General Astronomy

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The Solar System

The Sun

The temperature on the Sun's surface is 5800 degrees Kelvin, which though cool by stellar standards is extraordinarily hot by human standards. It is of medium size which allows for a steady, but slow, rate of conversion of matter into energy through nuclear fusion. This energy is what causes the Sun to shine by radiating light, much like electricity enables light bulbs to shine. Eventually, after some 5 billion years when the Sun has converted the hydrogen and helium in its center into energy, it will become a Red Giant -- its surface temperature will drop, and its size will increase.

The sun is an active star. It emits high energy particles (the solar wind) as well as gamma-ray, x-ray, ultraviolet, visible, infrared and radio radiation. The Earth's atmosphere filters out most of this radiation, which is harmful to life, and lets through the visible and infrared light. That's why one should not look directly at the sun -- a few minutes is enough to "burn" your eyes. Galileo did so while conducting his research and was consequently almost completely blind at the end of his lifetime.

Solar astrophysicists observe and investigate the Sun's properties using ground-based telescopes, space-based telescopes, and computer modeling and analysis. Almost 98% of the total mass in the solar system is in the Sun. The remaining 2 % is made up of the planets and their moons, comets and asteroids.

The Terrestrial Planets

The Terrestrial Planets are rocky, with weak magnetic fields and have either two, one or no moons. In order of their orbits' distance from the Sun they are: Mercury, Venus, Earth, and Mars.

Mercury

Mercury has almost no atmosphere and its surface is pockmarked with craters much like our Moon. It has no moons.

Venus

Venus is comparable in size to the Earth, and has a very dense atmosphere composed mainly of Carbon Dioxide. Although Venus is only 1/3 closer to the Sun than Earth, the temperature on Venus is many hundreds of degrees Kelvin, due to the Greenhouse Effect. It too has no moons.

Earth

Earth has an atmosphere, mostly composed on Nitrogen, and is cool enough to have liquid water on its surface in the form of oceans and lakes. It is the only planet to be hospitable to life. Earth has one moon.

Mars

Mars is slightly smaller and cooler than the Earth, and has a thin atmosphere, not dense enough to retain water, although it is considered that at some time in its past, Mars' surface may have had liquid water. Mars has two moons, Phobos and Deimos.

The Gas Giants

These planets are characterized by their gaseous nature, strong magnetic fields, and because they have rings and many moons. In order by orbital distance from the Sun they are: Jupiter, Saturn, Uranus, and Neptune.

Jupiter

Jupiter is the largest planet in the solar system. Because it is not solid, different parts of Jupiter rotate at different speeds causing the appearance of bands around the planet. The famous Great Red Spot is made by a hurricane on Jupiter that has persisted for at least 400 years and is the size of three Earths. Jupiter has at least 63 satellites, the most famous of which are the Galilean moons, Callisto, Ganymede, Europa and Io.

Saturn

Saturn is the next biggest planet and is well known for the extensive and beautiful ring system surrounding it. Saturn has such a low density that if you could find a body of water big enough, it would float in it. One of Saturn’s moon’s Titan is the only moon in the solar system that possesses a dense atmosphere(10 times denser than Earth's). Saturn has 57 confirmed moons, the best known of these are Titan, Thea, Dione, Tethys, Iapetus, Phoebe, Hyperion, Enceladus and Mimas.

Uranus

William Herschel discovered Uranus in March of 1781, but he thought at first it was a comet. Although Uranus was not known in ancient times, it had been observed and recorded on previous occasions (as early as 1690) but considered just another star. Uranus is one of two planets, Venus being the other that rotates east to west. Uranus' major moons are Puck, Miranda, Ariel, Umbriel, Titanis and Oberon.

Neptune

Because observations of Uranus showed perturbations (ie. deviated significantly from the ephemerid's tables) in its orbit, it was hypothesized that there must be an eighth planet. Neptune was discovered by Urbain Le Verrier, John Couch Adams and Johann Galle in September, 1846. Because Neptune is thirty times farther from the Sun than Earth it still has not completed a full orbit, which it will in 2011, since it was discovered in 1846. Neptune's two moons are Triton and Nereid.

