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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
SpaceEngineerDate: Thursday, 17.11.2011, 01:37 | Message # 16
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Quote (Hardts)
- Q1: Is there any place in the cosmos we know of where the temperature is lower than 0 K ?


Temperature below 0K is impossible. Temperature is a average atoms motion/vibration value. Zero point on Kelvin scale corresponds to zero motion energy, ie atoms full stops. What would happen at -10K? Atoms are already fully stopped. But 0K is unreachable. You can't stop atoms and say that they stopped with infinite accuracy, according to Heisenberg's uncertainty relation. BTW, all places in space have temperature at least 2.73K - this is the temperature of Cosmic Microwave Background Radiation.

Quote (Hardts)
- Q2: Is it possible to have liquid water below 0 C (excluding water current as a factor), and would atm. pressure have any say in this ?


It is possible in two ways:

1) Water has some substance mixed with, that reduces freezing point. For example, salt.
2) Water is under huge pressure. For example, deep water in Arctic could have temperature below -10°С.

Quote (Hardts)
- Q3: how would you calculate the terminal velocity of a spherical object (to simplify), given you have 'g' (gravity) and atmospheric pressure ? Can we (or is this perhaps the only option) ignore the makings of a given atmosphere (ie. procentage of different gasses) and just consider density as a multiplier to increasing friction? - How would we go about doing this calculation ?


The gas can stay on a planet, if mean velocities of it's molecules are lower than 6x escape velocity:

vmean < 6 v2

Mean velocity o molecules is:

vmean = sqrt(3.0 * R * T / mu)

where sqrt - square root, R - gas constans, mu - molar mass of gas molecule, T - temperature of planet's exosphere.

Escape velocity of planet is:

v2 = sqrt(2*G*M/R) = sqrt(2*g*R)

where G - Gravity constant, M - mass of the planet, R - radius of the planet, g - Gravitational acceleration.

I use better formula, that gives dissipation time, or decreasing of atmosphere's density with time:

t = vmeanv3 / (2*g2*R) * e(3*g[sup]2*R / vmeanv2)[/sup]

This is the time at which the gas concentration in the atmosphere decreases by e times.

As you can see, hydrogen has a big mean velocity, so it can leave Earth-sized planet, but can be stopped by gas giant gravity. Oxygen and Nitrogen can leave Moon, but stay on Earth and a Moon-sized Titan, because Titan is very cold and has a low exosphere temperature.

*





 
gpaw5765Date: Thursday, 17.11.2011, 20:00 | Message # 17
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Quote (Hardts)
- Q3: how would you calculate the terminal velocity of a spherical object (to simplify), given you have 'g' (gravity) and atmospheric pressure ? Can we (or is this perhaps the only option) ignore the makings of a given atmosphere (ie. procentage of different gasses) and just consider density as a multiplier to increasing friction? - How would we go about doing this calculation ?


Terminal Velocity depends on more factors than gravity and atmospheric pressure. Here's the formula:

Vt=sqrt((2*m*g)/(ρ*A*Cd))

Where:

Vt = terminal velocity,
m = mass of the falling object,
g = acceleration due to gravity,
Cd = drag coefficient,
ρ = density of the fluid through which the object is falling
A = projected area of the object.

The drag coefficient of a sphere is 0.47. The projected area would be pi*radius^2

You can check a detailed explanation here:
http://en.wikipedia.org/wiki/Terminal_velocity

Edit: The value of the Drag Coefficient can depend on several more variables. You can check them here:
http://en.wikipedia.org/wiki/Drag_coefficient

You calculate ρ (density) from the general formula of an ideal gas:
P*V=n*R*T
where
P is the pressure
V is the volume
n is the amount of substance of the gas (in moles)
R is the gas constant (8.314 J·K−1mol-1)
T is the absolute temperature

From which we can get:
ρ=(M*P)/(R*T)

Where M is the molar mass (mass of a mol of molecules)

Here you can get some more information and some typical values for density:
http://en.wikipedia.org/wiki/Density





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Edited by gpaw5765 - Thursday, 17.11.2011, 20:13
 
HardtsDate: Thursday, 17.11.2011, 21:38 | Message # 18
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Thank you both very much - excellent information!

I've got my mouth full now, need to understand these formulas.
The reason I wrote 'can you ignore the makings of the atmospher' / pretend it's like earth's - was that I assumed the building blocks of the atmosphere has a substantial say in determining the drag coefficient, and I choose a sphere because then I wouldn't have to mess with some complicated (and variable) projected area calculation. smile





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SpaceEngineerDate: Friday, 18.11.2011, 00:07 | Message # 19
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Wow, I mixed up the terminal velocity and escape velocity and gave you the answer to another qustion! Sorry for that smile

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TalismanDate: Friday, 18.11.2011, 07:39 | Message # 20
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I can't seem to get a clear answer but maybe I haven't studied the Brown Dwarf Wikipedia page hard enough,

What type of fusion is a brown dwarf undergoing? What type of heat/light does it emit (perhaps infra red?)

Does this mean every hot jupiter is actually a star emitting a small amount of light and heat?





