2020. 9. 10. 20:12ㆍSky observation
Messier 51, NGC 5194 - Whirlpool Galaxy
Messier 51 (M51, NGC 5194) is the famous "Whirlpool" galaxy in Canes Venatici. It is one of the most conspicuous and best-known spiral galaxies in the sky. M 51 is interacting with its much smaller neighbor, NGC 5195; the two galaxies may be seen with binoculars under very dark skies.
The Whirlpool Galaxy.
Discovery and History
Charles Messier discovered M 51 on 13 October 1773, and described it as a "very faint nebula, without stars". His friend Pierre Mechain discovered its companion, NGC 5195, in March 1781, and M 51 is mentioned in Messier's 1781 catalog as follows: "It is double, each has a bright center, which are separated 4' 35". The two 'atmospheres' touch each other, the one is even fainter than the other." Messier also added a sketch of the two "nebulae" in his personal copy of the catalog, confirming that he meant the designation M 51 to refer to the larger galaxy, NGC 5194. The smaller companion, NGC 5195, was later assigned its own catalog number by William Herschel: H I.186, and is also sometimes referred to as M 51B.
The Whirlpool's spiral pattern was not seen until 1845, when Lord Rosse discerned it in his 6-foot reflector at Parsonstown, Ireland. Lord Rosse made a very accurate painting of M 51; hence M 51 is sometimes referred to as Rosse's Galaxy or Lord Rosse's "Question Mark". At first its spiral pattern was thought to confirm Laplace's Nebular Hypothesis of solar system formation. This misconception was not dispelled until 1923, when it was finally recognized that spiral nebulae are in fact external galaxies, and much more remote that previous suspected.
Halton Arp included M 51 as No. 85, a "Spiral with Large High-Surface-Brightness Companion", in his Catalogue of Peculiar Galaxies.
Amateur Observation
M 51 is easy to find, 3.5° southeast of Eta Ursae Majoris, the easternmost star at the end of the Big Dipper's "handle". Its declination of +47° makes it a circumpolar object for most northern observers; it reaches high altitudes from the early hours in winter to the end of spring throughout the northern hemisphere.
M 51 is a showpiece if the sky is dark, but is quite sensitive to light pollution. With overall dimensions of 11' x 7', and a visual magnitude of 8.4, M 51 is visible in binoculars under dark sky conditions. M 51 has a bright core and a large, faint halo. Under very good conditions, suggestions of its spiral arms can be glanced with even a 4-inch telescope. Low magnification is best for viewing the pair.
With larger (12-inch) instruments, the spiral arms are quite visible, separated by dark swirls north and southwest of the core. The spiral arm east and northeast of the core is the most prominent. The various spiral bands and HII regions are visible with averted vision, as well as an apparent "bridge" between M 51 and NGC 5195. The latter has a core nearly as bright as, but smaller than, that of M 51. The true extent of M 51's structure can only be gathered from photographs; long exposures reveal a large halo extending beyond the visible circular appearance.
Bulk Properties
Two supernovae have been discovered in M 51 so far: SN 1994I and SN 2005cs. From observations of the 2005 supernova, M 51's distance is estimated at 23 million light-years. With this distance, and M 51's angular diameter of 11.2', the galaxy's bright circular disk has a radius of about 38,000 light-years. It is estimated to contain 160 billion solar masses.
M 51 is comparable to the Andromeda Galaxy (M 31) and to our own Milky Way in size, mass, and luminosity. And, like M 31 and the Milky Way, M 51 has a major satellite galaxy - NGC 5195. Visually, the two systems give the impression of being actually connected. But photographs reveal that this is not true, for they show that the dark dust lanes of the large spiral curve in front of the companion.
The Whirlpool Galaxy is the dominant member of a small group of galaxies that also includes M 63 (the Sunflower Galaxy), NGC 5023, and NGC 5229. This M 51 Group may actually be a "sub-clump" at the southeast end of a large, elongated group that includes the M 101 Group and the NGC 5866 Group; however, most catalogs identify the three groups as separate entities.
Structure and Evolution
The very pronounced spiral structure of the Whirlpool Galaxy is believed to be the result of the close interaction between M 51 and its companion galaxy NGC 5195. Decades ago, it was not known with certainty whether NGC 5195 was a true companion, or another galaxy passing at a distance. The advent of radio astronomy, and subsequent radio images of M 51, unequivocally demonstrated that their interaction is real.
Recent simulations support the hypothesis that M 51's spiral structure was caused by NGC 5195 passing through its main disk some 500 to 600 million years ago. In this model, NGC 5195 came from behind M 51, passed through the disk, and made another disk crossing 50 to 100 million years ago, moving to where we observe it now, slightly behind M 51.
Induced spiral structure in the larger galaxy isn't the only effect of the interaction. Significant compression of hydrogen gas leads to the development of starbirth regions. In pictures of M 51, these regions show up as bright blue knots throughout the spiral arms.
The Hubble Space Telescope has investigated the central region of M 51. A black hole, surrounded by a ring of dust, is thought to exist at the heart of the spiral. The dust ring stands almost perpendicular to the relatively flat spiral, and a secondary ring crosses the primary ring on a different axis.