Matter in the hearts of neutron stars – dense remnants of exploded massive stars – takes the most extreme form we can measure. Now, thanks to data from NASA’s Neutron star Interior Composition Explorer (NICER), an X-ray telescope on the International Space Station, scientists have discovered that this mysterious matter is less squeezable than some physicists predicted.

The finding is based on NICER’s observations of PSR J0740+6620 (J0740 for short), the most massive known neutron star, which lies over 3,600 light-years away in the northern constellation Camelopardalis. Previous observations place the neutron star’s mass at about 2.1 times the Sun’s.

At a neutron star’s surface, an atmosphere of hydrogen or helium rests on an iron crust. A mile or so down is the outer core, where atoms brake down into their building blocks: neutrons, protons, and electrons. Here, the immense pressure has crushed together protons and electrons to form a sea of mostly neutrons – packed together at up to twice the density of an atomic nucleus.

But what form does matter take in the inner core? Is it neutrons all the way down, or do the neutrons break into their own component parts, called quarks?

In traditional models of a typical neutron star, one with about 1.4 times the Sun’s mass, physicists expect the inner core to be mostly filled with neutrons. The lower density ensures that neutrons remain far enough apart to stay intact, and this inner stiffness results in a larger star.

In more massive neutron stars like J0740, the inner core’s density is much higher, crushing the neutrons closer together. It’s unclear whether neutrons can remain intact under these conditions or if they instead break down into their constituent parts, called quarks. Theorists suspect they shatter under the pressure, but many questions about the details remain. To get answers, scientists need a precise size measurement for a massive neutron star. A smaller star would favor scenarios where quarks roam freely at the innermost depths because the tinier particles can be packed more closely. A larger star would suggest the presence of more complex forms of matter.

Two teams used different approaches to model J0740’s size, getting results of around 15.4 miles (24.8 kilometers) and 17 miles (27.4 kilometers) across. The two results overlap significantly within their uncertainties, ranging from 14.2 to 17 miles (22.8 to 27.4 kilometers) and 15.2 to 20 miles (24.4 to 32.6 kilometers), respectively.

The J0740 result, combined with a previous NICER measurement of pulsar J0030+0451 and other multimessenger observations disfavor more squeezable models of neutron stars, including versions where the interior is a sea of quarks. J0740’s mass and size also pose problems for less squeezable models, which tend to be very neutron-rich.

Recent theoretical models propose some alternatives, such as inner cores containing a mix of neutrons, protons, and exotic matter made of quarks or new combinations of quarks. Future observations will help physicists narrow the possibilities even further.

Scientists using data from NASA’s Neutron star Interior Composition Explorer (NICER) telescope on the International Space Station have discovered X-ray surges accompanying radio bursts from the pulsar in the Crab Nebula. The finding shows that these bursts, called giant radio pulses, release far more energy than previously suspected. A pulsar is a type of rapidly spinning neutron star, the crushed, city-sized core of a star that exploded as a supernova. A young, isolated neutron star can spin dozens of times each second, and its whirling magnetic field powers beams of radio waves, visible light, X-rays, and gamma rays. If these beams sweep past Earth, astronomers observe clock-like pulses of emission and classify the object as a pulsar. Located about 6,500 light-years away in the constellation Taurus, the Crab Nebula and its pulsar formed in a supernova explosion. The neutron star spins 30 times each second, and at X-ray and radio wavelengths it is among the brightest pulsars in the sky. Out of more than 2,800 pulsars cataloged, the Crab pulsar is one of only a few that emit giant radio pulses, which occur sporadically and can be hundreds to thousands of times brighter than the regular pulses. And after decades of observations, only the Crab has been shown to enhance its giant radio pulses with emission from other parts of the spectrum. Previously seen in visible light, these enhancements now have been detected in X-rays for the first time. Between August 2017 and August 2019, researchers used NICER to repeatedly observe the Crab pulsar in X-rays. While NICER was watching, the team also studied the object using at least one of two ground-based radio telescopes in Japan. The team combined all of the X-ray data that coincided with giant radio pulses, revealing an X-ray boost of about 4% that occurred in synch with them. It’s remarkably similar to the 3% rise in visible light associated with the phenomenon, discovered in 2003. The researchers say that the total emitted energy associated with a giant pulse is dozens to hundreds of times higher than previously estimated from the radio and optical data alone. The enhancements suggest that giant pulses are a manifestation of underlying processes that produce emission spanning the electromagnetic spectrum, from radio to X-rays. And because X-rays pack millions of times the punch of radio waves, even a modest increase represents a large energy contribution. The researchers conclude that the total emitted energy associated with a giant pulse is dozens to hundreds of times higher than previously estimated from the radio and optical data alone.

