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There's a Hole in the Middle of It All
Apparition writes "CNN is reporting that the star at the center of our galaxy is actually a super-massive black hole. The article then claims that it occupies a volume of space about 3 times that of our solar system. If my math is correct, about 230 million suns could fit into that same volume, so it doesn't impress me that the claimed mass of the black hole is only between 2.6 and 3.7 million times that of the sun. So what is up here? Since when do black holes occupy so much space (I thought they were points)? And how can something with a density only 1/100 of our Sun be called super-massive?" I think the article is talking about a maximum possible size of the object, due to limitations on the resolution of our instruments. Nature has a no-registration story about the research. Update: 10/16 23:44 GMT by M : There's an article with more information on space.com, and a press release from the European Southern Observatory.
Yes, black holes are a point (that's called the singularity), but they're talking about the size of the event horizon, or point of no return. So this particular black hole has a mass of 2.6 to 3.7 million or whatever suns, but its event horizon is larger than the solar system.
With a black hole this big, you can actually cross the event horizon, and not be torn apart because the change in gravity over a certain distance (6 feet or so for your height) isn't great enough. Smaller black holes will rip you apart quicker though
According to my Astronomy course, Super-Massive black holes are less "violent" than their smaller brothers because most of the mass is concentrated at the center in a very very small space. Their event-horizons are very large because of this mass, which makes them seem not as dense as we would assume. With a small black hole, the event horizon is very small, and thus the effects near the point are much more drastic because mass that passes the event horizon is "consumed" immediately. I realize I am simplifying quite a bit, but hopefully you get the point.
The "size" of the black hole refers to the size of its event horizon (a.k.a the Schwarzschild Radius), which is R = GM/2c^2. For a huge value of M ("supermassive"), the event horizon is very large: once you cross this, there's no coming back, and our physics stops at the edge. But since R is so large, the tidal forces are small at the event horizon - much smaller than the tidal forces at the event horizon of a smaller black hole. (Chew on it for a second and it makes sense).
The "actual" naked singularity is in fact a point, but we have no way of probing anything inside the event horizon. So calculating the density of a black hole is misleading...
"I will take the Ring," he said, "though I do not know the way."
Theoretically the mass of the galaxy itself should be enough to hold it together. Even the black hole could have originally been formed from matter collecting at the center of gravity of the galaxy.
This would seem to imply that, in theory, a very large black hole could have rather low density inside the event horizon. It seems to me that a black hole could spontaneously form around a particularly dense cluster of stars if it was large enough and they all happened to clump together.
But my head starts to hurt thinking about what happens to physics when a region of normal space suddenly finds itself inside a black hole like that. I am definitely not a physicist, so I can't explain what goes on inside a black hole, or if my globular cluster black hole is even possible.
Crispin
----
Crispin Cowan, Ph.D.
Chief Scientist, WireX Communications, Inc.
Immunix: Security Hardened Linux Distribution
Available for purchase
It's true that often when the size of a black hole is mentioned, it is the Swartzchild radius or "Event Horizon" that is being mentioned, being it's apparent size to our instruments.
It is not however true that black holes are points. A black hole that became a point gravity source is what is referred to as a singularity. It was a singularity that became the big bang and if the "big crunch" theory is correct, it will probably be a singularity that the universe ends as, but under any other circumstances the creation of a singulairty would require a set of events so astronomically unlikely that it is not believed that any do have or will come into existence during the lifetime of the universe. So in fact black holes DO have a radius, but considering the tremendous size quoted here, I imagine they are in fact referring to the Swartzchild radius.
lysergically yours
> (and presumably unknowable)
You could go in and find out, but due to time dilation, you would see the rest of time flash before your eyes, and then witness the end of the universe. You wouldn't be able to tell anybody though, because no signals can escape unless they travel faster than the speed of light (which is of course impossible). You would also be dead, but that's another story.
-- Wibble
Comets can orbit the sun for a really long time; some smack into an object (like the sun, for instance), some escape their orbit, and some just keep orbiting. There's nothing that guarantees the star will get sucked in; it all depends on the orbital path, really. It may experience a slingshot effect and leave the black hole altogether.
As the above reply suggests, the galaxy's own mass should be able to hold it together. Just like the sun holds together the solar system and the Earth holds itself together.
/.-ers might be a recent paper that describes the universe as a cyclical entity (no "big bang"), by representing it as a pair of branes (world sheets, see string theory...). The end result is there's an event (a "bounce) that might look like the Big Bang, but it's really just a collision between the branes.
