Download Magnitude Scale

1
Magnitude Scale
• Method of scaling the apparent brightness
and luminosity of stars.
• Uses logarithms to create a scale that
handles a wide range of brightness
2
Apparent Magnitude
• Measure of the brightness of a star as seen from
the Earth (equivalent of apparent brightness).
• Denoted by m
• The smaller the number, the brighter the star
–m=0
–m=6
one of the brightest stars in the sky
eye can barely detect star in the sky
• Some stars or planets have negative magnitudes
-- these are even brighter than m = 0.
• With telescopes, we can detect stars as dim as
m = 27!
3
Absolute Magnitude
• Measure of the brightness of a star if
observed from 10 parsecs away (equivalent
of luminosity)
• Denoted by M or MV
• Absolute magnitude and apparent
magnitude are the same at a distance of 10
parsecs.
4
Magnitude Formula
m  M  5  log( d )  5
m = apparent magnitude
M = absolute magnitude
d  10
 m  M 5  5
d  10 10
- or -
 m M  5
d = distance in parsecs
(1 parsec = 3.26 light years)
Use this equation to find the distance to an object
if you know the apparent and absolute magnitudes
Color Index – Another way to
describe temperature
5
Note: small or negative numbers = brighter
Apparent Magnitude
(bright)
−5
U B
V
0
5
• Measure the
apparent magnitude
at two different
wavelengths bands
(usually B and V)
– U = low ultraviolet
– B = blue
– V = red
10
15
(dim) (short)
Wavelength ()
(long)
Color Index – Another way to
describe temperature
6
Note: small or negative numbers = brighter
Apparent Magnitude
(bright)
−5
U B
V
0
• Color Index = B − V
– The smaller (or more
negative) the number,
the more blue (and hot)
the star.
5
10
15
(dim) (short)
Wavelength ()
(long)
Color Index – Another way to
describe temperature
7
B − V = negative (blue, hot star)
Apparent Magnitude
(bright)
−5
U B
V
B − V ≈0 (yellow, medium
temp star)
B − V = positive (cool, red star)
0
5
10
15
(dim) (short)
Wavelength ()
(long)
8
Spectroscopic Parallax
• If apparent magnitude (apparent brightness) and
absolute magnitude (luminosity) are known
distance can be determined.
• Difficult to accurately measure luminosity for one
star
– Use spectra to get spectral type and class
• Can use a cluster of stars
• Distance to the cluster can be determined by
comparing the HR diagram of the cluster with a
template HR diagram
9
Spectroscopic Parallax
Observations
Template
0
-9
9
0
18
9
27
18
-0.4
hot
0.8
B-V
2.0
cooler
d  10 10
-0.4
hot
( m M ) / 5
0.8
B-V
2.0
cooler