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.
What Would You See As You Fell Into A Black Hole?
“But if you continue your fall towards the event horizon, you’ll eventually see the starlight compress down into a tiny dot behind you, changing color into the blue due to gravitational blueshifting. At the last moment before you cross over into the event horizon, that dot will become red, white, and then blue, as the cosmic microwave and radio backgrounds get shifted into the visible part of the spectrum for your last, final glimpse of the outside Universe, still assuming that nothing else falls in with you.”
When you fall into a black hole, terrible things happen to you. Your atoms get stretched apart in the terrifying phenomenon of spaghettification, you get sucked inevitably into the central singularity, and the entire outside Universe goes dark. But it doesn’t go dark all at once; instead, the front of the Universe gets cut off, but all the light paths converge directly behind you. What you see, as a couple of the videos within the article show, is that the entire outside Universe gets encoded in a light path that decreases to a shrinking circle directly behind you. As you try to avoid the central singularity, maximizing your survival time, you discover a terrifying phenomenon from inside the event horizon: that the singularity is everywhere, in all directions. At the last, you see the blueshifted, leftover glow from the Big Bang… and then nothing but darkness.
My words cannot do it justice (although I try!), but you’ll want to see the visualizations for yourself. They’re spectacular, and best of all, they’re accurate! Come see what you would see if you had the misfortune to fall into a black hole.
2018 Will Be The Year Humanity Directly ‘Sees’ Our First Black Hole
“No longer will we need to rely on simulations or artist’s conceptions; we’ll have our very first actual, data-based picture to work with. If it’s successful, it paves the way for even longer baseline studies; with an array of radio telescopes in space, we could extend our reach from a single black hole to many hundreds of them. If 2016 was the year of the gravitational wave and 2017 was the year of the neutron star merger, then 2018 is set up to be the year of the event horizon. For any fan of astrophysics, black holes, and General Relativity, we’re living in the golden age.”
Black holes have been dreamed up by theorists for centuries. Even in the aftermath of Newtonian gravity, a spectacular realization came about that if you gathered enough mass together into a small enough volume, the gravitational effects would be so pronounced that nothing, not even light, could escape. These black holes show up with very specific properties in General Relativity, and today in modern astrophysics, we know of three independent ways to make them. But despite observing their effects in many different wavelengths of light, such as the radio and X-ray, we’ve never imaged an event horizon directly. Although a telescope the size of Earth would be able to, that technology is out of reach. Owing to collaboration and human ingenuity, however, we’ve developed an array known as the Event Horizon Telescope that should reveal its first image next year!
Will we see the event horizon of a black hole for the first time? My bet is on yes. Come get the science as to why!
‘Direct Collapse’ Black Holes May Explain Our Universe’s Mysterious Quasars
“In a theoretical study published in March of this year, a fascinating mechanism for producing direct collapse black holes from a mechanism like this was introduced. A young, luminous galaxy could irradiate a nearby partner, which prevents the gas within it from fragmenting to form tiny clumps. Normally, it’s the tiny clumps that collapse into individual stars, but if you fail to form those clumps, you instead can just get a monolithic collapse of a huge amount of gas into a single bound structure. Gravitation then does its thing, and your net result could be a black hole over 100,000 times as massive as our Sun, perhaps even all the way up to 1,000,000 solar masses.”
Some of the most distant, luminous objects in our entire Universe, quasars, are a mystery. How does our Universe get an active, supermassive black hole that forms so early, especially given how relatively small the stars that make black holes are known to be? Even given the earliest, most massive stars that can theoretically form, you’d only expect seed black holes of a few hundred solar masses, yet these early quasars have almost a billion Suns worth of mass to begin with. You’d need a seed 1,000 times as massive to get there. Well, that’s exactly what the scenario known as ‘direct collapse’ could get you. If a massive galaxy is close by another cloud of gas, it can suppress the formation of stars all while that cloud collapses, potentially leading to a black hole directly, without any stars. That black hole could be up to a million solar masses, providing a path to the earliest quasars with no further hitches. With the technology coming online in the next few years, we might yet see this process in action for the first time.
Come learn about the huge strides we’ve made in direct collapse black holes and finding the origin of the Universe’s quasars. The way you view our Universe may never be the same!