Andromeda I is a dwarf spheroidal galaxy about 2.40 million light-years away in the constellation Andromeda. Andromeda I is part of a satellite galaxy of the Andromeda Galaxy, it is 3.5 degrees south and east of M31. As of 2005, it is the closest known dSph companion to M31 at an estimated projected distance of ~40 kpc or ~150,000 light-years. Andromeda I was discovered by Sidney van den Bergh in 1970 with the Mount Palomar Observatory 48-inch telescope. Further study of Andromeda I was done by the WFPC2 camera of the Hubble Space Telescope; this found that the horizontal branch stars, like other dwarf spheroidal galaxies were predominantly red. From this, the abundance of blue horizontal branch stars, along with 99 RR Lyrae stars detected in 2005, lead to the conclusion there was an extended epoch of star formation; the estimated age is 10 Gyr. The Hubble telescope found a globular cluster in Andromeda I, being the least luminous galaxy where such a cluster was found. Andromeda Galaxy Andromeda's satellite galaxies Andromeda I on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
The horizontal branch is a stage of stellar evolution that follows the red giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core and by hydrogen fusion in a shell surrounding the core; the onset of core helium fusion at the tip of the red giant branch causes substantial changes in stellar structure, resulting in an overall reduction in luminosity, some contraction of the stellar envelope, the surface reaching higher temperatures. Horizontal branch stars were discovered with the first deep photographic photometric studies of globular clusters and were notable for being absent from all open clusters, studied up to that time; the horizontal branch is so named because in low-metallicity star collections like globular clusters, HB stars lie along a horizontal line in a Hertzsprung–Russell diagram. After exhausting their core hydrogen, stars leave the main sequence and begin fusion in a hydrogen shell around the helium core and become giants on the red giant branch.
In stars with masses up to 2.3 times the mass of the Sun the helium core becomes a region of degenerate matter that does not contribute to the generation of energy. It continues to grow and increase in temperature as the hydrogen fusion in the shell contributes more helium. If the star has more than about 0.5 solar masses, the core reaches the temperature necessary for the fusion of helium into carbon through the triple-alpha process. The initiation of helium fusion begins across the core region, which will cause an immediate temperature rise and a rapid increase in the rate of fusion. Within a few seconds the core becomes non-degenerate and expands, producing an event called helium flash. Non-degenerate cores initiate fusion more smoothly, without a flash; the output of this event is absorbed by the layers of plasma above, so the effects are not seen from the exterior of the star. The star now changes to a new equilibrium state, its evolutionary path switches from the red giant branch onto the horizontal branch of the Hertzsprung–Russell diagram.
Stars between about 2.3 M☉ and 8 M☉ have larger helium cores that do not become degenerate. Instead their cores reach the Schoenberg-Chandrasekhar mass at which they are no longer in hydrostatic or thermal equilibrium, they contract and heat up, which triggers helium fusion before the core becomes degenerate. These stars become hotter during core helium fusion, but they have different core masses and hence different luminosities from HB stars, they vary in temperature during core helium fusion and perform a blue loop before moving to the asymptotic giant branch. Stars more massive than about 8 M☉ ignite their core helium smoothly, go on to burn heavier elements as a red supergiant. Stars remain on the horizontal branch for around 100 million years, becoming more luminous in the same way that main sequence stars increase luminosity as the virial theorem shows; when their core helium is exhausted, they progress to helium shell burning on the asymptotic giant branch. On the AGB they become cooler and much more luminous.
Stars on the horizontal branch all have similar core masses, following the helium flash. This means that they have similar luminosities, on a Hertzsprung–Russell diagram plotted by visual magnitude the branch is horizontal; the size and temperature of an HB star depends on the mass of the hydrogen envelope remaining around the helium core. Stars with larger hydrogen envelopes are cooler; this creates the spread of stars along the horizontal branch at constant luminosity. The temperature variation effect is much stronger at lower metallicity, so old clusters have more pronounced horizontal branches. Although the horizontal branch is named because it consists of stars with the same absolute magnitude across a range of temperatures, lying in a horizontal bar on a color–magnitude diagrams, the branch is far from horizontal at the blue end; the horizontal branch ends in a "blue tail" with hotter stars having lower luminosity with a "blue hook" of hot stars. It is not horizontal when plotted by bolometric luminosity, with hotter horizontal branch stars being less luminous than cooler ones.
