Milky Way Galaxy

Several hundred billion stars make up our galaxy, stretching over some 100,000 light years or about 30 kiloparsecs in a flattened disk which is about 10,000 light years ( 3 kpc) thick at the center. The sun is some 8 kiloparsecs out from the center, about two-thirds of the way out. About 6000 stars are visible with the naked eye.

Two small irregular galaxies called the "Magellanic Clouds" are relatively near the Milky Way. The Large Magellanic Cloud is at about 160,000 light years and the Small Magellanic Cloud is at about 200,000 light years from us. The Andromeda galaxy is the nearest large galaxy at about 900 kiloparsecs or 2.9 million light years.

Within our galaxy are some star clusters that may visually resemble galaxies because they form distinctly visible groups in a relatively small region of space.

Galaxy Geometry

There are many geometries of galaxies including the spiral galaxy characteristic of our own Milky Way. In the remarkable deep space photograph made by the Hubble Space Telescope, every visible object except for the one obvious foreground star seems to be another galaxy.

Elliptical galaxies usually have very little gas or dust and hence little evidence of new star formation. The spiral galaxies may have an abundance of gas and dust and show evidence of star formation in the form of lots of hot blue stars.

Seyfert Galaxies

The key to classifying a galaxy as a Seyfert Galaxy is the presence of broad emission lines from the bright, star-like nucleus. They are often strong radio and infrared sources. The first such active galaxies were discovered in 1943 by Carl Seyfert. Kaufmann cites the example of NGC 4151 which has 28% of its light concentrated in its emission lines. The emission lines include spectra of iron with 9 and 13 electrons stripped away, so the process involves very hot gases. The Seyfert galaxies exhibit variability and some have luminosities approaching the fainter quasars. Approximately 10% of the brightest galaxies in the sky are Seyferts. They are divided into Type 1 and Type 2 Seyferts.

Seyferts are thought to be spiral galaxies with a quasar-like center.

B L Lacertae Objects

B L Lacertae objects are active galaxies characterized by rapid (days) luminosity variations and a lack of emission lines in their spectra. When variability takes place in periods less than a day, variable objects are sometimes called blasars.

B L Lacertae objects are thought to be elliptical galaxies with quasar-like centers.

Blasars

A class of active galactic nuclei which show optical variability with periods on the order of a day or less are commonly referred to as blasars. They are further classified as radio-selected (RBL) and xray-selected (XBL). They are associated with the rapidly varying B L Lacertae objects.