In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines; each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary due to the temperature of the photosphere, although in some cases there are true abundance differences; the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are classified under the Morgan-Keenan system using the letters O, B, A, F, G, K, M, a sequence from the hottest to the coolest; each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, class D for white dwarfs; the full spectral class for the Sun is G2V, indicating a main-sequence star with a temperature around 5,800 K. The conventional color description takes into account only the peak of the stellar spectrum. In actuality, stars radiate in all parts of the spectrum; because all spectral colors combined appear white, the actual apparent colors the human eye would observe are far lighter than the conventional color descriptions would suggest. This characteristic of'lightness' indicates that the simplified assignment of colors within the spectrum can be misleading.
Excluding color-contrast illusions in dim light, there are indigo, or violet stars. Red dwarfs are a deep shade of orange, brown dwarfs do not appear brown, but hypothetically would appear dim grey to a nearby observer; the modern classification system is known as the Morgan–Keenan classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type. Other modern stellar classification systems, such as the UBV system, are based on color indexes—the measured differences in three or more color magnitudes; those numbers are given labels such as "U-V" or "B-V", which represent the colors passed by two standard filters. The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system. Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions.
Main-sequence stars vary in surface temperature from 2,000 to 50,000 K, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are listed from hottest to coldest; the spectral classes O through M, as well as other more specialized classes discussed are subdivided by Arabic numerals, where 0 denotes the hottest stars of a given class. For example, A0 denotes A9 denotes the coolest ones. Fractional numbers are allowed; the Sun is classified as G2. Conventional color descriptions are traditional in astronomy, represent colors relative to the mean color of an A class star, considered to be white; the apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, stars with particular spectral features such as carbon stars may be far redder than any black body.
The fact that the Harvard classification of a star indicated its surface or photospheric temperature was not understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated, this was suspected to be true. In the 1920s, the Indian physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to the solar chromosphere to stellar spectra. Harvard astronomer Cecilia Payne demonstrated that the O-B-A-F-G-K-M spectral sequence is a sequence in temperature; because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals; the Yerkes spectral classification called the MKK system from the authors' initial
A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations go back to prehistory. People used them to relate stories of their beliefs, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed over time. Many have changed in shape; some became popular. Others were limited to single nations; the 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac.
The origins of the zodiac remain uncertain. In 1928, the International Astronomical Union formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations; some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Examples of bright asterisms include the Pleiades and Hyades within the constellation Taurus or Venus' Mirror in the constellation of Orion.. Some asterisms, like the False Cross, are split between two constellations; the word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars".
The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects, it can specifically denote the recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms", yet the modern accepted astronomical constellations employ such a distinction. E.g. the Pleiades and the Hyades are both asterisms, each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism known as the Big Dipper or the Plough, composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major; the southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle.. A constellation, viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar.
From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, 23½° south. Although stars in constellations appear near each other in the sky, they lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy; the earliest evidence for the humankind's identification of constellations comes from Mesopotamian inscribed stones and clay writing tablets that date back to 3000 BC.
It seems that the bulk of the Mesopotamian constellations were created within a short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared in many of the classical Greek constellations; the oldest Babylonian star catalogues of stars and constellations date back to the beginning in the Middle Bronze Age, most notably the Three Stars Each texts and the MUL. APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age; the classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names. Biblical scholar, E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.
The biblical Book of Job also
Edwin Powell Hubble was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology and is regarded as one of the most important astronomers of all time. Hubble discovered that many objects thought to be clouds of dust and gas and classified as "nebulae" were galaxies beyond the Milky Way, he used the strong direct relationship between a classical Cepheid variable's luminosity and pulsation period for scaling galactic and extragalactic distances. Hubble provided evidence that the recessional velocity of a galaxy increases with its distance from the Earth, a property now known as "Hubble's law", despite the fact that it had been both proposed and demonstrated observationally two years earlier by Georges Lemaître. Hubble-Lemaître's Law implies. A decade before, the American astronomer Vesto Slipher had provided the first evidence that the light from many of these nebulae was red-shifted, indicative of high recession velocities.
Hubble's name is most recognized for the Hubble Space Telescope, named in his honor, with a model prominently displayed in his hometown of Marshfield, Missouri. Edwin Hubble was born to Virginia Lee Hubble and John Powell Hubble, an insurance executive, in Marshfield and moved to Wheaton, Illinois, in 1900. In his younger days, he was noted more for his athletic prowess than his intellectual abilities, although he did earn good grades in every subject except for spelling. Edwin was a gifted athlete, playing baseball, football and running track in both high school and college, he played a variety of positions on the basketball court from center to shooting guard. In fact, Hubble led the University of Chicago's basketball team to their first conference title in 1907, he won seven first places and a third place in a single high school track and field meet in 1906. His studies at the University of Chicago were concentrated on law, which led to a bachelor of science degree in 1910. Hubble became a member of the Kappa Sigma Fraternity.
