# 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

# M52

Messier 52 (also known as M 52 or NGC 7654) is an w:open cluster in the w:Cassiopeia constellation. It was discovered by w:Charles Messier in w:1774. M52 can be seen from w:Earth with w:binoculars.

## External links

* Messier 52, SEDS Messier pages

# M53

Messier 53 (also known as M53, or NGC 5024) is a w:globular cluster in the w:Coma Berenices constellation. It was discovered by w:Johann Elert Bode in w:1775. M53 is one of the more outlying globular clusters, being about 60,000 w:light-years away from the w:Galactic Center, and almost the same distance (about 58,000 light-years) from the w:Solar system.

## External links

* Messier 53, SEDS Messier pages
* Messier 53, Galactic Globular Clusters Database page

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 5024. Retrieved 2006-11-16.

# M54

Messier 54 (also known as M54 or NGC 6715) is a w:globular cluster in the w:constellation Sagittarius. It was discovered by w:Charles Messier in w:1778 and subsequently included in his catalog of w:comet-like objects.
Previously thought to be at a distance from w:Earth of about 50,000 w:light-years, it was discovered in w:1994 that M54 was most likely not part of the w:Milky Way, but actually part of the w:Sagittarius Dwarf Elliptical Galaxy, (SagDEG), making it the first extragalactic globular cluster ever discovered, even if it wasn't recognized as such for nearly two and a quarter centuries.
Modern estimates now place M54 at a distance of some 87,000 light-years, translating into a true radius of 150 light-years across. It is one of the denser of the globulars, being of class III (I being densest and XII being the least dense). It shines with the w:luminosity of roughly 850,000 times that of the w:Sun and has an w:absolute magnitude of -10.0.
M54 is easily found on the sky, being close to the w:star ζ Sagittarii. It is however, not resolvable into individual stars even with larger amateur w:telescopes.
In July of 2009, a team of astronomers reported that they had found evidence of an w:intermediate-mass black hole in the core of M54. This was the first report of such a black hole in any globular cluster.

## External links

* M54 @ SEDS Messier pages

## References

* 1 2 3 4 5 6 "SIMBAD Astronomical Database". Results for NGC 6715. Retrieved 2006-11-16.
* ↑ Gavin Ramsay and Kinwah Wu, Chandra observations of the globular cluster M54
* ↑ distance × sin( diameter_angle / 2 ) = 153 ly. radius
* ↑ Geisler, Doug; Wallerstein, George; Smith, Verne V.; Casetti-Dinescu, Dana I. (September 2007), "Chemical Abundances and Kinematics in Globular Clusters and Local Group Dwarf Galaxies and Their Implications for Formation Theories of the Galactic Halo", The Publications of the Astronomical Society of the Pacific, 119 (859): 939-961, doi:10.1086/521990{{citation}}: CS1 maint: date and year (link)
* ↑ Siegel, Michael H.; Dotter, Aaron; Majewski, Steven R.; Sarajedini, Ata; Chaboyer, Brian; Nidever, David L.; Anderson, Jay; Marín-Franch, Antonio; Rosenberg, Alfred; Bedin, Luigi R.; Aparicio, Antonio; King, Ivan; Piotto, Giampaolo; Reid, I. Neill (September 2007), "The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy", The Astrophysical Journal, 667 (1): L57-L60, doi:10.1086/522003{{citation}}: CS1 maint: date and year (link)
* ↑ Ibata, R.; Bellazzini, M.; Chapman, S. C.; Dalessandro, E.; Ferraro, F.; Irwin, M.; Lanzoni, B.; Lewis, G. F.; Mackey, A. D.; Miocchi, P.; Varghese, A. (July 2009), "Density and Kinematic Cusps in M54 at the Heart of the Sagittarius Dwarf Galaxy: Evidence for A $10^{4}M_{sun}$ Black Hole?", The Astrophysical Journal Letters, 699 (2): L169-L173, doi:10.1088/0004-637X/699/2/L169{{citation}}: CS1 maint: date and year (link)

# M55

Messier 55 (also known as M55 or NGC 6809) is a w:globular cluster in the w:constellation Sagittarius. It was discovered by w:Nicolas Louis de Lacaille in w:1751 and catalogued by w:Charles Messier in w:1778. M55 is at a distance of about 17,300 w:light-years away from w:Earth. Only about half a dozen w:variable stars have been discovered in M55.

## External links

* Messier 55, SEDS Messier pages
* Messier 55, Galactic Globular Clusters Database page

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 6809. Retrieved 2006-11-16.
* ↑ distance × sin( diameter_angle / 2 ) = 48 ly. radius

# M56

Messier 56 (also known as M56 or NGC 6779) is a w:globular cluster in the w:constellation w:Lyra. It was discovered by w:Charles Messier in w:1779. M56 is at a distance of about 32,900 w:light-years from w:Earth and measures roughly 84 light-years across.
The brightest w:stars in M56 are of 13th magnitude while it contains only about a dozen known w:variable stars like V6 (RV Tauri star; period: 90 days) or V1 (Cepheid: 1.510 days); other variable stars are V2 (irregular) and V3 (semiregular).

## External links

* Messier 56, SEDS Messier pages
* Messier 56, Galactic Globular Clusters Database page

## References

* 1 2 3 "SIMBAD Astronomical Database". Results for NGC 6779. Retrieved 2006-11-16.
* ↑ w:Valentin D. Ivanov et al., Near Infrared Photometry of Galactic Globular Clusters M56 and M15. Extending the Red Giant Branch vs. Metallicity Calibration Towards Metal Poor Systems
* ↑ distance × sin( diameter_angle / 2 ) = 42 ly. radius

# M57

The famously named "Ring Nebula" is located in the northern w:constellation of w:Lyra, and also catalogued as Messier 57, M57 or NGC 6720. It is one of the most prominent examples of the deep-sky objects called w:planetary nebulae (singular, planetary nebula), often abbreviated by astronomers as simply planetaries or PN.

