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Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,000s light years, typically separated by millions of light years distance. They come in a variety of flavors: Spiral, lenticular, elliptical and irregular. Besides simple stars, they typically contain various types of star clusters and nebulae.

A planet is a spherical ball of rock and gas that orbits a star. There are 8 planets; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune in order of increasing average distance from the Sun. Stars a ball of mostly hydrogen and helium gas that shines extremely brightly. Our Sun is a star. A star is so massive that its core is extremely dense and hot. At the high core temperatures of a star, atoms move so fast that they sometimes stick to other atoms when they collide with them, forming more massive atoms and releasing a great amount of energy. This process is known as nuclear fusion.

Mercury - Named after the Roman god of commerce, travel and thievery, Mercury is the closest planet to the Sun and the eighth largest. Mercury's existence has been known of since before the third century BC. The Greeks gave it two names, Apollo for when it appeared as a morning star and Hermes when it came as an evening star. Mercury has a large iron core which is most likely at least partially molten. The silicate outer shell is only 500 to 600 km thick.

Venus - Venus whose Greek equivalent was Aphrodite was the Roman goddess of love and beauty. It probably got its name from being the brightest object in the sky except for the Sun and the Moon. At one time, it was thought to be two separate bodies: the morning star (Eosphorus) and the evening star (Hesperus). It is the second planet from the Sun and the sixth largest.

Earth - Earth is the only planet whose name is not derived from Greek/Roman mythology. Earth comes from Old English and Germanic. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil, while the Greek goddess was Gaia, terra mater - Mother Earth.

Mars - Named for the Roman god of War, Mars probably got this name due to its red color. It is sometimes referred to as the Red Planet. The name of the month March derives from Mars. Known since prehistoric times, Mars is still a favorite of choice for human exploration.

Jupiter - Named after Jove, the chief god of Roman mythology, Jupiter is the fifth planet from the Sun and the largest planet in Earth's solar system. Besides the Sun, the Moon, and Venus, Jupiter is the brightest object in Earth's sky, often mistaken as a star. It is more than three times brighter than Sirius, the brightest star.

Saturn - Named after the Roman god of agriculture, Saturn is the sixth planet in order of distance from the sun, and the second largest in the solar system. Its most usually thought of for its ring system. The Italian scientist, Galileo was the first to spot Saturn's rings in 1610, using one of the first telescopes.

Uranus - When Uranus was first discovered in 1781 by William Herschel (brother to Caroline Herschel), he named it Georgium Sidus (Star of George) in honor of King George III of England. Later, astronomers changed the name to Uranus, for the ruler of Heaven and the Universe in Greek mythology. He was the father of Saturn and grandfather of Jupiter. Uranus is the seventh planet from the sun in our solar system. It is the third largest, by diameter.

Neptune - The Roman god Neptune, whose Greek equivalent was Poseidon was the god of the sea. His namesake planet is the eighth planet from the Sun and the fourth largest (by diameter). It is smaller in diameter but larger in mass than Uranus.