The speed of light (SoL) in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its precise value is 299.792.458 metres per second.
A light-year (LY), is a unit of length used informally to express astronomical distances. It is approximately 9 trillion kilometres (9.460.730.472.580 kilometres or 63.241 Astronomical Units). A light-year is the distance that light travels in vacuum in one Julian year (365.25 days).
The astronomical unit (AU) is a unit of length, roughly the distance from Earth to the Sun. It is now defined as exactly 149.597.870.700 metres (about 150 million kilometres). It is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars.
Gravity or gravitation is a natural phenomenon by which all things with energy are brought toward (or gravitate toward) one another, including stars, planets, galaxies and even light and sub-atomic particles.
A nebula (Latin for "cloud") is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula before the true nature of galaxies was confirmed in the early 20th century.