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Look Up

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As we learned last month, the Sun and stars alike are fascinating celestial objects. They produce the energy that allows us to exist, while varying greatly in size while doing so. To understand stars is to understand the lives that they lead. I will be breaking stars into two categories. First, I will lay out the life the Sun will have and stars similar to it, which will be denoted by “M8-.”

This means that any star from 0.8 of the Sun’s mass (has to be larger than 0.8 of the Suns mass; any smaller star dies before it gets the chance to live, becoming a brown dwarf) to eight times the mass of the Sun will be discussed first, denoted by “M8-”. While the big stars are anything that is bigger than eight times the mass of the Sun, which will be denoted by “M8+.” M8+ and M8- stars live much of their lives the same in the beginning phase, although, much later in life is when they differ. Let us discuss the smaller stars, M8-.

M8- stars, such as our Sun, start life as any other star, in an interstellar cloud. This interstellar cloud eventually starts to grow desnser. These dense clumps are usually caused by disturbances in the clouds from neighboring clouds, exploding stars, and other celestial events or objects. Pushing the material close enough till it is caught by neighboring materials’ gravitational pull. To understand it better, I think of how a rolling snowball will continue to collect snow, becoming larger and larger as it rolls down a hill. This gas clump begins to collapse due to the gravitational pull of the collecting material. The dense clump is then turned into a protostar when it grows big enough, beginning to rotate. The protostar then has bipolar outflows, one on each side of the magnetic poles, causing a clearing of most gas and dust around the infant star, which then goes into the T-Tauri phase. A star, like our own, uses its hydrogen to create helium through nuclear fusion. This particular type of nuclear fusion is what makes our star, the Sun, heat the Earth for as long as we are alive. After T-Tauri heats up enough, it starts its longest phase, the same phase our Sun is in, called the Main Sequence. Due to everything in our universe being temporary, an M8- star, like any other star, will run out of its supply hydrogen that is being fused into helium. While the M8- star tries to stay going, it then reaches for anywhere to use up the remaining hydrogen, which is not in the core anymore but in the layer surrounding the core. This causes the distinct bloating of a red giant. The M8- star can become 20 to 100 times bigger than our Sun due to the bloating of its layers. It is so big, in fact, that when this happens to the Sun (which it will in 5 billion or so years), it will reach far out to even Venus, possibly coming close to engulfing Earth. Scientists are actually still not sure if the Earth will be engulfed or if it will orbit dangerously close. No matter what the case, though, it is estimated that life on Earth will absolutely cease to exist in 5 billion years. The core of the red giant is still heating up, more than ever actually. The star is not in hydrostatic equilibrium as it once was in the main sequence phase; its core is now being compressed due to the lack of pressure against gravity, continuing to heat it up. After this, the M8- star then turns into a yellow giant. This is due to the depletion of hydrogen and its fusing of helium to keep it going. After its short yellow giant phase, it turns back into a red giant once more. The core is now reaching even greater temps from the increasing contraction. The red giant finally has no more hydrogen or helium left to fuse, and the star dies.

Due to the continuing contraction of the core, it ejects the outer layers of the star, leaving only what we call a white dwarf behind. In the blast, heavier elements are created such as carbon, which is an essential ingrediant for life. The white dwarf, now in hydrostatic equilibrium once more, is figuratively dead though, as alive as an asteroid in a sense. It is only luminous due to its residual heat from many years of nuclear fusion and past contraction of the core. The ejection of the outer layers creates a planetary nebula. This nebula can spread one light year across, and is the birth place of stars just like the one that just died.

The M8+ star’s life is much shorter due to it using up hydrogen much faster due to its increased temperature. Instead of the long-winded 10-billion-year cycle of the Sun, it spans a couple 100 million years, depending on the mass of the M8+ star, of course.

A M8+ star lives a similar life to any other star. Instead of a T-Tauri phase though, which is for stars of lower mass, high mass stars go through what is called a Herbig Ae/Be (HAeBe) phase.

Another important difference to note is that once a high-mass star leaves its main sequence, it goes straight into a yellow giant, while a M8- star goes from red giant, yellow giant, back to red giant before dying.

The M8+ star goes straight into the fusing of helium because of its increased temperature. After the depletion of its helium, it becomes a red giant, much larger than the red giants of M8- stars. Due to the increasing temperature, the star is able to fuse heavier elements than hydrogen and helium; this process is called nucleosynthesis. This occurs after it runs out of hydrogen and helium. The M8- star counterpart would turn into a white dwarf, surrounded by a planetary nebula, after running out of hydrogen and helium because it is not hot enough to fuse heavier elements, therefore, it dies. The M8+ fusing heavier elements causes it to be hotter and hotter. The core is then contracting more and more due to gravity and lack of hydrostatic equilibrium. The core becomes an iron core, iron being the heaviest element the star can fuse. As the inward contraction of gravity continues, the electrons and protons form a neutron core, and the star will go supernova due to its complete inability to support itself, blasting the heavier elements (and creating even heavier ones in the blast) into interstellar space in a catastrophic explosion.

