A galaxy is a set of stars, systems, stellar mass and stellar associations, gas and star dust linked to the force of gravity.
The name is derived from the greek noun γαλαξίας (galaxìas), which means “milky”; it is a clear allusion to the Milky Way, the Galaxy per excellence,which includes the Solar System.
The galaxies are very big objects; they vary from the smallest galaxies, named nano galaxies, which include few dozens of millions of stars to the giant galaxies, which include up to thousands of millions of stars, orbiting around a common center of mass.
Galaxies can have different shapes: elliptical, disc-shaped, spiral or unusual shapes, which are named peculiar galaxies. Probably there are more than 100 millions of galaxies across the universe; intergalactic space is partially filled with feeble gas.
THE SOLAR SYSTEM
Our solar system is made up of the sun, which is a star, eight planets, five nano planets and billions of smaller bodies, such as asteroids, comets and meteorites. It is made of the sun and four rocky planets. The sun is the only star and source of thermal energy within the solar system. It is believed that the sun and planets which orbit around the sun developed from a cloud of interstellar gases. The size of the solar system is difficult to define. With the exception of Mercury and Venus, all the planets of the solar system have satellites. Uranus, Jupiter, Neptune and Saturn are gas planets. Earth, Mercury, Mars and Venus are rocky planets. All the planets have a rotation period, as they rotate around their axis, and also a revolution period, as they revolve around the sun.
MERCURY: It is a rocky planet, very flat, rotates the opposite way to other planets and there are many craters on its surface.
VENUS: It is the warmest planet in the solar system, also named the morning star or the evening star, indeed it is the brighest natural object in the sky. Its atmosphere is composed of carbon dioxide, nitrogen and sulphuric acid. Its temperatures soar to 450°.
EARTH: It is the only planet of the solar system which hosts life. It has only one natural satellite, which orbits around the Earth, and this satellite is named the Moon. The Moon has been formed soon after the Earth. It has the same size of Mars. There is water, atmosphere, nitrogen and oxygen on its surface. It has a spherical shape, but it is a geodesic. The inner part of the Earth is mainly rocky.
MARS: Traces of water have been found on Mars, this is why Mars is red, a rusty planet due to the great amount of carbon oxide. Temperatures vary between -140 and +20. It has many similarities with the Earth, but also carbon dioxide, nitrogen, argon, water vapour, oxygen and carbon monoxide are present on Mars.
JUPITER: It resembles, as it is composed of hydrogen and helium. It is a rocky planet composed of carbon and iron silicates. It makes a complete revolution around the sun every 11,86 years.
SATURN: It is a planet classified as a gas giant and it is mainly composed of hydrogen and helium. Saturn has a ring system, which is composed of ice particles and silica dust. It makes a complete revolution around the sun every 29,458 years.
URANUS: It is named "ice giant", its atmosphere is composed of hydrogen and helium and it has a great amount of ice, it is the coldest planet of the solar system, its temperatures soar to -224 below zero. It has planetary rings, a magnetosphere and many satellites.
NEPTUNE: It is the smallest and thickest planet in the solar system, contains ice, its atmosphere is composed of hydrogen and helium, there are also strong winds on its surface. It makes a complete revolution around the sun every 164,79 years.
PLUTO: It was classified as the ninth planet on the 24th August 2006. Its atmosphere is composed of methane, argon, nitrogen, carbon monoxide and oxygen. Its surface is composed of water ice and methane. It has five natural satellites.