News | April 27, 2021

What's Inside a ‘Dead' Star?

Matter makes up all the stuff we can see in the universe, from pencils to people to planets. But there’s still a lot we don’t understand about it! For example: How does matter work when it’s about to become a black hole? We can’t learn anything about matter after it becomes a black hole, because it’s hidden behind the event horizon, the point of no return. So we turn to something we can study – the incredibly dense matter inside a neutron star, the leftover of an exploded massive star that wasn’t quite big enough to turn into a black hole.

The camera pans from right to left to show a glowing pale blue orb representing a neutron star.
Neutron stars, like the one here in this artist’s concept, are the dense remnants of massive stars that exploded in supernovae. Matter in their cores is on the verge of collapsing into a black hole. Credit: NASA’s Goddard Space Flight Center

NICER (Neutron star Interior Composition Explorer)is an X-ray telescope perched on the International Space Station. NICER was designed to study and measure the sizes and masses of neutron stars to help us learn more about what might be going on in their mysterious cores.

This brief time-lapse of NICER (Neutron star Interior Composition Explorer) shows how it scans the skies to study pulsars and other X-ray sources from its perch aboard the International Space Station. NICER is near the center of the image, a white box mounted on a platform with a shiny panel on one side and dozens of cylindrical mirrors on the opposite side. Around it are other silver and white instruments and scaffolding. NICER swivels and pans to track objects, and some other objects nearby move as well. The station’s giant solar panels twist and turn in the background. Movement in the sequence, which represents a little more than one 90-minute orbit, is sped up by 100 times.
This time-lapse loop shows NASA’s NICER (Neutron star Interior Composition Explorer) slewing to track pulsars and other X-ray sources from atop the International Space Station. Behind it, the station’s giant solar arrays track the Sun. The motion is sped up 100 times. Credit: NASA

When a star many times the mass of our Sun runs out of fuel, it collapses under its own weight and then bursts into a supernova. What’s left behind depends on the star’s initial mass. Heavier stars (around 25 times the Sun’s mass or more) leave behind black holes. Lighter ones (between about eight and 25 times the Sun’s mass) leave behind neutron stars.

This visualization opens on a star field with a bright star at the center of the image. The star then explodes, filling the frame with white light for a moment. As that bright light fades, we see there is a bubble of yellow and red material expanding away from the site of the exploded star.
When a star that is eight times larger than the sun ends its life, it does not go gentle into that good night. Shifting pressure in its core causes it to collapse and trigger a supernova, as shown here. The initial flash of light, which can outshine the star’s host galaxy, may last only seconds. But the resulting debris that is flung into space can be studied for millennia. Credit: Courtesy of ESA/Hubble/L. Calçada

Neutron stars pack more mass than the Sun into a sphere about as wide as New York City’s Manhattan Island is long. Just one teaspoon of neutron star matter would weigh as much as Mount Everest, the highest mountain on Earth!

A satellite view shows the island of Manhattan and the surrounding area. The map is split by a blue river that widens toward the bottom of the image with smaller tributaries feeding into it. The landscape is in shades of brown near the river, with some small white structures visible. The landscape fades to greens toward the edges. To the right of Manhattan is an illustration of a neutron star. The glowing orb is mottled with shades of pale and electric blue. The neutron star is roughly as wide as Manhattan is long.
A neutron star is the densest object astronomers can observe directly, crushing half a million times Earth's mass into a sphere similar in size to Manhattan Island, as shown in this artist’s concept. Credit: NASA's Goddard Space Flight Center

These objects have a lot of cool physics going on. They can spin faster than the blades in a blender, and they have powerful magnetic fields. In fact, neutron stars are the strongest magnets in the universe! The magnetic fields can rip particles off the star’s surface and then smack them down on another part of the star. The constant bombardment creates hot spots at the magnetic poles. When the star rotates, the hot spots swing in and out of our view like the beams of a lighthouse.

This simulation shows the possible arcs of magnetic field lines from a pulsar. At the center is a gray orb representing the pulsar. Close to the pulsar are various-sized ovals of orange that pierce the pulsar, looking like a tangled knot, though the lines do not actually cross each other. There are larger blue arcs that come off the screen, representing even more magnetic field lines.
This simulation shows a possible quadrupole magnetic field configuration for a pulsar with hot spots in the only the southern hemisphere. Credit: NASA's Goddard Space Flight Center

Neutron stars are so dense that they warp nearby space-time, like a bowling ball resting on a trampoline. The warping effect is so strong that it can redirect light from the star’s far side into our view. This has the odd effect of making the star look bigger than it really is!

