Neutron Stars — The Heaviest Objects in the Universe
The mysterious ancient stellar cores which populate the Cosmos
Throughout the vastness of the Milky Way galaxy, there are approximately 100 billion different stars, ranging from red dwarfs to supergiants. While neutron stars are estimated to account for only 1% of all the stars in our galaxy, on a cosmic scale 1% can still amount to quite a large quantity. Using this estimate, we are left with around one billion neutron stars — but what is a neutron star?
Burning at roughly 1 million degrees Kelvin (~1.8 million degrees Fahrenheit), neutron stars are some of the most violent astronomical bodies found in our universe. As the name suggests, a neutron star is a stellar body containing matter composed of 95% neutrons.
During the majority of its life, a star is stable. This is due to a balance between the outward pressure from expanding hot gasses and the inward force of gravity. This allows for a state called hydrostatic equilibrium. A star is classified as a “main sequence star” during this period. Most stars will burn hydrogen to helium for billions of years via a process called nuclear fusion. However, certain stellar properties begin to change when that hydrogen nears depletion. As a star reaches the end of its life, multiple things can happen depending on the mass of the given star. Medium-sized stars such as our sun go through a red giant phase once their hydrogen is exhausted. During this phase, the star’s volume begins to rapidly increase and helium begins to burn into carbon and oxygen. Over time, the star will gradually transition into the next phase of its existence: a white dwarf.
For stars with at least eight solar masses (1 solar mass = the mass of our sun), death occurs in a significantly more violent manner. After the given star’s helium has fused into carbon, enough pressure still exists for nuclear fusion to continue. Carbon will fuse into heavier elements such as oxygen, sodium, magnesium, and neon. Neon will fuse into oxygen and magnesium, with oxygen then fusing to silicon. Finally, as silicon burns to iron, there is no longer any energy to be released, and nuclear fusion can no longer take place. As nuclear fusion within the stellar core comes to a stop, the star will no longer actively expel energy.
Without nuclear fusion, there will not be any outward pressure to negate the immense gravitational forces. Millions of billions of trillions of tons of hot plasma will collapse inward towards the stellar core at 25% the speed of light, creating a shockwave of incomprehensible power. This is known as a supernova explosion. Following this massive release of energy, one of two scenarios will occur. If the mass and volume of the star in question have crossed a certain threshold, (the Schwarzschild radius) intense gravitational forces will cause an irreversible collapse. This is how a black hole is born.
Alternatively, if a star goes supernova and this threshold is not met, gravity will not succeed in forming a black hole. In this scenario, the pressure of the star’s collapse causes electrons and protons to fuse, forming neutrons. Neutrons are extremely dense and during this event, they are compressed together with extreme force. To put the scale of this compression into perspective, an iron core roughly the size of the Earth would be compressed into a sphere the size of a city. Hence, a neutron star is born. With a mass approximately one million times that of the Earth, condensed into a sphere roughly 12 miles in diameter, neutron stars are thought to be the densest objects in the universe. They are so dense that their gravitational forces cause light to bend around the body, creating an illusory magnification of the star. This effect also allows for an outside observer to simultaneously perceive both the front and back of the star from a fixed point.
In fact, a neutron star is so dense that if its mass was slightly greater, gravity would cause the stellar body to collapse in on itself, forming a black hole. Similar to planetary bodies, neutron stars have an atmosphere, a crust, an outer core, and an inner core. The crust of a neutron star is composed of elements left over from the supernova explosion, such as iron. Gravity is stronger closer to the core, fusing more and more protons and electrons into neutrons. At the base of the crust, nuclei are so close they start to touch.
Past the outer core, there exists an inner core. The conditions inside this region are so extreme, that the properties of its composition are unknown. Currently, only informed speculation is possible; many different theories exist, and as time passes discoveries will eventually be made. Any research that concerns bodies light-years away from our solar system is an undeniably difficult task to undertake, despite our technological advancements. Though we are currently far from deciphering the true nature of neutron stars, scientists continue to dedicate time, money, and resources to unravel yet another cosmic mystery.