He spent the three years at The Queen's College, Oxford after earning his bachelor's as one of the university's first Rhodes Scholars studying jurisprudence instead of science, added literature and Spanish, earning his master's degree. In 1909, Hubble's father moved his family from Chicago to Shelbyville, Kentucky, so that the family could live in a small town settling in nearby Louisville, his father died in the winter of 1913, while Edwin was still in England, in the summer of 1913, Edwin returned to care for his mother, two sisters, younger brother, as did his brother William. The family moved once more to Everett Avenue, in Louisville's Highlands neighborhood, to accommodate Edwin and William. Hubble was a dutiful son, who despite his intense interest in astronomy since boyhood, acquiesced to his father's request to study law, first at the University of Chicago and at Oxford, though he managed to take a few math and science courses. After the death of his father in 1913, Edwin returned to the Midwest from Oxford but did not have the motivation to practice law.
Instead, he proceeded to teach Spanish and mathematics at New Albany High School in New Albany, where he coached the boys' basketball team. After a year of high-school teaching, he entered graduate school with the help of his former professor from the University of Chicago to study astronomy at the university's Yerkes Observatory, where he received his Ph. D. in 1917. His dissertation was titled "Photographic Investigations of Faint Nebulae". In Yerkes, he had access to one of the most powerful telescopes in the world at the time, which had an innovative 24 inch reflector. After the United States declared war on Germany in 1917, Hubble rushed to complete his Ph. D. dissertation so he could join the military. Hubble volunteered for the United States Army and was assigned to the newly created 86th Division, where he served in 2nd Battalion, 343 Infantry Regiment, he rose to the rank of lieutenant colonel, was found fit for overseas duty on July 9, 1918, but the 86th Division never saw combat. After the end of World War I, Hubble spent a year in Cambridge, where he renewed his studies of astronomy.
In 1919, Hubble was offered a staff position at the Carnegie Institution for Science's Mount Wilson Observatory, near Pasadena, California, by George Ellery Hale, the founder and director of the observatory. Hubble remained on staff at Mount Wilson until his death in 1953. Shortly before his death, Hubble became the first astronomer to use the newly completed giant 200-inch reflector Hale Telescope at the Palomar Observatory near San Diego, California. Hubble worked as a civilian for U. S. Army at Aberdeen Proving Ground in Maryland during World War II as the Chief of the External Ballistics Branch of the Ballistics Research Laboratory during which he directed a large volume of research in exterior ballistics which increased the effective firepower of bombs and projectiles, his work was facilitated by his personal development of several items of equipment for the instrumentation used in exterior ballistics, the most outstanding development being the high-speed clock camera, which made possible the study of the characteristics of bombs and low-velocity projectiles in flight.
The results of his studies were credited with improving design and military effectiveness of bombs and rockets. For his work there, he received the Legion of Merit award. Hubble was raised as a Christian but some of his statements suggest uncertainty. Hubble married Grace Lilli
B-type main-sequence star
A B-type main-sequence star is a main-sequence star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are luminous and blue, their spectra have neutral helium, which are most prominent at the B2 subclass, moderate hydrogen lines. Examples include Regulus and Algol A; this class of stars was introduced with the Harvard sequence of stellar spectra and published in the Revised Harvard photometry catalogue. The definition of type B-type stars was the presence of non-ionized helium lines with the absence of singly ionized helium in the blue-violet portion of the spectrum. All of the spectral classes, including the B type, were subdivided with a numerical suffix that indicated the degree to which they approached the next classification, thus B2 is 1/5 of the way from type B to type A. However, more refined spectra showed lines of ionized helium for stars of type B0. A0 stars show weak lines of non-ionized helium.
Subsequent catalogues of stellar spectra classified the stars based on the strengths of absorption lines at specific frequencies, or by comparing the strengths of different lines. Thus, in the MK Classification system, the spectral class B0 has the line at wavelength 439 nm being stronger than the line at 420 nm; the Balmer series of hydrogen lines grows stronger through the B class peak at type A2. The lines of ionized silicon are used to determine the sub-class of the B-type stars, while magnesium lines are used to distinguish between the temperature classes. Type-B stars don't have a lack a convection zone in their outer atmosphere, they have a higher mass loss rate than smaller stars such as the Sun, their stellar wind has velocities of about 3,000 km/s. The energy generation in main-sequence B-type stars comes from the CNO cycle of thermonuclear fusion; because the CNO cycle is temperature sensitive, the energy generation is concentrated at the center of the star, which results in a convection zone about the core.
