|MadSci Network: Astronomy|
In 1863, Angelo Secchi found that he could order the spectra of stars into different types based on the strengths of certain spectral lines. This system was modified by Annie Cannon at Harvard Observatory. Initially, classifications were based on the strength of the a sequence of hydrogen absorption lines (called the Balmer lines), and the system was alphabetical from A to P (A had the strongest lines and P the weakest). With further study, some classifications were dropped and others rearranged so that the sequence is one of decreasing temperature, giving the spectral classification scheme in use today. The spectral classifications are O,B,A,F,G,K,M with R, N, and S recently added, where O stars have (surface) temperatures of about 50,000 K and M stars are approximately 3000 K. Each spectral type is divided into ten parts numbered from 0 to 9, so a B0 is hotter than a B5. Following the number and letter, is a Roman numeral which is indicative of the luminosity (inherent brightness) of the star. A bright supergiant is Ia, a faint supergiant is Ib, a bright giant is II, a normal giant is III, a subgiant is IV, and a normal dwarf or main-sequence star is V. Our sun is a G2 V, because its surface temperature is about 5800 K and it is a yellow, main sequence star. Temperature is what determines the spectral type of a star. [The system is also now being extended to L and T classes, though in some cases objects with these designations may be brown dwarfs, i.e., objects in which nuclear fusion does not occur in the core, rather than stars.]
Protostellar clouds are composed primarily of Hydrogen and Helium with trace amounts of all the other elements which if added together may come to as much as about 0.2%. All these other elements have been produced in the cores of stars during fusion reactions or in supernovae. The first stars to form would have had no heavy elements present and therefore could not have had planets around them. It is unclear what the metallicity of a system must be before there is sufficient material to form planetary bodies, but the metallicity undoubtedly influences how much planetary material can form in a star system. The Sun, for example, has a higher metallicity than most other stars of its age. When, in the age of the Universe, planetary systems could have formed is a current topic of research. [A recent, preliminary estimate is that the typical planetary system is older than ours, see Cosmological Constraints on Terrestrial Planet Formation.] Currently cloud material leading to the formation of a star is essentially the same for all stars with only small variations in the metallicity, or the sum of all elements heavier than He, from one molecular cloud to another. Assuming there is enough heavy material to form planets in the first place, we can move on to a discussion of the elemental composition of the planets.
The elemental composition of planets is not determined primarily by the composition of the original protostellar cloud. Rather, it is due primarily to temperature. Closer to the forming protostar, the gas and dust that make up the protoplanetary disk are warmer than in regions farther out. At the distance from Sun where Earth's material would have come from, the temperature of the protoplanetary material is estimated to have been about 800 to 1300 K. The temperatures of the protoplanetary disk where Mercury and Venus formed would have been somewhat higher. At these high temperatures, only refractory materials, such as aluminum, titanium, silicon, iron, nickel, magnesium, and sodium could condense out of the protoplanetary nebula. Farther out, near Jupiter's orbit, all the above elements condensed out of the nebula. In addition to these, it was cool enough for water molecules to condense out. Farther out, ammonia and methane could also condense out of the nebular material. That is why the outer solar system bodies have, in general, much more ice and other volatiles than inner solar system bodies. It was never cold enough for H and He to condense, but the giant planets grew to a large enough size that they could hold these atoms with their gravity. The presence of volatiles such as water on Earth was a subsequent addition due to cometary or asteroidal impacts.
The spectral type of a star can influence planet formation. For example, O and B type stars, which are very hot, have high stellar winds that may disperse planetary disk material before it can form planets. The biggest determination of the composition of a planetary body, however, is the temperature of the material in the nebula from which it formed.
For more information, see an introductory text book such as Introductory Astronomy & Astrophysics by Zeilik, StarDate's Solar System Guide, lecture notes on planetary system formation, or The Nine Planets site.
Refractory materials are those which evaporate at a high temperature. Volatile materials evaporate at low temperatures.
The condensation temperature is the temperature above which an atom or molecule is in the gas phase. Below this temperature, the atom or molecule is a solid.
Try the links in the MadSci Library for more information on Astronomy.