STARS: A BRIEF INTRODUCTION
        A star is a large burning ball of gas that is in space that gives off electromagnetic radiation, especially light, as a result of a nuclear reaction that goes on in inside the star called fusion. Our sun is a star. With the exception of the sun, all the stars appear to stay in the same position, maintaining the same pattern in the skies for years. In fact the stars are in rapid motion, but they are so far away form the earth that their changes in position can only be noticed over a period of centuries. The estimated number of stars visible to the naked eye from earth is about 8000, 4000 which are visible form the Northern Hemisphere and 4000 from the Southern Hemisphere. At any one time in either hemisphere, only 2000 stars are visible. The other 2000 are located in the daytime sky and are obscured by the much brighter light of the sun. The closest star to earth, besides the sun, is the triple star Proxima Centauri, which is 25 trillion miles from earth. In terms of speed of light, the common standard used by astronomers for expressing distance, this triple star system is about 4.29 light-years away. With light traveling at about 186,000 miles per second, it takes more than four years and three months for the light from this star to reach earth. So every time you look at the stars you are looking into the past!

DOUBLE STARS
        Many people believe that all the stars that they can see in the night sky are all just single stars. The truth is that over half the stars we can see at night are actually members of a double or multiple star systems. Some double stars can be detected through telescopic means, while most of them must be detected through spectroscopic means. A double star system is exactly what it sounds s like. It's two stars that orbit around each other. Double stars were first recognized by the British astronomer Sir William Herschel in 1803. Spectroscopic double stars, first identified in 1889, are not visually separable by the telescope but can nevertheless be recognized by means of doubling or broadening the spectrum lines* as the star pair revolves. When one star moves away from the earth and the other approaches it as they revolve in their orbit, the spectrum lines from the receding star shift toward the red, while those from the advancing star shift toward the violet. Another type of double star is the so-called eclipsing variable. Stars of this type are composed of a brighter star and a darker star. As seen from earth, when the orbit is such that the darker star eclipses the brighter one, the intensity of the light from the star fluctuates regularly. Investigation has shown that about two out of every three stars visible with a telescope of moderate size is a double star. Many thousands of visual double stars and many hundred spectroscope double stars have been studied intensely. Stars such as these are the main source of information about stellar masses.

PHYSICAL DESCRIPTION
        Our sun is a typical star, with a visible surface called a photosphere, an overlying surface of hot gases, and above them a more diffuse corona and an out flowing stream of particles called the solar (stellar) winds. Cooler areas of the photosphere, such as sunspots on the sun, are likely to be on other typical stars; their existence on some large nearby stars has been inferred by a technique called speckle inferometry. The make-up of the inside of a star can't be seen directly, but studies have indicated that convection (air rising and falling to do temp.) currents and layers of increasing density and temperature until the core is reached where the thermonuclear reaction (fusion) takes place. Stars consist mainly of hydrogen and helium, with varying amounts of heavier elements. The largest known stars are super giants with diameters that are more than 400 times that of the sun, whereas the small stars known as white dwarfs have diameters that may be only .01 times that of the sun. Giant stars are usually diffuse, however, it may be only forty times more massive than the sun, whereas white dwarfs are extremely dense and may have masses about .1 times that of the sun despite their small size. Super massive stars are suspected that could be 1000 times more massive than the sun and, at the lower range, hot balls of gases may exist that are too small to initiate nuclear reactions. Such possible brown dwarfs were first discovered and observed in 1987. Others have been detected since then. The brightest stars may be as much as one million times brighter than the sun; white dwarfs are about 1000 times less bright as the sun.

BIRTH, LIFE, AND DEATH
        Stars, being some of the most mysterious things in the universe can have several different endings, whereas they all begin the same.

BIRTH
        Stars begin their life as comparatively cool masses of gas. When gravity becomes strong enough, it starts to pull the gas together and the temperature begins to rise until the star reaches a value of about 1,800,000 degrees F. When it reaches this point, a nuclear reaction begins to take place in which the nuclei of hydrogen atoms combined with heavy hydrogen deuterons to form the nucleus of the inert gas helium. This reaction is called fusion. Fusion gives off great amounts of nuclear energy, which causes contraction of the star to halt. When the release of energy from the deuteron-hydrogen nucleus reaction ends, contraction begins anew, and the temperature of the star increases again till it reaches a point at which a nuclear reaction can occur between hydrogen and lithium and other light metals present in the body of the star. Again energy is released and contraction stops. When the lithium and other light materials are consumed, contraction resumes, and the star enters the final stage of development in which hydrogen is transformed into helium at extremely high temperatures through the catalytic action of carbon and nitrogen. This thermonuclear reaction is characteristic of the main sequence of stars and continues until all the available hydrogen is consumed.

