We can determine an amazing number of physical properties of stars,
usually based only on a little bit of light. Over the next several
lectures, ``we'' will talk about how the following properties
are determined for stars.
- BRIGHTNESS. This is not a fundamental property, but a combination
of the luminosity and distance to a star (and in some cases also dependent
on the amount of absorption in the direction of a star).
- DISTANCE. From trigonometric and spectroscopic parallaxes.
Determining distances to stars is how we figure out the scale of
things in the Galaxy and is CRUCIAL to understanding stars
because we can use the inverse square law for light dimming along with
apparent brightness of stars to figure out how much energy is
being produced and radiated away.
- LUMINOSITY. This is the amount of energy generated in the star
and released as electromagnetic radiation.
- RADIUS ``Size''. From Stephan's Law.
- CHEMICAL COMPOSITION. From the absorption line spectra. This
one is tied up in a semi-complicated way with the next one.
- TEMPERATURE. We have talked about Wien's Law
and using colors to derive stellar temperatures, but, there are
some complications. To REALLY get to surface temperatures of
stars, we need to learn about and understand stellar Spectral Types.
The Bigger Picture
First - let's get oriented. We live out in the suburbs of a pretty
good sized spiral galaxy that contains on the order of stars.
- The stars we see at night with the unaided eye are just
the very tip of a big iceberg. Go out on a clear summer night
somewhere where the city lights are not too bright and you
can see a swath of light across the sky. This is the
``Milky Way'' - the light from thousands of unresolved stars
in the plane of the Galaxy (galaxy with an uppercase ``G'' is the
galaxy we live in).
- The Galaxy (the one we live in is the Milky Way Galaxy) is
one of the senior members of a small group. There are
billions and billions of galaxies like the Milky Way out there.
What are the stellar Constellations?
Just people connecting dots. Many of the stars that are close together in
the plane of the sky (the projection of the three dimensional galaxy
into two dimensions) are quite far apart long the line of sight.
What about star names?
The brightest stars have various names - mostly from long ago. There are
no ``official'' names for stars. The often used convention in the West is
to use the Greek alphabet to identify the brightest stars belonging to some
- Sirius Canis Majoris brightest star in the constellation Canis Majoris
- Canis Majoris 2nd brightest star in the constellation Canis Majoris
- There are also some pretty standard catalogues for the designations of
bright stars and these are used world-wide.
- You can start your own business selling star names and people have done so!
- I will use brightness to be the apparent brightness. This
is different than luminosity with is an intrinsic property of
a star - luminosity is the luminous energy produced in the core of the
star. The brightness is a combination of the intrinsic brightness
of a star and its distance (and sometimes also the amount of
interstellar material in between). The brightest star in the sky is
- Astronomers use a logrithmic scale for the brightness of
stars called the magnitude system. Every 5 magnitudes is a
factor of 100 in intensity. It is confusing because it runs backwards.
For example the Sun has a brightness of -23 magnitude. The next
brightest star has a brightness of -1.4 magnitudes. Stars
a factor of 100 fainter than this have a brightness of 3.6 magnitudes.
In the city, this is about as faint a star as you can see. Out
at a darker site, you can see stars as faint as 6th magnitude
with the unaided eye. With a small telescope you can see
a factor of 10,000 fainter to around 16th magnitude and with the
big telescopes you can see another factor of 10,000 fainter to
26th magnitude stars.
Mon Jan 26 12:32:25 PST 1998