Why doesn't a globular cluster collapse?

Why doesn’t a globular star cluster collapse into a single body due to the mutual gravitational effects of the individual stars?
This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Image credit:  The Hubble Heritage Team (AURA/STScI/NASA)

This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Image credit: The Hubble Heritage Team (AURA/STScI/NASA)

Originally posted @ Medium!

Globular clusters are strange objects. They’re incredibly old collections of a huge number of stars — somewhere around a few hundred thousand on average, and they orbit around the very edges of galaxies, dotted spherically around. Our own Milky Way has a few hundred that we’ve found — Andromeda, the nearest other large galaxy we can check, has at least 60 some. (We’re likely to miss some around Andromeda, since they’re not the brightest things, and are quite small.) Even some of the Milky Way’s tiny nearby satellite galaxies have their own collection of globular clusters.

When I say that these clusters are incredibly old, we’re talking about as old as you can get. Some of the stars in a globular cluster are so old that they can help us to constrain the age of the universe. (If your globular cluster is nearly 13 billion years old, you can’t have a Universe younger than that.) We’re still not quite sure what conditions are needed for a globular cluster to form, or how they manage to survive for so long.

This illustration shows the location of the globular cluster M4 in our Milky Way Galaxy, which is depicted 'edge-on' or from the side. Globular clusters like M4 are the first pioneer settlers of the Milky Way. Many coalesced to build the hub of our galaxy and formed billions of years before the appearance of the Milky Way's magnificent pinwheel disk. Today, 150 globular clusters survive in the galactic halo. Image Credit:  NASA  /  ESA   and A. Feild

This illustration shows the location of the globular cluster M4 in our Milky Way Galaxy, which is depicted 'edge-on' or from the side. Globular clusters like M4 are the first pioneer settlers of the Milky Way. Many coalesced to build the hub of our galaxy and formed billions of years before the appearance of the Milky Way's magnificent pinwheel disk. Today, 150 globular clusters survive in the galactic halo. Image Credit: NASA/ESA and A. Feild

One thing you can take from their age is that they’re relatively stable. Which, as you have guessed, means that they can’t be in the process of collapsing down onto themselves, or we wouldn’t see so many of them looking so similar. So how do they manage to stay the same size over time?

At a very basic level, you might expect that if you suspended 100,000 stars in space, and let time roll forward, gravity would simply pull everything together. Some stars might get twirled around for a time, but gravity would ultimately crush everything into a mess at the very center of what used to be a globular cluster. And that’s exactly what would happen, if gravity were the only thing at play.

This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Image credit:  ESA/Hubble & NASA

This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Image credit: ESA/Hubble & NASA

But the stars are moving. And when you have motion, you can defeat gravity for a time. Each star is in orbit around the center of the globular cluster, and the motion of the star in a sideways fashion, combined with the pull of gravity, creates an oval-shaped orbit, and the star can circle the center of the globular cluster reasonably happily for quite some time. Because the star has enough energy to keep moving forward, the tug of gravity won’t be able to haul the star to the center of the cluster.

This is a bit of a simplified picture, because every single other star of the 100,000+ in the cluster is doing the exact same thing, and the orbits of the stars are effectively random, which is why the cluster looks so spherical. If the orbits of the stars were less perfectly scrambled, you would see the cluster look a little more elongated, like a disk (like a spiral galaxy does, from the side), instead of looking pretty much round. Elliptical galaxies, for instance, also have done a pretty effective job of scrambling the orbits of the stars within them, so they look pretty round as well — but contain many millions more stars than a globular cluster. If the stars of an object are in this sort of randomized configuration (instead of rotating nice and orderly in a disk) we call it pressure supported (as opposed to rotation supported). In this context, it’s not that there’s actually any physical push outwards that makes them pressure supported, but the random motions of the stars as they each orbit their center of gravity acts as a resisting force to gravity. It’s not pressure, but it’s not ordered rotation, and it still resists gravity, so: pressure. (Astronomers are bad at naming things.)

