Evolution of Stars, Origin of the Elements

"In the beginning, there was nothing at all. Earth was not found, nor Heaven above, a Yawning-Gap there was, but grass nowhere."

The Edda -- collection of Norse Myths dating to 1200

format for printing

In this lecture period we learn:

Evolution of stars and the H-R diagram (red giant, supernova, black hole)
Origin of elements (fusion and neutron capture)
Abundances of elements

: A powerpoint file on this material will be available on your Ctools site


Evolution of Stars

Eagle Nebula (Hubble Telescope)

New stars are being born in gaseous nebulae and have life cycles of over billions of years. The details of the life cycle depend on size and luminosity (brightness). As we saw earlier, the Sun acts as a black body with a temperature of ~6000 K. Other stars seen in the sky are hotter and cooler. Hotter stars are bluer and cooler stars are redder. Measurements show that stars are in the general range of 3000K to over 20,000 K. These measurements represent the star surface temperatures, not the temperatures inside, which are much hotter and lead to fusion reactions which power the star. 

Stellar temperatures and luminosity

Another characteristic is the brightness of a star - its luminosity. The most common classification scheme for stars plots surface temperature against luminosity: The Hertzprung-Russell (H-R) diagram. 


Hertzprung-Russell (H-R) diagram is a temperature vs. luminosity plot of stars. Some prominent stars are shown by name. The visible colors of stars are indicated at the top, and the different classes of stars are labeled in the diagram. The majority of stars are found to lie on the Main Sequence. Giants and supergiants do not behave as typical stars.

Main Sequence Stars

The majority of stars are on the main sequence burning hydrogen to make helium through fusion reactions. When the hydrogen is used up, the star will move away from the main sequence. A moderately sized star like the sun will become a red giant, growing in size to engulf the Earth and burning helium to make carbon. Following this stage, the outer layers will be thrown off, and the Sun will end up as a white dwarf, a dim object with a very small radius and high density. 
Solar system abundances of the elements. Note that the 1:4 ratio of helium to hydrogen (by weight) is what the Big Bang theory would predict. But other, heaver elements must come from other processes.
Abundances of the elements in Earth. Comparison with the figure above shows that Earth has lost most of its primordial hydrogen and helium.

Atoms -- a primer

The table below list basic characteristics of the building blocks of atoms.

Particle    Electrical Charge        Rest Mass               Molar Mass
            C                        kg                      g/mol
electron    -1.60217733(49) x E-19   0.91093897(54) x E-30   0.0005486
proton      +1.60217733(49) x E-19   1672.6231(10)  x E-30   1.0072697
neutron     0.0                      1674.9543(86)  x E-30   1.0086650
The atomic number, which is the number of protons, characterizes an atom. In the case of hydrogen, we have one positively-charged proton (the nucleus) that is orbited by one negatively-charged electron. In the other elements, the nucleus of an atom contains both positively-charged protons and neutrally-charged neutrons. The mass number of an atom is the combined weight of protons and neutrons. For our purposes it turns out that electrons have negligible mass compared to the nucleus. Some atoms can have different numbers of neutrons, called isotopes, but if we change the number of protons we change atomic species. The atomic number of H is 1 and its mass is ~1.67E-24 gram; atomic weight is typically expressed as reference mass, and is slightly more than 1 for H (it is 1.0079) because of the presence of other H isotopes. The next element in the periodic system, He, has atomic number 2 (two protons and 2 neutrons) and a mass of ~6.65E-24 gram (or atomic weight of about 4). To make one He atom, we therefore need four H atoms and some modifications. Assuming we are able to overcome the repelling force of the protons, the combined mass of 4 H atoms equals 6.696E-24gram, which exceeds the mass of He. The excess mass is released as energy following Albert Einstein’s well-know E=mc^2 equation; with m is (excess) mass and c is the speed of light. Small as the mass excess may seem, the energy release of only a very small amount of H that fuses to create He is tremendous. This H fusion process is the source of our Sun’s energy, and has also been used to create the most powerful weapon of destruction: the hydrogen bomb.

The consequence of the fusion process is a balance between gravitational contraction and expansion due to heat in the Sun. As long as H fusion continues heat will be generated, but what if all H has been converted to He? When H fusion stops, the sun cools and gravitational collapse occurs. This in turn creates heat from the restricted motion of atoms. At some point, enough heat may become available to fuse He atoms, for example into the element C. This releases excess mass in the form of energy as well. Then we have a He-fueled star. Why does H fuse before He? The answer lies in the structure of the nucleus, where a greater number of protons requires more energy to overcome the repelling forces. When He fusion occurs in the core of a star, sufficient heat is generated for H fusion to occur in outer parts of the star. The combined energy these two fusion processes generate causes the star to expand enormously, at which stage we call it a Red Giant. When our Sun reaches this stage 9n 4-5 billion years, its size will exceed the orbit of Earth.

Origin of "Heavy" Elements

Shell structure of a heavy star (~25 solar masses) at the end of its evolution, just prior to a supernova explosion. The fraction of the total mass contained in each shell and the principal elements present are shown.

Heavier stars evolve into supergiants, and it is the nuclear reactions in the interiors of these stars that gave rise to the heavy elements in the universe. The high temperatures (nearly 1,000,000,000 K) required for this process can only be reached in stars heavier than about 4 solar masses. When the core is composed of heavy elements near iron, no further nuclear fusion is possible and the tendency for the star to collapse under the tremendous gravitational forces is unchecked. As the star's heavy core implodes, huge shock waves break the star apart, creating a supernova. A recently observed supernova was seen in the neighboring Large Magellenic Cloud galaxy (a diffuse "satellite" galaxy of the Milky Way) early in 1987. Perhaps the most famous supernova was seen by Chinese astronomers in July 1054 and was visible for several weeks in broad daylight. The visible remnant of this huge explosion is now called the Crab Nebula; see Hubble Telescope image. This supernova remnant is 6,500 light years away. Another beautiful example of a supernova remnant is the Cygnus Loop, lying about 2,500 light years away.

The evolution of even more massive stars produce other objects in our universe, such as a black hole.  These object are invisible because all matter is unable to escape the gravitational attraction.  We use indirect means to recognize their presence and many galaxies, including our own Milky Way galaxy, may have a black hole in their core.

Self Test for the Evolution of Galaxies and Stars


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