# Messier Index/M51

The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting grand-design w:spiral galaxy located at a distance of approximately 31 million w:light-years in the w:constellation w:Canes Venatici. It is one of the most famous spiral galaxies in the sky. The galaxy and its companion (w:NGC 5195) are easily observed by amateur astronomers, and the two galaxies may even be seen with w:binoculars. The Whirlpool Galaxy is also a popular target for professional astronomers, who study it to further understanding of galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions.

## Discovery

What was later known as the Whirlpool Galaxy, was discovered in 1774 by w:Charles Messier, and is designated as M51. Its companion galaxy, w:NGC 5195, was discovered in 1781 by w:Pierre Méchain. It was however not until 1845 that the Whirlpool became the first to be recognized as a spiral. This was achieved by Lord Rosse employing a 72-inch (~1.83 m) reflecting telescope which he constructed at w:Birr Castle, Ireland. Sometimes M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).
In 2005 a w:supernova (w:SN 2005cs) was observed in the Whirlpool Galaxy, peaking at w:apparent magnitude 14.

## Properties

With the recent SN 2008cs derived estimate of 23 Mly distance, and an angular diameter of roughly 11.2′, it can be inferred that M51's bright circular disk has a radius of about ~38,000 light-years. Its w:mass is estimated to be 160 billion solar masses.
thumb|right|200px|The cross within the nucleus of M51 indicating two dust rings around the black hole at the center of the nebulaA w: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 nebula. A secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations. A pair of w:ionization cones extend from the axis of the main dust ring.

## Visual appearance

Located within the constellation w:Canes Venatici, M51 is found by following the easternmost star of the w:Big Dipper, w:Eta Ursae Majoris, and going 3.5° southeast. Its declination is +47°, making it a circumpolar for observers located above 43°N latitude; it reaches high altitudes throughout the northern hemisphere making it an accessible object from the early hours in winter through the end of spring season, after which observation is hindered in lower lattitudes.
M51 is visible through binoculars under dark sky conditions and can be resolved in detail with modern amateur telescopes. When seen through a 100 mm telescope the basic outlines of M51 and its companion are visible. Under dark skies, and with a moderate eyepiece through a 150 mm telescope, M51's intrinsic spiral structure can be detected. With larger (>300 mm) instruments under dark sky conditions, the various spiral bands are apparent with w:HII regions visible, and M51 can be seen to be attached to w:M51B.
As is usual for galaxies, the true extent of its structure can only be gathered from inspecting photographs; long exposures reveal a large nebula extending beyond the visible circular appearance.
In January 2005 the Hubble Heritage Team constructed a 11477x7965 pixel composite image (shown in the info box above) of M51 using Hubble's ACS instrument.

## Spiral structure

The very pronounced spiral structure of the Whirlpool Galaxy is believed to be the result of the close interaction between it and its companion galaxy w:NGC 5195.

## Star formation

Induced spiral structure in the larger galaxy isn't the only effect of the interaction. Significant compression of hydrogen gas occurs that leads to the development of starbirth regions. In pictures of M51 these show up as the bright blue 'knots' throughout the spiral arms.
Generally speaking, hydrogen gas is the most common component of the w:interstellar medium (the vast space between stars and planetary systems in galaxies). It exists primarily in its atomic and molecular form, and forms huge clouds throughout the entire galaxy. When large sources of gravitational pull pass nearby, such as other galaxies, gravitational interactions produce compression (density) waves that sweep through these hydrogen clouds. This causes some regions of the previously diffuse gas to compress into tight pockets of opaque and dense gas, these are dust lanes one so often sees in the spiral arms. In regions where the concentration and density of gas reaches a critical value, further collapse under its own gravitational pull occurs, and stars are born at the center of the collapse, where the gas is compressed so strongly that fusion initiates.
When this happens, these new-born stars gobble up huge amounts of gas causing them to expand, shine even hotter, and finally sweep away the surrounding layers of dust and gas by increasing efflux of the stellar wind. The gigantic proportions of the clouds out of which they are born means stars seldom, if ever, are created in isolation. Thus regions of several hot young stars emit sufficient light energy that they can be seen in the high resolution pictures of M51 across millions of lightyears distance.
For an example of such a formation in our own galaxy, see M16, the w:Eagle Nebula.

## Companion

Decades ago, it was not known with certainty whether the companion galaxy NGC 5195 was a true companion, or another galaxy passing at a distance. The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated the reality of the interaction.
Recent simulations bear out that M51's spiral structure was caused by NGC 5195 passing through the main disk of M51 about 500 to 600 million years ago. In this model, NGC 5195 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 Myrs ago until it is where we observe it to be now, slightly behind M51.

