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0. The Trigonometric Parallax. B. d. p. d = (1/p[arcsec]) parsec. B = 1 AU = 1.496*10 13 cm. 1 pc = 3.26 LY ≈ 3*10 18 cm. 0. The Moving Cluster Method. v. q. v f. v r. q. f. x. 0. - PowerPoint PPT Presentation
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The Trigonometric Parallax
Bp
B = 1 AU = 1.496*1013 cmd = (1/p[arcsec]) parsec
d
1 pc = 3.26 LY ≈ 3*1018 cm
The Moving Cluster Method
x
vvr
v
The magnitude scale system can be extended towards negative numbers (very bright) and
numbers > 6 (faint objects):
Sirius (brightest star in the sky): mv = -1.42
Full moon: mv = -12.5
Sun: mv = -26.5
Color and Temperature
Orion
Betelgeuze
Rigel
Stars appear in different colors,
from blue (like Rigel)
via green / yellow (like our sun)
to red (like Betelgeuze).
These colors tell us about the star’s
temperature.
Blackbody Radiation (I)
The light from a star is usually concentrated in a rather narrow
range of wavelengths.
The spectrum of a star’s light is approximately a thermal
spectrum called Blackbody Spectrum.
Blackbody Radiation
Fsurf = Teff4
= 5.67*10-5 erg/(cm2 s K4)
Wien’s displacement law:
max ≈ 0.29 cm / TK
(TK = temperature in Kelvin).
The Color Index (I)B band
V bandThe color of a star is measured by comparing its brightness in different wavelength bands:
The blue (B) band and the visual (V) band.
We define B-band and V-band magnitudes just as we did
before for total magnitudes.
Optical Wavelength Bands
U: 0 ≈ 3650 Å
B: 0 ≈ 4400 Å
V: 0 ≈ 5500 Å
The Color IndexWe define the Color Index
B – V(i.e., B magnitude – V magnitude)
The bluer a star appears, the smaller the color index B – V.The hotter a star is, the smaller its color index B – V.
B - V
Temperature
Example:For our sun:
Absolute V magnitude: 4.83
Absolute B magnitude: 5.51
=> Color index:
B – V = 0.68
From standard tables:
B – V = 0.68 => T ≈ 5800 K.
The Color-Color Diagram
B - V
U -
B
-0.5 1.51.00.50.0 2.01.5
1.0
0.5
0.0
-0.5
-1.0
Blackbody
B0
A0F0 G0
K0
M0
The Hertzsprung-Russell Diagram
B - V
Teff
Mbo
l
Log(
L)
Most stars are found along the Main Sequence
Zero-Age Main Sequence (ZAMS)
Radii of Stars in the Hertzsprung-Russell Diagram
10,000 times the
sun’s radius
100 times the
sun’s radius
As large as the sun100 times smaller than the sun
Rigel Betelgeuze
Sun
Polaris
Giants
Supergiants
White Dwarfs
Extinction and Reddening
Interstellar Extinction
Wavelength-Dependent Extinction