Astronomy Trivia Challenge: Can You Answer These Questions About the Night Sky?

[stuffy]

I wonder how long it would take that mirror to cool!

If astronauts were to dump a bucket of water on the moon's surface at lunar noon, roughly how long would it take all those water molecules to escape?

 

[Labguy]

Originally posted by stuffy
I wonder how long it would take that mirror to cool!

If astronauts were to dump a bucket of water on the moon's surface at lunar noon, roughly how long would it take all those water molecules to escape?

I haven't done any "Matherizing" (see my profile), but with the near-zero atmosphere (yes, the Moon has a small one), and the near-vacuum at the surface, I would say "roughly" less than two seconds if you mean that "escape" is to evaporate to its elemental gasses.

If you mean escape into space beyond the Moon's gravitational hold, I have no idea...

 

[stuffy]

This does deal with the escape velocity of the moon.

As a hint, it also has to do with the mean speed of the particles.

 

[marcus]

Originally posted by stuffy
This does deal with the escape velocity of the moon.

As a hint, it also has to do with the mean speed of the particles.

You said at "noon"
There are craters at the poles of the moon where it is always in shadow and quite cold. I expect ice could exist in a permanently dark crater at the moon's north or south pole.

But you said noon. Let's say at the equator, in vertical sunlight. Then the equilibrium temp is 394 kelvin, I think.

And escape speed from lunar surface is 2370 meters per second.

Let's say they really fling the water wide so it all ends up in intimate contact with hot rock and becomes 394 kelvin.

kT = 5.44 E-21 joules

kT = 0.034 eevee

A water molecule's mass is 3 E-26 kilogram

sqrt (2kT/m) = 600 meters per second (that is just a benchmark: the velocity that would have kT as kinetic energy)
there will be a spread of velocities roughly on that order of magnitude. the bulk will not have escape velocity of 2370 meters per second. so they don't all go off into space instantly

the story gets a mite complex-----UV in sunlight will start to dissociate the water in O and H-----but ignoring that complication...the thermal distribution of velocities will allow some fraction of the water molecules to escape, others will fall back to the surface and get reheated and have another chance.

someone else can have a go at setting up and solving the problem more rigorously-----one could put it in terms of a "half-life" of water at 394 kelvin on lunar surface

 

[stuffy]

You are definitely on the right path. My old Astronomy and Astrophysics book by Zeilik explains that the lifetime of a given gas is related to the value given by v_e/v_rms, probably to avoid the true complexity of the problem. I don't know if I want to give away the relation right now because that will certainly reveal the answer :)

 

[stuffy]

So much for that! v_esc/v_rms approx 3. Which means a lifetime of only a few years.

Next person to reply can ask a question.

 

[stuffy]

*rubs paddles* CLEAR! *shock*

 

[chroot]

Okay, okay, I'll ask one.

It might be depressing to learn that we will never be able to see all of the universe, no matter how patient we are. In fact, as the universe ages, the so-called "particle horizon," a 'boundary' beyond which we cannot see, will approach a specific size as time goes to infinity. (The particle horizon is, of course, not a physical boundary -- it is a mathematical surface that separates those points in space-time from which stars could send light to Earth in infinite time from those from which stars could not.)

The size of our observable universe will "max out" at this particular radius. What is this ultimate radius?

Bonus points: how big is the observable universe today?

- Warren

 

[StephenPrivitera]

oooh! I hope I get this one.

I remember reading about this in an old Astronomy magazine.

I believe that the current radius of the observable universe is 10 to 12 billion light years. It will max out at 40 billion light years. I've seen numbers as low as 20 billion LY, but my answer is 40. I trust my Astronomy writers.

 

[chroot]

Hmmm... sorry Stephen, those are not the correct figures -- at least not as understood from any recent data. Perhaps you could quote from the magazine?

- Warren

 

[StephenPrivitera]

See Astronomy, March 2003, page 45. It implicity states that the present radius of the observable universe is 14 billion light-years in the center caption. In a pamphlet produced by Astronomy (Solving Astronomy's 10 Greatest mysteries) that has no date whatsoever, it is explicitly stated that the max size currently is 10 to 12. Looking back at the March issue, page 44, last paragraph, it states, "in an accelerating universe, the speed of galaxies flying away from each other will one day overtake the speed of light - photons won't be able to catch up with the rapidly expanding space through which they travel." On page 45, first paragrah, it states, "there is a horizon . . . beyond which light cannot reach us." Further down the page, it mentions something about 40 billion light-years, but upon closer inspection, it says nothing about this figure being the "maximum" observable universe.

http://www.xs4all.nl/~carlkop/heelal.html
This site gives the radius of the visible universe as 12 billion light-years. See this section: ESA's Hipparcos satellite revises the scale of the cosmos. I have no idea of the date or validity of this source.

http://www.newageuniversity.org/universe.htm
This site gives it at 14.

