The Properties of the Stars
Read: Chapter 15
Stars shine for millions to billions of years, much
longer than a
Yet, we've been able to piece together how stars
are born, shine and eventually die.
brightness versus absolute
brightness or luminosity
apparent m 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:
Full circle = 360 X 60 X 60 = 1,296,000 arcseconds
- 360 degrees = full circle
- 60 arcminutes = 1 degree
- 60 arcseconds = 1 arcminute
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.
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.
The star Proxima Centauri has a parallax p = 0.77
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.
(corresponding to a distance d = 100 parsecs = 326 light years).
- From the ground, the smallest measurable parallax
is p = 0.01
(corresponding to a distance d = 500 parsecs = 1600 light
- From a satellite, the smallest measurable
parallax is p = 0.002
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
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
Originally classified as A,B,C,.. (before Wien), the
stellar spectra was recast into OBAFGKM
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
- Spectroscopic Binaries -- two stars are too
close to see as
separate stars, but spectrum shows absorption lines from two stars with
- Eclipsing Binaries -- one star disappears
when it passes
ANIMATION: Artist's conception
a visual binary
Summary of Stellar Properties:
|Mass (solar masses)