The Properties of the Stars

Read:  Chapter 15

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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   versus        absolute brightness or luminosity

apparent m  magnitude    versus      absolute magnitude


Inverse square law


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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

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.
             (corresponding to a distance d = 100 parsecs = 326 light years).               (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:







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