What Is A Star?
As we head out to the beginning of spring in 2024 to enjoy the outdoors, much of our gratitude is owed to the sun.
Summer is one of many people’s favorite seasons due to its longer days or the ability to do more outdoor activities. Yet, one of the biggest compelling factors is the weather. It is the hottest season, and the sun is out longer, bringing more happiness and joy as we feel its warmth on our cheeks. Nevertheless, people never take the chance to understand what our sun is even though we see this celestial body almost 12 hours a day. When people truly understand it, they’ll know how large, chaotic, and life-threatening it can be.
The physics of a star
When scientists looked at the sun in the early days of physics, they thought that it was burning fuel-like combustion, causing it to emit different parts of the electromagnetic spectrum such as infrared(heat), visible, and more. If this was the case, the sun should’ve burnt out a while ago. A couple of years later, German-American theoretical physicist Hans Bethe found out that energy produced from the sun comes from a process known as nuclear fusion or nuclear synthesis. Nuclear fusion is when atoms fuse to form a heavier atom. But to do this, you’d need lots of pressure. To be exact, you need 26.5 million gigapascals. The source of the pressure is from friend gravity.
As gravity tries to squeeze the star, nuclear fusion create outward gas pressure, helping it fight back against gravity
Have you ever been to the bottom of your local swimming pool and realized how painful it can get for your ears? That’s because when you go to the bottom of a 5ft deep swimming pool, you have about 5ft of water stacked on top of you! This causes lots of pressure, more pressure you’d experience outside the water. This happens to our sun as well but not with water. Our sun is very massive(about 865,000 miles) and has a mass of about 1.989 × 10^30 kg. When you have something this big, the center is going to have a lot of internal pressure and a high density because of gravity. The only job gravity has when it comes to stars is that it wants to squeeze everything making things smaller and denser. At the center of the star, also known as its core, the internal pressure is so great that atoms have lots of kinetic energy(movement of particles/objects). This causes them to move around at great speeds. Sometimes they collide with one another, causing fusion to occur. This seems counterintuitive to most people because every atom has an atomic nucleus that contains a positively charged particle called a proton. We know that like-charge repels so when you bring two atomic nuclei together they tend to repel. But due to such great internal pressure and a quantum mechanical phenomenon called quantum tunneling, the two atomic nuclei ignore this rule of like charges and will fuse, forming heavier atomic nuclei. This will release tremendous amounts of energy which is explained by Einstein's famous equation, E = MC^2 where something with very little mass can be converted into energy. But something strange also happens. When you add up the mass of the end atomic nuclei, it doesn’t add up! It’s like saying 1 + 1 = 0.8. Where did the extra mass go? As stated before, nuclear fusion releases energy but a question arises where did that energy also come from? Well, the mass was converted into energy. This phenomenon is known as mass defect. The energy prevents the star from collapsing(due to gravity) by creating an outward pressure through nuclear fusion. But what is mass? Most people confuse mass with weight, click below to truly understand what mass is!
Our sun is currently fusing millions of tons of hydrogen into millions of tons of helium every second. The process by which this happens is called the proton-proton chain. This process starts by fusing 1 proton and 1 neutron. This forms an isotope of hydrogen called deuterium. Isotopes are different versions of the same atom. The only difference between each isotope is the number of neutrons in the nucleus. During the reactions when deuterium is formed a positron(antimatter version of an electron) and an electron neutrino are emitted. Since the sun is made up of matter(protons, neutrons, and electrons), the positron will meet with an electron, and when they do they’ll annihilate each other, converting their matter into the form of gamma rays. The deuterium will then combine with another proton, becoming Helium-3(2 protons and 1 neutron) and in this process, it’ll emit gamma radiation. The final process of this p-p chain takes place when the Helium-3 combines with another Helium-3. This process will form Helium-4 and the other 2 protons will get kicked out. These two protons can then be used for the next p-p chain process. This entire process can be summoned up in a single formula which is: 4p + 2e- → 4He + 2ve + 6Ƴ. In summary, you need 4 protons and 2 electrons(to annihilate the positron at the end) which can give you Helium-4, 2 positrons, and 6 gamma rays.
What is a star made out of?
Most people will say that it is mainly made out of gas due to the high temperatures but that is incorrect. A star is made out of a state of matter called plasma. Plasma isn’t a liquid or a gas. It is composed of freely floating charges and atomic nuclei because the sun is so hot that the electrons can’t combine with protons to form basic atoms.
Real images of the sun taken by Extreme Ultraviolet Imager(EUI) on board of ESA/NASA’s Solar Orbiter on May 30, 2020
Hatfield, M. (2023, July 26). ESA/NASA's Solar Orbiter Returns First Data, Snaps Closest Pictures of the Sun - NASA. NASA. https://www.nasa.gov/science-research/heliophysics/esa-nasas-solar-orbiter-returns-first-data-snaps-closest-pictures-of-the-sun/
Our Sun
Our sun is one of the billions of stars in our galaxy. It's considered a G2V star. The letter “G” represents the spectral class of the sun. Scientists have observed stars for a while and decided to classify them. One way to do it is based on surface temperature and color. This classification system is called the Harvard spectral classification, as it was founded at Harvard University. The are 7 classes for this method which are: O, B, A, F, G, K, and M. “O” stars are the hottest of the category and are blue-purple while M stars a cooler and dark-red. Anything below M is considered a brown dwarf or a failed star that couldn’t ignite nuclear fusion. You can memorize this classification(from hottest to coldest) by using the common mnemonic, “Oh, Be A Fine, Girl/Guy Kiss Me”. The number “2” in G2V represents the subclassification of each of the letters. The subsection ranges from 0 to 9 where 0 is the hottest and 9 is the coldest of each class. For example, B1 is hotter than B2.
