10 things you don’t know about black holes

Ah, black holes. The ultimate shiver-inducer of the cosmos, out-jawing sharks, out-ooking spiders, out-scaring… um, something scary. But we’re fascinated by ‘em, have no doubt - even if we don’t understand a whole lot about them.

1) It’s not their mass, it’s their size that makes them so strong.

OK, first, a really quick primer on black holes. Bear with me!

The most common way for a black hole to form is in the core of a massive star. The core runs out of fuel, and collapses. This sets off a shockwave, blowing up outer layers of the star, causing a supernova. So the star’s heart collapses while the rest of it explodes outwards

As the core collapses, its gravity increases. At some point, if the core is massive enough (about 3 times the mass of the Sun), the gravity gets so strong that right at the surface of the collapsing core the escape velocity increases to the speed of light. That means that nothing can escape the gravity of this object, not even light. So it’s black. And since nothing can escape, well, read the quotation at the top of the page.

The region around the black hole itself where the escape velocity equals the speed of light is called the event horizon. Any event that happens inside it is forever invisible.

OK, so now you know what one is, and how they form. Now, the video showing why they have such strong gravity.

Sure, the mass is important, but sometimes it’s the little things that count.





2) They’re not infinitely small.

So OK, they’re small, but how small are they?

I was writing about black holes in my previous job, and we got in a fun discussion over just what we meant by black hole: did we mean the object itself that collapses down to a mathematical point, or the event horizon surrounding it? I said the event horizon, but my boss said it was the object. I decided she had a point (HAHAHAHAHA! A “point”! Man, I kill me), and made sure that when I wrote about the event horizon versus the black hole itself I was making myself clear.

Like I said above, to the collapsing core, its clock keeps ticking, so it sees itself collapsing all the way down to a point, even if the event horizon has some finite size.

What happens to the core? The actual mass that collapsed?

Out here, we’ll never know for sure. We can’t see in, and it sure enough isn’t gonna send any info out. But our math in these situations is pretty good, and we can at least apply them to the collapsing core, even when it’s smaller than the event horizon.

It will continue to collapse, and the gravity increases. Smaller, smaller… and when I was a kid I always read that it collapses all the way down to a geometric dot, an object with no dimensions at all. That really bugged me, as you can imagine… as well it should. Because it’s wrong.

At some point, the collapsing core will be smaller than an atom, smaller than a nucleus, smaller than an electron. It’ll eventually reach a size called the Planck Length, a unit so small that quantum mechanics rules it with an iron fist. A Planck Length is a kind of quantum size limit: if an object gets smaller than this, we literally cannot know much about it with any certainty. The actual physics is complicated, but pretty much when the collapsing core hits this size, even if we could somehow pierce the event horizon, we couldn’t measure its real size. In fact, the term “real size” doesn’t really mean anything at this kind of scale. If the Universe itself prevents you from measuring it, you might as well say the term has no meaning.

And how small is a Planck Length? Teeny tiny: about 10-35 meters. That’s one one-hundred quintillionth the size of a proton.

So if someone says a black hole has zero size, you can be all geeky and technical and say, not really, but meh. Close enough.

3) They’re spheres. And they’re definitely not funnel shaped.

The gravity you feel from an object depends on two things: the object’s mass, and your distance from that object. This means that anyone at a given distance from a massive object - say, a million kilometers - would feel the same force of gravity from it. That distance defines a sphere around an object: anyone on that sphere’s surface would feel the same gravity from the object at the center.

The size of an event horizon of a black hole depends on the gravity, so really the event horizon is a sphere surrounding the black hole. From the outside, if you could figure out how to see the event horizon in the first place, it would look like a pitch black sphere.

Some people think of black holes as being circles, or worse, funnel-shaped. The funnel thing is a misconception from people trying to explain gravity as a bending in space, and they simplify things by collapsing 3D space into 2D; they say the space is like a bed sheet, and objects with mass bend space the same way that a massive object (a bowling ball, say) will warp a bed sheet. But space is not 2D, it’s 3D (even 4D if you include time) and so this explanation can confuse people about the actual shape of a black hole event horizon.

4) Black holes spin!

It’s kind of an odd thought, but black holes can spin. Stars rotate, and when the core collapses the rotation speeds way, way up (the usual analogy is that of an ice skater who brings in his arms, increasing his rotation rate). As the core of the star gets smaller it rotates more rapidly. If it doesn’t quite have enough mass to become a black hole, the matter gets squeezed together to form a neutron star, a ball of neutrons a few kilometers across. We have detected hundreds of these objects, and they tend to spin very rapidly, sometimes hundreds of times a second!

