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.

Our Neutron star Interior Composition Explorer, or NICER, 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.

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.

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!

These objects have a lot of cool physics going on. They can spin faster than blender blades, 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.

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!

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

Scientists think neutron stars are layered a bit like a golf ball. 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? 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 building blocks, called quarks. And then some suggest that those quarks could recombine to form new types of particles that aren’t neutrons!

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, one 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.

NICER has now 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! Which 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.

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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Neutron Stars Are Even Weirder Than We Thought

Let’s face it, it’s hard for rapidly-spinning, crushed cores of dead stars NOT to be weird. But we’re only beginning to understand how truly bizarre these objects — called neutron stars — are.

Neutron stars are the collapsed remains of massive stars that exploded as supernovae. In each explosion, the outer layers of the star are ejected into their surroundings. At the same time, the core collapses, smooshing more than the mass of our Sun into a sphere about as big as the island of Manhattan.

Our Neutron star Interior Composition Explorer (NICER) telescope on the International Space Station is working to discover the nature of neutron stars by studying a specific type, called pulsars. Some recent results from NICER are showing that we might have to update how we think about pulsars!

Here are some things we think we know about neutron stars:

Pulsars are rapidly spinning neutron stars ✔︎

Pulsars get their name because they emit beams of light that we see as flashes. Those beams sweep in and out of our view as the star rotates, like the rays from a lighthouse.

Pulsars can spin ludicrously fast. The fastest known pulsar spins 43,000 times every minute. That’s as fast as blender blades! Our Sun is a bit of a slowpoke compared to that — it takes about a month to spin around once.

The beams come from the poles of their strong magnetic fields ✔︎

Pulsars also have magnetic fields, like the Earth and Sun. But like everything else with pulsars, theirs are super-strength. The magnetic field on a typical pulsar is billions to trillions of times stronger than Earth’s!

Near the magnetic poles, the pulsar’s powerful magnetic field rips charged particles from its surface. Some of these particles follow the magnetic field. They then return to strike the pulsar, heating the surface and causing some of the sweeping beams we see.

The beams come from two hot spots… ❌❓✔︎ 🤷🏽

Think of the Earth’s magnetic field — there are two poles, the North Pole and the South Pole. That’s standard for a magnetic field.

On a pulsar, the spinning magnetic field attracts charged particles to the two poles. That means there should be two hot spots, one at the pulsar’s north magnetic pole and the other at its south magnetic pole.

This is where things start to get weird. Two groups mapped a pulsar, known as J0030, using NICER data. One group found that there were two hot spots, as we might have expected. The other group, though, found that their model worked a little better with three (3!) hot spots. Not two.

… that are circular … ❌❓✔︎ 🤷🏽

The particles that cause the hot spots follow the magnetic field lines to the surface. This means they are concentrated at each of the magnetic poles. We expect the magnetic field to appear nearly the same in any direction when viewed from one of the poles. Such symmetry would produce circular hot spots.

In mapping J0030, one group found that one of the hot spots was circular, as expected. But the second spot may be a crescent. The second team found its three spots worked best as ovals.

… and lie directly across from each other on the pulsar ❌❓✔︎ 🤷🏽

Think back to Earth’s magnetic field again. The two poles are on opposite sides of the Earth from each other. When astronomers first modeled pulsar magnetic fields, they made them similar to Earth’s. That is, the magnetic poles would lie at opposite sides of the pulsar.

Since the hot spots happen where the magnetic poles cross the surface of the pulsar, we would expect the beams of light to come from opposite sides of the pulsar.

But, when those groups mapped J0030, they found another surprising characteristic of the spots. All of the hot spots appear in the southern half of the pulsar, whether there were two or three of them.

This also means that the pulsar’s magnetic field is more complicated than our initial models!

J0030 is the first pulsar where we’ve mapped details of the heated regions on its surface. Will others have similarly bizarre-looking hotspots? Will they bring even more surprises? We’ll have to stay tuned to NICER find out!

And check out the video below for more about how this measurement was done.

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Black Holes are NICER Than You Think!

We’re learning more every day about black holes thanks to one of the instruments aboard the International Space Station! Our Neutron star Interior Composition Explorer (NICER) instrument is keeping an eye on some of the most mysterious cosmic phenomena.

We’re going to talk about some of the amazing new things NICER is showing us about black holes. But first, let’s talk about black holes — how do they work, and where do they come from? There are two important types of black holes we’ll talk about here: stellar and supermassive. Stellar mass black holes are three to dozens of times as massive as our Sun while supermassive black holes can be billions of times as massive!

Stellar black holes begin with a bang — literally! They are one of the possible objects left over after a large star dies in a supernova explosion. Scientists think there are as many as a billion stellar mass black holes in our Milky Way galaxy alone!

