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Astr110 Photography Projects, Spring 2005

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Spiral Galaxy M101, Tina Boljevac

Spiral Galaxy M101

M101 was discovered by P. Mechain in 1781, and described in the same year by Charles Messier as a "nebula without a star." Admiral Smyth, to his credit, described it as "one of those globular nebulae that seem to be caused by a vast agglomeration of stars."

M101 is a relatively nearby spiral galaxy in the constellation of Ursa Major near the tip of the handle of the Big Dipper. It is located about 3 degrees northeast of Zeta Ursae Majoris and about 3 degrees northwest of Eta Ursae Majoris. M101 is the prototype of the multiple-arm galaxies of the Sc type. It has well defined spiral arms, each of which can be traced from a branching of two principal dust arms that begin in the bright nucleus, which dominates the central mass. Recent evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to circle the galaxy. These waves compress existing gas and cause star formation. One result is that M101 has several extremely bright star-forming regions spread across its spiral arms, which makes it the most luminous member of a rich galaxy group. M101 is so large that its immense gravity distorts smaller nearby galaxies. Evidence shows that the mass of all of the stars in M101 adds up to about 16 billion Suns!

Various website sources (listed below) indicate that M101 covers a large angular size, nearly as large as the full moon (more precisely, about 2/3 of the full moon, or 22 arc minutes). However, according to my own measurements by using the image above, its angular size is 5.25 arc minutes. Although the website sources indicate the linear diameter of over 170, 000 light years, my calculation indicate the linear diameter of 41, 230.37 light years. This discrepancy is most likely due to the fact that there are dimmer portions of the galaxy that extend beyond the image given above.The distance of M101 has been determined by the measurement of Cepheid variables with the Hubble Space Telescope in 1994/1995 to be about 24+/- million light years. According to the more recent recalibration of the Cepheid distance scale, the true distance of M101is closer to 27 million light years. M101 has a linear diameter of over 170, 000 light years and is thus among the biggest disk galaxies, with the total apparent visual brightness of 7.9 mag.







Right Ascension (J2000) 14:03.2
Declination (J2000) +54:21
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 300 seconds in BVRC
Date observed

April 1, 2005 (BVRC)