1. Generally describe the post main sequence evolution of stars more massive than 4-8 solar masses.

2. Describe the evolution to the red supergiant phase, including the position on the HR diagram, and the properties of a red supergiant star.

3. Describe the nucleosynthesis that takes place inside an evolving star leading up to the supernova phase.

4. Describe the evolution to the supernova phase, including the position on the HR diagram, and the properties of a supernova.

5. Discuss the difference between a Type I supernova and a Type II supernova.

6. Discuss the effect that ejection of mass from supernovae has on the chemistry of the ISM and future generations of stars.

¶ red supergiant

¶ nucleosynthesis

¶ iron formation

¶ iron core

¶ shells of lighter elements

¶ Type II supernova

¶ neutrino generation

¶ ejection of stellar mass

¶ supernova remnant

¶ neutron star

¶ pulsar

¶ black hole

¶ cosmic rays

¶ Supernova 1987a

¶ Main sequence stars which have masses greater than 8 solar masses (but possibly greater than only 4 solar masses) go through much more violent stages of evolution than lower mass stars. After a single massive star’s red giant phase, it will become a red supergiant, then a Type II supernova (losing much of its mass), and finally a neutron star or black hole.

¶ The most massive main sequence stars will go through the red giant phase just like stars of lower mass. However, in the most massive stars, nuclear fusion reactions in the stars’ cores can cause the production (or nucleosynthesis) of very heavy elements. These reactions go in stages with the heaviest elements, such as iron, forming last.

¶ The energy generation that occurs during the heavy element nucleosynthesis causes the outer layers of the star to expand even further. When this happens, the star becomes a red supergiant.

¶ As the nucleosynthesis of heavy elements proceeds, an onion-skin-like structure of elements in the star forms with the heaviest elements, like iron, residing in the core, followed by shells of lighter elements.

¶ When nuclear reactions no longer occur in the core, the core begins to collapse. The collapse takes place in a few milli-seconds (10^-3 sec).

¶ In massive stars the mass of the core is greater than the Chandrasekhar limit of 1.4 solar masses. As a result, the gravitational forces are so great that electrons are pushed into protons during the collapse to form a neutron core (massive amounts of neutrinos are generated in this reaction which mostly escape the star immediately).

¶ The energy generated as a consequence of the gravitational collapse of the massive core blows the outer part of the star (which contains most of the stellar mass) away from the core. This is called a Type II supernova. A supernova corresponds to an abrupt (few day) increase in luminosity by a factor of 100 million (10⁸).

¶ Note that type I supernovae occur in binary systems when large amounts of mass are transferred from a red giant to a white dwarf. When the mass of the white dwarf is made to exceed the Chandrasekhar limit of 1.4 solar masses, the white dwarf core itself becomes a type I supernova. The star completely explodes and no core is left behind.

¶ When a supernova occurs, most of the stellar mass gets ejected back into the gas and dust (ISM) in its galaxy. This is the gas, now enriched with heavy elements, from which future generations of stars will form. This is where the atoms that make up our bodies originated.

¶ The matter ejected from recent supernovae can often be observed. These structures are called supernovae remnants. The Crab Nebula, which resulted from a Type II supernova in 1054 AD, is the most famous supernova remnant. A neutron star in the form of a pulsar can be seen at its center.

¶ Cosmic rays are protons and the nuclei of elements heavier than hydrogen which are moving at velocities near the speed of light. They account for typically 20% of the high energy radiation people experience on Earth. Non-solar cosmic rays are believed to result when supernovae explosions accelerate particles. Genetic mutations of species on Earth are thought to be associated with cosmic rays.

¶ In 1987, a Type II supernova (called 1987a) was observed in the Large Magellanic Cloud, an irregular galaxy near to our own Milky Way Galaxy. Observations of this supernova allowed us to verify many of the theoretical predictions about the evolution of massive stars. The most significant result was the detection of bursts of neutrinos from the supernova.