Star classification started in early 20th century. It can be based on: Surface Temperature and Luminosity.
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Based on Surface Temperature
The classification is called “Morgan – Keenan spectral Classificatoin” which is already formed by Annie Cannon. Most of the early work on stellar spectra was done early in the 20th century at Harvard University. The principal figure in this story was Annie Jump Cannon. She joined Harvard as an assistant to Observatory Directory Edward C. Pickering in the 1890's to participate in the classification of spectra. She quickly became very proficient at classification examining several hundred stars per hour. She completed a catalogue of spectral types for hundreds of thousands of stars
In astrophysics, stars are classified by their surface temperature due to the effect of hydrogen line strength associated to specific spectral patterns. The spectral classification is composed of 7 main types: O, B, A, F, G, K and M which has the popular mnemonic of “ Oh, Be A Fine Girl, Kiss Me ”.
The classification is called “Morgan – Keenan spectral Classificatoin” which is already formed by Annie Cannon. Most of the early work on stellar spectra was done early in the 20th century at Harvard University. The principal figure in this story was Annie Jump Cannon. She joined Harvard as an assistant to Observatory Directory Edward C. Pickering in the 1890's to participate in the classification of spectra. She quickly became very proficient at classification examining several hundred stars per hour. She completed a catalogue of spectral types for hundreds of thousands of stars
In astrophysics, stars are classified by their surface temperature due to the effect of hydrogen line strength associated to specific spectral patterns. The spectral classification is composed of 7 main types: O, B, A, F, G, K and M which has the popular mnemonic of “ Oh, Be A Fine Girl, Kiss Me ”.
Morgan Keenan Spectral Classification
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O Type are relatively rare. They have a very high surface Temperature, in the range of 30,000 K and above, and are violet-blue in color. Some of the most massive stars lie within this spectral class. Class O stars frequently have complicated surroundings which make measurement of their spectra difficult.
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B Type is the first of the really populous classes. These stars are blue in color and burn hotly, with surface temperatures lying between 10,000 K —30,000 K. Their spectra have natural helium which are most prominent at the B2 subclass and moderate hydrogen lines.
A Type have surface temperatures in the range of 7,500 K — 10,000 K and are white or bluish white in color. Some of the brightest and most famous stars in the sky belong to this classification. They have a strong hydrogen lines at a maximum by A0 and lines of ionized metals at a maximum at A5. |
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F Type has a yellow-white or white color and surface temperatures between 6,000 K — 7,500 K. Neutral metals begin to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals.
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G Type, with temperatures ranging between 5,000 K — 6,000 K, have spectra that betray the existence of “metals” or “heavy elements” (any element heavier than Helium) and are yellow in color. They make up about 7.5% nearly one in thirteen of the main sequence stars in the solar neighborhood. This is where our Sun belongs to.
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K Type are occasionally referred to as Arcturian Stars, after the brightest of their type. Their surface temperatures are between 3,500 K — 5,000 K, which is a temperature low enough for simple molecules to form and are orange in color. They are orangish starts that are slightly cooler that the Sun. They have extremely weak hydrogen lines, if they are presnt at all, and mostly neutral metals. K spectrum stars may potentially increases the chances of life developing on orbiting planets that are within the habitable zone.
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M Type, the coolest of the common star types, these stars have very cool surface temperatures, below 3,500 K, which allows more complex molecules to form. These stars are red in color. Almost M stars are red dwarfs. The spectrum of a class M star shows lines belonging to oxide molecules in visible and all neutral metals but absorption lines of hydrogen are usually absent. Main sequence starts of this class have such low luminosities that results to not bright enough to be visible to see with the unaided eye.
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Due to presently discoveries of stars, a number of new spectral types have been taken as a record.
Class W represents the Wolf-Rayet Stars notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionized helium, nitrogen, carbon and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by stellar winds, thereby directly exposing their hot helium shells. Class W is further divided into subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers).