The same is true for a black hole. Even as the matter shrinks down smaller than the event horizon and is lost to the outside Universe forever, the matter is still spinning. It’s not entirely clear what this means if you’re trying to calculate what happens to the matter once it’s inside the event horizon. Does centrifugal force keep it from collapsing all the way down to the Planck length? The math is fiendish, but do-able, and implies that matter falling in will hit matter inside the event horizon trying to fall further but unable to due to rotation, This causes a massive pile up and some pretty spectacular fireworks… that we’ll never see, because its on the other side of infinity.





5) Near a black hole, things get weird

The spin of the black hole throws a monkey in the wrench of the event horizon. Black holes distort the fabric of space itself, and if they spin that distortion itself gets distorted. Space can get wrapped around a black hole - kind of like the fabric of a sheet getting caught up in a rotating drill bit.

This creates a region of space outside the event horizon called the ergosphere. It’s an oblate spheroid, a flattened ball shape, and if you’re outside the event horizon but inside the ergosphere, you’ll find you can’t sit still. Literally. Space is being dragged past you, and carries you along with it. You can easily move in the direction of the rotation of the black hole, but if you try to hover, you can’t. In fact, inside the ergosphere space is moving faster than light! Matter cannot move that fast, but it turns out, according to Einstein, space itself can. So if you want to hover over a black hole, you’d have to move faster than light in the direction opposite the spin. You can’t do that, so you have to move with the spin, fly away, or fall in. Those are your choices.

6) Approaching a black hole can kill you in fun ways. And by fun, I mean gruesome, horrifying, and really really ookie.

Sure, if you get too close, plop! You fall in. But even if you keep your distance you’re still in trouble…



Gravity depends on distance. The farther you are from an object, the weaker its gravity. So if you have a long object near a massive one, the long object will feel a stronger gravitational force on the near end versus a weaker force on the far end! This change in gravity over distance is called the tidal force (which is a bit of a misnomer, it’s not really a force, it’s a differential force, and yes, it’s related to why we have ocean tides on Earth from the Moon).

The thing is, black holes can be small - a BH with a mass of about three times the Sun has an event horizon just a few kilometers across - and that means you can get close to them. And that in turn means that the tidal force you feel from one can get distressingly big.



Let’s say you fall feet first into a stellar-mass BH. It turns out that as you approach, the difference in gravity between your head and your feet can get huge. HUGE. The force can be so strong that your feet get yanked away from your head with hundreds of millions of times the force of Earth’s gravity. You’d be stretched into a long, thin strand and then shredded.

Astronomers call this spaghettification. Ewwww.

So getting near a black hole is dangerous even if you don’t fall in. Evidently, there really is a tide in the affairs of men.



7) Black holes aren’t always dark

The thing is, black holes can kill from a long way off.


Matter falling into a black hole would rarely if ever just fall straight in and disappear. If it has a little bit of sideways motion it’ll go around the black hole. As more matter falls in, all this junk can pile up around the hole. Because of the way rotating objects behave, this matter will create a disk of material whirling madly around the hole, and because the gravity of the hole changes so rapidly with distance, matter close in will be orbiting much faster than stuff farther out. This matter literally rubs together, generating heat through friction. This stuff can get really hot, like millions of degrees hot. Matter that hot glows with intense brightness… which means that near the black hole, this matter can be seriously luminous.

Worse, magnetic and other forces can focus two beams of energy that go plowing out of the poles of the disk. The beams start just outside the black hole, but can be seen for millions or even billions of light years distant.

They’re bright.

In fact, black holes that are eating matter in this way can glow so brightly that they become the brightest continuously-emitting objects in the Universe! We call these active black holes.

And as if black holes aren’t dangerous enough, the matter gets so hot right before it makes the final plunge that it can furiously emit X-rays, high-energy forms of light (and the beams can emit even higher energy light than that). So even if you park your spaceship well outside the event horizon of a black hole, if something else falls in and gets shredded, you get rewarded by being fried by the equivalent of a gazillion dental exams.

Black holes are dangerous. Best to stay away from them.



8) Black holes aren’t always dangerous.



Having said that, let me ask you a question: if I were to take the Sun and replace it with Folgers crystals a black hole of the exact same mass, what would happen? Would the Earth fall in, be flung away, or just orbit like it always does?