Black holes are good candidates for causing a galaxy to accumulate. It can be kind of hard to explain what causes galaxies to form.
I'm getting off-topic, but I don't care...
One of the favorite explanation comes from irregularities in mass distribution as evidenced by perturbations in the cosmic microwave background. That's one of the reasons that the CMB became such a hot topic, it offers insight into the origin of large scale order in the universe.
Also of interest to
Like anything else in cosmology, it's all rather speculative (at least as compared to many other physical models).
Find the link on your own (/. might've even covered this topic).
An object can orbit a black hole just like a planet can orbit the Sun (or a star). The Sun will not swallow or pull in the Earth any time soon. Black Holes are not cosmic vacuum cleaners that "suck" up everything around them. If you're in a stable orbit, it would be just like orbiting a Sun.
That said, there is evidence from general relativity that due to graviton radiation (gravity particles), large orbiting bodies slowly move closer to each other. The gravitons leaving such a system take energy out of the system slowly bringing the orbiting bodies together. This effect is (AFAIK) theoretical, although many people are currently working on ways to detect this graviton radiation and show that it is coming from systems like this. So in this case, yes, eventually (think eons) the star and the black hole would slowly move towards each other (the star would move more since it the least mass of the two) and in this type of collision, the black hole wins.
Who said Freedom was Fair?
Radius = 2 * "Universal Gravitational Constant" * "mass inside event horizon" / pow("speed of light",2)
For a black hole the mass of our sun the radius is:
Radius = 2 * (6.67 * 10^-11m/kg/s^2) * (2 * 10^30 kg) / (3 * 10^8 m/s)^2 = 2.964 km
When you check my math make sure you get your units right. A black hole three times the size of our solar system would be quite massive, and you should be impressed.
Also, I saw a program on Discovery Channel a while ago (6 months+) which had an interview with an observational astronomer in which he claimed to have observed movement in the center of our Galaxy which was consistant only with a supermassive black hole. I guess he finally published.
Galium Arsenide is the material of the future, and always will be.
- r = 2GM/c^2
where G = 6.67e-11 m^3/s^2*kg (the gravitational constant) and c = 3e8 m/s (the speed of light, of course). Plug in 3 million sun-masses (the sun weighs 2e30 kg), and you have- r = 8.9e9 m = 5.5 million miles = 0.06AU
So unfortunately, the event horizon isn't three times as big as the solar system. The earth's orbit is 1AU (that's how the unit is defined). The event horizon barely stretches past the surface of the sun (7e8 meters)!So much for that idea!
"but really, wtf could hold an entire GALAXY together but a black hole?"
I am (or rather, was) an astrophysicist. The answer is the rest of the galaxy holds it together, a bit like the gravity of the Earth is what holds the Earth together. The galaxy has the mass of billions of stars - so any stars not at the center are being pulled towards the center.
In answer to the original poster, the 'size' of a black hole is its event horizon radius:
R = 2GM/c^2
where
G = universal gravitational constant
M = mass of the black hole
c = speed of light.
Quattuor res in hoc mundo sanctae sunt: libri, liberi, libertas et liberalitas.
Current theories in no way preclude the formation of a singularity.
;-)
True, but current theories also haven't proven that inside a black hole _is_ a singularity. Although it's been a while, I remember from an Astronomy class I took that due to the rate of spin outside the black hole, and that conservation of momentum would mean it would spin faster inside means that the odds of a true point singularity are relatively low.
But what do I know?
This is a small misunderstanding. Many people seem to think that a black hole has super gravity or extra strength power just because its a black hole. Actually, it all depends on the mass.
For example, if our sun suddenly turned into a black hole, we wouldn't get sucked in. We'd still orbit our new black hole sun the same way we orbited our old normal sun. Just because it became a black hole doesn't mean its mass changed. And since its mass didn't change, we would still orbit the same.
Ditto for our galaxy. If we didn't have this black hole at the center of the galaxy, but instead 3.7 million suns, everything would orbit just the same
---
A black hole is just God dividing by zero
"A language that doesn't affect the way you think about programming, is not worth knowing" - Alan Perlis
This is not conclusive proof of black hole theory, only conclusive proof of a supermassive object at the center of our galaxy. It does not answer the theoretical question as to whether black holes or gravastars best fit the observations.
Obviously, the scientists making this announcement would be in the black hole segment of the physics community.
Trying to think of something witty and clever to end this post with . . . . ah, screw it . . .
-- If any of the above made sense, I assure it was purely by accident.