Harvard Spectral Classification. (2013, June 12). Physics and Universe. https://physicsanduniverse.com/harvard-spectral-classification/#google_vignette
Life of a Star
In The Beginning
A star forms when there is a large cloud of dust and gas called a nebula collapses into a dense region. That place starts gaining more mass as gravitational pull gets stronger, attracting more matter. Over time, it gets hot as the core becomes more dense. This is called the protostar. Nuclear fusion can’t take place yet because the core is hot enough. After millions of years, the star has accumulated enough mass to ignite nuclear fusion(hydrogen to helium), and a star is born. Based on how much mass the star accumulated in the early stages, it can determine its future. If it collects lots of matter, the star will have a high mass and will deplete its fuels quicker because its cores will be even denser than our sun. If the mass of the star is less than our sun, it will last very long, possibly for trillions of years because the core is less dense due to the low mass. This will cause the star to slowly use up its fuel, lasting longer.
The life cycle of stars in our universe
What is the life cycle of a star from birth to death? | Socratic. (n.d.). Socratic.org. https://socratic.org/questions/what-is-the-life-cycle-of-a-star-from-birth-to-death
Main Sequence Stars
Every star spends most of its time in a stage called the main sequence stage. It is a time when after the star forms, it begins fusing hydrogen into helium(lighter atomic nuclei to heavier atomic nuclei) and a star does this for most of its life. This is where a star is found to be the most stable because the outward pressure caused by nuclear fusion balances out with gravity and this state is called hydrostatic equilibrium. Our sun is currently halfway through the main sequence stage. Scientists have used current data to organize each stage of a star. This is called the Hurtzsprung Russel Diagram. On the x-axis(horizontal), it tells the surface temperature of the star. The left part of the x-axis has a very high surface temperature and to the right is where you’ll find stars with cooler surface temperatures red dwarfs. On the Y-axis, it represents luminosity compared to the sun. The higher the Y-axis you are in, you’ll find bright stars like supergiants and at the bottom, you’ll find stars with a lower luminosity like white dwarfs. When looking at the graph you’ll see a trend which is that there is a diagonal line that stretches from the top left to the bottom right. This line represents the Main sequence of stars. Anything above it is considered a giant or supergiant. Anything below it will be celestial bodies with a small mass like a white dwarf.
A Hertzsprung Russel Diagram
AstroPages | HR Diagram | Western Washington University. (n.d.). https://astro101.wwu.edu/a101_hrdiagram.html
The Final Frontier For Stars
Stars, like everything else, aren’t immortal. Their fuel(hydrogen) will run out. When it runs out, the star collapses because gravity is always present and tries to collapse the star. But when it collapses for a short period, the core becomes even denser. This makes it possible to fuse helium into carbon. When fusing helium into carbon, it releases energy, returning it to hydrostatic equilibrium. The star itself will start to get hotter in it’s outer layer and will start fusing hydrogen in its outer layers that it stored up. Since the nuclear reaction is farther away from the core and closer to the edge of the star, the energy produced, will cause the outer layer to expand like a ballon. What we have now is a red giant. A red giant appears red because its expanded outer layers have cooled down, shifting the peak of its emitted light to the red part of the spectrum. The fuel of carbon will run out again and will begin fusing elements up to Iron 56 because it requires energy to fuse Iron. Stars are big energy-producing factories and Iron-56 requires energy. It is known as an endothermic reaction. This is known as the Iron Catastrophe. At this point the star will completely collapse, ending with a beautiful supernova that is 100x brighter than their host galaxy for a week and we will find a neutron star at the end which can later turn into a black hole. Sometimes a star might skip the neutron star stage and will directly become a black hole. Though this story may seem complete, most of the stars in our universe don’t follow this path, like our sun because only high-mass stars can go supernova and become neutron stars or black holes. Stars like our sun will become red giants and will consume the inner planets except for Mars. They will slowly shed off all their layers, leaving behind a dense core made up of only carbon and oxygen called a white dwarf. A white dwarf is the third densest object in the unverse but it is the size of the Earth. It is as bright as a full moon on a clear sky. The original layers of the sun will turn into a large cloud called a planetary nebula(this has nothing to do with planets though). If this scares you, scientists have done the math and found out this will happen around 5 billion years from now. Trillions of trillions of years after, the white dwarf will cool down enough that they become dark floating objects in the sky that once used to light up our universe called black dwarfs. Currently, there no black dwarfs in the observable universe because it takes there hasn’t been time for one to form.
An artist illustration of an hypothetical black dwarf
Plait, P. (2023, December 20). Black dwarf supernovae: The last explosions in the Universe. SYFY Official Site. https://www.syfy.com/syfy-wire/black-dwarf-supernovae-the-last-explosions-in-the-universe
Now you have a better understanding of that radiant object in our brilliant sky. So, the next time you are savoring a beautiful day, take a moment to contemplate the wondrous processes occurring within.
