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Galaxies Gallery :
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Pinwheel Galaxy - M101 (Wide-Field)
April 15, 2009
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. 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. It has a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses.
Another remarkable property of this galaxy is its huge and extremely bright H II regions, of which a total of about 3000 can be seen on photographs. HII regions usually accompany the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form. HII regions are ionized by large numbers of extremely bright and hot young stars. 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 amplified the density waves in the spiral arms of M101. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.
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Whilrpool Galaxy - M51 (Wide-Field)
April 1, 2009
The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici. It is one of the most famous galaxies in the sky. The galaxy and its companion (NGC 5195) are easily observed by amateur astronomers, and the two galaxies may even be seen with 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.
What was later known as the Whirlpool Galaxy, was discovered in 1774 by Charles Messier, and is designated as M51. Its companion galaxy, NGC 5195, was discovered in 1781 by 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 Birr Castle, Ireland. Sometimes M51 is used to refer the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).
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Whirlpool Galaxy - M51
January 29, 2009
The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici. It is one of the most famous galaxies in the sky. In 2005 a supernova (SN 2005cs) was observed in the Whirlpool Galaxy, peaking at apparent magnitude 14. With the recent SN 2005cs 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 mass is estimated to be 160 billion solar masses.
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 nebula. A secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations. A pair of ionization cones extend from the axis of the main dust ring. 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 NGC 5195. 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.
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Spiral Galaxy - NGC 891
January 28, 2009
NGC 891 is a fine edge-on spiral galaxy with a faint dust lane along its equator. It is about 30 million light-years away in the constellation Andromeda and it was discovered by William Herschel on October 6, 1784, and cataloged as H V.19. However, in the appendix to his first catalog, he confused it with his H V.18 (M110, NGC 205) when discussing the discoveries of his sister Caroline Herschel, in this case her entry No. 9; this remark was picked up by Admiral Smyth and later authors so that it was wrongly attributed to Caroline for a long time.
Gilbert A. Esquerdo and John C. Barentine have investigated NGC 891 in the infrared part of the electromagnetic spectrum, and suspect that this galaxy might have a bar (and thus be of Hubble type SBb) which is not seen in the visible image because of its edge-on orientation. NGC 891 is a member of a small group of galaxies, sometimes called the NGC 1023 group, which also contains NGCs 925, 949, 959, 1003, 1023, and 1058 as well as UGCs 1807, 1865 (DDO 19), 2014 (DDO 22), 2023 (DDO 25), 2034 (DDO 24), and 2259. Supernova 1986J was discovered in NGC 891 by van Gorkom, Rupen, Knapp, Gunn on August 21, 1986 and reached mag 14 (see IAUC 4248).
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Sombrero Galaxy - M104
January 28, 2009
The Sombrero Galaxy (also known as M 104 or NGC 4594 ) is an unbarred spiral galaxy in the constellation Virgo. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero. The galaxy has an apparent magnitude of +9.0, making it easily visible with amateur telescopes. The large bulge, the central supermassive black hole, and the dust lane all attract the attention of professional astronomers.
The Sombrero Galaxy was discovered in March of 1767 by Pierre Méchain, who described the object in a May 1767 letter to J. Bernoulli that was later published in the Berliner Astronomisches Jahrbuch. Charles Messier made a hand-written note about this and five other objects (now collectively recognized as M104 - M109) to his personal list of objects now known as the Messier Catalogue, but it was not "officially" included until 1921. William Herschel independently discovered the object in 1784 and additionally noted the presence of a "dark stratum" in the galaxy's disc, what is now called a dust lane. Later astronomers were able to connect Méchain's and Herschel's observations.
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Bode's Galaxy - M81
January 27, 2009
Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a spiral galaxy about 12 million light-years away in the constellation Ursa Major. M81 is one of the most striking examples of a grand design spiral galaxy, with near perfect arms spiraling into the very center. Because of its proximity to Earth, its large size, and its active galactic nucleus (which harbors a 70 million solar mass supermassive black hole) Messier 81 is a popular galaxy to study in professional astronomy research. The galaxy's large size and relatively low apparent magnitude (lower magnitude implies higher brightness) also make it a popular target for amateur astronomy observations.