The general theory of relativity predicts the formation of singularities, but when taken into consideration along with quantum theory as both Stephen Hawking and Roger Penrose have, they become astronomically unlikely(but not impossible). The formation of a black hole would require a mass at least as large as the one in the centre of our galaxy to form a true point singularity and it would have to compress in a mathematically exact symmetrical fashion. Most black holes should have a radius according to modern theories which use both relativity and quantum mechanics rather than ignoring one in favor of the other. Mind you, that radius should by phenomenally tiny.
The discussion you refer to is the one about Hawking radiation. Stephen Hawking has demonstrated that Black Holes do actually(counter to intuition) radiate an extroardinarily small amount of energy. There is considerable debate as to whether it is possible for this radiation to ever cause the black hole to dissipate.
lysergically yours
If my memory serves me, I believe that there is a nearly confirmed black hole at the center of M15 (a globular cluster). However the conditions for it's creation are probably still up for debate. A bunch of semi-simultaneous stellar collisions at the core is not out of the question though.
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"I think the article is talking about a maximum possible size of the object, due to limitations on the resolution of our instruments."
I'm sure this editorial comment was well-intentioned, but the article would have been much better off without it. What the article refers to corresponds closely quite nicely to the Schwarzschild radius of a supermassive black hole.
A very massive black hole will necessarily be much less dense than the Sun, and can even be less dense than the Earth.
The simple reason is that (assuming a static, spherically symmetric mass distribution) the mass of an object is directly proportional to its Schwarzschild radius. But density is proportional to mass divided by radius cubed.
So if you double the mass of a black hole, you must necessarily double its radius. By definition this increases its volume eight-fold, and so its density is decreased by a factor of four.
So as you consider larger and larger black holes, you must see that their densities are smaller and smaller.
If you are in the market for a comparatively easy textbook that will teach you more about general relativity, I recommend Exploring Black Holes by Taylor and Wheeler. If you have a firm grasp of calculus and freshman physics, you will be able to handle it. It is more expensive than a normal book, but cheaper than the average textbook.
Comment removed based on user account deletion
The author's confusion here seems to be regarding the differences between a blackhole and its singularity.
A black hole is just that -- a black hole. It is a region of space from which nothing can escape (approximately; black holes do very slowly radiate heat). In other words, the volume a black hole occupies is defined by the Schwartzchild radius: the point beyond which the escape velocity exceeds c.
A singularity is the "center" of a black hole; it is an infinitely dense point in space, of enormous mass.
Interestingly, black holes may have some useful properties for astronomers. Light heading towards a black hole will be refracted around it and bent; in essence, the black hole acts like a magnifying glass.
social sciences can never use experience to verify their statemen
It's true that often when the size of a black hole is mentioned, it is the Swartzchild radius or "Event Horizon" that is being mentioned, being it's apparent size to our instruments.
If general relativity is correct, there is no other radius that can be measured by an external observer (assuming some rather general conditions concerning the mass distribution).
So as long as you are using general relativity as a framework, from a scientific standpoint there is not much use in speculating about something that you can never measure.
The size issue: the companion star's orbit tells us the maximum possible size of the central object. If the orbit is 17 light hours across, the primary is at most that large. It can be smaller, just as our Sun's diameter is smaller than the orbit of Mercury.
The proof the central object is a black hole is that nothing else can fit millions of solar masses into a sphere 17 light-hours across. The black hole need not fill that volume. More precisely, the event horizon need not fill that volume.
Singularities, point masses, event horizons: the size of a black hole depends what you mean. The singularity is the postulated point of infinite density: outside observers can't see it because it's inside the event horizon. The event horizon is the point of no return; in classical terms, the escape velocity equals the speed of light at the event horizon. The gravitational force is finite at the event horizon, and need not be extreme if the black hole is very, very large. If the universe is closed, we are all inside a black hole now, and will experience singularity at the Big Crunch.
But it isn't useful to think about the inside of a black hole. Different physics might apply -- lots of smart people think so. From the outside, as another poster wrote, all you get to observe is the black hole's total mass, total charge and total angular momentum -- that's plenty to work with in astronomical observations.
As to matter 'spiralling in', or the entire galaxy being sucked in by 'infinite gravity': Earth isn't being sucked into our Sun, is it? Unless you're quite close to one, the gravitational field of a black hole essentially (asymptotically) follows an inverse square law, like the gravity from any object. (When you get close, in units of the Schwarzchild radius, you do indeed 'spiral in' because the field strength increases faster than inverse square. The precession of Mercury's orbit is used to measure the deviation from inverse-square near our Sun, and is one of the 'proofs' of Einstein's General Relativity.)
