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 5780 K

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