Transcript galaxy

Star birth occurs in a nebula. A star's initial
mass determines the star’s temperature,
luminosity, and diameter that ultimately
decides which life stages and type of death it
will experience.
The larger the star, the shorter its life will be!
Hydrostatic Equilibrium
 a star must be very hot and dense inside to balance its
own GRAVITY
TEMPERATURE inside a star governs the rate of nuclear
reactions WHICH determines its LUMINOSITY (energy
radiated by astronomical object seen by us as light)
The balance between gravity pushing inward and outward
pressure is maintained by heat due to nuclear reactions and
compression is called HYDROSTATIC EQUILIBRIUM
SO…..If a star expands or
contracts,
hydrostatic equilibrium Is
not in place.
NEBULA
• Large clouds of
gas and dust
that contract
due to
gravitational
force as it
becomes hotter
st
1
Stage of Life
Protostar:
hot condensed object of gas and dust
that is pulled together by gravity at
the center of a nebula
it generates energy and shines but is
NOT hot enough to ignite nuclear
fusion
Nuclear Fusion hydrogen atoms fuse into helium
creating the intense energy found in stars
2nd Stage
• Main Sequence (the Sun)
–Longest stage, stable star
–Hydrogen changes into helium
which creates enormous amounts
of energy
–The size of the star does not
change much
–90% of all stars
A Star of a Joke…
A woman at a gas station noticed a spaceship
landing in front of her. An alien stepped out of
the spaceship and started to pump gas into it.
The woman noticed the letters ''U.F.O.'' printed
on the side of the ship. She turned to the alien
and asked ''Does U.F.O. stand for Unidentified
Flying Object?''
The alien answered, ''No, it stands for Unleaded Fuel Only!''
rd
3
stage
Giants and Supergiants
– Main Sequence star
becomes a red giant
• Red giant star that
expands 50x their
normal size and cools
(giving it a reddish
color) it loses all its
hydrogen; appears
brighter
• Supergiant 
 Large star with high
mass that become
even hotter and
expanding even
more
 It is 100’s of times
hotter and more
luminous than a red
giant
Supernova
• gigantic explosion
in which a massive
star collapses
– Occurs after a
massive star
uses up its fuel
source
Supernova Remnant
White Dwarf
• small hot star
• Fuel is used up
–Final stage for
some stars
–Can shine for billions of
years before they
completely cool and
extinguish
Black Dwarf
• The remains
of a white dwarf
after it has
completely
cooled.
• Scientists do
not think universe
is old enough to
have any black
dwarfs yet
?
Neutron Star
A neutron star is a
type of remnant that
can result from the
gravitational collapse
of a massive star
during a supernova
event.
Small dense starone teaspoon would
weigh a billion tons
on Earth!
What is a Black Hole?
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• an object that is so massive that
light cannot escape its gravity
Did you know….?
Astronomers can detect black
holes by using X-ray telescopes
Pulsars
A neutron
star that
spins and
sends
out
beams of
radiation
is called a
pulsar
Hertzprung-Russell
Diagram
Shows the relationship between a star’s
surface temperature and absolute
magnitude
– Used to study the lives of stars
– Most stars lie along the main sequence
portion of the diagram
Our Place in Our Galaxy
Be careful about wishing on
stars….
What’s the Difference?
Solar System
Universe
Galaxy
Image credits: NASA, STScI
Where is our Solar System?
Visible stars,
gas, dust in
the Milky Way
Galaxy
Solar
System
here
Image credits: NASA, STScI
What Are Galaxies?
• A galaxy is a system of stars, dust, and
gas held together by gravity.
• Galaxies are large collections of stars…..
millions and billions of stars
• There are hundreds of billions of galaxies
in the Universe
• Millions to hundreds of billions of stars in
each galaxy
Galaxy Facts
•
•
•
The most distant galaxies ever photographed are as far as 10 billion to
13 billion light-years away.
Large galaxies have more than a trillion stars.
Only three galaxies outside the Milky Way are visible with the
unaided eye. People in the Northern Hemisphere can see the
Andromeda Galaxy, which is about 2 million light-years away. People
in the Southern Hemisphere can see the Large Magellanic Cloud,
which is about 160,000 light-years from Earth, and the Small
Magellanic Cloud, which is about 180,000 light-years away.
Remember….
A light-year is the distance that light travels in a vacuum in a year -- about 5.88
trillion miles (9.46 trillion kilometers).
An astronomical unit is 93 million miles or 150 million km (average distance
between Sun and the Earth)
The speed (velocity) of light is 300,000 km/sec.
Three Main Types of Galaxies
􀂄
Galaxies are classified based on shape
1. Spiral--rotating disk (with arms)
2. Elliptical—spheroid or oval disk of
stars with more chaotic motion
3. Irregular--none of the above
Examples of Three Main
Galaxy Types
Elliptical
Spiral
Irregular
Looking at the Milky Way
A Spiral Galaxy
 Viewed as a bright band of stars across the sky
 There are about 200-400 billion stars in the
Milky Way.
 Much of the Milky Way is blocked by dust seen as
a dark band through the middle of the Milky Way
􀂄
The Milky Way is…
• Gigantic! Its disk spans 100,000 light-years
• it’s about 14 billion years old, give or take a few
billion.
– Astronomers arrived at this age by measuring the age
of a single star. The galaxy can’t be any YOUNGER
than its oldest stars, so this technique yields a
MINIMUM age for the Milky Way.
– Astronomers determined the star’s age by measuring
its chemistry. They found that it contains only minute
traces of anything heavier than hydrogen and helium,
the two lightest elements.
One of the most important tracers of the
star’s age is a radioactive element called
thorium.The star contains only about half
as much thorium as expected.Thorium has
a half-life of 14 billion years. In other
words, in 14 billion years, half the star’s
thorium should have turned into other
elements. Since half of the thorium has
disappeared, astronomers deduce that the
star is about 14 billion years old — and so
is the Milky Way.
Spiral arms are a natural consequence of
some rotations.
If gas/dust in a disk
are rotating with the
same speed, stuff
further out will take
longer to go around
and will lag behind
Elliptical Galaxies
lack spiral
arms and dust
and
contain
stars that
are generally
identified as
being old
Irregular Galaxies
Irregular
galaxies
lack any
specific
form and
contain
stars, gas
and dust