Pinwheel Galaxy
Encyclopedia : P : PI : PIN : Pinwheel Galaxy
| Galaxy | List of galaxies |
|---|---|
|
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! style="text-align:left;" | Apparent magnitude (V)
| +7.5
|-
! style="text-align:left;" | Apparent dimensions (V)
| 27' × 26'
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! style="text-align:left;" | Constellation
| Ursa Major
|-
! colspan="2" style="background-color: #30D5C8; text-align: center;" | Physical characteristics
|-
! style="text-align:left;" | Radius
| 85,000
|-
! style="text-align:left;" | Absolute magnitude (V)
| -21.6
|-
! style="text-align:left;" | Notable features
| }
|-
! colspan="2" style="text-align:left;" | Other designations
|-
| colspan="2" |
|} The Pinwheel Galaxy (also known as Messier 101 or NGC 5457) is a spiral galaxy about 27 million light years away in the constellation Ursa Major. M101 is the brightest member of the M101 group of galaxies. It was discovered by Pierre Méchain on March 27, 1781, and he subsequently communicated his discovery to Charles Messier who verified its position and added it to the Messier Catalogue as one of the final entries. On February 28, 2006, NASA and the ESA released a very detailed image of Pinwheel Galaxy, which was the largest and most detailed image of a galaxy by Hubble at the time [link]. The image was composed from 51 individual exposures, plus some extra ground-based photos. The recessional velocity of M101 is estimated to be between 330 and 450 km/s.
Contents
DiscoveryPierre Méchain, the discoverer of M101, described it as a "nebula without star, very obscure and pretty large, 6' to 7' in diameter, between the left hand of Bootes and the tail of the great Bear. It is difficult to distinguish when one lits the [grating] wires."SEDS Historical Notes,[link]http://www.seds.org/messier/Mdes/dm101.htmlWilliam Herschel noted in 1784 that "[M101] in my 7, 10, and 20-feet reflectors shewed (sic) a mottled kind of nebulosity, which I shall call resolvable; so that I expect my present telescope will, perhaps, render the stars visible of which I suppose them to be composed."SEDS Historical Notes,[link]http://www.seds.org/messier/Mdes/dm101.html Lord Rosse observed M101 in his 72-inch Newtonian reflector during the second half of the 19th century. He was the first to make extensive note of the spiral structure and made several sketches. SEDS Historical Notes,[link]http://www.seds.org/messier/Mdes/dm101.html To observe the spiral structure in modern instruments requires a fairly large instrument, very dark skies, and a low power eye piece. Structure and CompositionM101 is a relatively large galaxy compared to the Milky Way. With a diameter of 170,000 light years it is twice the size of the Milky Way. Less is known about the mass of M101. A frequently cited number is an equivalent mass of about 16 billion solar masses. That value is almost certainly too low, and probably stems from M101's low surface brightness. New insights in its HII regions and rotational velocities have put the number between 100 and 1000 billion suns.NASA, ESA (2006),[link]NASA, ESA (2006)Another remarkable property of this galaxy are its huge and extremely bright HII regions, of which a total of about 3000 can be seen on photographs. HII regions are places in galaxies that contain enormous clouds of high density hydrogen gas contracting under its own gravitational force. Eventually, when the localized hydrogen contracts enough for fusion processes to begin, stars are born. Consequently, HII regions are places that often contain large numbers of extremely bright and hot young stars giving them their characteristic blue color. On photographs M101 can be seen to be asymmetrical on one side. It is thought that in the recent past (speaking in galactic terms) M101 underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this galactic encounter also caused large gravity waves (not Einstein's gravitational waves) through the spirals of M101 compressing the interstellar hydrogen gas in some places greatly. This tremendous compression of the interstellar hydrogen clouds is believed to be the cause of M101's gigantic and active HII and star birth regions. Possible hypernovaeRecent investigations with the space-bound Chandra X-ray Observatory has revealed the possibility of several hypernova remnants located in M101, but a closer examinationSnowden et al.(2001), [link]. Reconsidering the identification of M101 hypernova remnant candidates between the optical counterparts and the X-ray sources as given by Chandra has put doubt on this identification. Nevertheless, the softer X-ray sources in the Chandra imageChandra X-Ray (2004),[link]Chandra X-Ray image of M101 with temperatures in the range of one to four million degrees Celsius, have been suggested to constitute a new class of midrange massive black holes.Other imagesReferencesSee alsoExternal links
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