Scientists have reached a new frontier in our understanding of pulsars, the dense, whirling remains of exploded stars, thanks to observations from NASA’s Neutron star Interior Composition Explorer (NICER). Data from this X-ray telescope aboard the International Space Station has produced the first precise and dependable measurements of both a pulsar’s size and its mass. The pulsar in question, J0030+0451 (J0030 for short), is a solitary pulsar that lies 1,100 light-years away in the constellation Pisces. While measuring the pulsar's heft and proportions, NICER revealed that the shapes and locations of million-degree hot spots on the pulsar’s surface are much stranger than generally thought. Using NICER observations from July 2017 to December 2018, two groups of scientists mapped J0030’s hot spots using independent methods and converged on nearly identical results for its mass and size. One team, led by researchers at the University of Amsterdam, determined the pulsar is around 1.3 times the Sun’s mass, 15.8 miles (25.4 kilometers) across and has two hot spots — one small and circular, the other long and crescent-shaped. A second team found J0030 is about 1.4 times the Sun’s mass, about 16.2 miles (26 kilometers) wide and has two or three oval-shaped hot spots. All spots in all models are in the pulsar’s southern hemisphere — unlike textbook images where the spots lie on opposite sides other at each magnetic poles.

At about 10:04 p.m. EDT on Aug. 20, NASA’s Neutron star Interior Composition Explorer (NICER) telescope on the International Space Station detected a sudden spike of X-rays caused by a massive thermonuclear flash on the surface of a pulsar, the crushed remains of a star that long ago exploded as a supernova. The X-ray burst, the brightest seen by NICER so far, came from an object named SAX J1808.4-3658, or J1808 for short. The observations reveal many phenomena that have never been seen together in a single burst. In addition, the subsiding fireball briefly brightened again for reasons astronomers cannot yet explain. The data reveal a two-step change in brightness, which scientists think is caused by the ejection of separate layers from the pulsar surface, and other features that will help them decode the physics of these powerful events. The explosion, which astronomers classify as a Type I X-ray burst, released as much energy in 20 seconds as the Sun does in nearly 10 days. J1808 is located about 11,000 light-years away in the constellation Sagittarius, spins at a dizzying 401 rotations each second, and is one member of a binary system. Its companion is a brown dwarf, an object larger than a giant planet yet too small to be a star. A steady stream of hydrogen gas flows from the companion toward the neutron star, and it accumulates in a vast storage structure called an accretion disk. Hydrogen raining onto the pulsar's surface forms a hot, ever-deepening global “sea.” At the base of this layer, temperatures and pressures increase until hydrogen nuclei fuse to form helium nuclei, which produces energy — a process at work in the core of our Sun. The helium settles out and builds up a layer of its own. Eventually, the conditions allow helium nuclei to fuse into carbon. The helium erupts explosively and unleashes a thermonuclear fireball across the entire pulsar surface. As the burst started, NICER data show that its X-ray brightness leveled off for almost a second before increasing again at a slower pace. The researchers interpret this “stall” as the moment when the energy of the blast built up enough to blow the pulsar’s hydrogen layer into space. The fireball continued to build for another two seconds and then reached its peak, blowing off the more massive helium layer. The helium expanded faster, overtook the hydrogen layer before it could dissipate, and then slowed, stopped and settled back down onto the pulsar’s surface. Following this phase, the pulsar briefly brightened again by roughly 20 percent for reasons the team does not yet understand.

