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Transcript
Stellar Magnitude, Distance, and Motion
Apparent Magnitude
 How bright a star appears; the "what you see is what you get" magnitude
 Convolution of the true brightness and the effect of distance on the
observed brightness
 Every 5th magnitude is 100 times brighter than the one before
o A 1st magnitude star is 100 times brighter than a 6th magnitude star
o Makes it easy to compare star brightness ratios
Apparent Visual Magnitudes
Object
Sirius (brightest star)
Venus (at brightest)
Full Moon
The Sun
Faintest naked eye stars
Faintest star visible
from
Earth telescopes
Faintest star visible
from
Hubble Space
Telescope
Apparent
Visual
Magnitude
-1.5
-4.4
-12.6
-26.8
6-7
~25
~?
Absolute Magnitude
 Actual star brightness
 The apparent magnitude that a star would have if it were (in our
imagination) placed at a distance of 10 parsecs (which is 32.6 light years)
from the Earth
 Used to describe luminosity - The amount of energy a star gives off each second
The 20 Brightest Stars in the Sky
Common
Name
Luminosity
Solar Units
Distance Spectral
LY
Type
Proper Motion
arcsec / year
R. A.
hours min
Declination
deg min
Sirius
40
9
A1V
1.33
06 45.1
-16 43
Canopus
1500
98
F01
0.02
06 24.0
-52 42
Alpha Centauri
2
4
G2V
3.68
14 39.6
-60 50
Arcturus
100
36
K2III
2.28
14 15.7
+19 11
Vega
50
26
A0V
0.34
18 36.9
+38 47
Capella
200
46
G5III
0.44
05 16.7
+46 00
Rigel
80,000
815
B8Ia
0.00
05 12.1
-08 12
Procyon
9
11
F5IV-V
1.25
07 39.3
+05 13
Betelgeuse
100,000
500
M2Iab
0.03
05 55.2
+07 24
Achernar
500
65
B3V
0.10
01 37.7
-57 14
Beta Centauri
9300
300
B1III
0.04
14 03.8
-60 22
Altair
10
17
A7IV-V
0.66
19 50.8
+08 52
Aldeberan
200
20
K5III
0.20
04 35.9
+16 31
Spica
6000
260
B1V
0.05
13 25.2
-11 10
Antares
10,000
390
M1Ib
0.03
16 29.4
-26 26
Pollux
60
39
K0III
0.62
07 45.3
+28 02
Fomalhaut
50
23
A3V
0.37
22 57.6
-29 37
Deneb
80,000
1400
A2Ia
0.00
20 41.4
+45 17
Beta Crucis
10,000
490
B0.5IV
0.05
12 47.7
-59 41
Regulus
150
85
B7V
0.25
10 08.3
+11 58
 Here is a list of the 314 stars brighter than apparent magnitude 3.55 in both
hemispheres.
H-R Diagram
 Hertzsprung-Russell diagram
 A way to compare star temperatures & spectral types with their absolute
magnitude & luminosity
 Most stars fall along a patterned sequence - main sequence stars
 Size indicated by dwarf (like our Sun), giant, and supergiant
Stellar Distances
 Easiest way to tell distance = sight a star at different
locations and see how far the star moves relative to a
distant background = parallax
 Combining the distance of 1 A.U. (our distance from the
Sun), with the angle (in arc seconds) an object makes with
the Earth and the Sun, we define distance in Parsecs:
o A star that is 1 parsec from the Sun has a parallax
of one arc second
o d (parsecs) = (1/p)(seconds of arc)
1 A.U.
1 Parsec
1 Arc sec
 Example: If a star has a parallax angle of ¼ (or 0.25) arc
seconds, how many parsecs is it away from the Sun?
 Some common distances
o Light Year: the distance that light travels in one year (9.46 x 10^17
cm)
o Parsec (pc): 3.26 light years (or 3.086 x 10^18 cm).; also kiloparsec
(kpc) = 1000 parsecs and megaparsec (Mpc) = 1,000,000 parsecs
o Astronomical Unit (AU): the average separation of the earth and the
sun (1.496 x 10^13 cm)
 Some representative distances
o The Solar System is about 80 Astronomical Units in diameter.
o The nearest star (other than the sun) is 4.3 light years away.
o Our Galaxy (the Milky Way) is about 100,000 light years in diameter.
o Diameter of local cluster of galaxies: about 1 Megaparsec.
o Distance to M87 in the Virgo cluster: 50 million light years.
o Distance to most distant object seen in the universe: about 18 billion
light years (18 x 10^9 light years).
Stellar Motion
 Proper motion - the apparent change of position of a star on the celestial
sphere
o Denoted by the Greek
symbol  "mu"
o Is a velocity in units of
seconds of arc per year
o Proper motion is not
large.
 The star with the largest proper motion is called Barnard's Star.
 It moves 10.3 seconds of arc per year.
o Since the moon subtends about 1/2 of a degree (which is 1/2 x 60 x
60 = 1800 seconds of arc) on the celestial sphere, it takes Barnard's
star about 1800/10.3 ~ 180 years to change its position by the
angular diameter of the moon. All other stars have smaller proper
motions.
 The actual motion of stars involves a path in three space dimensions, so
the proper motion is just the projection of this true motion on the celestial
sphere.
 This true velocity of the star is called the space velocity
o Tangential velocity is responsible for the proper motion
 To determine this generally requires that we know the distance
to the star
o Radial velocity is the
motion of stars away from
Earth (outward
component)
 Creates a Doppler
shift of the spectral lines that can be used to determine it
directly
 The full space velocity of a star follows from Pythagoras' Theorem if both
the tangential and radial velocities are known.
 Typical values for the space velocities of stars are 20-100 km/s.