Galaxies are large collections of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. They range in size from dwarf galaxies with a few million stars to supergiants with one hundred trillion stars.
The Solar System orbits around the Sun in our Milky Way galaxy. It contains planets, moons, and asteroids. It also includes the Oort Cloud, a region of icy objects at the edge of our solar system.
Stars
Stars cannot form outside galaxies because the density of matter in intergalactic space is too low. But in some very specific circumstances, stars can form in voids between galaxies.
There is also an extensive population of ‘intergalactic stars’ – stellar interlopers that have ejected from galaxies and are now wandering around our universe. The Virgo galaxy cluster, for instance, contains over 10 per cent of its mass in the form of these stellar interlopers.
The Sun, the other stars, and all the gaseous nebulae in our galaxy revolve around a central point, moved by the gravitational attraction of the mass concentrated there. This makes the Milky Way a spiral galaxy, with planets and moons orbiting around the central mass.
The Solar System is a planetary system of eight planets and their moons, plus many other objects, all orbiting the Sun. The planets are divided into two main groups: those that revolve in the same direction as the Sun (gas giants) and those that do not.
Planets
A solar system consists of a star, the Sun, plus a host of orbiting bodies. There are eight known planets, including Earth; many asteroids (some with their own satellites); comets and vast reaches of highly tenuous gas and dust known as the interplanetary medium.
The four inner planets — Mercury, Venus, Earth and Mars — were formed from molten rocky material, while the outer planets were formed from frozen gases. As they got closer to the Sun, their strong winds blew away much of the ice and gases, leaving only their rocky surfaces.
Our Solar System was formed about 4.568 billion years ago, when the Sun sent out energy and particles in a steady stream, called stellar winds. As they moved through the galaxy, the winds shaped the Solar System.
Our planetary system contains eight rocky planets, four gas giants (Jupiter, Saturn, Uranus and Neptune) and three icy worlds — Pluto, Ceres and Eris. It also includes the Oort Cloud, a halo of comets that extends nearly halfway from our solar system to the nearest star.
Moons
Galaxies are collections of stars, dust and gases that form in the far reaches of our universe. They can be elliptical, spiral or a mixture of the two.
Our solar system is in one of these spiral arm-shaped galaxies. It is surrounded by a number of bright and faint galaxies, each with its own unique color and shape.
The Milky Way galaxy, or our home galaxy, is a spiral that is about 100,000 light-years across and has thousands to trillions of stars. It is named for its whitish appearance and is visible on winter nights and in places with little light pollution.
There are 181 moons in our solar system, many of which are a fraction the size of Earth. Most of them formed in the discs of gas and dust that circulate around planets, while others were captured into orbit by their larger host planets. Some of the moons have atmospheres. Some have hidden oceans of water underneath their surfaces, like Jupiter’s Io and Saturn’s Titan.
Comets
Comets are small bodies that orbit the Sun in elongated elliptical orbits. They can have periods that range from a few years to several millions of years.
A comet is made up of a nucleus, which is a solid structure no larger than a few kilometers in diameter, and a free-escaping atmosphere called a coma that extends hundreds to thousands of km in diameter. The coma is primarily made of volatile water ice that sublimates when it comes close to the Sun, but it can also be made of silicate and organic dust particles.
The coma is surrounded by a broad, curved tail that is composed of solar wind-pushed dust particles. The tail can end up millions of miles long, but it always points away from the Sun and never directly toward it.
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