Lecture 28:
Wednesday, March 31, 2004
The Properties of the Stars
Read: Chapter 16
Quiz Questions . Due
Monday, April 5, 2004.
Introduction:
Stars shine for millions to billions of years, much longer than a
human lifetime. Yet, we've been able to piece together how stars
are born, shine and eventually die.
DEFINITIONS:
Apparent brightness verus Absolute brightness or luminosity
apparent magnitude, absolute magnitude
Inverse square law
Same Luminosity, Twice as far away --> 4x dimmer
Measuring Distances to Stars: Parallax
Parallax is the change in the apparent position of an object which
results from a change in the observer's position.
Measuring small angles:
- 360 degrees = full circle
- 60 arcminutes = 1 degree
- 60 arcseconds = 1 arcminute
Full circle = 360 X 60 X 60 = 1,296,000 arcseconds
1 arcsecond = angular size of dime 2 kilometers away
Important Equation: Computing a Distance from a Parallax
d = 1/p
d = distance to star, measured in parsecs
p = parallax, measured in arcseconds
The parsec is defined as the distance at which a star
has a parallax of 1 arcsecond. In other units,
1 parsec = 3.26 light years = 206,000 AU.
Parsecs are the units most often used by professional astronomers in
measuring interstellar distances.
Example:
The star Proxima Centauri has a parallax p = 0.77 arcsecond.
d = 1/0.77 = 1.30 parsec = 4.23 light years
Because stellar parallaxes are so small, they can only be measured accurately
for relatively nearby stars.
- From the ground, the smallest measurable parallax is p = 0.01
arcsec (corresponding to a distance d = 100 parsecs = 326 light years).
- From a satellite, the smallest measurable parallax is p = 0.002
arcsec (corresponding to a distance d = 500 parsecs = 1600 light years).
For comparison, the distance from the Sun to the center of our galaxy
is about 8000 parsecs. Thus, we can only use stellar parallax to measure
distances in our immediate neighborhood, not for the entire galaxy.
Animation
Animation
Animation
Stellar Surface Temperatures
Measure the surface temperature of stars by taking a spectrum
of the star and using Wien's Law
In addition, the absorption lines in the stellar spectra are sensitive
to temperature.
Originally classified as A,B,C,.. (before Wien), the classification of
stellar spectra was recast into OBAFGKM by
Cecilia Payne-Gaposchkin.
Stellar Masses
Stellar masses are measured by observing binary stars, and using Kepler's
3rd Law to determine the mass of the stars from
the period of their orbit.
Types of Binary Stars:
- Visual Binaries -- direct image shows two stars orbiting each
other
- Spectroscopic Binaries -- two stars are too close to see as
separate stars, but spectrum shows absorption lines from two stars with variable
doppler shifts.
- Eclipsing Binaries -- one star disappears when it passes behind
the other
Visual Binaries:
ANIMATION: Artist's conception of
a visual binary
Eclipsing Binaries:
ANIMATION
ANIMATION
Spectroscopic Binaries:
ANIMATION
Summary of Stellar Properties:
SPECTRAL CLASS
|
Mass (solar masses)
|
Luminosity
(solar luminosities)
|
Temperature
degrees K
|
Radius
(solar radii)
|
O5
|
40
|
400,000
|
40,000
|
13
|
B0
|
15
|
13,000
|
28,000
|
4.9
|
A0
|
3.5
|
80
|
10,000
|
3.0
|
F0
|
1.7
|
6.4
|
7,500
|
1.5
|
G0
|
1.1
|
1.4
|
6,000
|
1.1
|
K0
|
0.08
|
0.46
|
5,000
|
0.9
|
M0
|
0.05
|
0.08
|
3,500
|
0.8
|