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Galaxies. The Basic Element of Cosmology. Warm Up. Please complete a K-W-L chart for the topic of galaxies. Warm Up-01/03/12. What was the Hubble “Deep Field” project? What did it discover? What type of galaxy is our Milky Way galaxy?. Edwin Hubble Webquest.

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The Basic

Element of Cosmology

warm up
Warm Up

Please complete a K-W-L chart for the topic of galaxies.

warm up 01 03 12
Warm Up-01/03/12

What was the Hubble “Deep Field” project?

What did it discover?

What type of galaxy is our Milky Way galaxy?

edwin hubble webquest
Edwin Hubble Webquest

warm up1
Warm Up
  • What is a galaxy?
  • What did Charles Messier do to link his name to galaxies?
  • Who was Edwin Hubble and what did he do?
  • What are the three types of galaxies?
  • What is the difference between an E0 elliptical galaxy and an E7?
  • What is the difference between an Sa spiral galaxy and an Sc?
  • Draw and label the Hubble Galaxy Classification scheme!
out beyond our galaxy
Out beyond our galaxy…

Out beyond our galaxy, the depths of space is filled with other star systems. Some are like our own.


We call these star systems galaxies. Galaxies come in many shapes and sizes. Some galaxies are egg-shaped with clouds of stars evenly spread throughout.


Some galaxies are completely irregular in shape.


Galaxies not only differ in size and shape, but the also differ in their content. Some galaxies are young and some are old.

what is a galaxy
A galaxy is an immense cloud of millions or billions of stars. Each star while following its own orbit is locked in place by the gravitational influence of all the other stars in its galaxy. Each galaxy is an independent and isolated star system.What Is a Galaxy?
early observation
Because galaxies are so distant, they are difficult to see with the unaided eye. The closest galaxy to our Milky Way is over 150,000 light years away. Al-Sufi, a Persian astronomer, noted observing the M31 (Andromeda) galaxy in the 10th century. Early Observation
early observation1
19th century Frenchman Charles Messier, studied comets. While looking for them he noticed a number of faint, diffuse patches of light. To avoid confusing them with comets in the future he began to assign them numbers. Galaxies are still referred to by their “M” or Messier number.Early Observation
early observation2
Other astronomers came along, such as William Herschel and John Dryer and created their own catalog. It became known as the New General Catalog or “NGC” for short. Some galaxies appear in both catalogs and therefore have two names. M82 is the same galaxy as NGC 3034Early Observation
edwin hubble
While today we know that galaxies are immense star systems, the idea was not widely accepted until the 1920’s. It was a graduate student at the University of Chicago that would define the nature of galaxies. That man was Edwin Hubble.Edwin Hubble
edwin hubble1
Edwin Hubble

Edwin Hubble noticed that when he looked at galaxies that they looked quite different. However, many shared certain characteristics. Hubble used these similarities the categorize all galaxies.

types of galaxies
Types of Galaxies

Hubble defines three basic types of galaxies: the spiral galaxy,the elliptical galaxy and the irregular galaxy. These are often denoted by the letters S, E and Irr, respectively.

subtypes of galaxies elliptical
Subtypes of Galaxies/Elliptical

Hubble subdivided the elliptical galaxies (denoted as E). He ranked them from E0 to E7. E0 galaxies had zero eccentricity while E7 are highly elliptical.

spiral galaxies
Spiral Galaxies

Spiral galaxies (S) consist of central bulges surrounded by sweeping spiral arms emanating from their centers. Hubble classified them from Sa to Sd. Sa galaxies have large central bulges and tightly wound arms while Sd galaxies have small central bulges with loosely wound spiral arms.

subtypes of galaxies barred
Subtypes of Galaxies/Barred

Hubble recognized subgroups within the major types as well. The first of these is the barred spiral galaxy (denoted SB). Barred spiral galaxies have arms that emerge from an elongated central region. Barred spirals are ranked from a to d, with SBa galaxies having large bulges and tightly wound arms to SBd galaxies having very small bulges with very loosely wound arms.

s0 galaxies
S0 Galaxies

S0 galaxies are those with nuclei surrounded by a disk-like structure without arms. The S0 galaxy shares properties of both spiral and elliptical galaxies and seems to bridge the gap between the two major types of galaxies.

Hubble introduced the S0 class long after his original classification scheme had been universally adopted largely because he noticed many highly flattened objects that otherwise had the properties of elliptical galaxies.

spiral galaxies2
Spiral Galaxies

Galaxies differ in ways other than shape, too. They differ in content as well. Spiral galaxies have a pretty even mixture of both old (population II) and new (population I) stars. Usually, spiral galaxies contain about 15% of their mass as gas and dust

elliptical galaxies
Elliptical Galaxies

Elliptical galaxies contain mostly older, population II stars. However, they generally retain about half their mass as dust and gas suggesting that they still have vast amounts of stars to create.


The Hubble “tuning fork” was not created to imply an evolutionary path for galaxies. However, astronomers have seen evidence of galaxies changing types. The collision between spiral galaxies of similar mass is believed to create elliptical galaxies.

warm up2
Warm Up

What is galactic cannibalism?