M8+ on the smaller size will then be formed into a neutron star. This is formed due to the pressure in the core of the M8+ star, so much heat that it squishes electrons and protons into neutrons. Bigger M8+ stars will also go supernova, blasting even heavier elements into space. These larger supernovas form the infamous black holes, celestial objects with so much mass that even the speed of light cannot escape it, light being the fastest thing in the known universe.

As previously stated, planetary nebulas are the birthplace of stars. In fact, usually at the death of any star, it gives back most of what it took to form years ago as a protostar. M8- stars are known for being a large contributor of carbon, oxygen, and nitrogen, which are essential for life.

Supernovas are important as well. Due to their fusion of heavier elements, they allow for planets like Earth to be born. Heavy elements are created such as zinc, iron, silver, tin, gold, mercury, lead, platinum, and uranium through these celestial events.

These elements and other heavy ones are what create planets such as the Earth, and all of us here on Earth. You, me, and my German shepherd puppy, sitting in her bed next to me as I type this, is all thanks to the death of massive stars and smaller ones alike. We are created by what occurs in the universe. If stars such as our own and bigger stars did not die, we would not be here today.

This fact was what got me initially interested in the world of astronomy. It made me feel so much more connected to the world around me and above me. The transiency of stars, giving us the ingedients to be alive through their deaths, is the way for the universe to explore and learn about itself through us.

I hope this does not scare you to know everything in this universe is temporary but, instead, makes you want to explore it further.

After all, just like the Pink Floyd song, “Fat Old Sun,” speaks to us about reflection on moments passing, so does the Sun as it goes down and disappears, reminding us of the passing of time. Look up to remind yourself not to always look into the future or into the past, but to take time to be in the moment, even though it may be fleeting.

Look Up

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by Mitchell Tester, College Student

““FAT OLD SUN””

The sun is a means for all living beings to exist, sitting at the center of our own solar system, 93 million miles from us here on Earth. Taking a trip around the Sun would take you 2.7 million miles, or 1,889 days, at a steady 60 miles per hour. On the contrary, a trip around Earth would take you only 17 days.

Although the Sun trumps the Earth in terms of size, the Sun is surprisingly quite small when compared to some stars of neighboring solar systems and galaxies. In fact, the biggest star in the known universe, UY Scuti, has a radius 1,700 times larger than the Sun. Therefore, the circumference of UY Scuti sits at a staggering 4,619,398,440 billion miles. A nice leisurely journey around UY Scuti would take 3 million days, or nearly 9,000 years, at 60 miles per hour.

Since driving on a star is impossible for many reasons, let’s instead say that you wanted to fly around UY Scuti in a spacecraft with speeds similar to the Apollo 11 mission, fitted with protection against the radiation and immense heat and with a breakneck speed of 24,790 miles per hour. Despite our more than impressive imaginary craft, it would still take us 21 years to fly around the star at a steady pace. This means, this big star is large enough to fit 6 quadrillion Earths inside of it, unlike the meager 1.3 million Earths that can fit inside our own star, the Sun.

The Sun started its life at the center of a solar nebula, basically an interstellar dust cloud, some 4 billion years ago. Interestingly enough, scientists have strongly theorized that our own planets and everything else in our solar system were formed in this same cloud. Disturbances by way of neighboring clouds, exploding stars, and other celestial occurrences are ways for the dust in these clouds to clump together, in addition to the force of their own gravity, growing larger and larger, until after millions of years, a protostar is formed from the core of that solar nebula. And for another 100,000 to 10 million years, it continues to collect mass until it finally stops collapsing in on itself. The time frame of a star’s life and its stages are heavily dependent on the mass of the star, so stay tuned for next month when I talk about the birth and death of stars in part two of “Fat Old Sun.”

For our protostar, eventually the immense pressure, extremely high temperatures, and accretion of material causes hydrogen to fuse into helium by way of nuclear fusion. When the atomic nuclei, hydrogen, starts to bond into a single heavier one, helium, the star is then born. At 25 million degrees Fahrenheit, it burns 600 million tons of hydrogen into helium every second, the equivalent of 10 billion hydrogen bombs.

The pressure that the star gives outward through its nuclear fusion is great enough that it is equal to the pressure inward, inward pressure being the collapsing of material because of gravity. This balancing act is something called hydrostatic equilibrium. One force holds the Sun together, making it spherical, while the other keeps it from collapsing in on itself. Like most things in life, balance is vital.

