The Journey of Stars
Ever wondered how star forms? Well, here’s how.
The celestial journey is an interesting one for mankind. For years, we looked up at the sky and asked it questions that have long been forgotten. However, what is most easily forgotten is that most material is made from star stuff. You, me, and Dupree (zinger)! As the great Carl Sagan said,
“We’re made of star stuff. We are a way for the cosmos to know itself.”
We must remember that after the Big Bang, the only elements prominent in our universe were simply hydrogen (75%) and helium (~24.5%). There was a little bit of lithium, but that’s about it. So, how did all these other elements form?
A star is born
In our early universe (380,000 years after the Big Bang), the universe was filled with random particles. One can imagine those particles floating through physical space like in Figure 1-a below. What we must keep track of is the hyperspace that is created by gravity as it bends spacetime, Figure 1-b. The ellipses simply represent that this happens in all directions.
Let’s imagine those black dots are Hydrogen atoms (1 proton & 1 electron). Over time, and mostly because of gravity, those atoms start pushing together, like the below figure indicates.
When these beautiful elements start amalgamating together, nebulas are born. Nebulas are pretty, so here’s a picture below.
What happens next is that all those black dots of hydrogen start colliding and start to create some friction (or heat). That heat starts acting as a resistive force against gravity; and, when an equilibrium is reached, a protostar is formed. There are four main types of resistive forces that keep stars afloat: thermal friction, thermal fusion, electron degeneracy pressure, and neutron degeneracy pressure. We will tackle them chronologically. The main concept of resistance is illustrated below.
When resistance and gravity reach an equilibrium, we can go back to our hyperspace to see what happens. Here, we see that the protostar creates a gravitational well; and, that other gas starts circulating around the newly found star. One can picture a bowling ball on a trampoline with marbles rolling around it to get a general picture.
From here, a star will start fusing elements to start the main sequence of its lifecycle. If fusion does not occur, then the star becomes a brown dwarf. Thus concludes the early stages of a star.
Main Sequence Activated — Small Stars
What happens during main sequence and beyond really depends on the size of the star, so we start with small stars. This includes anything less than ~8 solar masses when they are alive, and <1.4 solar masses at death. One solar mass, M☉, is the mass of our sun, 2x10³⁰ kg. It is evident that stars lose mass as they age, because they release energy; and E=mc² (i.e. if you lose energy in the form of light, then you’re also losing mass).
Ok, so elements start to fuse (i.e. hydrogen starts turning into helium) and that radiation becomes our new resistive force. In protostars, it was heat from atomic collisions that acted as the resistive force. In main sequence stars, fusion is the resistive force balancing gravitational contraction. Our new main sequence star is illustrated below.
The process of fusion keeps going on for a while until the star burns its hydrogen supply and gets bigger and redder. We call these red giants — great name. Below are the different parts of a red giant. The outer three are the star’s atmosphere.
After a while, all the hydrogen is used up and the star keeps contracting, because of gravity. Helium starts making carbon, and then the carbon starts making oxygen. However, after a certain point, the electrons in the electron cloud of the atom prevent any further collapse. This is called electron degeneracy pressure.
It is mostly due to the Heisenberg Uncertainty Principle, which states that one cannot know an object’s precise position and its momentum (mass times velocity). Therefore, if the electron’s cloud is squeezed, then the electron’s momentum must be greater (i.e. greater speed). The fact that the electron cannot go faster than the speed of light creates a new form of resistive force, thus keeping the star “alive”. This concept is illustrated below. Here v1 is less than v2 (i.e. v1<v2) and p2 is greater than p1 (i.e. p2<p1).
When electron degeneracy is reached, the core reaches equilibrium with gravitational contraction. The star is now a white dwarf. Now, what happens to the outer layers of the star? The outer layers are carried off by stellar winds and create beautiful planetary nebula. Now, the name is a complete misnomer, so don’t be fooled — there are no planets involved in creating these nebula. Instead, it is merely a cloud of gas that carries the remaining atoms fused by the dying star into the cosmos. Planetary nebulas are really pretty, so here’s a photo of the Helix Nebula by the illustrious Hubble Telescope.
