The night sky holds many wonders. Planets and galaxies are all masked by sunlight during the day, but are visible in darkness. Perhaps one of the most obvious nocturnal beauties are the stars. Where did they come from? This article gives a brief overview of how a star is born.
Stars are born in massive clouds of dust and gases called nebulae. When a nebula gets massive enough, dust and gas start to collapse and get hotter. If it gets hot enough, a protostar is formed. Everything depends on temperature. Most stars run on hydrogen fusion. This requires a lot of heat, and if a protostar is too cool for the hydrogen in its heart to ignite, it will not burn and will remain a brown dwarf, which is something like an incredibly massive Jupiter.
As the protostar continues to form, it continues getting both bigger and more dense until it has enough mass to ignite the hydrogen and begin fusion. Once it stops contracting and gets all its energy from fusion, it can be classified as a star.
Depending on the temperature of the star, it can live for millions or billions of years. Stars have a finite amount of fuel to burn through, and hotter stars burn through it faster. Temperature also affects the brightness and color of a star. The coolest stars are red, and the brightest are blue. Our sun is an average yellow.
The things holding the star together are the gravity of the outer layers of gases and the heat from the inner core of fusion. As the star dies, there is less fuel to be fused into helium, and the core expands. It continues expanding until there is no more hydrogen to burn, then it cools and contracts, shedding outer layers as it goes. The core of the star has become a white dwarf, which can take billions or trillions of years to cool down to the background temperature of the universe. The layers the star left behind are known as planetary nebulae.
As more massive stars (stars between 1.5 and 5 times the mass of the sun) die, their cores fuse hydrogen and helium into heavier elements. These stars expand and contract several times, eventually becoming neutron stars.
The most massive stars die spectacularly. As their cores burn hydrogen and helium and helium into heavier elements, their outer shells are expanding. However, the energy required to fuse helium into heavier elements is enormous. Up until iron, more energy is released from fusion than is put into it. That all changes with iron. When the star’s core hits the iron limit, it rapidly collapses, then explodes with such force that it rips the still expanding outer layers to shreds. If a star is more than 26 times as massive as our sun, the core will sink into itself and become a black hole.