Download Distances, magnitudes

The Trigonometric Parallax
B
d
p
d = (1/p[arcsec]) parsec
B = 1 AU = 1.496*1013 cm
1 pc = 3.26 LY ≈ 3*1018 cm
The Moving Cluster Method
v
q
vr
q
f
x
vf
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
Stars appear in
different colors,
from blue (like Rigel)
Orion
Betelgeuze
via green / yellow (like
our sun)
to red (like Betelgeuze).
These colors tell us
about the star’s
temperature.
Rigel
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 = s Teff4
s = 5.67*10-5 erg/(cm2 s K4)
Wien’s displacement law:
lmax ≈ 0.29 cm / TK
(TK = temperature in Kelvin).
The Color Index (I)
B band
The 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.
V band
Optical Wavelength Bands
U: l0 ≈ 3650 Å
B: l0 ≈ 4400 Å
V: l0 ≈ 5500 Å
The Color Index
We 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
B0
-1.0
U-B
-0.5
0.0
A0
F0
G0
0.5
K0
1.0
1.5
-0.5
0.0
0.5
M0
1.0
1.5
2.0
B-V
Log(L)
Mbol
The Hertzsprung-Russell Diagram
Most stars are
found along the
Main Sequence
Teff
B-V
Radii of Stars in the
Hertzsprung-Russell Diagram
Rigel
Betelgeuze
Polaris
Sun
100 times smaller than the sun
Extinction and Reddening
Interstellar Extinction
Wavelength-Dependent Extinction