Each star is born with a specific mass. This mass is the main factor in determining the star’s brightness, temperature, expected lifetime, type of death, and spectra. Stars are classified according to their spectra.

Astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral features.

Spectral lines are caused by different elements present in a star. The overall spectrum is strongly related to the surface temperature of the star. Stars are then categorized into classes distinguished by the strength of the spectral lines and their shape.

The 7 classes of stars are: O B A F G K M From O M: –Stars decrease in temperature –Stars generally become less massive (smaller)

O B A F G K M Oh Be A Fine Guy/Girl, Kiss Me! These are two of many mnemonic devices students and astronomers use to remember the proper order of stellar classification; from the hottest blue stars (O-type) to the coldest red stars (M-type). One Bug Ate Five Green Killer Moths

For analysis purposes, stars are placed on the H-R diagram according to their luminosity and their temperature (Spectral Class).

The Hertsprung-Russell or H-R diagram reveals that about 90% of all stars lie along a smooth diagonal curve called the main sequence with hot, luminous stars in the upper left and cool, dim stars in the lower right. Not all stars fall on the main sequence. Stars below the main sequence are called white dwarfs and those above it are called giants.

Since hotter stars are bluer, and cooler stars are redder, a White Dwarf is hotter than a Red Giant.

Color:blueColor: blue Temperature: 28, ,000 KTemperature: 28, ,000 K Size: very large and massiveSize: very large and massive Spectra: ionized helium, ultraviolet continuumSpectra: ionized helium, ultraviolet continuum Examples: 10 Lacertra, Zeta Puppis, and Iota Orionis AExamples: 10 Lacertra, Zeta Puppis, and Iota Orionis A The rock stars of the universe-they live fast, die young and leave a spectacular- looking corpse!

Color:blueColor: blue Temperature: 10, ,000 KTemperature: 10, ,000 K Size: large and massiveSize: large and massive Spectra: neutral helium lines,some hydrogenSpectra: neutral helium lines,some hydrogen Examples: Beta Centauri, Rigal, SpicaExamples: Beta Centauri, Rigal, Spica B stars are relatively rare, comprising only 0.1% of main sequence stars.

Color:whiteColor: white Temperature: 75, ,000 KTemperature: 75, ,000 K Size: moderate sized, very luminousSize: moderate sized, very luminous Spectra: strong hydrogen lines, ionized metalsSpectra: strong hydrogen lines, ionized metals Examples: Alpha Canis Majoris (Sirius) and Alpha Lyrae (Vega)Examples: Alpha Canis Majoris (Sirius) and Alpha Lyrae (Vega) A stars are amongst the most common naked eye stars.

Color: white-yellowColor: white-yellow Temperature: 6, ,000 KTemperature: 6, ,000 K Size: 1.2 to 1.6 times bigger than the SunSize: 1.2 to 1.6 times bigger than the Sun Spectra: weak hydrogen lines, strong Calcium and other ionized metalsSpectra: weak hydrogen lines, strong Calcium and other ionized metals Examples: Canopus, ProcyonExamples: Canopus, Procyon Often used as targets for extrasolar planet searches and SETI programs.

Color:Color: yellow Temperature: 5,000- 6,000 KTemperature: 5,000- 6,000 K Size: 0.8 to 1.1 times the mass of the SunSize: 0.8 to 1.1 times the mass of the Sun Spectra: weak hydrogen lines, neutral and ionized metalsSpectra: weak hydrogen lines, neutral and ionized metals Examples: Alpha Centauri A, CapellaExamples: Alpha Centauri A, Capella The best known example of a G star is our SUN!

Color: orangeColor: orange Temperature: 3,500- 5,000 KTemperature: 3,500- 5,000 K Size: smaller and cooler than the SunSize: smaller and cooler than the Sun Spectra: faint hydrogen lines, strong neutral metallic linesSpectra: faint hydrogen lines, strong neutral metallic lines Examples: Alpha Boötis (Arcturus) and Alpha Tauri (Aldebaran)Examples: Alpha Boötis (Arcturus) and Alpha Tauri (Aldebaran) Also used as targets for extrasolar planet searches like project Ozma in 1960.

Color: redColor: red Temperature: 2,500- 3,500 KTemperature: 2,500- 3,500 K Size: range from (main sequence- supergiants) times the mass of the SunSize: range from (main sequence- supergiants) times the mass of the Sun Spectra: strong metallic lines and wide titanium oxide bandsSpectra: strong metallic lines and wide titanium oxide bands Examples: Antares and BetelgeuseExamples: Antares and Betelgeuse Most common class by number of stars, since 90% of all stars are red dwarfs.