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Why don't black holes grow faster?

If black holes are infinitely dense and the laws of physics break down, why are they only as massive as the stars that created them? Why can’t they draw more mass from the bulk?
In this artist's illustration, turbulent winds of gas swirl around a black hole. Some of the gas is spiraling inward toward the black hole, but another part is blown away. Image credit:  NASA, and M. Weiss (Chandra X -ray Center)

In this artist's illustration, turbulent winds of gas swirl around a black hole. Some of the gas is spiraling inward toward the black hole, but another part is blown away. Image credit: NASA, and M. Weiss (Chandra X -ray Center)

Originally posted at Forbes!

A black hole is a rather complicated cosmic object, and one of the most common misconceptions about them is that they’re some kind of universal vacuum cleaner, constantly sucking material in towards their centers.

A black hole is formed when a star, which is at least 8 times more massive than our own sun, reaches the end of its lifetime and implodes on itself in a supernova. Supernovae release a huge amount of energy, and a large quantity of material is blown away from the star. The very core of the star, however, is undergoing a runaway collapse- the star is no longer producing energy in its core due to fusion (as our star currently is), so there’s no outward pressure to balance the inward crushing force of gravity. Gravity is able to overwhelm the repelling force of electrons between atoms (the same force that keeps you from falling through your chair), and also able to crush neutrons together, which is an even more difficult feat. The force of of gravity is so overwhelmingly dominant that the core of the old star is compressed to a density high enough that the escape velocity is greater than the speed of light.

At this point, the runaway collapse should continue, so that the core of the star collapses into a singularity, which is an object which takes up no space (we call it a point source) and is of infinite density. Infinities and physics currently do not play well together, so our descriptions of how space and time behave immediately surrounding the singularity begin to fail. (The singularity itself is currently most accurately described by a large number of question marks.)

Unfortunately for clarity, “black hole” has become a bit of a catch-all term that can apply to any part of the entire black hole. Astronomers tend to use it to refer to the entire area near and within the event horizon (the point of no return for light), which is fairly distant from the actual singularity.

If we are looking much further away from the singularity of the black hole (in other words, beyond the event horizon), physics is still able to describe the shape of the black hole’s distortion to space-time. At these distances, the black hole’s influence on the surrounding space is creating more of a gentle gravitational slope than a steep cliff. In fact, it has no more extreme an influence on the space surrounding it than a star of the same mass. If something happens to be in the area, it might roll towards the black hole, but if it doesn’t come close enough, it might just swing right on past, leaving the black hole untouched. If the material (gas, star, whatever it might be) wasn’t going to collide with the star, it probably won’t make it into the black hole.

Stars are usually relatively isolated – they may form within a cluster of other stars, but these clusters tend to be relatively loose, with large distances between stars. And since black holes form only at the end of an old star’s lifetime, any dust or gas that might have surrounded the star when it formed would have been blown away by a lifetime’s worth of stellar wind. By the time the black hole forms, there’s not going to be a lot of material hanging around for it to get close enough to trap. So there really isn’t an accessible “bulk” of material for the black hole to draw on!

Pictured above is an artistic impression of the Cygnus X-1 binary star system. It contains one of the best candidates for a black hole. Image credit:  ESA, Hubble

Pictured above is an artistic impression of the Cygnus X-1 binary star system. It contains one of the best candidates for a black hole. Image credit: ESA, Hubble

There is, however, one major exception to this idea; stars which are found in binary systems. These are systems where two stars are in orbit around each other, and typically pretty close in age. If one of the two stars goes supernova first, the other star (which is probably also reaching the end of its life) may become a source of fuel to the black hole as it expands. Our star’s atmosphere will reach out to the orbit of Mars by the time it reaches full red giant phase; a more massive star would extend even further. If the ballooning star reaches out far enough, its outer edges might begin to fall towards the black hole instead of staying attached to the star itself. Even then, though, black holes are very, very bad at getting material that’s handed to them this way to get all the way down past the event horizon, which would be the moment the black hole can actually grow in mass.

So, not only are the black holes formed from a single star generally lacking access to a “cosmic bulk” of material, but even their gathering of the closest material is so inefficient that it’s very hard for them to grow quickly.

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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Does the Sun have a magnetic North and South pole?

Does the Sun have a magnetic north and south pole like the Earth or a bar magnet?

It does!

Bar magnets are some of the simplest magnetic objects that are out there - with a positive end and a negative end, and a smooth connection of field lines connecting top to bottom, cylindrically encasing the whole magnet. If you’ve ever played with any of those “doodle a hairdo” games that are made of iron filings, you’ve seen the shape of the magnetic field lines.


The Earths’ magnetic field is pretty straightforward as magnetic fields on planets go - its magnetic field is fueled by the motion of our iron core. If you have a magnetic material moving (as our core is), you can produce a magnetic field pretty straightforwardly. Unlike a bar magnet, though, the magnetic north and south poles aren’t as fixed to a single position on the surface of the Earth. The bar magnet is a magnet because we’ve produced the metal to contain a specific property. But the Earth’s magnetic field is produced on the fly, and is influenced by the exact content of the metals flowing around in the core and mantle. If you’re near the poles, you can watch Magnetic North drift by a few degrees every year. Magnetic North is distinct from Geographic North. Geographic north is the physical point where no stars would rise or set, since you’re exactly at the top of the spinning Earth. Magnetic North is the top end of the magnetic field, and is often not at the same place.

The Sun is even more complex. It also forms its magnetic field on the fly, through internal motion, and creates a magnetic field that surrounds it, and at first, it looks like your standard bar magnet diagram again. But, unlike the Earth, the sun doesn’t rotate as a solid body. The equator of the sun rotates faster than the poles do, and it pulls the magnetic field along with it. Over time (and we’re talking years, here, not billions of years) the magnetic field of the sun gets increasingly tangled, which eventually causes the magnetic field to twist up on itself, forming sunspots. After 11 years, the Sun’s magnetic field resets entirely. But it resets by flipping over - the positive end of the magnet, had it been pointing up, is now pointing down. After another 11 years, it’ll reset again, and flip back up the way it had been previously. This 11 (or 22) year cycle has been a consistent feature of our Sun for as long as we’ve been observing it!


So, while the sun does have a magnetic field, with both a north and south pole like a bar magnet, it’s also very unlike a magnet and the Earth, because that magnetic field is complex, and changes dramatically over time.

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