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Galaxies
&
Stars
What is a galaxy?
• A galaxy is a huge collection of gas, dust
and billions of stars.
• These stars are held together by the force of
gravity.
• Earth and our solar system are part of the
Milky Way Galaxy
• Most of the stars you see at night are part of
the Milky Way Galaxy
THE MILKY WAY GALAXY
Our Home...
• The Milky Way is a
spiral galaxy
• Our solar system
is located on the
Orion Arm of the
Milky Way Galaxy
The galaxy is believed to be home to
100 billion stars!!!!
Our Galactic Neighbour
• If you look out towards the constellation of
Andromeda you are looking towards our
neighbouring galaxy Andromeda which is
29 million light years away!!!
Star clusters
• Star clusters are groups of stars that are
close together and that travel together
Pleiades
M15 Globular Cluster
Some popular star
clusters!
More…M15
Galaxies are divided into five different types,
determined by their shape:
• Elliptical (shape similar to a football)
– Most common
• Spiral (shape similar to a flat pinwheel)
– Arms are composed of a lot of gas and dust
and young blue stars (I.e. star formation is
ongoing)
More elliptical galaxies
Some spiral galaxies
Types of Galaxies (Cont’d)
• Lenticular galaxies - Disc-shaped with a
bulge in the middle
Types of Galaxies (Cont’d)
• Irregular (no particular shape)
– Smaller and less common than spiral or
elliptical
• Barred-spiral galaxy
– A type of spiral galaxy
B
A
Elliptical Galaxy
Spiral Galaxy
C
Spiral Galaxy
D
Irregular Galaxy
• Galaxies are moving outwards in the
Universe. Sometimes two galaxies collide
and it appears that the big galaxy eats the
smaller galaxy.
Unusual Galaxies
• Quasars
– Very distant objects,
look like faint stars
– Brightest objects in
the universe
– Give off 100 times
more energy than
Milky Way
Life and Death of Stars
Stars
• The biggest things in
the universe are the
stars like our sun
What makes it go
• Stars are giant balls of gas that burn by a
process called nuclear fusion
• This process combines two hydrogen
atoms into one helium atom giving off a
lot of heat.
Star Colour and Temperature
• The temperature of a star determines its
colour.
Red → Yellow → White → Blue
Coolest
Hottest
• Our Sun is a yellowish colour so it is
considered a medium-cool star.
Life and Death of a Star
Draw this diagram!!!
Star Birth
• Stars are created in a
nebula as the gas
contracts because of
gravity
• As they become larger
they heat up until they
reach a temperature
when fusion begins
and they “turn on”
Star Life
• Our star is now in what is called the main
sequence where it is stable and consistent.
• A cooler smaller star like our sun can last
for about 8 billion years
• Fast burning blue stars only last for a
million years
The Fate of Stars
Stellar Nebula
– cloud of interstellar gas and dust
Red Giant
- a large, cool, bright star
Red Supergiant
- an extremely large, cool, bright star
Planetary Nebula
- a bright shell of gas ejected from an old, low-mass star
Supernova
- stellar explosion where a star becomes about a million times brighter
White Dwarf
- a low-mass star that has run out of fuel and condensed to the size of
about Earth
Neutron Star
- what’s left of a star, is very compact and consists of mostly neutrons
Star Death
• Stars live until the hydrogen in
the core that fuels them runs out
• After that they collapse inward
since there is nothing opposing
gravity any more.
• The new heat in the core fuses
the helium into carbon
• The outer shell still has some
hydrogen and burns expanding
to form a red giant or
supergiant star
• Once the core fuses into lead
fusion stops and the star
collapses.
Dwarf Stars
• When the star core
reaches lead it can not
undergo fusion any
longer
• This white hot ball of
lead “white dwarf”
cools to form a “black
dwarf” star
Death of a Giant Star
Neutron Stars
• Sometimes a star is so
heavy that even lead
cannot support its weight.
• The atoms collapse even
further creating a ball of
neutrons.
• This forms a rapidly
spinning neutron star that
is only a few kilometers in
size.
• We see neutron stars from
the radiation that shoots
out of either end.
Black Hole
- an object whose gravity is so strong that nothing can escape it
Black holes
• Sometimes the star is so heavy
that not even the neutrons stay
apart and crash together
forming a black hole
• After that we are not sure as
this creates a black hole
• A black hole is an object so
dense that not even light can
escape it.
• We can find black holes by
looking for objects in space
orbiting seemingly empty
space.
The End???