Like almost everything in the universe, stars born, live their lives and then die in the span of million 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. The existing model of the stellar evolution gives us a clear idea about the life cycle of the stars including our Sun.
How a star end its life depends ultimately on its one characteristic: mass. If it’s a low mass star, then it would end as a white dwarf, or as a black hole if its a massive star, but anything in between would collapse into a neutron star.
So what is a neutron star? 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.
A neutron star results from a supernova explosion (occurs during the last stages of stellar life) assisted with 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. 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 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 phenomena 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 special sub-type of neutron star that exhibit 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 the 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 X-ray spectrum.
Sub types of X-ray pulsars include millisecond pulsars, which are also known as 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 600000 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 1 quadrillion times and the gravitational field is about 200 billion times stronger than the Earth’s. However, the reason behind their powerful magnetic field is still a mystery.
8. The Closest Neutron Star
Artististic 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 is quite possible that it could 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 does 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 about 100 million to one billion neutron star present in our Milky Way galaxy. However, astronomers have discovered only less than two thousand pulsars (most common type of neutron star) till date.
This massive contrast in numbers could be due to their age. Neutron stars are generally billions of years old, which give 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. Fastest Rotating Neutron Star
Newly born neutron stars can achieve an extremely high rotation rate due to the conservation of angular momentum. The fastest rotating neutron star ever recorded till date is PSR J1748-2446ad and it’s located in the constellation Sagittarius about 18,000 light years away from the Earth.
The distant pulsar is rotating at a furious rate of 716 times per seconds 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 less 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 oblate spheroid. These changes are mainly caused by the interaction between the star’s magnetosphere and the plasma.
While this phenomena was first observed in few X-ray pulsars such as Centaurus X-3 and Hercules X-1, it is now 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 to a sudden increase in rotational speed of a pulsating neutron star. This sudden increase is believed to be caused by a phenomena known as starquake – a sudden change in a star’s crust, however it is not proven. A starquake causes star’s equatorial radius to shrink even more and since the angular momentum is conserved, its speed is increased.
However, 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 such binary pulsar system known to us. The two pulsars are designated as PSR J0737−3039A and PSR J0737−3039B.
2. Neutron Stars Can 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 till 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 planet 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 merger of black holes which do not emit any significant electromagnetic signal. Shortly after the merger, the Fermi Gamma ray space telescope observed a short gamma ray burst designated GRB 170817A.
Few Short Facts
5. 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.
4. 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. This phenomenon is only observed in a magnetar. Researchers are still unable to find the underlying cause of such behavior since it is not predicted by the current models of neutron stars.
3. 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.
2. 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 the neutron was vaguely observed by Sir James Chadwick in 1932.
1. 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 to be included 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.