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.