I'll change my answer to 14 billion light-years.
But that was only the bonus question. I can't find an answer to your other question. Maybe next time!

_____edit
http://www-astronomy.mps.ohio-state.edu/~ryden/ast162_10/notes43.html
a more reliable source, citing the radius of the OU at 14 billion light-years

 

[chroot]

Well, as usual in cosmology, I'm afraid you're getting confused by some definitions of distance.

The universe is 13.7 billion years old, according to recent experiments. As a result, you might think that the observable universe is exactly 13.7 billion light-years in radius -- but this is not true. Because the universe used to be smaller, we can actually see much further than 13.7 billion light-years.

When I say "how big is the observable universe," what I mean is: what is the comoving distance to the furthest objects which are causally connected to us (i.e. within our past light-cone)?

Perhaps this question is just going to get mired in the confusing definitions that run amuck in cosmology... argh.

- Warren

 

[marcus]

No No!

Do not despair or say aaargh!

It is a very clear question. The answer is extremely interesting and depends on recently discovered acceleration in expansion of universe!

Terrific question


Lineweaver's online introduction to cosmology is the greatest.

Wait, I will get it.

Originally posted by chroot
Well, as usual in cosmology, I'm afraid you're getting confused by some definitions of distance.

The universe is 13.7 billion years old, according to recent experiments. As a result, you might think that the observable universe is exactly 13.7 billion light-years in radius -- but this is not true. Because the universe used to be smaller, we can actually see much further than 13.7 billion light-years.

When I say "how big is the observable universe," what I mean is: what is the comoving distance to the furthest objects which are causally connected to us (i.e. within our past light-cone)?

Perhaps this question is just going to get mired in the confusing definitions that run amuck in cosmology... argh.

- Warren

 

[marcus]

shucks, I got so interested reading Lineweaver I forgot I was
going to answr the question.

anybody at all interested in cosmology should download
this new (May 2003) tutorial

Inflation and the Cosmic Microwave Background
Charles Lineweaver

it is in arxiv: astro-ph/0305179

and it is also at some Caltech site with a name like
Level 5 knowledge base

the current distance of objects whose light (emitted in the past) we are now seeing can be maximum 47 billion lightyears

Lineweaver's figure 1 on page 6 is the main thing to look at.

because of accelerating expansion the present distance of the farthest objects we will EVER see (even if we live to infinity)
is maximum about 60 billion lightyears.

(this is assuming cosmological constant, causing the acceleration in the expansion rate, is in fact constant. if it is not all bets are off)

 

[marcus]

fundamental idea is that light travel time, the time it took light to get here, is not a very workable idea of distance because of different rates of expansion at different times in the past.
so many things effect the light travel time

but there is a really clear idea of the PRESENTDAY distance to an object, measured this instant by observers at rest with respect to the expansion process that the universe is undergoing

that is the same as being at rest with respect to the Cosmic Microwave Background, something extremely easy to check by doppler measurement. We know the solar system's speed relative to CMB very accurately, and can allow for it.

So there is a well-defined current distance to objects

as a general rule the light travel time "distance" would not match either the instantaneous distance of the object at present or the instantaneous distance to it at the time the light was emitted.
the instantaneous distance at some definite moment (like a time in the past) is called sometimes "proper distance". With the term "comoving distance" reserved for the proper distance at this moment in year 2003. Terminology seem to me still a bit unsettled, so I just say current or presentday distance because that's what it seems to me that it is, whatever technical term they decide to call it.

 

[chroot]

You got it, Marcus. Lineweaver's little 30-pager is a great overview of the state of cosmology right now, you're right! It inspired my question. I'm not sure who sent the link to me -- it was probably you. :)

Anyway, it is a shame that Astronomy magazine and others are so haphazard with their details. It's pretty much unacceptable for a magazine like Astronomy to assert in March 2003 that the size of the observable universe is 14 billion light years. Even if you ignore the recent findings about the accelerating expansion, this is patently wrong. Light travel time is just not a good way to describe distances in a universe that has changed in size.

The accelerated expansion, by the way, is the reason why the particle horizon maxes out at 62 billion light-years -- if the expansion were constant, we would eventually be able to see everything in the entire universe.

Your turn, marcus.

- Warren

 

[marcus]

Originally posted by chroot


The accelerated expansion, by the way, is the reason why the particle horizon maxes out at 62 billion light-years -- if the expansion were constant, we would eventually be able to see everything in the entire universe.

Right! And that is hard to grasp too----without accelerated expansion even objects receding at greater than c could emit light that eventually reaches us----infinity being as it were a long time The idea of being able to see everything if you wait long enough is staggering. But with accelerated expansion it is only a finite piece that we will ever get to see no matter how long we wait. Cool ideas they get to deal with, these cosmologists.

