Stars have always filled people with wonder. The Egyptians incorporated the sun as a big part of their religion. The ancient Greeks named the constellations. The sky was a mysterious place with strange objects scattered throughout it. As technology evolved, the ability to answer people’s curiosity improved. It became possible to figure out the nature of stars. A hint came in 1860 when it was observed that the dark lines in the spectrum of light coming from stars were caused by different elements absorbing specific wavelengths. So much has been discovered about the complexity of stars, and yet, scientists still have so much to learn.
“The sky was a mysterious place with strange objects scattered throughout it. As technology evolved, the ability to answer people’s curiosity improved.”
In fact, the prominent discovery of how stars are formed was made so by accident. In 1965 New Jersey, radio astronomers Arno Penzias and Robert Wilson were trying to calibrate their antenna. No matter what they did, every direction they pointed it in they got a consistent level of noise. At first, they thought a family of pigeons nesting near the antenna might be to blame, but after the birds were evicted, the problem persisted. They finally realized that the noise was due to weak microwaves present from the creation of the universe. These microwaves are not completely uniform, which means that some parts of the galaxy were hotter, others cooler. This was caused by parts of the universe being denser than others, which caused those parts to expand less. Eventually, this led to pockets of space being substantially denser than its surroundings, which became dense enough to pull in the surrounding gas. This process created the first stars through nuclear fusion. Fusion is the merging of nuclei to form a new heavier atom, while releasing a bunch of energy. This process takes place in the cores of stars.
There are many different types of stars, one being the main sequence star, which is the most common star in the universe, making up about 90%. It is an umbrella term that has seven different spectral types classified under it by plotting stellar luminosity against temperature on a line called the main sequence. The sun is a main sequence star; another one is Alpha Centauri, the closest star to the solar system. It was formed from a clump of dust and gas.
When the core of main sequence stars heats up to millions of degrees, the process of nuclear fusion starts. This causes two hydrogen protons to merge and become a helium nucleus. This is called a star’s hydrogen–burning phase. Fusion releases energy which heats the star, creating pressure which resists the force of gravity. A star gets named a main sequence star if it fuses hydrogen atoms into helium in its core. They can be a tenth to 200 times the sun’s mass and can have life spans ranging from millions to billions of years.
Another type of star is the red giant. These are formed when a main sequence star between half to less than eight times the sun’s mass runs out of hydrogen in its core and collapses in on itself as fusion was the only force resisting gravity’s pull. The added pressure to the core causes it to heat up, which causes the fused helium to fuse into carbon, which also releases energy. This is called a star’s helium–burning phase. In the time before a red giant star is formed, a main sequence star’s core will continue to shrink more and more as its ability to perform fusion falters, which will cause its temperature to increase, and draw in the hydrogen surrounding the core closer. Because of this, the rest of the star starts to expand to a much bigger size, creating a red giant. This will also cause the star to change to a more orange–reddish color. Eventually it will become unstable and lead to all of its layers being blown away, which creates what is called a planetary nebula. This is predicted to happen to the sun in 5 billion years.
The next star is called a white dwarf. They form from red giants at the end of their lifetime, when all its fuel is finally exhausted. As red giants cool down and shed their shells, only its core remains. They are characterized by a low luminosity, are similar in mass to the sun, and size to the Earth. Because of this, they are extremely dense, stemming from the death of the main sequence star where the core collapses in on itself. They are usually around 100,000 Kelvin, and despite their name they can appear blue–white to red. This is the last stage for low to medium mass stars, and eventually the sun will become this.
97% of stars in the Milky Way will eventually become white dwarfs. White dwarfs have exhausted all of its nuclear energy, so it only gives off residual thermal energy of ions in its core, a process that continues for an additional billions of years.
The neutron star is very fascinating. The diameter of this star is about as long as New York City’s Manhattan Island, but they pack more mass than the sun. They form when a main sequence star with eight to twenty times the mass of the sun, considered a medium star, runs out of hydrogen in its core. A neutron star replaces a white dwarf for super giant stars as a phase. The neutron star is the remaining core of those stars. For bigger stars that form neutron stars, they fuse hydrogen to helium, and helium to carbon just like the smaller stars, but when they run out helium they shrink and heat up and then start fusing carbon into neon, neon into oxygen, oxygen into silicon, and finally silicon into iron.
All these processes create energy that keeps the core from collapsing, but as the star starts fusing heavier elements, it buys less time for itself. When it gets to fusing silicone into iron, its energy only lasts for days. The next step would obviously seem to be fusing iron into a heavier element, but this process requires energy and doesn’t release it, so the core collapses and becomes a neutron star and then rebounds in a supernova, which is a huge explosion that occurs at the death of stars about 5 times the size of the sun. The pressure leading to this is so intense that all the electrons and protons get crushed together into a neutron, but if the star were bigger, this force would not be enough to stop the collapsing core, and it would become a stellar mass black hole. They are the densest objects known to exist in the universe. They usually are spinning wildly due to their intense magnetism.
Red dwarf stars are a kind of main sequence star, and are the most plentiful star in the universe, and are the smallest star to use hydrogen fusion. Their masses can range from about 0.08 to 0.6 times that of the sun. These stars are much lighter than the previous stars, and because of that, the helium has an easier time going to the star’s surface and sinking back down, taking hydrogen with it. Due to this process, more of the star’s hydrogen can be used compared to other stars, allowing it to last for up to 14 trillion years.
Stars are fascinating phenomena that formed many of the elements heavier than hydrogen. With more space surveying technology being developed, more information on stars will be revealed.
