Astronomy 100 -- The Hertzsprung-Russell Diagram

The H-R Diagram is a graph of the Surface Temperature (or Spectral Class) of stars versus their Luminosity (or Absolute Magnitude).

Here are some more examples

The H-R diagram proved to be of interest as stars are not scattered everywhere in the diagram. Instead, the stars are located in four distinct regions of the diagram.

• The Main Sequence (MS):
  
  This is the band that stretches from the upper left corner to the lower right corner.
  
  
• Red Giant Region:
  
  Grouping in the middle right-hand side (redward, or on the cooler side of the Main Sequence.)
  
  
• Supergiant Region:
  These are the stars scattered across the top.
  
  
• White Dwarf Region:
  Area below the Main Sequence.

Another page with a good description of the H-R diagram.

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I. THE MAIN SEQUENCE

The basic property of Main Sequence (MS) stars is that they are ALL producing energy by FUSING HYDROGEN INTO HELIUM.

Main Sequence stars are very stable and are in a state known as

"HYDROSTATIC EQUILIBRIUM"

which means there is a balance between the gravitational pull in and the pressure pushing out (the Sun is a very hot gas!).

This is true for ALL MS stars and these stars will keep this same structure as long as there is energy generated in their cores by the fusion of hydrogen into helium.

= = = = = = = = = = = = = = = = =

Relationships Along the Main Sequence

• 1) Temperature sequence (high to low)
• 2) Luminosity sequence (high to low)
• 3) Mass sequence (high to low)
  
  O-stars up to 100 solar masses.
  M-stars down to 1/10 solar mass.
• 4) Lifetime sequence (short to long)
  
  O-stars live about 10 million years.
  M-stars live up to 2 trillion years!
  The very long lifetime of K and M stars explains why 90% of all stars around us are K and M stars. As the lifetimes of these stars are longer than the age of the Universe, every K and M star that has ever formed is still around. This is not the case for massive stars, such as O and B stars.

Property of all MS stars: Energy is generated by fusing 4 hydrogen atoms into 1 helium atom.

• 1) For stars less than 1.5 solar masses, the dominant fusion reaction is
  Proton-Proton (P-P) cycle.
• 2) For stars greater than 1.5 solar masses, the dominant fusion reaction is the
  Carbon-Nitrogen-Oxygen (CNO) cycle.

Mass limits for Main Sequence Stars:

Lower Limit = 0.08 solar masses.
If the mass is lower than 0.08, the core never becomes hot enough for nuclear fusion to begin. May become a planet (such as Jupiter).

Upper Mass Limit: about 100 solar masses.
The exact value is uncertain, but if a star larger than 120 solar masses tries to form, it blows itself apart before it can reach the MS.

The MASS-LUMINOSITY Relation

For Main Sequence stars, the mass of the star determines the luminosity of the star.

Luminosity = Mass^3.5
or

Best to use solar units, where the luminosity is measured in solar luminosities and masses are measured in solar masses. As an example, for a 4 solar mass star, the luminosity will be 4 x 4 x 4 x 2 = 128 times the luminosity of the Sun.

The MASS-LIFETIME Relation

The Main Sequence lifetime, t, of a star is also determined by the mass of the star,

t_MS = Mass^-2.5
or
t_MS = 1 / Mass^2.5
or

Again, use solar units! Doing so will give the lifetime in units of the lifetime of the Sun (about 9 billion years) So, for that 4 solar mass star, the lifetime will be 1 / (4 x 4 x 2) = 1/32 only 1/32 the lifetime of the Sun.

II. THE RED GIANT REGION

Stars in this region are relatively cool in surface temperature (3000 K to 5000 K), and so classified as K and M stars, yet they are very luminous. For these stars to be more luminous than MS K and M stars, they must be larger in diameter! These stars have diameters between 10 and 100 times the diameter of the Sun and this is why they are called GIANTS

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III. THE SUPERGIANTS

These stars are the most luminous stars we observe. However, since they are not really hotter than other stars (in fact, many are K and M stars), they can only have such large luminosities by being even larger in diameter than the red giants. Hence, they earned the name of SUPERGIANTS. Supergiants have diameters between 100 and 1000 times the diameter of the Sun!

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IV. THE WHITE DWARF REGION

The white dwarf region lies below the Main Sequence. While these stars have very hot surfaces, ranging from 5000 K up to 20.000 K, they are not very luminous. For the giants, in order to increase the luminosity, the stars had to have large diameters. So, while white dwarfs have high temperatures, in order for these stars to have low luminosities, they must be very small in diameter. White dwarfs have diameters similar to that of the Earth, which is only about 1/100 that of the Sun!

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Basic Properties of Stars

The Process of Star Formation.

Back to The Front Page.

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