Its all about Physics.

  Galaxy : Introduction: A galaxy is a collection of stars, stellar remnants, interstellar gas, dust, and dark matter that is gravitationally bound. The name comes from the Greek word galaxias, which means "milky" and refers to the Milky Way galaxy, which houses the Solar System. Galaxies range in size from dwarfs with fewer than one hundred million (108) stars to the largest galaxies known as supergiants with one hundred trillion (1014) stars, each orbiting its galaxy's centre of mass. Galaxies are thought to contain an average of 100 million stars.   Galaxy According on their optical shape, galaxies are classified as elliptical,[5] spiral, or irregular. It's believed that the centres of several of them contain supermassive black holes. Sagittarius A*, the name of the Milky Way's primary black hole, is four million times more massive than the Sun. GN-z11 is the oldest and furthest away galaxy as of March 2016. It can be seen as having existed only 400 million yea...

  Solar System : Introduction: The gravitationally bound system comprising the Sun and the asteroids that orbit it is known as the Solar System. A massive interstellar molecular cloud gravitationally collapsed to make it 4.6 billion years ago. The Sun contains 99.86% of the system's mass, with Jupiter holding the majority of the remaining mass. Mercury, Venus, Earth, and Mars are the four terrestrial planets in the inner solar system, and they are made mostly of rock and metal.The four enormous planets in the outer solar system dwarf the terrestrial planets in both size and mass. The next two, Uranus and Neptune, are ice giants, being primarily composed of volatile substances with relatively high melting points compared to hydrogen and helium, such as water, ammonia, and methane. Jupiter and Saturn, the two largest, are gas giants, being primarily composed of hydrogen and helium. All eight planets have approximately circular orbits that are located close to the ecliptic, the plane ...