The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000K. This is far beyond. Theories to explain these stars include binary interactions, "late thermal pulses", where a thermal pulse that Asymptotic giant branch stars experience occurs after fusion has ceased and the star has entered the superwind phase; these stars are "born again" with unusual properties. Despite the bizarre-sounding process, this is expected to occur for 10% or more of post-AGB stars, although it is thought that only late thermal pulses create extreme horizontal-branch stars, after the planetary nebular phase and when the central star is cooling towards a white dwarf. Globular cluster CMDs show horizontal branches that have a prominent gap in the HB; this gap in the CMD incorrectly suggests that the cluster has no stars in this region of its CMD. The gap occurs at the instability strip, so many stars in this region pulsate; these pulsating horizontal-branch stars are known as RR Lyrae variable stars and they are variable in brightness with periods of up to 1.2 days.
It requires an extended observing program to establish the star's true apparent color. Such a program is beyond the scope of an investigation of a cluster's color
In astronomy, metallicity is used to describe the abundance of elements present in an object that are heavier than hydrogen or helium. Most of the physical matter in the Universe is in the form of hydrogen and helium, so astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium"; this usage is distinct from the usual physical definition of a solid metal. For example and nebulae with high abundances of carbon, nitrogen and neon are called "metal-rich" in astrophysical terms though those elements are non-metals in chemistry; the presence of heavier elements hails from stellar nucleosynthesis, the theory that the majority of elements heavier than hydrogen and helium in the Universe are formed in the cores of stars as they evolve. Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars, it follows that older generations of stars, which formed in the metal-poor early Universe have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars. These became known as Population I and Population II stars. A third stellar population was introduced in 1978, known as Population III stars; these metal-poor stars were theorised to have been the "first-born" stars created in the Universe. Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest; some methods include determining the fraction of mass, attributed to gas versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the Sun. Stellar composition is simply defined by the parameters X, Y and Z. Here X is the mass fraction of hydrogen, Y is the mass fraction of helium, Z is the mass fraction of all the remaining chemical elements.
Thus X + Y + Z = 1.00. In most stars, nebulae, H II regions, other astronomical sources and helium are the two dominant elements; the hydrogen mass fraction is expressed as X ≡ m H / M, where M is the total mass of the system, m H is the fractional mass of the hydrogen it contains. The helium mass fraction is denoted as Y ≡ m He / M; the remainder of the elements are collectively referred to as "metals", the metallicity—the mass fraction of elements heavier than helium—can be calculated as Z = ∑ i > He m i M = 1 − X − Y. For the surface of the Sun, these parameters are measured to have the following values: Due to the effects of stellar evolution, neither the initial composition nor the present day bulk composition of the Sun is the same as its present-day surface composition; the overall stellar metallicity is defined using the total iron content of the star, as iron is among the easiest to measure with spectral observations in the visible spectrum. The abundance ratio is defined as the logarithm of the ratio of a star's iron abundance compared to that of the Sun and is expressed thus: = log 10 star − log 10 sun, where N Fe and N H are the number of iron and hydrogen atoms per unit of volume respectively.