The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy.  Image credit:  ESA/Hubble & NASA

The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy.  Image credit: ESA/Hubble & NASA

Just because these systems are reasonably old and reasonably stable doesn’t mean they don’t change over time, and one thing that does occur with time is that the stars within the cluster will get close enough to each other to interact gravitationally. Not all the stars in the cluster are the same size, so the way they interact will depend on their mass. If the two stars are (for simplicity) roughly equal in mass, but one happens to be moving a bit faster, then the faster star will donate some of its energy to the slower star, giving them both a similar speed when they leave each other’s company, under most circumstances. (There’s no requirement for them to be going the same direction.) If, however, the stars are very different in mass, and they split their total energy, the less massive star will wind up going much faster. Kinetic energy is equal to the mass times the velocity squared — a large mass means a much smaller velocity for the same amount of energy, and vice versa.

This means that over time, the massive stars will tend to slow down, and the light stars will tend to speed up. If a star is slowing down, then gravity gets to take over, and does indeed pull that star down, closer to the core of the cluster. You wind up with some globular clusters collecting all their high mass stars in their very centers, with the lightest stars zooming around the outskirts, going much faster. This pattern of energy allows the cluster to sort itself from heaviest to lightest. (The technical term for this end result is called ‘mass segregation’.)

This image shows a globular cluster known as NGC 104 — or, more commonly, 47 Tucanae, since it is part of the constellation of Tucana (The Toucan) in the southern sky. After Omega Centauri it is the brightest globular cluster in the night sky, hosting tens of thousands of stars. Image credit:  ESA/Hubble & NASA

This image shows a globular cluster known as NGC 104 — or, more commonly, 47 Tucanae, since it is part of the constellation of Tucana (The Toucan) in the southern sky. After Omega Centauri it is the brightest globular cluster in the night sky, hosting tens of thousands of stars. Image credit: ESA/Hubble & NASA

The other thing that can cause change in a globular cluster is if its orbit takes it too close to the massive galaxy it sits near. Each globular cluster is, as a whole, orbiting a fairly massive galaxy, particularly in comparison to the cluster. This orbit takes a very long time to complete, and it seems that many globular clusters can successfully orbit the galaxy without getting too near — but if the cluster does get too near, then the cluster will experience an intense tidal force because of the gravitational pull of the galaxy. This tidal force can shear off the outer layers of the cluster (if it’s a mass-segregated cluster, this means it looses the smallest stars). These outer layers get pulled away into a long stream of stars, barely detectable in the night sky even with a powerful telescope, and leaving an even denser nucleus of stars behind; at least four such globulars exist in the interior of our own Milky Way.

Although globular clusters don’t collapse, it’s conceivable that the most massive stars — which give rise to black holes — will segregate the fastest, giving rise to an intermediate mass black hole at the centers of globulars. On the other hand, it’s conceivable that black holes are ejected too frequently, as massive stars tend to give rise to far less massive black holes. While stellar-mass black holes have been observed inside a few globular clusters, there’s no consensus as to whether they can contain larger ones at their centers or not; that’s still an open question!

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Could black holes be the engines of new galaxies?


Originally posted @ Medium!

A very good question! Black holes are interesting objects, and still somewhat scientifically perplexing ones, particularly when it comes to determining how they interact with the galaxy they live within.

As a first filter, though, we can ignore all of the small, stellar mass black holes that hang out throughout the disk of a galaxy. These small black holes, while they’re still fascinating objects, and can produce pretty impressive jets and energy outputs, are nowhere near as energetic as you would need if you want to influence the whole galaxy. Really, we’re only interested in the class of black holes which are found at the very centers of galaxies; supermassive black holes.

Supermassive black holes are typically about a billion times more massive than our sun, which is the main distinction from stellar mass black holes, which are only a few times larger than our Sun. Despite how much larger they are, these supermassive black holes are not uncommon — it seems that every massive galaxy has a supermassive black hole nestled at its core. All our observations of galaxies and their black holes suggest that the black holes and their galaxies grow in lockstep with each other; smaller supermassive black holes are found in smaller galaxies, and larger supermassive black holes in larger galaxies.

