Dark Stars ??

[InvaderXan]

Cool! Cheers for that, I'll have a search sometime.

And another thing about having a black hole in your car -- gravitational redshifting would explain why traffic lights always seem to be red.

Apologies if I'm ever too technical, by the way -- anyone who wants me to explain anything I say please do say so. I'd be more than happy to!

[ngc2403]

:laughing3: :laughing3: :laughing3:

I am very impressed,

but as you say not very practical.

but as use full as the lane-Emmden Equation of stellar structure where n=0 is the solution

ally

[dogfish]

Keep it coming, technical or not, I'm enjoying this thread I know what Steve is saying but thicko's like me :? can just skip the maths.

Martin

[beamish]

here here :hello1:

[acey]

The Wikipedia entry for supermassive black holes

http://en.wikipedia.org/wiki/Supermassive_black_hole

says they can have a density less than air. The "size" used to calculate this density is given by the Schwarzschild radius (event horizon): you could argue that the black hole proper is a tiny thing at the centre of this big spherical region.

For smaller black holes the density is bigger, and tends towards infinity - which is what you would expect, because the singularity (thing in the middle of a black hole) is a point of infinite density.

For bigger and bigger black holes the density falls further. I don't have a spare envelope on whose back to work it out, but if you took the radius to be that of the observable universe, you could work out the density of matter required to make the whole universe a black hole. And I assume this comes out to be more than the actual density of about 3 protons per cubic metre - otherwise we shouldn't be here. When our (observable) universe was younger and smaller, its density was higher, but so too was the critical density that would have made the universe collapse back on itself.

Extrapolating black hole formulae to stuff like air or sand is obviously not particularly realistic, as others have pointed out - it's just a bit of fun. A bit like the one about Saturn floating on water. Like, how do you find a bucket with the required tensile strength? Not to mention a super-planet to stand it on.

But I realise I've wandered a bit off-topic given that the original question concerned the theoretical upper limit on stellar mass. As a humble theorist, I'd better that leave that one to the astrophysicists.

[InvaderXan]

Oh, ok. So yes, if you consider the black hole to be everything within the event horizon then I can see how you'd get such a low density, especially as the event horizon's radius would increase with mass, while the black hole's physical volume would (theoretically) remain infinitessimal -- so the more massive the black hole, the lower it's relative density.

It still seems rather counterintuitive, IMHO. Rather like considering the Sun's Hill Sphere to be part of the Sun.

Technically too (I may be going out on a limb here), seeing as you can't see within the event horizon, you could describe the black hole's mass, using Heisenberg's Uncertainty Principle, as a Probability Density Function. In other words, any part of the black hole could be anywhere within the event horizon at any time.

There's probably some reason why that can't actually be the case, mind you... And you're right, this thread is veering wildly off topic.

[acey]

Regarding the location of the gravitating mass inside a black hole, if it were anywhere off centre then there would be unequal attraction outside the event horizon, and this would be measurable.

[beamish]

Since I started the thread, feel it'd be okay to widen it a tad I suppose that there is also a theoretical limit to the size of planets(rocky) obviously, since gaseous ones would I assume would simply grow to become stars?

Karlo

[InvaderXan]

Since I started the thread, feel it'd be okay to widen it a tad I suppose that there is also a theoretical limit to the size of planets(rocky) obviously, since gaseous ones would I assume would simply grow to become stars?

Karlo

In my understanding, the threshold actually lies between terrestrial planets and gaseous planets. Earth (for example) never became a gas giant, because hydrogen is too light and energetic to be retained by the Earth's relatively small gravitational pull.

Once a terrestrial planet has strong enough gravity that it can retain gaseous hydrogen (the most abundant molecule in the universe!) -- then there would be nothing to stop the planet accreting successively more hydrogen and turning into a gas giant. At least, hypothetically speaking.

A planet would have to be massive enough that it's escape velocity was greater than the average speed of an atmospheric hydrogen molecule. Anyone care to do the calculation?

Oh, and the theoretical upper mass limit for gas giants is around 13 jupiter masses. Past that, they start fusing deuterium and become brown dwarfs.

[beamish]

marvelous cheers. I do have that escape velocity somewhere ! No seriously !!

[dogfish]

marvelous cheers. I do have that escape velocity somewhere ! No seriously !!

Get out of town - I'm feeling thicker by the minute (but still enjoying it)

Martin

[Talitha]

I'm feeling thicker by the minute (but still enjoying it)

Martin

Ditto! Actually, I understand some of it but get lost in the equations. Interesting discussion though.. what a fun way to learn!

[Cloudwatcher]

I'm feeling thicker by the minute (but still enjoying it)

Martin

Ditto! Actually, I understand some of it but get lost in the equations. Interesting discussion though.. what a fun way to learn!

Double Ditto!

(Get your video now! )

CW

Click as usual.

[ngc2403]

How have the rest of you learned about this things? most of want i am telling you about is 1st, 2nd & some 3rd year Astronomy at university level. if you follow have of it you can thing of yourself as being easily good enough to do an astronomy degree. (someone told me that only the top 1% of people in the UK can understand this course sooooo stop putting yourself down

:hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1: :hello1:

Once a terrestrial planet has strong enough gravity that it can retain gaseous hydrogen (the most abundant molecule in the universe!) -- then there would be nothing to stop the planet accreting successively more hydrogen and turning into a gas giant. At least, hypothetically speaking.

