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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Element of Cosmology
Please complete a K-W-L chart for the topic of galaxies.
What was the Hubble “Deep Field” project?
What did it discover?
What type of galaxy is our Milky Way 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.
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.
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.
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 (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.
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 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.
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 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.
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?
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.
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.
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
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.
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.
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.
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.
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.
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”).
Hubble defined the relationship with the Hubble law which says:
V = Recessional Velocity, in kilometers/second
D = Distance, in parsecs (3.26 million light-years) and
H = Hubble Constant (70 km/sec/Mpc)
To determine the diameter of a galaxy, use the formula:
d = 2pAD where:
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.
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 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.