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Chapter 2
Stars and Galaxies
Where are you?
• The Earth circles the sun
• The sun is one of billions of billions of
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stars.
To measure distances between stars we
a distance measurement called the
Light- year
1 light-year is the distance light travels
in one year.
Light-Year
• Light moves at 300,000 km/sec
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 That’s 186,000 mile/sec
It would reach the sun in about 5
minutes
How far would it go in a year?
Nearest star is 4.3 light years away
Binary Stars
• Most stars are found in pairs
• These stars revolve around each other
• If a dim star passes in front of a bright
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star, it will block its light.
Called an eclipsing binary
Algol dims every 69 hours
The closest star -Alpha Centari is
actually a triple star system
Binary System
Top
Side
Constellations
• Groups of stars that appear to stay
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together
Zodiac
Named after gods, animals, and heroes
Stars are not necessarily near each
other
Nova
• A star getting suddenly brighter
• Occurs in a binary star system
• Gases from one star are pulled into the
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other.
Causing a nuclear explosion.
Clusters
• Smaller groups of stars within a galaxy
• Globular Clusters- Spherical shaped
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with may (up to 100,000 stars)
Open clusters- less organized- with
fewer stars ( hundreds )
Nebula
• Gas and dust clouds in space.
• Most can’t be seen
• If they reflect light from nearby stars
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they can be seen
Probably the birthplace of new stars
Ring Nebula
Galaxies
• Huge collections of stars
• each may contain hundreds of billions of
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stars
The major feature of the universe
Maybe as many as 100 billion galaxies
Types of Galaxies
• Elliptical - round, from flat disks to
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spheres - contain older stars
Spiral- Flattened arms that spin around
a center
Irregular- no definite shape -less
common
Andromeda
Large Magellanic Cloud
M51
M83
Milky Way
• Our galaxy
• almost all the stars
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you can see in the
sky
100,000 light-years
across
15,000 top to bottom
100 to 200 billion
stars
Spectrum
Prism
• White light is made
up of all the colors
of the visible
spectrum.
• Passing it through
a prism separate it.
If the light is not white
• Stars give off different
colors of light
• Passing this light
through a prism does
something different.
• How we know what
stars are made of.
• Spectra from stars will have lines
missing
Doppler Effect
• Change in wavelength caused by the
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apparent motion of the source.
Cars moving by you
Same things happen to light
Light from objects coming toward you is
compressed looks more blue
Light from objects away looks more red
Red Shift
• Light from galaxies moving away
Blue Shift
• Light from galaxies moving toward us
A big surprise
• No Galaxies showed blue shift
• All galaxies showed red shift.
• Which means
 All galaxies were moving away
 The universe is expanding
The Big Bang Theory
• The universe started with a
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concentrated area of matter and
energy.
15-20 billion years ago
Then it exploded and has been
expanding ever since
Faster moving stuff traveled farther
Explained red shift
Big Bang Theory
• Predicts energy should be evenly
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distributed
Astronomers did find it
Called background radiation
Evenly spread throughout the
universe.
Gravity
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Force of attraction
All objects attract each other.
Pulled matter into clumps
These clumps became bigger
became galaxies
Open or closed?
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Two possible results of big bang.
Open universe will continue expanding
Stars will eventually lose all energy
end of universe is emptiness.
In a few hundred billion years
Closed Universe
• Gravity will eventually pull all the
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galaxies back together.
Eventually all matter will come back
together at the center of the galaxy
Blue shift
Packed into a area as small as a period.
Then another big bang
Every 80 to 100 million years.
