What Happens When Planets, Stars, And Black Holes Collide?
“Brown dwarf collisions. Want to make a star, but you didn’t accumulate enough mass to get there when the gas cloud that created you first collapsed? There’s a second chance available to you! Brown dwarfs are like very massive gas giants, more than a dozen times as massive as Jupiter, that experience strong enough temperatures (about 1,000,000 K) and pressures at their centers to ignite deuterium fusion, but not hydrogen fusion. They produce their own light, they remain relatively cool, and they aren’t quite true stars. Ranging in mass from about 1% to 7.5% of the Sun’s mass, they are the failed stars of the Universe.
But if you have two in a binary system, or two in disparate systems that collide by chance, all of that can change in a flash.”
Nothing in the Universe exists in total isolation. Planets and stars all have a common origin inside of star clusters; galaxies clump and cluster together and are the homes for the smaller masses in the Universe. In an environment such as this, collisions between objects are all but inevitable. We think of space as being extremely sparse, but gravity is always attractive and the Universe sticks around for a long time. Eventually, collisions will occur between planets, stars, stellar remnants, and black holes.
What happens when they run into one another? Unbelievably, we not only know, we have the evidence to back it up!
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.
For the first time, we’ve measured that pressure distribution inside the proton, paving the way to understanding why massive neutron stars don’t all form black holes.
This Is Why The Event Horizon Telescope Still Doesn’t Have An Image Of A Black Hole
“Of all the black holes visible from Earth, the largest is at the galactic center: 37 μas.
With a theoretical resolution of 15 μas, the EHT should resolve it.
Despite the incredible news that they’ve detected the black hole’s structure at the galactic center, however, there’s still no direct image.”
Last year, data from the South Pole Telescope, a 10-meter radio telescope located at the South Pole, was added to the Event Horizon Telescope team’s overall set of information. Here we are, though, half a year later, and we still don’t have a direct image of the event horizon for the galactic center’s black hole. There aren’t any problems; the issue is that we have to successfully calibrate and error-correct the data, and that takes time and care to get it right. Science isn’t about getting the answer in the time you have to get it; it’s about getting the right answer in the time it takes to get things right. From that point of view, there’s every reason this is worth waiting for.
The Event Horizon Telescope team is on the right track; here’s where we are right now in our quest to create the first image of a black hole’s event horizon!
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.
Come get the fascinating physics behind how the most massive stars die. You might think “supernova” every time, but the Universe is far more intricate and complex than that!
This Is Why Our Universe Didn’t Collapse Into A Black Hole
“The level to which the expansion rate and the overall energy density must balance is insanely precise; a tiny change back then would have led to a Universe vastly different than the one we presently observe. And yet, this finely-tuned situation very much describes the Universe we have, which didn’t collapse immediately and which didn’t expand too rapidly to form complex structures. Instead, it gave rise to all the wondrous diversity of nuclear, atomic, molecular, cellular, geologic, planetary, stellar, galactic and clustering phenomena we have today. We’re lucky enough to be around right now, to have learned all we have about it, and to engage in the enterprise of learning even more: the process of science. The Universe didn’t collapse into a black hole because of the remarkably balanced conditions under which it was born, and that might just be the most remarkable fact of all.”
The Universe is a vast and complex place, full of a diversity of structure from the smallest scales to the largest. And yet, by many accounts, it’s a wonder that it came to be this way at all.
If things were just a little bit different at the very beginning, the Universe could have recollapsed in on itself in a mere fraction-of-a-second after the Big Bang. That very clearly didn’t happen, but why not? And, if it did happen, would we have formed a black hole? There must have been an incredibly perfect, finely-tuned balance between the initial expansion rate and the energy density of everything within the Universe, or this careful balance wouldn’t have existed, and we never would have arisen in this Universe.
Yet, here we are! So, what factors conspired to allow us to exist, exactly as we are? Come find out for yourself!
Einstein’s Ultimate Test: Star S0-2 To Encounter Milky Way’s Supermassive Black Hole
“The largest, closest single mass to Earth is Sagittarius A*, our Milky Way’s supermassive black hole, weighing in at 4,000,000 solar masses.
The star S0-2 makes the closest known approach to this black hole, reaching a minimum distance of just 18 billion kilometers.
That’s only three times the Sun-Pluto distance, or a meager 17 light-hours.”
After a 16 year wait, the closest star to the Milky Way’s supermassive black hole, S0-2, will make its closest approach later this year. At its closest, it should be moving at a whopping 2.5% the speed of light, enabling us to test out Einstein’s relativity in an entirely new regime. We should, for the first time, be able to measure the gravitational redshift from our galactic center, and to track the relativistic “kick” that Einstein’s theory predicts when an orbit gets modified by traveling close to an extremely large mass. New studies have recently shown that S0-2 doesn’t appear to have a binary companion, which makes it even more interesting for such an observation, which won’t come again until the year 2034. As a bonus, scientists hope to shed light on how stars form in the harsh environment of the galactic center at all.