The unit used for metallicity is the dex, contraction of "decimal exponent". By this formulation, stars with a higher metallicity than the Sun have a positive logarithmic value, whereas those with a lower metallicity than the Sun have a negative value. For example, stars with a value of +1 have 10 times the metallicity of the Sun. Young Population I stars have higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than −6.0, that is, less than a millionth of the abundance of iron in the Sun. The same notation is used to express variations in abundances between other the individual elements as compared to solar proportions. For example, the notati
Principal Galaxies Catalogue
The Catalogue of Principal Galaxies is an astronomical catalog published in 1989 that lists B1950 and J2000 equatorial coordinates and cross-identifications for 73,197 galaxies. It is based on the Lyon-Meudon Extragalactic Database, started in 1983. 40,932 coordinates have standard deviations smaller than 10″. A total of 131,601 names from the 38 most common sources are listed. Available mean data for each object are given: 49,102 morphological descriptions, 52,954 apparent major and minor axis, 67,116 apparent magnitudes, 20,046 radial velocities and 24,361 position angles; the Lyon-Meudon Extragalactic Database was expanded into HyperLEDA, a database of a few million galaxies. Galaxies in the original PGC catalogue are numbered with their original PGC number in HyperLEDA. Numbers have been assigned for the other galaxies, although for those galaxies not in the original PGC catalogue, it is not recommended to use that number as a name. PGC 6240 is a large lenticular galaxy in the constellation Hydrus.
It is located about 106 million parsecs away from Earth. PGC 39058 is a dwarf galaxy, located 14 million light years away in the constellation of Draco, it is nearby, however it is obscured by a bright star, in front of the galaxy. Category:Principal Galaxies Catalogue objects Astronomical catalogue PGC info at ESO's archive of astronomical catalogues PGC readme at Centre de Données astronomiques de Strasbourg
Dwarf spheroidal galaxy
A dwarf spheroidal galaxy is a term in astronomy applied to small, low-luminosity galaxies with little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are companions to the Andromeda Galaxy. While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are spheroidal in shape and have lower luminosity. Despite the radii of dSphs being much larger than those of globular clusters, they are much more difficult to find due to their low luminosities and surface brightnesses. Dwarf spheroidal galaxies have a large range of luminosities, known dwarf spheroidal galaxies span several orders of magnitude of luminosity, their luminosities are so low that Ursa Minor and Draco, the known dwarf spheroidal galaxies with the lowest luminosities, have mass-to-light ratios greater than that of the Milky Way. Dwarf spheroidals have little to no gas with no obvious signs of recent star formation.
When it comes to the Local Group, dSphs are found near the Milky Way and M31. The first dwarf spheroidal galaxies discovered were Sculptor and Fornax in 1938; the Sloan Digital Sky Survey has resulted in the discovery of 11 more dSph galaxies as of 2007 By 2015, many more ultra-faint dSphs were discovered, all satellites of the Milky Way. Nine new dSphs were discovered in the Dark Energy Survey in 2015; each dSph is named after constellations they are discovered in, such as the Sagittarius dwarf spheroidal galaxy, all of which consist of stars much older than 1-2 Gyr that formed over the span of many gigayears. For example, 98% of the stars in the Carina dwarf spheroidal galaxy are older than 2 Gyr, formed over the course of three bursts around 3, 7, 13 Gyr ago; the stars in Carina have been found to be metal-poor. This is unlike star clusters because, while star clusters have stars which formed more or less the same time, dwarf spheroidal galaxies experience multiple bursts of star formation.
Because of the faintness of the lowest-luminosity dwarf spheroidal galaxies and the nature of the stars contained within them, some astronomers suggest that dwarf spheroidal galaxies and globular clusters may not be separate and distinct types of objects. Other recent studies, have found a distinction in that the total amount of mass inferred from the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the stars themselves. Studies reveal that dwarf spheroidal galaxies have a dynamical mass of around 10 7 solar masses, large despite the low luminosity of dSph galaxies. Although at fainter luminosities of dwarf spheroidal galaxies, it is not universally agreed upon how to differentiate between a dwarf spheroidal galaxy and a star cluster. In the current predominantly accepted Lambda cold dark matter cosmological model, the presence of dark matter is cited as a reason to classify dwarf spheroidal galaxies as a different class of object from globular clusters, which show little to no signs of dark matter.
