There are three main types of galaxies: irregular galaxies with no defined shape, elliptical galaxies that are flattened balls containing only old stars, and spiral galaxies with a central bulge and rotating disk containing gas, dust, and younger stars. Stars form within giant clouds of gas and dust called nebulae, and their lifetime depends on their mass - smaller stars live much longer than larger ones. The universe contains these galaxies, nebulae, and stars, all bound together by gravity.

1. The Universe…
…is space and everything in it.

2. Galaxies
A galaxy is a cluster of
stars, gas, and dust that
are held together by
gravity.
 Thereare three main
types of galaxies:
 Irregular
 Elliptical
 Spiral

3. Types of Galaxies – Irregular…
 Some galaxies do
not have definable,
regular shapes and
are known as
irregular galaxies.
 They contain young
stars, dust, and
gas.

4. Types of Galaxies – Elliptical...
Ellipticalgalaxies look like
flattened balls.
These galaxies contain billions
of stars, but have little gas and
dust between the stars.
Because of the lack of gas and
dust, new stars cannot form in
most elliptical galaxies, and so they
contain only old stars.

5. Types of Galaxies –Spiral…
Spiral galaxies consist of a
flat, rotating disk with stars, gas,
and dust and a central
concentration of stars, known
as the bulge. The bulge is
surrounded by a much fainter
halo of stars.
Contains middle aged stars

6. Types of Galaxies – Barred Spiral…
Our Milky Way galaxy has
recently (in the 1990s) been
confirmed to be a type of
spiral galaxy known as a
barred spiral galaxy.
Our Sun is located at the
edge of the Milky Way
galaxy.

7. Nebulae (plural for Nebula)
 Stellar Nebula- A
gigantic cloud of gas and
dust from which stars are
made; sometimes called
a “Stellar Nursery”
 Planetary Nebula- When
a star runs out of fuel to
burn, its outer layers of
gas are blown outward in
the shape of a ring.

8. Stars
A star contains hydrogen gas that forms helium
through a process known as nuclear fusion. Stars
form when gravity pulls together gas and dust
from stellar nebula.
 Itreleases enormous amounts of energy which
causes the star to become very hot and glow.
 Vary in mass, size, and temperature
 Held together by gravity.

9. A Star is Born…
A star is made up of a large amount of gas,
in a relatively small volume.
A stellar nebula, on the other hand, is a
large amount of gas and dust,
spread out in an immense
volume.
 All
stars begin their lives as parts of stellar
nebulas.

10. A Star is Born…
 Gravitycan pull some of the gas and dust
in a nebula together.
 The contracting cloud is
then called a protostar.
A star is born when the contracting gas
and dust become so hot that nuclear
fusion begins.

11. Lifetimes of Stars…
 Beforethey can tell how old a star is,
astronomers must determine its mass.
 Medium mass stars, such as the Sun, exist
for about 10 billion years.
 These are known as
main sequence stars.

12. Lifetimes of Stars…
 Starswith more mass have shorter lives
than those with less mass.
 Small stars use up their fuel more slowly
than large stars, so they have much longer
lives, about 200 billion years.

13. Lifetimes of Stars…
Stars form in a stellar
nebula, from collapsing
clouds of interstellar gas
and dust. This is called a
protostar. The rest of the life
cycle depends on
the mass of the
When a star runs out of star.
fuel, it will become a white
dwarf, neutron star, or
black hole.

14. Main Sequence Stars…
 Main
Sequence is the stage in which stars
spend the majority of their lifetime.
 Small
and medium stars first
become red giants.
 Theirouter layers expand during the red
giant phase.
 Eventually, the outer parts grow bigger still
and drift out into space.

15. Main Sequence Stars...
 Theblue, white, hot core is left
behind causing a white dwarf.
 When there is no more energy, it
becomes a black dwarf. It is the remaining
burnt-out cinder left, as the star goes out.

16. A dying giant, or supergiant star, can
suddenly explode. Within hours, the star
blazes millions of times brighter.
 The explosion is called a supernova.

17. Neutron Stars…
 After
a star explodes, some material
from the star is left behind. This
material may become part of a
planetary nebula.
 Thecore will compress
and form a neutron star.
 Neutron stars are even
smaller and more dense
than white dwarfs.

18. Black Holes…
 Themost massive stars may have more
than 40 times the mass of the Sun. One
might have more than five times the
mass of the Sun left, after it becomes a
supernova.
 The gravity of this mass is so strong that
the gas is pulled inward, packing it into a
smaller and smaller space. These
massive stars become black holes when
they die.