Black holes are actually frequently referred to as “galactic engines,” as they can produce an absolutely stupendous amount of energy: literally millions of times the energy output of the Sun. This energy is usually a byproduct of the extraordinary inefficiency with which it manages to absorb external material in order to grow. The energy produced by the black hole often manifests as a jet of material, flung outwards along magnetic field lines at speeds of a significant fraction of the speed of light. The engine metaphor reflects the black hole’s ability to take a certain material (generally gas, dust, or in some cases, stars) and convert a good chunk of that material into energy; in the case of a car engine, we take gasoline and burn it, converting the gasoline into energy that can power the car.


If we run with the car engine metaphor a little longer — cars function (pretty dramatically) differently if their engine is on or off. So if the black hole is “on” — which for black holes means that they’re trying to accrete new material — we might expect the galaxy it sits in to react somehow. There’s so much energy being produced that it seems only natural to think the galaxy should somehow care. This is still an active field of research, and we haven’t quite converged to a consensus on how the black hole should feed back into the galaxy, or if there should be a dramatic impact on the number of stars that galaxy can form, for instance.

But what about black holes in newly forming galaxies? To examine those, we must look at galaxies which are at extremely large distances from the Earth, where light has taken so long to reach us that we look billions of years into the past. At these distances we discovered an interesting object: a quasar.


Quasar is short for “quasi-stellar radio source” (also known as QSOs, or quasi-stellar objects, because they don’t all emit in the radio, after all). Quasars are extremely bright point sources on the sky like a star (often saturating the camera, causing flares and other unfortunate effects), but when spectra were taken of these objects, their light looked nothing like that of a star. Quasars were rather mysterious for a number of years — because they blasted the camera with light, it was extremely difficult to determine if these objects were actually within a galaxy, and if so, where they were — central or off-center? If they were isolated, and there was no additional galaxy surrounding them, what could cause them?


Observing techniques developed, and it became clear that these quasars were indeed in the centers of galaxies, but the galaxies were incredibly faint relative to the brightness of the quasar (hence why we couldn’t spot them before). A number of theories were put forth: merging neutron stars, interacting stellar mass black holes, flares from collisions, etc. But thanks to more advanced techniques, we determined the light from quasars to be produced by the central black hole of these faint galaxies, dramatically outshining the rest of the galaxy. These objects are also much more common at large distances from us than they are nearby — so they tend to exist much more commonly in the youngest galaxies in our Universe.

But how did they get so bright? To understand that, we need to poke our noses a little closer in to the black hole. An active black hole (in that it is trying and mostly failing to accrete new material and grow) is always surrounded by a superheated disk of material — an accretion disk — which is swirling around the black hole, gradually losing the angular momentum it needs to maintain a stable orbit, and slowly falling towards the black hole. This innermost accretion disk is often surrounded by a larger donut-shaped ring of dust and gas, which is not quite as hot, and not nearly as close. Perpendicular to the inner disk (and to the dusty donut surrounding it), a jet will form. This jet is mostly electrons and protons fired out at speeds close to the speed of light, and these particles can radiate their energy away through X-rays and radio waves.


If you happened to be pointed in a direction that has you looking straight down the jet, you’d find yourself staring down a very bright portion of the black hole, instead of being able to see the glare from a less direct path. It seems that with quasars, we happen to be looking down the jet, or very nearly directly down the jet.

It doesn’t seem to be just an orientation effect, though — if it were just orientation, we’d see them nearby, in older galaxies as well, but they seem to be quite rare in the nearby universe. So there must be a secondary thing at work as well; very likely this is the amount of gas hanging around near the center of the galaxy. If there’s not enough gas in the galaxy to hang around near the black hole in that innermost disk, the black hole simply won’t have any material to fling outwards into a powerful jet. In the local universe, galaxies tend to be mostly stars, with less than 25% gas. But much younger galaxies have the percentages reversed; they can easily be more than 60% gas. This means that even without any external effects from things like collisions between galaxies (also more common at these times, and will also help to push gas to the centre of the galaxy), it’s much easier for the black hole to have an easy fuel source to power its huge jets, streaming away from the galaxy!