Come find out how the newest test of Einstein could push us past the limits of relativity, or confirm it in an entirely new way!
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.
Thanks to LISA, launching in the 2030s, that’s all going to change. Suddenly, we’ll be able to predict these events weeks, months, or even years in advance! Here’s how.
Black Holes Must Have Singularities, Says Einstein’s Relativity
“The thing is, there’s a speed limit to how fast these force-carriers can go: the speed of light. If you want an interaction to work by having an interior particle exert an outward force on an exterior particle, there needs to be some way for a particle to travel along that outward path. If the spacetime containing your particles is below the density threshold necessary to create a black hole, that’s no problem: moving at the speed of light will allow you to take that outward trajectory.
But what if your spacetime crosses that threshold? What if you create an event horizon, and have a region of space where gravity is so intense that even if you moved at the speed of light, you couldn’t escape?”
Usually, when physicists first start teaching about black holes, the attitude they’re met with is skepticism. People can accept that as you compress a large mass into a smaller and smaller volume, it gets harder to escape its gravitational pull. As you go from a star to a white dwarf to a neutron star, you have to move closer to the speed of light to leave it’s surface. If you go even denser, you’ll create an event horizon: a region of space where the gravitational pull is so strong that nothing, not even light can escape. People are okay with that, but when you go to the next step and declare that anything that crosses the event horizon eventually falls into a central singularity, suddenly they’re not okay. Why, they reason, couldn’t there be some denser, exotic, degenerate form of matter than what we presently know? Why couldn’t that lie inside a black hole, rather than a singular point or ring? It’s a good question and an interesting bit of intuition, but there’s an answer for that.
If you wanted to hold anything up against collapse to a singularity inside a black hole, the force-carriers governing the interaction would have to travel faster than light, which is a no-go. Find out the full story on why black holes must have singularities!
Black Hole Mergers Might Actually Make Gamma-Ray Bursts, After All
“If there is a gamma-ray signal associated with black hole-black hole mergers, it heralds a revolution in physics. Black holes may have accretion disks and may often have infalling matter surrounding them, being drawn in from the interstellar medium. In the case of binary black holes, there may also be the remnants of planets and the progenitor stars floating around, as well as the potential to be housed in a messy, star-forming region. But the central black holes themselves cannot emit any radiation. If something’s emitted from their location, it must be due to the accelerated matter surrounding them. In the absence of magnetic fields anywhere near the strength of neutron stars, it’s unclear how such an energetic burst could be generated.”
In 2015, the very first black hole-black hole merger was seen by the LIGO detectors. Interestingly, the NASA Fermi team claimed the detection of a transient event well above their noise floor, beginning just 0.4 seconds after the arrival of the gravitational wave signal. On the other hand, the other gamma-ray detector in space, ESA’s Integral, not only saw nothing, but claimed the Fermi analysis was flawed. Subsequent black hole-black hole mergers showed no such signal, but they were all of far lower masses than that very first signal from September 14, 2015. Now, however, a reanalysis of the data is available from the Fermi team themselves, validating their method and indicating that, indeed, a 3-sigma result was seen during that time. It doesn’t necessarily mean that there was something real there, but it’s suggestive enough that it’s mandatory we continue to look for electromagnetic counterparts to black hole-black hole mergers.
The Universe continues to be full of surprises, and the idea that black hole mergers may make gamma-rays, after all, would be a revolutionary one! Come get the full story today.
Ask Ethan: How Do Hawking Radiation And Relativistic Jets Escape From A Black Hole?
“Everything you read about a black indicates that “nothing, not even light, can escape them”. Then you read that there is Hawking radiation, which “is blackbody radiation that is predicted to be released by black holes”. Then there are relativistic jets that “shoot out of black holes at close to the speed of light”. Obviously, something does come out of black holes, right?”
When it comes to black holes, the cardinal rule is that there exists an event horizon: a region from which nothing inside can ever escape. Once you cross over, you can never get out. No matter how fast you move, how quickly or what direction you accelerate in, or even if you travel at the speed of light, your inevitable destiny lies at the central singularity. So how, then, are things like relativistic jets and Hawking radiation emitted from black holes? The key to understanding them lies in examining the conditions that occur outside the event horizon, in the region near (but not exactly at) the black hole itself. This is the critical environment where spacetime is curved, matter achieves relativistic speeds, and the quantum fields themselves are affected by relativity.
Hawking radiation and relativistic jets may be real, but they don’t break the laws of physics to exist! Find out how they really do escape on this edition of Ask Ethan.