 
HardtsDate: Friday, 18.11.2011, 12:04 | Message # 21
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Quote (SpaceEngineer)
Wow, I mixed up the terminal velocity and escape velocity and gave you an answer to another qustion! Sorry for that


Heheh - np, yeah I did wonder a bit biggrin





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HarbingerDawnDate: Wednesday, 23.11.2011, 16:09 | Message # 22
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Quote (Talisman)
What type of fusion is a brown dwarf undergoing?

Low mass brown dwarfs fuse deuterium (an isotope of hydrogen), and high mass brown dwarfs also fuse lithium.

Quote (Talisman)
What type of heat/light does it emit (perhaps infra red?)

Brown dwarfs emit their radiation almost entirely in infrared, though younger or more massive brown dwarfs (class L) would perceptibly glow with a deep red light. Class T and Y dwarfs would likely emit no noticeable visible light.

Quote (Talisman)
Does this mean every hot jupiter is actually a star emitting a small amount of light and heat?

No. Hot jupiters are normal planets that orbit very close to their host stars. The light and heat we observe from them come from the star, and is not generated in the planet itself. Whatever light that comes from those planets that isn't reflected from the star is radiated from the planet due to it being heated by the star. The deep interior of the planet would likely be similar to Jupiter.





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Edited by HarbingerDawn - Wednesday, 23.11.2011, 16:11
 
Space_NavyDate: Sunday, 22.01.2012, 04:48 | Message # 23
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You sure are a smarty-pants SpaceEnginner. smile

I have a couple questions as well.. concerning orbital paths.

We know that moons orbit planets, and planets orbit stars. And this leads to my questions:

Q1: Do stars move in an orbital path similar to planetary orbit? I may answer my own question but would they not orbit the galactic core? Which leads to my 2nd question..

Q2: Galaxies have got to be moving (considering the Andromeda galaxy is expect to merge with us), but I'm curious on where exactly they are going? Just drifting about in the universe?





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TwoNybbleDate: Sunday, 22.01.2012, 07:38 | Message # 24
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Quote (SpaceEngineer)
Quote (Hardts)
- Q2: Is it possible to have liquid water below 0 C (excluding water current as a factor), and would atm. pressure have any say in this ?


It is possible in two ways:

1) Water has some substance mixed with, that reduces freezing point. For example, salt.
2) Water is under huge pressure. For example, deep water in Arctic could have temperature below -10°С.


It is actually possible in one more way: Supercooling. This occurs when the water is pure and has no substance causing nucleation to occur. At standard pressure, water can be cooled all the way down to its crystal homogeneous nucleation point of about -48.3 degrees Celsius without freezing.

Quote (Space_Navy)

Q1: Do stars move in an orbital path similar to planetary orbit? I may answer my own question but would they not orbit the galactic core? Which leads to my 2nd question..


The stars in our galaxy and most others orbit around a central supermassive black hole in the galactic core, so you are partly right. This black hole is approximately an incredible 4.1 million solar masses. Astronomers are fairly certain of its existence because of the overwhelmingly high amount of mass in a very small section of the Milky Way at the core.

If you substitute the Sun with this black hole and the Planets with the stars in the galaxy it would be an accurate depiction of the galaxy. It is the same idea, on a much larger scale. The stars move in elliptical orbits around this central black hole much like the planets in our solar system. Our Sun's orbit around the Milky Way takes approximately 250 million years for one revolution. Compare that to the 1 year it takes Earth to orbit the Sun!

Quote (Space_Navy)
Q2: Galaxies have got to be moving (considering the Andromeda galaxy is expect to merge with us), but I'm curious on where exactly they are going? Just drifting about in the universe?


Galaxies do not decide where they "want" to go. If you drop a bag of marbles on the ground, they are going to spread out with random velocities with no particular reason why they are going in those directions. But where do these velocities come from in the first place in the case of the Universe? Well, perhaps some of it can be explained with gravitational forces between relatively close galaxies (Like Andromeda and our own). Galaxies like these influence each other and accelerate towards one another.

However, this is not a complete explanation. Hubble discovered that most galaxies have a "red shift" in their light frequencies when viewed in a telescope. This red-shift is caused by the Doppler Effect, when an object is moving rapidly (at relativistic speeds to be noticeable) away from the viewer. This would imply that most of the galaxies in the Universe, discounting our Local Group, are moving away from our own galaxy. What would cause all galaxies to be moving away from each other like this? The only explanation is from the Big Bang. Inflation from this event still exists today (it is actually accelerating!) and is causing galaxies to move apart at very fast speeds.

What is the fate of all the galaxies in the Universe if they are rapidly moving apart from one another? One day in the far, far future, it is quite possible that all other galaxies in the Universe will have moved outside our visible "bubble," obscuring them from view. Only the nearby galaxies in our Local Group will remain in the night sky at this point. However, if this does happen, it will be long after the Milky Way-Andromeda collision and the Sun's own demise.

To answer as simply as possible, some galaxies do orbit one another in clusters and groups much like stars and planets. Overall, however, almost all other galaxies are rapidly accelerating away from one another. They are drifting with the inflation from the Big Bang.


