Stars are composed mainly of hydrogen and helium, with trace amounts of metals. They have extremely hot, dense cores which emit radiation in the form of a continuous spectrum (from the photosphere, the deepest layer which can be observed) which can be well approximated as a blackbody. This radiation then passes through the upper layers of the atmosphere, which are still hot but of lower density, and which absorb radiation preferentially at key wavelengths determined by the atomic composition of the atmosphere. The observed absorption lines depend primarily upon the exitation and ionization of the atmosphere, rather than its chemical composition.

We classify stars according to the characteristics of their spectral line features. Most differences (see this line plot, and these two-dimensional optical spectra) are caused by changes in temperature. A sub-class number (spectal type) further orders each alphabetic type from 0 (hottest) to 9 (coolest).

The spectral sequence (based on temperature): O B A F G K M L T

• O: (30,000 - 50,000 K) Massive, bright, hot stars, the bluest end (and the rarest members) of the Main Sequence. Hydrogen and metals are ionized, so there are only a few, weak H absorption lines but strong He+ lines. Sub-classified by comparing strength of He I and He II features.

• B: (11,000 - 30,000 K) Hot stars, but covering a wide range of masses and temperatures; often associated with O stars. Hydrogen is ionized, so H lines are weak but He lines strong (He increasing from B0 through B9, HeI strengthens from B0 to B2 and then fades, HeII first appears at B0), a few weak metal lines (highly ionized Si, O, Ca). B stars dominate the sky to the casual observer, and make up the seven brightest members of the Pleides.

• A: (7,500 - 11,000 K) Hydrogen is neutral and excited, so H lines are strong; weak Ca+ lines in later (A9) types.

• F: (5,900 - 7,500 K) H lines get weaker and weaker from F0 through F9, Ca+ gets stronger, and weak metal lines appear.

• G: (5,200 - 5,900 K) Hydrogen is cool, so hydrogen lines are weak (and weaken with subclass); Ca+ and Fe+ features strong, other metals appear.

• K: (3,900 - 5,200 K) H weak and begins to disappear with subclass; strong metal lines, weak hydrocarbon (CH and CH) molecular bands begin to appear

• M: (2,500 - 3,900 K) Includes both red dwarf and red giants. Strong molecular absorption bands (TiO and VO) and neutral metals, H disappears.

• L: (1,300 - 2,500 K) Strong metal-hydride molecular bands (CrH and FeH) and neutral metals (K, Cs, Rb), but TiO and VO bands are nearly absent. Sub-classifications are detailed here.

• T: ( < 1,300 K) Strong bands of methane (CH4), like Jupiter; low-mass brown dwarfs below the hydrogen fusion limit.

[NMSU, N. Vogt]

The MK spectral classification was originally based on the appearance of pairs of spectral lines in the blue region, at a resolution of 2A. Here are the critical ratios. Why do you think these data were used?

Class	Line pairs
O5 - O9	4471 He I	4541 He II
B0 - B1	4552 Si III	4089 Si IV
B2 - B8	4129 Si II	4121 He I
B8 - A2	4471 He I	4481 Mg II
B8 - A2	4026 He I	3934 Ca II
A2 - F5	4032 Mn I	4130 Mn I
A2 - F5	4300 CH	4385 CH
F2 - K	4300 Gband	4340 H
F5 - G5	4045 Fe I	4101 H
F5 - G5	4226 Ca I	4340 H
G5 - K0	4144 Fe I	4101 H
K0 - K5	4226 Ca I	4325 Ca I
K0 - K5	4290 Ca I	4300 Ca I

The secondary classification depends upon luminosity, and is designated by the Roman numerals I thru V, in order of decreasing luminosity:

Class	Name	Description
Ia	Bright supergiants	The brightest of the very massive and luminous stars, near the end of their lives.
Ib	Supergiants	Very massive and luminous stars near the end of their lives (wide range of temperatures, narrow range of luminosities). Very rare - 1 in a million stars is a supergiant.
II	Bright giants	Stars with a luminosity between giant and supergiant class.
III	Giants	Mainly low-mass stars at the end of their lives that have swelled to become giants (large but cool); also some high mass stars evolving on their way to supergiant status.
IV	Subgiants	Stars which have begun evolving to giant or supergiant status.
V	Dwarfs	All normal hydrogen-burning stars; stars spend most of their lives in this category before evolving up the scale. O and B stars in this category are very luminous, brighter than most giants.
	(Main Sequence)

How are spectral measurements of the following elements and molecules useful in determining temperatures, luminosities, and stellar composition: Ca, Mg, Na, H, CN, CH, MgH, CaH, TiO?

• HeI (3820A) indicates stellar types earlier than B5
• H and H indicate lines B through F types
• FeI (4384A) indicates G and K types
• molecular bands point towards cool K and M types

U	B	V	R	I	J	H	K	L	M
3650 A	4450 A	5510 A	6580 A	8060 A	1.22 u	1.63 u	2.19 u	3.45 u	4.75 u