Basic Information

Vocabulary

Gravity	Every object in the Universe exerts a force of attraction upon the objects around it. This force is known as gravity. The Earth is retained by the Sun because of the Sun's gravity; the Moon is retained by the Earth because of the Earth's gravity, etc.
Billion	In most English-speaking countries a billion equals one thousand million; you can write it as 10⁹. Likewise, a trillion equals one million millions, and you can write it as 10¹².
Light year	Distances in the Universe are huge, and so they are measured in huge units. A light year is a distance unit that equals about 9.5 trillion km. It is the distance covered by the light in one year.
Astronomical unit	It is another distance unit. It is the distance between the Sun and the Earth, and equals some 150 million km.

Galaxies and Clusters

The Universe is about 14 billion years old, and is formed by more than 100 billion galaxies. A galaxy is a huge system of stars, interstellar gas and dust. Typical galaxies contain from ten million to one trillion stars, all orbiting a common centre of gravity. Some galaxies are elliptical shape, some are spiral, others are irregular. Galaxies are usually separated from others by distances on the order of millions of light years. Groups of galaxies gravitationally attracted between themselves are called galactic clusters.

The Solar System is located in the Milky Way galaxy, a spiral galaxy with a diameter estimated at about 100,000 light years, containing approximately 200 billion stars. The Milky Way belongs in a cluster of over 30 galaxies known as the Local Group. The galaxy of Andromeda is the nearest to the Milky Way and the biggest one in the Local Group.

The Solar System resides in one of the Milky Way's spiral arms, known as the Orion Arm, at about 27,000 light years from the galactic centre. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years.

Stars

Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are alone in the sky (the Sun, the North Star), others have companions (Sirius, which is a binary star system, and Alpha Centauri, which is a ternary star system). The nearest star to the Sun is Alpha Centauri C or Proxima Centauri. The brightest star in the Northern hemisphere's night sky is Sirius.

Not all stars are the same: stars come in all sizes, brightnesses, temperatures and colours:

· The colour of a star is due to its temperature. A blue or white star is hotter than a yellow star, which is hotter than a red star.
· The brightness of a star depends on two factors: (a) its distance from us and (b) its luminosity: how much energy it puts out in a given time, which, in turn, depends on its size. Think on a street lamp, which puts out more light than a hand torch. That is, the street lamp is more luminous. But if that street lamp is 1 km away from you, it will not be as bright, because light intensity decreases with distance. In this case, a hand torch held 10 m away from you would be brighter than the street lamp. The same is true for stars.

Constellations

Constellations are groups of stars visibly related to each other in a particular pattern. Some well known constellations contain familiar patterns of bright stars. Examples are Ursa Major (containing the Big Dipper or Plough), Orion (that resembles the figure of a hunter) and Cassiopeia (with the shape of a "W"). The stars of a constellation, although appearing to be very near, may be millions LY away one to another.

The Solar System

The Solar System is the stellar system formed by the Sun and the group of celestial objects gravitationally bound to it:

· the eight planets and their 162 known moons,
· five dwarf planets and their six known moons, and
· thousands of small solar system bodies (SSSB).

The Sun is the main component of the Solar System, a star that contains 99.9% of the Solar System's mass. The Sun releases enormous amounts of energy in the form of electromagnetic radiation, which includes visible radiation (light), ultraviolet radiation and infrared radiation.

The planets are the biggest objects orbiting the Sun. Their orbit is almost circular. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky planets; the four outer planets are gaseous giant planets with a small rocky core. All planets but the two first are orbited by natural satellites (usually called "moons"). The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology. The following table shows some major magnitudes measured relative to the Earth:

	Diameter _{(relative to the Earth)}	Mass _{(relative to the Earth)}	Distance to the Sun _{(in AU)}	Revolution period _{(in Earth's years)}	Rotation period _{(in Earth's days)}	Moons
Mercury	0.4	0.06	0.4	0.24	59	0
Venus	0.95	0.8	0.7	0.6	243	0
Earth	1	1	1	1	1	1
Mars	0.5	0.1	1.5	1.9	1	2
Jupiter	11.2	318	5.2	11.9	0.38	65
Saturn	9.4	95	9.5	29.5	0.4	62
Uranus	4	15	19	84	0.7	27
Neptune	3.8	17	30	165	0.7	13

The dwarf planets are also rocky objects orbiting the Sun, smaller than the planets, but bigger than asteroids. There are currently (20-sep-2008) five dwarf planets in the Solar System; the two best known of which are:

· Ceres, formerly considered the Solar System's biggest asteroid;
· Pluto, a trans-neptunian object formerly considered the Solar System's ninth planet;

The SSSBs comprise several types of celestial bodies, the best known of which are:

· Asteroids = planetoids = minor planets. They're the smallest rocky bodies orbiting the Sun. Unlike planets and dwarf planets, they are not spherical, but irregularly shaped. Most of them occupy orbits between the ones of Mars and Jupiter, and make up the asteroid belt. The biggest asteroid is called Vesta.
· Comets, very small icy trans-neptunian objects that orbit the Sun in very eccentric orbits. When a comet approaches the Sun, its icy surface begins to boil away, creating two long tails, one of gas and another of dust, which are often visible with the naked eye. Two well known comets are Halley and Hale-Bopp.