Thus concludes our track of smaller stars, which can be seen on the left portion of the graphic below (which is the same one as the top one added for convenience).
Main Sequence Activated — Massive Stars
Massive stars start out as stars bigger than 8 Solar Masses while alive, as indicated by the above graphic. They follow a very similar pattern compared to smaller stars with the 3 main differences being: (a) the process lasts millions of years as opposed to billions of years, (b) a star’s core contains a more diverse set of atoms, and (c) the star is much, much bigger.
The process lasts a shorter amount of time, because, since the star is bigger, it burns its fuel more rapidly. We see something similar in humans and dogs, where the smaller ones tend to outlive the larger ones. Furthermore, the core is more diverse, because the increased mass of the star increases the gravitational contraction; thereby increasing the rate of fusion between particles into heavier and heavier elements. In massive stars, all the elements between hydrogen and iron can be formed. These concepts are illustrated below:
Now, after a while, the star gets bigger and turns into a red supergiant — an even better name! When running out of fuel, the star, especially the core, continues contracting; and therefore, continues fusing elements up to iron. What happens next is called neutron degeneracy pressure. Here, since gravity is so strong, the electrons get pushed into the protons and form neutrons. From there, all that is left in the core are atoms made up of only neutrons — a neutron soup of sorts! The concept is the same as electron degeneracy pressure, except the main resistive force is happening with neutrons instead. If the star is between ~1.5–3 solar masses at death, then the core forms a neutron star. A spinning neutron star is called a pulsar and is defined like so by Kip Thorne:
“A pulsar is a magnetized, spinning neutron star that emits a beam of radiation (radio waves and sometimes also visible light and X-rays). As the star spins, its beam sweeps around like the beam of a turning spotlight; each time the beam sweeps past Earth, astronomers receive a pulse of radiation.”
When the neutron star forms, the outer layers of the massive star collapse on the core and bounce into the cosmos — a supernova occurs. Supernovas are extremely bright — around 10⁸ (100 million) times more luminous than our sun and can outshine a whole galaxy! Furthermore, supernova allows for the creation of various heavier elements above iron (element 26). That’s a lot of elements considering there’s 118 known elements! The catch is that elements between 26–118 are much less abundant than some of the lighter elements (<26). Supernovas are pretty too — let’s show them some love.
Now, what about bigger stars? These are stars that are more massive than ~3 solar masses at death and greater than ~20–30 solar masses during its lifetime. Well, for those stars, the gravitational contraction overwhelms all the resistive forces of the star and creates a “rip” in the fabric of spacetime. This rip is so crazy that it forms what is commonly known as a black hole — a term coined by the great John Wheeler. A black hole is illustrated below.
Conclusion
Overall, the formation of stars is a very interesting one. It starts from simple hydrogen atoms (around 10^-13 cm in size, weighing ~1.7x10^(–27) kg) combining together to form larger and larger objects, like massive stars which can weigh up to 200 solar masses (i.e. 4x10³² kg)! As with everything in life, gravity seems to be the driving force for their creation, life, and ultimate death. We sum up this chapter with a couple graphics to really seal everything in. Again, words only do so much, which is why I littered this article with pictures. The first figure shows the complete lifecycle of stars — yet again.
Again, M☉ signifies 1 solar mass (i.e. the mass of our sun, 2x10³⁰ kg). So, 0.08 M☉ is 0.08 solar masses. Furthermore, note that the planetary nebula and the supernova remains can be recycled to form new nebula and produce new stars — nature doesn’t waste anything! Sometimes, it’s fun to think that all humans are really star stuff. In the early universe there was only hydrogen and some helium. Without stars, we wouldn’t have carbon — the main building block for all of life. Some say you should “count your lucky stars”. The second graphic shows a scaled version of the universe filled with various types of stars. Enjoy and as always — thanks for reading!