This animation illustrates the way strong gravity bends light from a neutron star, and that the smaller and denser the star, the more distortion. On the right is a cartoon representation of the NICER telescope. On the left is a mottled circle in shades of dark and light blue, representing the neutron star. Straight lines poke out from the surface, one points straight toward NICER from the right side of the star, then moving up the star, the lines point at increasingly higher angles up and the right, away from NICER. These lines are mirrored on the bottom of the star as well. As the animation plays, the star shrinks to represent a smaller, denser neutron star. As it shrinks, the lines bend over, curving near the surface of the neutron star, until more and more of them are visible to the NICER telescope.
NICER observes X-ray light from the surfaces of neutron stars. In these strong-gravity environments, shown in this artist’s concept, light paths are distorted so that NICER can see emission from the star's far side, especially for smaller, denser stars. Credit: NASA's Goddard Space Flight Center Conceptual Image Lab

NICER uses all the cool physics happening on and around neutron stars to learn more about what’s happening inside those stars, where matter lingers on the threshold of becoming a black hole. (We should mention that NICER also studies black holes!)

Scientists think that neutron stars are layered a bit like golf balls. At the surface, there’s a really thin (just a couple centimeters high) atmosphere of hydrogen or helium. In the outer core, atoms have broken down into their building blocks – protons, neutrons, and electrons – and the immense pressure has squished most of the protons and electrons together to form a sea of mostly neutrons.

But what’s going on in the inner core of neutron stars? Physicists have lots of theories. In some traditional models, scientists suggested the stars were neutrons all the way down. Others proposed that neutrons break down into their own, even smaller building blocks, called quarks. And then some suggest that those quarks could recombine to form new types of particles that aren’t neutrons!

This animated GIF opens on a cartoon representation of a neutron star at the center, shown in shades of dark and light blue and surrounded by a thin pale blue line. A wedge appears in the neutron star, as though cutting into a ball to reveal the interior. Inside are concentric rings of blue that are dark in the center and get successively lighter toward the exterior. The outside of the neutron star has just a thin layer of the mottled dark blue. Lines appear with text to label each ring. The outermost pale blue outline is labeled “Atmosphere, hydrogen, helium, carbon.” The thin dark blue layer is labeled “Outer crust, ions, electrons.” Inside that is a thicker layer of pale blue that is labeled “Inner crust, ions, superfluid neutrons.” Next is a thick layer of slightly darker blue that is labeled “Outer core, superconducting protons.” Finally, the dark blue ball at the center is labeled “Inner core, unknown."
This stylized animation shows the structure of a neutron star. The states of matter at neutron stars' inner cores remain a mystery. NICER will confront nuclear physics theory with unique measurements, exploring the exotic states of matter within neutron stars through rotation-resolved X-ray spectroscopy. Credit: NASA's Goddard Space Flight Center Conceptual Image Lab

NICER is helping us figure things out by measuring the sizes and masses of neutron stars. Scientists use those numbers to calculate the stars’ density, which tells us how squeezable matter is!

Let’s say you have what scientists think of as a typical neutron star, weighing about 1.4 times the Sun’s mass. If you measure the size of the star, and it’s big, then that might mean it contains more whole neutrons. If instead it’s small, then that might mean the neutrons have broken down into quarks. The tinier pieces can be packed together more tightly.

This animation shows how particles that break down into smaller pieces can take up a smaller volume. The animation opens with a large circle that is filled with blue balls. The balls then change so that each one becomes three smaller red circles. These red circles have room between them. They then collapse down, moving closer together, until they are closely packed. At the same time, the circle shrinks.
This animation shows how neutron stars with the same mass could be larger or smaller depending on the state of the particles inside. If the star is made of neutrons, the star will be larger. But if the same mass was instead made of smaller pieces, like quarks, then they can be packed into a tighter region. Credit: NASA’s Goddard Space Flight Center

NICER measured the sizes of two neutron stars, called PSR J0030+0451 and PSR J0740+6620, or J0030 and J0740 for short.

J0030 is about 1.4 times the Sun’s mass and 16 miles across. (It also taught us that neutron star hot spots might not always be where we thought.) J0740 is about 2.1 times the Sun’s mass and is also about 16 miles across. So J0740 has about 50% more mass than J0030 but is about the same size! This tells us that the matter in neutron stars is less squeezable than some scientists predicted. (Remember, some physicists suggest that the added mass would crush all the neutrons and make a smaller star.) And J0740’s mass and size together challenge models where the star is neutrons all the way down.

In this animation, the surface of neutron star J07040 is represented by a blue wireframe sphere. The wireframe lines run around the sphere creating crisscrossing latitude and longitudinal lines. The sphere is tilted with the top just to the right of center. Two pink spots come into and out of view as the sphere turns. One is near the star’s equator, the other is in the southern hemisphere.
This model shows J0740, a neutron star that is about 1.4 times the mass of the Sun. Researchers found that it has two circular hot spots almost directly opposite each other. Credit: NASA’s Goddard Space Flight Center

So what’s in the heart of a neutron star? We’re still not sure. Scientists will have to use NICER’s observations to develop new models, perhaps where the cores of neutron stars contain a mix of both neutrons and weirder matter, like quarks. We’ll have to keep measuring neutron stars to learn more!

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