Don't Judge a Star by its Size

In 1967, Jocelyn Bell detected the first evidence of what we now consider to be the most dense object known — a neutron star. A star's fate is almost predetermined by its initial size. So, how big is a star to generate a neutron star? How or why does it become so dense?

Black Hole (left) and neutron star (right): artist's conception of a black hole orbited by a cracked neutron star. Copyright and credit to Caltech/R. Hurt (IPAC).

Neutron star formation

 Stars about 8 times the mass of the Sun die faster because their higher rate of nuclear fusion resists the gravitational force. Once it consumes all its fuel, it collapses in on itself, and the star explodes into a supernova. If the remaining core has a mass of approximately 1-3 solar masses, it will collapse to a neutron star. A body with the same composition as an atom's nucleus. But that's not what truly makes them interesting. What scientists are impressed by is its mass-radius relation and its density. For reference, our Sun has a radius of almost 700,000 km. And, with only a 10 km radius, a neutron star holds a mass greater than our Sun's.

How do we find them

Neutron stars have high temperatures even though they don't undergo nuclear fusion. Much of their energy comes from the supernova they underwent. The surface eventually cools, but sufficiently high temperatures allow scientists to observe the star using X-ray telescopes.

 That is just an example; however, their observed wavelengths vary with their state or classification (each of which is explained in the sections below). A neutron star can be found in binary systems– partnered with ordinary or other neutron stars–, as pulsars, or as magnetars. The three "formats” are most commonly found, respectively, via X-rays, radio signals, and gamma rays.

Binary systems

In case it is paired with an ordinary star, the powerful gravitational force of the neutron star causes it to accrete material from the partner. In other words, it can strip/pull matter from the other star's surface. That material is then pulled towards the neutron star due to its magnetic field. Simultaneously, the elements being drawn are heated to the point of radiating X-rays, which are subsequently detected by telescopes. 

There is also the chance of a binary composed of two different neutron stars. The type of binary system, however, does not change the fact that, in the event of a collision, they generate "kilonovas". So, if a neutron star and its stellar partner collapse due to their gravity, they produce short-duration gamma-ray bursts (kilonova). An event that is powerful enough to begin nuclear fusion, which leads to heavier elements (reaching gold) than those seen in usual stellar processes (reaching iron).

Pulsars and Magnetars

Most neutron stars are what is called a “pulsar”. The name derives from their constant rotation and the emission of radiation at regular intervals. Similar to what is seen in a lighthouse. Pulsars have strong magnetic fields that, in the form of two jets, channel particles/matter. Because of that, what we see are flashes of light captured when that jet is funneled in our direction. On the other hand, magnetars are simply neutron stars with extreme magnetic fields. While a normal neutron star would have a magnetic field about a trillion times Earth's, a magnetar's is 1000 times stronger.

Related links:

Nasa: https://imagine.gsfc.nasa.gov/science/objects/neutron_stars1.html

https://imagine.gsfc.nasa.gov/science/objects/neutron_stars2.html

ESA: https://esahubble.org/wordbank/neutron-star/

Harvard: https://www.cfa.harvard.edu/research/topic/neutron-stars-and-white-dwarfs