BBC2 Documentary - Swallowed by a black hole

MisterDrak

Hello,

Over from the Athletics forum, where I usually hang out, so excuse my lack of knowledge...

Watched the BB2 Documentary last night, about a super massive black hole about to swallow a gas cloud 3 times the size of earth.

Iv read about super massive black holes being at the centre of most Galaxies including our own. The early part of the documentary seemed to suggest that we would have a "grand stand view" of this event, and the telescopes and astronomers of earth would gain valuable information from seeing this...

Given that the Galaxy is 100K light years and that (roughly speaking) we are 50K light years from the core (and the black hole), how can we possibly see an event such as this? given we have difficulty seeing Exo-planets in relatively close solar systems, and i would expect Exo-planets would be many times larger that this gas cloud???

Any and all comments welcome !

ps200306

I haven't seen the documentary, but I recorded it and hopefully will get a look. But I read from the documentary's blurb that they are talking about a gas cloud "three times the size of the earth". Now, I would assume this means three times the mass of the earth.

We can do a quick back-of-an-envelope calculation on the volume of this. One mole of hydrogen atoms weighs one gram and contains atoms. The earth weighs kg. So three earth masses in hydrogen atoms is:



Now let's assume a density of 1 atom per cubic centimetre (which we'll revisit later) and assume a spherical cloud. The radius of a sphere with this volume is km ... about a hundred times the size of our solar system. This is pretty enormous -- vastly bigger than any exoplanet.

What about our density assumption? Well, it's possible for a gas cloud to be much less dense than we've assumed. We know from the type of light that is emitted by gas clouds some distance from black holes in other galaxies that it can only be produced in densities less than a millionth of what we've assumed. This would make our cloud radius a hundred times bigger. But closer to the black hole it may be safer to assume a HII region that is perhaps a thousand times denser than our assumption, making the cloud ten times smaller. It's still ten times the size of our solar system!

At any rate, we won't be seeing the cloud itself. At the distance of the galactic centre, ten times the width of our solar system subtends an angle of about 0.06 arc second. That's within the resolving power of the Hubble Space Telescope ... but only barely. What we will be seeing is the light emitted as the gas cloud is heated by friction on its journey toward the black hole. That will cover a broad range of wavelengths and we may be able to tell from its spectrum and from Doppler broadening of some of its peaks the sort of motion that it is undergoing.

MisterDrak

PS200306,

Many thanks for the detailed response.

The except from the program reads as follows, n.b it quotes Size rather that Mass.

"A gas cloud three times the size of our planet has strayed within the gravitational reach of our nearest supermassive black hole. And across the globe, telescopes are being trained on the heart of our Milky Way galaxy, some 27,000 light years from Earth, in the expectation of observing this unique cosmic spectacle"...

However in the light of your expert response, the above does not make to much sence... Ever think about applying for a job on the Beeb ?

Thanks again.

ps200306

Something three times the size of the earth by volume wouldn't make much sense. At the density of a HII region, it would weigh about as much as a decent sized car. Or, if we're talking about three times the earth's diameter, the mass of a small truck.

I can hardly take the frustration of listening to a Horizon program, let alone working on one. They tend to have about as much information as you could cram into a three minute talk, interspersed with misleading graphics that look like fireworks filmed with a shaky camera.

Rubecula

MisterDrak wrote: »

Hello,

Over from the Athletics forum, where I usually hang out, so excuse my lack of knowledge...

Watched the BB2 Documentary last night, about a super massive black hole about to swallow a gas cloud 3 times the size of earth.

Iv read about super massive black holes being at the centre of most Galaxies including our own. The early part of the documentary seemed to suggest that we would have a "grand stand view" of this event, and the telescopes and astronomers of earth would gain valuable information from seeing this...

Given that the Galaxy is 100K light years and that (roughly speaking) we are 50K light years from the core (and the black hole), how can we possibly see an event such as this? given we have difficulty seeing Exo-planets in relatively close solar systems, and i would expect Exo-planets would be many times larger that this gas cloud???

Any and all comments welcome !

Forgive me for butting in but I think you are asking how we can see it? Well the answer is we can't see it directly with our own mark 1 eyeballs, even with telescopes. What we can and will see is through radio telescopes. As the black hole "eats" the gas the gas will emit hard radiation that can be "seen" with the proper equipment.

(Hope that clears it up for you.)

ThunderCat

Looks like this is on again on bbc2 in 10 mins for anyone interested

Zubeneschamali

ps200306 wrote: »

They tend to have about as much information as you could cram into a three minute talk, interspersed with misleading graphics that look like fireworks filmed with a shaky camera.

That's very unfair to the black hole show. They didn't have firework graphics, they used moody footage of astronomers walking about, staring into the distance, and paddling a kayak on a foggy river.

Three minutes worth of info is about right, though.

ps200306

Zubeneschamali wrote: »

That's very unfair to the black hole show. They didn't have firework graphics, they used moody footage of astronomers walking about, staring into the distance, and paddling a kayak on a foggy river.

Three minutes worth of info is about right, though.

I just watched it at last. Holy bajoley, that was stultifying stuff. I have to take issue with you though. There was definitely a couple of shaky camera fireworky thingies. And kayaks. And the usual camera circling the nostrils of moody astronomers. And somebody throwing themselves off a cliff. (I wonder was that a subliminal message about what you can do with yourself if you're actually bored to death). Oh yeah, there was the mandatory bloke scribbling equations on a black board (the formulas for gravitational potential energy, and the Schwarzschild radius).

Program synopsis: a cloud of gas is falling into the Milky Way's central black hole. That's it. Three second's worth, at best. A bit of padding about the discovery of black holes, the location of the Milky Way's black hole, the behaviour of matter in an accretion disk, and the relationship between galaxy size and black hole size. That probably took it up to about three minutes worth. The rest was shaky camera action and the usual grimly ponderous narration that would be better fitted to "the earth is being devoured by hoards of alien nanobots, there's nothing we can do, and we'll all be dead in thirty minutes" (which is kind of what this program makes you wish for).

What there wasn't was any information about what we actually hope to see. (I had to replay the last ten minutes to check again, 'cos I was snoring my head off at that stage). Apart from "a black hole sitting down to dinner", there was nothing else. I did manage to ascertain the gas cloud is three times the mass of the earth -- it mentions it 27 minutes in. I would think what we're going to get is something similar to what we get from quasars, but at higher spectral resolution. Perhaps a spectrum with a nice power-law continuum, which is caused by synchrotron radiation due to the whirling around of the gas cloud in the accretion disk. This program leaves us none the wiser.

All joking aside, the level of these programs nowadays is shockingly bad. I know it's a bit much to expect advanced details for a TV audience, but look at the OP's post -- it doesn't provide basic information like: how big is it and how can we see it from where we are?; how bright will it be?; how often does this happen?; how long will it last?; what's the cloud made of and where did it come from?; etc. etc. etc.

1stimpressions

Is there anything you would recommend watching, be it old or new.

ps200306

A decent book. Science on TV is a dead loss.

ps200306

Forget Horizon and the BBC shaky camera effects, here's some real pictures of the gas cloud heading for the black hole:

http://www.eso.org/public/videos/eso1332c/

... and here's the paper on which the documentary appears to have been based:

http://www.eso.org/public/archives/releases/sciencepapers/eso1151/eso1151.pdf