Lives of the Stars

• Stars come into being from the collapse of giant molecular clouds that consist mainly of hydrogen. Such clouds are commonly found in the spiral arms of galaxies, and examples of these clouds in the Milky Way have long been known as diffuse nebulae. An easily accessible example is M42, the great nebulae in the sword of Orion. The nebula is visible to the naked eye, more interesting in a pair of binoculars, and is quite striking in a small telescope. William Huggins established that this nebula was a cloud of hydrogen gas in 1864 using a spectroscope.
• The mutual gravitational attraction of the particles in the cloud pulls matter together and the conservation of angular momentum insures that the proto-star rotates more rapidly as it decreases in size. While still in the fluid state, a small amount of material around the equator is spun off. This is available for forming planets later.
• As matter falls through the ever-strengthening gravitational field, heat is generated. In particular, the pressures generated by the in fall of matter cause the temperature of the central core to rise rapidly. When a core temperature of about 15 million K is reached, a thermonuclear reaction is ignited, fusing hydrogen nuclei (one proton) into helium nuclei (two protons and two neutrons) and generating energy in the form of electromagnetic radiation as a byproduct. This process of fusing lighter nuclei into heavier nuclei is known as nucleosynthesis.
• Eventually a state of hydrostatic equilibrium is reached where the radiation pressure arising from the thermonuclear reaction in the core exactly counterbalances the gravitational pressure forcing the star to collapse. In this state a star neither contracts nor expands – it remains the same size and radiates at a constant luminosity. Stars in hydrostatic equilibrium all lie along the Main Sequence of the HR diagram.
• The luminosity and surface temperature of a main sequence star depends its mass. The greater the mass, the hotter the core, the faster thermonuclear reaction proceeds, and the greater the energy produced.
• Once the all of the hydrogen in the core has been fused into helium, the thermonuclear reactions stop and the star, no longer in hydrostatic equilibrium, moves off the main sequence.
• The core of the star gets hotter (because the gravitational pressures are no longer counterbalanced) and the outer layers expand (because the molecules move with higher velocities due to the increased heat). The star becomes a red giant.
• The details of what happens next depend on the mass of the star. In all cases the gravitational pressure causes the temperature of the core to increase until further thermonuclear reactions take place. Broadly, this entails (1) burning hydrogen in regions immediately outside the core and (2) fusing helium into heavier elements.
• If the mass of the star is about eight times the mass of the sun or less, the core eventually becomes hot enough to synthesize elements up to oxygen (eight protons), but no higher. Ultimately, the outer layers of the star drift away into the interstellar medium and a small, very dense star known as a white dwarf is left behind. A white dwarf is prevented from collapsing by what is called degenerate electron pressure. This condition occurs when all available levels where electrons can exist in a system are filled. Think of it as eggs in a crate – once every receptacle in the egg crate is filled, there is room for no more eggs. The crate material cannot be stretched or altered in anyway to accommodate more eggs, thus maximum egg density is achieved.
• If the mass of the star is greater than eight solar masses, the core reaches temperatures capable of synthesizing elements up to iron (26 protons). The stability of nuclei is such that no further synthesis beyond iron is possible. However, for these massive stars the outer layers do not drift away but continue to apply pressure to the core. Eventually the gravitational pressures become so great that the counterbalancing degenerate electron pressure is not sufficient to prevent further collapse. Return to the egg crate analogy. How can you make the egg crate smaller when all the egg-holding slots are filled? Just sit on it! When this point is reached in the star, electrons are forced into the protons, the core collapses into a much smaller ball and the star explodes in what is called a supernova. The supernova explosion is so powerful that all the remaining elements on the periodic chart are synthesized and dissipated into interstellar space. In the spiral arms of galaxies, the remains of the star are then available for new star formation.
• The Chandrasekhar Limit: If the remnant core of a supernova is greater than 1.4 times the mass of the sun, it cannot become a white dwarf. Gravitational pressure will over ride the electronic pressure and the core becomes either a neutron star or a black hole. This value of 1.4 solar masses is known as the Chandrasekhar Limit after the Indian-American astronomer who predicted it theoretically in the 1930s. How does this relate to what has just been stated about stars with 8 times the mass of the sun? Well, research in the latter half of the twentieth century has established that if a star has a total mass of 8 times the sun or greater, then after it loses much of its original material in a supernova explosion, its remnant core will always be greater than 1.4 solar masses. Thus, a star with a total mass of 8 solar masses or greater will ultimately wind up with a core that exceeds the Chandrasekhar limit.
• The remnants of massive stars can be either a neutron star or a black hole, depending on the original mass. If the mass of the original star was less than 25 solar masses, then the final remnant will be a neutron star. If the original star had a mass greater than 25 solar masses, the final remnant will be a black hole.
• A neutron star is an extremely dense (nearly the density of the atomic nucleus) star consisting mostly of neutrons. Young neutron stars rotate rapidly and project duel beams of electromagnetic radiation that are observed on Earth as high frequency pulses. These young neutron stars are known as pulsars.
• A black hole is a dimple in the fabric of space-time consisting only of mass, charge and angular momentum. The escape velocity of a black hole is greater than the speed of light, thus no radiation can escape.

Important Milestones

· (1755) Immanual Kant published “Universal Natural History and Theory of the Heavens: An essay on the Constitution and Mechanical Origin of the Whole Universe According to Newton’s Principles”. In this book he outlined the basic principles of the nebular hypothesis, the idea that the solar system was formed from a cloud of gas following the dictates of Newtonian mechanics.

· (1796) Pierre-Simon Laplace presents a more detailed version of Kant’s nebular hypothesis.

· (1864) William Huggins showed by spectroscopic analysis that some nebulae are clouds of glowing hydrogen gas.

· (1865) The Whirlpool Nebula (M51) discovered by Lord Rosse in Ireland was thought to represent a solar system forming out of gas, as suggested by Kant and Laplace. This interpretation was found to be in error by the end of the 19^th century.

· (1935) Arthur Eddington showed that hydrostatic equilibrium of stars implies a core temperature sufficient to ignite a thermonuclear reaction. A star can stay in hydrostatic equilibrium as long as there is hydrogen in the core.

· (1967) Joceyln Bell discovered pulsars, rapidly varying radio sources that were later identified as rapidly rotating neutron stars. Also, many of these objects were found to be associated with material associated with supernova explosions.

· (1984) Beta Pictoris found to be imbedded in dust cloud, in accordance with models of stellar evolutionary models.

· (1987) Supernova 1987A in the Large Magellanic Cloud was the closest supernova since the invention of the telescope. This explosion enabled astronomers to confirm that a large number of neutrinos are produced during a supernova event, in accordance with theoretical models.

· (1995) Michel Mayor and Didier Queloz discovered the first generally recognized planet orbiting another star (51 Pegasi).