Messier 81 was first discovered by Johann Elert Bode in 1774. Consequently, the galaxy is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, which was subsequently listed in the Messier Catalogue.Most of the emission at infrared wavelengths originates from interstellar dust. This interstellar dust is found primarily within the galaxy's spiral arms, and it has been shown to be associated with star formation regions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are very effective at heating the dust and hence enhancing the infrared dust emission from these regions.
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Galaxy Group - M81 and M82
January 19, 2009
Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a spiral galaxy about 12 million light-years away in the constellation Ursa Major. Messier 82 (also known as NGC 3034 or the Cigar Galaxy) is a prototype starburst galaxy about 12 million light-years away in the constellation Ursa Major. Together they form a gravitational pair that are separated by 150,000 light years. This Galaxy Group is the nearest Galaxy Group to our own Local Group of galaxies. Our Local Group comprises about 30 galaxies (including dwarf galaxies), with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy (M31).
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Pinwheel Galaxy - M101
January 15, 2009
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. 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. It has a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses.
Another remarkable property of this galaxy is its huge and extremely bright H II regions, of which a total of about 3000 can be seen on photographs. HII regions usually accompany the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form. HII regions are ionized by large numbers of extremely bright and hot young stars. 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 amplified the density waves in the spiral arms of M101. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.
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Spiral Galaxy - M74
December 1, 2008
Messier 74 (also known as NGC 628) is a face-on spiral galaxy in the constellation Pisces and was discovered by Pierre Méchain in 1780. Méchain then communicated his discovery to Charles Messier, who listed the galaxy in his catalog. The galaxy contains two clearly-defined spiral arms and is therefore used as an archetypal example of a 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 spiral arm structure and spiral density waves..
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Spiral Galaxy - NGC 891
September 27, 2008
NGC 891 is a fine edge-on spiral galaxy with a faint dust lane along its equator. It is about 30 million light-years away in the constellation Andromeda and it was discovered by William Herschel on October 6, 1784, and cataloged as H V.19. However, in the appendix to his first catalog, he confused it with his H V.18 (M110, NGC 205) when discussing the discoveries of his sister Caroline Herschel, in this case her entry No. 9; this remark was picked up by Admiral Smyth and later authors so that it was wrongly attributed to Caroline for a long time.
Gilbert A. Esquerdo and John C. Barentine have investigated NGC 891 in the infrared part of the electromagnetic spectrum, and suspect that this galaxy might have a bar (and thus be of Hubble type SBb) which is not seen in the visible image because of its edge-on orientation. NGC 891 is a member of a small group of galaxies, sometimes called the NGC 1023 group, which also contains NGCs 925, 949, 959, 1003, 1023, and 1058 as well as UGCs 1807, 1865 (DDO 19), 2014 (DDO 22), 2023 (DDO 25), 2034 (DDO 24), and 2259. Supernova 1986J was discovered in NGC 891 by van Gorkom, Rupen, Knapp, Gunn on August 21, 1986 and reached mag 14 (see IAUC 4248).
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Fireworks Galaxy - NGC 6946
September 27, 2008
NGC 6946, also known as the "Fireworks Galaxy", is an intermediate spiral galaxy about 10 million light-years away, on the border between the constellations Cepheus and Cygnus. It was discovered by William Herschel on September 9, 1798. NGC 6946 is highly obscured by interstellar matter of the Milky Way galaxy, as it is quite close to the galactic plane. Nine supernovae (SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et, and SN 2008S) were observed in NGC 6946.
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Triangulum Galaxy - M33
September 25, 2008
The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a spiral galaxy approximately 3 million light-years away in the constellation Triangulum. The galaxy is also sometimes informally referred to as the Pinwheel Galaxy by some amateur astronomy references and in some public outreach websites. However, the SIMBAD Astronomical Database, a professional astronomy database that contains formal designations for astronomical objects, indicates that the name "Pinwheel Galaxy" is used to refer to Messier 101, and several other amateur astronomy resources and other public outreach websites also identify Messier 101 by that name.
With a diameter of about 50,000 light-years, it is the third largest galaxy in the Local Group, a group of galaxies which also contains the Milky Way Galaxy and the Andromeda Galaxy, and it may be a gravitationally bound companion of the Andromeda Galaxy. Estimates indicate that Triangulum may be home to between 30 and 40 billion stars, compared to 200-400 billion for the Milky Way, and 1 trillion stars for Andromeda. The Pisces Dwarf (LGS 3), one of the small Local Group member galaxies, is possibly a satellite of Triangulum.
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