The Surprising Reason Why Neutron Stars Don’t All Collapse To Form Black Holes
“The measurements of the enormous pressure inside the proton, as well as the distribution of that pressure, show us what’s responsible for preventing the collapse of neutron stars. It’s the internal pressure inside each proton and neutron, arising from the strong force, that holds up neutron stars when white dwarfs have long given out. Determining exactly where that mass threshold is just got a great boost. Rather than solely relying on astrophysical observations, the experimental side of nuclear physics may provide the guidepost we need to theoretically understand where the limits of neutron stars actually lie.”
If you take a large, massive collection of matter and compress it down into a small space, it’s going to attempt to form a black hole. The only thing that can stop it is some sort of internal pressure that pushes back. For stars, that’s thermal, radiation pressure. For white dwarfs, that’s the quantum degeneracy pressure from the electrons. And for neutron stars, there’s quantum degeneracy pressure between the neutrons (or quarks) themselves. Only, if that last case were the only factor at play, neutron stars wouldn’t be able to get more massive than white dwarfs, and there’s strong evidence that they can reach almost twice the Chandrasekhar mass limit of 1.4 solar masses. Instead, there must be a big contribution from the internal pressure each the individual nucleon to resist collapse.
How Do The Most Massive Stars Die: Supernova, Hypernova, Or Direct Collapse?
“When we see a very massive star, it’s tempting to assume it will go supernova, and a black hole or neutron star will remain. But in reality, there are two other possible outcomes that have been observed, and happen quite often on a cosmic scale. Scientists are still working to understand when each of these events occurs and under what conditions, but they all happen. The next time you look at a star that’s many times the size and mass of our Sun, don’t think “supernova” as a foregone conclusion. There’s a lot of life left in these objects, and a lot of possibilities for their demise, too. We know our observable Universe started with a bang. For the most massive stars, we still aren’t certain whether they end with the ultimate bang, destroying themselves entirely, or the ultimate whimper, collapsing entirely into a gravitational abyss of nothingness.”
How do stars die? If you’re low in mass, you’ll burn through all your fuel and just contract down. If you’re mid-ranged, like our Sun, you’ll become a giant, blow off your outer layers, and then the remaining core will contract to a white dwarf. And the high-mass stars can take an even more spectacular path: going supernova to produce either a neutron star or a black hole at their core. But that’s not all a high-mass star can do. We’ve seen supernova impostors, hypernovae that are even more luminous than the brightest supernova, and direct collapse black holes, where no explosion or even ejecta exists from a star that used to be present and massive. The science behind them in incredible, and while there are still uncertainties in predicting a star’s fate, we’re learning more all the time.
Black Hole Mergers To Be Predicted Years In Advance By The 2030s
“When we detect black hole-black hole events with LIGO, it’s only the last few orbits that have a large enough amplitude to be seen above the background noise. The entirety of the signal’s duration lasts from a few hundred milliseconds to only a couple of seconds. By time a signal is collected, identified, processed, and localized, the critical merger event has already passed. There’s no way to point your telescopes — the ones that could find an electromagnetic counterpart to the signal — quickly enough to catch them from birth. Even inspiraling and merging neutron stars could only last tens of seconds before the critical “chirp” moment arrives. Processing time, even under ideal conditions, makes predicting the particular when-and-where a signal will occur a practical impossibility. But all of this will change with LISA.”
The past few years have ushered in the era of gravitational wave astronomy, turning a once-esoteric and controversial prediction of General Relativity into a robust, observational science. Less than a year ago, with three independent detectors online at once, the first localizations of gravitational wave signals were successfully performed. Multi-messenger astronomy, with gravitational waves and an electromagnetic follow-up, came about shortly thereafter, with the first successful neutron star-neutron star merger. But one prediction still eludes us: the ability to know where and when a merger will occur way in advance.
Ask Ethan: How Does Spinning Affect The Shape Of Pulsars?
“[S]ome pulsars have incredible spin rates. How much does this distort the object, and does it shed material this way or is gravity still able to bind all of the material to the object?”
If you spin too quickly, the matter on the outskirts of your surface will fly off. If you’re in hydrostatic equilibrium, your shape will simply distort until your equatorial bulge and your polar flattening result in the most stable, lowest-energy configuration. For our Earth, this means the best place to launch a rocket is near the equator, and our planet’s polar diameter is a little more than 20 km shorter than its equatorial diameter. But what about for the fastest-rotating natural object we know of: a neutron star. While most neutron stars rotate a few times a second, the fastest one makes 766 rotations in that span, meaning that a neutron on the surface moves at about 16% the speed of light. Much faster, and could it escape? Or, perhaps, is the pulsar’s shape highly distorted, either due to that rotation or to the incredibly strong magnetic fields inside? Neutron star matter is very different from anything we’re used to, so don’t bet on any of those.
Seeing One Example Of Merging Neutron Stars Raises Five Incredible Questions
“1.) What is the rate at which neutron star-neutron star mergers occur? Before this event was observed, we had two ways of estimating how frequently two neutron stars would merge: from measurements of binary neutron stars in our galaxy (such as from pulsars), and from our theoretical models of star formation, supernovae, and their remnants. That gave us a mean estimate of around 100 such mergers every year within a cubic gigaparsec of space.
