Stellar Classification

Stellar Classification Lab 4

Classification of Stars • Based on spectral characteristics • This gives information about temperature in a different way • Absorption lines can be observed only for a certain range of temperatures • The range involved shows atomic energy levels which have been populated

So it is complicated….. • Difference in stars is not just their chemical make up but their surface temperature AND size • Spectra of two stars with same temperature but different sizes is not the same • Also, larger star will have higher luminosity

Spectral Types • Spectral type of a star gives information about temperature, luminosity, and color • From this information, the distance, mass, surrounding environment, and past history of the star can be deduced • Spectral classification is basic to evolution of stars • An early schema (from the 19th century) ranked stars from A to P, which is the origin of the currently used spectral classes.

Note! • While these descriptions of stellar colors are traditional in astronomy, they really describe the light after it has been scattered by the atmosphere • The Sun is not in fact a yellow star, but has essentially the color temperature of a black body of 5780K

Standard Classes

Spectral Types • Class O stars are very hot and very luminous, being strongly blue in color • 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 • Naos (in Puppis) shines with a power close to a million times solar

Class B • Class B stars are again extremely luminous • Rigel (in Orion) is a prominent B class blue supergiant • Their spectra have neutral helium and moderate hydrogen lines • As O and B stars are so powerful, they live for a very short time and tend to cluster together in OB1 associations, which are associated with giant molecular clouds • The Orion OB1 association is an entire spiral arm of our Galaxy and contains all the constellation of Orion.

Class A • Class A stars are amongst the more common naked eye stars • Deneb in Cygnus is another star of formidable power, while Sirius is also an A class star, but not nearly as powerful • As with all class A stars, they are white. • Many white dwarfs are also A. • They have strong hydrogen lines and also ionized metals.

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

Class G • Our Sun is of this class. • They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals. • 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 it is an unstable place

Class K • Class K are orange stars which are slightly cooler than our Sun. • Some K stars are giants and supergiants, such as Arcturus, while others like Alpha Centauri B are main sequence stars. • They have extremely weak hydrogen lines, if at all, and mostly neutral metals.

Class M • Class M has the most number of stars • All red dwarfs are Class M • More than 90% of stars are red dwarfs, such as Proxima Centauri. • M is also host to most giants and some supergiants such as Antares and Betelgeuse. • The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. • Titanium oxide can be strong in M stars. • The red color is deceptive, and is due to the dimness of the star. • An equally hot object like a halogen lamp (3000˚ K) which is white hot, appears red at a few km away

Other Spectral Types • W: Up to 70,000˚K - Wolf-Rayet stars • L: 1,500 - 2,000˚K - Stars with masses insufficient to run the regular hydrogen fusion process (brown dwarfs).Also contain lithium which is rapidly destroyed in hotter stars. • T: 1,000˚K - Cooler brown dwarfs with methane in the spectrum. • C: Carbon stars. • R: Formerly a class on its own representing the carbon star equivalent of Class K stars • N: Formerly a class on its own representing the carbon star equivalent of Class M stars • S: Similar to Class M stars, but with zirconium oxide replacing the regular titanium oxide. • D: White dwarfs

Odd Arrangement of Letters • The reason for the odd arrangement of letters is historical • When people first started taking spectra of stars, they noticed that stars had very different hydrogen spectral lines strengths • So they classified stars based on the strength of the hydrogen Balmer series lines from A (strongest) to Q (weakest) • Then other lines of neutral and ionized species then came into play (H&K lines of calcium, sodium D lines etc) • Later it was found that some of the classes were actually duplicates and so were removed

Divisions and subdivisions • It was only much later that it was discovered that the strength of the hydrogen line was connected with the surface temperature of the star. • These classes are further subdivided by numbers (0-9) • A0 denotes the hottest stars in the A class and A9 denotes the coolest ones • The sun is classified as G2.

Energies in Electron Volts • Room temperature thermal energy of a molecule: 0.04 eV • Visible light photons: 1.5-3.5 eV • Energy for the dissociation of an NaCl molecule into Na+ and Cl- ions: 4.2 eV • Ionization energy of atomic hydrogen: 13.6 eV • Approximate energy of an electron striking a color television screen: 20,000 eV • High energy diagnostic medical x-ray photons: 200,000 eV • I electron volt = 1 eV = 1.6x10-19 joules

Review of Basic Units • A joule is a unit of energy. • Four joules is the amount of energy needed to raise the temperature of a gram of water by 1 degree Celsius • 4 joules ~ 1 calorie • A calorie is also a measure of energy • 1 calorie = 4.186 joules.

Joules and eV • Another way of visualizing the joule is the work required to lift a mass of about 102 g (like a small apple) for one meter under the earth's gravity • One joule is also the work required to move an electric charge of 1 coulomb through an electrical potential difference of 1 volt

Stellar Classification