Hi Jenna,
Great question! By measuring the properties of stars, astronomers
can learn lots about how they work, about the environments in which
stars are found (clusters, galaxies, dust clouds, etc.), and some
things about the whole universe (such as its expansion rate). By
studying stars that are older and younger than our sun, we can make
some guesses as to how our sun will behave as it gets older.
The easiest thing to measure about a star is how bright it appears
to us. Back in the old days, the best light detectors anyone had were
their own eyes, and people ranked stars brightnesses comapring them to
other stars. These days, we have electronic equipment, such as
photometers and charge coupled devices (CCD's) which can read out with
high precision exactly how much light is hitting them per second. A CCD
is the same thing that's found in video cameras which reads out
electrical pulses which indicate how much light strikes a tiny surface.
It's important to collect only light from one star at a time (or do as
best a job as possible), and to collect as much of it as possible, so
to do a good job with this measurement, large telescopes are used.
The next easiest thing to measure about a star is its color. Some
stars actually look red or blue or yellow to they eye, and again modern
telescopes and equipment make the study of color more precise. They use
diffraction gratings, finely etched parallel lines on metal, to split
the light into its different colored components, in much the same way a
prism does. Electronic equipment (it used to be film, but nowadays
electronics is faster, more reliable, and more convenient for data
storage) reads out the light thus split. By looking at absorption lines
in the spectrum, astronomers can determine the chemical composition of
the star's surface and immediate surroundings. By measuring the shift
in the frequencies of light corresponding to the absorption lines,
astronomers can calculate, using Doppler's law, how fast the star is
moving towards us or away from us. This has been used to gauge the
expansion of the universe, as well as to measure how fast galaxies are
turning.
The temperature of the star can be deduced from the spectrum of
light measured with the diffraction grating. It is known just how much
light at each color you expect to get from a large, hot ball of gas
like a star. Hotter stars are more blueish, and colder stars are more
reddish. Some stars, called "brown dwarfs", are so cold they emit
rather little light at all. This temperature is only the temperature of
the "surface" of the star (that is, the photosphere). We cannot tell
easily the temperature in the middle of a faraway star, although we can
use our knowledge of how gases work under high heat and pressure
conditions to make educated guesses.
To get the size of a star usually involves estimating the distance
to the star. Stars that are not too too far away can have their
distances measured using parallax. As the earth moves around the sun,
we see the stars from a different vantage point every night.
Observations made six months apart have the biggest difference in
vantage point. If we compare where we see a nearby star, as seen in a
background of very faraway stars, at one observation and six months
later, we may find that its apparent position has shifted a tiny amount
relative to the faraway background stars. By knowing the size of the
earth's orbit and this little angle, we can estimate the distance to
the star.
Some stars are close enough to be seen as a disk in high-power
telescopes (I believe Betelgeuse is big enough and close enough to see
the disk). Most stars appear as points of light in even the largest
telescopes, however. To estimate the size of a star where the disk is
visible, we simply make a triangle with one angle is the angular radius
of the disk, and the side is the distance to the star. The far side is
the radius of the star.
This only works for a couple of stars. A more common way to do it
is to measure the brightness, the temperature, and the distance. We
know what colors a hot gas will emit at a particular temperature, and
we also know how much light energy will be emitted per unit time from
the surface of the star. By knowing the distance to the star and how
bright it seems to us, we can calculate the total light energy coming
from the star per unit time (the "luminosity"). The energy put out by
the star is proportional to its area -- the bigger the star is at its
temperature, the more light energy it will emit. We can then deduce the
size from that.
Most stars are too far away to use parallax to estimate their
distances. Some special kinds of stars, called "Cepheid variable
stars", are stars that periodically change their brightness. The time
it takes for these stars to dim and get bright again depends on their
intrinsic brightness, as explained .
This is calibrated for nearby Cepheid variable stars and used for
faraway ones. By knowing how intrinsically bright a faraway Cepheid
variable star is, and by knowing how bright it appears to us, we can
estimate how far away it is. If we know other stars are nearby the
Cepheid variable, we can use the same distance estimate.
Tom
(published on 10/22/2007)