Thanks to the observation of this event, we now have our first observational rate estimate, and it’s about ten times larger than we expected. We thought we would need LIGO to reach its design sensitivity (it’s only halfway there) before seeing anything, and then on top of that we thought that pinpointing the location in at least 3 detectors would be unlikely. Yet we not only got it early, we localized it on the first try. So now the question is, did we just get lucky by seeing this one event, or is the true event rate really so much higher? And if it is, then what is it about our theoretical models that are so wrong?”
Now that we’ve observed merging neutron stars for the first time, in many different wavelengths of light as well as in gravitational waves, we’ve got a whole new world of data to work with. We’ve independently confirmed that gravitational waves are real and that we can, in fact, pinpoint their locations on the sky. We’ve demonstrated that merging neutron stars create short gamma ray bursts, and shown that the origin of the majority of elements heavier than the first row of transition metals comes primarily from neutron star-neutron star mergers. But the new discovery raises a ton of questions, too. Seeing this event has presented theorists with a number of new challenges, ranging from the event rate being some ten times as great as expected to much more matter being ejected than we’d thought. And what was it that was left behind? Was it a neutron star? A black hole? Or an exotic object that’s in its own class?
Gravitational waves, Light and Merging neutron stars
Unlike black hole mergers (gif-1), when two neutron stars merge (gif-2) they give off a huge blast of light in addition to the gravitational wave.
Today LIGO announced that they were able to detect the gravitational waves from the merger of two neutron stars and the revolutionary thing about this is that with the help of telescopes situated across the globe we were to able to confirm this.
5 Facts We Can Learn If LIGO Detects Merging Neutron Stars
“We have already entered a new age in astronomy, where we’re not just using telescopes, but interferometers. We’re not just using light, but gravitational waves, to view and understand the Universe. If merging neutron stars reveal themselves to LIGO, even if the events are rare and the detection rate is low, it’s means we’ll have crossed that next frontier. The gravitational sky and the light-based sky will no longer be strangers to one another. Instead, we’ll be one step closer to understanding how the most extreme objects in the Universe actually work, and we’ll have a window into our cosmos that no human has ever had before.”
Two years ago, advanced LIGO turned on, and in that brief time, it’s already revealed a number of gravitational wave events. All of them, to no one’s surprise, have been merging black holes, since those are the easiest class of events for LIGO to detect. But beyond black holes, LIGO should also be sensitive to merging neutron stars. Even though the range over which LIGO can see them is much smaller, if there are enough neutron star-neutron star mergers happening, we might have a chance. A little over a week ago, a rumor broke that LIGO may have seen one, which would be a phenomenal occurrence. Not only would we have a new type of event that we detected in gravitational waves, we would, for the first time, have the capability of correlating the gravitational and electromagnetic skies. Astronomy, for the first time ever, could view the very same object in gravitational waves and through telescopes.
Beyond Black Holes: Could LIGO Have Detected Merging Neutron Stars For The First Time?
“We are present at an incredible time in history: at the birth of the observational science of gravitational wave astronomy. The coming decades will reveal a series of “firsts,” and that should include the first binary neutron star merger, the first pinpointing of a gravitational wave source, and the first correlation between gravitational waves and an electromagnetic signal. If nature is kind to us, and the rumors are true, we may have just unlocked all three.”
It seems like an eternity ago, but it’s been under two years since LIGO first began the science run that would first detect merging black holes. Their latest scientific data run is scheduled to end in just two days, and thus far, they’ve announced a total of three black hole-black hole merger discoveries, along with a fourth probable candidate. Yet thanks to the Twitter account of renowned astrophysicist J. Craig Wheeler, a bit of information has leaked: LIGO may have discovered merging neutron stars for the first time. They’d be approximately ten times lighter than the black holes we’ve witnessed merging, which means the signals are only 10% as strong. In order to get the same amplitude, they’d need to be only 10% as distant, cutting the search volume down to 0.1% the volume. But still, neutron stars may be much more abundant, so we might have a chance. Just yesterday, Hubble observed a galaxy with a binary neutron star inside, just 130 million light years away.
“Why can suns grow to… many different sizes? That is, ranging from somewhat larger [than] Jupiter up to suns exceeding Jupiter’s orbit?”
“Bigger mass makes a bigger star,” you might be inclined to say. The smallest stars in size should be small because they have the least amount of material in them, while the largest ones of all are the largest because they’ve got the most material to make stars out of. And that’s a tempting explanation, but it doesn’t account for either the smallest stars or the largest ones in the Universe. As it turns out, neutron stars and white dwarfs are almost all larger in mass than our own Sun is, and yet the Sun is hundreds or even many thousands of times larger than they are. The most massive star known is only 30 times the physical size of our Sun, while the largest star of all is nearly 2,000 times our Sun’s size. As it turns out, there’s much, much more at play than mass alone.
Everyone knows the recipe for a black hole: create a massive enough star, allow it to burn through the fuel it its core, and wait. After enough time, the core will collapse, creating a type II supernova and a runaway fusion reaction. The outer layers explode while the core implodes, leaving behind a black hole if it’s massive enough. Alternatively, merge two failed black holes – i.e., neutron stars – together, and you get a black hole, too. But there ought to be a third way: through direct collapse. We haven’t seen enough supernovae for the stars that exist, and we don’t have a great explanation, otherwise, for super-early supermassive black holes. For the first time ever, we’ve witnessed a massive star simply wink out of existence. We may have just caught a direct collapse black hole red-handed.