The other mechanism for 'spiralling in' is loss of orbital energy due to friction, as in the accretion disk around neutron stars, for example.
That is all. Return to your homes and families. :-)
je ne suis pas un fou
I found this cool earth orbit physics toy and demonstration while reading one of my favorite web logs, Sensible Erection. (I, uh... Read it for the articles.)
/drag the screen to put a satelite in orbit.. see how long it lasts...
"This is the coolest this i have seen all week, click
pull off a moon only orbit for maximum kudos"
The physics for object orbits are incredible. This is a great demonstration of the exact effects you describe, and should apply to the questions and comments about orbits around a black hole.
Enjoy!
P.S.: You have no idea what a breath of fresh air it is to be able to visit cool links that aren't being slashdotted to hell and back.
Please mod down all the people who are currently at +5 claiming that the size of the object is really the event horizon, which is very large due to it being a supermassive black hole. This is a true statement, but it still doesn't explain the claimed size of the black hole in the article.
If you work out the schwartzchild radius of the sun using r=2GM/c^2 it comes out to around 3000 m. For the upper limit of 3.7 million solar masses that would mean that the black hole had a schwartzchild radius of around 1 x 10^10 m. This is about a factor of 14 larger than the radius of the sun which is 7 x 10^8 m.
This is no where near as large as the "volume of space around 3 times larger than the solar system" which is in the article. The poster of the article was also correct that the density was way too low. It is correct that supermassive black holes have large event horizons which are larger than the radii of typical stars like the sun. However, the average density inside of that event horizon is still denser than a neutron star.
I wish I had the 5 moderator points I had last week, I'd go to town on this story...
Some of the more avant guard sting theorists are advancing the notion that black holes are simply really really big (as in high energy) elementary particles (i.e. strings). It'll be interesting to see if this particular theory holds any water, because it might mean high energy physicists may one day be trying to sling black holes at each other ;).
BlackGriffen
How is this story ground-breaking news? I posted this story over a year ago on Sept 6, 2001. The original post's article even states that they estimate the mass to be 2.6 million times that of the sun. Nice repeat.
This limitation excludes any other explanations, such as a dense cluster of stars or a cloud of stellar material. That much mass in that little space would inevitable collapse and become a black hole.
Regarding discussions about whether the "volume" of the article implied the Event Horizon, that's what I thought it was at first also. But then I came up with some numbers that don't seem to correspond to those of the CNN article. I then checked out the original paper. The paper is formally on the observation of a star that seems to be orbiting the galaxial center, and this radius of orbiting is what they are pinning down as the a putative upper limit of the size of the supermassive object.
It would seem that the original poster's comment was correct in that this was the _Upper Limit_ of the radius of the supermassive object, and not the Event Horizon radius.
Let me clarify,
The Schwarzschild radius (Or Event Horizon) is given by
r_SCH = 2 G M / c^2
where G is gravitational constant, M is mass of object, and c is speed of light. If we use, as per CNN article (yeah, I know, good source)
M = 3 x 10 ^ 6 * mass of sun
mass of sun = 2 x 10 ^ 30 kg
s.t. M = 6 x 10 ^ 36 kg
and G = 6.67 x 10^ -11 Nm^2/kg^2
and c = 3 x 10^8 m/s^2
then r_SCH = 12 x 10 ^ 36 * 6.67 x 10 ^-11/9 x 10^16
r_SCH ~ 1 x 10^10 meters.
I looked up some values of Pluto's radius, and got about 3000 million miles, or 5 x 10^9 km, or about 5 x 10^12 m.
So this galaxial blackhole seems to have a radius 100-1000 times less than the solar system radius.
And indeed, in the final page of the Schodel paper, there is a mention that the observed radius of the orbiting star is ~ 2000 times the Schwarzschild radius, and not the actual Schwarzschild of the star. i.e. the observed radius of orbit is much much larger than the putative Schwarzchild radius.
These researchers are popular here on campus at UCLA. Also, check out some nifty pictures here.
> if the "big crunch" theory is correct,
It's not. Astronomers have known for a while that the universe was expanding, but didn't know the rate. They recently discovered that the rate was accelerating!
