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When a massive star dies, only two things might possibly remain — and both are among the most bizarre and fascinating objects in the universe.
If the dying star is large enough, its core collapses under its own gravity into a singularity, or black hole — an infinitely dense geometric point in space where the standard laws of physics break down.
The other possible remnant of a dying giant? It's called a neutron star, and it's extreme in just about every way.
Though the word "star" might suggest something big, bright, and gaseous, neutron stars are small, compact balls of exotic matter. These ancient star cores spin blindingly fast, glowing in X-rays rather than visible light, at temperatures 100 times hotter than the solar surface. And though they average just 12 miles in diameter, they've got as much mass as half a million Earths — almost twice as much as our sun.
In other words, neutron stars are staggeringly dense. A single sugar-cube–sized piece of neutron star matter ("neutronium") would weigh nearly a billion tons. That's as much as Mount Everest, or all the cars in the United States. If you were to somehow extract a pinch of neutronium and return with it to Earth, it would be so heavy that it would fall through the entire planet.
Because neutron stars are so massive and so small, their surface gravity is a hundred billion times stronger than the Earth's. If you were to drop an object — say, an eggplant — from three feet above the surface of a neutron star, it would accelerate to 1,250 miles per second in the microsecond before the collision. Then, the eggplant would explode with more force than an atomic bomb, shattering its atoms' nuclei and merging with the neutron orb.
This intense gravity makes neutron stars some of smoothest and most perfectly spherical objects in the universe. The outer crust is made of atomic nuclei packed into a crystalline lattice that's 10 billion times stronger than steel. Gravitational pressure prevents irregularities, so surface bumps rarely exceed a quarter inch.
Within the crust, atomic nuclei are morphed into strange, pasta-like shapes, like strands, tubes, and sheets. Deeper in, the meat of the star is nothing but ultradense neutrons. Scientists aren't totally sure what's at the core, but they think it's a frictionless superfluid made of more elementary particles called quarks and gluons. Quark-gluon plasma is the same primordial soup of particles that existed a millionth of a second after the Big Bang.
Now, this weird, degenerate matter can only form under the extreme heat and pressure inside a giant collapsing star. As the dying star burns through the last of its fuel, the inward force of gravity begins to overpower the outflow of radiation, compressing its core into denser and denser states. Atoms are crushed, their electrons fusing with their protons to form neutrons. And unlike atoms, which are 99.9 percent empty space, pure neutrons can be squeezed by gravity until every particle is flush.
Ultimately, the dying star's heart implodes, shrinking from more than 5,000 miles across to just a dozen in a thousandth of a second. The collapse is catastrophic: Outer layers rush inward at one-fourth the speed of light, rebounding off the hardened neutron core. The shockwave generated ejects octillions of tons of superheated matter in a titanic explosion called a supernova, outshining all the hundreds of billions of other stars in the host galaxy for weeks at a time.
Supernovae are lopsided explosions, and they send newly formed neutron stars careening across space at mind-numbing speeds. These cosmic cannonballs have been clocked moving at more than three million miles per hour, or nearly 1,000 miles per second.
As they hurtle through the void, runaway neutron stars pile up the rarefied interstellar gas that's in their way like the snow in front of a snowplow. The stars' intense electromagnetic fields heat the gas, forming beautiful fiery wakes called bow shocks. Some hypervelocity neutron stars trail comet-like jets of material spanning dozens of light-years.
Neutron stars are also born spinning rapidly. They conserve their progenitor stars' angular momentum, but since they're so much smaller, they spin much, much faster. Think of a figure skater tucking in her arms as she pirouettes. Many neutron stars spin more than once a millisecond, and the fastest ever discovered spins 716 times per second. At its equator, it's rotating at more than 40,000 miles per second.
In 1967, astronomers detected regular pulses of radio waves coming from across the galaxy. Some thought it might be a beacon from a distant alien civilization. What they found instead was a pulsar, or pulsating radio star. These are highly magnetized neutron stars that beam narrow cones of radio waves from their magnetic poles as they spin. The frequency of pulses from each of these cosmic lighthouses is unique, and it remains perfectly steady over eons.
Before NASA launched the Pioneer spacecraft into interstellar space, they had Carl Sagan design a golden plaque inscribed with a message for any extraterrestrials that might find it. The plaque featured a map of our sun's place within the immensity of the Milky Way, which it indicated in relation to 14 pulsars. Should any aliens find our message in a bottle, they can triangulate our location using those pulsars.
There's another subtype of neutron star — magnetars — of which only two dozen have been identified. As their name suggests, magnetars boast the strongest magnetic fields in the universe — around a quadrillion times stronger than the field surrounding Earth. But strong magnets are one thing. Magnetars are monsters. If a rogue magnetar were to fly past us, it would lift metal objects off the surface of the Earth from 100,000 miles away. If it came within 10,000 miles, its magnetism would kill us all by stopping the electrical nerve impulses that make our hearts beat. And if it grazed us at 1,000 miles, it would rip all of the iron out of our bloodstreams (though we'd already be dead anyway). Thankfully, the closest magnetar is 15,000 light-years away.
But that doesn't mean they're harmless. Because the outer shells of neutron stars are so rigid, they can crack under the stresses caused by magnetars' intense interior force fields. These ruptures are called starquakes, and they're similar to earthquakes in that there's a sudden and violent seismic reconfiguration of the crust. But starquakes shift the crust just a fraction of an inch, triggering an enormous magnetically powered explosion. The luminous bursts of radiation discharged by these quakes can be seen across the galaxy.
The biggest starquake ever detected released more energy in a tenth of a second than the sun does in 150,000 years. The source was 50,000 light-years away — halfway across the galaxy — but the awesome power of its enormous flare still physically affected the Earth, ionizing and inflating the atmosphere.
The scale of such forces is so utterly beyond our realm of experience that it's impossible for our puny simian brains to truly grasp. But it's still fun to try. The universe is filled with bizarre objects and phenomena, most of which we can't see. Neutron stars were discovered just 50 years ago. Who knows what we'll find next?
What if I told you that recent experiments have revealed a revolutionary new method of propulsion that threatens to overthrow the laws of physics as we know them? That its inventor claims it could allow us to travel to the Moon in four hours without the use of fuel? What if I then told you we cannot explain exactly how it works and, in fact, there are some very good reasons why it shouldn’t work at all? I wouldn’t blame you for being sceptical.
The somewhat fantastical EMDrive (short for Electromagnetic Drive) recently returned to the public eye after an academic claimed to have recorded the drive producing measurable thrust. The experiments from Professor Martin Tajmar’s group at the Dresden University of Technology have spawned numerous overexcited headlines making claims that—let’s be very clear here—are not supported by the science.
The idea for the EMDrive was first proposed by Roger Shawyer in 1999 but, tellingly, he has only recently published any work on it in a peer-reviewed scientific journal, and a rather obscure one at that. Shawyer claims his device works by bouncing microwaves around inside a conical cavity. According to him, the taper of the cavity creates a change in the group velocity of the microwaves as they move from one end to the other, which leads to an unbalanced force, which then translates into a thrust. If it worked, the EMDrive would be a propulsion method unlike any other, requiring no propellant to produce thrust.
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