Scientists have mapped the environment surrounding a black hole that is 10 times the mass of the Sun using NASA’s Neutron star Interior Composition Explorer (NICER) payload aboard the International Space Station. NICER detected X-ray light from a recently discovered black hole, called MAXI J1820+070 (J1820 for short), as it consumed material from a companion star. Waves of X-rays formed "light echoes" that reflected off the swirling gas near the black hole and revealed changes in the environment’s size and shape. A black hole can siphon gas from a nearby star and into a ring of material called an accretion disk that glows in X-rays. Above this disk is the corona, a region of subatomic particles that glows in higher-energy X-rays. Astrophysicists want to better understand how the inner edge of the accretion disk and the corona change in size and shape as a black hole accretes material from its companion star. If they can understand how and why these changes occur in stellar-mass black holes over a period of weeks, they could shed light on how supermassive black holes evolve over millions of years and how they affect the galaxies in which they reside. One method used to chart those changes is called X-ray reverberation mapping, which uses X-ray reflections in much the same way sonar uses sound waves to map undersea terrain. From 10,000 light-years away, the scientists estimated that the corona contracted vertically from roughly 100 to 10 miles — that’s like seeing something the size of a blueberry shrink to something the size of a poppy seed at the distance of Pluto.

This time-lapse video, obtained June 8, 2018, shows the precise choreography of NASA’s Neutron star Interior Composition Explorer (NICER) as it studies pulsars and other X-ray sources from its perch aboard the International Space Station. NICER observes and tracks numerous sources each day, ranging from the star closest to the Sun, Proxima Centauri, to X-ray sources in other galaxies. Movement in the movie, which represents a little more than one 90-minute orbit, is sped up by 100 times. One factor in NICER’s gyrations is the motion of the space station’s solar arrays, each of which extends 112 feet (34 meters). Long before the panels can encroach on NICER’s field of view, the instrument pirouettes to aim its 56 X-ray telescopes at a new celestial target. As the movie opens, the station’s solar arrays are parked to prepare for the arrival and docking of the Soyuz MS-09 flight, which launched on June 6 carrying three members of the Expedition 56 crew. Then the panels reorient themselves and begin their normal tracking of the Sun. Neutron stars, also called pulsars, are the crushed cores left behind when massive stars explode. They hold more mass than the Sun in a ball no bigger than a city. NICER aims to discover more about pulsars by obtaining precise measures of their size, which will determine their internal make-up. An embedded technology demonstration, called Station Explorer for X-ray Timing and Navigation Technology (SEXTANT), is paving the way for using pulsars as beacons for a future GPS-like system to aid spacecraft navigation in the solar system — and beyond.

Scientists analyzing the first data from the Neutron Star Interior Composition Explorer (NICER) mission have found two stars that revolve around each other every 38 minutes. One of the stars in the system, called IGR J17062–6143 (J17062 for short), is a rapidly spinning, superdense star called a pulsar. The discovery bestows the stellar pair with the record for the shortest-known orbital period for a certain class of pulsar binary system. The data from NICER also show J17062’s stars are only about 186,000 miles (300,000 kilometers) apart, less than the distance between Earth and the Moon. Based on the pair’s breakneck orbital period and separation, scientists involved in a new study of the system think the second star is a hydrogen-poor white dwarf. The researchers were also able to determine that J17062’s stars revolve around each other in a circular orbit, which is common for this type of system. The white dwarf donor star is a “lightweight,” only around 1.5 percent of our Sun’s mass. The pulsar is much heavier, around 1.4 solar masses. The stars orbit a point around 1,900 miles (3,000 km) from the pulsar, almost as if the donor star orbits a stationary neutron star, but NICER can is sensitive enough to detect a slight fluctuation in the neutron star’s X-ray emission due to the tug from the donor star.

The Neutron Star Interior Composition Explorer (NICER) payload, destined for the exterior of the space station, will study the physics of neutron stars, providing new insight into their nature and behavior. These stars are called “pulsars” because of the unique way they emit light – in a beam similar to a lighthouse beacon. As the star spins, the light sweeps past us, making it appear as if the star is pulsing. Neutron stars emit X-ray radiation, enabling the NICER technology to observe and record information about their structure, dynamics and energetics. The payload also includes a technology demonstration called the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) which will help researchers to develop a pulsar-based space navigation system. Pulsar navigation could work similarly to GPS on Earth, providing precise position and time for spacecraft throughout the solar system.

The 2-in-1 mission launched on June 3, 2017 aboard SpaceX's eleventh contracted cargo resupply mission with NASA to the International Space Station. The payload arrived at the space station in the Dragon spacecraft, along with other cargo, on June 5, 2017.

Here's just some of what we already know about neutron stars. An upcoming NASA mission will further investigate these unusual objects from the International Space Station. The Neutron star Interior Composition Explorer mission, or NICER, will study the extraordinary environments — strong gravity, ultra-dense matter, and the most powerful magnetic fields in the universe — embodied by neutron stars. NICER is a two-in-one mission. The embedded Station Explorer for X-ray Timing and Navigation Technology, or SEXTANT, demonstration will use NICER data to validate, for the first time in space, pulsar-based navigation.

