Solar system and nebulaes
The Solar System consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons, three dwarf planets (Ceres, Pluto, and Eris and their four known moons), and billions of small bodies. This last category includes asteroids, Kuiper belt objects, comets, meteoroids, and interplanetary dust.
In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets are in turn orbited by natural satellites, usually termed "moons" after Earth's Moon, and each of the outer planets is encircled by planetary rings of dust and other particles. All the planets except Earth are named after gods and goddesses from Greco-Roman mythology.
We divide planets into two groups: inner and outer:
The four inner – Mercury, Venus, Earth and Mars have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes.
The four outer planets, or gas giants (sometimes called Jovian planets) – Jupiter, Saturn, Uranus, Neptune - collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn's atmospheres are largely hydrogen and helium. Uranus and Neptune's atmospheres have a higher percentage of “ices”, such as water, ammonia and methane. Some astronomers suggest they belong in their own category, “ice giants.” All four gas giants have rings, although only Saturn's ring system is easily observed from Earth.
A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle. We know three main types of the nebulae: emission, reflection and dark nebulae.
Emission nebulae are clouds of high temperature gas. These nebulae are usually red because the predominant emission line of hydrogen happens to be red. Emission nebulae are usually the sites of recent and ongoing star formation.
Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.
Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across.
The nebula Snail is circa 650 ly from the Earth. It is the nearest and the brightest nebula to our Earth. Its diameter is circa 1.5 ly and arose 25,000 years ago.
The Tarantula Nebula is more than 1,000 light-years across - a giant emission nebula. If Tarantula is in our Galaxy, it would be visible through day and it occupies ¼ of sky.
Perhaps the most well known nebula is the Orion Nebula, also known as M42. It is one of the very few that can be seen with the naked eye. It is a bright emission nebula over 30 light-years in diameter.