The Pinwheel Galaxy (also known as Messier 101, M101 or NGC 5457) is a face-on spiral galaxy distanced 21 million light-years (six megaparsecs) away from Earth in the constellation Ursa Major. Discovered by Pierre Méchain on March 27, 1781, it was communicated to Charles Messier who verified its position for inclusion in the Messier Catalogue as one of its final entries.
On February 28, 2006, NASA and the European Space Agency released a very detailed image of the Pinwheel Galaxy, which was the largest and most detailed image of a galaxy by Hubble Space Telescope at the time. The image was composed of 51 individual exposures, plus some extra ground-based photos.
The Pinwheel Galaxy, as taken by Hubble Space Telescope
|Observation data (J2000 epoch)|
|Right ascension||14h 03m 12.6s|
|Declination||+54° 20′ 57″|
|Helio radial velocity||241 ± 2 km/s|
|Distance||20.9 ± 1.8 Mly (6.4 ± 0.5 Mpc)|
|Apparent magnitude (V)||7.86|
|Number of stars||1 trillion (1012)|
|Size||~170,000 ly in diameter|
|Apparent size (V)||28′.8 × 26′.9|
|Messier 101, M101, NGC 5457, UGC 8981, PGC 50063, Arp 26|
Pierre Méchain, the discoverer of Messier 101, 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."
William Herschel noted in 1784 that "...in my 7, 10, and 20-feet [focal length] 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 diameter 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 eyepiece.
M101 is a large galaxy, with a diameter of 170,000 light-years. By comparison, the Milky Way has a diameter of 258,000 light years. It has around a trillion stars, twice the number in the Milky Way. It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses.
M101 has a high population of H II regions, many of which are very large and bright. H II regions usually accompany the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form. H II regions are ionized by large numbers of extremely bright and hot young stars; those in M101 are capable of creating hot superbubbles. In a 1990 study, 1264 H II regions were cataloged in the galaxy. Three are prominent enough to receive New General Catalogue numbers - NGC 5461, NGC 5462, and NGC 5471.
M101 is asymmetrical due to the tidal forces from interactions with its companion galaxies. These gravitational interactions compress interstellar hydrogen gas, which then triggers strong star formation activity in M101's spiral arms that can be detected in ultraviolet images.
In 2001, the x-ray source P98, located in M101, was identified as an ultra-luminous X-ray source - a source more powerful than any single star but less powerful than a whole galaxy - using the Chandra X-ray Observatory. It received the designation M101 ULX-1. In 2005, Hubble and XMM-Newton observations showed the presence of an optical counterpart, strongly indicating that M101 ULX-1 is an x-ray binary. Further observations showed that the system deviated from expected models - the black hole is just 20 to 30 solar masses, and consumes material (including captured stellar wind) at a higher rate than theory suggests.
M101 has five prominent companion galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585, and Holmberg IV. 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 probably distorted the companion galaxy NGC 5474. M101 and its companion galaxies comprise most or possibly all of the M101 Group.
On August 24, 2011, a Type Ia supernova, SN 2011fe, initially designated PTF 11kly, was discovered in M101. The supernova was visual magnitude 17.2 at discovery and reached magnitude 9.9 at its peak. This was the fourth supernova recorded in M101. The first, SN 1909A, was discovered by Max Wolf in January 1909 and reached magnitude 12.1. SN 1951H reached magnitude 17.5 in September 1951 and SN 1970G reached magnitude 11.5 in January 1970. On February 10, 2015, a luminous red nova, known as M101 OT2015-1 was discovered in the Pinwheel Galaxy.
Messier 83 or M83, also known as the Southern Pinwheel Galaxy and NGC 5236, is a barred spiral galaxy approximately 15 million light-years away in the constellation Hydra. Nicolas Louis de Lacaille discovered M83 on February 23, 1752 at the Cape of Good Hope. Charles Messier added it to his catalogue of nebulous objects (now known as the Messier Catalogue) in March 1781. This is one of the closest and brightest barred spiral galaxies in the sky, making it visible with binoculars. Its nickname of the Southern Pinwheel derives from its resemblance to the Pinwheel Galaxy (M101).
This is a massive, grand design spiral galaxy.
The morphological classification of NGC 5236 in the De Vaucouleurs system is SAB(s)c, where the 'SAB' denotes a weak-barred spiral, '(s)' indicates a pure spiral structure with no ring, and 'c' means the spiral arms are loosely wound. The peculiar dwarf galaxy NGC 5253 lies near M83, and the two likely interacted within the last billion years resulting in starburst activity in their central regions.The star formation rate in M83 is higher along the leading edge of the spiral arms, as predicted by density wave theory. NASA's Galaxy Evolution Explorer project reported finding large numbers of new stars in the outer reaches of the galaxy – 20 kpc from the center. It had hitherto been thought that these areas lacked the materials necessary for star formation. Six supernovae have been observed in M83: SN 1923A, SN 1945B, SN 1950B, SN 1957D, SN 1968L and SN 1983N.
M83 is at the center of one of two subgroups within the Centaurus A/M83 Group, a nearby galaxy group. Centaurus A is at the center of the other subgroup. These are sometimes identified as one group and sometimes as two. However, the galaxies around Centaurus A and the galaxies around M83 are physically close to each other, and both subgroups appear not to be moving relative to each other.Messier 99
Messier 99 or M99, also known as NGC 4254, is a grand design spiral galaxy in the northern constellation Coma Berenices approximately 15 megaparsecs (49 megalight-years) in distance from the Milky Way. It was discovered by Pierre Méchain on March 17, 1781. The discovery was then reported to Charles Messier, who included the object in the Messier Catalogue of comet-like objects. Messier 99 was one of the first galaxies in which a spiral pattern was seen. This pattern was first identified by Lord Rosse in the spring of 1846.This galaxy has a morphological classification of SA(s)c, indicating a pure spiral shape with loosely wound arms. It has a peculiar shape with one normal looking arm and an extended arm that is less tightly wound. The galaxy is inclined by 42° to the line-of-sight with a major axis position angle of 68°. Four supernovae have been observed in this galaxy: SN 1967H (type II), 1972Q, 1986I (type II), and 2014L (type Ic).A bridge of neutral hydrogen gas links NGC 4254 with VIRGOHI21, an HI region and a possible dark galaxy. The gravity from the latter may have distorted M99 and drawn out the gas bridge, as the two galaxy-sized objects may have had a close encounter before they went their separate ways. However, VIRGOHI21 may instead be tidal debris from an interaction with the lenticular galaxy NGC 4262 some 280 million years ago. It is expected that the drawn out arm will relax to match the normal arm once the encounter is over.
While not classified as a starburst galaxy, M99 has a star formation activity three times larger than other galaxies of similar Hubble type that may have been triggered by the encounter. M99 is likely entering the Virgo Cluster for the first time and is located at the periphery of the cluster at a projected separation of 3.7°, or around one megaparsec, from the cluster center at Messier 87. The galaxy is undergoing ram-pressure stripping as it moves through the intracluster medium.
New General Catalogue 5000 to 5499