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G star A yellowish star of spectral type G, with a surface temperature in the range 5,000 to 5,800°C and mass of 0.8 to 1.1 solar masses. G stars have spectra containing many absorption lines of neutral and ionized metals, together with some molecular absorption bands. The H and K lines of ionized calcium are particularly strong. Main sequence G stars, of which the Sun and Alpha Centauri A are examples, are the prime targets of searches for extrasolar planets and targeted SETI programs. G-type giant stars, such as Capella, are slightly cooler but more luminous than their main sequence counterparts, while G-type supergiants have a mass of 10 to 12 solar masses and a luminosity of 10,000 to 300,000 times that of the Sun. Picture: Our Sun
Types of Stars Stars can be classified by their surface temperatures as determined from Wien's Displacement Law1, 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 are: Spectral TypeTemperature of surfaceType of Star O 30000 - 60000 K Blue stars B 10000 - 30000 K Blue - white stars A 7500 - 10000 K White stars F 6000 - 7500 K Yellow - white stars G 5000 - 6000 K Yellow stars K 3500 - 5000 K Yellow - orange stars M less than 3500 K Red stars
The commonly used mnemonic for the sequence of these classifications is "Oh Be A Fine Girl, Kiss Me". Apart from spectral type the scientists specify so-called luminosity class. Luminos Ia Very luminous supergiants Ib Less luminous supergiants II Luminous giants III Giants IV Subgiants V Main sequence stars (dwarf stars) VI Subdwarf VII White Dwarfs
O stars A massive blue luminous star, of spectral type O, with a surface temperature of about 30,000 to more than 50,000°C and a mass of 20 to about 100 solar masses. Its spectrum is dominated by emission lines of singly ionized helium (these lines forming the so-called Pickering series). Most other lines are from at least doubly ionized elements, though H and He I lines are also present. O stars are powerful ionizers of the surrounding interstellar medium, giving rise to large, bright emission nebulae and virtually defining the spiral arms of the Milky Way. Most O stars are very fast rotators. They have lifetimes of only 3 to 6 million years. Well-known examples include Alnitak (Zeta Orionis) and Naos (Zeta Puppis). Oe stars have prominent hydrogen emission lines. Oef stars are early-type O stars that show double emission lines in He II at 4686 Å. Of stars are peculiar O stars in which emission features at 4634 to 4641 Å from N III and 4686 Å from He II are present. They have a well-developed absorption spectrum, which implies that the excitation mechanism of the emission lines is selective, unlike that of Wolf-Rayet stars. The spectra of Of stars are usually variable, and the intensities of their emission lines vary in an irregular way. Of stars belong to extreme Population I. All O stars earlier than O5 are Of. Picture: Alnitak is the left-hand star in Orion's Belt. Image: NASA
B star A large, luminous, blue-white star of spectral type B with a surface temperature of 10,200 to 30,000°C. The spectrum is characterized by absorption lines of neutral or singly ionized helium, with lines of atomic hydrogen evident, especially at the cooler end of the range. Main sequence B stars, examples of which include Spica and Regulus, have a mass in the range 3 to 20 solar masses and a luminosity of 100 to 50,000 times that of the Sun. Often they are found together with O stars in OB associations since, being massive, they are short-lived and therefore do not survive long enough to move far from the place where they were formed. Their brief main sequence careers, measured in tens of millions of years, probably allows too little time for even the most primitive forms of life to develop on any worlds that circle around them (assuming that life could exist at all in such an environment). B-type supergiants, of which Rigel is a familiar example, may be up to 25 times as massive and 250,000 times as luminous as the Sun. Picture: Rigel (right) and the Witch Head Nebula.
A star A star of spectral type A, white in color, with a spectrum dominated by the Balmer series of hydrogen. Lines of heavy elements, such as iron, are noticeable at the cooler end of the range. Main sequence A stars have surface temperatures of 7,500 to 9,900 K, luminosities of 7 to 80 Lsun, and masses of 1.5 to 3 Msun; familiar examples include Sirius, Vega, and Altair. A-type supergiants, such as Deneb, may be as hot as 11,000 K and have masses up to 16 Msun and luminosities of up to 35,000 Lsun. Among A-type peculiar stars are Ae stars, Am stars, and Ap stars. Also, two of the main kinds of pulsating variables, RR Lyrae stars and Delta Scuti stars, have surface temperatures in the A-star range. Picture: Altair - The brightest star in the constellation Aquila, the twelfth brightest star in the sky.
