PROPERTIES OF STARS
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:
This is determined from trigonometric and spectroscopic parallaxes.
Determining distances is CRUCIAL to understanding stars because
we can use distances to figure out
- the scale of things in the Galaxy and
- how much energy stars produce and radiate away -
by using the
inverse square law for light dimming along with apparent
This is the amount of energy generated in the star and released as
This is not a fundamental property but a combination of the luminosity
and distance to a star (and in some cases it is also dependent on the
amount of absorption in the direction of a star).
"Size" of the star - calculated from Stephan's Law.
- CHEMICAL COMPOSITION.
This is determined from absorption line spectra; it is tied up
in a semi-complicated way with 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 1011 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 Milky Way Galaxy).
- The Sun and Earth are out in the disk of the Galaxy and
we therefore have a view that is edge-on. Even at that the view
in visible light is obscured in many directions (particularly
toward the center of the Galaxy) by dust so it was quite difficult to
figure out the shape of the Galaxy. As we will learn in a week or so,
at infrared E-M radiation penetrates much further through
dust. The image below (click on it to get a bigger version) is
what the Milky Way looks like imaged in infrared light. This is a
mosaic generated by the COBE satellite.
The Galaxy seen
in the infrared. This image was obtained by the COBE satellite
which mapped the distribution of ``3 degree'' background radiation
left over from the Big Bang. (Click for a larger version)
- Below is an image of NGC 891, a galaxy located in the Virgo
cluster that we think is very similar
to the Milky Way Galaxy. This is probably what the Galaxy looks like
seen from the ``side'' in visible light. The dark lanes in through
the center along the long axis are caused by dust in the
plane of the galaxy.
NGC 841 is a galaxy in the Virgo cluster that we think is very similar to the
Milky Way Galaxy as seen edge on. This image is a combination of three
optical bandpasses and is similar to what you would see with your
eye if looking through a large telescope.(Click for a larger version)
- The Milky Way Galaxy is one of the senior members of a small
group of galaxies we call the ``Local Group''. The Local Group
is on the outskirts of a population of thousands of galaxies
called the Virgo Supercluster. In the observable Universe,
there are billions and billions of galaxies.
What are the stellar Constellations?
Constellations are 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
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 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 mean the apparent brightness. This
is different than luminosity which 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 logarithmic 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 magnitudes. The next
brightest star has a brightness of -1.4 magnitudes. Stars
a factor of 100 fainter than this have a brightness of (-1.4+5 =) 3.6
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