NICER is planned for launch aboard the SpaceX CRS-11, currently scheduled for June 1, 2017. Learn more about the mission at nasa.gov/nicer.

This video previews the Neutron star Interior Composition Explorer (NICER). NICER is an Astrophysics Mission of Opportunity within NASA’s Explorer program, which provides frequent flight opportunities for world-class scientific investigations from space utilizing innovative, streamlined and efficient management approaches within the heliophysics and astrophysics science areas. NASA’s Space Technology Mission Directorate supports the SEXTANT component of the mission, demonstrating pulsar-based spacecraft navigation. NICER is an upcoming International Space Station payload scheduled to launch in June 2017.

Learn more about the mission at nasa.gov/nicer.

Several cameras on the International Space Station (ISS) have eyes on NICER. Since arriving to the space station on June 5 – aboard SpaceX’s eleventh cargo resupply mission – NICER underwent robotic installation on ExPRESS Logistics Carrier 2, initial deployment, precise point tests and more. This video shows segments of NICER’s time in space. Scientists and engineers will continue to watch NICER, using these cameras, throughout the mission’s science operations.

The Neutron star Interior Composition Explorer (NICER) payload undergoes electromagnetic testing at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Electromagnetic testing serves to verify that NICER’s electrical subsystems do not interfere with each other or with International Space Station electrical systems through, for example, conducted or transmitted emissions. This test also verifies that NICER is not susceptible to malfunction due to the electromagnetic environment of the space station. Two time-lapse videos show the NICER payload deploy during electromagnetic testing and return to its stowed configuration following the tests.

Time-lapse of NICER's box-shaped X-ray Timing Instrument (XTI), with attached flight electronics and the payload's pointing system, being lifted in a clean-tent at NASA's Goddard Space Flight Center. The XTI was lifted and positioned onto the flight Adapter Plate, NICER's interface to the International Space Station-provided hardware for installation on station.

Time-lapse of the Neutron star Interior Composition Explorer (NICER) range of motion test was taken on April 11, 2016, at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The Neutron star Interior Composition Explorer (NICER) mission was built and tested at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

In addition to NASA Goddard scientists and engineers, the mission team includes the Massachusetts Institute of Technology and commercial partners, who provided spaceflight hardware.

Imagine a technology that would allow space travelers to transmit gigabytes of data per second over interplanetary distances or to navigate to Mars and beyond using powerful beams of light emanating from rotating neutron stars. The concept isn't farfetched.

In fact, Goddard astrophysicists Keith Gendreau and Zaven Arzoumanian plan to fly a multi-purpose instrument on the International Space Station to demonstrate the viability of two groundbreaking navigation and communication technologies and, from the same platform, gather scientific data revealing the physics of dense matter in neutron stars.

Two views of a Neutron Star: First, a closeup view of a neutron star cycling before, during and after a gamma ray burst and second, crossing a Protoplanetary Nebula toward an elusive Neutron Star

A pulsar is a neutron star which emits beams of radiation that sweep through the earth's line of sight. Like a black hole, it is an endpoint to stellar evolution. The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis. Pulsars pulse because the rotation of the neutron star causes the radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period. External viewers see pulses of radiation whenever this region above the the magnetic pole is visible. Because of the rotation of the pulsar, the pulses thus appear much as a distant observer sees a lighthouse appear to blink as its beam rotates. The pulses come at the same rate as the rotation of the neutron star, and, thus, appear periodic.

A pulsar is a rapidly spinning and highly magnetized neutron star, the crushed core left behind when a massive sun explodes. Most were found through their pulses at radio wavelengths, which are thought to be caused by narrow, lighthouse-like beams emanating from the star's magnetic poles.

When it comes to gamma-rays, pulsars are no longer lighthouses. A new class of gamma-ray-only pulsars shows that the gamma rays must form in a broader region than the lighthouse-like radio beam. Astronomers now believe the pulsed gamma rays arise far above the neutron star.

This animation takes us into a spinning pulsar, with its strong magnetic field rotating along with it. Clouds of charged particles move along the field lines and their gamma-rays are beamed like a lighthouse beacon by the magnetic fields. As our line of sight moves into the beam, we see the pulsations once every rotation of the neutron star.