Download Introduction to Astronomy

Announcements
• Pick up graded homework
• Lecture slides on web site have been
updated
• Take the test W, Th, or F before class
• Physics seminar today, 1:00, SL 121: The
Utah Science Center, Joe Andrade
Properties of Stars
25 October 2006
Today:
• More properties of stars: brightness,
temperature, size
• A survey of the stars in our
neighborhood
Brightness of Stars
• True brightness (or “luminosity”) is a star’s actual rate
of energy output, measured (for example) in watts.
The sun’s luminosity is about 4 x 1026 watts.
• Apparent brightness is determined by the “intensity” of
starlight striking a detector. It is measured (for
example) in watts per square meter. The sun’s
apparent brightness from earth’s location is about
1400 watts per square meter.
Formula:
Apparent brightness =
True brightness
4π(distance)2
Brightness of Stars
Formula:
Apparent brightness =
True brightness
4π(distance)2
How far away are the stars?
• A clue: Compare brightness of our sun to brightness of
stars . . . It’s the difference between night and day!
Magnitude system for brightness
• Smaller numbers imply brighter stars.
• “Apparent magnitude” is a measure of apparent
brightness. Antares has mag. 1; Polaris has mag. 2;
naked eye limit is about 6. Sirius has mag. –1.5.
• “Absolute magnitude” is a measure of true brightness.
It’s what the apparent magnitude would be if the star
were 33 light-years away. Sun’s absolute magnitude
is about 5.
• The formulas that relate magnitudes to brightnesses
(in watts or W/m2) are complicated and not so
important.
Names of Stars
• Brightest stars have actual names, like Sirius,
Canopus, Betelgeuse, Rigel, Vega, Polaris.
• Prominent stars in each constellation have Greekletter designations, like Alpha Centauri, Epsilon
Eridani, Tau Ceti.
• For dimmer stars, must resort to various numbering
schemes.
The Nearest Stars
Name
Distance
(light years)
Sun
Apparent Luminosity
magnitude (compared to sun)
–26.7
1
Alpha Centauri A
4.4
0
1.5
Alpha Centauri B
4.4
1.4
0.44
Alpha Centauri C
4.3
11
0.00006
Barnard’s star
5.9
9.5
0.00042
Wolf 359
7.6
13.5
0.00002
HD 05735
8.3
7.5
0.0055
Luyten 726-8A
8.4
12.5
0.00006
Luyten 726-8B
8.4
13.0
0.00004
Sirius A
8.6
–1.4
21.8
Sirius B
8.6
8.3
0.003
Ross 154
9.4
10.5
0.00048
Ross 248
10.3
12.3
0.00011
The Brightest Stars
(as viewed from earth)
Name
Distance
(light years)
Sun
Apparent Luminosity
magnitude (compared to sun)
–26.7
1
Sirius A
8.6
-1.4
21.8
Canopus
310
-0.6
14,000
Arcturus
37
-0.1
110
Alpha Centauri A
4.4
0.0
1.5
Vega
25
0.0
48
Capella
42
0.1
130
Rigel
770
0.2
40,000
Procyon
11.4
0.4
7.0
Betelgeuse
430
0.5
9400
Achernar
144
0.5
1070
Hadar (Beta Cen)
525
0.6
12,000
Star colors and temperatures
Calculating sizes of stars
Luminosity = (constant) x (4πR2) x (temperature)4
Suppose two stars (A and B) have the same temperature,
but A is 100 times more luminous than B. How do their
sizes compare?
A must have 100 times as much surface area, but this
means its radius (or diameter) is only 10 times greater.
A
B