Shine bright like a diamond

Gabe Sidel

17 Jan 2024 — 5 min read

The Life of a Star

Stars are all around us, from the shining sun that makes life on planet Earth possible, to the twinkling lights in the night sky. A star is a luminous ball of gas, mainly made up of hydrogen and helium, that is held together by its own gravity. Let's talk about how it's made.

Within a stellar nebula, a huge cloud of dust in space, pieces of dust begin to come together, forming a dust cloud. As the cloud gains mass, it begins to collapse under the attraction of its own gravity and accumulate even more materials, in a process known as accretion. As it gains mass and becomes more dense, gasses are pulled to the core and heat up due to gravitational energy. Then, radiation emitted from this process gets trapped due to the density, causing it to heat up even faster than before, and turning it into a protostar. However, during this process, fragments of the gas surrounding the protostar may begin to accumulate and become their own protostar, and eventually their own star. So, this new star and the original star will orbit around each other and their common center of mass. This is known as a binary system, and approximately 85% of stars in the universe are in binary systems. Two stars in a binary system that are indiscernible to the naked eye are known as a visual binary. Although the process of becoming a protostar may seem rapid, the transition from a nebula to a protostar can actually take up to 10 million years! After a protostar reaches a temperature of 15 million Kelvin, from gaining more mass and heating up even more, it begins to efficiently experience nuclear fusion in its core, which turns its hydrogen atoms into helium atoms, creating energy. This new star is known as a main sequence star, and most of the stars in the universe, including our very own Sun, fall into this category. During this stage in a star's life cycle, the force of gravity pushing in on the star, and the pressure from the energy of fusion pushing out on the star put it in equilibrium, so it's neither shrinking nor growing.

December 31, 2023 capture of the Sun by the Solar Dynamics Observatory

The Fork in the Road

However, in the case of a smaller star, after it converts all of the hydrogen in its core to helium, this balance starts to shift, and it begins the next phase in its life. With no more hydrogen in its core to produce helium, the force of the star's own gravity makes it collapse in on itself. As the star now begins to get denser and hotter because of this, it can begin to use helium to fuel its fusion reactions, instead of hydrogen. The helium fuses into elements such as carbon and oxygen. Since this new fusion reaction creates more energy, the star begins to expand, getting bigger and bigger. This new, huge star, is known as a red giant. In a red giant, the outer layers become increasingly far from the center as the heat of fusion pushes them away, so the force of gravity on them begins to weaken, and they start to drift away from the core of the star. As they do this, a beautiful planetary nebula is formed.

Planetary Nebula NGC 6543, also known as the Cat's Eye Nebula

Once all of the outer layers have drifted away, only the dying core, known as a white dwarf, remains. The reason that this dense core doesn't collapse in on itself is because of electron degeneracy pressure, which is exerted by the electrons in atoms pushing back against the matter closing in on them. Although there is no more fusion, the core is still hot. After millions of years, it will cool down such that it stops being hot or emitting light, thus becoming a black dwarf, which is essentially just a compact ball of ash. However, stars take so long to reach this point that it's believed that there are no black dwarfs in the entire universe.

On the Other Hand...

As described previously, smaller stars are only capable of fusing smaller elements such as helium, into carbon and oxygen, and once they do so, will turn into red giants and then planetary nebula. With more massive stars, it's a different story.

Stars with four or more solar masses, one solar mass being equal to the mass of 333,000 Earths, are able to fuse heavier elements, going from hydrogen, helium, carbon, oxygen, all the way up to fusing silicon into iron. Once the star has formed an incredibly dense, heavy iron core, it will no longer have any way to undergo fusion and thus resist the force of gravity that is squeezing it. Within seconds, the pressure of the gravity of the star becomes so immense that it collapses in on the iron core. This releases an incredible amount of energy, which actually pushes all of the other matter of the star out. This explosion is known as a supernova. The energy emitted from this supernova is so great, that for a few days its light may outshine all of the other stars in the galaxy put together.

After a supernova occurs, the remaining core of the star may become either a neutron star or a black hole, depending on its mass. If the core is between 1 and 3 solar masses, it will become a neutron star.

Neutron Star

During the supernova explosion, the star's core gets so crushed and squeezed that its protons and electrons crash together, becoming neutrons, and forming a neutron star. Neutron stars are so incredibly dense that a sugar cube cut out of one would weigh 1 trillion kilograms. However, the reason that the star doesn't continue to collapse from its gravity is due to neutron degeneracy pressure, which is the outward pressure exerted by neutrons pushing up on each other and other matter.

Rotating neutron stars are known as pulsars, because they are described as emitting periodic bursts of energy and radiation. Since neutron stars have such incredibly dense cores, they have strong magnetic fields, and spin rapidly, sometimes hundreds of times in a second. This also happens due to the preservation of angular momentum, which states that if the radius of a sphere shrinks while maintaining the same mass, its angular velocity must increase. So, when these neutron stars form from the shrinking supernova, they must spin much faster. This same phenomena is seen in ice skaters; when they pull their arms in, they spin faster, but slow down when they extend their arms back out. The magnetic fields of the stars funnel particles out of each pole quickly enough that they create beams of electromagnetic radiation. Since the poles may not be exactly aligned with the star's axis of rotation, the light beams aren't static, and sweep across space like a lighthouse. Only when the radiation reaches us can we sense it, so we perceive the star as pulsing.

Neutron stars typically have magnetic fields trillions of times as strong as Earth's. However, when they have magnetic fields that are even 1000 times stronger than that, they are classified as magnetars. Since the magnetic field of these stars is so powerful, when there is a slight movement in the star's crust, it ripples through the magnetic field and causes a huge explosion, which sends out energy in the form of electromagnetic radiation. In 2004, magnetar SGR 1806-20 experienced such a powerful explosion that in one tenth of a second, it released the same amount of energy that the sun had in 100,000 years!