Like almost everything in the universe, stars born, live their lives, and then die in the span of millions and sometimes billions of years. It took decades for researchers to identify and catalog the different types of stars, how they form, and their evolutionary sequence.
How a star ends its life depends ultimately on its one characteristic: mass. If it’s a low mass star, then it would end as a white dwarf, a black hole if it’s a massive star, but anything in between would collapse into a neutron star.
So a neutron star is basically a remnant core of a collapsed star. They are small and extremely massive. A typical neutron star has a radius between 10-13.5 km and mass ranging from 1.4 and 2.16 solar masses.
Neutron stars result from a supernova explosion (occurs during the last stages of stellar life) assisted with the gravitational collapse that squeezes the stellar core so hard that it reaches the density of atomic nuclei. Over time, they can evolve further by various means.
Here, we have compiled 15 interesting facts about neutron stars that every space geek should know.
10. There are Three Types of Neutron Stars
Based on their unique characteristics, neutron stars can be divided into three sub-types; X-ray pulsars, Magnetars, and radio pulsars. Radio pulsars or simply pulsars are the most common type of neutron stars that emits powerful electromagnetic pulses. However, they are extremely difficult to detect.
Since pulsars emit electromagnetic radiation from their magnetic poles, they can only be observed when the radiation beam is pointed toward the Earth. From Earth, this beam would appear as if coming from a fixed point in space. This phenomenon is also known as the lighthouse effect.
These pulsars, if found in a ‘special state’ can provide us with invaluable knowledge about the universe.
A magnetar is a unique sub-type of neutron star that exhibits extremely powerful magnetic fields. While other characteristics such as radius, temperature, and density of magnetars are similar to other neutron stars, they are differentiated from others by their strong magnetic fields and a slightly higher rotation rate.
Artist’s impression of a magnetar | Image Courtesy: ESO/L. Calçada
X-ray pulsars are also known as accretion-powered pulsars, which generally exists in a binary star system where a neutron star is in orbit with another stellar companion. They emit energy in the X-ray spectrum.
Subtypes of X-ray pulsars include millisecond pulsars (recycled pulsars), low-mass X-ray binaries, intermediate-mass X-ray binaries, and high-mass X-ray binaries.
9. They are Very Hot and Extremely Dense
The surface temperature of almost every observable neutron star is around 600,000 K, and it is even higher in newly formed ones. In comparison, the Sun has a surface temperature of approximately 5, 775 K, whereas Sirius, a white dwarf, has a surface temperature of 9,940 K.
A neutron star is compact and so dense that a spoon full of sample material from the star would weigh well over a billion tons. Its density is highly variable that increases with the depth. Near the core, a neutron star becomes denser than an atomic nucleus.
Furthermore, their magnetic field is about one quadrillion times, and the gravitational field is about 200 billion times stronger than the Earth’s. However, the reason behind their powerful magnetic field remains a mystery.
8. The Closest Neutron Star
Artistic concept of an “isolated neutron star” | Image Courtesy: Casey Reed/Penn State University
Back in 2007, a group of researchers discovered a peculiar X-ray source in the constellation of Ursa Minor about 250-1000 light-years away, which they later identified as a neutron star. It could, possibly, be the closest neutron star to Earth.
Officially designated as 1RXS J141256.0+792204, the neutron star is nicknamed Calvera after the antagonist of the popular 1960s movie “The Magnificent Seven.” Unlike most of the observable stars, Calvera belongs to a rare group of isolated neutron stars that do not have any supernova remnant and a companion star.
7. There are About Two Thousand Known Pulsars in the Milky Way
According to an estimation based on the number of supernova explosions, there should be at least 100 million neutron stars present in our Milky Way galaxy. However, astronomers have discovered only less than two thousand pulsars (a most common type of neutron star) to date.
This massive contrast in numbers could be due to their age. Neutron stars are generally billions of years old, which gives them adequate time to cool down. Without the needed energy to emit at different wavelengths, many pulsars become almost invisible to our satellites. Even the young pulsars can go undetected because of their narrow field of emission.
6. The Fastest Neutron Star Rotates At a Rate of 716 Times Per Second
Newly born neutron stars can achieve an extremely high rotation rate due to the conservation of angular momentum. The fastest rotating neutron star recorded to date is PSR J1748-2446ad, located in constellation Sagittarius, about 18,000 light-years away from the Earth.
The distant pulsar is rotating at a furious rate of 716 times per second or 43,000 rotations per minute. Studies have confirmed that the star has a mass slightly less than two solar masses and a radius of fewer than 16 km.
5. Their Rotational Speed can Increase Further
In some cases, a neutron star in a binary system can start absorbing accreted matter or plasma from its companion star. This process can significantly increase the rotation speed of the neutron star and can also change its shape to an oblate spheroid. These changes are triggered by the interaction between the star’s magnetosphere and the plasma.