Edited by TwoNybble - Sunday, 22.01.2012, 07:41
 
SpaceEngineerDate: Sunday, 22.01.2012, 13:32 | Message # 25
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Quote (Space_Navy)
Q1: Do stars move in an orbital path similar to planetary orbit? I may answer my own question but would they not orbit the galactic core? Which leads to my 2nd question..


This is a very approximate explanation.

Quote (TwoNybble)
The stars in our galaxy and most others orbit around a central supermassive black hole in the galactic core, so you are partly right.


You are wrong. Compare 4.1 millions solar masses with 100 billions - the mass of all stars in our galaxy. The Sun and other stars are moved in the gravity field created by all these stars. As long as the stars tend to concentrate at the core and the disk, the path of stars that are far away form the core (like our Sun) is approximate and consists of two motions:
1) rotation around the core in nearly circular orbit;
2) oscillating up and down relative to the galactic disc plane.


http://www.thedailystar.net/newDesign/news-details.php?nid=205023

Only stars within several tens of light years from the central black hole are orbiting it as a "central sun", because its mass dominates.

The Sun lives in a so-called "co-rotation zone" of our galaxy. This means that the Sun's orbital period (250 My) is close to rotation period of the spiral arms, so the Sun never crosses the spiral arms. Stars that are closer to the galaxy center can cross the spiral arms, and maybe broken down by their gravity field, that leads to reducing their orbital radius. Every time they cross the spiral arm, they get closer to the core.

Our galaxy has another generation of stars; called type II generation. These stars fly in a random direction in the galactic halo. Their motion is similar to the motion of stars in globular clusters: The star is moved in a random direction, but then it leaves the dense center part of the cluster (or galactic core), it decelerates by its gravity and is returned back. So we can't say that these stars have some regular "orbit" or "orbital period": it's almost random, but tends to be similar to an elliptical orbit around galaxy center.

Quote (Space_Navy)
Q2: Galaxies have got to be moving (considering the Andromeda galaxy is expect to merge with us), but I'm curious on where exactly they are going? Just drifting about in the universe?


In clusters of galaxies they move like star in star clusters, but with one distinction: galaxies can merge with each other and settle to a cluster center, forming a giant elliptical galaxy. The clusters themselves, and the isolated galaxies, don't move anywhere, they just participate in the universe expanding, and slowly move to nearest clusters due to local gravity forces.

Quote (TwoNybble)
This red-shift is caused by the Doppler Effect, when an object is moving rapidly (at relativistic speeds to be noticeable) away from the viewer.


You are wrong again smile Cosmological red shift is not a Doppler effect. It is caused by "expanding" of photons. The distance between waves of electromagnetic field increases together with expanding of space. So photons become "reddish" as they travel to us. Galaxies themselves are approximately fixed (neglecting slow local moving caused by local grvity fields from neighboring galaxies and clusters), but space is expanding. Take a rubber sheet draw coordinate grid on it and draw some small dots - galaxies. Draw a wavy curve representing photons. Then start stretching the sheet evenly in all directions. Galaxies stay fixed (their coordinates don't change), but the distance between them increases. Wavy curve stretches too - wavelength of photons increased. This is a good model of expanding space.

*





 
TwoNybbleDate: Sunday, 22.01.2012, 17:19 | Message # 26
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Well, of course the reality is much more complicated than what I explained. I was providing a simple explanation rather than get into the technicalities of these extremely complex relationships.

At some point, the stars near the center of our galaxy do orbit the central black hole. After some distance, the gravitational influence of the hole alone is not what is holding the galaxy together. But slightly further stars orbit the mass produced by the dense group of orbiting stars. And even further stars orbit the mass produced by these. The interactions of gravity are very complex at these stages, and it is much easier to explain by simply saying that we orbit the central black hole. There has to be a point where we stop to provide a simple explanation, or else we would be writing books about the subject.

Quote (SpaceEngineer)

Cosmological red shift is not a Doppler effect.


Thanks for the correction. I assumed they were the same effect at large distances, but I did not account for the expansion of space which is not speed, but "stretching."
 
Space_NavyDate: Sunday, 22.01.2012, 17:45 | Message # 27
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Thank you TwoNybble and SpaceEngineer for your incredible answers! smile
I have a better understanding of this now thanks to you two!





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SpaceEngineerDate: Sunday, 22.01.2012, 19:44 | Message # 28
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Quote (Space_Navy)
The interactions of gravity are very complex at these stages, and it is much easier to explain by simply saying that we orbit the central black hole. There has to be a point where we stop to provide a simple explanation, or else we would be writing books about the subject.

I don't agree with you here. This is the same as saying that Earth orbits the Sun's core - the most dense part of the Sun.

*





 
matty406Date: Sunday, 22.01.2012, 20:40 | Message # 29
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This thread, oh yes. I'll find some time to cram all this info.
 
TwoNybbleDate: Sunday, 22.01.2012, 21:36 | Message # 30
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Quote (SpaceEngineer)
I don't agree with you here. This is the same as saying that Earth orbits the Sun's core - the most dense part of the Sun.


Fair point. I just thought it would be easier to draw parallels with my simple explanation though.
 
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