## Galaxy group information

The Whirlpool Galaxy is the brightest galaxy in the w:M51 Group, a small w:group of galaxies that also includes M63 (the Sunflower Galaxy), w:NGC 5023, and w:NGC 5229. This small group may actually be a subclump at the southeast end of a large, elongated group that includes the w:M101 Group and the w:NGC 5866 Group, although most group identification methods and catalogs identify the three groups as separate entities.

## M51 in Fiction

This galaxy is featured in the "w:Homeworld" RTS Franchise, which is home to a human-like race of beings called "Hiigarans". This galaxy is also in "w:The Genesis Quest" and "w:Second Genesis" by w:Donald Moffitt, and is the home galaxy for the aliens known as "The Nar".

## References

* ↑ R. W. Sinnott, editor (1988). The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer. Sky Publishing Corporation and Cambridge University Press. ISBN 0-933-34651-4. {{cite book}}: |author= has generic name (help)
* 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 "NASA/IPAC Extragalactic Database". Results for NGC 5194. Retrieved 2006-12-06.
* ↑ Takáts, K.; Vinkó, J. (2006). "Distance estimate and progenitor characteristics of SN 2005cs in M51". Monthly Notices of the Royal Astronomical Society, Online Early. 372: 1735. doi:10.1111/j.1365-2966.2006.10974.x.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* 1 2 H. Arp (1966). "Atlas of Peculiar Galaxies". Astrophysical Journal Supplement. 14: 1–20. doi:10.1086/190147.
* ↑ D. M. Elmegreen, B. G. Elmegreen (1987). "Arm classifications for spiral galaxies". Astrophysical Journal. 314: 3–9. doi:10.1086/165034.
* ↑ Nemiroff, Robert (2000-07-24). "Astronomy Picture of the Day". nasa.gov. Retrieved 2007-04-22. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
* ↑ "Catalogue des Nébuleuses & des amas d'Étoiles." by Charles Messier, Connoissance des Temps for 1784 (published 1781), pp. 227-267 (page 246) [Bibcode: 1781cote.rept..227M]
* ↑ MacRobert, Alan M. (w:August 24 w:2005). "Supernova in M51". Sky Tonight. Sky and Telescope. Retrieved August 7, 2006. {{cite web}}: Check date values in: |date= (help)
* ↑ Bishop, David. "Supernova 2005cs in M51". supernovae.net. Retrieved August 7, 2006.
* ↑ "NASA's Hubble Space Telescope Resolves a Dark "x" Across the Nucleus of M51". News Center. HubbleSite. w:June 8 w:1992. Retrieved August 7, 2006. {{cite web}}: Check date values in: |date= (help)
* ↑ "Out of This Whirl: the Whirlpool Galaxy (M51) and Companion Galaxy". News Center. HubbleSite. w:April 25 w:2005. Retrieved August 7, 2006. {{cite web}}: Check date values in: |date= (help)
* ↑ Salo, Heikki; Laurikainen, Eija (1999). "A Multiple Encounter Model of M51". Astrophysics and Space Science. 269/270: 663–664. doi:10.1023/A:1017002909665.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ R. B. Tully (1988). Nearby Galaxies Catalog. Cambridge: Cambridge University Press. ISBN 0-521-35299-1.
* ↑ P. Fouque, E. Gourgoulhon, P. Chamaraux, G. Paturel (1992). "Groups of galaxies within 80 Mpc. II - The catalogue of groups and group members". Astronomy and Astrophysics Supplement. 93: 211–233.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ A. Garcia (1993). "General study of group membership. II - Determination of nearby groups". Astronomy and Astrophysics Supplement. 100: 47–90.
* ↑ G. Giuricin, C. Marinoni, L. Ceriani, A. Pisani (2000). "Nearby Optical Galaxies: Selection of the Sample and Identification of Groups". Astrophysical Journal. 543: 178–194. doi:10.1086/317070.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ L. Ferrarese, H. C. Ford, J. Huchra, R. C. Kennicutt Jr., J. R. Mould, S. Sakai, W. L. Freedman, P. B. Stetson, B. F. Madore, B. K. Gibson, J. A. Graham, S. M. Hughes, G. D. Illingworth, D. D. Kelson, L. Macri, K. Sebo, N. A. Silbermann (2000). "A Database of Cepheid Distance Moduli and Tip of the Red Giant Branch, Globular Cluster Luminosity Function, Planetary Nebula Luminosity Function, and Surface Brightness Fluctuation Data Useful for Distance Determinations". Astrophysical Journal Supplement. 128: 431–459. doi:10.1086/313391.{{cite journal}}: CS1 maint: multiple names: authors list (link)

## External links

* SEDS: Spiral Galaxy M51
* NASA's APOD: The Whirlpool Galaxy in Dust and Stars (4/10/01)
* NightSkyInfo.com - The Whirlpool Galaxy
* Whirlpool Galaxy at ESA/Hubble
* The Whirlpool Galaxy (Messier 51(a)/NGC 5194)
* M51 The Whirlpool Galaxy