Supernova

Supernova->> STUDYING SUPERNOVAS

Chinese astronomers recorded supernovas visible from Earth as far back as ad 185. Probably the most well-known ancient supernova is the one that created the Crab Nebula in 1054. From Chinese and Japanese records, astronomers estimate that it was about 20 times as bright as any other star in the night sky. It was visible even during the day for several weeks after it first appeared.

The last time a supernova in the Milky Way galaxy became visible from Earth was October 1604. It was bright enough to be seen at night with the naked eye for more than a year. German mathematician Johannes Kepler made detailed observations of the supernova and carefully measured its position. Since then, astronomers have not seen any supernovas in the Milky Way. A number of supernovas have appeared in other galaxies, however.

One of the most important supernovas of the 20th century, and the brightest in the sky of the northern hemisphere since 1937, burst into view on March 28, 1993, in the galaxy M 81. Astronomers noticed the strange behavior of the parent star—a huge red-colored star called a red giant—before it exploded and were able to track its changes as it became a supernova.

On February 24, 1987, one of the closest supernovas in centuries occurred in the Large Magellanic Cloud, only 160,000 light-years from Earth. A light-year is a measure of distance equal to the distance that light travels in a year, or 9.5 trillion km (5.9 trillion mi). This supernova was visible from the southern hemisphere. Since this eruption, scientists have learned that its parent star may have once been a hot blue star with a mass about 20 times that of the Sun. The star probably swelled into a red giant star before it exploded.

Scientists are continually searching for and studying supernovas. Astronomers learn about the final evolutionary paths of massive stars from supernovas. In addition, supernovas give clues to the origin of the chemical elements that make up stars, planets, and even life. A supernova in a distant galaxy can even help astronomers measure the distance to the galaxy. To do this, astronomers examine the radiation emitted by the shell of material from the star’s atmosphere and use the information they gain to develop models of how wide the shell is. They then compare the width of their model to the apparent width of the shell as viewed from Earth to estimate the distance to the supernova remnant and to its parent galaxy.

Star

Star (astronomy), massive shining sphere of hot gas. Of all the stars in the universe, our Sun is the nearest to Earth and the most extensively studied. The stars visible to the naked eye all belong to the Milky Way Galaxy, the massive ensemble of stars that contains our solar system (the Sun and its nine planets).

About 5,000 stars can be seen with the naked eye, although not all of these stars are visible at any given time or from any given place. With a small telescope, hundreds of thousands of stars can be seen. The largest telescopes disclose millions of galaxies, which may each contain over 200 billion stars. Modern astronomers believe there are more than 1 x 1022 stars in the universe (this number is very large, a 1 followed by 22 zeros). The largest stars, if placed at the Sun's position, would easily engulf Earth, Mars, Jupiter, and Saturn. The smallest white dwarf stars are about the size of Earth, and neutron stars are less than about 20 km (about 10 mi) in diameter.

All stars are composed of hot glowing gas. The outer layers of some stars are so empty that they can be described as red-hot vacuums. Other stars are so dense that a teaspoonful of the material composing the outer layers would weigh several tons. Stars are made chiefly of hydrogen and a smaller amount of helium. Even the most abundant of the other elements present in stars—oxygen, carbon, neon, and nitrogen—are generally present in very small quantities.

The Sun, our nearest star, is about 150 million km (about 93 million mi) from Earth. It appears different from the stars visible in the night sky because it is about 250,000 times closer to Earth than the next closest star. The next nearest star is Proxima Centauri, which is more than 30 trillion km (20 trillion mi) from Earth. While light from the Sun takes only about eight minutes to reach Earth, the farthest stars are so distant that their light takes billions of years to reach Earth.

The color of stars—ranging from the deepest red through all intermediate shades of orange and yellow to an intense white-blue—depends directly on their temperature. The coolest stars are red and the hottest stars are blue. Most stars make light by several different kinds of thermonuclear fusion, a process in which the nuclei of atoms combine to form a heavier element and release energy (see Nuclear Energy). One of the most common thermonuclear fusion processes occurs in stars when four hydrogen atoms combine into a helium atom, releasing energy that is transformed into light and heat.

In the 1990s astronomers discovered planets orbiting stars outside our solar system. Planets outside our solar system are difficult to detect, because they are much fainter than stars are. However, astronomers located these planets by measuring the wobble of a star’s motion created by the slight gravitational pull that is exerted on the star by a planet. Although scientists can only speculate how many Earthlike planets with continents and oceans exist in the universe, they believe that many stars have planetary systems (See also Gravity).

Planet

Planet, any major celestial body that orbits a star and does not emit visible light of its own but instead shines by reflected light. Smaller bodies that also orbit a star and are not satellites of a planet are called asteroids or planetoids. In the solar system, there are nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Planets that orbit stars other than the Sun are collectively called extrasolar planets. Some extrasolar planets are nearly large enough to become stars themselves. Such borderline planets are called brown dwarfs. See also Planetary Science.

Meteor

Meteor, in astronomy, small solid body known as a meteoroid that enters a planet's atmosphere from outer space and is raised to incandescence by the friction resulting from its rapid motion. Brilliant meteors, known as fireballs, occur singly and generally consist of a luminous head, followed by a cometlike train of light that may persist for several minutes; some, called bolides, have been seen to explode with a sound like thunder. Fainter meteors, called shooting or falling stars, usually occur singly and sporadically. At intervals, however, hundreds of such meteors occur simultaneously and appear to emanate from a fixed point. These swarms are called meteor showers and are named after the constellation in which they seem to have their point of origin. Some appear annually on the same days of each year and are called periodic showers; others occur infrequently at varying intervals. The periods of meteor showers generally coincide with those of certain comets. Most meteors are dissipated in flight and fall to the earth as dust; a meteor that reaches the surface of the earth or another planet is called a meteorite.

Meteoroid

Meteoroid, solid body orbiting the sun, becoming a meteor, or shooting star, if it enters the earth's atmosphere. The vast majority of meteroids are the size of grains of dust, but they range upward in size without any definite limit. The largest can have masses of thousands of tons. See Asteroids; Comets.