Most people think the Earth would fall in, sucked inexorably down by the black hole’s powerful gravity. But remember, the gravity you feel from an object depends on the mass of the object and your distance from it. I said the black hole has the same mass as the Sun, remember? And the Earth’s distance hasn’t changed. So the gravity we’d feel from here, 150 million kilometers away, would be exactly the same! So the Earth would orbit the solar black hole just as nicely as it orbits the Sun now.

Of course, we’d freeze to death. You can’t have everything.




9) Black holes can get big.

Q: What happens if two stellar-mass black holes collide?

A: You get one bigger black hole.

You can extrapolate from there. Black holes can eat other objects, including other black holes, so they can grow. We think that early on in the Universe, when galaxies were just forming, matter collecting in the center of the nascent galaxy can collapse to form a very massive black hole. As more matter falls in, the hole greedily consumes it, and grows. Eventually you get a supermassive black hole, one with millions or even billions of times the mass of the Sun.

However, remember that as matter falls in it can get hot. It can be so hot that the pressure from light itself can blow off material that’s farther out, a bit like the solar wind but on a much grander scale. The strength of the wind depends on many things, including the mass of the black hole; the heftier the hole, the windier the, uh, wind. This wind prevents more matter from falling in, so it acts like a cutoff valve for the ever-increasingly girthy hole.

Not only that, but over time the gas and dust around the black hole (well, pretty far out, but still near the center of the galaxy) gets turned into stars. Gas can fall into a black hole more easily than stars (if gas clouds collide head-on their motion relative to the black hole can stop, allowing them to fall in; stars are too small and too far apart for this to happen). So eventually the black hole stops consuming matter because nothing more is falling into it. It stops growing, the galaxy becomes stable, and everyone is happy.


In fact, when we look into the Universe today, we see that pretty much every large galaxy has a supermassive black hole in its heart. Even the Milky Way has a black hole at its core with a mass of four millions times that of the Sun. Before you start running around in circles and screaming, remember this: 1) it’s a long way off, 26,000 light years (260 quadrillion kilometers), 2) its mass is still very small compared to the 200 billion solar masses of our galaxy, and therefore 3) it can’t really harm us. Unless it starts actively feeding. Which it isn’t. But it might start sometime, if something falls into it. Though we don’t know of anything that can fall into it soon. But we might miss cold gas.



Anyway, remember this as well: even though black holes can cause death and destruction on a major scale, they also help galaxies themselves form! So we owe our existence to them.


10) Black holes can be low density.

Of all the weirdnesses about black holes, this one is the weirdest to me.

As you might expect, the event horizon of a black hole gets bigger as the mass gets bigger. That’s because if you add mass, the gravity gets stronger, which means the event horizon will grow.

If you do the math carefully, you find that the event horizon grows linearly with the mass. In other words, if you double the black hole’s mass, the event horizon radius doubles as well.

That’s weird! Why?

The volume of a sphere depends on the cube of the radius (think way back to high school: volume = 4/3 x ? x radius3). Double the radius, and the volume goes up by 2 x 2 x 2 = 8 times. Make the radius of a sphere 10 times bigger and the volume goes up by a factor of 10 x 10 x 10 = 1000.

So volume goes up really quickly as you increase the size of a sphere.

Now imagine you have two spheres of clay that are the same size. Lump them together. Is the resulting sphere twice as big?

No! You’ve doubled the mass, but the radius only increases a little bit. Because volume goes as radius cubed, to double the radius of your final clay ball, you’d need to lump together eight of them.

But that’s different than a black hole. Double the mass, double the size of the event horizon. That has an odd implication…

Density is how much mass is packed into a given volume. Keep the size the same and add mass, and the density goes up. Increase the volume, but keep the mass the same, and the density goes down. Got it?

So now let’s look at the average density of matter inside the event horizon of the black hole. If I take two identical black holes and collide them, the event horizon size doubles, and the mass doubles too. But volume has gone up by eight times! So the density actually decreases, and is 1/4 what I started with (twice the mass and eight times the volume gives you 1/4 the density). Keep doing that, and the density decreases.

A regular black hole - that is, one with three times the Sun’s mass - with have an event horizon radius of about 9 km. That means it has a huge density, about two quadrillion grams per cubic cm (2 x 1015). But double the mass, and the density drops by a factor of four. Put in 10 times the mass and the density drops by a factor of 100. A billion solar mass black hole (big, but we see them this big in galaxy centers) would drop that density by a factor of 1 x 1018. That would give it a density of roughly 1/1000 of a gram per cc… and that’s the density of air!