## Observation

M57 is located in w:Lyra, south of its brightest star w:Vega. w:Vega is the northeastern vertex of the three stars of the w:Summer Triangle. M57 lies about 40% of the angular distance from β Lyrae to γ Lyrae.
M57 is best seen through at least a 20 cm (8-inch) w:telescope, but even a 7.5 cm (3-inch) telescope will show the ring. Larger instruments will show a few darker zones on the eastern and western edges of the ring, and some faint nebulosity inside the disk.
This nebula was discovered by w:Antoine Darquier de Pellepoix in January, 1779, who reported that it was "...as large as Jupiter and resembles a planet which is fading." Later the same month, w:Charles Messier independently found the same nebula while searching for w:comets. It was then entered into his catalogue as the 57th object. Messier and w:William Herschel also speculated that the nebula was formed by multiple faint stars that were unable to resolve with his telescope.
In 1800, Count w:Friedrich von Hahn discovered the faint central star in the heart of the nebula. In 1864, w:William Huggins examined the spectra of multiple nebulae, discovering that some of these objects, including M57, displayed the spectra of bright w:emission lines characteristic of fluorescing glowing gases. Huggins concluded that most planetary nebulae were not composed of unresolved stars, as had been previously suspected, but were nebulosities.

## Evolution of planetary nebulae

Planetary nebulae are formed after medium or low mass stars, such as the Sun, exhaust their hydrogen fuel in the stellar core. At this point the structure of the star changes so it can achieve a new equilibrium condition in which it can continue to burn; the outer layers of the star expand and it becomes a w:red giant. Further internal temperature instabilities develop from the fusion reactions, causing the outer atmosphere to be expelled by hot superwinds either continuously or in several energetic pulses. This expanding gaseous shell forms the spherical nebula, brightly illuminated by w:ultraviolet energy from the central star.

## Properties

The nebula is located at 0.7 kpc (2,300 w:light-years) from w:Earth. It has a visual magnitude of 8.8v and w:photographic magnitude of 9.7p. Photographically, over a period of 50 years, the rate of nebula expansion is roughly 1 w:arcsecond per w:century, which corresponds from spectroscopic observations to 20–30 km$^{-1}$). M57 is illuminated by a central w:white dwarf or planetary nebula nucleus (PNN) of 15.75v visual magnitude, whose mass is approximately 1.2 MΘ (in w:solar masses.)
All the interior parts of this nebula have a blue-green tinge that is caused by the doubly-ionized w:oxygen w:emission lines at 495.7 and 500.7 nm. These observed so-called "w:forbidden lines" occur only in conditions of very low density containing a few atoms per cubic w:centimeter. In the outer region of the ring, part of the reddish hue is caused by w:hydrogen emission at 656.3 nm, forming part of the w:Balmer series of lines. Forbidden lines of ionized w:nitrogen or [N II] contributes to the reddishness at 654.8 and 658.3 nm.

### Nebula structure

M57 is an example of the class of planetary nebulae known as bipolar nebulae, whose thick equatorial rings visibly extend the structure through its main axis of symmetry. It appears to be a w:prolate spheroid with strong concentrations of material along its w:equator. From Earth, the symmetrical axis is viewed at about 30°. Overall, the observed nebulosity has been currently estimated to be expanding for approximately 1,610±240 years.
Structural studies find this planetary exhibits knots characterized by well developed symmetry. However, these are only silhouettes visible against the background emission of the nebula's equatorial ring. M57 may include internal w:N II emission lines located at the knots' tips that face the PNN; however, most of these knots are neutral and appear only in extinction lines. Their existence shows they are probably only located closer to the ionization front than those found in the Lupus planetary w:IC 4406. Some of the knots do exhibit well-developed tails which are often detectable in optical thickness from the visual spectrum.

### Planetary nebula nucleus (PNN)

The central PNN was discovered by Hungarian astronomer Jenő Gothard on September 1, 1886 from images taken at his observatory in Herény, near Szombathely (now part of Szombathely). Within the last two thousand years, the central star of the Ring Nebula has left the w:asymptotic giant branch after exhausting its supply of w:hydrogen fuel. Thus it no longer produces its energy through w:nuclear fusion and, in evolutionary terms, it is now becoming a compact w:white dwarf star.
The PNN now consists primarily of w:carbon and w:oxygen with a thin outer envelope composed of lighter elements. Its mass is about 0.61–0.62 w:solar mass, with a surface temperature of 125,000±5,000 K. Currently it is 200 times more luminous than the w:Sun, but its w:apparent magnitude is only +15.75.

## Notes

    ^a Radius = distance × sin(angular size / 2) = 2.3$^{+1.5}$$_{−0.7}$ kly * sin(230″ / 2) = 1.3$^{+0.8}$$_{−0.4}$ ly
    ^b 9 apparent magnitude - 5 * (log$_{10}$(700$^{+450}$$_{−200}$ pc distance) - 1) = -0.2$^{+0.7}$$_{−1.8}$ absolute magnitude