Supermassive black holes have remained rather mysterious in comparison. Data suggest that supermassive black holes could be created when multiple black holes merge and make a bigger one. Or that these black holes formed during the early stages of galaxy formation, born when massive clouds of gas collapsed billions of years ago. There is very strong evidence that a supermassive black hole lies at the center of all large galaxies, as in our Milky Way.

Imagine an object 10 times more massive than the Sun squeezed into a sphere approximately the diameter of New York City — or cramming a billion trillion people into a car! These two examples give a sense of how incredibly compact and dense black holes can be.

Because so much stuff is squished into such a relatively small volume, a black hole’s gravity is strong enough that nothing — not even light — can escape from it. But if light can’t escape a dark fate when it encounters a black hole, how can we “see” black holes?

Scientists can’t observe black holes directly, because light can’t escape to bring us information about what’s going on inside them. Instead, they detect the presence of black holes indirectly — by looking for their effects on the cosmic objects around them. We see stars orbiting something massive but invisible to our telescopes, or even disappearing entirely!

When a star approaches a black hole’s event horizon — the point of no return — it’s torn apart. A technical term for this is “spaghettification” — we’re not kidding! Cosmic objects that go through the process of spaghettification become vertically stretched and horizontally compressed into thin, long shapes like noodles.

Scientists can also look for accretion disks when searching for black holes. These disks are relatively flat sheets of gas and dust that surround a cosmic object such as a star or black hole. The material in the disk swirls around and around, until it falls into the black hole. And because of the friction created by the constant movement, the material becomes super hot and emits light, including X-rays.  

At last — light! Different wavelengths of light coming from accretion disks are something we can see with our instruments. This reveals important information about black holes, even though we can’t see them directly.

So what has NICER helped us learn about black holes? One of the objects this instrument has studied during its time aboard the International Space Station is the ever-so-forgettably-named black hole GRS 1915+105, which lies nearly 36,000 light-years — or 200 million billion miles — away, in the direction of the constellation Aquila.

Scientists have found disk winds — fast streams of gas created by heat or pressure — near this black hole. Disk winds are pretty peculiar, and we still have a lot of questions about them. Where do they come from? And do they change the shape of the accretion disk?

It’s been difficult to answer these questions, but NICER is more sensitive than previous missions designed to return similar science data. Plus NICER often looks at GRS 1915+105 so it can see changes over time.

NICER’s observations of GRS 1915+105 have provided astronomers a prime example of disk wind patterns, allowing scientists to construct models that can help us better understand how accretion disks and their outflows around black holes work.

NICER has also collected data on a stellar mass black hole with another long name — MAXI J1535-571 (we can call it J1535 for short) — adding to information provided by NuSTAR, Chandra, and MAXI. Even though these are all X-ray detectors, their observations tell us something slightly different about J1535, complementing each other’s data!

This rapidly spinning black hole is part of a binary system, slurping material off its partner, a star. A thin halo of hot gas above the disk illuminates the accretion disk and causes it to glow in X-ray light, which reveals still more information about the shape, temperature, and even the chemical content of the disk. And it turns out that J1535’s disk may be warped!

This isn’t the first time we have seen evidence for a warped disk, but J1535’s disk can help us learn more about stellar black holes in binary systems, such as how they feed off their companions and how the accretion disks around black holes are structured.

NICER primarily studies neutron stars — it’s in the name! These are lighter-weight relatives of black holes that can be formed when stars explode. But NICER is also changing what we know about many types of X-ray sources. Thanks to NICER’s efforts, we are one step closer to a complete picture of black holes. And hey, that’s pretty nice!

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Five Famous Pulsars from the Past 50 Years

Early astronomers faced an obstacle: their technology. These great minds only had access to telescopes that revealed celestial bodies shining in visible light. Later, with the development of new detectors, scientists opened their eyes to other types of light like radio waves and X-rays. They realized cosmic objects look very different when viewed in these additional wavelengths. Pulsars — rapidly spinning stellar corpses that appear to pulse at us — are a perfect example.

The first pulsar was observed 50 years ago on August 6, 1967, using radio waves, but since then we have studied them in nearly all wavelengths of light, including X-rays and gamma rays.

Typical Pulsar

Most pulsars form when a star — between 8 and 20 times the mass of our sun — runs out of fuel and its core collapses into a super dense and compact object: a neutron star. 

These neutron stars are about the size of a city and can rotate slowly or quite quickly, spinning anywhere from once every few hours to hundreds of times per second. As they whirl, they emit beams of light that appear to blink at us from space.

First Pulsar

One day five decades ago, a graduate student at the University of Cambridge, England, named Jocelyn Bell was poring over the data from her radio telescope - 120 meters of paper recordings.

She noticed some unusual markings, which she called “scruff,” indicating a mysterious object (simulated above) that flashed without fail every 1.33730 seconds. This was the very first pulsar discovered, known today as PSR B1919+21.