While this phenomenon was first observed in few X-ray pulsars such as Centaurus X-3 and Hercules X-1, it is now being observed in other similar pulsars. On a different note, a long term decrease in the pulse period of the Centaurus X-3 is also recorded.
4. Neutron Stars can Sometimes Undergo a “Glitch”
An artist’s concept of a “stellar quake” | Image Courtesy: NASA
A glitch in astronomical terms denotes a sudden increase in the rotational speed of a pulsating neutron star. This sudden increase is believed to be caused by a phenomenon known as starquake – a sudden change in a star’s crust. However, it is not scientifically proven. A starquake causes star’s equatorial radius to shrink even more, and since the angular momentum is conserved, its speed is increased.
A number of recent studies have indicated that the level of energy released during a starquake would not be sufficient enough to cause a glitch. Instead, a new theory has been put forward in which these glitches can be explained with the help of disturbances in hypothetical superfluid core of a pulsar.
3. Can Exist in A Complex Binary System
Most of the observable neutron stars exist in a binary system, where they are either paired with white dwarfs, main-sequence stars, red giants, or another neutron star. Researchers have also theorized the possibility of a neutron star- black hole system, which, if found, could be the holy grail of physics.
But in 2003, an international team of radio astronomers at the Parkes Observatory, Australia discovered a binary system with two pulsars, i.e., two pulsating neutron stars in a gravitationally bound system. This is the only binary pulsar system known to us. The two pulsars are designated as PSR J0737−3039A and PSR J0737−3039B.
2. Neutron Stars Can Also Host Planets
Artist’s concept of PSR B1257+12 system
Like others, neutron stars can also host planets and even have a well defined planetary system. Theoretically, these exoplanets can either be indigenous, captured, or exists in the circumbinary form (a planet in a binary star system).
Furthermore, a pulsating neutron star in a binary system can remove the atmosphere of its companion star entirely, leaving behind just the bare celestial mass. These masses can be interpreted either as a planet or a stellar object.
Only two such planetary systems have been confirmed to date. The first one is comprised of three planets, namely Poltergeist, Phobetor, and Draugr, revolving around PSR B1257+12. The second system contains only one extrasolar world, and it’s rotating around PSR B1620-26.
Cool Fact: Draugr was the smallest exoplanet discovered during the time of its discovery.
1. A Collision of Two Neutron Stars
On 17 August 2017, about 70 different observatories around the world, including Virgo and LIGO, detected a gravitational wave signal now known as GW170817. This gravitational wave was produced during the last few minutes of the coalescence of two neutron stars. Although this was not the first one detected, it is considered as a breakthrough discovery in astronomy.
The reason behind this is that all previously recorded gravitational wave signals were caused by a merger of black holes that do not emit any significant electromagnetic signal. Shortly after the collision, the Fermi Gamma-ray space telescope observed a short gamma-ray burst designated GRB 170817A.
Few Short Facts
15. Hulse-Taylor binary or PSR B1913+16 is a pulsar, which, along with a neutron star, forms a binary star system. After its discovery in 1972, it became the first-ever binary pulsar to be observed and proved to be crucial in the study of gravitational waves. The discovery and further analysis earned Russell Alan Hulse and Joseph Hooton Taylor, Jr., the Nobel Prize in Physics in 1993.
14. In contrast to a glitch, a neutron star can also experience “anti-glitch.” During an “anti-glitch” phase, a sudden decrease in the rotational speed of the neutron star can be observed. So far, this phenomenon is only seen in a magnetar. Researchers are still unable to find the underlying cause of such behavior since the current models of neutron stars do not predict it.
13. Comparable to the Chandrasekhar limit (maximum mass at which a white dwarf can remain stable), Tolman–Oppenheimer–Volkoff limit is the upper ceiling to the mass of a neutron star after which the dead star further collapses into a black hole. Its value ranges from 1.5 to 3.0 solar mass.
12. The existence of neutron stars was predicted by astronomers Walter Baade and Fritz Zwicky in 1934, more than three decades before they were confirmed for the first time. Their prediction came in less than two years after Sir James Chadwick vaguely observed the neutron in 1932.
11. The Magnificent Seven is a name given to a group of young and isolated neutron stars, which are located between a distance of 390 to 1630 light-years away and are closest to the Earth. The first neutron star in the group was RX J1856.5-3754, which was discovered in 1992 and then confirmed in 1996.
The remaining six stars in the group are RX J0806.4-4132, RX J0720.4-3125, RBS1556, RBS1223, RX J0420.0-5022 and 1RXS J214303.7+065419. Each of the seven X-ray sources are detected by ROSAT satellite.