ASTR
123, Robert L Zimmerman
Assignment
#2: Properties of the Milky Way
Assigned
Oct 13, 2009---------Due Oct 20
Return the following images
from the WWW and answer the questions. Place everything in a word processor and
include under each picture a brief description of the image.
Questions:
1.
How does the density wave cause star birth? Why are the regions
between the spiral arms dark?
2.
Explain how self-propagating star formation can also make partial spiral arms.
3.
Draw the galactic rotation curve (Fig 23.21) and explain what it means. What
does the difference between the two curves tell us about dark matter?
What are the masses inside 40 kpc (the limit of measurement), 15 kpc (the edge
of the visible galaxy) and the mass inside 8 kpc?
Images
4.
The bulge at the center of the Milky Way is a region that contains a
high-density of stars and lies in the direction of the constellation
Sagittarius. There is evidence for a very massive black hole at its center
called SgrA*. Return a radio, infrared or X-ray image of our galactic
center near Sgr A*. Include a description of the image. The URL is
http://antwrp.gsfc.nasa.gov/apod/ap051023.html
5. This
three-paneled image--- Galactic Center X-ray Binaries---
shows Chandra's latest results from the Galactic Center. Include a description of the image. The URL is
http://chandra.harvard.edu/photo/2005/gctr_bin/more.html
6
The galaxy's central square degree is seen in radio and infrared light. The
region is known as the Central Molecular Zone. Include a description of the
image. The URL is
http://antwrp.gsfc.nasa.gov/apod/ap081110.html
The
long parallel rays slanting across the top of the above radio image are known collectively as the Galactic Center Radio Arc and jut straight out
from the Galactic plane. Include a description of the
image. The URL is
http://antwrp.gsfc.nasa.gov/apod/ap080427.html
Misc
remarks by Prof Zimmerman
The
Milky Way
The Milky Way is a barred spiral galaxy with at least 200 billion stars that
orbit the Galactic Center. Its total mass is probably 600 billion or so solar
masses. Its diameter is about 100,000 light years (30 kpc). Our solar system is
situated in the disk about 28,000 light years (8 kpc) from the Galactic Center
and orbits the Galactic Center on a nearly circular orbit. We are moving at
about 250 km/sec, and need about 225 million years to complete one orbit. The
Solar System has orbited the Galactic Center about 20 or 21 times since its
formation about 4.6 billion years ago. Our galaxy has a disk component
exhibiting a spiral structure, a nuclear region, and a halo component.
http://antwrp.gsfc.nasa.gov/apod/ap980523.html
Halo
The halo is a spherical region that surrounds the disk. It has a low density of
old stars located mainly in globular clusters (these cluster have between
10,000 - 1,000,000 Population II stars). Our galaxy has about 200
globular clusters, of which we know about 150. From their apparent
distribution in the sky, Harlow Shapley concluded that the center of the Milky
Way lies in the direction of Sagittarius and the Sun is not at the
center. The halo is composed mainly of dark matter, which extends well
beyond the edge of the disk.
Charles
Messier
During
the years from 1758 to 1782 Charles Messier, a French astronomer (1730 - 1817),
compiled a list of approximately 100 diffuse objects that were difficult to distinguish
from comets through the telescopes of the day. Discovering comets was the way
to make a name for yourself in astronomy in the 18th century -- Messier's aim
was to catalog the objects that were often mistaken for comets. Charles
Messier was a comet ÒhunterÓ but he is best known for his catalog
of Deep Sky Objects with their "Messier" numbers. Another
designation for deep sky objects is NGC---New General Catalogue of Nebulae and
Clusters of Stars. J.L.E. Dreyer made this catalog in 1888. http://antwrp.gsfc.nasa.gov/apod/ap980128.html