Astronomy 100 -- Basic Properties of Stars

STARS! STARS! STARS! STARS! 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 away the 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 accurately.
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.

STELLAR LUMINOSITY

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?

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 star.

Use the Stefan-Boltzmann Law to determine how much energy comes from each square meter.

So, these observations have given us:
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
SPECTRAL CLASSES
O-B-A-F-G-K-M 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%?
a) more hydrogen and helium.
b) "Metals" (everything besides H and He)

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:
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.
But, it is possible to compare stars of the same temperature to see which one has more "metals".

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:
Temperature
Luminosity
Radius, and the
Lifetime.

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 Nearest Star: The Sun
The Hertzsprung-Russell Diagram
Back to The Front Page.