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.