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Stellar Classification. 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.
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Stellar Classification 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.
Stellar Classification • 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.
Stellar Classification • The 7 classes of stars are: O BAF G K M • From O M: • Stars decrease in temperature • Stars generally become less massive (smaller)
O BA F G K M Oh Be AFine 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
H-R Diagram For analysis purposes, stars are placed on the H-R diagram according to their luminosity and their temperature (Spectral Class).
H-R Diagram • TheHertsprung-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.
O Stars • Color:blue • Temperature: 28,000- 50,000 K • Size: very large and massive • Spectra: ionized helium, ultraviolet continuum • Examples: 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!
B Stars • Color:blue • Temperature: 10,000- 28,000 K • Size: large and massive • Spectra: neutral helium lines, some hydrogen • Examples: Beta Centauri, Rigal, Spica B stars are relatively rare, comprising only 0.1% of main sequence stars.
A Stars • Color:white • Temperature: 75,000- 10,000 K • Size: moderate sized, very luminous • Spectra: strong hydrogen lines, ionized metals • Examples: Alpha Canis Majoris (Sirius) and Alpha Lyrae (Vega) A stars are amongst the most common naked eye stars.
F Stars • Color: white-yellow • Temperature: 6,000- 75,000 K • Size:1.2 to 1.6 times bigger than the Sun • Spectra: weak hydrogen lines, strong Calcium and other ionized metals • Examples: Canopus, Procyon Often used as targets for extrasolar planet searches and SETI programs.
G Stars • Color: yellow • Temperature: 5,000- 6,000 K • Size:0.8 to 1.1 times the mass of the Sun • Spectra: weak hydrogen lines, neutral and ionized metals • Examples: Alpha Centauri A, Capella The best known example of a G star is our SUN!
K Stars • Color: orange • Temperature: 3,500- 5,000 K • Size:smaller and cooler than the Sun • Spectra: faint hydrogen lines, strong neutral metallic lines • Examples: Alpha Boötis (Arcturus) and Alpha Tauri (Aldebaran) Also used as targets for extrasolar planet searches like project Ozma in 1960.
M Stars • Color: red • Temperature: 2,500- 3,500 K • Size: range from 0.5-25 (main sequence- supergiants) times the mass of the Sun • Spectra: strong metallic lines and wide titanium oxide bands • Examples: Antares and Betelgeuse Most common class by number of stars, since 90% of all stars are red dwarfs.
