1. Galaxies

2. Objectives
Identify three types of galaxies.
Describe the contents and characteristics of galaxies.
Explain why looking at distant galaxies reveals what young galaxies looked like..

3. What is a Galaxy ? Solar System Distance from Earth to Sun
= 93,000,000 miles
= 8 light-minutes
To define what a galaxy is, we start with something close to home - our own solar system - and its place in our galaxy.
The distance from the Earth to the Sun is 93 million miles (150 million km), or 8 light-minutes. That is, the time it takes light from the sun to reach Earth is 8 minutes.
The entire solar system is 5.5 light-hours across. That is, light takes 5.5 hours to cross the solar system.
Size of Solar System
= 5.5 light-hours

4. What is a Galaxy? Stellar Region 30 light-years Sun (solar system
too small to be
seen on this scale) 30
light-years
Neighboring stars can be grouped into stellar regions. The solar neighborhood is about 30 light-years across. A light year is the distance light travels in one year, and is about 6 trillion miles (9.5 trillion km)
[Note that in some regions stars can be clustered quite closely together. Thus, some neighborhoods may be more crowded than others.]

5. What is a Galaxy? A Galaxy is: a massive collection of stars, gas, and
Sun’s Stellar Region
A Galaxy is:
a massive
collection of
stars, gas, and
dust kept together
by gravity
A stellar region is just a small portion of a galaxy. A galaxy is a massive collection of stars, gas, and dust held together by its own gravity. Galaxies can range in size from 6,000 to 350,000 light-years across. Our own galaxy, the Milky Way, is about 105,000 light-years in diameter.
The galaxy pictured here is the Andromeda Galaxy, which is the nearest large galaxy to our own. We think the Milky Way looks very much like Andromeda.
100,000 light-years

6. What is a Galaxy? If our solar system was the size of a cell in
the human body,
our galaxy
would still measure over
one mile across.
Galaxies are enormous in size. This slide shows the scale of our galaxy by analogy to the size of a cell in the human body. [A cell in the human body is about 50 microns (or meters) across.]
Using this same analogy, the Andromeda Galaxy would be 22 miles away.

7. Contents of Galaxies
Gas Clouds A large clouds of gas and dust in interstellar space is called a nebula.
Star Clusters A globular cluster is a tight group of stars that looks like a ball and contains up to 1 million stars.
An open cluster is a group of stars that are close together relative to surrounding stars.

9. Contents of Galaxies
Quasars

10. There are two main classes of galaxies organized by Hubble
ELIPTICAL
Spherical galaxies classified on a scale of
0 to 7
The number indicates how egg-shaped the ellipse is - 0 means a ball shape, and 7 looks like a dish
SPIRAL
Central nucleus surrounded by arms of stars
Spiral or barred spiral
The bar is a dense area of stars that extends along the diameter of the galaxy

11. Types of Galaxies Spiral Barred Spiral Elliptical Irregular Peculiar
disk-like appearance with arms of stars and dust forming a spiral pattern
similar to spirals but with a bright bar of stars and gas through the center
elliptically-shaped, with less gas and dust than spirals; no disk or “arms”
distorted form of one of the above types, often due to collision with another galaxy or similar catastrophic event
Barred Spiral
neither elliptical nor spiral in shape; gas and dust as in spirals but no defined “arms”
Elliptical
Irregular
There are different types of galaxies, each with different characteristics.
This slide presents the different galaxy types and their major features, one at time. The slide ends by showing a few additional examples of each type of galaxy.
[This version works properly in Powerpoint for Mac OS X. The large image of the galaxy is now grouped with its descriptive text.]
Peculiar

12. Where did they come from?
Galaxies first formed in the densest spots in the Universe.
By the time the Universe was a few hundred million years old, the densest regions had ceased expanding and began to collapse
Matter in these areas did not expand rapidly with the big bang
These areas formed dense groups of rotating stars which became galaxies

13. Where did they come from?
A gigantic star formed at the center of the star cluster from the inflowing matter.
Stars containing more than about 100 times the mass of the Sun are unstable. If slightly compressed, they collapse faster and faster until a black hole forms.
In every observed galaxy, supermassive black holes have been found.

