Why haven't galaxies been consumed by their black holes?

If there is a black hole at the center of every galaxy, how come those galaxies have not been consumed by them over the millennium?
Artist concept of matter swirling around a black hole.  Image credit : NASA/Dana Berry/SkyWorks Digital

Artist concept of matter swirling around a black hole. Image credit: NASA/Dana Berry/SkyWorks Digital

We tend to think of black holes as some kind of cosmic vacuum cleaner, constantly sucking in all the material around it. And while it’s true that if you managed to very carefully drop an object into a black hole, you’d never ever get that object back, under normal circumstances, black holes are actually remarkably bad at pulling material that close.

There are two reasons for this; the first is that black holes aren’t actually attractive to anything for any reason other than gravity. Much like our solar system is in a stable orbit around the sun, the vast majority of a galaxy is in a stable orbit around the black hole, with no real reason to go plunging towards the very centre of the galaxy.

The second reason that black holes are bad at being astronomical vacuum cleaners is that they’re really, really, inefficient at getting material close enough to them to cross the event horizon and add to the mass of the black hole. Even small black holes, which exist in great numbers in a galaxy, are much better at tearing a companion star apart than they are at actually growing their own size by consuming the other star.

Material near a black hole tends to form what’s called an accretion disk- a thin, rapidly rotating disk outside the event horizon of the black hole. The gas trying to get to the black hole will speed up the closer it gets to the black hole, and any jostling between gas particles will heat the gas to incredibly high temperatures. At these temperatures, the gas will start glowing in X-rays, which flow out vertically away from the disk. Sometimes this process also causes huge galactic winds, which pushes material vertically away from the galaxy. A significant fraction of the material which could otherwise have made it to the black hole will get pushed straight back out again before it gets particularly close.

But that’s assuming that there’s a lot of material near the black hole, actively falling towards the event horizon. The supermassive black holes in the centers of galaxies have an additional problem - there might not even be any material around in the first place. The Milky Way’s central black hole, for instance, seems to be surrounded by stars, but almost no gas, so there’s no accretion disk around our black hole. In order to be shredded by a black hole, a star would have to come very, very close to the black hole. The star that orbits the black hole in the centre of the Milky Way orbits once every fifteen years (this is really short) and we’ve been (incredibly) able to watch it move around the black hole. It comes within a light-day of the event horizon, and that’s still not close enough to get torn apart or sucked in. (There are videos of the orbiting stars. Go watch them. Here’s the actual data and here’s the physical model from that data. They are both super cool.)

The fastest way for a black hole to grow in size - at least, as far as we know right now - is by crashing into another galaxy. When that happens, after things settle down, the heaviest objects will wind up in the centre, which for two galaxies will be the two black holes. Over time, the two black holes will lose enough energy while orbiting each other to merge into a single black hole. If the other galaxy was about the same mass as the original galaxy, this should double the mass of the black hole in one fell swoop - much more efficient than than by trying to build mass with gas.

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Why do we only see one side of the moon?

Lunar libration.  Image credit : wikimedia user  Tomruen .

Lunar libration. Image credit: wikimedia user Tomruen.

The simple answer (and one that you’ve probably heard before) is that we only see one side of the moon because the moon rotates around the Earth at the exact same speed as it rotates around its own axis, so that the same side of the moon is constantly facing the surface of the earth.  This means that one full ‘day’ of the moon (meaning the length of time it takes for the moon to rotate around itself once) is about 4 weeks long.  If the moon didn’t rotate at all, we would see all of its sides; the only way for us to see such a constant face of the moon is if it’s also rotating. There’s a great visualization of this below.

Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. Except for libration effects, this results in it keeping the same face turned towards the Earth, as seen in the figure on the left. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon didn't spin at all, then it would alternately show its near and far sides to the Earth while moving around our planet in orbit, as shown in the figure on the right.  Image credit : Wikimedia user  Stigmatella aurantiaca

Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. Except for libration effects, this results in it keeping the same face turned towards the Earth, as seen in the figure on the left. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon didn't spin at all, then it would alternately show its near and far sides to the Earth while moving around our planet in orbit, as shown in the figure on the right. Image credit: Wikimedia user Stigmatella aurantiaca

If you watch the way the moon spins (or doesn’t), you can see that only the left side has a consistent side facing the surface of our planet (which, we must note, is not even a little bit to scale here).

However, the underlying reason why the moon rotates at this exact speed, forcing us to only see a single side of it, is because the moon has been tidally locked to the earth.  Tidal locking is a stable configuration, and relatively easy to get to, given enough time, so many of our solar system’s moons are found to be tidally locked, including the dwarf planet Pluto and its largest moon Charon, which are both tidally locked to each other.

The “lock” part of this name refers to the way that an object - like the Moon - is apparently fixed in position, with one side always facing the other object.  Any object which is found to be tidally locked will always have one side of itself facing the surface of the planet it’s orbiting.  The amount of time it takes to orbit around the planet will vary from object to object (Phobos, one of the moons of Mars, is tidally locked and orbits Mars every 8 hours - way faster than our Moon), but as long as the object is tidally locked, the rotation will match the length of time it takes to orbit.

However, it’s the “tidal” part of the tidal locking that gives us the real key to why tidal locking happens at all.

We’re most familiar with tides as the effect of our oceans rising and falling due to the position of the moon.  The Moon’s gravity pulls on the earth, and the water on the surface of the Earth closest to the moon responds to that pull by elongating towards the moon. The water on other parts of the earth feels the Moon’s gravitational pull as weaker, with the water on the opposite side of the earth feeling the weakest pull. However, these tidal forces also have another effect - they resist rotation.