Between the two factors — orientation, and the larger presence of gas near these central black holes — quasars are very much the powerhouses of the early universe!

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How can we tell the universe is expanding evenly?

I’ve heard the idea that spacetime needs to be constantly created in the universe. Could this happen uniformly throughout the universe, or would it start from a definite point like a whirlpool in reverse. How would we tell which of these is true?

Originally posted @ Medium!

To properly answer this question, we have to drift backwards in time a bit, to understand where the idea that the universe is expanding comes from in the first place. The story really starts in the era of Lorentz and Hubble, when our vision of the Universe’s scale was expanding hugely. Scientists were beginning to comprehend the scale of the universe — the Milky Way was not an isolated entity encompassing all that there was.

Extragalactic astronomy — the study of things outside our own galaxy — was born in the first decades of the 20th century. We were starting to take measurements of (what we now know to be) other galaxies, trying to determine how far they were exactly.

With every measurement that was taken of these spiral and elliptical “nebulae” in the skies, it became clear that each one appeared to be moving away from us. This is a relatively straightforward thing to determine. Galaxies (if they are forming new stars) tend to glow in very specific colors, due to the atoms within them. Hydrogen, for instance, has a very bright, unique pink-red glow, and oxygen glows green in these environments. The colors are due to the exact wavelength of light that the atoms produce when an electron loses energy, and we know these wavelengths quite precisely.

If these colors are shifted in any direction from where they would be if you measured them in a lab on the Earth, it means that the object we’re looking at is moving along our line-of-sight. If it’s moving towards us, they will shift towards the blue, as the peaks and troughs of the light wave are pushed closer together, and if they are shifted towards the red, the light wave will be stretched. Cunningly, we have termed this redshift and blueshift. It’s exactly the same phenomenon as doppler shift changing tone of a siren as it passes you.

So when we looked out at the night sky, and found that everything was redshifted and not blueshifted, that implied that everything was moving away from us. And if everything was moving away from us, then that required an adjustment to the simple, static picture of the universe. If the universe was in balance, with no expansion or contraction, you wouldn’t expect to see everything else in the universe fleeing from you, as though you’re at the top of a hill, watching everything else roll down a hill away from you.

So either the universe wasn’t in balance (in this case it would have to be expanding), or the Earth, our vantage point, was in a special place, like the top of our metaphorical hill. Historically, scientists haven’t liked arguments that mean we have to exist in a statistically improbable location in the universe, and nowadays we have another argument at our sides that argues against this.

No matter where we look in the sky, everything looks pretty much the same. Sure, the small things change — there might be five galaxies in a group here, 13 in a group there, but on average, things are distributed nearly evenly across the sky. And, looking at the distribution of things, it shouldn’t matter where we’re standing — if we were in a totally different galaxy, in a vastly different part of the universe, we should see a universe that looks pretty much (at least on a statistical level) the same as ours. The technical terms for this are “homogenous” (the same in every direction you look) and “isotropic” (the same from any point you could be standing).

So how would we know if the Universe was expanding from everywhere, or from a fixed point in space? Fortunately, these two scenarios would reflect their changes in both the way we see galaxies moving away from us, and in the overall distribution of galaxies in the sky.

If the universe is expanding evenly (our current theory), from everywhere, gradually expanding the space between galaxies, then we would actually expect every galaxy to appear to be moving away from us. If all things are separating, but we are stuck in a fixed location, then relative to us, all things are moving further away. Notably, this would appear to be true no matter which galaxy you happened to find yourself on — you’d always observe everything to be receding from you.

If, on the other hand, space spooled out from some kind of vortex over to the left, we’d notice a difference in the way that galaxies were spread in the universe, and a difference in redshifts as we look around at different parts of the sky.