Best Known Pulsar

Before long, we realized pulsars were far more complicated than first meets the eye — they produce many kinds of light, not only radio waves. Take our galaxy’s Crab Nebula, just 6,500 light years away and somewhat of a local celebrity. It formed after a supernova explosion, which crushed the parent star's core into a neutron star. 

The resulting pulsar, nestled inside the nebula that resulted from the supernova explosion, is among the most well-studied objects in our cosmos. It’s pictured above in X-ray light, but it shines across almost the entire electromagnetic spectrum, from radio waves to gamma rays.

Brightest Gamma-ray Pulsar

Speaking of gamma rays, in 2015 our Fermi Gamma-ray Space Telescope discovered the first pulsar beyond our own galaxy capable of producing such high-energy emissions. 

Located in the Tarantula Nebula 163,000 light-years away, PSR J0540-6919 gleams nearly 20 times brighter in gamma-rays than the pulsar embedded in the Crab Nebula.

Dual Personality Pulsar

No two pulsars are exactly alike, and in 2013 an especially fast-spinning one had an identity crisis. A fleet of orbiting X-ray telescopes, including our Swift and Chandra observatories, caught IGR J18245-2452 as it alternated between generating X-rays and radio waves. 

Scientists suspect these radical changes could be due to the rise and fall of gas streaming onto the pulsar from its companion star.

Transformer Pulsar

This just goes to show that pulsars are easily influenced by their surroundings. That same year, our Fermi Gamma Ray Space Telescope uncovered another pulsar, PSR J1023+0038, in the act of a major transformation — also under the influence of its nearby companion star. 

The radio beacon disappeared and the pulsar brightened fivefold in gamma rays, as if someone had flipped a switch to increase the energy of the system. 

NICER Mission

Our Neutron star Interior Composition Explorer (NICER) mission, launched this past June, will study pulsars like those above using X-ray measurements.

With NICER’s help, scientists will be able to gaze even deeper into the cores of these dense and mysterious entities.

For more information about NICER, visit https://www.nasa.gov/nicer

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Neutron Stars Are Weird!

There, we came right out and said it. They can’t help it; it’s just what happens when you have a star that’s heavier than our sun but as small as a city. Neutron stars give us access to crazy conditions that we can’t study directly on Earth.

Here are five facts about neutron stars that show sometimes they are stranger than science fiction!

1. Neutron stars start their lives with a bang

When a star bigger and more massive than our sun runs out of fuel at the end of its life, its core collapses while the outer layers are blown off in a supernova explosion. What is left behind depends on the mass of the original star. If it’s roughly 7 to 19 times the mass of our sun, we are left with a neutron star. If it started with more than 20 times the mass of our sun, it becomes a black hole.

2. Neutron stars contain the densest material that we can directly observe

While neutron stars’ dark cousins, black holes, might get all the attention, neutron stars are actually the densest material that we can directly observe. Black holes are hidden by their event horizon, so we can’t see what’s going on inside. However, neutron stars don’t have such shielding. To get an idea of how dense they are, one sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth—about as much as a mountain. That is what happens when you cram a star with up to twice the mass of our sun into a sphere the diameter of a city.

3. Neutron stars can spin as fast as blender blades

Some neutron stars, called pulsars, emit streams of light that we see as flashes because the beams of light sweep in and out of our vision as the star rotates. The fastest known pulsar, named PSR J1748-2446ad, spins 43,000 times every minute. That’s twice as fast as the typical household blender! Over weeks, months or longer, pulsars pulse with more accuracy than an atomic clock, which excites astronomers about the possible applications of measuring the timing of these pulses.

4. Neutron stars are the strongest known magnets

Like many objects in space, including Earth, neutron stars have a magnetic field. While all known neutron stars have magnetic fields billions and trillions of times stronger than Earth’s, a type of neutron star known as a magnetar can have a magnetic field another thousand times stronger. These intense magnetic forces can cause starquakes on the surface of a magnetar, rupturing the star’s crust and producing brilliant flashes of gamma rays so powerful that they have been known to travel thousands of light-years across our Milky Way galaxy, causing measurable changes to Earth’s upper atmosphere.

5. Neutron stars’ pulses were originally thought to be possible alien signals

Beep. Beep. Beep. The discovery of pulsars began with a mystery in 1967 when astronomers picked up very regular radio flashes but couldn’t figure out what was causing them. The early researchers toyed briefly with the idea that it could be a signal from an alien civilization, an explanation that was discarded but lingered in their nickname for the original object—LGM-1, a nod to the “little green men” (it was later renamed PSR B1919+21). Of course, now scientists understand that pulsars are spinning neutron stars sending out light across a broad range of wavelengths that we detect as very regular pulses – but the first detections threw observers for a loop.

The Neutron star Interior Composition Explorer (NICER) payload that is soon heading to the International Space Station will give astronomers more insight into neutron stars—helping us determine what is under the surface. Also, onboard NICER, the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) experiment will test the use of pulsars as navigation beacons in space.

Want to learn even more about Neutron Stars? Watch this...

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