Cheers
Assuming a football field is 100m long
(sorry, I'm British):
size of event horizon (36 light second)
= 100 million football fields
closest approach distance of star to hole
(17 light hours)
= 184 billions football fields
The world is everything that is the case
Roger Penrose talks about it in his book `the emperor's new mind', and here is an excellent link
The Schwartzchild radius is the radius, for a given mass, that will form a singularity. For a ten solar mass star, that is about 30 kilometers.
The Chandrasekhar limit gives the size limit for a star to collapse and produce a white dwarf. Most stars end their lives with a gravitational collapse, but electron degeneracy pressure (from the Pauli exclusion principle) prevents further collapse. However, for stars above ~1.2 solar masses, the gravitational collapse will overcome fermion repulsion, and the collapse will continue. Once the star's density has reached a certain point, it will collapse into a singularity. That density times the star's mass determines the Schwartzchild radius.
The event horizon is delineated by those light rays that will neither fall in nor escape from, the black hole. However, just because you cross the event horizon does not necessarily mean you will strike the singularity. Instead, it depends upon the type of black hole you've encountered.
In actual reality, you'll be fried by the blue shifted radiation coming from the accretion disk around the hole, but let's ignore that quibble.
Black holes have mass, spin, and charge. No other properties are discernable behind the event horizon. The fact that the above properties can be determined without a world-line (that is, information also does not propagate faster than light, and hence cannot escape) says something fundamental about those properties.
An uncharged, unspinning black hole is called a Schwartzchild hole. Once you cross the event horizon, you will unavoidably strike the singularity and perish.
In the other types of black holes, such as the Kerr black hole (uncharged, spinning), Reisnner-Nordstrom (charged, zero angular momentum), and the Kerr-Newman black hole (charged, spinning) it is possible to cross the event horizon without striking the singularity. Instead, you can pass into another universe.
Indeed, it's theoretically possible that you will pass through many universes. This is a one-way trip, however. If you try to get back to where you were, you will encounter the singularity and die.
Actual solution of the Einstein field equations for the holes listed above, however, produce perturbations. These perturbations, so far, cancel out the ability to miss the singularity and enter another universe.
Moving on, Hawking demonstrated that black holes evaporate. Hawking radiation is produced when half of a virtual particle pair appears inside the event horizon. Since both particles are no longer available to disappear under the Heisenberg time limit, the remaining particle acquires real energy. This energy comes from the black hole.
Since the rate of evaporation is proportional to surface area/mass, smaller black holes evaporate explosively. Indeed, no black holes smaller than a proton could exist from the big bang.
Finally, recent research shows that the universe is inflating, due to Einstein's cosmological constant (which, he ironically labelled as his "worst mistake"). That is, Hubble's constant is increasing. There will be no Big Crunch. The universe will expand at a faster and faster rate into nothingness.
There are a lot of good books on cosmology. General Relativity is undergoing a renaissance right now because of all of this important, new information.
"Invincibility is in oneself, vulnerability in the opponent." --Sun Tzu
I bet integral will be perfect for further studies:
http://www.esa.int/export/esaMI/Integral/
3 hours until launch.. now i have to install realplayer *shrug*
Live rm feed at http://esa.capcave.nl/esa/integral/info.html
From Gsu.edu Astrophysics: ... The Schwarzschild radius (event horizon) just marks the radius of a sphere past which we can get no particles, no light, no information.
Any mass can become a black hole if it collapses down to the Schwarzschild radius
R= 2(MG)/ c^2
Therefore at 3.7 million solar masses...
the Schwarzschild radius is
1.0919401548997975x10^10 M
Which is much smaller than our solar system (the earth orbits at 150,000,000 KM).
But I imgine that they would measure the Acreation Disk.....
The Schwarzschild radius calulation is fun. One can plot density verses radius and it becomes clear that something the size of our galaxy with density of water would be a black hole...
Space is an empty place!
DJMD - The fourth man - Planetary
one of the three properties a black hole can have - size, charge and angular momentum
IIRC BH also have entropy.
Working for necessity's mother.
The BBC have a similar story
Enjoy
Eclectic beats from Leeds, UK
handmadehands.co.uk
Photons must have mass otherwise space sails couldn't work. you can't steal kinetic energy from a massless particle.
o n. html
Maybe you want to verify your knowledge of physics before posting a response?
"Spinor fields describing particles of zero rest mass satisfy the so-called zero rest mass equations. Examples of zero rest mass particles include the neutrino (a fermion ) and the gauge bosons (as long as gauge symmetry is not violated) such as the photon or Higgs boson. "
http://mathworld.wolfram.com/ZeroRestMassEquati
With great power comes great electricity bills.