Question

1. Briefly describe the three nuclear energy production methods in stars, making sure to give the inputs (fuels) and products of each process.
2. How is the end of a high mass star's life different from that of a low mass star?
3. Give a few properties of each and tell whether each comes from a low-mass or high-mass star: white dwarfs, neutron stars, and black holes.
4. In which type(s) of galaxy is one likely to find new star formation? Why?

Answer 1

1. The energy produced by the stars is generated from uncontrolled Nuclear Fusion Reactions. For stars of masses similar to the Sun, which have internal core temperatures less than about fifteen million kelvins, the dominant nuclear fusion process is the Proton-Proton Fusion process. For more massive stars, which can be of higher temperatures, the Carbon Cycle Fusion becomes the dominant process. And, for older stars which are collapsing at the center, the inner core temperature can exceed one hundred million kelvins and initiate the Helium Fusion process, also called the Triple-Alpha process.

In the Proton-Proton Cycle nuclear fusion process, hydrogen nuclei fuse to give helium nuclei and result in huge amount of energy. To be more precise, the fuel is (for one reaction) 4 protons, i.e., 4 hydrogen nuclei and the products are one alpha particle, with the release of two positrons and two neutrinos (which changes two of the protons into neutrons) and a huge amount of energy.

In the CNO Cycle (for carbon–nitrogen–oxygen), unlike the above process, reactions proceed via the catalyst isotopes carbon, nitrogen and oxygen. It is the dominant process in those stars that are more than 1.3 times as heavy as the Sun. In one reaction, 4 protons fuse, using carbon, nitrogen and oxygen isotopes as catalysts, to produce one alpha particle, two positrons and two electron neutrinos. The positrons will annihilate with electrons, releasing energy again in the form of gamma rays. The neutrinos escape from the star carrying some energy. One nucleus goes on to become carbon, nitrogen and oxygen isotopes through a number of transformations and go on like this till the next reaction, then the next and so on.

In the triple-alpha process, a series of nuclear fusion reactions happen by which 3 of the alpha particles are transformed into a carbon nucleus in one step. Already there is lot of helium around due to either of the above two processes. Even further nuclear fusion reactions of helium nuclei with proton or another alpha particle produce lithium-5 and beryllium-8, respectively. Both of these two products are highly unstable and decay almost immediately to smaller nuclei, unless a third alpha particle fuses with a beryllium-8 nucleus before that time to produce a stable carbon-12 nucleus. Obviously a lot of energy is also produced in all these reactions.

2. Once a low mass star like our Sun has exhausted all of its nuclear fuel, its core collapses into a dense white dwarf and rest of the outer layers are expelled into the inter-planetary nebula. On the other hand, high mass stars (with about ten or more times the mass of the Sun) undergoes a dramatic fate - they can explode into a bright Supernova as their inner iron cores collapse into an extremely dense neutron star or if the mass is too high, into a black hole, emitting super powerful gamma ray bursts called quasars.

3. As said above, white dwarfs originate from low-mass stars, like our Sun and neutron stars originate from high-mass stars whereas black holes originate from very high-mass stars. A few characteristic properties of them are listed below:

( i ) White Dwarfs : White dwarfs are stable in nature because the inward gravitational pull is balanced by the degeneracy pressure of the star's electrons --- a consequence of the Pauli Exclusion Principle. Electron degeneracy pressure provides a weak limit against further compression, therefore, for a given chemical composition, white dwarfs of higher masses have a smaller volume. With no more fuel left to burn in the core, the star radiates its remaining heat energy into space for billions of years.

( ii ) Neutron Stars : When the stellar core of a high-mass star collapses, the huge huge pressure causes electrons and protons to fuse by electron capture forming neutrons. With no electrons to provide the electron degeneracy pressure, the whole core collapses into a small super dense ball of radius of about 10 km to 100 km. Now, the neutrons provide the necessary degenercy pressure against further collapse. The ball is a phenomenally dense, super packed globe of neutron, one teaspon of it can weigh heavier than the Sun. Their time period of rotation shortens dramatically as the stars collapse (due to the conservation of angular momentum) - observed rotational time periods of neutron stars range from about 1.5 milliseconds to few seconds.

( iii ) Black Holes : If the mass of the very high-mass stellar remnant is high enough, the neutron degeneracy pressure will now be insufficient to prevent the gravitational collapse below the Schwarzschild radius. The stellar remnant thus becomes what is known as a black hole. Black holes are predicted by the General Theory of Relativity. According to Classical General Relativity, no matter or information, not even light itself, can escape from the black hole to an outside observer, although quantum effects may allow such deviations as recently suggested.

New star formation is most likely to be found in spiral galaxies with dense inter-stellar nebulae because the dense gas clouds help in the quenching process leading to the birth of new stars.