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The H-R Diagram
© Copyright 2002, Jim Loy
On the left is a version of the Hertzsprung-Russell (H-R) diagram.
Stars are plotted on the graph, using their color (temperature) on the
horizontal scale, and their luminosity (the sun = 1) on the vertical scale. The
colors are coded O (blue), B (blue), A (blue-white), F (white), G (yellow), K
(orange), and M (red), and students learn this sequence with the mnemonic "Oh
be a fine girl (guy), kiss me." And a few obvious patterns emerge. Most stars
are on a diagonal band from blue giants on the upper left to red dwarfs on the
lower right. This band is called the main sequence. Many of the brightest stars
are red and orange giants and super giants, in the upper right. And white
dwarfs appear in the lower left. The sun is a G star, on the main sequence.
A star's color is mostly from heat (red = cool, blue = hot), known as
black body radiation. But some stars get some of their color from other
features. Examining a star's spectrum allows astronomers to find its adjusted
temperature. If the star is not a red giant or a white dwarf, then it is almost
certainly near the main sequence on the graph. So its luminosity can be
deduced, and from that it is easy to find the star's approximate distance from
us. There are more accurate methods for determining distance to some stars, but
the H-R diagram is one of the many methods. If a star is a red giant or super
giant, the same diagram can help determine its distance, in the same way.
There is also an equation called the mass-luminosity relationship, which
relates to stars on the main sequence. Thus, if you know a star's luminosity,
you very likely can deduce its approximate mass. The H-R diagram helps us do
that.
Stars change (evolve) over the ages. The H-R diagram, and many other
kinds of evidence tells us quite a bit about that. Most stars are currently on
the main sequence because most of them remain there for a very long time. When
a star's core ignites in hydrogen fusion, the star quickly becomes a main
sequence star. Much evidence shows that a star of the mass of the sun or
smaller then remains on the main sequence for a very long time, billions of
years. Then it runs out of hydrogen fuel and collapses; then the helium in the
core ignites a second fusion reaction, and the star becomes a giant. The star
has moved off the main sequence into the upper right part of the diagram. It
stays there for a while (maybe a few million years) changing from red giant to
variable star, and then dies again, becoming a white dwarf. Stars less massive
than the sun (the many red dwarfs) go through a similar life cycle, but remain
on the main sequence much longer.
A star about five times the mass of the sun lives a much shorter life,
as it burns its fuel faster. Then it will become a red giant, then a Cepheid
variable, then a supergiant. A much more massive star will explode after the
first collapse, when the hydrogen fuel is used up. Some of these which are part
of binary star systems will become novas, before becoming white dwarfs or
neutron stars. Even more massive stars become supernovas, which for a few weeks
outshine all of the stars in the whole galaxy. In just a few weeks, a supernova
will release more energy than the sun does in 10 billion years. Inside an
expanding cloud of debris, the original star will become a neutron star (see
Supernova 1054). A small percentage of stars are
certainly massive enough to become black holes after exploding.
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