A galaxy is a large collection of stars, gas, dust and dark matter bound together by gravity. There are three main types of galaxies: elliptical, spiral and irregular. Elliptical galaxies have an oval or spherical shape without distinct features. Spiral galaxies have a central bulge and spiral arms. Irregular galaxies lack a distinct shape. Our galaxy, the Milky Way, is a spiral galaxy containing over 100 billion stars and other material.

1. GALAXY

2. A galaxy is a gravitationally bound system
of stars, stellar remnants, interstellar gas, dust,
and dark matter.The word galaxy is derived
from the Greek galaxias literally "milky", a
reference to the Milky Way

3. Elliptical
Galaxies
Spiral
Galaxies
Irregular
Galaxies

4. ELLIPTICAL
GALAXIES
Elliptical galaxies are shaped like a spheriod, or
elongated sphere. In the sky, where we can only
see two of their three dimensions, these
galaxies look like elliptical, or oval, shaped
disks. The light is smooth, with the surface
brightness decreasing as you go farther out
from the center. Elliptical galaxies are given a
classification that corresponds to their
elongation from a perfect circle, otherwise
known as their ellipticity. The larger the
number, the more elliptical the galaxy is. So, for
example a galaxy of classification of E0

5. M89 - an E0
galaxy
M32 - An E2
galaxy
NGC 4621 - an E5
galaxy
Examples:

6. Irregular galaxies have no regular or
symmetrical structure. They are divided into
two groups, Irr I and IrrII. Irr I type galaxies have
HII regions, which are regions of elemental
hydrogen gas, and many Population I stars,
which are young hot stars. Irr II galaxies simply
seem to have large amounts of dust that block
most of the light from the stars. All this dust
makes is almost impossible to see distinct
stars in the galaxy.

7. NGC 6822: NGC 1427A PGC 36867
Examples:

8. SPIRAL
GALAXIES
Spiral galaxies have three main components: a
bulge, disk, and halo (see right). The bulge is a
spherical structure found in the center of the
galaxy. This feature mostly contains older stars.
The disk is made up of dust, gas, and younger
stars. The disk forms arm structures. Our Sun is
located in an arm of our galaxy, the Milky Way.
The halo of a galaxy is a loose, spherical
structure located around the bulge and some of
the disk. The halo contains old clusters of stars,
known as globular clusters.

9. NGC 5457 NGC 628 NGC 1187
Examples:

10. The Milky
Way

11. WHAT IS MILKY
WAY?
The Milky Way is shaped like a huge whirlpool that rotates once every
200 million years. It is made up of at least 100 billion stars, as well as dust
and gas. It is so big that light takes
100 000 years to cross from one side to the other.
The centre of the Galaxy is very hard to see because clouds of gas and
dust block our view. Scientists think that it contains a supermassive black
hole that swallows anything passing too close.
Outside the main spiral are about 200 ball-shaped clusters of stars. Each
'globular cluster' is very old and contains up to one million stars. The
Milky Way belongs to a cluster of at least 40 galaxies. The so-called Local
Group has two large spiral galaxies – the Milky Way and Andromeda.

12. ORIGIN OF MILKY WAY
The Greek name for the Milky Way (Galaxias) is
derived from the word for milk (, gala). One legend
explains how the Milky Way was created by
Heracles when he was a baby.[2] His father, Zeus,
was fond of his son, who was born of the mortal
woman Alcmene. He decided to let the infant
Heracles suckle on his divine wife Hera's milk
when she was asleep, an act which would endow
the baby with godlike qualities. When Hera woke
up and realized that she was breastfeeding an
unknown infant, she pushed him away and the
spurting milk became the Milky Way

13. Pictures of
Milky Way

14. STARS

15. A star is a luminous ball of gas, mostly
hydrogen and helium, held together by its own
gravity. Nuclear fusion reactions in its core
support the star against gravity and produce
photons and heat, as well as small amounts of
heavier elements. The Sun is the closest star to
Earth.

16. The scientific name for the twinkling of stars is stellar scintillation (or
astronomical scintillation). Stars twinkle when we see them from the Earth's
surface because we are viewing them through thick layers of turbulent (moving)
air in the Earth's atmosphere.
Stars (except for the Sun) appear as tiny dots in the sky; as their light travels
through the many layers of the Earth's atmosphere, the light of the star is bent
(refracted) many times and in random directions (light is bent when it hits a
change in density - like a pocket of cold air or hot air). This random refraction
results in the star winking out (it looks as though the star moves a bit, and our
eye interprets this as twinkling).

18. Sirius Betelgeuse Arcturus

19. Rigel Deneb Vega
Example Of Stars