The Moon was almost certainly not tidally locked when it first formed - at that time, it would have rotated at a faster speed, which meant that had any observer been on the early Earth, they could have seen all sides of the moon as it spun.  However, the gravitational pull from the Earth - which like the tides due to the Moon, pulls on the side of the Moon closest to the earth more than the far side, resisted this faster rotation. This resistance due to the gravitational pull of the Earth gradually slowed down the faster spin of the Moon until the Moon was no longer rotating faster than it was orbiting.  Once the Moon’s rotation had slowed so much that a single face was always facing the surface of the Earth, it had officially been tidally locked, and has stayed in this configuration ever since.

The Moon also has the same influence on the Earth, but since the Moon is so much less massive than the Earth, this resistance to rotation takes a much longer time to impact the Earth's spin.  However, it’s still a measurable effect! The Moon is slowing down the rotation of the Earth by about 15 microseconds every year, gradually lengthening our days.


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Are all nebulae/galaxy photos false colour? Even NASA ones?

Most of the nebulae and galaxy photos are what we’d call false colour, yes - although it’s probably much more fair to the people who make these images to call them “exaggerated colour”, or perhaps “reconstructed colour”. These images do not usually reflect what we would see if we looked at them ourselves.

The human eye has a really bizarre sensitivity pattern to light. We’re pretty good at seeing things in the yellow-green range, orange we can usually do, but once you get into reds and blues, our eye suddenly gets really bad at registering the deep reds and dark purples, and our brain translates those colours into “black”, or more accurately as, “there are no photons here that I can deal with”. To anything outside the range of visible light we are completely blind. This odd sensitivity pattern means that it’s really hard to make a camera with exactly the same sensitivity as our eye. This is the same reason why it’s sometimes hard to get your camera to pick up the colours we can see by eye. Most cameras have settings nowadays to help change the sensitivity towards a specific colour, but they won’t perfectly replicate the eyeball.

Furthermore, from a scientific standpoint, replicating the eyeball isn’t an incredibly useful thing to do. We’re usually more interested in either a specific colour of light (usually one that corresponds to the colour certain atoms release) which helps us tell how much of that atom is present in the nebula or galaxy. Alternately, we can go after broader “colour” - the relative contribution of blue versus red light tells us things about the stars and dust in a galaxy. If we’re interested in these total values, and in trying to compare red and blue, then introducing the handicaps of the human eye into the equation will only serve to complicate our situation more than necessary.

Given that we’re detecting light at much better sensitivities than the human eye, and that we’re usually doing it discrete chunks instead of one (very complex) curve as the eye does, putting these chunks of light back into a single image is tricky business. Even when all the light was taken from the narrow range of light that we could see, it must still be reconstructed and tweaked to reflect the brilliance of the nebula in the colours we’ve observed. Hubble has produced many beautiful images (such as the one above) labeled as ‘visible light images’. What this means is that the narrow ranges of colours that Hubble observed all fall within the the visible range - but they have still been patched together, the colour of each set of data overlaying on top of each other to build an image in full colour. This particular image had 6 colours to work with, and it’s made a lovely and vivid image, but it is still only six colours. The colours here aren’t really “false”, but they have been “reconstructed” from six black and white images.

“Exaggerated colour” images can be used to extend our sight much beyond what we can actually see. Perhaps a galaxy is rather unimpressive in visible light, but has an impressive brilliance in the ultraviolet or X-Ray - to our eyes this is dark; but if the telescope can look at ultraviolet or X-Ray light, we can put it into our image, and reconstruct an image that we will never see with our own eyes.

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Does our galaxy orbit anything?

The Moon orbits the Earth, the Earth orbits the Sun, the Sun orbits the center of the Milky Way, but does our galaxy orbit anything?

Our galaxy does indeed! The Milky Way is one of two large galaxies that make up what’s called the Local Group, which contains some fifty-odd galaxies. The other large galaxy involved is Andromeda, our closest galactic neighbor; our galaxy and Andromeda are slowly orbiting each other. The rest of the Local Group are mostly small things, like the Large or Small Magellanic Clouds, which are gravitationally tied to either the Milky Way or Andromeda, and orbit the larger galaxy to which they’re bound. Andromeda weighs in somewhere between 700 billion solar masses and a trillion solar masses. This is approximately the same mass as our own Milky Way, which is also usually considered to have about a trillion solar masses worth of stuff hanging around.

If you want to figure out how any two objects are going to orbit each other, you want to know their mass, how far apart they are, and how fast they’re moving relative to each other. With this information, you can determine what path the two objects will take relative to each other. The main thing we need to be concerned with right now is the mass. The masses of your two objects - in this case, the Milky Way and Andromeda - determine the point around which both objects will orbit. This is called the centre of mass, and is defined as the point in space that has an equal distribution of mass around it. For a system like the Sun and the Earth, the Sun contains almost all the mass - the mass of the Earth being so far away doesn’t really change the center of mass very much. The Earth only pulls the Earth-Sun center of mass the tiniest bit away from the centre of mass of the Sun itself. But if the two objects are close to each other in mass and widely separated, the center of mass is actually between the two, in empty space! This is the case for Pluto and its moon Charon, which are close enough in mass that they both technically orbit a point above the surface of Pluto.

The same is true of the Milky Way and Andromeda. They’re both extremely massive objects and very far away from each other, but they’re gravitationally tied to each other. Since neither can escape the gravitational pull of the other, they are in orbit around a point somewhere near the middle of the space between the Milky Way and Andromeda. This point - the center of mass between the two - is where our galaxy and Andromeda will eventually collide. (Not to worry, it’ll be another 3-4 billion years.)

You could scale up your question - does the Local Group as a whole orbit anything? And it does - the Local Group is part of the Virgo Supercluster, which in turn is in motion relative to even larger structures in the universe. Nothing is truly at rest, after all.

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