A billion solar mass black hole would have an event horizon 3 billion km in radius - roughly the distance of Neptune to the Sun.

Are black holes at the center of galaxies?

Bubbles of dark matter could be masquerading as supermassive black holes at the centres of galaxies. If so, they could explain the puzzling pattern of X-ray emissions from the heart of the Milky Way.

Cosmologists know that most galaxies host a compact, supermassive object at their centre and they believe these must be black holes. Such a black hole is thought to be responsible for the X-ray flares coming from the middle of our galaxy, which would be caused by the black hole devouring surrounding matter. But recent observations show that these flares fire roughly every 20 minutes – a regularity that is hard to explain in terms of the behaviour of a black hole.

Now Anatoly Svidzinsky, a physicist at Texas A&M University in College Station, Texas, thinks that hypothetical particles called axions could solve the mystery. Axions have very little mass and no electric charge, and they barely interact with other particles. They were originally proposed to fix a problem with the strong force in particle physics, but have more recently been considered as possible candidates for dark matter, the unseen stuff thought to make up nearly 90 per cent of a galaxy's mass.

In the 1990s, computer simulations of clouds of dark matter made of axions showed that giant bubbles of these particles would burst out from the clouds. Svidzinsky thinks that such bubbles exist at the centre of galaxies. His model shows that the axion bubbles would expand and contract with a period of 20 minutes – matching the period of infrared and X-ray flares from Sagittarius A*, the location of the supermassive compact object at the centre of our galaxy. The model predicts that stable axion bubbles would weigh between about 1 million and 2.5 billion times the mass of the sun – exactly the mass range observed for compact objects at the centres of galaxies (www.arxiv.org/ astro-ph/0607179).

"The proposal looks quite intriguing," says Tim Sumner, who is leading the search for galactic dark matter, including axions, at Imperial College London in the UK. "But it obviously needs a lot more evidence and assessment before it can really displace the more established scenarios."

One big assumption in Svidzinsky's model is that gravity starts to repel as the gravitational field gets stronger – a tweak to general relativity proposed by physicist Huseyin Yilmaz in the 1990s. And this is what causes the bubbles to oscillate. As the bubble grows, its surface tension pulls it back. As it collapses, its gravity eventually becomes repulsive and the bubble expands again.

The fact that Svidzinsky's model relies on this controversial version of gravity doesn't necessarily count against it, says Konstantin Zioutas of the particle physics laboratory CERN in Geneva, Switzerland. "There are various studies in progress around the world which suggest that Einstein did not speak the last word on gravity," says Zioutas. For example, extra dimensions can change the way that gravity behaves in extreme cases.

The other important question is whether axions really exist. There have been attempts to create them in the lab (New Scientist, 14 July, p 35) and even a possible indirect sighting in the sun's halo by Zioutas (New Scientist, 17 April 2004, p 8). "Until their existence is confirmed, axions will appear to be a deus ex machina," says Zioutas.

Still, Zioutas adds, if Svidzinsky is correct, his idea could solve another mystery perplexing astronomers. As well as X-ray flares, astronomers can see diffuse X-rays emanating from our galactic centre. "We don't know what can be causing these," says Zioutas. "Any gas that would be hot enough to emit this radiation would be moving too fast to be held in our galaxy." Dark matter axions, however, could be releasing these X-rays as they decay, he says.

Evidence one way or the other may be just around the corner. "Within a few years astronomers will be able to resolve compact objects at the centre of galaxies with radio interferometers," says Svidzinsky. Black holes will have a constant size, whereas an axion bubble's radius will oscillate, he says. Zioutas is looking forward to the answer. "There is so much at stake here – rewriting both Einstein and dark matter," he says.

Black Hole Gives Up Some Secrets

ATLANTA - Three

separate teams of researchers have unlocked some longstanding secrets of two stars that have puzzled astronomers for more than 20 years.

The studies focus on the pair of objects known as binary system SS 433, some 16,000 light-years from Earth. The pair consists of an old, faint star locked in a tight orbit with a stellar corpse -- either a black hole or dense neutron star. The presumed black hole constantly strips gas from its companion, channels it into a flat "accretion disk" and then later spits the material out in opposing, polar jets that shoot out at 90-degree angles to the disk.

