The Universe…
…is space and everything in it.
Galaxies

A galaxy is a cluster of stars,
gas, and dust that are held
together by gravity.

There are three main types
of galaxies:
 Irregular
 Elliptical
 Spiral
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.
Types of Galaxies –
Elliptical

Elliptical galaxies 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.
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 b y a much fainter
halo of stars.

Contains middle aged stars
Types of Galaxies –
Barred Spiral

Our Milky Way galaxy has
recently (1990’s) 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.
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.
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.

It releases enormous amounts of energy which causes the
star to become very hot and glow.

Vary in mass, size, and temperature

Held together by gravity
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 nebulae.
A Star is Born…
 Gravity
can pull some of the
gas and dust in a nebula
together.
 The
contracting star is then
called a protostar.
A
star is born when contracting
gas and dust becomes so hot
that nuclear fusion begins.
Lifetimes of Stars
Before
they 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.
Lifetimes of Stars
 Stars
with more mass have
shorter lives that 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
(or longer).
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 star.

When a star runs out of fuel, it will
become a white dwarf, neutron star, or
black hole.
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.

Their outer layers expand during the red
giant phase.

Eventually, the outer parts grow bigger still
and drift out into space.
Main Sequence Stars
 The
blue-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.
A
dying giant, or supergiant star, can suddenly
explode. Within hours, the star blazes millions of
times brighter.
 The
explosion is called a supernova.
Neutron Stars
 After
a star explodes, some material
from the star is left behind. This
material may become part of a
planetary nebula.
 The
core will compress and form a
neutron star.
 Neutron
stars are even smaller and
more dense than white dwarfs.
Black holes
 The
most massive stars may have more
than 40 times the mass of the Sun. One
might have more than 5 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.