Stellar classification

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Stars can be classified by their surface temperatures as determined from Wien's Displacement Law, but this poses practical difficulties for distant stars. Spectral characteristics offer a way to classify stars which gives information about temperature in a different way - particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. The standard classes and their surface temperatures are:

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[edit] Stellar Classification

Star Type Mass Temperature Radius Apparent Color
Type O >16 >30,000 >6.6 Blue
Type B 2.1 - 16 10,000-30,000 1.8 - 6.6 Blue-white
Type A 1.4 - 2.1 7,500-10,000 1.4 - 1.8 White
Type F 1.04 - 1.4 6,000-7,500 1.15 - 1.4 Yellow-white
Type G 0.8 - 1.04 5,200-6,000 0.96 - 1.15 Yellow
Type K 0.45 - 0.8 3,700-5,200 0.7 - 0.96 Yellow-orange
Type M 0.08 - 0.45 2,300-3,700 <0.7 Red
Brown Dwarf 0.002 - 0.08 <2,300 <0.7 Magenta

More recently, the classification was extended into O B A F G K M L T, where L and T are extremely cool stars or brown dwarves.

Class O stars are very hot and very luminous, being strongly blue in colour. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Class O stars emit most of their radiation in ultra-violet.

Class B stars are again extremely luminous. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area which they were formed in as they don't have the time. They, then, tend to cluster together in what we call OB1 associations, which are associated with giant molecular clouds.

Class A stars are amongst the more common naked eye stars. As with all class A stars, they are white. Many white dwarves are also A. They have strong hydrogen lines and also ionized metals.

Class F stars are still quite powerful but they tend to be main sequence stars. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow.

Class G stars are probably the most well known for only the reason that the Sun is of this class. They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be.

Class K is slightly cooler than our Sun, they're orange stars. Some K stars are giants and supergiants. They have extremely weak hydrogen lines, if it's present at all, and mostly neutral metals.

Class M is by far the most common class if we go by the number of stars. All our red dwarves go in here and they are plentiful, more than 70% of stars are red dwarves. M is also host to most giants and some supergiants. The spectrum of an M star shows lines belonging to molecules, neutral metals but hydrogen is usually absent. Titanium oxide[?] can be strong in M stars.

The new class L are stars that are a very dark red in colour, they are brightest in infra red. Their gas is cool enough to allow metal hydrides and alkali metals to be prominent in the spectrum.

Right at the bottom of the scale is class T. These are stars barely big enough to be stars and others that are substellar, being of the brown dwarf variety. They are black, emitting little or no visible light but being strongest in infrared. Their surface temperature is a stark contrast to the fifty thousand degrees or more for O stars, being a cool 700 degrees Celsius. Complex molecules can form, evidenced by the strong methane lines in their spectra.

T and L could be more common than all the other classes combined, if recent research is accurate. From studying the number of propylids (clumps of gas in nebulae from which stars are formed) then the number of stars in the galaxy should be several orders of magnitude higher than what we know about. It's theorised that these propylids are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other propylids in the vicinity, stripping them of their gas. The victim propylids will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long (no star below 0.8 solar masses has ever died in the history of the galaxy) then these smaller stars will accumulate over time.

Also occasionally used are the stellar classifications R, N and S. R and N stars are carbon stars (that is, giants) which run parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier C, with N0 starting at roughly C6. S stars have ZrO lines rather than TiO, and are in between the M stars and the carbon stars. S stars have carbon and oxygen abundances are almost exactly equal, and both elements are locked up almost entirely in CO molecules. For stars cool enough for CO to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" on the normal main sequence, "leftover carbon" on the C sequence, and "leftover nothing" on the S sequence.

In reality the relation between these stars and the traditional main sequence suggest a rather large continuum of carbon abundance and if fully explored would add another dimension to the stellar classification system.

[edit] Yerkes spectral classification

The Yerkes spectral classification, also called the MKK system, is a system of stellar spectral classification introduced in the Yerkes Observatory.

This classification is based on spectral lines sensitive to stellar surface gravity which is related to luminosity, as opposed to the Harvard classification which is based on surface temperature. Since the radius of a giant star is much larger than a dwarf star while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf. These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured.

Six different luminosity classes are distinguished:

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