1. Notes: Galaxies

2. So, What is a Galaxy? A galaxy is a large system of:
Stars.
Gas – mostly Hydrogen.
Dust.
Dark matter.
They orbit a common center and are bound together by gravity.
It’s widely believed that the center of many are black holes.
Galaxies are HUGE.
The Milky Way is 100,000 light years in diameter.
The Sun is 28,000 light years from the center.
Galaxies a VERY far apart.
M31 is the closest to us.
It’s 2.2 million light years away.

3. What Are The Parts of a Galaxy?
Galactic disk: location of most of the stars. Broken up into 3 parts:
Nucleus: the center of the disk.
Bulge: the area around the nucleus.
Spiral arms: extend outward from the center.
We are in one of the spiral arms of our galaxy.
Globular Clusters: groups of old stars scattered around the galactic disk.
Halo: Large, dim region surrounding the entire galaxy.
Made of hot gas and (possibly) dark matter.

5. How do Galaxies Form? We don’t really know, but we have an idea:
Shortly after the Big Bang, gravity would’ve cause gas and dust clouds to collapse.
This may have led to the early formation of galaxies.
Interactions between galaxies (they can crash into each other) seem to play an important role in their formation.
Because many galaxies are so far away from us, it was thought that they formed early in the universe’s history.
Recent discoveries have shown that many may have formed fairly recently.

6. Galaxies are Clumped in Space
Galaxies are not randomly dispersed throughout the universe.
They occur in groups called Clusters.
The galaxies in a cluster are bound by gravity to each other and can collide.
Our Galaxy belongs to a Cluster called “The Local Group.”
Clusters of galaxies occur in bigger groups called Superclusters.
Most Superclusters are not gravitationally bound units like Clusters are.
All the Clusters are moving away from each other as the Universe expands.

8. Colliding Galaxies
Galaxies that belong to the same Cluster can collide.
Sometimes these galaxies pass through each other, and sometimes they combine.
The individual stars in the colliding galaxies do not hit each other, but the huge clouds of gas and dust do collide.
Pairs of colliding galaxies often exhibit long “antennae” of stars ejected by the collision.
NGC 2207 (right) and IC 2163 are orbiting and gravitationally distorting each other.

9. Type of Galaxies
In 1936 Hubble used the Hubble Sequence to classify galaxies into 3 main categories.
He classified them based on their morphology (shape).
The classifications were:
Elliptical
Spiral
Lenticular
There was a 4th category called irregular that contains galaxies whose shapes didn’t fit into any particular cateogory.
The Hubble Sequence is still the most often used system for classifying galaxies.

10. Elliptical Galaxies An elliptical galaxy is a oval-shaped galaxy.
They have no gas or dust.
They have no visible bright stars or spiral patterns.
They have no galactic disk.
In the Hubble Sequence they are classified with an ‘E’ followed by a number.
E0 = circular; E7 = most elliptical.
About 60% of the galaxies in the universe are elliptical.
Most are small (1% the size of the Milky Way). Some are huge.

11. Spiral Galaxies
Sombrero Galaxy - Sa
Consists of a flattened disk, with stars forming a spiral structure, and a central concentration of stars called a bulge.
They are the brightest galaxies, but only make up 20% of the galaxies in the universe.
There are two types:
Normal Spiral: have an obvious disk shape with a bright center and well defined spiral arms.
In the Hubble sequence they are classified with a ‘S’ followed by a lower case letter.
The lowercase letter describes how tightly wound their spirals are.
Barred Spiral: similar to normal spirals, but their bulge is more elongated, making it look like they have a bar.
In the Hubble sequence they are classified with a ‘SB’ followed by a lower case letter.
The Milky Way is a barred spiral – designated SBc
Whirlpool Galaxy - Sbc
Pinwheel Galaxy - Scd
NGC SBbc

12. Lenticular Galaxies
Consist of a bright central bulge (like a spiral galaxy) with a shape like an elliptical galaxy.
They are found on the Hubble tuning fork where the two spiral branches separate.
They are classified as “S0”.
They have no visible spirals.
There is no evidence of new star formation.
They can be very hard to distinguish from E0 galaxies.
Spindle Galaxy
Cartwheel Galaxy
NGC 4866

13. Irregular Galaxies Have no defined structure.
They don’t fit anywhere in the Hubble Sequence.
Hubble usually referred to them as Irr I and Irr II.
They tend to be small (1-25% the size of the Milky Way).
They include the Magellanic Clouds.
Two irregular galaxies orbiting the Milky Way.
Important navigational tools for Pacific Islanders.
Named such because they were made known to Europeans after Magellan’s voyage.
Large Magellanic Cloud (LMC)
NGC 1427A – Irr I

14. Small Magellanic Cloud (SMC)
Large Magellanic Cloud (LMC)