# Q/As about space

#### Warped

##### New member
If you had a jar of air and a jar of nothing (vacuum), and you threw both in a pool, would they both float or would the vacuum jar sink?

#### Fizyk

##### Member
Urwumpe said:
No, there are things that are as close to a black hole as possible from our point of view. So close, that we can tell that, beyond the veil of relativity, there has to be a real black hole already.
You fell into a trap here "Already" would suggest that it happened, but that's the point, in our reference frame it didn't happen and never will. From our perspective, we have a collapsing star, which will never collapse completely. But, on the other hand, if you went there and flew into this star, you would fall into a completely formed black hole, because your frame of reference would be then very different from ours.

Jarvitä said:
But how does that fit into the current trend of describing everything from our point of view, eg the "50 year old black hole" that's millions of light years away?
I'd say it's a simplification. It's easier to talk about a black hole, than about an almost-black-hole-that-will-never-form-completely-from-our-point-of-view-but-you-would-still-be-able-to-fall-into-it.

#### Urwumpe

##### Not funny anymore
Addon Developer
Donator
Also, if something is from our point of view irreversible on the way to a black hole, it is pretty hard to not call it one.

#### dgatsoulis

##### ele2png user
If you had a jar of air and a jar of nothing (vacuum), and you threw both in a pool, would they both float or would the vacuum jar sink?

If a vessel filled with air can float in water, then an identical vessel wth the air sucked out of it, will float too. In fact it will float higher than the one with the air.

It all comes down to [ame="http://en.wikipedia.org/wiki/Density"]Density - Wikipedia, the free encyclopedia[/ame].

If your mass per unit of volume is larger than the water's, you will sink.
Since the jar with the air, weighs a little bit more than the one that doesn't have air in it, it will sink a bit more.

#### blixel

##### Donator
Donator
I have a question. I've wondered - if you have a hypothetical universe that is completely empty (like in a computer simulation) and you put an object in it ... say a sphere of solid iron 5 meters in diameter, what kinds of objects could orbit it? Would it have enough gravitation pull for a penny to orbit around it?

The reason I've wondered this is because I've heard that if you put two dice in space a centimeter apart, they will attract each other.

So that idea has made me wonder what kind of things you could see with your own eyes at a small scale. Suppose you had a solid iron ball 1 centimeter in diameter and you wanted to put it in orbit around a much more massive solid sphere of iron ... what would the mass of the larger iron sphere have to be?

#### Artlav

##### Aperiodic traveller
Addon Developer
Beta Tester
if you have a hypothetical universe that is completely empty (like in a computer simulation) and you put an object in it ... say a sphere of solid iron 5 meters in diameter, what kinds of objects could orbit it?

...

Suppose you had a solid iron ball 1 centimeter in diameter and you wanted to put it in orbit around a much more massive solid sphere of iron ... what would the mass of the larger iron sphere have to be?
Any object.
Technically, any object less massive than the sphere.

Gravity is universal, so in absence of other forces (empty universe), anything less massive would orbit anything more massive, given proper velocities.

#### Grover

##### Saturn V Misfire
Addon Developer
Donator
i thought of a question during physics today:

how small yould you have to compress the earth, to turn it into a black hole with a significant event horizon. IE the point where after you compressed it, it would further compress itself due to its own gravitational pul accross itself

and: if start use hydrogen as their fuel (followed by "short" bursts of using helium as fuel, then C + O and so on), doesnt that mean that eventually, all hydrogen and helium in the universe will eventually be depleted, and no stars will be able to form? or at least until a sufficient body of other materials forms to make a short-lived star fuelled by other means (probably carbon/oxygen IMHO)

thanks

#### agentgonzo

##### Grounded since '09
Addon Developer
i thought of a question during physics today:

how small yould you have to compress the earth, to turn it into a black hole with a significant event horizon. IE the point where after you compressed it, it would further compress itself due to its own gravitational pul accross itself
[ame="http://en.wikipedia.org/wiki/Schwarzschild_radius"]9mm[/ame]

and: if [stars] start use hydrogen as their fuel (followed by "short" bursts of using helium as fuel, then C + O and so on), doesnt that mean that eventually, all hydrogen and helium in the universe will eventually be depleted, and no stars will be able to form? or at least until a sufficient body of other materials forms to make a short-lived star fuelled by other means (probably carbon/oxygen IMHO)

thanks
Not really. Once stars go Nova and explode at the end of their lives, they spray out a lot of unfused hydrogen (and helium and other stuff) into the local area. As the universe expands, not all of this hydrogen can globulate (good word) under gravity to begin fusion again so you get a lot of hydrogen spread over the universe in an unfusing smear.

