Lesson 41: Properties of Stars
In this lesson, we will look at three fundamental properties of stars that
are studied by astronomers – luminosity, surface temperature, and mass – and
how these properties are calculated or measured.
There are three fundamental properties of a star that
astronomers seek to measure: luminosity, temperature, and mass.
The apparent brightness of a star -- how bright it looks in
our sky -- is based on its luminosity (the amount of light a star actually
emits) and its distance.
The inverse square law for light tells us the relationship between distance,
luminosity, and apparent brightness. It states that the apparent brightness of
a light source declines according to an inverse square law, so if we look at
the Sun from twice the Earth's distance, its apparent brightness would be
dimmed by a factor of 4, and if we look at it from three times the distance,
its apparent brightness would dim by a factor of 9.
The apparent brightness of a star is determined by measuring the amount of
light per square meter we receive from the star. Then if we know its distance,
we can use the inverse square law to calculate its luminosity, or if we know
its luminosity, we can calculate its distance.
We have learned about stellar parallax before. It refers to
the small shifts in a star's annual position when we look at it from different
positions in Earth's orbit around the Sun.
By measuring the angle of this change when we observe the star, we get its
parallax angle. Stars that are further away will appear to shift less, so their
parallax angles will be smaller. If we measure this angle in arcseconds, then 1
divided by this angle will give us the distance of the star in parsecs.
For example, if a star has a stellar parallax angle of one-tenths of an
arcsecond, then its distance will be 10 parsecs. A parsec is 3.26 light-years,
so 10 parsecs would be 32.6 light years.
Our Sun falls roughly in
the middle when it comes to the luminosities of stars. There are stars that
are much dimmer, such as Proxima Centauri, whose luminosity is a tiny fraction
of our Sun's. Meanwhile, the brightest stars can be as much as a million times
as luminous as our Sun.
Dim stars appear to
be more common than bright stars, so even though there are stars that are much
brighter than our Sun, the vast majority of stars are dimmer.
In addition to luminosity,
astronomers seek to determine a star's surface temperature.
As we learned previously,
a star's temperature can be determined by looking at its color. A cooler star
looks red because it emits a lot more red light than blue light while a hotter
star emits more blue light than red light and looks blue.
A star's spectrum can also be used to determine its surface temperature. When
using spectral lines to determine temperature, scientists classify a star by
spectral type, ranging from hottest to coolest -- O, B, A, F, G, K, and M.
Cool, red stars of spectral type M are much more common than hot,
blue stars of spectral type O.
The third fundamental property of a star is its mass, and
is generally more difficult to
measure than its surface temperature or luminosity.
The most dependable method for determining a star's mass is using Newton's
version of Kepler's third law. This law requires us to be able to observe one
object orbiting another, so for stars, it is generally only used to measure
masses in binary star systems -- systems in which two stars orbit each other.
Fortunately, about half of all stars appear to be members of binary star systems.