Discover

The 'Pinwheel Galaxy' (also known as 'Messier 101' or 'NGC 5457') is a face-on spiral galaxy about 27 million light-years away in the constellation Ursa Major.
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 Space Telescope at the time.^[1] The image was composed from 51 individual exposures, plus some extra ground-based photos.

Contents

Discovery

Structure and Composition

Possible hypernovae

Companion galaxies

See also

External links

References

Discovery

Pierre 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."^[2]
William Herschel noted in 1784 that "[M101] in my 7, 10, and 20-feet reflectors shewed 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."
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.
To observe the spiral structure in modern instruments requires a fairly large instrument, very dark skies, and a low power eye piece.

Structure and Composition

The Pinwheel Galaxy. Credit: Scott Anttila.

M101 is a relatively large galaxy compared to the Milky Way. With a diameter of 170,000 light-years it is nearly 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.^[3]
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 encounter also amplifies the density waves in the spiral arms of M101. The amplification of these waves also leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.

Possible hypernovae

Recent investigations with the space-bound Chandra X-ray Observatory has revealed the possibility of several hypernova remnants located in M101, but a closer examination^[4] 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 image^[5] 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.

Companion galaxies

M101 has five prominent companion galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585, and Holmberg IV Carnegie Atlas of Galaxies, A. Sandage, J. Bedke, , , Carnegie Institution of Washington, 1994, ISBN 0-87279-667-1 . As stated above, the gravitational interaction between M101 and its satellites may have triggered the formation of the grand design pattern in M101. M101 has also problably distorted the companion galaxy NGC 5474. M101 and its companion galaxies comprise most or possibly all of the M101 Group^[6][7][8][9].

See also

★ Messier 74 - ''a similar face-on spiral galaxy''

★ Messier 83 - ''a similar face-on spiral galaxy that is sometimes referred to as the Southern Pinwheel Galaxy''

★ Messier 99 - ''a similar face-on spiral galaxy''

★ A Wrinkle in Time - ''features Uriel, a fictional planet in the Pinwheel Galaxy''

External links

★ Messier 101, SEDS Messier pages

★ Extremely detailed picture of the Pinwheel Galaxy at HubbleSite.org

★ 'WIKISKY.ORG', SDSS image of M101

★ Pinwheel Galaxy at ESA/Hubble

★ Spiral Galaxy Messier 101 (Pinwheel Galaxy) at the astro-photography site of 'Mr. Takayuki Yoshida.'

References

1. http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/10/
2. SEDS Historical Notes, [1]
3. NASA, ESA (2006), [2] NASA, ESA (2006)
4. Snowden et al.(2001), [3]. Reconsidering the identification of M101 hypernova remnant candidates
5. Chandra X-Ray (2004), [4]. Chandra X-Ray image of M101
6. Nearby Galaxies Catalog, R. B. Tully, , , Cambridge University Press, 1988, ISBN 0-521-35299-1
7. Groups of galaxies within 80 Mpc. II - The catalogue of groups and group members, P. Fouque, E. Gourgoulhon, P. Chamaraux, G. Paturel, , , Astronomy and Astrophysics Supplement, 1992
8. General study of group membership. II - Determination of nearby groups, A. Garcia, , , Astronomy and Astrophysics Supplement, 1993
9. Nearby Optical Galaxies: Selection of the Sample and Identification of Groups, G. Giuricin, C. Marinoni, L. Ceriani, A. Pisani, , , Astrophysical Journal, 2000