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:Color: blueblue• Temperature:Temperature: 28,000- 28,000-

50,000 K50,000 K• Size:Size: very large and very large and

massivemassive• Spectra:Spectra: ionized helium, ionized helium,

ultraviolet ultraviolet continuumcontinuum

• Examples:Examples: 10 Lacertra, 10 Lacertra, Zeta Puppis, and Zeta Puppis, and Iota Orionis AIota Orionis A

The rock stars of the

universe-they live fast, die young and

leave a spectacular-

looking corpse!

• Color:Color: blueblue• Temperature:Temperature: 10,000- 10,000-

28,000 K28,000 K• Size:Size: large and large and massivemassive• Spectra:Spectra: neutral helium neutral helium

lines,lines,some some hydrogenhydrogen

• Examples:Examples: Beta Beta Centauri, Rigal, Centauri, Rigal, SpicaSpica

B stars are relatively

rare, comprising only 0.1% of

main sequence

stars.

• Color:Color: whitewhite• Temperature:Temperature: 75,000- 75,000-

10,000 K10,000 K• Size:Size: moderate sized, moderate sized,

very luminousvery luminous• Spectra:Spectra: strong strong

hydrogen lines, hydrogen lines, ionized ionized metalsmetals

• Examples:Examples: Alpha Canis Alpha Canis Majoris (Sirius) Majoris (Sirius)

and and Alpha Lyrae (Vega)Alpha Lyrae (Vega)

A stars are amongst the

most common

naked eye stars.

• Color:Color: white-yellow white-yellow• Temperature:Temperature: 6,000- 6,000-

75,000 K75,000 K• Size:Size: 1.2 to 1.6 times 1.2 to 1.6 times

bigger than the Sunbigger than the Sun• Spectra:Spectra: weak hydrogen weak hydrogen

lines, strong lines, strong Calcium Calcium and other and other ionized ionized metalsmetals

• Examples:Examples: Canopus, Canopus,ProcyonProcyon

Often used as targets for extrasolar

planet searches and SETI

programs.

• Color:Color: yellow• Temperature:Temperature: 5,000- 5,000-

6,000 K6,000 K• Size:Size: 0.8 to 1.10.8 to 1.1 times times the the

mass of the Sunmass of the Sun• Spectra:Spectra: weak hydrogen weak hydrogen

lines, neutral and lines, neutral and ionized metalsionized metals

• Examples:Examples: Alpha Alpha Centauri A, CapellaCentauri A, Capella

The best known

example of a G star is our SUN!

• Color:Color: orange orange• Temperature:Temperature: 3,500- 3,500- 5,000 5,000

KK• Size:Size: smaller and cooler smaller and cooler

than the Sunthan the Sun• Spectra:Spectra: faint hydrogen faint hydrogen

lines, strong neutral lines, strong neutral metallic linesmetallic lines

• Examples:Examples: Alpha Boötis Alpha Boötis (Arcturus) and Alpha (Arcturus) and Alpha Tauri (Aldebaran)Tauri (Aldebaran)

Also used as targets for extrasolar

planet searches like project Ozma

in 1960.

• Color:Color: red red• Temperature:Temperature: 2,500- 2,500-

3,500 K3,500 K• Size:Size: range from 0.5-25 range from 0.5-25

(main sequence-(main sequence-supergiants) times supergiants) times the mass of the the mass of the

SunSun• Spectra:Spectra: strong metallic strong metallic

lines and wide lines and wide titanium oxide titanium oxide bandsbands

• Examples:Examples: Antares and Antares and BetelgeuseBetelgeuse

Most common class by

number of stars, since 90% of all

stars are red dwarfs.