Class W represents the Wolf-Rayet Stars notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionized helium, nitrogen, carbon and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by stellar winds, thereby directly exposing their hot helium shells. Class W is further divided into subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers).
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Class L is cooler than M stars. Some have masses large enough to support hydrogen fusion, but some are of substellar mass. They are very dark red in color and brightest in infrared. Their atmosphere is cool enough to allow metal hydrides and alkali metals to be prominent in their spectra. They have a temperature of 1,300 – 2400 Kelvin.
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Class T are cool brown dwarfs with the surface temperatures between 700 and 1300 Kelvin. Their emission peaks in the infrared. Methane is prominent in their spectra.
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Class Y are cooler than those of spectral class T and have qualitatively different spectra from them. The surface temperature is approximately less than 600 Kelvin.
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Class C are known to be Carbon Stars. These are red giants near the end of their lives in which there is an excess carbon in the atmosphere.
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Class S have zirconium monoxide lines in addition to those of titanium monoxide and are in between the class M starts and the carbon stars.
Class D (Degenerate) is the modern classification used for white dwarfs – low mass stars that are no longer undergoing nuclear fusion and have shrunk to planetary size, slowly cooling down.
Class D (Degenerate) is the modern classification used for white dwarfs – low mass stars that are no longer undergoing nuclear fusion and have shrunk to planetary size, slowly cooling down.
Based on Luminosity
Over the years, astronomers have developed a system for classifying stars according to the widths of their spectral lines. Because the line width depends on pressure in the stellar photosphere, and because this pressure in turn is well correlated with luminosity, this stellar property has come to be known as Luminosity Class. The information about the stars luminosity is given by a Roman numeral from I to VII. This luminosity class is simply appended to the spectral class, for example the star Betelgeuse is a “M2 I” star, which means it has a “M2” spectral class and a “I” luminosity class.
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Supergiants are extremely massive and luminous stars, usually nearing the end of their lifespan. These are subdivided into:*
- 0 or Ia+ - Hypergiants or Extremely luminous supergiants (Cygnus OB2#12)
- Ia - Luminous supergiants (Eta Canis Majoris)
- Iab - Intermediate luminous supergiants (Gamma Cygni)
- Ib - Less luminous supergiants (Zeta Persei)
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Bright Giants is a relatively rare group of giant stars that are very luminous. As an example some of them are a thousand times brighter than our own Sun.
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Giants are typically a hundred times more luminous than our own Sun, but considerably more massive.
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Subgiants are far more massive and luminous than our own Sun but fall short of the true giants.
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Main Sequence is a very numerous class of main sequence stars, whose mass and luminosity is generally comparable with that of our own Sun.
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Subdwarf is denoted by “sd” under Yerkes spectral classification system. They are They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On an Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence. The term "subdwarf" was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs". The explanation of their underluminosity lies in their low metallicity: these stars are unenriched in elements heavier than helium. The lower metallicity decreases the opacity of their outer layers and decreases the radiation, resulting in a smaller, hotter star for a given mass.
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White Dwarfs is also called as a degenerate dwarf. It is a stellar remnant composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun, and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy.
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Yellow Dwarf is also looks like a main sequence star but it is smaller in size. An example of this is our sun.
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Brown Dwarf is also called a “failed star”. This is sub stellar objects that never accumulated enough material to become stars. They are too small to generate the heat required for hydrogen fusion. It constitutes the midpoint between the smallest red dwarf stars and massive planets like Jupiter. They are the same size as Jupiter, but to qualify as a brown dwarf, they must be at least 13 times heavier. Their cold exteriors emit radiation beyond the red region of the spectrum, and to the human observer they appear magenta rather than brown. As brown dwarfs gradually cool, they become difficult to identify, and it is unclear how many exist.
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Neutron Star is a stellar remnant. It is form when stars larger than about 10 solar masses exhaust the fuel and the core collapse.