Download The H-R Diagram

The H-R Diagram
The Herzsprung Russel Diagram (HRD) is a systematic way of arranging stellar data. It plots the
Absolute Magnitude (MV) or Luminosity (L/L¤) versus the Spectral Type, Surface Temperature or
Color. The brightest stars are at the top and the hottest stars at the left. The radius increases diagonally
towards the upper right. Only for main sequence stars (m.s. stars) the mass increases diagonally to the
upper left. The stars in different areas have distinctly different physical properties and are in different
stages of their life.
The Main Sequence:
The main sequence goes from the upper left to the lower right. The stars at the upper left are hotter,
larger, and more massive than the Sun. The Sun is near with an absolute magnitude of MV=+5 and a
spectral type G2. Like all m.s. stars it is of luminosity class V. Those stars to the lower right are less
massive, cooler, and smaller than the Sun. The main sequence contains about 80% of all stars. Stars
spend most of their life on the main sequence, although stars in the upper left have very short lifetimes
compared with those in the lower right. Stars on the main sequence are converting Hydrogen (H) to
Helium (He) in their cores.
Supergiants:
The Supergiants have luminosity class Ia and Ib. They form an almost horizontal sequence across the
top of the H-R diagram, appearing in all Spectral Classes. All are very massive; the mass is greater for
hotter supergiants (on the left!); the radius is greater for cooler supergiants. Supergiants probably have
two or more shells of nuclear reactions and will eventually explode as supernova.
Giants:
The giants are in a roughly horizontal sequence near MV = 0. Their luminosity classes range from II to
IV. They are generally red or orange colored (cooler than 6000 K). They have masses roughly
comparable to those in the middle of the main sequence. They are very large (although not as large as
supergiants). Giants have converted all the hydrogen in their cores and are now converting Helium to
Carbon or Oxygen in their cores and Hydrogen to Helium in a shell outside the Helium core.
White Dwarfs:
White dwarfs are very hot, very small, faint stars. They are faint because they are so small, even though
they are hot (remember the luminosity is the flux emitted from the surface of the star and WD’s are
small). They occur in a diagonal strip across the lower left of the H-R Diagram, roughly parallel to the
main sequence. Most white dwarfs have approximately the mass of the sun, but a radius about 0.01 to
0.001 of the radius of the sun (roughly about the size of a the earth). Their average density is about 106
to 108 solar density. They have exhausted all of their nuclear fuel, are no longer generating energy, and
are at the end of the stellar evolution cycle. They are contracting and cooling and will eventually become
"black dwarfs", emitting less and less energy.
The HRD showing the location of some of the
most well known and closest stars.
Note that Betelgeuse and Rigel are both
supergiants. They are roughly 100,000 times
as luminous as the sun (that means that you
have to have 100,000 suns to get the same
light output).
The Sun is just an average, yellow, main
sequence star. It has finished roughly half of its
life as a main sequence star, and will then
become a red giant star and eventully die as a
white dwarf. As a red giant it will emit roughly
100 times as much light as today, but as a
white dwarf, it will only be one hundredth
times as luminous…
The HRD showing the location of stars
according to type, in particular the main
sequence, the giants and super-giants and
white dwarfs.
Note that the equal-radii-lines are drawn.
Super-giants are roughly 50 to 1000 times
larger than the sun, giants are roughly 5 to 100
times larger, and white dwarfs are one
hundredth the size of the sun.
Betelgeuse is as big as the orbit of Jupiter
around the sun, and Sirius B, a white dwarf, is
as big as the Earth.