Say we’re looking directly at our spooling space vortex, and it’s pushing new space out into existence. That wouldn’t change the distance between our galaxy and the galaxy behind us, away from the vortex. So the galaxy behind us would appear to be stationary relative to us. In fact, most galaxies on “our side” of the vortex would appear to be moving either slowly, or not at all, relative to us. Things on either side of the vortex wouldn’t appear to be drifting forwards or backwards, but they’d have some pretty solid apparent sideways motion — at high speeds, this might drive a measurable parallax, where you could see its motion against the background galaxies. Meanwhile, galaxies on the other side of the vortex would appear to recede from us very rapidly!

I said we’d also notice in the distribution of galaxies — and that’s because in this scenario, the vortex is forming empty space; so it’s creating a perfect bubble — an absolutely empty sphere of space, growing rapidly with time. And if we were to take large sky surveys of the galaxy population, such a bubble would surely stand out as unusual. We’ve actually looked for signs that we might be inside such a cosmic bubble (though, clearly, it’s not as empty as the vortex-bubble would be), and so far we’ve found not even a hint of evidence that we’re in one.

Instead, measurements like galaxy counts, temperature evolution over time and density measurements out to very large distances point towards the expanding, isotropic, homogeneous universe picture, and not the Earth at the privileged center of a vortex (or explosion) picture.

So if there were a cosmic vortex somewhere, spooling out spacetime from a specific point, ultimately we would expect the way we observe the universe to be very different. It’s only because space is being created between all the objects which exist currently that we observe all galaxies to be moving away from us, and our observations that the universe is distributed evenly across the sky, so no matter where we stand, we should see something similar — which wouldn’t hold if there were a giant vortex in the sky.

Have your own space question? Feel free to ask!

Why do galaxies have two spiral arms?


Originally posted at Medium!

Only a small fraction of galaxies actually have two spiral arms! Even if we ignore the entire class of galaxies dubbed “ellipticals”, which have no spiral arms at all, there are a ton of spiral galaxies with a number of arms not equalling two.

Spiral galaxies are one half of the larger galaxy population; the overall population is generally divided into spirals and ellipticals. Spirals are also known as “late type” galaxies, and ellipticals as “early type” galaxies. This, along with many other unfortunate naming conventions in astronomy, is for historical reasons. Edwin Hubble was one of the first people to try to classify galaxies by any method, and he developed a system that we’ve since dubbed the “tuning fork diagram”, which puts the smooth, featureless ellipticals one one side, and spirals on the other side. The two tines of the tuning fork are for those galaxies with bars, versus those galaxies without. In a footnote of a paper in 1926, he assigns the elliptical galaxies the moniker “early” and spirals the term “late”, as a way of marking a progression from simple to complex structures.

As we have since learned, early-type galaxies are actually the oldest galaxies in existence at any point in time, and are thought to be created through the destruction of numerous late-types, so this ‘early’ and ‘late’ naming system is very precisely backwards. (But professional astronomers still use it.)

Looking at the spiral galaxies, a few things will pop out; the first being this large division between bars and no bars. All the barred galaxies on this diagram are drawn with only two arms, but this isn’t always the case. Our own galaxy, the Milky Way, has a central bar, and well more than two spiral arms. In fact, if you look at our best maps of the Milky Way, not only do we have more than two spiral arms, but we also have these smaller branches (sometimes labeled “spurs” between the galaxies, as the arms themselves fork and divide. It’s within one of these little offshoots that our solar system resides, about two thirds of the way out from the center of the galaxy.

The Milky Way is hard to look at, since we’re embedded within it, but galaxies which are close to us are easier to observe. So Andromeda, a galaxy which is slightly larger than our galaxy, and very nearby, ought to be easier to observe. Andromeda also doesn’t appear to have strong spiral arms; it shows dark dust lanes, but not the prototypical idea of strong arms, winding around a bright nucleus.

If we look back to our Hubble diagram, on the top left, there’s a kind of many-armed sea-star looking galaxy; this is actually a remarkably common style of galaxy, and the non-barred counterpart to our own galaxy.