Quasars
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Quasi - stellar radio source
Quasi- means “something like”
stellar means “star”
Most distant objects in the universe -12
billion light years
Give off tremendous energy as x-rays
and radio waves
as much as 100 galaxies
Quasars
• 1 sec, enough for 1 billion years
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electricity for Earth
At the edge of the universe
At the very beginning of the universe
Another Tool
• Spectroscope
• Breaks the light of a star up into its
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colors
Called a spectrum
Kind of spectrum tells scientists
 what the star is made of
 which way and how fast it is moving
Stars
• Are formed by the same forces
• Have different
 Size
 Composition
 Temperature
 Color
 Mass
 Brightness
Size
• 5 main categories
• Medium sized - like our sun
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 from 1/10 size of sun to 10 times it’s
size
Giant stars- 10 to 100 times bigger than
the sun
Supergiant stars- 100 to 1000 times
bigger than the sun
Size
• White dwarfs- smaller than 1/10 the size
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of the sun
Neutron stars - smallest stars - about 16
km in diameter
Composition
• Determined with a spectroscope
• by the colors of light it gives off
• The lightest element Hydrogen makes
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up 60 - 80 % of a star
Helium is second most
96-99 % is hydrogen and helium
rest is other elements -
Temperature
• Color also
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Blue
indicates
temperature
White
hottest surface
50000 °C
Yellow
coolest 3000°C
Red-orange
Red
35,000 °C
10,000 °C
6,000 °C
5,000 °C
3,000 °C
Brightness
• Magnitude - measure of brightness
• Apparent magnitude - how bright it
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looks from earth
Absolute magnitude - how bright it really
is
Variable stars - brightness changes
from time to time
Cephid variables - pulsating variableschange both brightness and size
Hertzsprung-Russell diagram
• Found that as temperature increased,
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so did absolute magnitude
90% of stars followed this pattern
Called main sequence stars
Other 10% were once main sequence
stars but have changed over time
Absolute Magnitude
Supergiants
50000
Giants
20000
10000 6600
6000
5000
3000
Distance to stars
• One method is parallax
• Apparent change in position as the
earth goes around the sun
Measure the angle to the star
Wait half a year
Measure the angle to the star
Triangle tells distance
Distance to stars
• Parallax works only to 100 light-years
• More than 100 light-years they use a
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complicated formula based on apparent
and absolute magnitude.
More than 7 million light-years they use
the red shift
Why Stars Shine
• Stars are powered by nuclear fusion
• Hydrogen atoms join to form helium
• Happens because gravity pulls the atoms in
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the core so close together
The sun turns 600 billion kilograms of
hydrogen to 595.8 kilograms of helium every
second
The 4.2 billion kilograms of mass are turned
to energy -light, heat, UV, x-rays
• E= mc2
The Sun
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An average star
Over 1 million earth’s would fit inside
1/4 the density of the Earth
made of 4 layers
CoronaOutermost layer
•Temp1,700,000ºC
•Few particles
Chromospheremiddle of
atmosphere
•Temp-27,800ºC
•1000’s of km thick
Corona
Chromosphere
Photosphere•Temp-6000ºC
•550 km thick
•Surface of the
sun
Core1,000,000ºC
15,000,000ºC
Activity on the Sun
• Storms on the sun
• Prominences- Loops or arches of gas that
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rise from the chromosphere
Solar Flares- Bright bursts of light, huge
amounts of energy released
Sunspots- Dark areas on the suns surface
in the lower atmosphere
Motion shows the rotation of the sun
Interferes with radio
Solar Prominence
Solar Wind
• Continuous stream of high energy
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particles.
Can also interfere with radio and TV
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Star Life Cycles
Stars change over time
New stars form from nebulae
Gravity pulls the dust and gas together
Mostly hydrogen
Forms a spinning cloud
Hydrogen atoms hit each other and
heat up
Star Life Cycle
• When the temperature reaches
15,000,000 °C fusion begins
• Makes a protostar - a new star
• What determines the life cycle of the star
is how much mass it starts with.
Medium-Sized stars
• Shine for a few billion years as
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hydrogen turns to helium.
When hydrogen is used up, the core is
almost all helium.
Helium core shrinks and heats up
Makes outside expand and cool
Gives off red light
Becomes red giant
Medium-Sized Stars
• Helium in core turns to carbon
• Last of hydrogen gas drifts away to
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become a ring nebula or a planetary
nebula.
When last of helium is used up the core
collapses and becomes a white dwarf
Incredibly dense- a teaspoon will weigh
tons
How long
• It depends on the mass.
• The smaller a star starts out, the longer
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it takes
From a few to 100 billion years for
medium sized stars
The sun will take about 10 billion years
Massive Stars
• Start with at least 6 times the mass of
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the sun.
Like medium stars up until they become
red giants.
The helium in the core becomes carbon,
but it keeps getting hotter.
Carbon atoms for heavier elements like
oxygen and nitrogen and even iron
Massive stars
• Can’t go further than iron.
• Iron absorbs energy until it explodes in
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a supernova
Temperatures up to 100,000,000,000°C
• Then heavier atoms can form
• Explosion results in a new nebula,but
with the new elements
Neutron Stars
• If the star started out 6 to 30 times the
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mass of the sun, the core of the
exploding star becomes a neutron star.
As massive as the sun, but only 16 km
across.
Neutron stars spin rapidly and give off
pulses of radio waves
If these radio waves come in pulses it is
called a pulsar
Black holes
• If the star was bigger than 30 times the
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mass of the sun
The left over core becomes so dense
that light can’t escape its gravity.
Becomes a black hole.
Grab any nearby matter and get bigger
As matter falls in, it gives off x-rays.
That’s how they find them