LIFE
        The life of a star is pretty boring unless it collides with another huge object or when it nears its death. The fusion mentioned above takes place throughout the life of the star until all hydrogen is used up, then it will swell into a red giant. It obtains its greatest size when all its central hydrogen has been converted into helium. If it is to continue shining, its temperature at the center must rise high enough to cause fusion of the helium nuclei. During this process, the star probably becomes much smaller and denser. When it has exhausted all possible sources of nuclear energy, it may contract further and become a white dwarf. The average life span of a star is estimated about ten billion years. Our sun is about five billion years old.

DEATH
        The death of a star is probably the most exciting part of its life. Depending on its size, it can go out in a big bang called a super nova and become a nebula, it can become a black hole, an extremely dense star called a neutron star, or it can burn itself out and become particles floating in space. Because this has so many possibilities, we will make each one a category with lots of detail like the others. While on the subject of the death of a star, the background picture of a sun sized star ending its life. Pretty cool huh?

BLACK HOLE
        A black hole is an extremely dense celestial body that has been theorized to exist in the universe. Gravitational field of a black hole is so strong that nothing can escape from it. Not even light! Black holes may form during the death of a star. As nuclear fuels are exhausted in the core of a star, the pressure associated with their heat is no longer available to resist contraction of the core to ever higher densities. Two new types of pressure arise at densities a million and a million billion times that of water, and a compact white dwarf or neutron star may form. If the core mass exceeds about 1.7 solar masses, however, neither neither electron nor neutron pressure is sufficient to prevent collapse to a black hole. So far astronomers using the Hubble Space Telescope have discovered three black holes. A black hole’s gravitational force is so strong, it distorts light and reality, therefore, if you were sucked into a black hole and you looked up, you would see the future of the universe flash before your eyes. Scientists and astronomers have theorized that if you could build a space capsule strong enough to withstand the extreme gravity you could use the black hole as a time machine. The only problem with this theory is that if you can make it thought the black hole, it would throw you anywhere in time and space, leaving you stranded. Of course, this is just a theory and there is know way to test it right now, so don't put in a report that black holes are time machines cause we don't know!

NOVAS AND SUPERNOVAS
        Another way that a star can end its life is to become a nova or a supernova. A nova is a star that suddenly flares up and slowly fades, but can continue to exist for some time. A supernova acts the same way, but the explosion destroys of profoundly changes the star. Supernovas are much rarer than novas, which are seen rather frequently in the pictures taken from the heavens.

NOVAS
        Before the time of modern technology, a star that suddenly showed up out of nowhere was called a nova, or "new star." This is incorrect, because the stars that where involved had existed long before they became visible to the naked eye. By the use of the latest technology, astronomers estimate that about 12 novas happen a year in the Milky Way, or earth’s galaxy, but two to three of them are to far away to be seen or are obscured by interstellar material. Though, novas are much easier to observe in other nearby galaxies than the earth’s galaxy. Novas are named according to the year in which they occurred and what constellation they appear in. Normally a nova flares up several thousand times its original brightness in a matter of days or hours. Next, it enters a transition stage during which it may fade and grow bright again and then fade gradually to or near to its original brightness. Novas are considered variable stars in the late part of its life. They apparently behave as they do because their outer layers have built up excess of helium gas through nuclear reactions and expand too rapidly to be contained. The star explosively emits a small fraction of its mass as a shell of gas- the cause of the increase in brightness- and then settles down. A star such as this is typically a white dwarf and is commonly thought to be the smaller member of a binary (two-star) system, subject to a continuous infall of matter form the larger star. This is perhaps the always the case with white dwarf novas, which erupt repeatedly at regular intervals of a few to hundreds of days. Novas in general show a relationship between their maximum brightness and the time they take to fade a certain number of magnitudes. By means of measurements of nearer novas in other galaxies as indicators of the distance to those galaxies.

SUPERNOVAS

        Supernovas. Supernovas! A supernova is one of the most exciting things in the universe! They are HUGE explosion that is far more spectacular and destructive than a nova and much rarer. Events so spectacular and wonderful as these only occur no more than once every few years in the Galaxy; and despite their increase in brilliance by a factor of billions, only a few are ever observable to the naked eye. Until 1987, only three had been positively identified in recorded history, the best known one that occurred in AD 1054 and is now known as the Crab nebula. Supernovas, like novas, are often seen more in other galaxies. So the most recent supernova, which appeared in the Southern Hemisphere on February 24, 1987, was found in a companion galaxy, the Large Magellanic Cloud. This supernova, which exhibits some unusual traits, is now the object of intense astronomical scrutiny. The mechanisms that produce supernovas are less certain than those of novas, particularly in the case of stars approximately as massive as the earth’s sun. Stars that are much more massive sometimes explode in the late stages of their rapid life as a result of gravitational collapse, when the pressure created by nuclear processes within the star is no longer able to withstand the weight of the star’s outlying layers. Little may remain after the explosion except the expanding shell of gases. The Crab nebula has left behind a pulsar, or rapidly rotating neutron star. Supernovas are significant contributors to the interstellar material that forms new stars.

NEUTRON STARS