There is a fun class of galaxy which is (really, in all seriousness) technically called “flocculent spirals”. These galaxies are defined by a total lack of strong arms; they have tufts and wispy bits, but nothing so dramatic as a spiral arm. (Flocculent, in case you are curious, is defined to mean “wool-like”, which is a rather pleasing mental image for describing the disk of a galaxy.)

After all this, I’d forgive you for thinking that this distinct spiral arm thing is complete gibberish, but there are still plenty of galaxies out there with strong, definite spiral arms, with very little filling in the gaps between those arms. These galaxies have the delightful name of “grand design spirals.” Grand design spirals (which make up only about 10% of galaxies) tend to have 2 or 4 strong spiral arms, and they’re often extremely photogenic and pleasingly symmetric objects in the sky. But the method of forming and maintaining those arms has been remarkably hard to explain.

The problem with explaining spiral arms lies fundamentally in that the components of a galaxy rotate at different speeds; the inner part of the galaxy rotates faster than the outer parts. So the easiest explanation — that the arms are actually just areas of the galaxy that are physically more dense, and a fixed association — doesn’t work. The problem is easily demonstrated with some string (or a cable you don’t care about), by winding it up around your finger. If the arms are a physical thing, like the cable, the inner part of the galaxy will rotate many more times than the outer part will, so the spiral arm will end up “tightly wound”. The cable/string wound around your finger doesn’t look so much like a spiral arm as it does a series of circles, and so tightly wound that you wouldn’t be able to distinguish the spaces between the arms.

So any solution to this problem has to mean that the spiral arms can’t be a physical thing. So far the best explanation is called Density Wave Theory, which is effectively what happens if you have a traffic jam going in circles. The wave is a compression of the material that already exists in the galaxy, but stars aren’t fixed to a location “inside” or “outside” of the spiral arm. Like cars passing through a traffic jam, while the stars are within the arm, they’re in a very tightly packed region of space (and therefore quite bright), but they’re not stuck there forever, and eventually they will pass onwards, as the density wave moves past them.

There’s one other reason you can see galaxies with two spiral arms, and it’s a reason close to my science heart. Interactions between galaxies can cause the galaxy to be strongly distorted, due to intense gravitational tides. Like the tides on Earth, this stretches out the galaxy, and pulls material out into two “arms”.

Unlike the quieter grand design spirals, these are not formed by density waves but by gravity, and so these arms are dubbed “tidal arms”. These tidal arms are often quite dramatic, stretching across many hundreds of thousands of light years. They are markers of a near collision in the past, and often foreshadow a more dramatic collision with a companion in the relatively near future, where it and its companion will merge into a single, newly built object.

Have your own space question? Feel free to ask!

How does gravity escape a black hole?

I’ve heard that gravity “moves” at the speed of light; if it does, then how can a black hole’s influence extend outside of the event horizon? If the answer is that “it’s not light, it ‘moves’ in a different way that’s not subject to the same rules as light,” then why does it move at the same speed?

Originally posted @ Medium!

There’s a few things tangled up in here, but let’s see if we can untangle them.

The first is about how gravity affects space-time in general. Normally you see the gravitational distortion of, say, the Earth, represented like the above. Or you might have run into the explanation that gravity acts like a bowling ball on a rubber sheet, with heavier objects causing deeper indentations in the sheet…


Or you might have run into the explanation that gravity acts like a bowling ball on a rubber sheet, with heavier objects causing deeper indentations in the sheet.

These are reasonable explanations, though simplified, since the indentations happen in all directions, so these ‘indentations’ are really three dimensional distortions, or condensations, of space itself. But these simplifications illustrate an important point — fundamentally, gravity is a distortion to space itself. Often we speak of this distortion in terms of a ‘well’, which goes back to this two dimensional sheet metaphor, since it’s easy to think of things rolling downhill into a divot, formed by the presence of a large amount of mass.

Mathematically, the influence of gravity is written out as directly proportionate to the mass of the object, and inversely proportional to the square of the distance between you and that object. This distance dependence is what creates the particular smooth curve away from the center of the object. If you’re very close to the object, the gravitational well is deep, and you feel a strong gravitational pull. If you’re further away, gravity can’t pull space that far out of shape, so you feel a much weaker gravitational force.

