Abstract
Stars are born of gas and dust in the interstellar medium and evolve throughout their lifetimes from early to final stages. They are massive luminous spheres of plasma composed of the most abundant elements in space—hydrogen and helium—held together by their own gravity. The evolution of the various types of stars is traced along the Hertzsprung-Russell diagram. The life cycle of a star, from birth in the giant molecular clouds, through active phase involving nuclear fusion as an energy source, and ultimate death, principally depends on its mass. The final stages of low-mass and high-mass stars are also different: low-mass stars die forming a red giant and then a white dwarf, while high-mass stars explode as supernovae, leaving behind a neutron star or black hole.
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Notes
- 1.
The Salpeter function is the rate of formation of stars of different masses in the Galaxy inferred from observations of stars of different luminosities and defining the process of stellar evolution. This concept is important in theories of star formation.
- 2.
In order to determine the rotation rate of a star, methods such as spectroscopic measurements or tracking of the rotation rate of starspots are used.
- 3.
The surface gravity can influence the appearance of a star’s spectrum: the higher surface gravity pertinent to compact stars causes a broadening of the absorption lines, while the opposite is the case for giant pre-main sequence stars having a lower surface gravity.
- 4.
In astronomy all chemical elements heavier than helium are considered to be “metals,” and the relative abundance of these elements in a star is called the star’s metallicity (also designated Z ). The metallicity affects its lifetime, magnetic field formation, and stellar wind strength. The younger stars of rather high metallicity and older stars of substantially less metallicity are called Population I and Population II stars, respectively. Population III first generation halo stars are also distinguished in protogalaxies when they began to form and contract. These massive huge stars appeared to consist almost entirely of hydrogen and quickly became supernovae, reionizing the surrounding neutral hydrogen and releasing the first heavy elements into the interstellar medium.
- 5.
Many stars exhibit small variations in luminosity; for example, the energy output of our Sun varies by about 0.1 % over an 11-year solar cycle.
- 6.
During the helium burning phase, very high-mass stars with more than nine solar masses expand to form red supergiants.
- 7.
Some very hot (T ~ (30–200) × 103 K) and very massive (over 20 M o ) highly luminous (~106 L o) evolved stars are losing mass rapidly (a billion times faster than the Sun!) by means of a very strong stellar wind with a speed about five times more than the average speed of the solar wind. These stars (which also have some specific features in their spectra have characteristic lifespans only in the order of a few million years and this is still sufficient time for their stellar winds to carry away a significant proportion of the total stellar mas - a few tens of solar masses. Such stars are called Wolf-Rayet (WR) stars in honor of their discoverers Charles Wolf and Georges Rayet.
- 8.
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron, thereby changing a nuclear proton to a neutron with the simultaneous emission of an electron neutrino. Inverse beta decay is an alternate decay mode of electron capture for radioactive isotopes with sufficient energy to decay by positron emission.
- 9.
A degenerate state of matter, as a quantum mechanics entity, is defined as a collection of free, noninteracting particles (such as electrons, neutrons, protons, fermions) with a pressure and other physical characteristics. It arises at the extraordinarily high density in compact stars’ interiors (or at extremely low temperatures) and is different from an ideal gas in classical mechanics. When degenerate electrons cannot move to the already filled lower energy levels according to the above-mentioned Pauli exclusion principle, a degeneracy pressure is generated in fermion gas which strongly resists further compression although no thermal energy is extracted.
- 10.
Basically, supernovae can be triggered by either the sudden reignition of nuclear fusion in a degenerate star or by the collapse of the core of a massive star, and they are difficult to predict.
- 11.
Basically, the maximum (Chandrasekhar) mass is expected to be below ~2.5 M O depending on the stiffness of the nuclear equation of state (EoS), but it could be lower if phase transitions take place. Observations of large neutron star masses of order ~2.3 M O would therefore restrict the EoS severely for dense matter.
- 12.
A pulsar is sometimes also called an X-ray burster. Pulsars with extremely high magnetic fields are called magnetars.
- 13.
Let us recall that lile electrons, neutrons belong to the particles called fermions. They provide neutron degeneracy pressure to support a neutron star against collapse. An additional pressure is assumed to be provided by repulsive neutron-neutron interactions.
- 14.
Actually, from an outside observer’s viewpoint, the spacecraft would never penetrate inside a black hole; whereas for the astronauts it would happen nearly instantaneously and they would see its interior of infinite density (the singularity).
- 15.
Very recently, the world-renowned British physicist Stephen Hawking, based on quantum theory rather than gravity, suggested that leakage of information from a black hole is possible, proposing that because of the quantum effects of space-time fluctuations in the wide range, no clear horizon boundary around a black hole exists. Instead of an event horizon, he introduced a “visible horizon,” a surface where the light leaving a black hole is temporarily retained.
- 16.
This method allowed us to discover the first strong candidate for a black hole, Cygnus X-1, as early as 1972.
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Marov, M.Y. (2015). Stars: Birth, Lifetime, and Death. In: The Fundamentals of Modern Astrophysics. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8730-2_6
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DOI: https://doi.org/10.1007/978-1-4614-8730-2_6
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