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for Messier 57. Retrieved 2006-12-19.
* 1 2 3 O'Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (2002). "Knots in Nearby Planetary Nebulae". The Astronomical Journal. 123 (6): 3329–3347. doi:10.1086/340726.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ Harris, Hugh C.; Dahn, Conard C.; Canzian, Blaise; Guetter, Harry H.; Leggett, S. K.; Levine, Stephen E.; Luginbuhl, Christian B.; Monet, Alice K. B.; Monet, David G.; Pier, Jeffrey R.; Stone, Ronald C.; Tilleman, Trudy; Vrba, Frederick J.; Walker, Richard L. (February 2007). "Trigonometric Parallaxes of Central Stars of Planetary Nebulae". The Astronomical Journal. 133 (2): 631–638. doi:10.1086/510348.{{cite journal}}: CS1 maint: date and year (link) CS1 maint: multiple names: authors list (link)
* ↑ Murdin, P. (2000). "Ring Nebula (M57, NGC 6720)". Encyclopedia of Astronomy and Astrophysics, Edited by Paul Murdin, article 5323. Bristol: Institute of Physics Publishing, 2001. Http://eaa.iop.org/abstract/0333750888/5323. doi:10.1888/0333750888/5323. {{cite journal}}: External link in |journal= (help)
* 1 2 Crossen, Craig (2004). Sky Vistas: Astronomy for Binoculars and Richest-Field Telescopes. Springer. ISBN 3211008519. OCLC 52424007. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
* ↑ Garfinkle, Robert A. (1997). Star-hopping: Your Visa to Viewing the Universe. Cambridge University Press. ISBN 0521598893. OCLC 37355269.
* ↑ Messier, Charles (1780). Catalogue des Nébuleuses & des amas d'Étoiles, Connoissance des Temps for 1783. pp. 225–249.
* ↑ Frommert, Hartmut. "William Huggins (February 7, 1824 - May 12, 1910)". Students for the Exploration and Development of Space. Archived from the original on 2008-04-21. Retrieved 2008-04-11. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
* ↑ Huggins, W. (1863–1864). "On the Spectra of Some of the Nebulae. And On the Spectra of Some of the Fixed Stars". Proceedings of the Royal Society of London. 13: 491–493. doi:10.1098/rspl.1863.0094. Retrieved 2007-02-04. {{cite journal}}: Check date values in: |year= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
* ↑ De Loore, C. W. H. (1992). Structure and Evolution of Single and Binary Stars. Springer. ISBN 0792317688. OCLC 25711985. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
* 1 2 Karttunen, Hannu (2003). Fundamental Astronomy. Springer. p. 314. ISBN 3540001794.
* 1 2 O'Dell, C. R. (2007). "The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula". The Astronomical Journal. 134 (4): 1679–1692. doi:10.1086/521823. Retrieved 2008-03-31. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
* ↑ O'dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (2003). "Knots in Planetary Nebulae". Winds, Bubbles, and Explosions: a conference to honor John Dyson, Pátzcuaro, Michoacán, México, September 9-13, 2002 (Eds. S. J. Arthur & W. J. Henney) Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) (http://www.astroscu.unam.mx/~rmaa/). 15: 29–33. {{cite journal}}: External link in |journal= (help)CS1 maint: multiple names: authors list (link)

## External links

* SEDS: Messier Object 57
* Infrared Ring Nebula, APOD 2005 March 11
* NightSkyInfo.com - The Ring Nebula
* M57 The Ring Nebula in Lyra
* M57 at ESA/Hubble
* M57 imaged with a semiprofessional amateur-telescope

# M58

Messier 58 (also known as M58 and NGC 4579) is a w:barred spiral galaxy located approximately 68 million w:light-years away in the w:constellation Virgo. It was discovered by w:Charles Messier in w:1779. M58 is one of the brightest galaxies in the w:Virgo Cluster. Two w:supernovae, w:SN 1988A and w:SN 1989M, have been observed in M58.

## External links

* SEDS Messier: M58
* Spitzer Space Telescope page on Messier 58

## 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 "NASA/IPAC Extragalactic Database". Results for NGC 4579. Retrieved 2006-10-05.
* ↑ G. Gavazzi, A. Boselli, M. Scodeggio, D. Pierini and E. Belsole (1999). "The 3D structure of the Virgo cluster from H-band Fundamental Plane and Tully-Fisher distance determinations". Monthly Notices of the Royal Astronomical Society. 304: 595–610. doi:10.1046/j.1365-8711.1999.02350.x.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ Burnham, Robert Jr (1978). Burnham's Celestial Handbook: Volume Three, Pavo Through Vulpecula. Dover. pp. 2086–2088. ISBN 0-486-23673-0.
* ↑ "Messier Object 58". Archived from the original on 1996-12-25. Retrieved 2006-11-18.

# M59

Messier 59 (also known as M59 or NGC 4621) is an w:elliptical galaxy in the constellation Virgo.

## History

Messier 59 and the nearby elliptical galaxy w:Messier 60 were both discovered by w:Johann Gottfried Koehler in April w:1779 during observations of a w:comet in the same part of the sky. w:Charles Messier listed both in the w:Messier Catalogue about three days after Koehler's discovery.

## Virgo Cluster membership

M59 is a member of the w:Virgo Cluster.

## External links

* Elliptical Galaxy M59 @ SEDS Messier pages

## 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 "NASA/IPAC Extragalactic Database". Results for NGC 4621. Retrieved 2006-11-18.
* ↑ J. L. Tonry, A. Dressler, J. P. Blakeslee, E. A. Ajhar, A. B. Fletcher, G. A. Luppino, M. R. Metzger, C. B. Moore (2001). "The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances". Astrophysical Journal. 546 (2): 681–693. doi:10.1086/318301.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* 1 2 K. G. Jones (1991). Messier's Nebulae and Star Clusters (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-37079-5.

# M60

Messier 60 (also known as NGC 4649) is an w:elliptical galaxy approximately 55 million w:light-years away in the w:constellation Virgo.

## History

Messier 60 and the nearby galaxy w:Messier 59 were both discovered by w:Johann Gottfried Koehler in April w:1779 during observations of a w:comet in the same part of the sky. w:Charles Messier listed both in the w:Messier Catalogue about three days after Koehler's discovery.

## NGC 4647

w:NGC 4647 appears approximately 2′.5 away from Messier 60; the optical disks of the two galaxies overlap. Although this overlap suggests that the galaxies are interacting, photographic images of the two galaxies do not reveal any evidence for gravitational interactions between the two galaxies as would be suggested if the two galaxies were physically close to each other. This suggests that the galaxies are at different distances and are only weakly interacting if at all.

## Virgo Cluster membership

M60 is the third-brightest giant elliptical galaxy of the w:Virgo cluster of galaxies, and is the dominant member of a subcluster of four galaxies, which is the closest-known isolated compact group of galaxies.

## Supernovae

A w:supernova (w:SN 2004W) was observed in Messier 60.