Because of the large amounts of dark matter in dwarf spheroidal galaxies, they may deserve the title "most dark matter-dominated galaxies."Further evidence of the prevalence of dark matter in dSphs includes the case of Fornax dwarf spheroidal galaxy, which can be assumed to be in dynamic equilibrium to estimate mass and amount of dark matter, since the gravitational effects of the Milky Way are small. Unlike the Fornax galaxy, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way. A topic of research is how much the internal dynamics of dwarf spheroidal galaxies are affected by the gravitational tidal dynamics of the galaxy they are orbiting. In other words, dwarf spheroidal galaxies could be prevented from achieving equilibrium due to the gravitational field of the Milky Way or other galaxy that they orbit. For example, the Sextans dwarf spheroidal galaxy has a velocity dispersion of 7.9±1.3 km/s, a velocity dispersion that could not be explained by its stellar mass according to the Virial Theorem.
Similar to Sextans, previous studies of Hercules dwarf spheroidal galaxy reveal that its orbital path does not correspond to the mass contained in Hercules. Furthermore, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way
Andromeda IV is an isolated irregular dwarf galaxy. The moderate surface brightness, a blue color, low current star formation rate and low metallicity are consistent with it being a small dwarf irregular galaxy similar to Local Group dwarfs such as IC 1613 and Sextans A. Arguments based on the observed radial velocity and the tentative detection of the RGB tip suggest that it lies well outside the confines of the Local Group. Further study using the Hubble space telescope has shown it to be a solitary irregular dwarf galaxy; the galaxy is between 22 and 24 million light years from Earth, so is not close to the Andromeda Galaxy at all. The galaxy is isolated; the Holmberg diameter is 1880 parsecs, but neutral atomic hydrogen gas extends more than eight times further out in a disk. The galaxy is dark, the baryonic mass to dark matter ratio is 0.11. It was discovered by Sidney van den Bergh in 1972. List of Andromeda's satellite galaxies Andromeda IV on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
The Local Group is the galaxy group that includes the Milky Way. Its has a total diameter of 3 Mpc, a total mass of the order of 2×1012 solar masses, it consists of two clusters of galaxies in a "dumbbell" shape, the Milky Way and its satellites on one hand, the Andromeda Galaxy and its satellites on the other. The two clusters are separated by about 0.8 Mpc and move towards one another with a velocity of 123 km/h. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster; the total number of galaxies in the Local Group is unknown but known to exceed 54, most of them being dwarf galaxies. The two largest members, the Andromeda Galaxy and the Milky Way, are both spiral galaxies with masses of about 1012 solar masses each, each have their own system of satellite galaxies: The Andromeda Galaxy's satellite system consists of Messier 32, Messier 110, NGC 147, NGC 185, Andromeda I, And II, And III, And V, And VI, And VII, And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.
The Milky Way's satellite galaxies system comprises Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy, Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I, Leo II, Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies. The Triangulum Galaxy is the third largest member of the Local Group, at about 5×1010 M☉, the third spiral galaxy, it may not be a companion to the Andromeda Galaxy. Pisces Dwarf Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either; the membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy, is uncertain due to extreme distances from the center of the Local Group. The other members of the group are gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf Galaxy, Leo A, Tucana Dwarf Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte, Aquarius Dwarf Galaxy, Sagittarius Dwarf Irregular Galaxy.
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. There, he described it as "a typical small group of nebulae, isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147, he identified IC 10 as a possible part of Local Group. By 2003, the number of known Local Group members had increased from his initial 12 to 36. Smith's Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth and 8,000 light years from the disk of the Milky Way galaxy HVC 127-41-330, a high-velocity cloud, 2.3 million light-years from Earth Monoceros Ring, a ring of stars around the Milky Way, proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy Galaxy cluster List of nearest galaxies List of galaxy clusters IC 342/Maffei Group, the group of galaxies nearest to the Local Group Local Supercluster List of Andromeda's satellite galaxies List of Milky Way's satellite galaxies The Local Group of Galaxies, SEDS Messier pages A Survey of the Resolved Stellar Content of Nearby Galaxies Currently Forming Stars, Lowell Observatory van den Bergh, Sidney.
"Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific. 112: 529–536. ArXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. Doi:10.1086/316548