Name ______________________________________ Date / /
Earth Science Mrs. Kravchuk
Chapter Twenty Eight
Stars and Galaxies
Key Ideas
Topic 1
A Closer Look at Light
The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light,
ultraviolet, X-rays, and gamma rays. Astronomers analyze stellar spectra to determine
stars’ composition and movement.
Topic 2
Stars and Their Characteristics
Stars differ in mass, size, and surface temperature. Surface temperature affects the color
of a star. Hydrogen and helium are the two most abundant elements in stars. Apparent
magnitude, luminosity, and absolute magnitude all describe the brightness of stars.
Distances in space are measured in astronomical units, light-years, and parsecs.
Topic 3
Life Cycles of Stars
The Hertzsprung-Russell diagram plots a star’s luminosity against its surface
temperature. The diagram’s groups of stars represent life-cycle stages of stars. A star’s
fate depends on its mass. Most stars are main-sequence stars.
Topic 4
Galaxies and the Universe
Galaxies contain millions or billions of stars. The big bang model is a hypothesis about
the origin of the universe.
Key Terms
absolute magnitude apparent magnitude absorption spectrum astronomical unit
black hole big bang model constellation Cephid variables
Doppler Effect white dwarf galaxy supernova
giant light-year main-sequence nebula
neutron star parallax parsec planetary nebula
pulsar quasar supergiant emission spectrum
electromagnetic spectrum
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Topic 1
A Closer Look at Light
Light is more than what we get when we turn on a lamp. It also refers to a form of
radiation that stars and other celestial objects emit. Most of what we know about the
universe is what we learned from analyzing the light that reaches us from distant stars.
electromagnetic radiation
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All types of electromagnetic radiation travel in the form of waves, at a speed of about
300,000 kilometers per second—the speed of light.
electromagnetic spectrum
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Visible white light is actually made up of light of many colors, each with a different
wavelength. These are the colors seen in a rainbow. Red light has the longest
wavelength, and violet the shortest. Astronomers use spectra of distant stars to learn
more about them. A spectroscope, which uses a prism to split the light gathered by
a telescope is used.
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Spectroscopes break light into 3 different types of spectra. By analyzing these,
astronomers can figure out what makes up the atmosphere of stars and the planets.
Continuous spectrum is an unbroken
band of colors. It shows that its source
is emitting light of all visible colors.
Light bulbs, glowing liquids (molten iron),
and hot, compressed gases inside stars
are examples.
Emission spectrum is a series of unevenly
spaced lines of different colors and brightness.
Every element has its own unique emission
spectrum.
9 is a continuous spectrum
crossed by dark lines. These lines form as light
from a glowing object passes through a cooler
gas, which absorbs some of the wavelengths.
This is used to determine what elements are
present in the cooler gas.
Analyzing spectra allows scientists to tell how stars move in relation to the Earth. This
movement is called the Doppler effect. Stars moving farther away from Earth
exhibit a red shift. The wavelength it emits becomes longer, meaning that the light waves
become longer, shifting to the red. Those moving closer exhibit a blue shift. The light
waves become shorter, shifting to the blue. Evidence of the red shift or blue shift helps
astronomers determine whether a star is moving toward or away from Earth. An
observable red shift helps give credence to the theory that the universe is expanding.
HOMEWORK
Read Textbook pages 612 to 616
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Topic 2
Stars and Their Characteristics
Life on earth is dependent on our nearest star, the sun. The sun is just one among many
stars. If you look in the sky on a clear night, you can see thousands. Use a telescope and
you can see millions. Watching the stars is one of the oldest of human pursuits. Ancient
peoples gave groups of stars names based on mythology. Many of these
constellation names are still used today.
circumpolar
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These stars are found close to Polaris and are always visible in the night sky, in the
Northern Hemisphere. The Little Dipper constellation is circumpolar.
Most stars look like tiny dots of light from the night sky, but if you look closely, you will
see that they vary in color. They also vary in size and mass, composition, temperature,
distance from the earth, and brightness. Some have only 1/50 of our sun’s mass, while
large stars have more than 50 times its mass.
