The Milky Way is a barred spiral galaxy that is warped due to gravitational interactions with its neighboring dwarf galaxies, the Large and Small Magellanic Clouds. It is composed of stars, gas, dust, and contains over 200 billion stars. The Milky Way is home to our solar system and planet Earth.

1. Galaxies
far far away. We see
clusters of stars. These make
a galaxy. Then we see clusters
of galaxies and clusters of clusters
of galaxies.
And clusters of clusters of clusters
of clusters of galaxies.

2. What is a Galaxy?
• a system of millions or billions of stars,
together with gas and dust, held
together by gravitational attraction.

3. Classification System
• Elliptical:
• 1. 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.
• a. The light is smooth, with the surface brightness
decreasing as you go farther out from the center.
• b. Elliptical galaxies are given a classification that
corresponds to their elongation from a perfect circle,
otherwise known as their ellipticity.
• 1. The larger the number, the more elliptical
the galaxy. So, for example a galaxy of classification
of E0 appears to be perfectly circular, while a
classification of E7 is very flattened. The elliptical
scale varies from E0 to E7.

4. Elliptical Examples

5. Spiral
• 1. Spiral galaxies have three main components: a
bulge, disk, and halo.
• a. The bulge is a spherical structure found in the
center of the galaxy. This feature mostly contains
older stars.
• b. 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.
• c. 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.

6. Spiral
• 1. Spiral galaxies are classified into two groups, ordinary and barred.
• a. The ordinary group is designated by S or SA.
• 1. In ordinary spirals the arms originate directly from the nucleus,
or bulge.
• b. The barred group is designated by SB.
1. In the barred spirals there is a bar of material that runs
through the nucleus that the arms emerge from.
• 2. Both of these types are given a classification according to how
tightly their arms are wound.
• The classifications are a, b, c, d ... with "a" having the tightest
arms.
• In type "a", the arms are usually not well defined and form almost a
circular pattern.
Sometimes you will see the classification of a galaxy with two lower
case letters. This means that the tightness of the spiral structure is
halfway between those two letters.

7. Spiral Examples

8. S0 (zero) Galaxies
• S0 galaxies are an intermediate type of
galaxy between E7 and a "true" spiral
Sa. They differ from ellipticals because
they have a bulge and a thin disk, but
are different from Sa because they
have no spiral structure. S0 galaxies are
also known as Lenticular galaxies.

9. Lenticular Galaxy
But despite the
fact that it
does have a
central bulge
similar to that
of Spiral
galaxies, it is
much larger, on
average, than
traditional
Spirals.

10. More on Lenticular Galaxies
• In terms of content - such as stars and gas content –
Lenticular galaxies have the most in common with
Elliptical galaxies.
• 1. They have mostly old, red stars with very few hot
blue stars. This is an indication that star formation
has slowed significantly, or is non-existant.
• 2. Lenticular and Elliptical galaxies both have a
distinct lack of interstellar gas, a key component for
continued star formation.
• But: unlike Ellipticals, the Lenticular galaxies still
contain a significant amount of interstellar dust.

11. Irregular Galaxies
• Irregular galaxies have no particular shape.
They are among the smallest galaxies and are
full of gas and dust. Having a lot of gas and
dust means that these galaxies have a lot of
star formation going on within them. This can
make them very bright. The Large and Small
Magellanic Clouds are examples of irregular
galaxies. They are two small galaxies which
orbit around our own Milky Way Galaxy. About
20% of all galaxies are irregulars.

12. The Hubble Classification System
• Spiral Galaxies
• As their name implies, spiral galaxies have outstretched, curving arms suggestive of a
whirlpool or pinwheel. Hubble distinguished different sub-classes according to the
tightness of the arms and the size of the nucleus. He called these Sa, Sb, and Sc. In terms
of the arms, Sa is the tightest wound while Sc is the most open. In terms of the nucleus, Sa
has the largest while Sc has the smallest. The galaxies that appear to have a spiral disc but
no visible arms are called S0.
• Barred Spirals
• Barred spirals show the same spiral structure as normal spirals, and also a prominent bar
through the nucleus. The spiral arms emerge from the end of the bar. The sub-
classifications are the same as for normal spirals.
• Irregulars
• Certain galaxies lack either an obvious spiral structure or nuclear bulge, appearing instead as
a random collection of stars with no obvious order. They are distinguished from ellipticals
by their lack of symmetry.
• Elliptical Galaxies An elliptical galaxy shows no spiral structure and can vary from almost
round (what Hubble called E0) to almost cigar shaped (called E7). This classification is
based on our perspective from Earth and not on the actual shape.

13. Hubble Classification Scheme

14. Let’s Classify These

15. Fill in the table.

16. The Milky Way
The Milky Way
Galaxy is an immense
and very interesting
place. Not only does it
measure some
100,000–120,000
light-years in
diameter, it is home
to planet Earth, the
birthplace of
humanity. Our Solar
System resides
roughly 27,000 light-
years away from the
Galactic Center, on
the inner edge of one
of the spiral-shaped
concentrations of gas
and dust particles
called the Orion Arm.

17. The Milky Way is Warped
•
For starters, the Milky Way is a disk about 120,000 light years across
with a central bulge that has a diameter of 12,000 light years. The disk
is far from perfectly flat though, as can be seen in the picture below.

18. LMC and SMC
• It is warped in shape, a fact which astronomers
attribute to the our galaxy’s two neighbors -the
Large and Small Magellanic clouds.
• These two dwarf galaxies — which are part of our
“Local Group” of galaxies and may be orbiting the
Milky Way — are believed to have been pulling on the
dark matter in our galaxy like in a game of galactic
tug-of-war. The tugging creates a sort of oscillating
frequency that pulls on the galaxy’s hydrogen gas, of
which the Milky Way has a great deal.

19. LMC and SMC
• The Large Magellanic Cloud lies about 163 thousand light-years
from Earth. Its companion, the Small Magellanic Cloud is about
200,000 light-years away.
• For many years astronomers thought the Magellanic Clouds
orbited the Milky Way. Recent measurements may prove that
they could be moving too fast for that.
• The Magellanic Clouds are gas-rich, meaning they have a higher
portion of their mass as gas. They also have less portion of their
mass bound up in metallic elements.
• The Magellanic Clouds have both had their shapes distorted by
gravitational interactions with the Milky Way. As these galaxies
pass near the Milky Way, their gravitational pull also misshapes
the outer bars of our galaxy.

20. Large Magellanic Cloud

21. LMC
• The Large Magellanic Cloud contains a
highly active starbirth region called the
Tarantula Nebula. It is part of a larger
cloud of gas and dust, and its high rate
of star formation may be caused by
compression of interstellar gas and dust
by the collision of the cloud with the
interstellar medium. The 1987a
supernova exploded not far from this
region.

22. Small Magellanic Cloud
This

23. SMC
• Recent studies of the Small Magellanic
Cloud indicate that it might be a former
single galaxy split into two remnants.
Gravitational interactions with the LMC
may have broken that galaxy apart.

24. 1. The Milky Way is warped.
2. It has a halo, but you can’t
directly see it.
• 3. It has over 200 billion stars
• 4. Its really dusty and gassy.
• 5. It was made from other galaxies.
• 6. Every picture you’ve seen of the
Milky Way from above is either
another galaxy or an artist’s
interpretation.

25. 7. There is a black hole at the
center.

26. • 8. It’s almost as old as the Universe
itself.
• 9. It’s part of the Virgo
Supercluster, a group of galaxies
within 150 million light years.
• 10. It’s on the move.