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STARS - Jim Colyer (Posted: Jul 14)
The stars are always there, even in the daytime when they are blotted out by the sun. That is something we do not understand as children. As the earth revolves in its yearly orbit, different parts of the stellar panorama are exposed in the night sky. Seasons are identified with constellations.

The constellations are two dimensional. They are illusions. They existed in the minds of ancients who created them. Stars are three dimensional. They are at various distances. Some appear bright because they are close. Others appear dim because they are far away.

The most identifiable constellations are the Big Dipper (Ursa Major), Orion, Scorpius and the Southern Cross.

The pointers in the bowl of the Big Dipper point to the North Star, Polaris. Polaris is overhead at the north pole and retains its fixed position as the earth spins. It is a tiny bit off. This causes precession of the equinoxes and a shift away from Polaris as the North Star. Given enough time, 26,000 years, the earth wobbles like a top.

The Zodiac consists of Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces. The sun, moon and planets remain against the background of the Zodiac because of the flatness of the solar system. It is all in the same plane. Flatness is caused by spinning.

The most elaborate story in the sky is that of Perseus and Andromeda, told by the autumn constellations. There are Andromeda's parents, Cepheus and Cassiopeia, and the monster, Cetus. Pegasus is there, the winged horse ridden by Perseus. In Perseus' hand, is the head of evil Medusa as represented by Algol.

Constellations are a product of Greek and Roman mythology. Even the Greeks borrowed from the Babylonians. Despite their relative distances, stars forming the constellations are in the vicinity of the sun when we view the entire Milky Way.

The Greeks saw the Milky Way in poetic fashion. Legend had it that Hercules was born of an affair between Zeus and a mortal. When Zeus tried to have his wife, Hera, suckle the baby, she pushed it away and her milk flowed across the sky.

Visible stars range from 1st to 6th magnitude. The brightest stars are Sirius, Canopus, Alpha Centauri, Arcturus, Vega and Capella. Arabs named them. Stars twinkle because of our atmosphere. We see 4000 at any one time. Stars are so far away that they appear only as points of light even in our largest telescopes.

The stars are trillions of miles away. Their distances are measured in light-years. A light-year is the distance light travels in a year, 6 1/2 trillion miles. Light is the fastest thing in the universe (Or is it?) at 186,000 miles per second. The closest star is Proxima Centauri, a companion of Alpha Centauri, 4 light-years or 25 trillion miles out.

Double stars are the rule. Algol is a double star. So is Mizar, the second star in the handle of the Big Dipper. Another double is Albireo at the end of Cygnus the Swan. The contrast between the blue star and its yellow companion is striking. Castor in Gemini is a system of 6 stars.

Stars are like people. They are born. They age. They die. Stars are born when gaseous nebulas shrink under their own gravity. Mass determines whether a celestial body will be a star. If the mass is great enough, pressure and temperature at the core will be great enough for nuclear reactions to start.

Stars form in clusters. The Pleiades are stars condensing from surrounding gas. The Orion Nebula is a stellar nursery. So is the Trifid Nebula. Nebulas glow because they reflect starlight. The Horsehead Nebula is dark but outlined by starlight behind it.

Stars come in colors. Blue-white stars are the youngest and hottest. They are in the spiral arms of the Milky Way. Red giants are the oldest and coolest. Yellow stars like our sun are in the mid-temperature range. Walter Baade studied star populations.

Stars die in two ways. Average stars like our sun become red giants. They die peacefully by exhausting their fuel. Antares and Betelgeuse are red giants.

When a star uses all its hydrogen, it burns helium to make carbon. Elements are created in the cores of stars. Our bodies are made from remnants of stars.

A dying star is called a planetary nebula when it gives off a shell of gas. The Ring Nebula in Lyra is a planetary nebula. It is a bad name because it has nothing to do with planets.

A dying star shrinks to become a white dwarf. A white dwarf is the core of a red giant. White dwarfs can be red or brown but are still known as white dwarfs.

Massive stars die by becoming supernovas and blowing up. A supernova was seen in the Large Magellanic Cloud in 1987. Supernovas become pulsars. Pulsar can become a black holes. Black holes are collapsed stars whose gravity is so great that even light can not escape them. They may be at the centers of galaxies. Cygnus X-1 is a black hole. A quasar is a galaxy falling into a black hole.

ASTRONOMY http://jimcolyer.com/papers/entry?id=2