Straightforward enough question.

Can anyone enlighten me? - Does it look like a pancake on it's end or something else?

Oh yeah, lets see what it "looks" like ;)

Well you must "see" something...

I think it can only be seen by the effect it has on material drawn into it. I heard that they rotate and therefore some of the material coming towards it instead gets thrown off in the direction of the axis of rotation.

Hmmm, ok, this may be a silly question but is it possible to get "behind" a black hole? - If so what would you see from there?

Would you see just a "sheet of blackness"?

For a Schwarzschild black hole (spherically symmetric, uncharged, nonrotating) against a background of distant stars, and you being a distant observer, you'd see a circle of blackness void of stars. A sphere has no "behind", so it would look the same from all angles.

I think it can only be seen by the effect it has on material drawn into it. I heard that they rotate and therefore some of the material coming towards it instead gets thrown off in the direction of the axis of rotation.

Absolutely. It's called an 'accretion disk'. Say a black hole is part of a binary system, it's captured a gas giant, and they orbit each other (google up cataclysmic variables and accretion for pics), the black hole pulls matter from the star, into the plane of rotation, which spirals down, accelerating, and giving off masses of X-Rays. A fair few of these systems have been detected.

So a black hole looks like a planet with a ring system, kind of, if it's accreting.

For a solitary black hole, well, yeah, it's a sphere, and looks the same from all angles, although it bends light around it, and will 'lens' the background, so you may see distortions of the stars behind it.

That's just great folks - gives me a whole new angle to look more into black holes now.

:D

Technically its just a teeny weeny sphere that has almost infinite gravity due to its incredicle density. Think of a pea with immense gravity.

Oh, wait I just thought of something. Why does the accretion disk rotate on one axis? Why isn't it an accretion cloud?

what phlogistician said. my science lecture hall had some awsome shots of accretion disks up on the wall. blurry and small, but still amazing looking.



renderings and photos:
http://images.google.com/images?q=black+hole&hl=en&lr=&c2coff=1&client=firefox-a&rls=org.mozilla:en-US:official&tab=ni&ie=ISO-8859-1&sa=N

this is the one image that always caught my attention:

http://www.saskschools.ca/~gregory/space/sp/blkhole.jpg

accretion disk, with a central point of radition release where particles split as they reach the event horizon, as predicted by Steven Hawking

Why does the accretion disk rotate on one axis?Mmmmmmmm. A leprechaun once told me it had something to do with averaging out angular momentum. Oh well, maybe I was dreaming.

what phlogistician said. my science lecture hall had some awsome shots of accretion disks up on the wall. blurry and small, but still amazing looking.



renderings and photos:
http://images.google.com/images?q=black+hole&hl=en&lr=&c2coff=1&client=firefox-a&rls=org.mozilla:en-US:official&tab=ni&ie=ISO-8859-1&sa=N

I don't know if you have noticed or not, but the images that show a 'black'
hole in the center of the disks are 'artistic impressions', not real images.
The ones with a swirl of matter and a bright white center are the 'true' images. Theory states there should be an event horizon in the center, within
the circumference of which is black. I am not aware of any images that
actually show the black area, but there may be some. Also, event horizons
(within which is black) are not tiny for massive black holes, only the postulated infinity within the event horizon. The infinity arises because
theory 'breaks down' within the event horizon, it is a product of mathematics
which may not actually exist. Gravastars would also look like the 'true' images,
as would any very intense gravitational source. Also, many of the 'images'
are not visible light images, but are artifically colored from x-ray, or other
non-visible light sources. I am not saying there is no such thing as a black
hole, only that I am unaware of any true visible light images that actually
show a 'black' area within an accretion disk.

Question: How and why are jets ejected from the black hole? They are actually ejected from OUTSIDE the black hole, I assume, so they can get out, but why would this happen?

Yep,
They are “<i>actually ejected from OUTSIDE</i>”…

The particles probably get quite hot (<i>ie fast</i>) as they spiral in…

As a side issue I suspect that the event horizon (<i>schwarzschild surface</i>) isn’t actually black.
It emits Hawking radiation (<i>If it bleeds, it can die…</i>) which I suppose is radiated equally throughout the <b>electromagnetic spectrum</b>; but the less energetic ones would not escape as much as the Ultraviolet/x-ray high energy end would…

<b>The black hole is blue…</b>

(if I’m mistaken , please correct me – as I quite like pink)

...Ok, so they get hot and fast "as they spiral in", why does it switch directions and shoot out?

