Astronomy 100 -- Basic Properties of Stars
THE DISTANCES TO STARS
Stars are really,
really,really, far away, so rather than use miles or kilometers, we will use some new units for distance.
Astronomers use two ways of measuring distance:
1. The LIGHT-YEAR (LY).
This is the distance light travels in a year.
This distance is about 6 million, million miles!
The light-year is handy because if a star is 10 light-years
away, the light from that star takes 10 years to reach us.
2. The PARSEC (pc).
One parsec is equal to 3.26 light-years.
We also have the
kiloparsec (kpc) = 1,000 pc and the
megaparsec (Mpc) = 1,000,000 pc.
DETERMINING DISTANCES TO NEARBY STARS
For nearby stars, astronomers use the
PARALLAX of the star.
When astronomers take pictures of the stars at different times of
the year (like June and December) nearby stars shift in
position with respect to the distant background stars. This
shift is an angle that is part of a triangle. Since we know the
base of the triangle, we can determine the distance to the star.
The size of the parallax angle (or shift) is related to how far
star is: closer stars have a larger parallax than stars further
away. This method can only be used for stars closer than
about 60 LY, which is between 1000 and 2000 stars. For stars
beyond 60 LY, the parallax angle becomes too small to measure
Recently, the Hipparcos Satellite in orbit around the Earth has
been able to provide data to determine the parallax distance for
stars as distant as 1500 LY, about 120,000 stars! Two different
teams of astronomers are now working analyzing these data.
Stars emit energy at all wavelengths, but
a lot of the energy is in visible light.
The Bolometric Luminosity:
This is the total amount of energy radiated each second at
ALL wavelengths. The bolometric luminosity is
directly related to the absolute magnitude.
Recall the Stefan-Boltzmann Law: the amount
of energy radiated from a surface of one
square meter depends only on the
temperature of the object.
How can we use what we measure?
So, these observations have given us:
- Measure the apparent brightness of a star.
- If the distance is known, we can find it's absolute brightness,
which is true measure of how much total energy the star emits.
- Using Wien's Law, we can determine the temperature of the
- Use the Stefan-Boltzmann Law to determine how much energy
comes from each square meter.
1) The total amount of emitted energy.
2) The amount of energy emitted from each square meter.
Dividing the total energy by the energy from each square meter
(the sigma T to the fourth part)
will give the total number of square meters (the 4 pi R squared
part), which is
the surface area of the star!
This provides the RADIUS
of the star!!!
WHAT WE LEARN ABOUT STARS
FROM STELLAR SPECTRA
- The Surface Temperature
- The Composition
- The Rotation Speed (how fast it's spinning)
- The Atmospheric Pressure
- The Radial Velocity (how fast it's moving- towards or away)
I. The Temperatures of Stars
From Wien's Law, we know that if we can determine the peak of the
spectrum, we can determine the surface temperature of a star.
Stars are put into different
depending on their surface temperatures.
|T (K) ||Spectral Class ||.
|25,000 || O || Oh |
|15,000 || B || Be |
|10,000 || A || A |
| 7,000 || F || Fine |
| 5,000 || G || Girl/Guy|
| 4,000 || K || Kiss|
| 3,000 || M || Me|
II. The Composition of Stars
Stars are like the Sun being
98% hydrogen and helium.
What is the last 2%?
Absorption lines in the spectrum of a star indicate the
elements present. So by looking at the absorption lines,
we can tell what elements are in the atmosphere of the star.
But, the exact amounts are difficult to determine, here's why:
a) more hydrogen and helium.
b) "Metals" (everything besides H and He)
But, it is possible to compare stars of the same temperature
to see which one has more "metals".
Measures amounts just in the atmosphere,
not the whole star. We have to make educated guesses at what's
deeper inside the star.
- Absorption line strengths depend on temperature
and pressure of the star's atmosphere as well
as amount of "metals".
- Amounts depends on atomic properties and model stars,
which are not perfect.
III. The Rotation Speed of Stars
IV. The Atmospheric Pressure of Stars
V. Stellar Motions
THE MASSES OF STARS
The masses of stars have been determined from
observations of stars in binary (double stars) systems
using Newton's Law of Gravity.
*** Important result! ***
Many stars (90%) of the same spectral class
are found to have the same mass!
The Russell-Vogt Theorem
"The equilibrium structure of an ordinary star is UNIQUELY
determined by its Mass and Chemical Composition."
This means the MASS and the COMPOSITION of a star determine all
the other properties of a star:
Radius, and the
Since stars are relatively similar in composition
(remember that stars are nearly all H and He),
it should be obvious that biggest influence is MASS.
The Hertzsprung-Russell Diagram
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