## Black Hole

At the center of M60 is a w:black hole of 4.5 billion solar masses, one of the largest ever found.

## External links

* Elliptical Galaxy M60 @ SEDS Messier pages

## 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 "NASA/IPAC Extragalactic Database". Results for NGC 4649. Retrieved 2006-12-13.
* ↑ J. L. Tonry, A. Dressler, J. P. Blakeslee, E. A. Ajhar, A. B. Fletcher, G. A. Luppino, M. R. Metzger, C. B. Moore (2001). "The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances". Astrophysical Journal. 546 (2): 681–693. doi:10.1086/318301.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* 1 2 K. G. Jones (1991). Messier's Nebulae and Star Clusters (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-37079-5.
* 1 2 A. Sandage, J. Bedke (1994). Carnegie Atlas of Galaxies. Washington, D.C.: Carnegie Institution of Washington. ISBN 0-87279-667-1.
* ↑ http://arxiv.org/abs/0910.4168 The Supermassive Black Hole and Dark Matter Halo of NGC 4649 (M60)

# M61

Messier 61 (also known as M61 or NGC 4303) is a w:spiral galaxy in the w:Virgo Cluster. It was discovered by w:Barnabus Oriani on w:May 5, w:1779.
M61 is one of the larger members of the Virgo Cluster.
Six supernovae have been observed in this galaxy:
* SN 2008in
* SN 2006ov
* SN 1999gn
* SN 1964F
* SN 1961I
* SN 1926A

## External links

* www.seds.org/messier/m/m061.html

## References

* ↑ Tschöke, D. (2000). "X-rays from the barred galaxy NGC 4303". Astronomy and Astrophysics. EDP Sciences. 360 (2): 447–456. ISSN 0004-6361. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
* 1 2 3 4 5 6 7 8 9 10 "NASA/IPAC Extragalactic Database". Results for M 61. Retrieved 2006-11-18.
* ↑ "List of Supernovae". w:Central Bureau for Astronomical Telegrams. Retrieved 30 December 2008.

# M62

Messier 62 (also known as M62 or NGC 6266) is a w:globular cluster in the w:constellation w:Ophiuchus. It was discovered in w:1771 by w:Charles Messier.
M62 is at a distance of about 22,500 w:light-years from w:Earth and measures some 100 light-years across. From studies conducted in the 1970s it is known that M62 contains the high number of 89 w:variable stars, many of them of the w:RR Lyrae type. The globular also contains a number of w:x-ray sources, thought to be close w:binary star systems, and millisecond w:pulsars in binary systems.

## External links

* Messier 62, SEDS Messier pages
* Messier 62, Galactic Globular Clusters Database page
* M62 on willig.net

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 6266. Retrieved 2006-11-17.
* ↑ distance × sin( diameter_angle / 2 ) = 49 ly. radius

# M63

The Sunflower Galaxy (also known as Messier 63, M63, or NGC 5055) is an w:unbarred spiral galaxy in the w:Canes Venatici w:constellation. It is a w:flocculent spiral galaxy, consisting of a central disc surrounded by many short spiral arm segments. The Sunflower Galaxy is part of the w:M51 Group, a group of galaxies that also includes the Whirlpool Galaxy (M51).

## History

The Sunflower Galaxy was discovered by w:Pierre Méchain on w:June 14 w:1779. The galaxy was then listed by w:Charles Messier as object 63 in the w:Messier Catalogue.
In the mid-1800s, w:Lord Rosse identified spiral structure within the galaxy, making this one of the first galaxies in which such structure was identified.

## External links

* Sunflower Galaxy @ SEDS Messier pages

## References

* 1 2 3 4 5 6 7 "NASA/IPAC Extragalactic Database". Results for NGC 5055. Retrieved 2006-10-10.
* ↑ Frommert, Hartmut & Kronberg, Christine (2002). "Messier Object 63". Retrieved Dec. 26, 2006
* 1 2 K. G. Jones (1991). Messier's Nebulae and Star Clusters (2nd edition ed.). Cambridge: Cambridge University Press. ISBN 0-521-37079-5. {{cite book}}: |edition= has extra text (help)

# M64

The Black Eye Galaxy (also called Sleeping Beauty Galaxy; designated Messier 64, M64, or NGC 4826) was discovered by w:Edward Pigott in March 1779, and independently by w:Johann Elert Bode in April of the same year, as well as by w:Charles Messier in 1780. It has a spectacular dark band of absorbing dust in front of the galaxy's bright nucleus, giving rise to its nicknames of the "Black Eye" or "Evil Eye" galaxy. M64 is well known among amateur astronomers because of its appearance in small w:telescopes. It is a w:spiral galaxy in the Coma Berenices constellation.

## General information

At first glance, M64 seems to be a fairly normal spiral galaxy. As in the majority of galaxies, all of the stars in M64 are orbiting in the same direction, clockwise as seen in the Hubble image.
However, recent detailed studies have led to the remarkable discovery that the interstellar gas in the outer regions of M64 rotates in the opposite direction from the gas and stars in the inner regions. The inner region has a radius of only approximately 3,000 light-years, while the outer section extends another 40,000 light-years. This pattern is believed to trigger the creation of many new stars around the boundary separating the two regions.
A collision of two galaxies has left a merged star system with an unusual appearance as well as bizarre internal motions. Astronomers believe that the oppositely rotating gas arose when M64 absorbed a satellite galaxy that collided with it, perhaps more than one billion years ago. Active formation of new stars is occurring in the shear region where the oppositely rotating gases collide, are compressed, and contract.
Particularly noticeable in the image are hot, blue young stars that have just formed, along with pink clouds of glowing hydrogen gas that fluoresce when exposed to w:ultraviolet light from newly formed stars. It is approximately 17 million light years from earth.
The small galaxy that impinged on its neighbour has now been almost completely destroyed, its stars either merged with the main galaxy or scattered into space, but signs of the collision persist in the backward motion of gas at the outer edge of M64.