F star A white or yellowish-white star of spectral type F whose spectrum shows strong absorption lines of ionized calcium which are more prominent than the hydrogen lines. Moderately strong lines due to iron and other heavier elements are also in evidence. Main sequence F stars, of which Procyon is an example, have a surface temperature of 5,800 to 6,900°C, a mass of 1.2 to 1.6 solar masses, and a luminosity of 2 to 6 times that of the Sun. Relatively nearby, late-type members of this category are generally included in the list of target stars for extrasolar planet searches and targeted SETI programs. F-type supergiants, of which Canopus and Polaris (the Pole Star) are examples, have a mass up to 12 solar masses and a luminosity up to 32,000 times that of the Sun. Picture: Canopus - The brightest star in the constellation Carina, the second brightest star in the sky, and one of the principal stars used in spacecraft navigation. It was known in antiquity as the
G star A yellowish star of spectral type G, with a surface temperature in the range 5,000 to 5,800°C and mass of 0.8 to 1.1 solar masses. G stars have spectra containing many absorption lines of neutral and ionized metals, together with some molecular absorption bands. The H and K lines of ionized calcium are particularly strong. Main sequence G stars, of which the Sun and Alpha Centauri A are examples, are the prime targets of searches for extrasolar planets and targeted SETI programs. G-type giant stars, such as Capella, are slightly cooler but more luminous than their main sequence counterparts, while G-type supergiants have a mass of 10 to 12 solar masses and a luminosity of 10,000 to 300,000 times that of the Sun. Picture: Our Sun
K star An orange-red star, of spectral type K. The spectra of K stars are dominated by the H and K lines of calcium and lines of neutral iron and titanium, with molecular bands due to cyanogen (CN) and titanium dioxide (TiO) becoming increasingly prominent at the cooler end of the range. K-type main sequence stars (that is, K-tye dwarfs) are intermediate in size and temperature between M-type red dwarfs and Sun-like stars (type G), with a mass of 0.5 to 0.8 Msun, a temperature of 3900 to 5200 K (3600 to 4900°C), and a luminosity 0.1 to 0.4 Lsun. Nearby examples include Epsilon Indi, Epsilon Eridani, and Tau Ceti, the latter two having been the target stars of Project Ozma. Early type main sequence K stars within a few tens of light-years of the Sun are generally included in the list of target stars (see FGK stars) of searches for extrasolar planets and targeted SETI programs, since if they have planets orbiting within their habitable zones there is the possibility that these worlds support life of some kind. Giant K types are typically 100 to 400 K cooler, and have luminosities of 60 to 300 Lsun and masses of 1.1 to 1.2 Msun. Familiar examples of K-type giant stars include Arcturus (K1), Aldebaran (K5), and Pollux. Picture: Pollux - The brightest star in the constellation Gemini and the 17th brightest in the sky.
M star A cool, red star, of spectral type M, with a surface temperature of less than 3,600°C. Molecular absorption bands are prominent in the spectrum, with bands of titanium oxide becoming dominant at the lower end of the temperature range. Main sequence M stars, known as red dwarfs, have a mass of less than 0.5 Msun and a luminosity of less than 0.08 Lsun; examples include the nearby Proxima Centauri and Barnard's Star. M-type giant stars, known as red giants, occur in the mass range 1.2 to 1.3 Msun and may have luminosities exceeding 300 Lsun. The largest stars of all are M-type supergiants, such as Betelgeuse and Antares, of mass of 13 to 25 Msun and luminosity of 40,000 to 500,000 Lsun. Picture: Antares (upper left), Rho Ophiuchi (blue), and Sigma Scorpii ( red).
R star - an obsolete name for what is now classed as a hotter carbon star and N star - an obsolete name for what is now classified as a cool carbon star (classes C6 to C9). A red giant whose spectrum is dominated by strong absorption bands of carbon-containing molecules; the Swan bands of C2 are especially prominent, with absorption by CN, CH, C3, SiC2, and CaII present to varying degrees, with often a strong sodium D line. Carbon stars, also known as C stars, have carbon/oxygen ratios that are typically four to five times higher than those of normal red giants and show little trace of the light metal oxide bands that are the usual red giant hallmark. They resemble S stars in their relative proportion of heavy and light metals, but contain far more carbon in their upper layers. The carbon is likely the dredged-up ashes of nuclear helium burning in the stellar interior. Carbon stars lose a significant fraction of their total mass in the form of a stellar wind which ultimately enriches the interstellar medium – the source of material for future generations of stars. Carbon stars were previously classified as stars of spectral type R (hotter, with surface temperatures of 4,000 to 5,000 K) and N (up to 10 times more luminous but cooler, with a temperature of about 3,000 K). They are typically associated with some circumstellar material in the form of sooty shells, disks, or clouds. S star A red giant of spectral type S, similar to an M star except that the dominant oxides in its spectrum are those of the metals of the fifth period of the periodic table (zirconium, yttrium, etc.) instead of the third (titanium, scandium, vanadium). S stars also have strong cyanogen (CN) molecular bands and contain spectral lines of lithium and technetium. Pure S stars, also called zirconium stars, are those in which zirconium oxide bands are very strong and titanium oxide bands are either absent or only barely detectable. Almost all S stars are long-period variables. So-called SC stars appear to be intermediate in type between S stars and carbon stars (C stars) and have a carbon/oxygen abundance ratio near unity.
