All of the highlighted words in the Space Talks are defined in the glossary.

Variable Stars

Variable stars-stars that change in brightness-are divided into two major groups, extrinsic and intrinsic. Extrinsic variables change in brightness either by the eclipse of one star by another, or by the effects of stellar rotation. An eclipsing binary system is created when two stars are orbiting each other and one star, from our perspective on Earth, happens to pass in front of and then behind the other star. These magnitude changes result in a distinct pattern that is observable. One example of an eclipsing binary is beta Persei (Algol).

Our own Sun is also a type of extrinsic variable star! Our Sun has sunspots, which are related to its magnetic activity. Sometimes there is a large area of dark sunspots on the Sun's surface, and as the Sun rotates, the sunspots rotate also-sometimes facing the Earth, sometimes facing away from the Earth. The Sun's apparent magnitude increases when the sunspots face away from the Earth, and decreases when the sunspots face towards the Earth. Other stars also have "starspots" that produce changes in magnitude as they rotate.

Intrinsic variable stars change in magnitude due to internal physical changes that cause them to periodically brighten and fade. Pulsating variables are one type of intrinsic variable star. Stars are luminous balls of gas held in equilibrium by two forces operating in opposing directions-gravitational force directed towards the center of mass of the star, and radiation pressure from the thermonuclear fusion process directed from the core towards the surface of the star. Some stars pulsate because a small imbalance between these two forces stops the star from reaching equilibrium. When the star pulsates, it expands past its equilibrium point until the expansion is slowed and reversed by the force of gravity, and it then contracts. It then overshoots its equilibrium point again until the contraction is slowed and reversed by the increased radiation pressure from the core of the star. The mechanism responsible for the continued pulsations or oscillations of most variable stars does not originate in the core, but in regions of instability within the stellar atmosphere. Delta Cephei, in the constellation Cepheus, belongs to one type of pulsating variable called Cepheid variables.

Another group of intrinsic variable stars are eruptive variables. There are several types of eruptive variables which undergo eruptions or explosions instead of pulsations. The most spectacular are the catastrophic supernovae explosions which occur in massive dying stars. The thermonuclear fusion process in stellar cores consists of the conversion of hydrogen to helium. When the supply of hydrogen is exhausted, the nuclear fires start to sputter and the star begins to collapse. The resulting stages of stellar evolution for dying stars depends upon their initial mass. When stars twice as massive as our Sun begin to die, heavier and heavier elements are produced by the fusion process. Eventually, in the most massive stars, the nuclear fires burn so hot during the final stages of collapse that iron starts to fuse. All elements lighter than iron produce energy during the fusion process, but iron consumes energy. When iron starts to fuse, the stage is set for complete disaster-nothing can stop the total destruction of the star. In a fraction of a second, a star that has existed for millions of years will cease to exist in the visible universe. The unimaginably violent death leaves behind nebulae-beautiful layers of atmospheric material thrown from the surface during the explosion-sometimes the only evidence of the star's previous existence. Supernovae display light increases of 20 magnitudes or more and can outshine all other stars in a galaxy.

Betelgeuse (alpha Orionis) is a luminous red supergiant in the constellation Orion. It is a semiregular variable star, having periods of regular pulsations interrupted by periods of irregular light variation. Betelgeuse is five times more massive than the Sun and is in a binary system with a 14`'' magnitude companion star. Betelgeuse is 410 light-years away, and will eventually become a supernova. Here on Earth, we will not know of the destruction of Betelgeuse until 410 years after its core has evolved into a neutron star, leaving its atmospheric layers behind. Betelgeuse does not have enough mass to become a black hole.

Another example of eruptive variables are novae. Novae result from stars in two different evolutionary stages orbiting each other in close binary systems. For example, a star with its atmosphere bloated during the red giant stage may be orbiting a dense, hot white dwarf. The loosely-held outer atmospheres of a red giant sometimes whirl into a disk and spiral onto the surface of the more dense white dwarf, triggering nuclear reactions on the surface. The increase in brightness can range from 5 to 20 magnitudes. A white dwarf has an extremely dense carbon core, the end result of stellar evolution for low-mass stars like the Sun. Before the final collapse into the white dwarf stage, these stars go through a red giant phase, during which a planetary nebula is sometimes ejected from the star. The ejection of a planetary nebula is not as violent as a supernova explosion, and after approximately 50,000 years planetary nebulae become too thin and tenuous to be seen. In approximately 5 billion years, the Sun will evolve through a red giant phase and its bloated surface will extend beyond the orbit of Mars, incinerating the inner planets. It may eject a planetary nebula before settling down as a white dwarf, slowly radiating its heat energy into space. Eventually, with all its heat dissipated, the Sun will become a black dwarf, a cold dense chunk of carbon, still accompanied by its frozen planetary family.