50 Fun Facts About Neutron Stars

A neutron star is the gravitationally collapsed core of what kind of object?

Stars that end up as neutron stars typically start out with 10 to 25 times the mass of the Sun.

Roughly what is the radius of a typical neutron star?

That is roughly the size of a city, yet the star packs in around 1.4 times the mass of the Sun.

Which class of objects is the only one denser than neutron stars?

Neutron stars are the second-smallest and second-densest known class of stellar objects.

What kind of event compresses a stellar core into a neutron star?

The core is squeezed past white-dwarf density all the way to the density of atomic nuclei.

About how many neutron stars are thought to exist in the Milky Way?

The estimate comes from counting how many stars have gone supernova, and most of them have long since cooled and gone dark.

A normal-sized matchbox of neutron-star material would weigh roughly how much?

That is the same weight as a half-cubic-kilometre chunk of the Earth's surface, a cube about 800 metres on each edge.

A teaspoon of neutron-star material has a mass roughly 900 times that of which famous structure?

At the same density the entire Earth would fit into a sphere 305 metres across, about the size of the Arecibo Telescope.

Which pair of physicists proposed the existence of neutron stars at a December 1933 meeting, less than two years after the neutron was discovered?

They also correctly proposed that the release of gravitational binding energy is what powers a supernova.

Who discovered the neutron in 1932, setting up the idea of neutron stars?

Two Caltech astronomers proposed neutron stars less than two years later.

Which graduate student first noticed the signal of the first pulsar in 1967 while analysing chart recordings from a Cambridge radio telescope?

She sometimes reviewed as much as 96 feet of paper chart data per night, and had helped build the telescope over the previous two years.

What playful nickname did the discoverers give the first pulsar signal?

It stood for 'little green men', because the regularity briefly suggested an alien beacon, until a second pulsar turned up elsewhere in the sky.

How long is the period of PSR B1919+21, the first pulsar ever discovered?

It sits in the constellation Vulpecula and was originally catalogued as CP 1919, for Cambridge Pulsar.

Which band put a plot of the first pulsar's radio pulses on the cover of its 1979 debut album Unknown Pleasures?

The image was created by radio astronomer Harold Craft and has since become one of the most reproduced graphics in music.

Which of Bell's colleagues shared the 1974 Nobel Prize in Physics for the discovery of pulsars, while she was left out?

Fred Hoyle publicly argued that Bell should have been a co-recipient; the omission is still debated.

What did Bell Burnell do with the $3 million from her 2018 Special Breakthrough Prize?

The fund, run by the Institute of Physics, helps female, minority and refugee students become research physicists.

In which part of the UK was pulsar discoverer Bell Burnell born?

She grew up in Lurgan, County Armagh, and was encouraged by staff at the nearby Armagh Observatory.

The Crab Pulsar is the remnant of a supernova that was widely observed on Earth in which year?

Discovered in 1968, it was the first pulsar to be connected with a supernova remnant.

How many times per second does the Crab Pulsar rotate?

Its 33-millisecond period is lengthening by 38 nanoseconds a day as energy pours out in the pulsar wind.

The measurement of the Crab Pulsar's 33-millisecond period in 1968 finally convinced scientists that pulsars were rotating neutron stars rather than what?

Lovelace and colleagues made the measurement using the Arecibo Observatory.

What is a magnetar?

Their fields are roughly a hundred million times stronger than any man-made magnet, and decay after about 10,000 years.

Which two scientists proposed the existence of magnetars in 1992?

They were trying to explain the mysterious transient gamma-ray sources now known as soft gamma repeaters.

Compared with ordinary radio pulsars, how fast do most observed magnetars rotate?

Typical neutron stars seen as radio pulsars turn one to ten times per second.

On 27 December 2004, radiation from a starquake on which magnetar produced the brightest event ever seen from outside the Solar System up to that time?

In a tenth of a second it released more energy than the Sun puts out in 150,000 years, and it briefly expanded Earth's ionosphere.

Which cosmic phenomenon has been linked to magnetars by 2020 findings from Australia's ASKAP array?

Out of roughly 3,000 known neutron stars, only 31 were confirmed magnetars as of January 2021.

PSR J1748-2446ad, the fastest-spinning pulsar known, rotates roughly how many times per second?

Its equator moves at roughly a quarter of the speed of light, more than 70,000 kilometres per second.

The fastest-spinning known pulsar lives in which globular cluster in Sagittarius?

The cluster is about 18,000 light-years away, and the pulsar has a bloated companion that eclipses it regularly.