## External links

* Messier 64, SEDS Messier pages
* Hubblesite description and high resolution images
* ESA/Hubble images of M64
* APOD for August 2, 2007
* a real photo by Pete Albrecht

## References

* ↑ J. L. Tonry, A. Dressler, J. P. Blakeslee, E. A. Ajhar, A. B. Fletcher, G. A. Luppino, M. R. Metzger, C. B. Moore (2001). "The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances". Astrophysical Journal. 546 (2): 681–693. doi:10.1086/318301.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ 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 "NASA/IPAC Extragalactic Database". Results for NGC 4826. Retrieved 2006-11-06.
* ↑ "Centre de Données astronomiques de Strasbourg". Results for M 64 -- Seyfert Galaxy. Retrieved 2008-07-09.

# M65

Messier 65 (also known as NGC 3623) is an w:intermediate spiral galaxy about 22 million w:light-years away in the w:constellation Leo. It was discovered by w:Charles Messier in w:1780. M65, M66, and w:NGC 3628 comprise the famous w:Leo Triplet, a small w:group of galaxies.

## Discovery

M65 was discovered by Charles Messier and included in his w:Messier Objects list. However, w:William Henry Smyth accidentally attributed the discovery to w:Pierre Méchain in his popular 19th century astronomical work A Cycle of Celestial Objects (stating "They [M65 and M66] were pointed out by Méchain to Messier in 1780"). This error was in turn picked up by w:Kenneth Glyn Jones in w:Messier's Nebulae and Star Clusters. This has since ramified into a number of other books by a variety of authors.

## Star Formation

The galaxy is low in dust and gas, and there is little star formation in it, although there has been some relatively recently in the arms. The ratio of old stars to new stars is correspondingly quite high. In most wavelengths it is quite uninteresting, though there is a radio source visible in the w:NVSS, offset from the core by about two arc-minutes. The identity of the source is uncertain, as it has not been identified visually, or formally studied in any published papers.

## Interaction with Other Galaxies

To the eye, M65's disk appears slightly warped, and its relatively recent burst of star formation is also suggestive of some external disturbance. Rots (1978) suggests that the two other galaxies in the Leo Triplet interacted with each other about 800 million years ago. Recent research by w:Zhiyu Duan suggests that M65 may also have interacted, though much less strongly. He also notes that M65 may have a central bar -- it is difficult to tell because the galaxy is seen from an oblique angle -- a feature which is suggestive of tidal disruption.

## External links

* SEDS Messier: Spiral Galaxy M65

## References

* 1 2 3 4 5 6 7 8 "NASA/IPAC Extragalactic Database". Results for. Retrieved 2006-10-21.

# M66

Messier 66 (also known as NGC 3627) is an w:intermediate spiral galaxy about 36 million w:light-years away in the w:constellation Leo. It was discovered by w:Charles Messier in w:1780. M66 is about 95 thousand light-years across with striking dust lanes and bright star clusters along sweeping spiral arms.
M66 is part of the famous w:Leo Triplet, a small w:group of galaxies that also includes M65 and w:NGC 3628.

## History

Gravitational interaction from its past encounter with neighboring NGC 3628 has resulted in:
* An extremely high central mass concentration;
* A high molecular to w:atomic mass ratio;
* A resolved noncorotating clump of w:H I material apparently removed from one of the w:spiral arms.
This third result shows up visually as an extremely prominent and unusual spiral arm and dust lane structures as originally noted in the w:Atlas of Peculiar Galaxies.

## External links

* SEDS: Spiral Galaxy M66
* Spiral Galaxy M66

## References

* 1 2 3 4 5 6 "NASA/IPAC Extragalactic Database". Results for NGC 3627. Retrieved 2006-08-31.
* ↑ G. de Vaucouleurs, A. de Vaucouleurs, H. G. Corwin, R. J. Buta, G. Paturel, P. Fouque (1991). Third Reference Catalogue of Bright Galaxies. New York: Springer-Verlag.{{cite book}}: CS1 maint: multiple names: authors list (link)
* ↑ 36.2 mly × tan( 9′.1 ) = ~96 kly. radius
* 1 2 Zhang, Xiaolei; Wright, Melvyn; Alexander, Paul (1993). "High-Resolution CO and H i Observations of the Interacting Galaxy NGC 3627". Astrophysical Journal. 418: 100. doi:10.1086/173374.{{cite journal}}: CS1 maint: multiple names: authors list (link)

# M67

Messier 67 (also known as M67 or NGC 2682) is an w:open cluster, or galactic cluster, in the constellation Cancer. M67's w:Trumpler class is variously given as II 2 r, II 2 m, or II 3 r. It was discovered by w:Johann Gottfried Koehler in w:1779. Its age is estimated at between 3.2 and 5 billion years. The most recently estimated age of four billion years appears to be the most reliable; thus the stars of M67 are most likely slightly younger than the w:Sun.
M67 is not the oldest known open cluster, but there are very few in the galaxy known to be older. M67 is an important laboratory for studying w:stellar evolution, since all its stars are at the same distance and age, except for approximately 30 anomalous w:blue stragglers, whose origins are not fully understood.
M67 is one of the most-studied open clusters, yet estimates of its physical parameters such as age, mass, and number of stars of a given type, vary substantially. Richer et al. estimate its age to be 4 Gyr, its mass to be 1080 solar masses, and the number of w:white dwarfs to be 150. Hurley et al. estimate its current mass to be 1400 solar masses and its initial mass to be approximately 10 times as great.
M67 has more than 100 stars similar to the Sun, and many red giants. The total star count has been estimated at over 500. The cluster contains no w:main sequence stars bluer than spectral type F, other than perhaps some of the blue stragglers, since the brighter stars of that age have already left the main sequence. In fact, when the stars of the cluster are plotted on the w:Hertzsprung-Russell diagram, there is a distinct "turn-off" representing the stars which are just about to leave the main sequence and become red giants. As the cluster ages, the turn-off will move down the main sequence.
It appears that M67 does not contain an unbiased sample of stars. One cause of this is w:mass segregation, the process by which lighter stars (actually, systems) gain speed at the expense of more massive stars during close encounters, which causes the lighter stars to be at a greater average distance from the center of the cluster or to escape altogether.