A planet would have to be massive enough that it's escape velocity was greater than the average speed of an atmospheric hydrogen molecule. Anyone care to do the calculation?

there was an interesting theory months ago in the astronomy now mag. which said that the planets could have formed by the top down model or top up. i can't remember which was which but they said that the rocky planets formed like rocky planets form a collection of dust and ice.

however the gaseous planets are said to have formed by gas joining together not rocks which then got large enough to capture gas.

another thing to consider is that the gaseous planet Lay farther from the sun where the gas is cooler and this means that the Temperature(which is the definition of kinetic energy and so the speed of the gas) is lower then gas closer to the sun. hence allowing the gas to collect together and form planets. example nitrogen is gas on earth but solid on the moons of Neptune and saturn. going with this idea would mean that the bigger moons could have hydrogen on them though they are smaller 'lighter' then earth.

[beamish]

Books, mags, web, forum...... Lots of everything ,nothing in depth !

[Paxo]

How have the rest of you learned about this things? most of want i am telling you about is 1st, 2nd & some 3rd year Astronomy at university level. if you follow have of it you can thing of yourself as being easily good enough to do an astronomy degree. (someone told me that only the top 1% of people in the UK can understand this course sooooo stop putting yourself down

PhD, though not in astronomy but physics related (looking at quantum effects and electron free path sizes in nanomaterials), and spending the last 6 years as a senior science advisor following 10 years in a few top research jobs in industry. I do read a lot of astronomy related research materials and texts though (a hobby), I pick these up day-to-day during my work, and as Karlo has said, through books, mags, web and other unpaid professional astronomers.

Steve..

[dogfish]

I've actually got two doctorates* but both are in psychology (PhD and DClinPsychol), so I know I'm not that thick. I enjoy reading cosmology stuff and like the others my understanding so far is from books, mags and web articles, but also discussions like these on forums (?fora?) and in person with my fellow enthusiasts.

String theory is beyond me, I've found, but I cope with most of the physics I've read, as long as I skip the maths :withstupid: .

Cheers guys, Martin

*(mental picture conjured up of the guy on chewing the fat doing that thing with his hands under his chin - apologies if this is lost on those south of the border)

[beamish]

"Le Penseur" G Rodin ?? ( the Thinker) One of my favourite sculptors- You ought to visit the museum in Paris.

I think I get String theory ? Branes are hard.

I've always had a tendency towards science, as a kid I used to love watching stuff with my old man, such as James Burke's Connections and the like, still watch the The Royal Society Xmas lectures( though it's more likely to be Easter rpts)

[ngc2403]

:oops: :oops: :oops: :oops: :oops: :oops: :oops:

well i see that your all very educated gentlemen, of the world.

i like that fact that we are having this discussion over lots of topics each one in it's own right a life time of work and at the cutting edge of research just now

so who is going to teach me string theory i have never read or took to much of and interest in it because of the fact that as i understand it there is no maths that exists to properly describe it

[dogfish]

"Le Penseur" G Rodin ?? ( the Thinker) One of my favourite sculptors- You ought to visit the museum in Paris.

Hi Karlo,

This is what I meant http://www.youtube.com/watch?v=cteGri5LAz0 It's from a Scottish TV show and it's something people do when someone's being pretentious. V. funny series.

Cheers, Martin

[ngc2403]

:laughing3: :laughing3: :laughing3: :laughing3: :laughing3:

brillant martin

[beamish]

OOOps there was i thinking we were being quite high brow- very funny Martin !!

so who is going to teach me string theory i have never read or took to much of and interest in it because of the fact that as i understand it there is no maths that exists to properly describe it

I don't think I'd be up to the job but I love your excuse there is no maths that exists to properly describe it

I just find it a plausible way to describe how the properties of matter can be explained as different frequencies of vibrating strings-some open some closed. So having had a glass or two of wine and not the motivation to go and look it up right now I'm afraid that's all I'll say about it. I will add that as I understand it strings also open up the possibility of multiverses. I remember reading a good few yrs ago now how scientists had observed matter "twinkling " in and out of existance, as if phasing. Given that matter =energy=matter , this variable phasing /frequency is very interesting.

[ngc2403]

So having had a glass or two of wine and not the motivation to go and look it up right now I'm afraid that's all I'll say about it.

that is ok by me i don't think i could hand the whole thing in one night

maybe two or three

is the idea of particles coming an going from one universe to another not what hawking radiation is based on? and the stuff on imaginary particles from everybodies favourite part of maths complex numbers!

[beamish]

the stuff on imaginary particles from everybodies favourite part of maths complex numbers!

You got me ! As for Hawking Radiation-that does sound familiar. I AM an amatuer

[ngc2403]

i think that the imaginary particles thought came from the idea that there is two types of numbers real and imaginary. the real numbers are easy to use and are the numbers that you will use every day. imaginary numbers have real parts and imaginary parts and they can be used to describe rotation. the real part is like the x-axis and the imaginary part is like the y-axis to make an imaginary number real y must be zero. i think the idea is that the imaginary part of the particle changes causing it to slip in and out of different universes or something like that