Astronomers have seen SS 433 do some strange things since the system was first discovered in the 1960s. Not only are its jets detectable in infrared and X-rays, but also visibly too. And the light spectrum seen by astronomers seems to change over time.

The system "remains unique and hard to understand 25 years after its discovery," said Bruce Margon, associate director for the Space Telescope Science Institute, which runs the Hubble Space Telescope. "For example, why is this the only star system we see with these relativistic jets?"

Relativistic is a term used to describe material moving at a significant fraction of the speed of light.

The jets were the target of researchers at Massachusetts Institute of Technology (MIT), who used the Chandra X-ray Observatory to study SS 433s emissions, which stream from the object at a roughly one-fourth of light's speed. Their results may help develop new ways to measure masses of black holes.

The system resembles the output of giant galaxies in the making, objects called quasars that exist mostly far away and in the early universe.

"We believe its very much like a quasar jet," MITs Herman Marshall, lead investigator for his team, said of SS 433. "Its a jet laboratory thats closer to home."

Another effort by astronomers with the National Radio Astronomy Observatory (NRAO) made daily observations of SS 433s apparent black hole, compiling the images into a movie that allowed them to track individual ejections of jet material as they spewed forth. The movie helped researchers determine the source of brightness variations in the jets.

A separate team led by astronomer Todd Hillwig of Georgia State University made the first observations the companion star in SS 433.

The results of all three studies were presented here Monday during the 203^rd meeting of the American Astronomical Society.

Honing in on a microquasar

SS 433 is what astronomers call a microquasar, a system where the central massive object -- a neutron star or black hole -- mimics the behavior of quasars, which are thought to consist of gigantic black holes at the center of distant, developing galaxies and which consistently spit bright jets of materials from their poles.

But since microquasars are closer to Earth than their quasar cousins -- which can sit 10 billion light-years in the distance -- they are easier to observe and changes occur more noticeably over time.

"If you were trying to [see] this with an extragalactic jet, it would take years, even decades" said Amy Mioduszeweski, who led one study for the NRAO.

NRAO astronomers used the Very Large Baseline Array, a network of 10 radio telescopes arranged across 5,000 miles (8,046 kilometers) from Hawaiis Mauna Kea to St. Croix in the U.S. Virgin Islands to track SS 433s jets.

Observations of past microquasars have shown that their jets typically get fainter as they taper out into space. But SS 433 doesnt. As the jets shoot out, they sometimes flare up to become even brighter.

The NRAO study also detected other, non-jet related radio waves emanating from SS 433s microquasar core, leading researchers to believe the objects accretion disk forms a wind that interacts with a denser wind from the black holes companion star, causing the added emissions. The disk wind may also interact with jet material to cause the belated jet flare-ups.

"The most obvious place for it to come from is from the center of this system," Mioduszeweski said. "But we dont know for sure where this outflow is coming from."

The system is in the constellation Aquila, the Eagle.

A matter of timing

Hillwigs team had a hard time catching a glimpse of the SS 433s companion star. The accretion disk and bright jets of the black hole are so bright they blot out any chance of seeing the companion star, except during a time of eclipse, when the star crosses in front of the black hole as seen by observers on Earth.

"We think its a matter of timing," explained Douglas Gies, a Georgia State astrophysicist and team member who presented the study. "The best time to see to see the mass-donor star is when the accretion disk wobbles in precession during an eclipse. This set of requirements exists just twice each year."

Although the star passes in front of the black hole every 13 days, only twice a year does it do so at an angle high enough above the black hole to be noted by ground-based instruments.

After a failed first attempt, Hillwigs team successfully identified the star from its companion using the 13-foot (four-meter) telescope at Arizonas Kitt Peak National Observatory. What the team found was a light pattern suggesting SS 433s star is an old, swollen supergiant with a surface temperature of about 13,000 Fahrenheit. It has a mass 11 times greater than that of Earths Sun. The large mass provides a feast for SS 433s much more compact black hole.

"Apparently, the black hole cant digest the overwhelming amount of gas its companion star is giving," Gies said. "So it may be funneling the excess into these powerful jets."

How much less massive the black hole is than its neighbor is up for debate. Hillwigs team claimed the black hole was roughly three solar masses while the MIT study originally found it significantly larger, some 16 solar masses. MIT researchers later reduced that estimate to about eight solar masses. The two teams plan to work together in the future to hone in on a more definitive mass for the black hole.