show where
temperatures differ from the average background
temperature, “ripples” become apparent.
• These ripples are irregularities caused by small
fluctuations in the distribution of matter in the early
universe.
• The ripples are thought to indicate the first stages in the
formation of the universe’s first galaxies.
Stars, Galaxies, and the Universe
A Theory Emerges, continued
Timeline of the Big Bang
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Stars, Galaxies, and the Universe
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A Universe of Surprises
Dark Matter
• Analysis of the ripples in the cosmic background
radiation suggests that the matter that humans, the
planets, the stars and the matter between the stars
makes up only 4% of the universe.
• About 23% of the universe is made up of a type of matter
that does not give off light but that has gravity. This type
of matter is called dark matter.
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A Universe of Surprises, continued
Dark Energy
• Most of the universe is made up of an unknown material
called dark energy.
• Scientists think that dark energy acts as a force that
opposes gravity. Many scientists think that some form of
undetectable dark energy is pushing galaxies apart.
• Because of dark energy, the universe is not only
expanding, but the rate of expansion also seems to be
accelerating.
Stars, Galaxies, and the Universe
Maps In Action
The Milky Way
Section 1