When one galaxy absorbs another, what evidence do you see?

If two spiral galaxies combine, what is the end product?

What is the closest galaxy to our own?

What is the period-luminosity relationship?

How does the period-luminosity help to determine how far another galaxy is away from us?

creation of galaxies
Creation of Galaxies

Merger seems to play an important role in the formation of galaxies. Primarily amount star clusters. This may be one way to explain the vast difference in the ages of stars contained in spiral galaxies. As a smaller galaxy is absorbed by another, its stars are attracted to the central portion of the larger bulge. Galactic collision. It’s called galactic cannibalism, the big eating the small.

the causes of galactic types
The Causes of Galactic Types

Astronomers are still uncertain as to the exact causes of galaxy types. Spiral disks rotate rapidly when compared to elliptical halos. However, halos and bulges in spiral galaxies rotate much more slowly. Thus astronomers believe that more than a galaxy’s rotation determines its type.

Many astronomers believe that galaxies form from merger of like galaxies followed my galactic cannibalism. Streams of stars can be seen trailing stretching from our galactic halo. Astronomers think theses stars are the remnants of smaller galaxies absorbed by our Milky Way.
galactic cannibalism
Galactic Cannibalism

This beautiful, eerie silhouette of dark dust clouds against the glowing nucleus of the elliptical galaxy NGC 1316 may represent the aftermath of a 100-million-year-old cosmic collision between the elliptical and a smaller companion galaxy

can merger explain the differences between spiral and elliptical galaxies
Can Merger Explain the Differences between spiral and Elliptical Galaxies?

Many astronomers say yes. Both observation and computer modeling are compelling astronomers to propose a new hypotheses for the origin of spiral and elliptical galaxies. The theory should say that new galaxies are born as disk-like systems lacking a central bulge.

Subsequent collisions and mergers with other star systems create elliptical galaxies. This theory has added attractions as well.

Many astronomers once believed that galaxies were formed from one creation event. But, looking at the bulge of our galaxy, you see stars of various ages throughout.

A massive black hole hidden at the center of nearby galaxy, Centaursus A, feeds on a smaller galaxy in a spectacular collision.
merger model
Merger Model

With each merger, the left over gas from the cannibalized galaxy would gravitate to the center and begin to form new stars. This accounts for the stars of various ages existing there.

merger model1
Merger Model

Another feature of the merger model is that it also explains large spiral galaxies. When a large spiral galaxy merges with an elliptical galaxy, the elliptical galaxy becomes the bulge of the new spiral galaxy.

measuring the properties of galaxies
Galaxies are separated by enormous distances. The closest galaxy to our own is the Large Magellanic Cloud about 150,000 light years away. The distances are so great that using parallax will not work to determine distance.Measuring the Properties of Galaxies
candle power
Candle Power

One of the techniques used to measure the size of the Milky Way was “standard candles” of luminosity, discussed in Chapter 15 (The Milky Way). The most reliable sources of known standard candles is the “Cepheid variable” (Chapter 13).


Cepheids are convenient to use for several reasons. One, the are very bright, averaging about 1 million times the Sun’s luminosity. They can be seen a very long way away.

Because Cepheids are variable, astronomers can use the period-luminosity relationship to determine exactly how bright the variables are.Cepheids
red shift and the hubble constant
We know that when we look at other galaxies, the spectrum from those galaxies is red-shifted. This is due primarily to the Universe expanding. This was first noticed by Vesto Slipher.Red-shift and the Hubble Constant
red shift and the hubble constant1
Red-shift and the Hubble Constant

Others, like Hubble, also noticed that the dimmer the star (i.e. the farther away the star), the larger the red-shift. The speed that a galaxy moves away from us (“V”), its recessional velocity, increases with distance (“D”).

the hubble law
The Hubble law

Hubble defined the relationship with the Hubble law which says:

V=HD, where

V = Recessional Velocity, in kilometers/second

D = Distance, in parsecs (3.26 million light-years) and

H = Hubble Constant (70 km/sec/Mpc)

the hubble constant
The Hubble Constant created lots on controversy initially. To calibrate the formula, you have to know both the distance and velocity of at least a few galaxies. The solution was to simply assign the number 70 km/sec/Mpc. The Hubble Constant
measuring the diameter of a galaxy
Measuring the Diameter of a Galaxy

To determine the diameter of a galaxy, use the formula:

d = 2pAD where:

360 d=diameter

A=angular size


measuring the mass of a galaxy
Measuring the Mass of a Galaxy

We can use Kepler’s third law. The orbital velocity tells you the mass. There is one problem though. When you add up all the mass that we see, it does not equal the mass indicated by our galactic motion. It is always less.

why less
Why Less?

How is it that our galaxy is more massive than the sum of all that we see? Easy, there is stuff out there we can’t see. It’s called dark matter!

dark matter
Dark Matter

Dark matter is what scientists believe accounts for the discrepancy between the mass we see and the mass needed to make our galaxy move as it does. How much are we talking here? Maybe 10 times more massive. We are only seeing on average about 10% of a galaxies mass.