14. Classification of Galaxies
Hubble Tuning Fork

15. The Hubble Tuning Fork
After he discovered what galaxies really were, Edwin Hubble became the first person to classify galaxies.
Astronomers use his system, called the "Hubble Tuning Fork," even today.
First, Hubble divided the galaxies into two general categories: elliptical and spiral galaxies.
Elliptical galaxies are shaped like ellipses, and spiral galaxies are shaped like spirals, with arms winding in to a bright center.

16. The Hubble Tuning Fork
Elliptical galaxies are classified by how round or flat they look.
An E0 galaxy is very round and an E7 galaxy is very flat.
In detail, the number after the "E" is determined by the galaxy's ellipticity - the ratio of the ellipse's major axis to minor axis.
Galaxies with higher ellipticities have higher numbers.

17. The Hubble Tuning Fork
Hubble noticed that some spiral galaxies have a bright line, or bar, running through them.
He called these galaxies "barred spiral galaxies."
Galaxies with spiral arms, but without the bar, are just called "spiral galaxies."

18. The Hubble Tuning Fork
Spiral galaxies are further classified by how tightly their arms are wound.
Type a galaxies have their arms wound very tightly and have large central bulges.
Type c galaxies have very their arms would loosely and have small central bulges.

19. The Hubble Tuning Fork
Some galaxies are a transition type between the elliptical and spiral galaxies, labeled S0 on the tuning fork.
These are called "lenticular galaxies."
Lenticular galaxies have a central bulge and a disk but no spiral arms.

20. The Hubble Tuning Fork
The third class of galaxies is irregular galaxies.
Irregular galaxies are neither spiral nor elliptical, and can have any number of shapes.
They are frequently the product of two galaxies colliding with each other, or at least affecting each other through the force of gravity.

22. Galaxies 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
For each slide, write the type of galaxy as classified by the Hubble Tuning Fork diagram.

23. Spiral

24. Spiral

25. Lenticular Spiral

26. Barred Spiral

27. Elliptical

28. Milky Way galaxy - Spiral

29. Barred Spiral

30. Barred Spiral

31. Elliptical

32. Irregular

33. Elliptical

34. Spiral

35. Irregular

36. Lenticular Spiral

37. Spiral

38. Milky Way Galaxy = Our Home
Our solar system is located in a spiral galaxy called the Milky Way.
From the side, the Milky Way appears to be a narrow disk with a bulge in the middle.
The galaxy’s spiral structure is visible only from above or below.

39. The Milky Way Galaxy The Milky Way might contain one trillion stars.
The visible disk of stars is about 100,000 light-years across.
The Sun is located about 26,000 light-years from the galaxy's center in one of the spiral arms.

40. The Milky Way Galaxy
In the galaxy, all stars orbit around a central region, or core.
It takes about 225 million years for Sun to orbit the center of the Milky Way.
The last time our solar system was in its current position was during the Triassic Period, when dinosaurs first appeared on Earth!

41. The Milky Way Galaxy
The Milky Way often is classified as a normal spiral galaxy.
However, recent evidence suggests that it might be a barred spiral.

42. The Milky Way Galaxy
You can, see the Milky Way stretching across the sky as a misty band of faint light.
You can see the brightest part of the Milky Way if you look low in the southern sky on a moonless summer night.
Like many other galaxies, the Milky Way has a supermassive black hole at its center.
This black hole might be more than 2.5 millions times as massive as the Sun.

43. The Milky Way Galaxy
Evidence for the existence of the black hole comes from observing the orbit of a star near the galaxy's center.
Additional evidence includes X-ray emissions detected by the Chandra X-ray Observatory.