For 24 years the spin record belonged to PSR B1937+21, discovered by Don Backer in 1982 as the first example of which type of object?

It spins 642 times a second, a value that placed fundamental limits on how big and heavy neutron stars can be.

A 'glitch' in a neutron star is a sudden small change of what kind?

One explanation is a starquake: as the crust cracks and the star becomes more spherical, conservation of angular momentum speeds it up.

Which 1974 discovery by Hulse and Taylor consisted of two neutron stars orbiting each other?

Its orbit has decayed exactly as general relativity predicts for gravitational-wave emission, earning the pair the 1993 Nobel Prize.

How long does it take the two neutron stars of the Hulse-Taylor system to orbit each other?

At closest approach they are only about 1.1 solar radii apart, and their periastron swings round by 4.2 degrees a year.

Which telescope did Hulse and Taylor use to discover their binary pulsar?

They noticed the pulses arriving slightly early and late in a 7.75-hour cycle, the signature of an orbit.

GW170817, the first gravitational-wave detection from merging neutron stars, was observed on which date?

The name encodes the date; the announcement came two months later on 16 October 2017.

In which galaxy did the neutron star merger GW170817 take place?

The galaxy is roughly 140 million light-years away in the constellation Hydra.

How many observatories saw the aftermath of GW170817 across the electromagnetic spectrum?

They were spread over seven continents and space, a landmark for multi-messenger astronomy that Science named its 2017 Breakthrough of the Year.

How long after the gravitational wave from GW170817 did the Fermi and INTEGRAL spacecraft detect a short gamma-ray burst?

The coincidence confirmed the long-standing idea that neutron star mergers are a major source of short gamma-ray bursts.

What name is given to the glowing cloud of radioactive debris left after two neutron stars collide?

The term was coined in 2010 because its peak brightness reaches about a thousand times that of a classical nova.

Which astronomer accidentally scooped the GW170817 announcement by tweeting 'New LIGO. Source with optical counterpart. Blow your sox off!'?

He deleted the tweet and apologised, but others quickly noticed telescopes interrupting schedules to observe NGC 4993.

The first planets ever found outside the Solar System were discovered in 1992 orbiting what kind of object?

The host, PSR B1257+12, spins hundreds of times a second; tiny shifts in its timing revealed the planets.

Which nearby X-ray-emitting neutron star, about 400 light-years away, belongs to the group nicknamed the Magnificent Seven?

Another fast-moving neighbour was named Calvera, after the villain in the 1960 film The Magnificent Seven.

Which threshold, named for three physicists and first estimated at about 0.7 solar masses, is the maximum mass a neutron star can have before collapsing into a black hole?

Their tiny first value assumed a non-interacting neutron gas; modern estimates put the ceiling nearer two solar masses.

Observations suggest PSR J0952-0607, the heaviest neutron star known, has a mass of about how many solar masses?

It is slowly consuming a very low-mass companion star.

According to one estimate, if you dropped an object from one metre above the surface of a neutron star 12 km in radius, how fast would it hit the ground?

Tidal forces would spaghettify any ordinary object into a stream of material before it even landed.

Because of gravitational time dilation, roughly how many years pass on a neutron star's surface while ten years pass on Earth?

That ignores the extra dilation from the star's very rapid spin.

A 2020 study suggested that 'mountains' on a neutron star's surface may be no taller than what?

That is hundreds of times smaller than earlier predictions, which helps explain why spinning neutron stars have not shown up in gravitational-wave searches.

A pulsar whose companion has less than a tenth of the Sun's mass is nicknamed after which creature?

Together with 'redback' pulsars, which have heavier companions, they form the family of 'spider pulsars'.

A hypothetical object formed when a neutron star merges into a red giant is named after which two physicists?

No Thorne-Żytkow object has been confirmed, though several candidates have been proposed.

A main-sequence star needs an initial mass above roughly how many solar masses to become a neutron star?

Such stars build an iron-rich core that eventually exceeds the Chandrasekhar limit and collapses.

What sits beneath a neutron star's millimetres-thin atmosphere, above its solid crust?

The ocean is only metres to tens of metres deep, and its dynamics are controlled by the star's rotation and magnetic field.

Who examined Scorpius X-1 in 1967 and correctly concluded its radiation came from an accreting neutron star?

It was the same year Bell Burnell and Hewish found the first pulsar, but the two discoveries came from X-ray and radio work respectively.

What is an 'anti-glitch' in a neutron star?

One was seen in the magnetar 1E 2259+586 alongside a 20-fold jump in X-ray brightness, behaviour current models do not predict.