PHYSICAL PROPERTIES OF A STAR
Objective
1. To plot the H-R diagram for stars and use it to estimate the temperature and luminosity
of a star, given its spectral class.
2. To determine the mass, radius, and lifetime o
...
PHYSICAL PROPERTIES OF A STAR
Objective
1. To plot the H-R diagram for stars and use it to estimate the temperature and luminosity
of a star, given its spectral class.
2. To determine the mass, radius, and lifetime of a star, using the appropriate equations
and graphs.
Equipment
Scientific calculator, pencil, highlighter, and semi-logarithmic graph paper, which is
provided at the end of this document.
Introduction
There are six physical quantities, which are used to define a star:
1. Photospheric Temperature
2. Luminosity
3. Mass
4. Radius and Volume
5. Average Density
6. Lifetime and chemical composition
Let us examine how each of these quantities can be deduced.
Photospheric Temperature
The photospheric temperature (T) is measured in K. This can be calculated by direct
observation from Earth. The photosphere of a star emits a continuous spectrum observable
from the Earth. By dispersing the spectrum and graphing its Planck curve, the maximum
wavelength can be determined using Wien's Law, T = 2.898 x 106 K-nm / λmax where the
maximum wavelength is measured in nanometers.
Another method used to determine the temperature of a star is by interpreting its spectral
signature. Astronomers have correlated the spectral lines observed with the degree of ionization
present in the star’s photosphere. Since temperature determines the degree of ionization, once
the spectral class of a star is identified, it is possible to use a table like the one below to
determine a star’s temperature. Remember the spectral sequence is O, B, A, F, G, K, M, with
the O stars being the hottest. Each letter category is in turn divided into 10 sub-categories,
ranging from zero to nine. A star with the classification B9 is therefore slightly cooler than B8,
but hotter than A0.
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