All stars’ are born from cold and dense gas clouds termed nebulae. Hydrogen gas is the primary constituent of a nebula. Nebulae remain in a fixed stable position for millions of years. Then, when turbulences such as a nearby star erupt, nebulae begin to crumble to their gravity and spin. Then, nebulae create cores (called protostars) in their densest areas, and the leftover gas twirls around, forming a disk around a spherical object. As the protostars rotate faster and faster, they heat up to the point that nuclear fusion can be created. During the process, the protostars grow and gather more mass from their surrounding disk. Eventually, when the protostars gain enough mass to become stable, they enter the Main Sequence stage. Depending on the mass, there are generally two paths for a protostar: the lightweight main sequence and the heavyweight main sequence. Stars spend most portion of their life in this phase.

 A star's stability is determined by the balance of gravity pulling inward and hot gases’ pressure pushing outward. Stars with less than eight solar masses are classified as lightweight stars. For billions of years, these stars stay stable, converting hydrogen into helium. However, at some point, they run out of hydrogen. Consequently, gravity takes over, and the stars shrink down. Due to the internal pressure caused by gravitational compression and hot temperatures, helium begins to fuse. Afterward, the stars expand tremendously - larger than their previous state - and become red giants. The phase of a red giant is short-lived compared to the main sequence since helium reacts quicker than hydrogen. Helium is then burned into heavier elements - particularly carbon and oxygen - depending on the red giant’s mass. When red giants run out of helium, they shrink again. This time, the cores of the stars are unstable, and no further reaction is possible. The stars then enter their last stage: the white dwarf stage. The white dwarfs are city-sized and extremely heavy, and their inward gravitational pull is so strong that a human would be squashed if placed on them. 

The factor that alters the course of a heavyweight star from a lightweight star is mass. Heavyweight stars are commonly larger, brighter, and heavier, and possess greater than or equal to eight solar masses. Heavyweight stars follow a similar main sequence path to lightweight stars, except they burn hydrogen faster and have a relatively shorter life span than lightweight stars. Another difference is, in rare cases, if a heavyweight star is heavy enough, under various circumstances the protostar transforms into a blue supergiant instead of a star in the main sequence phase. This does not apply to every heavyweight star and happens only once in a while. 

After the main sequence, stars, along with blue supergiants, evolve into red supergiants. Helium reacts quicker in a red supergiant than in a red giant, so a red supergiant has a shorter lifetime. The star then explodes as a supernova. A supernova typically has two passages: a supernova with three or fewer solar masses results in a neutron star, while a supernova with more than three solar masses ends up being a black hole. A neutron star is an immensely dense, city-sized core. A neutron star is denser and heavier than a white dwarf. Black holes are not something we are familiar with. They are invisible as they don’t emit visible light although they can be detected through other means. A black hole’s gravitational pull is so strong that it usually sucks anything within two to three miles of it.  No one would survive if one entered. They are still dark to us.