Classification of Stars
Color Surface temperature (C°) Examples
Blue Above 30,000 10 Lacertae
Blue-white 10,000 – 30,000 Rigel, Spica
Blue-white 7,500 – 10,000 Vega, Sirius
Yellow-white 6,000 – 7,500 Canopus, Procyon
Yellow 5,000 – 6,000 Sun, Capella
Orange 3,500 – 5,000 Arcturus, Aldebaran
Red Less than 3,500 Betelgeuse, Antares
The Hubble Space telescope has made viewing the stars easier, eliminating atmospheric
interference. A spectrometer is used to detect the different colors of the different
elements that make up the stars. A display of colors and lines passing through the
spectrometer produces a specific spectra for each star. Each element produces its own
spectra, the same as the elements found on earth. In stars, hydrogen and helium are the
most abundant elements. Carbon, oxygen, nitrogen, and calcium make up the remainder
of the mass. Surface temperature is indicated by color. Blue stars burn the brightest,
while red stars are the coolest.
Two kinds of motion are associated with stars, actual motion and apparent motion.
Because of the extreme distance of stars, their actual motion can only be measured by
high-powered telescopes and other instruments. Apparent motion is visible with the
unaided eye, and is due to the motion of the earth. Leaving a camera shutter open shows
the path of the star across the sky over time. Our revolution around the sun causes the
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stars to appear to move, making different stars visible at different times of the year. Stars
appear to move across the sky and disappear below the western horizon.
Most stars have at least 3 actual motions:
1. _____________________________________
2. _____________________________________
3. _____________________________________
The colors of the spectrum of a star moving toward the earth appear to shift toward blue
because the light waves from a star appear to have shorter wavelengths a the star moves
toward earth. Those moving away experience a red shift. The wavelength of light
appears to be longer. Most distant galaxies have red shift spectra because they are
moving away from the earth as the universe expands.
Because the universe is so vast, distance is measured in large units
light year
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Light travels about 9.5 trillion km in one year. Light from the sun takes 8 minutes to
reach the earth. The star system nearest us, Proxima Centauri, is 4.5 light years away,
almost 300,000 times the distance from the earth to the sun. Sirius, the brightest star seen
from earth, is about 9 light years away. Polaris is 700 light years away.
parallax
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parsec
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A star is a sphere of super hot gases, mostly helium and hydrogen. About one or two
percent is actually oxygen, carbon, nitrogen and sodium. No two stars contain exactly
the same percentages of elements, giving each star a unique spectrum.
Stars vary greatly by mass, size, and density. Mass is expressed in terms of Solar Mass.
This is the number of times greater or less their mass is to our sun.
Comparison of Star Sizes
Star Type of Star Mass Radius
(Solar Masses) (Solar Radii)
Sirius Main Sequence 2.3 2.5
Rigel Blue Supergiant 20 36
Betelgeuse Red Supergiant 20 1000
Aldeberan Red Giant 5 20
Deneb Yellow Supergiant 14 60
Capella Red Giant 3.5 13
Pollux Red Giant 4 8
Altair Main Sequence 2 1.5
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1 Solar Mass = 1 x 10 kg = 330,000 Earth Masses
Astronomers estimate how bright a star is by measuring its spectrum. They compare the
estimate of true brightness with the apparent brightness of the star. These two
measurements are used to calculate the distance of a star from the earth.
Some stars serve as distance indicators. A Cephid variable star brightens and fades in
a regular pattern. This change is caused as the star shrinks and swells. Most have a
regular cycle ranging from 1 to 100 days. The cycle is measured and its true brightness is
estimated. Using true and apparent brightness, the distance to the star is calculated, this,
in turn, tells scientists the distance to the Cephid’s galaxy.
About 6,000 stars are visible to the eye, over 3 billion can be seen with telescopes, over a
trillion can be seen by the Hubble. The ability to see a star depends on its distance from
the earth and its brightness. Two scales are used to measure brightness, how bright it
appears from the earth, and how bright it would appear if all the stars were the same
distance from the earth.
apparent magnitude
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Apparent Magnitude vs. Absolute Magnitude
How do apparent magnitude and absolute magnitude compare? Here are the apparent
magnitudes and absolute magnitudes of some of the brightest stars.