The Dwarf Planets

A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

Currently, there are four celestial bodies that have been redefined by the IAU as dwarf planets: Ceres, Pluto, Charon, Eris.

Ceres

Ceres has been classified as a dwarf planet that might also be classified as an asteroid. Discovered on January 1, 1801, by Giuseppe Piazzi, Ceres has a diameter of about 950 kilometers and is by far the largest and most massive known body in the asteroid belt, as it contains approximately a third of the belt's total mass. The classification of Ceres has changed several times. Even though it was classified as a planet when it was first discovered, because it resembled similar bodies in the asteroid belt it was reclassified as an asteroid for over 150 years.

Pluto and Charon

Although there has been some controversy over whether or not it should be classified as a planet, the International Astronomical Union (IAU) officially considers Pluto a Dwarf planet.

The IAU, is the international body in charge of naming celestial objects.

Pluto was discovered by astronomer Clyde Tombaugh at the Lowell Observatory in Arizona on February 18, 1930. Tombaugh was searching for a "Planet X" to explain discrepancies in the predicted orbit of Neptune. It is now known these discrepancies were an artifact of the slightly incorrect value then assumed for the mass of Neptune.

Pluto is a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. Pluto and Charon have two known moons, Hydra and Nix. At about 1,186 km (737 miles), Charon's diameter is a little more than half of Pluto's. Charon was discovered in 1978, while two additional moons Hydra and Nix, were discovered in 2005. NASA launched its New Horizons spacecraft to Pluto and Charon in January 2006, and it should arrive in 2015, becoming the first spacecraft to visit them.

But...Isn't Charon a moon of Pluto, which should disqualify it? But no, it gets in under a loophole. Every moon and its planet orbit each other, technically, swinging mutually around a common center of gravity. In every case but this, though, that center of gravity is within, though not at the very center of, the planet. Charon and Pluto's center of gravity is much closer to Pluto, but still out in space. Ergo, says the IAU, it's a double planet.

Eris

In July 2005, Astronomer Mike Brown of CalTech and his team announced the discovery of yet another Kuiper Belt Object - this one larger than Pluto. The new dwarf planet has a diameter of 3,000 km (1,850 miles) which is 700 km (435 miles) larger than Pluto. The new dwarf planet is the most distant object ever seen in orbit around the sun, even more distant than Sedna, the Kuiper Belt discovered in 2003. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest planet Pluto, and takes more than twice as long to orbit the sun as Pluto.

The Milky Way

Our solar system is located in the Milky Way, a spiral galaxy and one of at least 240 billion galaxies in the universe. Our galaxy is shaped like a flat pancake (" the disk") with a ping-pong ball ( "the bulge") in the center; the spiral arms are in the disk. It contains approximately 400 billion stars; the Sun is about two-thirds out from the center.

The Milky Way is part of the Local Group, a small cluster of thirty nearby galaxies. The Andromeda Galaxy is the largest in the Local Group, and the Milky Way is the second largest. Other constituents of the Local Group are the Large and Small Magellanic Clouds, which are irregular galaxies visible with the naked eye from the southern hemisphere.

Star Names

The bright stars, those that can be seen directly by eye, have been given common names, or names corresponding to the constellation in which they are found. Usually the brightest stars in constellation have names like, Spica, in the constellation Virgo, Polaris in the Little Dipper (Ursa Minor). Most often, the bright stars are called e.g. Alpha Centauri -- the brightest star in the constellation Centaurus, Kappa Draconis -- the 10th brightest star in the constellation Draco, or simply 106 Virgo. Although there are many ways of naming stars, the official method is according to their position in the sky, so Spica is also IDS 13200-1038 A (IDS is the Index Catalog of Double Stars)

The IAU, is the international body in charge of naming celestial objects.

The Universe

The Universe is about 14 billion years old. It has been expanding since its origin in the Big Bang, and is infinite in size. Everything that we see -- planets, stars, galaxies -- is a very small fraction, about 4%, of the total content of the Universe. Dark Matter makes up 26% and Dark Energy is the largest component at 70% of the total content. We don't know much about dark matter -- except that is not like the ordinary matter we are familiar with, and we know even less about dark energy, except that it is responsible for the acceleration in the Universe's expansion.

Cosmologists study the history of the Universe.