Spectral Type

In Astronomy, the star S show four principal characteristics: their Temperature of surface, them revolved on the surface, them Mass and them Luminosity. These characteristics are not independent from/to each other and are not directly measurable.

If you observe the sky attentively, you will notice that the stars all are not white: some are red, the other bluish ones. They are presented in a variety of colors determined by their temperature of surface. The hot stars are blue while the cold stars are red. In an order ascending of temperature, a star will be red, orange, yellow, white, blue and violet. This order of color can seem strange: one often associates the red with the heat and blue cold. But physics shows us the reverse. The hotter one body is, the more the photons which escape from it have energy, and the shorter their wavelength is.

A priori, one could classify stars according to their Température of surface by using the Loi of Wien, but this raises some difficulties. The characteristics spectral S make it possible to classify stars differently, by indirectly using information which relates to their temperature or their gravity. Indeed, the absorption lines present in the electromagnetic Specter of stars can be observed only in one certain range of temperature because it is only in this range that the levels energy S atomic relative to these lines are populated. In the same way, the width of the absorption lines depends on gravity on the surface of star and thus of its luminosity.

Diagram of Hertzsprung-Russell

At the beginning of the 20th century, Ejnar Hertzsprung and Henry Norris Russell studied the relation between the luminosity and the temperature of surface of stars. They independently concluded that the majority of stars are in a precise area of graphics luminosity-temperature. One now indicates such a graph “Diagramme of Hertzsprung-Russell” (or more simply “diagram HR”).

Indeed, 80% of stars are located on a diagonal tape of the graph. One calls this band “principal sequence”. It shows a relation of proportionality between the temperature and the luminosity. The majority of stars are found there since they pass there most of their life.

The stars located apart from the principal sequence are is at the beginning or the end of their life. It is, except for the dwarf white ones, of the transitional stages of more or less short duration. Thus, a star moves on the diagram. At the end of its life, it leaves the principal sequence and becomes a dwarf giantess then white (see the article evolution of the stars).

Classification of Harvard

The classification of Harvard east that which allots a spectral type to a star, and corresponds overall to a scale of temperature. The classification of Yerkes is that which allots a Classe of luminosity to a star, and corresponds overall, on a scale of ray (see Loi of Stefan-Boltzmann) for a given temperature.

This method was developed with the observatory of Harvard at the beginning of the 20th century by Henry Draper. After death To drape, its widow bequeathed to the observatory an amount of money to continue the work of classification. Most of this work was carried out by the “girls” of the observatory, mainly Annie Jump Canon and Antonia Maury while being based on the work of Williamina Fleming. This work was completed by the publication of the Henry Draper Catalog (HD) between 1918 and 1924. The catalog contained 225  000 stars until the ninth magnitude. The classification of Harvard is based on absorption lines which are especially sensitive to the temperature rather than with the gravity of surface. The various classes and their temperature are the following ones:

To memorize the order of the spectral types (OBAFGKM), the english-speaking use the sentence “Oh, Be has Fine Girl/Guy, KIS Me! ” , which results in “Oh! Would be nice a fille/un nice guy, kisses me”; there are many alternatives. In French, one could say: “Observe well with the firmament: imposing multicoloured kaleidoscope! ” The reason of the strange arrangement of the letters is historical. When the first star spectra were taken, it was noticed that the line of the Hydrogène varied much and one classified stars according to the intensity of the line of Balmer: of has, strongest, with the Q , weakest. Then the lines of others elements chemical came concerned: the lines H and K of the Calcium, the line D of the Sodium, etc Later, it appeared that many of these classes overlapped and were withdrawn. It is only well later still that it was discovered that the intensity of the lines depended primarily on the temperature of surface of star.

Currently, these classes are subdivided using the figures (0-9): A0 for the hottest stars of the class has and A9 for the least hot. For example, our Sun is a star of the type G2 . More recently, classification was extended in W O B HAS F G K MR. L T and R NR C S , where W is the stars Wolf-Rayet, L and T represents extremely cold stars: brown dwarf , and R NR C S are used for the carbonaceous stars.

Stars WR (or W)

See detailed article: Star Wolf-Rayet.

Classify O

The stars of classes O are very hot (temperature of surface: 35.000 K for delta Ori ) and very luminous and of blue color. For example, Naos, in the Constellation of the Poop, is shining nearly a million times stronger than the Sun. These stars have intense lines of Hélium and rather weak lines of Hydrogène, they emit mainly in the Ultraviolet. These stars are so energy that they develop a strong stellar wind and thus lose matter which then forms envelopes giving of the emission lines (standard Oe for the emissions in the Hydrogène, Of type for the emissions in HeII, NIII).