## External links

* Messier 67, SEDS Messier pages

## References

* ↑ S1280 and S1284: Two Oscillating Blue Stragglers in the Open Cluster M67, Xiao-Bin Zhang, Rong-Xian Zhang and Zhi-Ping Li, Chin. J. Astron. Astrophys. Vol. 5 (2005), No. 6, 579–586.
* ↑ The White Dwarf Cooling Age of M67, Harvey B. Richer, Gregory G. Fahlman, Joanne Rosvick, Rodrigo Ibata, Astrophysical Journal Letters v.504, p.L91 (1998)
* ↑ A Complete N-body Model of the Old Open Cluster M67, Jarrod R. Hurley, Onno R. Pols, Sverre J. Aarseth, Christopher A. Tout, Mon.Not.Roy.Astron.Soc. 363 (2005) 293-314
* ↑ [ http://www.aanda.org/articles/aa/pdf/2003/26/aah4157.pdf] Mass segregation in M67 with 2MASS, Ch. Bonatto and E. Bica, (2003) Astron. Astrophys. 405, 525

# M68

Messier 68 (also known as M68 or NGC 4590) is a w:globular cluster in the w:Hydra constellation. It was discovered by w:Charles Messier in w:1780. M68 is at a distance of about 33,000 w:light-years away from w:Earth.

## External links

* Globular Cluster M68 @ SEDS Messier pages
* Messier 68, Galactic Globular Clusters Database page

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 4590. Retrieved 2006-11-17.
* ↑ distance × sin( diameter_angle / 2 ) = 53 ly. radius

# M69

Messier 69 (also known as M69 or NGC 6637) is a w:globular cluster in the w:constellation Sagittarius. It was discovered by w:Charles Messier in August 31, w:1780, the same night he discovered M70. At the time, he was searching for an object described by LaCaille in 1751-2 and thought he had rediscovered it, but it is unclear if LaCaille actually described M69.
M69 is at a distance of about 29,700 w:light-years away from w:Earth and has a spatial radius of 42 light-years. It is a close neighbor of w:Globular Cluster M70, 1,800 light-years separating the two objects, and both these clusters lie close to the Galactic Center. It is one of the most metal-rich globular cluster known.

## External links

* Globular Cluster M69 @ SEDS Messier pages
* Messier 69, Galactic Globular Clusters Database page

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 6637. Retrieved 2006-11-17.
* ↑ distance × sin( diameter_angle / 2 ) = 42 ly. radius

# M70

Messier 70 (also known as M70 or NGC 6681) is a w:globular cluster in the w:constellation Sagittarius. It was discovered by w:Charles Messier in w:1780.
M70 is at a distance of about 29,300 w:light years away from w:Earth and close to the w:Galactic Center. It is roughly the same size and w:luminosity as its neighbour in space, w:Globular Cluster M69. Only 2 w:variable stars are known within this cluster.

## External links

* Messier 70, SEDS Messier pages
* Messier 70, Galactic Globular Clusters Database page

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 6681. Retrieved 2006-11-17.
* ↑ distance × sin( diameter_angle / 2 ) = 34 ly. radius

# M71

Messier 71 (also known as M71 or NGC 6838) is a w:globular cluster in the w:constellation w:Sagitta. It was discovered by w:Philippe Loys de Chéseaux in 1746 and included by w:Charles Messier in his catalog of w:comet-like objects in 1780. It was also noted by Koehler at w:Dresden around 1775.
M71 is at a distance of about 12,000 w:light years away from w:Earth and spans some 27 light years across. The irregular w:variable star Z Sagittae is a member of this cluster.
M71 was long thought (until the 1970s) to be a densely packed w:open cluster and was classified as such by leading w:astronomers in the field of star cluster research due to its lacking a dense central compression , its stars having more "metals" than is usual for an ancient globular cluster, and further its lacking the RR Lyrae "cluster" variable stars that are common in most globulars. However, modern photometric photometry has detected a short "w:horizontal branch" in the H-R diagram of M71, which is characteristic of a globular cluster. The shortness of the branch explains the lacking of the RR Lyrae variables and is due to the globular's relatively young age of 9-10 billion years. The relative youth of this globular also explains the abundance of "metals" in its stars. Hence today, M71 is designated as a very loosely concentrated globular cluster, much like M68 in Hydra. M71 has a luminosity of around 13,200 suns.

## External links

* Messier71 @ SEDS Messier pages
* Messier 71, Galactic Globular Clusters Database page
* Messier 71, LRGB CCD image based on two hours total exposure

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 6838. Retrieved 2006-11-17.
* ↑ distance × sin( diameter_angle / 2 ) = 13 ly. radius

# M72

Messier 72 (also known as M72 or NGC 6981) is a w:globular cluster in the Aquarius constellation discovered by w:Pierre Méchain in w:August 29 w:1780. w:Charles Messier looked for it on the following w:October 4 and 5, and included it in his catalog. Both decided that it was a faint nebula not a cluster as is now believed. Using 10-inch (250 mm) telescopes, viewing the cluster is difficult resulting in only a view of a faint blurry picture, However using Kopernicks' 20-inch (510 mm) telescope resolution is highly increased. M72 is located at about 53,000 w:light-years away from w:Earth and lies in a considerable distance beyond the w:Galactic Center. Another source states that the cluster is 62,000 light-years away, with a diameter of 42 light-years. Generally considered a young cluster, the cluster has several blue giants, yet star clusters generally contain the oldest stars.

## External links

* Messier 72, SEDS Messier pages
* Messier 72, Galactic Globular Clusters Database page
* Messier 72, LRGB CCD image based on two hours total exposure
* M-72 Information

## References

* 1 2 3 "SIMBAD Astronomical Database". Results for NGC 6981. Retrieved 2006-11-16.

