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 Temperature

O 30,000 - 60,000 K Blue stars

B 10,000 - 30,000 K Blue-white stars

A 7,500 - 10,000 K White stars

F 6,000 - 7,500 K Yellow-white stars

G 5,000 - 6,000 K Yellow stars (like the Sun)

K 3,500 - 5,000K Yellow-orange stars

M < 3,500 K Red stars

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