Classify B

The stars of class B are also very luminous and hot (temperature of surface: 13.000 K); Rigel, in the constellation of Orion is a blue Supergéante of class B . Their spectrum has neutral helium lines and the hydrogen lines are rather weak (they are called lines of Balmer ). The stars of the type O and B are so powerful that they live only very little of time. They thus deviate only little the place where they were formed. Thus, they tend to be assembled in what is called associations OB1 which gather these stars within immense a molecular Nuage. Association OB1 of Orion forms a whole arm of the Milky Way and contains all the constellation of Orion. It should be noted that it is the presence of very brilliant stars and not their number which make that the arms of the Galaxie S appear more brilliant. One can add that among the 100 most brilliant stars, 1/3 are of stars of the type B. Certain stars B show lines in emission in their spectrum. According to whether the lines are prohibited lines (in) or normal lines, one speaks about stars “B” or “Be” (the “E” for emission, to see the detailed article.).

Classify has

The stars of class has are among most common visible with the naked eye. Alpha Cygni (Deneb) in the Constellation of the swan and Sirius the most brilliant star of the sky in the visible one, are two stars of class has . Like all those of this type, they are white, their spectrum has rather intense hydrogen lines (lines of Balmer) and more slightly shows the presence of metals ized Ion (line K of ionized calcium).

Some of them show noted characteristics remarkable Am or a. They belong to stars with strong magnetic field (spots) or presenting strong concentration of certain metals (by levitation of with the radiative forces) reinforcing the spectral lines with these chemical elements.

Classify F

The stars of class F are still very luminous (temperature of surface: 7200 K to 6.000 K), and are in general stars of the principal Séquence, like Fomalhaut in the Constellation of the southern Poisson, Canopus, the Pole star, Procyon A. Their spectrum is characterized by hydrogen lines weaker than in stars has and the presence of lines of neutral and ionized metals (FeI, FeII, TiII, CaI, Ca II, MgI, etc).

Classify G

The stars of class G are the best known ones, for the only reason that our Sun is of this class. They have hydrogen lines even weaker than those of class F and ionized or neutral metal lines. The lines of CaII H & K are very marked. The type G is one of the last (in addition to K and M, below) where one still distinguishes (being given the temperature of surface from 5.000 to 6.000 K) from the molecular lines still rather strong (CH, CN, C2, OH). Besides they owe their name “G” with the CH molecule which presents a strong absorption towards 4300 identified by Fraunhofer by the letter G. Centauri alpha has is a star G.

Classify K

The stars of class K are stars of orange color, slightly less less hot than the Sun (temperature of surface: 4.000 K). Some, like Antarès, are giant S reds whereas others, like Alpha Centauri B, are stars of the principal sequence. They have hydrogen lines very weak, even non-existent, and especially of the neutral metal lines. Some molecular compounds are visible there: CH, CN, CO, as well as broad band of TiO (Oxide of Titanium) for coldest.

Classify M

The stars of class M are most numerous (temperature of surface: 2.600 K). All the dwarf reds, is 90% of existing stars, are of this type, such as for example Proxima Centauri. Bételgeuse, just as the variable stars of type Mira is also of this type. Their spectra show lines corresponding to Molécule S (CN, CH, CO, TiO, VO, MgH, H2, etc) and neutral metals, the lines of the Oxyde of Titane can be very intense and the lines of hydrogen miss generally.

Classify L

The stars of the new class L are of red color very dark and illuminate especially in the Infrarouge. Their Gaz is rather cold so that the Hydrure S of metals and the alkaline metals prevail in their spectrum.

Classify T

The stars of class T are at the end of the scale. It is either of stars hardly enough massive to be able to carry out reactions of nuclear Fusion, or brown dwarf (quasi-stars deprived of nuclear fusion). They emit little or not visible Lumière, but only of the infra-reds. Their temperature of surface can be as low as 600 C, which allows the formation of Molécule S complexes, as confirms it the observation of lines of Méthane in the spectrum of some of these stars.

Nebula E from which stars are formed) then the number off stars in the Galaxy should Be several orders off Magnitude higher than what we know butt. It' S theorized that thesis propylids are in has race with each other. The first one to form will become has Proto-star, which are very violent one objects and will disrupt other propylids in the vicinity, stripping them off to their gas. The victim propylids will then probably go one to become hand sequence stars gold brown dwarf stars off the L and T classes, invisible goal quite to custom. Live Since they so long (No star below 0.8 solar Mass be has ever died in the history off the galaxy) then thesis smaller stars will accumulate over time. -->

Classify R, NR, S and C

The stars of class R , NR S and C are the carbonaceous stars, of giant stars having a strong proportion in Carbone. They correspond to a classification in parallel with stars of class G with M and were recently unified in a single class C . The stars of class S are located halfway between carbonaceous stars and those of class M and have in their spectrum of the lines of Oxyde of Zinc rather than of Titane. They have an abundance in Oxygène and almost identical Carbone, both element S being almost exclusively in the form of Carbon monoxide (CO). When a star is rather cold so that CO can be formed, this one consumes a maximum of oxygen and of carbon and there remains nothing any more but the element in excess: oxygen in stars of the principal sequence, carbon in carbonaceous stars and about nothing in stars class S .

Actually, there exists a continuity between stars of the principal sequence and the stars carbonaceous which would require another dimension in classification to be correctly treated.

See too

Internal bonds

External bond

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