These distortions to space are always present — for instance, all the planets are in their own distortion of space, with moons that float in ellipses within this distortion. The Sun has its own, much deeper distortion that all the planets are circling. The Sun follows the distortion of the galaxy. And, critically, each distortion was present in a milder form before each of these objects collapsed into their current form, so these distortions didn’t spontaneously form at any stage — they simply contracted and deepened as density increased. A small gas cloud could have the same mass as a small planet, but the planet’s gravitational well will be steeper than the gas cloud’s. The steeper the gravitational well, the faster you need to go to escape it, but you need both extreme mass and extreme concentration of that mass before you make it to black hole territory.

The event horizon of a black hole, in this context, describes the location beyond which, the black hole’s gravitational well is so steep, that even light can’t escape it. If you take one of these rubber sheet diagrams, you could place down a circle to describe it’s location. But the event horizon itself doesn’t describe a physical boundary to the influences of gravity. The gravitational well itself exists continuously outside and inside of the event horizon — outside of the event horizon it’s just slightly less steep. You could place another circle outside the event horizon that describes the location at which you need to go half the speed of light to escape — there’s no change or boundary in the physical distortion at this circle, it’s just a descriptive line that might prove useful to understand or describe the object’s influence on space.

The stars which eventually turn into black holes are the most massive stars that form, which tend to burn bright blue, extremely hot, and burn out quickly. Typically the threshold for the mass of the star that can leave behind a black hole is given at around eight solar masses, though this is probably a slightly fuzzy boundary. These stars have had their own gravitational distortion since they were a cloud of dense gas, well before they were stars. The stars contract, and light up, and their gravitational twist of space gets more intense, but it’s not a sudden change. It’s more of a gradual shift towards a more and more dramatic gravitational well.

The ‘movement’ of gravity at the speed of light limit can be considered entirely separately; while gravity has a broad extent across a wide swath of space, the speed of light imposes a limit on how quickly information about changes can travel across that space.

The common example is that if the Sun were to spontaneously vanish — not explode, but vanish completely — the Earth wouldn’t notice anything different until about 8 minutes later, when the sunshine would vanish. But the Earth would also continue to orbit around the physically nonexistent Sun for another 8 minutes, because the changes to the distortion of space due to the Sun’s presence also haven’t reached us yet. If you think of the speed of light more as an information speed limit than as a particle speed limit, this makes it slightly easier to think about.

Information doesn’t always travel at the speed of light, though — depending on the environment that the information is traveling through, and the form of that information (which is not always light), the speed of information can proceed at speeds that are much slower than the speed of light. The speed of light in a vacuum seems to be a hard upper limit that nothing can surpass, but if your information is in the form of a compression wave, like sound, then the information travels at the speed of sound in that medium.

Think of a lightning strike — unless you’re right underneath the strike, you hear the thunder several seconds after the lightning strike. Light travels faster through air than sound does, so even though they were created at the same time, it’s the light’s information that reaches you first, and this discrepancy grows the further away you are.
The speed of light in water is even slower than through air (1.3 times slower, in fact). Effectively, the more dense the material you’re working with, the slower information goes through it.

Going back to black holes, changes to the shape of the gravitational distortion are also information traveling through a medium, though the medium of space is generally almost a perfect vacuum, so we’re working close to our absolute speed limit. As far as we know, gravity doesn’t function in a way that would slow it down in a vacuum, so it should also pass along information at the speed of light.

Gravity is one of the least fundamentally understood forces of nature; we have a very good descriptive understanding of when we expect it to be important, and a very accurate description of its strength, but we don’t know exactly how it functions. How does it interact with matter? Is there a particle mediating its interactions? We have to wait a bit longer to answer these questions, but hopefully we’ll have better answers as we build detectors able to observe tiny fluctuations in the gravitational field that surrounds us!

Have your own space question? Feel free to ask!