Are all stars the same? Not in the least! Some stars are just beginning
to form in nebulae, others are enjoying middle age along the main
sequence, and some have begun to die. The life cycle of a star can
be compared to the life cycle of humans. Find
out what stage you would be in if you were a star. The Hertzsprung-Russell
Diagram is a tool that shows relationships and differences between
stars. It is something of a "family portrait." It shows stars of
different ages and in different stages, all at the same time. But
it is a great tool to check your understanding of the star life
cycle. Letís go over the basics before we check your understanding.
In the Hertzsprung-Russell (HR) Diagram, each star is represented
by a dot. There are lots of stars out there, so there are lots of
dots. The position of each dot on the diagram tells us two things
about each star: its luminosity (or absolute magnitude) and its
The vertical axis represents the starís luminosity or absolute
magnitude. Luminosity is technically the amount of energy a star
radiates in one second, but you can think of it as how bright or
how dim the star appears. Depending upon the textbook you use, the
labels on the HR diagram could be a little different. Luminosity
is a common term, as is absolute magnitude
. Absolute magnitude is the intrinsic brightness of a star. In either
case, the scale is a "ratio scale" in which stars are compared to
each other based upon a reference (our sun).
The horizontal axis represents the starís surface temperature (not
the starís core temperature Ė we cannot see into the core of a star,
only its surface)! Usually this is labeled using the Kelvin temperature
scale. But notice : In most graphs and diagrams,
zero (or the smaller numbers) exist to the left on the diagram.
This is not the case here. On this diagram, the higher (hotter)
temperatures are on the left, and the lower (cooler) temperatures
are on the right. Some HR diagrams include the color of stars as
they can be seen through filters on spectrophotometers. This is
also a "ratio scale."
So how do you read the HR diagram? Well, letís look at some basic
regions on it. A star in the upper left corner of the diagram would
be hot and bright. A star in the upper right corner of the diagram
would be cool and bright. The Sun rests approximately in the middle
of the diagram, and it is the star which we use for comparison.
A star in the lower left corner of the diagram would be hot and
dim. A star in the lower right corner of the diagram would be cold
Letís see how good you are at reading the HR diagram. Click the
diagram below, we have five stars. How would you describe the temperature
of Star A? (Comparing the stars to each other) And how would you
describe the luminosity (brightness) or Star A? Look at Stars B
through E and describe each of them.
Are there any stars that seem out of place? For example, are there
any stars that are really hot but not very bright? Are there any
stars that are not very hot but they shine very brightly? What do
you think could account for these differences in stars that do not
fit the pattern? Stars that do fit the pattern are called
main sequence stars. Letís look at those before
we deal with the exceptions.
Most of the stars lie within a region called the main sequence.
It looks kind of like a curved line sloping from the upper-left
to the lower-right of the HR diagram. There are some stars that
are not on the main sequence, but for now letís concentrate on the
main sequence stars. Why is there a main sequence region anyway?
Well, there is a predictable relationship between the brightness
and size of a star. This shows up on the HR diagram. We know that
hotter things are brighter. A hotter temperature means that more
energy is radiated into space. Bigger stars are brighter. A bigger
surface area means that more energy is radiated into space. The
sun is used as a reference, so letís use some logic to figure out
how this works.