how far away are the stars?
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How far away are the stars?
Parallax
Measuring the Distance to the Stars with Parallax
angleparallax 1 parsecsin star a todistance
pd 1
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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