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Astronomy 363
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Astronomy 263/264
Stellar Properties and H-R Diagrams
Summary
How can we determine the various properties of stars such as their
size, energy output, and temperature? One of the most interesting
questions in stellar astronomy concerns whether or not there is a
relationshop between these properties. This lab will attempt to answer
these questions, and familiarize you with the concepts listed below.
Concepts
Stars are good examples of
blackbody radiation. A blackbody
is an imaginary object that absorbs all the light that hits it, then
heats up and radiates this energy away. The radiation always takes a
certain form that we call the blackbody curve. This blackbody curve
has a peak at some wavelength, which determines what color the
blackbody appears to be. In the figure below, three blackbody curves
are shown for three stars. Each has a different peak wavelength that
corresponds to the temperature of the star.
The relationship of the peak wavelength to the temperature of the star
is known as Wien's Law. Wien's law
says that the higher the temperature of the star,
the shorter the peak wavelength of the blackbody curve. Cool
stars appear redder than hot stars (which are more blue).
We describe how bright a star appears to be when viewed from the Earth
as it's apparent magnitude. The
apparent magnitude is not how bright the star actually is, however, becase
the farther away the star is, the dimmer it will appear to be to us.
We can calculate the absolute magnitude
of a star if we know the distance to the star, and it's apparent magnitude.
The absolute magnitude is how bright the star would appear to be, if we
moved the star closer to earth and placed it 10 parsecs away.
The Stefan-Boltzmann law allows us to
calculate the size of the star, if we know its luminosity and
temperature. In general, brighter and cooler stars will be bigger than
dimmer and hotter stars. From Wien's Law and the Stefan-Boltzmann law,
we see that if we measure the star's color and luminosity, then we can
calculate its temperature and size.
If we plot a group of stars on a graph, called an
H-R Diagram, we can learn more about
the stars. On the H-R diagram, stars are plotted based on their
Temperature and their Luminosity. Then, we see that most stars lie along
a smooth curve that we call the Main Sequence
. The Sun lies along this line, so we can call it a Main
Sequence star.
We break stars into spectral classes based on their temperature. There are
seven main classes: O, B, A, F, G, K, M.
This lists the classes in order, from O (hot and blue) to M (cold and red).
(You may use the mnemonic "Oh, Be A Fine Girl/Guy, Kiss Me!" to remember
the order, from hot to cold.) O stars tend to be younger, and M stars
tend to be older. The Sun is a type G
star.
The stars can be further broken down into size classes:
Supergiant, Giant, and Dwarf. The
Sun is a dwarf star. The relative
sizes are shown in the picture below:
Last updated on February 15, 2000.
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