How far away are the stars?

Parallax

Measuring the Distance to the Stars with Parallax

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Remember that 1 parsec = 3.26 light years

Visual magnitude scale• Originally, the brightest stars in

the sky were considered first magnitude, the next brightest group second magnitude, and so on

• Now, magnitude is more precisely defined, so that a m=+1.0 star is 100 times as bright as a m=+6.0 star

• Each magnitude is a factor of 2.5

Star Names• Most of the brightest stars

have names (e.g., Betelgeuse, Sirius)

• Up to 24 stars in each constellation have a name with a Greek letter (e.g., β Tauri)

• The rest may only be known by a catalog number (e.g., HDE 226868)

Absolute magnitude• Equivalent to apparent magnitude

of the star if it was at a distance of 10 parsecs

• The sun is M=+4.8

Luminosity• Measure of the amount of light a

star gives off• L =3.83 x 1026 W• The brightest stars (M=-10) have

luminosities of 106 L• The dimmest stars (M=+17) have

luminosities of 10-5 L

The Nature of Stars

A star’s color reveals its surface temperature

“Cool”, red star Warm, yellow star Hot, blue star

Photometry• Technique used to accurately measure

the color of stars using UBV filters• U - ultraviolet wavelengths• B - blue wavelengths• V - visible wavelength

• astronomers compare combinations of U minus B (U-B) and B minus V (B-V)

• these combinations are called color indices

Stars are classified by their spectra as

O, B, A, F, G, K, and M spectral types

Stars are classified by their spectra as

O, B, A, F, G, K, and M spectral types• O B A F G K M• hottest to coolest• bluish to reddish• An important sequence to

remember:• Our Best Astronomers Feel Good

Knowing More• Oh Boy, An F Grade Kills Me• Oh Be a Fine Girl (or Guy), Kiss Me

The Hertzsprung-Russell diagram

identifies a definite relationship

between temperature and

absolute magnitude

Absolute magnitude vs temperature

or

luminosity vs spectral type

Determining the Sizes of Stars from an HR Diagram

• Main sequence stars are found in a band from the upper left to the lower right

• Giant and supergiant stars are found in the upper right corner

• Tiny white dwarf stars are found in the lower left corner of the HR diagram

Luminosity classes• Class I includes

all the supergiants

• Class V includes the main sequence stars

• For example, the Sun is a G2 V

How do we know the mass of a star?

• Binary star systems• Recently, also from extrasolar

planets

Binary stars orbit their common center of mass

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