# M73

Messier 73 (also known as M73 and NGC 6994) is an asterism of four w:stars in the constellation of Aquarius. An asterism is comprised of physically unconnected stars that appear close to each other in the sky as seen from Earth. M73 is one of the best-known asterisms in the sky, and it has been carefully studied scientifically.

## History

M73 was discovered by w:Charles Messier on w:October 4, w:1780, who originally described the object as a cluster of four stars with some nebulosity. Subsequent observations by w:John Herschel, however, failed to reveal any nebulosity. Moreover, Herschel noted that the designation of M73 as a cluster was questionable. Nonetheless, Herschel included M73 in his General Catalogue of clusters, w:nebulae, and w:galaxies, and John Dreyer included M73 when he compiled the w:New General Catalogue.

## Scientific Research: Asterism or Open Cluster?

M73 was once treated as a potential sparsely populated w:open cluster, which consists of stars that are physically associated in space as well as on the sky. The question of whether the stars were an asterism or an open cluster generated a small, interesting debate.
In 2000, L. P. Bassino, S. Waldhausen, and R. E. Martinez published an analysis of the colors and luminosities of the stars in and around M73. They concluded that the four bright central stars and some other nearby stars followed the color-luminosity relation that is also followed by stars in open clusters (as seen in a w:Hertzsprung-Russell diagram). Their conclusion was that M73 was an old open cluster that was 9 arcmin wide. G. Carraro, however, published results in 2000 based on a similar analysis and concluded that the stars did not follow any color-luminosity relation. Carraro's conclusion was that M73 was an asterism. Adding to the controversy, E. Bica and collaborators concluded that the chance alignment of the four bright stars seen in the center of M73 as well as one other nearby star was highly unlikely, so M73 was probably a sparse open cluster. The controversy was solved in 2002, when M. Odenkirchen and C. Soubiran published an analysis of the high resolution w:spectra of the six brightest stars within 6 arcmin of the central position of M73. Odenkirchen and Soubiran demonstrated that the distances from the Earth to the six stars were very different from each other, and the stars were moving in different directions. Therefore, they concluded that the stars were only an asterism.
Although M73 was determined to be only a chance alignment of stars, further analysis of asterisms is still important for the identification of sparsely populated open clusters. Such clusters can be important for demonstrating how open clusters are ripped apart by the gravitational forces in the w:Milky Way.

## Location

The asterism's location in the sky is shown in the following map of the constellation Aquarius:

## External links

* Messier 73, SEDS Messier pages
* Messier 73, LRGB CCD image based on two-hours total exposure

## References

* 1 2 3 4 5 "SIMBAD Astronomical Database". Results for Messier 73. Retrieved 2006-12-21.
* ↑ K. G. Jones (1991). Messier's Nebulae and Star Clusters (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-37079-5.
* ↑ L. P. Bassino, S. Waldhausen, & R. E. Martinez (2000). "CCD photometry in the region of NGC 6994: The remains of an old open cluster". Astronomy & Astrophysics. 355: 138–144.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ G. Carraro (2000). "NGC 6994: An open cluster which is not an open cluster". Astronomy & Astrophysics. 357: 145–148.
* ↑ E. Bica, B. X. Santiago, C. M. Dutra, H. Dottori, M. R. de Oliveira, & D. Pavani (2001). "Dissolving star cluster candidates". Astronomy & Astrophysics. 366: 827–833. doi:10.1051/0004-6361:20000248.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ M. Odenkirchen & C. Soubiran (2002). "NGC 6994: - clearly not a physical stellar ensemble". Astronomy & Astrophysics. 383: 163–170. doi:10.1051/0004-6361:20011730.

# M74

Messier 74 (also known as NGC 628) is a face-on w:spiral galaxy in the w:constellation Pisces. The galaxy contains two clearly-defined w:spiral arms and is therefore used as an archetypal example of a w:Grand Design Spiral Galaxy. The galaxy's low surface brightness makes it the most difficult Messier object for amateur astronomers to observe. However, the relatively large angular size of the galaxy and the galaxy's face-on orientation make it an ideal object for professional astronomers who want to study w:spiral arm structure and spiral density waves.

## History

M74 was discovered by w:Pierre Méchain in w:1780. Méchain then communicated his discovery to w:Charles Messier, who listed the galaxy in his catalog.

## Supernovae

Two w:supernovae have been identified in M74: w:SN 2002ap and w:SN 2003gd.
SN 2002ap has attracted considerable attention because it is one of the few Type Ic supernovae (or w:hypernovae) observed within 10 Mpc in recent years. This supernovae has been used to test theories on the origins of similar Type Ic supernovae at higher distances and theories on the connection between supernovae and w:gamma ray bursts.
SN 2003gd is a Type II-P supernova. Type II supernovae have known luminosities, so they can be used to accurately measure distances. The distance measured to M74 using SN 2003gd is 9.6 ± 2.8 Mpc, or 31 ± 9 million ly. For comparison, distances measured using the brightest supergiants are 7.7 ± 1.7 Mpc and 9.6 ± 2.2 Mpc. Ben E. K. Sugerman found a "light echo" - a reflection of supernova explosion that appeared after the explosion itself - associated with SN 2003gd. This is one of the few supernovae in which such a reflection has been found. This reflection appears to be from dust in a sheet-like cloud that lies in front of the supernova, and it can be used to determine the composition of the interstellar dust.

## Galaxy group information

M74 is the brightest member of the w:M74 Group, a group of 5-7 galaxies that also includes the peculiar w:spiral galaxy w:NGC 660 and a few w:irregular galaxies. Although different group identification methods may consistently identify many of the same member galaxies in this group, the exact group membership is still uncertain.

## Star formation

* M74 as observed with the w:Spitzer Space Telescope as part of the w:Spitzer Infrared Nearby Galaxy Survey. The blue colors represent the 3.6 micrometre emission from stars. The green and red colors represent the 5.8 and 8.0 micrometre emission from w:polycyclic aromatic hydrocarbons and possibly dust.