This results in a steady mixing of the hydrogen fuel with the helium byproduct of the nuclear fusion. Many B-type stars have a rapid rate of rotation, with an equatorial rotation velocity of about 200 km/s. Spectral objects known as "Be stars" are massive yet non-supergiant entities that notably have, or had at some time, 1 or more Balmer lines in emission, with the hydrogen-related electromagnetic radiation series projected out by the stars being of particular scientific interest. Be stars are thought to feature unusually strong stellar winds, high surface temperatures, significant attrition of stellar mass as the objects rotate at a curiously rapid rate, all of this in contrast to many other main-sequence star types. Though the related terminologies are confusingly ambiguous, spectral objects known as "B" or "B stars" are distinct from Be stars since said B entities are in possession of distinctive neutral or low ionization emission lines that are considered to have'forbidden mechanisms', something denoted by the use of brackets or parenthesis.
In other words, these particular stars' emissions appear to undergo processes not allowed under 1st-order perturbation theory in quantum mechanics. The definition of a "B star" can include objects that are large enough to be in Blue giant and Blue supergiant territory, beyond the size of standard main-sequence stars; the revised Yerkes Atlas system listed a dense grid of B-type dwarf spectral standard stars, however not all of these have survived to this day as standards. The "anchor points" of the MK spectral classification system among the B-type main-sequence dwarf stars, i.e. those standard stars that have remain unchanged since at least the 1940s, are upsilon Orionis, eta Aurigae, eta Ursae Majoris. Besides these anchor standards, the seminal review of MK classification by Morgan & Keenan listed "dagger standards" of Tau Scorpii, Omega Scorpii, 42 Orionis, 22 Scorpii, Rho Aurigae, 18 Tauri; the Revised MK Spectra Atlas of Morgan, Abt, & Tapscott further contributed the standards Beta2 Scorpii, 29 Persei, HD 36936, HD 21071.
Gray & Garrison contributed two B9 V standards: omega For A and HR 2328. The only published B4 V standard is 90 Leonis, from Lesh. There has been little agreement in the literature on choice of B6 V standard; some of the B-type stars of stellar class B0–B3 exhibit unusually strong lines of non-ionized helium. These chemically peculiar stars are termed helium-strong stars; these have strong magnetic fields in their photosphere. In contrast, there are helium-weak B-type stars with understrength helium lines and strong hydrogen spectra. Other chemically peculiar B-types stars are mercury–manganese stars with spectral types B7-B9; the aforementioned Be stars show a prominent emission spectrum of hydrogen. B-type stars known to have planets include the main-sequence B-types HIP 78530 b, the subgiants Kappa Andromedae b and a few B-type subdwarfs. Herbig Ae/Be star Stellar classification, Class B Star count
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
Star clusters are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars contain fewer than a few hundred members, are very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space though they are no longer gravitationally bound. Star clusters visible to the naked eye include the Pleiades and the Beehive Cluster. Globular clusters are spherical groupings of from 10,000 to several million stars packed into regions of from 10 to 30 light years across, they consist of old Population II stars—just a few hundred million years younger than the universe itself—which are yellow and red, with masses less than two solar masses. Such stars predominate within clusters because hotter and more massive stars have exploded as supernovae, or evolved through planetary nebula phases to end as white dwarfs.
Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions. In our galaxy, globular clusters are distributed spherically in the galactic halo, around the galactic centre, orbiting the centre in elliptical orbits. In 1917, the astronomer Harlow Shapley made the first reliable estimate the Sun's distance from the galactic centre based on the distribution of globular clusters; until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However improved distance measurements to globular clusters using the Hipparcos satellite and accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less. Our galaxy has about 150 globular clusters, some of which may have been captured from small galaxies disrupted by the Milky Way, as seems to be the case for the globular cluster M79.
Some galaxies are much richer in globulars: the giant elliptical galaxy M87 contains over a thousand. A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age; the brightest globular cluster in the northern hemisphere is Messier 13 in the constellation of Hercules. Super star clusters are large regions of recent star formation, are thought to be the precursors of globular clusters. Examples include Westerlund 1 in the Milky Way. Open clusters are different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane, are always found within spiral arms, they are young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67 for example. They form from H II regions such as the Orion Nebula. Open clusters contain up to a few hundred members, within a region up to about 30 light-years across.
Being much less densely populated than globular clusters, they are much less gravitationally bound, over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can result in the ejection of stars, a process known as'evaporation'; the most prominent open clusters are the Hyades in Taurus. The Double Cluster of h+Chi Persei can be prominent under dark skies. Open clusters are dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die. Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are used as standard candles. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe. Indeed, the open cluster NGC 7790 hosts three classical Cepheids which are critical for such efforts.
Embedded clusters are groups of young stars that are or encased in an Interstellar dust or gas, impervious to optical observations. Embedded clusters form in molecular clouds, when the clouds begin to form stars. There is ongoing star formation in these clusters, so embedded clusters may be home to various types of young stellar objects including protostars and pre-main-sequence stars. An example of an embedded cluster is the Trapezium cluster in the Orion Nebula. In ρ Ophiuchi cloud core region there is an embedded cluster; the embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through radiation pressure, stellar winds and outflows, or supernova explosions. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered leading to the disruption of a star cluster.
Most young embedded clusters disperse shortly after the end of star formation. The open clusters fou