Star Apparent Magnitude Absolute Magnitude
Aldeberan +0.87 -0.65
Algol +2.09 -0.15
Antares +1.06 -5.38
Betelgeuse +0.45 -5.09
Capella +0.08 -0.48
Polaris +1.97 -3.59
Procyon +0.41 +2.62
Rigel +0.18 -6.75
Sirius -1.44 +1.42
Spica +0.98 -3.55
The most powerful telescopes can detect stars with an apparent magnitude of 29, a star
visible with the eye has an apparent magnitude of 6. This is called a sixth magnitude star.
A first-magnitude star is one of the brightest stars in the sky. Some stars, planets, the
moon, and our sun have negative magnitudes because they are brighter than first-
magnitude stars.
absolute magnitude
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This is how bright the star would appear if it were seen from a distance of 32.6 light
years.
Stars have both an apparent and absolute magnitude. The relationship between the two
depends on the distance between the earth and the object. Stars less than 32.6 light years
away appear brighter than they would if they were farther away. This gives them an
apparent magnitude lower than their absolute magnitude.. The sun is only a fraction of a
light year away. It has an apparent magnitude of -26.8 and an absolute magnitude of +5.
HOMEWORK
Read Textbook pages 617 to 624
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Topic 3
Life Cycles of Stars
H- R diagram
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Plotting star surface temperatures against their absolute magnitude reveals an interesting
pattern. This graph was named for Ejnar Hertzsprung and Henry Russel, the astronomers
who discovered the pattern. Most of the stars fall within the range of the band running
through the diagram
main-sequence star
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The sun and many of the visible stars are main-sequence stars.
giant
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Arcturus is a giant. Some are so large that they are known as super-giants. Antares,
with a diameter of more than 2.7 million km is a super-giant.
super giant
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white dwarf
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Stars live for billions of years, making them impossible to study throughout their life
span. Instead, theories have been developed by studying stars in different stages.
nebula
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A nebula is usually 70% hydrogen, 28% helium, and 2% heavier elements. The particles
have a weak gravitational pull for one another. When a force compresses these particles,
parts of the nebula contract. According to Newton’s law of gravitation, force increases as
distance decreases. Therefore, as density of the particles increases, their gravitational
attraction increases, allowing a denser region of matter to build up within the cloud.
Gravity causes these dense regions to contract. As this occurs, the mass begins to spin
more rapidly.
protostar
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As gravity pulls more material toward the center, pressure inside builds. Increased
pressure causes the temperature to rise. Eventually, the gas becomes so hot that it
becomes plasma, the fourth state of matter in which electrons have been removed from
their parent atom and move freely. Neon light is in a weak plasma state. Temperatures
in the protostar continue to rise until they reach 10,000,000 °C, and nuclear fusion
begins, releasing enormous amounts of energy. This process will continue for billions of
years. A nebula may produce more than a single star. It may also produce planets which
can revolve around a central star. It is believed that our solar system formed in this way.
The second and longest stage in the life of a star is the main-sequence stage. Energy is
generated in the core of a star releasing radiant energy. This energy moves outward, but
the star does not expand due to gravity. This makes the star stable in size. A star enters
its third stage when all the particles in its core have fused into helium atoms. Without
hydrogen as a source of fuel, the core of the star contracts due to its own gravity. This
increases the core’s interior temperature, causing the helium atoms in the core to fuse into
carbon. Energy is released, causing the star to expand, and cool. It is now called a giant
(10 times bigger than our sun) or a super-giant (100 times bigger than our sun.) The end
of helium fusion marks the end of the giant stage of the star’s evolution, becoming a
medium-sized star. No energy remains for fusion, the outer gases are gone, and the core
is revealed.
planetary nebula
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Gravity causes the last of the matter in the star to collapse inward. The hot, dense core
remaining is called a white dwarf. It will continue to shine for billions of years
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before it begins to cool completely. As it cools, it shines less and less. When it no longer
emits light, it is dead and called a black dwarf. They probably do not exist yet as
scientists believe that the universe is not yet old enough to produce them.
nova
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The explosion can cause the star to become thousands of times brighter. This occurs for
a short period of time, several days, and then it returns to normal brightness. A white
dwarf may become a nova several times.
supernova
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These are produced by stars that are more massive than the stars that produce novas.
