Celestial hemisphere:  Northern  ·  Constellation: Andromeda (And)  ·  Contains:  NGC 404  ·  The star Mirach (βAnd)
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Ghost of Mirach (NGC 404), Nucdoc
Ghost of Mirach (NGC 404)
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Ghost of Mirach (NGC 404)

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Description

Beta Andromedae (Beta And, β And, β Andromedae) is the Bayer designation for a prominent star in the northern constellation of Andromeda. It has the traditional name Mirach, which is also spelled Merach, Mirac, Mirak.[12] β Andromedae is located northeast of the Great Square of Pegasus and is theoretically visible to all observers north of 54° S. The galaxy NGC 404, also known as Mirach's Ghost, is visible seven arc-minutes away.[13]

This star has an average apparent visual magnitude of 2.05,[2] which makes it the brightest star in the constellation. However, the luminosity varies slightly from magnitude +2.01 to +2.10.[3] Based upon parallax measurements, it is located at a distance of roughly 197 light-years (60 parsecs) from Earth.[1] The apparent magnitude of this star is reduced by 0.06 from extinction caused by the gas and dust along the line of sight.[7]

Properties[edit]

Beta Andromedae is a red giant with a stellar classification of M0 III.[4] Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[14] It is suspected of being a semiregular variable star whose apparent visual magnitude varies from +2.01 to +2.10.[3] At this stage of the star's evolution, the outer envelope has expanded to around 100 times the size of the Sun.[10] It is radiating 1995[10] times the luminosity of the Sun at an effective temperature of 3842 K.[11]

NGC 404 is a field galaxy[5] located about 10 million light years away in the constellation Andromeda. It was discovered by William Herschel in 1784, and is visible through small telescopes.[6] NGC 404 lies just beyond the Local Group and does not appear gravitationally bound to it. It is notable for being within 7 arc-minutes of second magnitude star Mirach, making it a difficult target to observe or photograph and granting it the nickname "Mirach's Ghost".[6][7]

Physical characteristics[edit]

NGC 404 is a very isolated dwarf lenticular galaxy, a bit more luminous and smaller than the Small Magellanic Cloud[8] and unlike many other early-type galaxies is very rich in neutral hydrogen, most of it concentrated on a pair of large rings around it.[9] It also has star formation both on its center[10] and on its outermost regions, albeit at a low level[11]

Both the outer gas disk and its star formation are assumed to have been triggered by one or several mergers with smaller galaxies roughly 1 billion years ago[11] and it has been proposed NGC 404 is a former spiral galaxy that was transformed into a lenticular one by that event[10]

LINER emission[edit]

NGC 404 contains a low-ionization nuclear emission-line region (LINER), a type of region that is characterized by spectral line emission from weakly ionized atoms.[12] A nuclear star cluster is also present as well as -likely- a supermassive black hole, with a mass of several tens of thousands solar masses[13]

Distance measurements[edit]

At least two techniques have been used to measure distances to NGC 404. The infrared surface brightness fluctuations distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to NGC 404 using this technique in 2003 is 9.9 ± 0.5 Mly (3.03 ± 0.15 Mpc).[2]

However, NGC 404 is close enough that red supergiants can be imaged as individual stars. The light from these stars and knowledge of how they should compare to nearby stars within the Milky Way galaxy allows for direct measurement of the distance to the galaxy. This method is referred to as the tip of the red giant branch (TRGB) method. The estimated distance to NGC 404 using this technique is 10.0 ± 1.2 Mly (3.1 ± 0.4 Mpc).[3] Averaged together, these distance measurements give a distance estimate of 10.0 ± 0.7 Mly (3.07 ± 0.21 Mpc).[a]

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Ghost of Mirach (NGC 404), Nucdoc