I had a question ready, which can serve here in the game and also I posted it out in the "lineweaver" thread. Answers there don't count in the game. Here it is:

-----------------

This question is based on Lineweaver's Figure 1, top section'
There are three diagrams in Figure 1, I mean the top one.

If you go to the IPAC-Caltech site then you can enlarge the diagrams to fill the screen.

http://nedwww.ipac.caltech.edu/leve...r_contents.html

It's also convenient to print it out which you can do from the Los Alamos archive site-----tho the diagrams are smaller printed out.

http://arxiv.org/abs/astro-ph/0305179

Either way, what you see in the figure is a tear-drop shaped lightcone

And also you see a curve showing the extent of the "HUBBLE SPHERE" and he explains what he means by that.

Question 1: What are meant by lightcone and Hubble sphere?

There is a point in the diagram where the Hubble sphere and lightcone intersect. The Hubble sphere line crosses the side of the teardrop shaped lightcone.

Question 2: What does the intersection signify? Why does it come at the widest point of the lightcone----where the side of the lightcone is vertical?

 

[marcus]

I just noticed the rules Nicool posted at the beginning of the thread say I should post a new question since this one wasnt answered in 3 days

Originally posted by marcus

Question 1: What are meant by lightcone and Hubble sphere?

There is a point in the diagram where the Hubble sphere and lightcone intersect. The Hubble sphere line crosses the side of the teardrop shaped lightcone.

Question 2: What does the intersection signify? Why does it come at the widest point of the lightcone----where the side of the lightcone is vertical?

Just to conclude, the Hubble radius is the current distance to where space is receding at c ( Lineweaver, caption to fig. 1).
Within the Hubble sphere space is receding at speeds < c. Outside it is receding at > c. the radius of the h. sphere changes with time as shown in the figure.

The past lightcone essentially shows the paths along which light now reaching us has traveled----in a simple 2D spacefime diagram the lightcone line is the path of a ray of light coming towards us for the whole history of the universe and reaching us today.

Where the two curves intersect is where light started out towards us in a region of space receding from us (at that time) at speed c------so for the first few days the light didnt get any closer to us! It's speed towards us was canceled by the Hubble flow away from us. So its time-line goes vertically up the diagram parallel to ours. But after a while it begins making progress "swimming upstream" against the current of the Hubble flow (another term for the expansion of space).

Below that intersection light starting out towards us is initially swept back (teardrop shape of lightcone, below its widest point)

I'll think of another and post it.

 

[marcus]

another question
background to question:
the expanding universe model of cosmology was used
in 1948 (Alpher/Herman) to predict the CMB
over 10 years later the CMB was found and it had roughly
the temperature they had predicted-----a classic longshot.
They had predicted 5 kelvin and it turned out to be 2.726.


Another background, with a different temperature, is also
predicted by the model. (Lineweaver p23 section 7.4)

This other cosmic background temp has not as yet been measured.

what is this other background and what (roughly) is its predicted temperature?

 

[subtillioN]

the 2.7k CMB is simply the ambient temp of the ubiquitous intersteller Hydrogen. This temp was actually more correctly predicted by the steady state model as I recall.

See this link:
http://www.Newtonphysics.on.ca/COSMIC/Cosmic.html

" Abstract.
It is recalled that one of the most fundamental laws of physics leads to the prediction that all matter emits electromagnetic radiation. That radiation, called Planck's radiation, covers an electromagnetic spectrum that is characterized by the absolute temperature of the emitting matter. From astronomical observations we observe that most matter in the universe is in the gas phase at 3 K. Stars of course are much hotter. The characteristic Planck's spectrum, corresponding to 3 K, is actually observed in the universe exactly as required.
However, in the standard model of the universe, the simple fundamental Planck's law has been ignored. It is claimed that the observed radiation comes from a combination of complicated hypotheses, involving an elaborate "creation mechanism" called the Big Bang. After this event, the radiation would have been emitted at a single instant when matter became decoupled from radiation. Finally, that radiation would have been shifted increasing its wavelength about 1000 times. We show that the 3 K radiation spectrum observed is simply the Planck's radiation emitted by gaseous matter at 3 K. "

 

[marcus]

Originally posted by subtillioN
the 2.7k CMB is simply the ambient temp of the ubiquitous intersteller Hydrogen. This temp was actually more correctly predicted by the steady state model as I recall.

See this link:
http://www.Newtonphysics.on.ca/COSMIC/Cosmic.html

a stable unexpanding universe in equilibrium at 3 kelvin
is a charming thought but the question was about
something predicted by the prevailing model
there is this other background that mainstream cosmology
predicts is there and that it has a certain black body temperature,
the question is about what temp is predicted by the model

BTW when they do measure the temp it will be a good test
of the standard cosmic model, right?

so what is this other background I am referring to?
and what is the predicted temp?

