Most stars on the main sequence are relatively average: not too big and not too small. But every so often, a star begins its life as an absolute monster: a supergiant.
These supergiants do join the main sequence, but due to the sheer amount of gravitational force and pressure, they burn through the hydrogen in their cores in a fraction of the time that smaller stars do. They quickly leave the main sequence and expand in size to become thousands of times larger than the sun. Like smaller stars, these supergiants begin helium fusion in their cores and begin hydrogen fusion in shells around the core. But unlike smaller stars, which stop their fusion at this point, supergiants form several layers throughout the star of differently fusing gases, giving it an onion-type look.
Fusion in the core will eventually reach iron. At this point, nuclear fusion no longer produces energy, and so it stops. Without outward radiation pressure, the intense gravity causes the star to collapse. Protons and electrons get forced together in the core, turning it into a rigid sphere of neutrons. As the outer layers reach the limits of the neutron core, they rebound off and get propelled outward with huge amounts of energy. These shockwaves tear the star apart in a supernova.
Credit: NASA, ESA, and G. Bacon (STScI)
All that remains is the rigid sphere of neutrons, known as a neutron star. However, some supergiants are so massive that even the neutron star continues to collapse, creating an object so dense that it creates a singularity, also known as a black hole. Supergiants are rare in our universe, but their existence is crucial. Supernovas are so energetic that they create all of the elements heavier than iron, and many star systems, including our own, are made from the remnants of these explosions. Without them, life itself wouldn’t exist.
When a protostar’s core reaches 15,000,000 degrees Celsius, nuclear fusion begins in its core. This ignition marks the star’s birth as it becomes a main sequence star.
Main sequence stars have a ton of variety. They range from cooler red stars to hotly burning blue ones, and their size can range from a fraction of our sun’s mass up to several hundred times as large. The only thing that matters for the main sequence is the presence of hydrogen fusion in the core. Hydrogen fusion takes hydrogen ions and turns them into helium, creating massive amounts of energy in the process. The outwards radiation pressure resists the force of gravity, preventing the star from collapsing any further.
But once the core runs out of hydrogen, the star starts to contract again briefly, until a shell of hydrogen around the core becomes hot enough to fuse into helium. When this happens, the radiation pushes the outer layers of the star far out into space, turning the star into a red giant. The core continues to collapse, however, continuing to heat up until it reaches 200,000,000 degrees Celsius. At this point, the helium that now makes up the core begins to fuse into carbon. Eventually, the helium will also run out. When this happens, the outer layers of the star continues to expand and cool down until finally all that is left is a planetary nebula with the remnant of the core at the center. We call this remnant a white dwarf.
You may be surprised to not hear the word “supernova” being thrown around. This is because supernovae only occur in incredibly large stars. For most of the main sequence stars, their deaths will be relatively calm and quiet, going out not with a bang, but with a sigh.
In the vast emptiness of space, there are floating clouds of gas and dust called nebulae. These clouds are stellar nurseries, filled with material that will one day become multiple solar systems. When part of a nebula is slightly more densely packed than the rest, gravity is stronger in that region. It begins to pull the surrounding gas inwards. This increases gravity, which pulls even more stuff in. As this happens, the gas molecules rub against one another, heating up due to friction. When the gas heats up, it begins to give off light, illuminating the nebula with brilliant colors. Eventually, this region of the cloud takes on a rough spherical shape. The gas begins to ionize, separating electrons from their nuclei as the sphere heats up even further as it continues to collapse due to the massive gravitational forces at play. This spherical shape is known as a protostar! This protostar will continue to collapse and heat up, eventually becoming a main-sequence star. For that to happen, nuclear fusion must begin in the core. When it starts, outwards radiation pressure is so strong that it prevents gravity from causing the star to collapse any further. We’ll talk about the life — and death — of main-sequence star in a future blog post, so keep your eyes open!Written By: Scott Yarbrough
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