## Suspected black hole

In w:March 22, w:2005, it was announced that the w:Chandra X-ray Observatory had observed an w:ultraluminous X-ray source (ULX) in M74, radiating more X-ray power than a w:neutron star in periodic intervals of around two hours. It has an estimated mass of around 10,000 w:Suns. This is an indicator of an w:intermediate-mass black hole. This would be a rather uncommon class of black holes, somewhere in between in size of stellar black holes and the massive black holes theorized to reside in the center of many galaxies. Because of this, they are believed to form not from single supernovae, but possibly from a number of lesser stellar black holes in a star cluster. The X-ray source is identified as CXOU J013651.1+154547.

## Amateur astronomy observation information

Messier 74 is located 1.5° east-northeast of w:Eta Piscium. As stated above, the galaxy has the lowest surface brightness of all the Messier objects. It may be very difficult to see unless the sky is dark and clear, and it may be difficult to see in locations affected by w:light pollution. The galaxy may be best viewed under low magnification; when highly magnified, the diffuse emission becomes more extended and appears too faint to be seen by many people. Additionally, the galaxy may be more easily seen when using w:averted vision when the eyes are fully dark adapted.

## External links

* Spiral Galaxy M74 @ SEDS Messier pages

## 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 "NASA/IPAC Extragalactic Database". Results for NGC 628. Retrieved 2006-08-12.
* 1 2 3 M. A. Hendry, S. J. Smartt, J. R. Maund, A. Pastorello, L. Zampieri, S. Benetti, M. Turatto, E. Cappellaro, W. P. S. Meikle, R. Kotak, M. J. Irwin, P. G. Jonker, L. Vermaas, R. F. Peletier, H. van Woerden, K. M. Exter, D. L. Pollacco, S. Leon, S. Verley, C. R. Benn, G. Pignata (2005). "A study of the Type II-P supernova 2003gd in M74". Monthly Notices of the Royal Astronomical Society. 359: 906–926. doi:10.1111/j.1365-2966.2005.08928.x.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ A. Sandage, J. Bedke (1994). Carnegie Atlas of Galaxies. Washington, D.C.: Carnegie Institution of Washington. ISBN 0-87279-667-1.
* 1 2 3 4 S. J. O'Meara (1998). The Messier Objects. Cambridge: Cambridge University. ISBN 0-521-55332-6.
* 1 2 3 4 5 6 K. G. Jones (1991). Messier's Nebulae and Star Clusters (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-37079-5.
* ↑ S. Nakano, R. Kushida, Y. Kushida, W. Li (2002). "Supernova 2002ap in M74". IAU Circular. 7810.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ R. Evans, R. H. McNaught (2003). "Supernova 2003gd in M74". IAU Circular. 8150.
* ↑ P. A. Mazzali, J. Deng, K. Maeda, K. Nomoto, H. Umeda, K. hatano, K. Iwamoto, Y. Yoshii, Y. Kobayashi, T. Minezaki, M. Doi, K. Enya, H. Tomita, S. J. Smartt, K. Kinugasa, H. Kawakita, K. Ayani, T. Kawabata, H. Yamaoka, Y. L. Qiu, K. Motohara, C. L. Gerardy, R. Fesen, K. S. Kawabata, M. Iye, N. Kashikawa, G. Kosugi, Y. Ohyama, M. Takada-Hidai, G. Zhao, R. Chornock, A. V. Filippenko, S. Benetti, M. Turatto (2002). "The Type Ic Hypernova SN 2002ap". Astrophysical Journal. 572: L61–L65. doi:10.1086/341504.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* 1 2 S. J. Smartt, P. M. Vreeswijk, E. Ramirez-Ruiz, G. F. Gilmore, W. P. S. Meikle, A. M. N. Ferguson, J. H. Knapen (2002). "On the Progenitor of the Type Ic Supernova 2002ap". Astrophysical Journal. 572: L147–L151. doi:10.1086/341747.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* 1 2 A. Gal-Yam, E. O. Ofek, O. Shemmer (2002). "Supernova 2002ap: the first month". Monthly Notices of the Royal Astronomical Society. 332: L73–L77. doi:10.1046/j.1365-8711.2002.05535.x.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ S. D. Van Dyk, W. Li, A. V. Filippenko (2003). "On the Progenitor of the Type II-Plateau Supernova 2003gd in M74". Publications of the Astronomical Society of the Pacific. 115: 1289–1295. doi:10.1086/378308.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* 1 2 B. E. K. Sugerman (2005). "Discovery of a Light Echo from SN 2003gd". Astrophysical Journal. 632: L17–L20. doi:10.1086/497578.
* ↑ S. D. Van Dyk, W. Li, A. V. Filippenko (2006). "The Light Echo around Supernova 2003gd in Messier 74". Publications of the Astronomical Society of the Pacific. 118: 351–357. doi:10.1086/500225.{{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.
* ↑ A. Garcia (1993). "General study of group membership. II - Determination of nearby groups". Astronomy and Astrophysics Supplement. 100: 47–90.
* 1 2 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)
* ↑ Chandra :: Photo Album :: M74 :: 22 Mar 05

# M75

Messier 75 (also known as M75 or NGC 6864) is a w:globular cluster in the w:constellation Sagittarius. It was discovered by w:Pierre Méchain in w:1780 and included in w:Charles Messier's catalog of w:comet-like objects that same year.
M75 is at a distance of about 67,500 w:light years away from w:Earth and its apparent size on the sky translates to a true radius of some 67 w:light years. It is classified as class I, meaning it is one of the more densely concentrated globular clusters known. The w:absolute magnitude of M75 is about -8.5 or some 180,000 more luminous than the w:Sun.

## External links

* Messier 75 @ SEDS Messier pages
* Messier 75, Galactic Globular Clusters Database page

## References

* 1 2 3 4 "SIMBAD Astronomical Database". Results for NGC 6864. Retrieved 2006-11-16.
* ↑ distance × sin( diameter_angle / 2 ) = 67 ly. radius