Stars begin their life-cycle, sustain planetary orbits, and collapse as a result of gravity. They follow regular patterns of maturation which are characterized by their mass.
Gas and the Interstellar Medium
Between stars and galaxies there is gas and dust which is collectively referred to as the Interstellar Medium. Stars begin their birth when Jeans Instability causes gravity to collapse this medium into Giant Molecular Clouds (GMCs) such as the Orion Nebula.
The Orion Nebula is an example of an Emission Nebula, and it glows because stars formed within it energize its internal gases. Sunlight reflects off of dust within Reflection Nebulae, and hydrogen absorbs light entering Dark Nebulae.
Gases within GMCs fragment, and in doing so produce Gravitational Potential Energy in the form of heat. Pressure concentrates gases inside of the cloud into a protostar.
The Birth of a Star
Brown dwarfs are protostars which do not have the mass to begin the nuclear fusion, they never ignite. They are larger than planets, but they do not reach the mass of Main Sequence Stars.
Stars vary in luminosity. The brightness of Variable Stars, such as Polaris, changes due to inner pulsations.
During star formation, a protoplanetary disk is produced around the center. This disk feeds the maturing star, and it may give rise to planets (the process of planetary formation is not yet fully understood).
Red dwarfs, such as our nearest neighboring star Proxima Centauri, are the smallest stars. This class constitutes most of the stars in our galactic neighborhood, the Milky Way.
They are relatively cool and slow-burning, and even at their brightest only have a luminosity about a tenth of our sun. The life-cycles of these stars takes trillions of years, and is therefore explored using simulations.
Red Giants and White Dwarfs
In about 5 billion years our sun will evolve into a Red Giant. Once a red giant has exhausted its fuel it will collapse into a White Dwarf.
A white dwarf will continue radiating heat until it settles into a Black Dwarf. No black dwarfs exist yet (the universe is too young).
Universal Light Shows
High-mass stars do not fade out in the relatively peaceful way that stars with low-mass do. These high-mass stars live for a much shorter time, and their core has so much pressure that iron is produced.
When iron fuses into heavier elements, energy is lost instead of gained. This lowers the core's temperature, resulting in its collapse.
Neutrons are formed from this collapse, and neutrinos are released. This tremendous release of energy can outshine galaxies for weeks.
A Type Ia Supernova is produced in binary systems containing a white dwarf, where the companion stars collide and the resulting mass raises above the Chandrasekhar Limit (critical mass). This increase causes the core collapse.
Type II Supernova are produced outside of binary systems, and Neutron Stars (being composed primarily of neutrons) are all that is left of the stellar core. Pulsars are neutron stars which rotate rapidly, and emit energy in pulses.
Some massive stars are very hot and luminous, and rapidly lose mass in stellar winds in their later stages. These are classified as Wolf-Rayet Stars.
Who Turned Out the Lights
If a star is large enough, it will completely collapse into itself. This stellar result is what is referred to as a black hole.
Gravity pulls together stars, dust, and gases into systems known as Galaxies. It is believed that Supermassive Black Holes are located at the center of most galaxies (a single galaxy can consist of trillions of stars).
