Milky Way Galaxy
This is a photo toward the center of our galaxy taken by the COBE satellite in the infrared. In infrared you can see through some of the dust surrounding the galactic center and show the flattened shape of the galaxy with its central bulge. How many stars are in the Milky Way? Estimates are around 300 billion or 3 x 1011, perhaps plus or minus a 100 billion. Estimates are obtained by obtaining a measure of mass by analyzing rotation times and subtracting dark matter, then obtaining an average star mass. Uniform projections from the Hubble Ultra Deep Field lead to an estimate of 100 billion (1011) galaxies in the universe.
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. They may be open clusters such as the Pleiades or globular clusters such as M30. Sources
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Index Galaxy concepts Reference Gonzalez & Richards Ch. 8 | |||||
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Galaxy GeometryThere are many geometries of galaxies including the spiral galaxy characteristic of our own Milky Way. The above image is a segment of the remarkable deep field photograph made by the Hubble Space Telescope, every visible object except for the one obvious foreground star seems to be another galaxy. An even deeper view was obtained in the Hubble Ultra Deep Field in 2003-04. The Extreme Deep Field dated 2012 added greater resolution. 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.
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Index U. Iowa Hubble's tuning fork, skyserver Hubble sequence, Wiki Galaxy concepts | ||||||||||||||
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Seyfert GalaxiesThe 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. |
Index Galaxy concepts Reference Kaufmann | ||
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B L Lacertae ObjectsB 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. |
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BlasarsA 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. |
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