 

[marcus]

Originally posted by Nicool002
Hi guys! Most of you know how this works but for the newcomers:

The rules are this: someone will ask a question and if the question is not answered correctly within 3 days then a new question will be posted. If an answer to a question is posted and the person that posted the question does not respond to the answer within 2 to 3 days, then the first person to have answered the question will then be able to post their own question. HAVE FUN AND LEARN!

I will start:

Question: What is the brightest star in the Northern Sky? (excluding the sun)

This game can be a lot of fun and interesting.
Since no one has answered in 3 days I think it is up for grabs.
Anyone can ask a question
or so I think from looking at Nicool's rules

 

[marcus]

I asked the last question in the Q/A game and it has gone unanswered for more than 3 days so I will answer it myself and maybe pose another.

Originally posted by marcus
another question
background to question:
the expanding universe model of cosmology was used
in 1948 (Alpher/Herman) to predict the CMB
over 10 years later the CMB was found and it had roughly
the temperature they had predicted-----a classic longshot.
They had predicted 5 kelvin and it turned out to be 2.726.


Another background, with a different temperature, is also
predicted by the model. (Lineweaver p23 section 7.4)

This other cosmic background temp has not as yet been measured.

what is this other background and what (roughly) is its predicted temperature?

The other background (besides the microwave background) which the expanding universe model predicts is the NEUTRINO BACKGROUND and this also has a thermal distribution with a definite temperature, just like the microwave background does, but the model predicts a DIFFERENT TEMPERATURE!

The microwave temperature of space has been measured at 2.73 kelvin, corresponding to a redshift z = 1100.

The predicted neutrino background temperature is 1.9 kelvin.

The fact that the two temps differ the way they do provides for a sensitive test of the model at some time in the future when the neutrino background will have been well-enough studied to determine its temperature. I think Lineweaver's discussion of this on page 24 is pretty interesting. It explains why T_CMB should be hotter than T_neutrino by a certain amount.

 

[marcus]

A new question for the Q/A game

We know that Aristarchus of Samos (who flourished around 270 BC) maintained that the Earth travels around the sun and that the cycle of day and night is caused by the Earth's rotation.

The main reason we know about Aristarchus and his heliocentric model is because it is described in a book written a few years later by somebody else.

(Unfortunately Aristarchus original writings proposing the heliocentric model have been lost, although his calculation of the relative sizes of the earth, moon, and sun has been preserved)

What is the book which is our primary source about this and who wrote it?


For "extra credit", why did the Greek contemporaries of Aristarchus mostly reject his proposed heliocentric model?

 

[Shadow]

was it De architectura by virtruvis?

 

[marcus]

Originally posted by Shadow
was it De architectura by virtruvis?

No, the book was by Archimedes.

Archimedes described the heliocentric model (later popularized by Copernicus) around 250 BC in a book called the "Sand-Reckoner."

Copernicus knew about this and made a reference to it in one of his manuscripts, giving Aristarchus the credit for the discovery.

In "Sand-Reckoner" Archimedes describes Aristarchus model of the universe as a sphere with the sun in the middle and he calculates the number of grains of sand that would be needed to fill it.

There is an odd similarity between this and the present excitement among modern cosmologists about how much matter and energy of various sorts the universe contains.

the rules of this "Astronomy" Q/A game suggest that if a question goes unanswered for 3 days or more (which this has) then another should be provided, so I will think of one unless someone else gets to it before I do

 

[marcus]

the most distant quasar

What is the most distant object known to astronomers?
I mean a localized object----a certain quasar in fact---rather than diffuse radiation like the microwave background and ancient neutrinos.

What is its observed redshift?

 

[Shadow]

Well there is a baby galaxy that Astronomers have found and it took 122 billion years for it's light to reach Earth but that is, in fact NOT a quasar... when you say local, do you mean in this galaxy?

 

[chroot]

It's a quasar (I can find neither its name nor its location) at redshift 6.4, discovered by the Sloan DSS.

- Warren

 

[marcus]

Originally posted by chroot
It's a quasar (I can find neither its name nor its location) at redshift 6.4, discovered by the Sloan DSS.

- Warren

You're right! It is your turn

We both probably saw the announcement in Ned Wright's "news of the universe"-----the quasar was discussed by Bob Becker (of the Sloan sky survey) around November 2002 at a talk that Wright referred to

The direction to it is very close to the star in the Big Dipper called "gamma UMa"

You teach astronomy, I know, but for anybody who doesn't know which star is "gamma" in that constellation, imagine the dipper in standard dipper position and picture where the handle joins on to it: that star is delta and gamma is the one just beneath it.

like, gamma is the lower left corner star of the dipperbowl

the quasar is about a degree below that corner star
and indeed the reshift is a huge 6.4, imagine wavelengths stretched out to more than seven times original length!

Think of a good one Warren.