The nuclear fusion that is occurring in our Sun is occurring in the core, where the pressure and the temperature are so high that the nuclei can be fused together rather than be repelled from one another (which they naturally do).

The energy that is created by the heart of the Sun is called photons, packets of energy that move in waves. The packets of energy, photons, are created by the excitement of an electron and its journey back to a stable state (they like to be stable), that energy needs to go somewhere, so it goes outward. So, the photons’ journey then begins, saying goodbye to their atomic friend, they then move toward the radiative layer of the Sun, to be absorbed and emitted many times throughout their journey.

Inside the radiative layer, the energy created in the core is carried by photons outward, through thermal radiation, which occurs from the high temperatures that cause kinetic energy (energy that is created by movement) of random movements of atoms and molecules in the matter of the Sun. This is at the (literal) core of what heats us here on Earth.

As energy is carried through the radiative layer, it then moves to the convection layer of the Sun, still being absorbed and emitted over and over. Heat rises, which means the Sun, in a figurative sense, lives by the same rules as heating your house does. Due to heat rising, it means the coolness sinks to make way for the heat. This rotation in temperature causes convection—heated gas rises and the denser cooler gas sinks.

The photons continue their journey outward through the hot gas, making their way into the photosphere, which is the visible surface of the Sun. This photosphere is the coolest layer, although still reaching temps of 10,000 degrees Fahrenheit. If you look at an image of the Sun, you will notice a granulation texture seen on the surface. That texture is the before-mentioned clumps of hot gas rising, and the darker (in comparison observed as black) clumps of cool gas sinking inward. When I say gas, I am specifically talking about a superheated gas called plasma, often referred to as the fourth state of matter. Plasma is what happens when extreme conditions are met, superheated matter (in this case, gas) is stripped of its electron(s), electrically charging it (referred to as becoming ionized) and turning it into plasma.

The photons move onward, continuing their journey towards Earth, then reaching the Sun’s atmosphere’s first layer, the inner layer, the chromosphere, a thin layer of red-colored plasma (chromo meaning color). The chromosphere is usually hidden from our view, although it can be seen by special satellite telescopes and even during a solar eclipse. While the photons continue to rise or move outward, they reach the corona, the outermost layer of the Sun. The corona is the least dense layer of the Sun, in addition to carrying the least amount of energy. Despite this, it is extremely hot, upwards of 2 million degrees Fahrenheit, even hotter than the chromosphere. There are many theories as to why the outer layer is hotter, although there is no definitive answer yet.

Photons, moving outward in all directions, passing through the outer reaches of the Sun’s corona, having been emitted and absorbed countless times throughout the Sun’s layers, are now in space, traveling away from the Sun. Interestingly enough, the distance that photons can reach is not finite, continuing on forever as long as there is no interruption, which can occur by way of a planet, black hole, or anything else that absorbs light. This is how we see the light of stars that are trillions and trillions of miles away. Our photons, after completing their journey through the Sun, are then carried through the vacuum of space, by electromagnetic radiation at the speed of light, to our home, the Earth. It actually takes light, or photons, roughly 8 minutes to travel the 93,000,000 miles to Earth. While from the core to the corona, the journey can take upward of around 10,000 to 170,000 years.

The photons, nearing at the end of their journey, are now coming close to contact with Earth’s atmosphere. Before telling you what happens to our photon friends when contact is made with our Earth’s atmosphere, I must explain one thing: Photons are made up of everything in the electromagnetic spectrum, which includes all the colors of visible light. As you may know, all those colors together make white light. Think of Isaac Newton and his experiment of letting light travel through a prism—white light in, all the colors of the rainbow out.

Have you ever found yourself looking up and wondering why the sky is blue? Photons do indeed play a role in that, although photons alone are not the reason our sky is blue. Our sky is blue due to nitrogen and oxygen making up 99 percent of our atmosphere, therefore, effectively scattering the blue and violet light from the photons, while the other colors of the white light spectra make their way down to Earth, mostly unphased. This is also why the Sun appears a yellowish orange to us here on Earth; while, if you were in space, the Sun would be white because it is white light. Although here on Earth, due to nitrogen and oxygen effectively scattering blue light, the lack of blue light (because it is being scattered in the atmosphere) leaves the Sun looking a yellowish orange. If you are wondering why the Sun appears red when it is close to the horizon, that is due to the light going through more of the atmosphere, in return scattering more blue light than normal, leaving the Sun reddish orange rather than yellow.

So, now you know, as you walk outside in the morning to see that (technically white) fat old sun rising, feeling the warmth on your face, the particles that gave way to this feeling took 10,000 to 170,000 years (and 8 minutes) to reach you.