As far as I can work out, all light and matter that reflects or emits light at the time right before it crossed over the event horizon, will remain visible right up until the point when the black hole expires - no matter how long this may take.

And, since that light is red-shifted with respect to a stationary observer at a safe viewing distance, it will appear on the low end of the spectrum, probably not in the visible spectrum at all, but if it did remain in the visible spectrum, I believe it would be dark, dark red.

The only way, as far as I can tell, that light can escape a black hole, is if that black hole is rotating rapidly and there is a subsequent white hole, creating a wormhole (as this would cancel the affect of time and space, subsequently ejecting the matter from the black holes influence before it had a chance to reach the singularity, with the end result being the matter as ejected with respect to the white hole being symmetrical to that of how it entered the black hole; barring the consequence of disturbing the balance between the black and white holes that is.

However, though this is mathematically possible, I seriously doubt such a situation exists in nature, as there is not an object (as far as I know of) in our observable universe that has a fast enough rotational speed with respect to its mass to cause a black hole to rotate fast enough to produce a worm hole and subsequently, a white hole. With observable nature and predictable nature, any black holes that currently exist, or that could be produced by the most prime candidates, will produce black holes that, if you were to cross the event horizon, your would inevitably hit the singularity.

I hope that made sense :p - I’m short of time. :(

As far as switching directions, yuri, apply the first paragraph of this post to your thinking and imagine the object taking the path of an ‘s’ as it travels between the black hole and the white hole singularities, never actually reaching the center of either.

Cool,
(er, a for a feeding black hole rather than a non feeding one)

So, everything is redshifted - the gamma rays would be redshifted to, say, blue? the blue to red... etc
Would we notice?

(ie the light looks just the same ...)

So for a feeding black hole it would be white???

With respect to a black hole that is capturing matter and energy, gamma rays should also be subject to a red-shift below the visible frequency upon the most proximal location to the outside of the event horizon, again, with respect to a stationary observer at a safe viewing distance.

The reason for this is because the event horizon is traveling away from the singularity at the speed of light; therefore, any light approaching the event horizon will leave behind a radiation pattern as the entering light struggles to travel back to the stationary observer.

I know this explanation is scant on the details and I can elaborate more at a later time should you need me to, however I am very short on time now.

Thanks for explaining that.

There aren't any white holes, according to Hawking; any mass passing through the bridge would cause it to collapse.

As far as the appearance of black holes goes, there are three types. The smallest holes, from picogrammes up to millions of tonnes, would be brilliant objects radiating via Hawking radiation. However all the naturally occuring mini-black holes would have evaporated long ago, so we won't see these at all.
Stellar black holes are about 6 kilometers across, and would be almost invisible; they would resemble black spheres, with a optical distortion around the edge called Einstein rings.

http://www.daviddarling.info/images/black_hole_210704.jpg

If the black hole was formed from one star of a binary pair, an accretion disk might form; this would be thousands of kilometers across, and the hole would be invisible in the middle. However the magnetic field of the hole would almost certainly be very strong, and would lead to the formation of polar jets to north and south.
A black hole with an accretion disk would look like this

http://observe.arc.nasa.gov/nasa/space/stellardeath/graphics/ob971203_sm_a.jpg

A supermassive black hole at the centre of a galaxy would be as big as a solar system; it might have an accretion disk, like this one: (an actual image, not a graphic; by Hubble Space Telescope)

http://schoolcentral.com/Willoughby4/Ebus/Work-2000-2/black%20hole%20in%20the%20middle%20of%20a%20galexi e.jpg

but some will have cleared an empty space around them, and would appear as black spheres surrounded by Einstein rings. Every few days a wandering star could be ripped apart when it came to close to the supermassive hole; this would be a brilliant, supernova like event, but most of the mass of the star would probably be ejected in hyperbolic escape orbits-
black holes are messy eaters.

ok, could you explain in a little more detail how...
"However the magnetic field of the hole would almost certainly be very strong, and would lead to the formation of polar jets to north and south."
these jets work? I can't wait a week for my teacher (Mr. Smith, who was actually taught by wheeler!) to explain it =]

Hum,
a quick explanation is that the lines of the magnetic field get <i>twisted</i> up (`cos the black holes spinning ) ..like rubber-bands getting twisted...and they <b>snap</b>... and reconnect, er, with other broken `lines`...the lines ping outwards, and any matter that gets in the way, gets caught up with it, and gets chucked out at great speed...

Thats why we just see two `jets` (from both poles)...

"<i>I suppose they call it dynamic magnetic reconnection… </i>"