After the super-giant stage, carbon in the core fuses to become magnesium, then iron,
then fusion stops. Energy is absorbed, the core collapses, and the outside explodes
neutron star
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A spoonful of matter from a neutron star would weigh more than 100 million tons on
earth. A neutron star with more mass than the sun would have a diameter of only about
30 km, and would rotate very rapidly.
pulsars
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Radiation from pulsars is in the form of radio waves.
black holes
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The gravity of a black hole is so great that light can not escape from it. Since they do not
emit light, finding them is difficult. They are often found by observing their effect on a
companion star. Matter from the companion star is pulled into the black hole, giving off
x-rays as it enters the black hole. Scientists believe that there is a black hole in the
constellation Cygnus. It is theorized that there are black holes at the center of every
universe.
Homework
Read Textbook pages 626 to 630.
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Topic 4
Galaxies and the Universe
The stars you see in the sky are only a few thousand of the billions that actually exist.
Only one in four that you see is actually a single star, many are double or triple star
systems, some are even clusters of stars.
Constellations are used to divide the sky into sectors. This is helpful when trying to
locate a particular star. Stars within each constellation are labeled by magnitude. The
brightest is labeled alpha, α, the second brightest is beta, β, and so on. The brightest star
in Scorpious is called Alpha Scorpii, the second brightest is called Beta Scorpii. Alpha
scorpii is also called Antares, meaning ―rival of Mars,‖ because it glows red.
galaxies
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These large-scale groups of stars are held together by the force of gravity. A typical
galaxy is about 100,000 light-years in diameter. Besides stars, galaxies also contain dust
clouds and gas, or nebulae. Some nebulae are bright and glow from hot gases within or
shine by reflecting light from nearby stars. Others are dark and located amidst a group of
stars. Dark nebulae absorb light from the more distant stars behind them. It is estimated
that there are between 50 billion and 1 trillion galaxies in the known part of the universe.
The Large Magellanic Cloud and Small Magellanic Cloud are the closest galaxies to our
Milky Way Galaxy. They are 150,000 light-years away. Within the next 3 million light-
years, there are about 17 other galaxies. The Milky Way and these others are called the
Local Group.
There are three types of galaxies:
1. _________________________
2. _________________________
3. _________________________
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As you look into the night-time sky, you will see a band of stars that stretch across the
sky called the Milky Way. It is the disk of the Milky Way Galaxy. The sun is just one of
the billions of stars that make up the galaxy. The star orbit around the center. The
diameter is about 100,00 light years. At its center, the galaxy is about 2,000 light years
thick. Out sun is about 30,000 light years from the center and is located on one of the
arms of the galaxy. The galaxy rotates. The sun revolves around the center of the galaxy
at a speed of about 250 km/s. It completes one revolution every 200 million years.
Star clusters can be open or globular. Open star clusters look like big groups spread out.
Globluar clusters look like small piles all grouped together. Binary stars are two stars
located next to each other.
Multiple star systems have more than one star. Two revolve rapidly around a common
center of gravity, and the third revolves more slowly at a greater distance. These systems
are important because they are used to help determine stellar mass.
According to the Big Bang Theory, all the matter and energy in the universe was once
concentrated in an extremely small volume. When the bang occurred, matter and energy
moved out from the center, causing it to expand. Gravity began to have an effect forming
galaxies that continue to move outward.
quasars
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These were first observed in 1960. They were among the first objects formed after the
big bang and seem to have evolved into galaxies.
Homework
Read Pages 631 to 635.
The Lab, “Is the Sun an Average Star?,” is due tomorrow.
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