Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 March 10 
Posted on 03/09/2003 9:34:42 PM PST by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 March 10
Explanation: Why do many galaxies appear as spirals? A striking example is M101, shown above, whose relatively close distance of about 27 million light years allow it to be studied in some detail. Recent evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to orbit the galaxy center. These waves compress existing gas and cause star formation. One result is that M101, also called the Pinwheel Galaxy, has several extremely bright star-forming regions (called HII regions) spread across its spiral arms. M101 is so large that its immense gravity distorts smaller nearby galaxies.
| Right Ascension | 14 : 03.2 (h:m) |
|---|---|
| Declination | +54 : 21 (deg:m) |
| Distance | 27000 (kly) |
| Visual Brightness | 7.9 (mag) |
| Apparent Dimension | 22.0 (arc min) |
Spectacular photos like the above one are always timed exposures. If you look at M101 through a telescope, say an 8" reflector, you will see something more like this:
Same galaxy M101 in ultraviolet light. It looks so different because this one shows mainly clouds of gas containing newly formed stars many times more massive than the sun, which glow strongly in ultraviolet light. In contrast, visible light pictures of galaxies tend to be dominated by the yellow and red light of older stars.
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