Sol
Sol is the scientific name for the star circled by Planet Earth.
The Solar system is the scientific name for the planets, asteroids and comets that circle Sol. Some people incorrectly use the term "other Solar systems". There is only one Solar system - around Sol. Similar systems, around other stars, are stellar systems.
Sol, and its satellites, are believed to have formed from a cloud of dust and gas approximately 4.5 billion years ago. Sol is classified as a G-type star, meaning it casts yellowish light, in the visible spectrum. Sol is firmly on the "main sequence", which means it is powered by the fusion of Hydrogen into Helium
This classification system where yellow stars are classed as G-type is scaled upon the star's surface temperature. Main sequence stars hotter than Sol, are bluer, hotter, and consume their Hydrogen fuel more quickly than Sol. Main sequence stars cooler than Sol are redder than Sol, and consume the Hydrogen fuel more slowly than Sol.
Stars like Sol fuse Hydrogen to Helium at their core. The rate at which Hydrogen fuses to Helium within a star's core is related to a star's mass. A star twice as massive as Sol has to fuse Hydrogen to Helium eight times as quickly, to generate enough energy to fight the mass of the overlying gas.
As a star like Sol fuses Hydrogen to Helium the size of the Helium rich core grows, and the boundary layer where Hydrogen fuses gets larger. This means a star like Sol will slowly a bit brighter, as it gets older.
Astronomers believe that stars the size of Sol, and stars larger than Sol, have layers of circulation, and that Hydrogen in their outer layers will never reach the layer where Hydrogen fuses to Helium. The smallest, dimmest stars, will be very long lived, trillions of years, for two reasons. First, due to their smaller mass, they will fuse their Hydrogen very slowly. Second, it is believed they lack the multiple layers of circulation, so they will fuse almost all their Hydrogen.
When stars the size of Sol exhaust the Hydrogen available to fuse their inner core gets hot enough and dense enough for Helium to fuse to Carbon and higher elements. Stars that begin to fuse Helium in their core expand their surface layer, and turn into giant stars. They are said to leave the main sequence.
Stars spend a relatively period as giant stars, when their available Hydrogen has been fused. Fusing Helium and higher elements releases relatively less energy than Hydrogen fusion. And these stars increase in Luminosity.
Astronomers predict that, when Sol is in its giant phase, it will balloon out past the orbit of Venus, possibly as far as to Earth's orbit.
Stars that are significantly more massive than Sol can blow up in a Supernova, leaving a Neutron stars or black hole. But stars as massive as Sol have less catastrophic ends. When they run out of fuel to fuse, and there is no energy to keep them as large as an active star - they shrink. During this period they are known as a white dwarf. The remains of a burnt out star can shrink to the size of a planet like Earth. These white dwarfs can remain very hot, for tens of billions of years, but they are very dim, due to their small size. Eventually, after tens or hundreds of billions of years they can slowly radiate away all their heat. They would then be known as a black dwarf. However, our Universe is less than 15 billion years old, and no white dwarfs have had a chance to radiate away their heat.
