As I stepped outside the other morning, I was shocked by the lingering warmth of the Sun. Mad as I was, I always take a moment to appreciate the importance of our star and others like it. Without the Sun, we would have no heat, no light, no solid ground to walk on, not even the life to appreciate our position in space. To be sure, our Sun didn’t create the elements we walk on, nor did it create the necessary building blocks for life. For that, we must look at stars that are massive enough to fuse ions into iron.
In the first three minutes of the universe, every proton and neutron that currently exists was formed. Lone protons were hydrogen and paired pairs of protons and neutrons formed helium. The super-hot gas more or less uniformly filled the universe. However, tiny fluctuations in the density caused small groupings of particles, which in turn caused larger clumps, to form and increased the local gravity causing a larger clump (and then more gravity and a larger clump, etc.) At its onset, those clumps formed by two protons hitting at such an angle to stick and not ricochet. Then they hit in just the right angle to cause the pair to spin. As more and more atoms were added, they maintained the same spin. Soon, the whole nebula began to turn and the high mass region in the center started to turn spherical. While the center of the gas cloud turns more rapidly, the lower massed regions lag behind and flatten out into a disk, which eventually coalesces into the planets. For the moment, however, let’s ignore the rockier bits and head back into the center of the rotating mass.
A rotating nebula collapsing into a proto-planetary disk which eventually becomes a star. Photo via astro.umass.edu
Around that time, the ever-increasing gravity at the center starts to crush the hydrogen nuclei into as small as region as possible. The protons in the center are incredibly hot and moving very fast, which in such a high density region causes atomic collisions. It isn’t the same type that caused them to stick together originally. Those atoms collide with such force they knock the charge out of one of the protons, leaving one of the original protons bound to a chargeless neutron. The element is a heavy form of hydrogen, an isotope called deuterium which is double the mass of normal hydrogen (protons and neutrons weigh about 1 unified atomic mass unit, or 1/12 of neutral carbon-12.) Though the end goal of this first stage of fusion is helium, the still-shrinking mass is star because any type of fusion occurred in the first place.
After the deuterium is formed, it and another proton combine to form a light isotope, helium-3. The last step in this chain reaction is when two light heliums collide. The contact sheds off two protons (which then are free to start a new reaction) and leaves two protons and two neutrons, the natural isotope of helium-4.
The proton-proton chain method of helium fusion. Sun-like stars produce energy this way. Photo via Wikimedia Commons user Borb.
Each time the elements smash together, energy is released in the form of gamma rays. The high-energy radiation is what pushes out against the stellar collapse. When both forces are exactly equal, the star is said to be in hydrostatic equilibrium and remains almost the same size until it changes its main fusion type. In the case of the Sun, this will take about 4 billion years. Red dwarf stars can take trillions of years to exhaust their fuel.
The evolution of the Sun. Graphic via ucfilespace.uc.edu.
In astronomy, anything heavier than lithium is considered to be a metal. Because the first stars were only fusing hydrogen into helium, they had extremely low metal content and are called Population III. While those stars had little to no heavier elements, they are responsible for making them. When a star is massive enough, it will fuse helium and beyond up until it begins to make iron. The problem is, iron is a nuclei that actually absorbs energy to fuse rather than emitting it. Without an actively fusing core, there is nothing to prevent the recollapse of the star. One of two things can happen at this point: a neutron star or a black hole.
In the case of the former, as all of the mass between 10 and 29 solar masses comes crushing inward, the iron core gets squeezed so tightly that its electrons and protons fuse together into incredibly dense neutrons. The incredible density comes from what is known as neutron degeneracy pressure, or the force that the neutrons exert to not be crushed any further. A more well known type of degeneracy pressure is that of electrons. Some people might be familiar with that type as it’s what prevents you from sinking through the floor or collapsing in on yourself. Elements are separated by their electron clouds, which are a great deal of empty space around a tightly packed nucleus.
The denser the object, the more warped the space around it becomes. Photo via imgur.
The most common analogy of an atom is picturing the nucleus as an apple on the 50 yard line of a football stadium and the surrounding stadium as the electron cloud. So the most tightly packed two atoms can be are a full stadium apart. Now imagine someone throws all of the apples into one basket and one teaspoon of the apples weighs 10 million tons. All things considered, neutron stars shove twice the Sun’s mass into a sphere about the size of Athens and are incredibly interesting objects.
Eventually the star will squeeze inward as much as it can until neutron degeneracy pressure wins out and the in falling matter rebounds and hurtles out into space, which we see as a supernova. Supernovae are responsible for making elements heavier than iron and the violent spreading of material distributes these elements throughout the interstellar medium and nebulae that will eventually form Population II (low metallicity) and Population I (high metallicity) stars.
The Crab Nebula in Taurus. It is a supernova remnant from 1054 AD that formed a neutron star. Photo via Wikimedia Commons user HubbleSite.
If the collapsing star is of an even higher mass, not even the neutron degeneracy pressure can withstand the weight. The star gets crushed smaller and smaller until its mass is within its Schwarzschild radius. It is at this point that matter is so dense that its escape velocity exceeds that of light, and a black hole is formed. The Sun’s Schwarzschild radius would be about 2 miles, while Earth’s is less than an inch.
Earth’s Schwartzchild radius, courtesy of Little Planet Factory.
