• All of the stars you can see in the sky without a telescope belong to our own galaxy, the MILKY WAY GALAXY
• The Milky Way contains about 100 billion stars
• The Milky Way consists of three parts:
  • A flattened disk
  • A bulge
  • A spherical halo
• The total optical extent of the Milky Way is about 100,000 light years diameter

An optical image of the spiral galaxy NGC 891. This galaxy, seen from the side, may resemble the Milky Way. Notice the dust lane in the disk.

• Seen from face-on, the Milky Way has a prominent spiral pattern in its disk.
• The Sun is about 25,000 light years away from the center of the Galaxy

• There are three basic types of galaxies: spirals, ellipticals, and irregulars

The Sombrero galaxy (M104), a spiral galaxy with a large bulge, seen edge-on. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory.


M100, a face-on galaxy. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory.


The Andromeda galaxy (M31), the closest large spiral to the Milky Way. The two companion galaxies are M32 and NGC 205, two dwarf elliptical galaxies. M32 is the closer galaxy to M31. Images from the Messier Catalog web page.

• Some spirals have a bar in the center

  
  The barred spiral galaxy, NGC 2442. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory.

ELLIPTICAL GALAXIES

• Elliptical galaxies are flattened spheroids

  
  The giant elliptical M87. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory.

  
  Dwarf elliptical galaxies near the spiral galaxy M100. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory.

IRREGULAR GALAXIES

• Irregular galaxies are irregularly shaped galaxies without prominent bulges or spiral arms

  
  The Large Magellanic Cloud. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory/Royal Observatory, Edinburgh.

  
  The Small Magellanic Cloud. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory/Royal Observatory, Edinburgh.

• Do galaxies CHANGE from one type to another?????
  • Perhaps through gravitational interactions?
• Galaxies are NOT `Island Universes': they interact with their neighbors
• About 6 percent of galaxies are obviously peculiar: they do not fit the standard galaxy types
  • They are tidally distorted

  
  Left: IRAS 03119+1448. Right: Arp 295. (IRAS 03119+1448 image from Beverly Smith at East Tennessee State University.) Arp 295 image from John Hibbard at the National Radio Astronomy Observatory.)

• In 1972: Alar and Juri Toomre created computer simulations of galaxy interactions that were able to reproduce these peculiar stuctures.

  
  Left: A simulation by the Toomres of a low mass companion passing close to a disk galaxy. Right: An optical image of Arp 82, an interacting pair of galaxies. (Image from Beverly Smith at East Tennessee State University.)

  
  Left: an optical image of the Antennae galaxies (NGC 4038/9). Right: a computer model of the Antennae galaxies by the Toomres. Image from the Digitized Sky Survey.

  
  The same model of the Antennae pair, seen from a different direction.

The Mice (NGC 4676). Left: An optical image. right: A computer model. (Image from John Hibbard at the National Radio Observatory. Model from John Hibbard at the National Radio Observatory and Josh Barnes at the Institute for Astronomy.)

Click here to see a simulation of this interaction.
Click here to rotate the model, to see the three dimensional structure. (These movies are from John Hibbard at the National Radio Observatory and Josh Barnes at the Institute for Astronomy.)

• The interstellar gas in an interacting galaxy also gets disturbed
• Since the gas in a spiral often extends beyond the observable stars, maps made in the 21 cm HI line often show the most dramatic tails and bridges

Left: An optical image of the M81/M82/NGC 3077 group. M81 is the brightest galaxy, M82 is at the top of the image, and NGC 3077 is to the right. Middle: A 21 cm HI map of the same region. Note that the three galaxies are connected by bridges. Right: A computer model of this system. (These images are from Min Yun at the National Radio Astronomy Observatory)

Click here to see a movie of this interaction. (movie from Min Yun at the National Radio Astronomy Observatory)

• A head-on collision between a low mass galaxy and a disk galaxy can create rings

  
  A model illustrating ring formation. (model by A. Toomre)

  
  A Hubble Space Telescope image of the Cartwheel galaxy.

• An off-center collision can create a partial ring

  
  A Hubble Space Telescope image of the interacting galaxy pair NGC 7714/5. Notice the partial ring and the bridge connecting the two galaxies. Also notice the young clusters of stars in the bridge. (Images from Beverly Smith at East Tennessee State University.)

  Click here to see a computer model of this encounter. (Movie from Beverly Smith at East Tennessee State University.)

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  GALAXY MERGERS

• In 1977, Alar Toomre also speculated that spiral galaxies may merge to form elliptical galaxies

  Click here to see a simulation of two disk galaxies merging. Movie from Chris Mihos at Case Western Reserve University.

  
  Left: NGC 6090, two spirals in the process of merging. Note that there are still two disks. Right: NGC 2623, a merging pair of galaxies: only one body is evident. (Images from Beverly Smith at East Tennessee State University.)

  
  Centaurus A, a peculiar elliptical. This may have merged with a spiral galaxy. Image from the Anglo-Australian Observatory. Copyright Anglo-Australian Observatory.

• Even `normal' galaxies interact with their neighbors
• All galaxies have `quirks'
• The Milky Way is interacting with the Large Magellanic Cloud and the Small Magellanic Cloud.

An HI map of the region near the LMC and SMC. Data obtained by Mary Putnam and her collaborators using the Australian Telescope National Facility.

• The Milky Way is warped by the passage of the Magellanic Clouds

• The Magellanic Clouds may eventually merge with the Milky Way.

Click here to see a computer model showing the merger of a low mass companion with a disk galaxy. (Movie from Chris Mihos at Case Western Reserve University.)

• NOTE: When two galaxies collide, the individual stars don't collide because the space between the stars is huge compared to the sizes of the star
• When two disk galaxies collide, the gas clouds DO collide because gas clouds are big compared to the space between them
• This may cause the gas clouds to gravitationally collapse, and form new stars

A Hubble Space Telescope image of the main disks of the Antennae galaxies (image from Whitmore and Schweizer 1995).

• When galaxies collide, they can get bent out of shape
• Collisions can cause galaxies to change from one type to another
• These collisions can cause new stars to form in the galaxies