Most Stars are Simple!

Key points: H-R diagram; main sequence; overall simplicity of stars; evolution off main sequence

Whenever we get a mass of interstellar gas between 0.08 and 100 M and held tightly by gravity, we get a star. Yet stars are basically very similar to each other - it is only a slight oversimplification to say that the status of a star is totally determined by its

  1. ) mass;
  2. ) age;
  3. ) initial composition.

The simplicity of stars is sometimes called "the Russell-Vogt Theorem." For the stars near the sun, even the initial composition is usually very similar, so their properties depend on just mass and age! The evolution of stars is then just a question of determining how ones of a given mass change their properties as they age.

The H-R Diagram shows the effects of aging on stars of different mass.  
ngc2264a.jpg (46219 bytes) Very young clusters of stars have all or most of their members above the main sequence, since they are still contracting and have not reached stability with all power derived from thermonuclear fusion. Eventually they settle on the main sequence.

(From U. Tenn,


hr_diagram1.gif (10967 bytes)

Why is there a main sequence?

If there were no thermonuclear fusion of hydrogen into helium, a star would continue to shrink under gravitational attraction as it released energy. There would be no stable period in the life of the star, and if we looked at stars over a range of ages and masses we would expect them to be scattered all over the H-R Diagram. Thermonuclear fusion heats the inside of the star, creating pressure that stops the collapse and producing a long period of great stability under hydrostatic equilibrium that defines the main sequence. (From B. Kellet,

Artist's concept of a red dwarf rising on a mythical planet The "bottom" of the main sequence -- its end toward low temperature and low luminosity -- is where the least massive stars lie. These stars have very strong convection and hence a lot of flare activity as shown in this artist's concept of how one might look from one of its planets. (by Don Dixon)

A mass less than 0.08 times the mass of the sun never develops enough pressure and a high enough temperature for hydrogen fusion, so it slowly radiates its trapped gravitational energy away and disappears. These low mass objects are brown dwarfs.

The most massive stars, 60-80 times as massive as the sun, lie on the main sequence at the very high luminosity and high temperature tip. The great outpouring of photons from stars more massive than about 100 times the sun tears them apart, so they never manage to become stable.

Why is the main sequence so well definedbutton.jpg (6796 bytes)

Evolution Off the Main Sequence

Interior structure of a main sequence star What happens when the hydrogen fuel in the core begins to run out? (From Chaisson & McMillan, Astronomy Today)



These stars are found in other parts of the H-R diagram:
hrgiant.gif (45815 bytes) Stars that have exhausted their core supply of hydrogen swell up and become very luminous. They are called red supergiants and giants, and their large size, relatively cool temperatures, and high luminosities mean they are plotted on the HR diagram above and to the right of the main sequence. Fundamentally, they do not lie on the main sequence because they are no longer powered by core burning of hydrogen.

4He + 4He ---> 8Be + photon

8Be + 4He ---> 12C + photon

Interior structure of a helium-burning star That is, these stars start to burn helium as well as hydrogen and build up the combustion product carbon in their cores. Here is a sketch of the interior structure of the star at this stage. (From Chaisson & McMillan, Astronomy Today)

These reactions produce a large burst of energy when they start -- the He in a star's core may "burn" in just a few minutes (or even seconds). This is called the helium flash. The energy released takes thousands of years to diffuse outwards and leave the star.

starclock.gif (34345 bytes) Here is an example of this evolution for a star five times the mass of the sun.buttonbook.jpg (10323 bytes) (from Leos Ondra,

Evolution of Very Massive Stars

If the star has enough mass, the core will contract, heat up, and trigger further reactions after the helium in the core has all been converted to carbon. A star must have a mass greater than 8 M for the following reactions to take place buttonex.jpg (1228 bytes)

12C + 4He ---> 16O

12C + 16O ---> 28Si

16O + 16O ---> 28Si + 4He

28Si + 28Si ---> 56Fe

Interior structure of a star with an iron core This sketch shows the structure of the star when it is accumulating an iron core, as in the last of the reactions listed above. (From Chaisson & McMillan, Astronomy Today)

The star resembles an onion -- the core might be converting Si to Fe with each step outward yielding a layer burning another set of elements.

Because Fe (iron) is the most stable atomic nucleus, reactions cease when 56Fe is formed.

Relative Rates of Evolution

Mass Luminosity Relation

By compiling data on many pairs of stars, we know that the luminosity of a star on the main sequence increases rapidly with mass: larger masses imply higher luminosities because higher mass stars have higher central temperatures and hence convert H to He faster.

A star's lifetime on the main sequence is also related to its mass buttonbook.jpg (10323 bytes)

High mass stars burn H rapidly.

Therefore, they have short main sequence lifetimes.

Low mass stars burn H slowly.

Therefore, they have long main sequence lifetimes

Higher mass stars have higher surface temperatures that are a reflection of their higher internal temperatures -- the more massive the star, the larger the gravitational pressure at the center and the higher the central temperature and pressure. Nuclear reactions proceed more rapidly if the temperature is higher so massive stars burn their nuclear fuel more quickly and evolve more quickly.buttonbook.jpg (10323 bytes)

The Fate of the Sun

The sun will go through the same evolution as other stars, and in about 5 billion more years will have become a red giant:

Below is an artist's conception of how the stages might look from the same vantage point on Earth (from Cosmos by C. Sagan, paintings by A. Schaller).

endearth1.JPG (153379 bytes) "The last perfect day", several billion years in the future.
Dying Earth  

The waters recede and most life is extinguished as the sun starts to swell and its luminosity rises.

Dead planet The oceans have evaporated and the atmosphere has escaped into space
Red giant glowers over a dead Earth  

The sun, now a red giant, fills the sky over a dead planet. As we see in the next section, the red giant will eventually throw off its outer layers and become a white dwarf.

Test your understanding before going onbuttongrad.jpg (11232 bytes)

Bright star Alcyone from the inlaid star map at Hoover Dam, (G. Rieke)

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Postage stamp celebrating Chandrasekhar's theory of white dwarfs

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

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