You might have heard of the “Milky Way”. This is the galaxy in which our solar system, like many others, is found. But what is a galaxy? A galaxy is a massive, gravitationally bound system that consists of stars, stellar remnants, planets, gas, dust, and an important but poorly understood component tentatively dubbed dark matter. In our milky way alone, it contains about 200-400 billion stars and is estimated to have at least 50 billion planets.
Stars are born within the clouds of dust and scattered throughout the galaxy. A familiar example of such as a dust cloud is the Orion Nebula, revealed in vivid detail in Fig 2. Turbulence deep within these clouds gives rise to knots with sufficient mass such that the gas and dust can begin to collapse under its own gravitational attraction. As the cloud collapses, the material at the center begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing cloud that will one day become a star. However, not all of the surrounding material ends up as part of a star – the remaining dust can become planets, asteroids, comets or remain as dust.
As the stars formed, they are fueled by the nuclear fusion* of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and nuclear fusion also provides the energy by which the star shines.