Galaxies and Stars
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
a ball that has been flattened.

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 by a much fainter
halo of stars.

Contains middle aged stars
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.
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.

Stars release enormous amounts of energy which
causes the them to become very hot and glow.

Stars vary in mass, size, temperature, and age.

Stars are 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
nebulas.
A Star is Born…

Gravity can 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.
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
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.
Lifetimes of Stars…
Stars form in a stellar
nebula, from collapsing
clouds of interstellar gas
and dust. This is called a
protostar.
When a star runs out of
fuel, it will become a white
dwarf, neutron star, or
black hole.
The rest of the life
cycle depends on
the mass of the
star.
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 hot, blue, white, core is left
behind resulting in 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.

As a giant or supergiant star dies, it 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 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.