10 Interesting Facts about Neutron Stars

10 Interesting Facts about Neutron Stars
Image Credit: Jupe/Alamy

Simply put, a neutron star is the collapsed and highly compressed remains of a relatively massive star that died in a supernova event. Neutron stars are also the smallest stars known to exist, with their typical radius being only about 10-20 km, and weighing on average about twice as much as the Sun. Below are 10 more interesting facts about neutron stars you may not have known.

Most neutron stars are very fast rotators

Since the conservation of angular momentum following a supernova explosion transfers the progenitor star’s rate of rotation to the remnant that is only about 20 km (12.5 miles) in diameter, the result is that the neutron star rotates very rapidly when it is formed. Most known neutron stars rotate several hundred times per second, but the fastest rotator yet discovered, the neutron star designated PSR J1748-2446ad, is known to rotate 716 times per second, which translates into 43,000 rotations per minute, or 24% of the speed of light at the star’s equatorial surface.

Neutron stars are very hot, and very dense

Neutron stars that can be observed are typically very hot, with surface temperatures that can be as high as 60,000K, compared to say around 6,000K for the Sun. They are also very dense, with a piece of a typical 10-km-diameter neutron star no bigger than a normal matchbox weighing about 3 billion tons, or as much as a cube of Earth that has 800-meter-long sides. Note, however, that the density of neutron stars decreases as their diameters increase.

Neutron stars are not very massive

Despite their very high densities, neutron stars are limited to between 1.1 and 3 solar masses, with the most massive observed weighing only 2.1 times as much as the Sun. Neutron stars have such strong gravity that protons and electrons are crushed together to form neutrons, and it has been proposed that an overdense neutron star with more mass could evolve into a quark star, representing a hypothetical intermediate stage between neutron stars and black holes. Up until now, however, no theoretical quark stars and electroweak stars with masses between 3 and 5 times solar have been found, but what is certain is that above 10 solar masses, the stellar remnant will collapse into a black hole.

Neutron Stars

The Milky Way has about 100 million neutron stars

The figure of 100 million is based on standard models of galactic evolution, and is further derived from the estimated number of supernova event that have occurred in the history of the Milky Way galaxy. Only relatively young neutron stars are easy to detect, though, as most are cold, or slow rotators, or are not accreting material from companion stars, thus making them virtually undetectable. Nonetheless, the Hubble Space Telescope has managed to detect a few neutron stars recently that are emitting only thermal radiation.

Neutron stars and pulsars are often the same thing

All neutron stars have extremely strong magnetic fields typically in the order of 104 to 1011 teslas, which is several billion times stronger than Earth’s magnetic field of only 25 to 65 microteslas. It is thought that the magnetic pulses given off by some neutron stars result from large build-ups of electrostatic fields near the stars’ magnetic poles. Electrons are then accelerated along magnetic lines, and if the stars magnetic field is not aligned with the stars’ rotational axis, this magnetospheric radiation becomes detectable by an observer if the stars’ magnetic axis point toward the observer. In all known cases, the periodic magnetospheric pulses coincide with the stars’ rotational period, thus, a neutron star becomes a pulsar, pulsating radiation that can be detected as originating from the star.

Neutron stars are powerful gravitational lenses

While all massive objects are known to be able to bend light rays, neutron stars take this to a whole new level. On average, the gravitational field of a neutron star is about 200 billion times stronger than that of the Earth, which means that the light emitted from the side of a neutron star that faces away from us, is bent to the point where parts of the normally invisible side of the star becomes visible. In extreme cases, a neutron stars’ gravitational field can be so strong that light emitted from it cannot escape, since it is caught in an orbit around the star. In such a case, the entire surface of the star is visible from just a single point of observation.

Neutron stars can spin up, and then down again

In cases where a neutron star accretes matter from a normal companion, the matter is sucked up like a sponge, which can increase a neutron star’s rate of rotation by as much as 100 revolutions per second in the case of millisecond pulsars. The increased rate of rotation can deform some neutron stars into an oblate spheroid, but as the star slows down over time, the crust tends to revert to its original spherical state, which causes “star-quakes”, and a decrease in the star’s spin rate. That said, recent studies have shown that star quakes are likely not energetic enough to reduce a neutron star’s spin rate, and some investigators are now suggesting that, in some cases at least, the reduction is caused by different rates of rotation of the crust and the interior of the star.

Neutron stars can host planets

At least two neutron stars are known to have planets, although the origin of these planets is not certain. It is thought that in some cases, the planets may be original, in the sense that they survived the formation of the neutron star, or that they may have been captured. According to some models, a neutron star is perfectly capable of completely stripping the outer layers off of a main-sequence companion star, which would leave a planetary-mass object. One planet, named Draugr, which orbits the neutron star PSR B1257+12 along with the planets Poltergeist and Phobetor, is the smallest exoplanet discovered to date, with a mass of only two times that of our Moon.

Time dilation between Earth and a neutron star is measurable

Due to the exceedingly high gravity on a neutron star, the difference in time displayed between identical atomic clocks placed on the star and Earth would be significant. For instance, a period of eight years will (according to the clock on the star) pass while ten years will pass on Earth, with the difference being caused by the effect of gravity on the vibrating cesium atom in the clock. To put the strength of the gravitational field of a neutron star in perspective, consider this; if an object were dropped from a height of just one meter above the surface of a neutron star with a 12 km radius, the object will strike the surface of the star at a speed of 1.4 million meters/sec.

New neutron stars cool down fairly rapidly

While the temperature inside a newly formed neutron star is estimated to be around 10 billion kelvin, the newly formed neutron star emits so many neutrinos that much of this heat is carried away by the escaping neutrinos. In fact, a young, isolated neutron star emits such large quantities of neutrinos that its temperature falls by as much as 100 to about only 10 million kelvin in only a few years. At this reduced temperature, the neutron star emits almost all of its radiation in X-ray frequencies.

Related Articles