See also [ame="http://en.wikipedia.org/wiki/Future_of_an_expanding_universe"]what will happen to our universe[/ame]

#### Grover

##### Saturn V Misfire
Addon Developer
Donator
but surely, since huge amounts of hydrogen were fused, all the way up to the heaviest of metals, it should be depleted, yes, some unfused hydrogen/helium is ejected along with the stars outer layers (anything but the core), but alot of it is used to keep the star going for a few billion years

#### agentgonzo

##### Grounded since '09
Addon Developer
but surely, since huge amounts of hydrogen were fused, all the way up to the heaviest of metals, it should be depleted, yes, some unfused hydrogen/helium is ejected along with the stars outer layers (anything but the core), but alot of it is used to keep the star going for a few billion years
Yes, but the outer layers can still have a large proportion of unfused hydrogen as it's just the hydrogen in the core that fuses. The lighter hydrogen atoms not in the core will 'float' to the surface of the star and be kept from fusing. I'm having trouble finding the proportion of hydrogen left in a main sequence star a the end of its life though.

---------- Post added at 09:46 ---------- Previous post was at 09:40 ----------

Found it (well, one reference at least).
A typical star burns about 12 percent of its hydrogen fuel while on the main sequence
http://adsabs.harvard.edu/full/1973ApJ...185..937H

#### Grover

##### Saturn V Misfire
Addon Developer
Donator
yes, but the hydrogen that IS fused never returns to its original state in any significant process, so EVENTUALLY all the hydrogen in the universe wil be fused into heavier atoms, leaving helium as the lightest atom in the universe (naturally occurring anyway). then the only suns to form will be HUGE and caused by massive bodies of other gasses/dust,... until we get to iron being the lightest atom, then there wont be amy more fusion ("normal") stars ever again unless something fissions the new heavier matter into hydrogen again (possibly a naturally occurring phenomenon that we cant possibly imagine with our current understanding of matter?)

#### agentgonzo

##### Grounded since '09
Addon Developer
yes, but the hydrogen that IS fused never returns to its original state in any significant process, so EVENTUALLY all the hydrogen in the universe wil be fused into heavier atoms
No.

If you spread the hydrogen out thin enough across the universe (which is what will happen with the expanding universe) then this remaining hydrogen cannot coalesce into stars and fuse into helium or heavier elements.

#### Grover

##### Saturn V Misfire
Addon Developer
Donator
No.

If you spread the hydrogen out thin enough across the universe (which is what will happen with the expanding universe) then this remaining hydrogen cannot coalesce into stars and fuse into helium or heavier elements.

ah, so either way, the stars go out, but not through lack of hydrogen, but for lack of large bodies of hydrogen that would collapse into a sun

#### agentgonzo

##### Grounded since '09
Addon Developer
ah, so either way, the stars go out, but not through lack of hydrogen, but for lack of large bodies of hydrogen that would collapse into a sun
Yes.

#### sitha241

##### Member
Mass

If sun is burning fuel and loosing mass is there a possibility that planet orbits are becoming larger for small amount?

#### Izack

##### Non sequitur
Addon Developer
If sun is burning fuel and loosing mass is there a possibility that planet orbits are becoming larger for small amount?
I suppose yes, we're picking up mass from the solar wind. It's really negligible, though. The grand majority of ejected/radiated mass is flung off into empty space, and the very very little that actually encounters our magnetic field is mostly in the van Allen belts. For what actually makes it to the surface, it's really just a particle here and there.

#### MaverickSawyer

##### Acolyte of the Probe
The bigger source of mass reduction is actually the solar wind, which carries away massive (by our standards) amounts of material every second. However, the practial change is quite negligable over the lifespan of humans as a species. After all, the Sun accounts for 98% (or was it 99%? Can't recall) of the mass in our solar system. A few exatons of lost gas is nothing to it.

#### Urwumpe

##### Not funny anymore
Addon Developer
Donator
Yes, the planetary orbits are actually slowly increasing. But we are talking about geological timescales here. The sun does not even loose one percent of its mass in one million years.

Tnx for replies

#### NovaSilisko

##### The Siliskoiest of Siliskos
Addon Developer
I would think that the other planets have a greater effect on orbits than the changing mass of the sun would.

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