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• Users: cragwolf
• In Astronomy and Astrophysics
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1. What is the efficiency of the Sun, really?

The implication of the Wikipedia article is 100%, real-time and not just eventually, i.e. it's on a Joules/sec (Watts) basis. For if you accept their number of 9.2 x 1037 proton-proton chain reactions per second, and ignore the CNO cycle, you come up with an energy-generated number that exactly...
2. What is the efficiency of the Sun, really?

Does all (i.e. 100%) of the energy generated in the Sun's core via nuclear fusion eventually escape out into space (as photons, neutrinos, ... and umm solar wind?), or is some energy lost to internal processes? Surely the photons must lose energy as they take their long, indirect path from the...
3. Estimating the ratio of flux of sunlight to moonlight (at full moon)

Indeed you're right, about me and the Moon. Thanks for the hint, I'm going to look at it again when I get home from work.
4. Estimating the ratio of flux of sunlight to moonlight (at full moon)

I'm trying to estimate the ratio of the flux of sunlight to moonlight (at full moon). What I'm interested in is if I'm doing the physics right. I know what the solar flux is at the surface of the Earth (actually, just outside the atmosphere): roughly 1400 Watts/m^2. I can actually calculate that...
5. Possible Explanations To Fermi's Paradox?

Possibly, but not necessarily. There's a book that speculates on answers to Fermi's question, by Stephen Webb, but I haven't read it and I don't know how good it is. It's an entertaining question to think about, but you will not find any definitive answers. I suspect that highly intelligent...

How big is the universe? We don't know. Is there a physical edge to it? We don't know. How did something come from nothing? If indeed that is what happened ... you guessed it ... we don't know. We have theories (for which there is good evidence) that describe a part of those questions (e.g...
7. A redshift derivation question

Neither can I. I get cross-terms which I can't get rid of. My idea was that the difference in redshifts between the gas cloud and the quasar is due solely to the (longitudinal) relativistic doppler effect.
8. Cosmological Principle

You are right and wrong. You are right in saying that the cosmological principle can't apply in a universe with a boundary. You are wrong in saying that a cosmological universe can't apply in a finite universe. Why? Because a finite universe does not necessarily have a boundary. A closed...
9. Universal Translator needed?

Stanislaw Lem challenged the standard assumptions about ET communication 37 years ago in His Master's Voice (and later, in other works like A Perfect Vacuum). As usual, his deep thoughts on the subject were largely ignored by scientists, philosophers and Star Trek fanboys. The first and/or last...
10. History of Universe (Baez timeline)

Please do not expect a mathematical physicist to get his details on human history 100% correct.
11. What is meant by expanding space

The stick isn't expanding. The solar system isn't expanding. Our galaxy isn't expanding. Even our galaxy cluster isn't expanding. You need to consider the universe on the scale at which it appears (roughly) homogenous. So in your raisin analogy, just consider the raisins as galaxy clusters. The...
12. Why can't we go to the center of the galaxy?

What distance to the centre of the Galaxy did you use to calculate this time? Most estimates range around 8 kpc. If you input that value into the equations for a rocket accelerating at 1g for half the distance (and then decelerating at 1g for the remainder) then you should get approximately...
13. Any questions?

Has anybody had sex in space?

15. Astronomy Q&amp;amp;A Game

Assuming that's true ( :biggrin: ) , why does the atmosphere expand when the Helium ionizes (and contract when it recombines). Also why doesn't this happen to the Sun?
16. Astronomy Q&amp;amp;A Game

Cepheids Here is my question (I hope it hasn't been asked before): Cepheid variable stars pulsate in a very regular and stable manner. What causes this pulsation? Please include in your answer an explanation of why the Sun doesn't pulsate in this way.
17. Astronomy Q&amp;amp;A Game

here is my guess: tholin (my post has to be longer than ten words for some reason.)
18. 1000 GRBs a day

How did they calculate 1000 per day? And if we live in a spatially infinite universe, shouldn't that be a number density instead of a number?
19. A&C reference library

A very nice website for amateurs like me (I especially like the FAQs): http://www.astronomycafe.net Ned Wright's calculator has been mentioned, but here are links to his brilliant tutorial and FAQ: http://www.astro.ucla.edu/~wright/cosmolog.htm...
20. Why Space Travel?

Fortunately, this sick corporate mentality hasn't taken over scientific organisations just yet.
21. First victim of moon base/Mars mission?

Here's a recent article on the subject of lunar astronomical observatories: Does the Lunar Surface Still Offer Value As a Site for Astronomical Observatories? by Daniel F. Lester, Harold W. Yorke, and John C. Mather Here's the abstract:
22. Universe Infinite

Even if Omega was precisely equal to one, you could still have a finite universe: in this case, its topology would have to be multiply-connected. For example, a 3-torus, a kind of 3-dimensional version of the surface of a doughnut, is flat everywhere, but its volume is finite. General relativity...
23. GEMS - a rich trove of information about galaxy evolution

This has been theorized by astronomers studying the formation of globular clusters. See this review article, for example: http://arxiv.org/abs/astro-ph/0107297
24. Why Space Travel?

Well, some good points from Artman, but the reason we do space exploration is not to cure cancer on Earth, not to eliminate poverty, not to save frogs from extinction. We do it out of curiosity, the desire to know. No astronomer I knew became an astronomer because he wanted to save the world. Of...
25. Do you think astronomy is a useful science?

That's why it's so good. More of us should be engaged in these useless endeavours, and less of us in the useful endeavours (which are mostly about exploiting or harming other people). The other thing, of course, is that what is considered useful, varies with culture and era. Why should I be...
26. Communication with Extraterrestrial Life

I think that the sole utility of Drake's equation is to help us to distinguish in our minds between those factors which we are completely uncertain about and those factors which we are "orders-of-magnitude" uncertain about.
27. Communication with Extraterrestrial Life

Stanislaw Lem, science fiction writer, philosopher. The link below is a very good introduction to his work: http://www.themodernword.com/scriptorium/lem.html [Broken]
28. Communication with Extraterrestrial Life

Thanks Nereid: a very intelligent post. Well, I would like to see them spell antidisestablishmentarianism, build a car, count to 1,296, play chess ... well, you get my drift. These animals are simply not as intelligent as us. The idea that ET communication optimists have is that if the...
29. What does one calorie per cubic mile mean to you?

You've come across another way of explaining why the sky is dark at night: there's just not enough energy out there (per unit volume one must be careful to note) to make it bright.
30. Alternative forms of the Friedmann equations

Wow, 0.85 joules per cubic kilometre! Space is pretty empty. I actually prefer units where c=1. It reinforces the idea of time as a dimension (measured in metres, like distances), and the equivalence of mass, energy, momentum, and pressure (measured in kg). But I'm not so comfortable with G=1 or...
31. Moving galaxy

Only that the list may be incomplete. Perhaps the Great Attractor is being attracted to an even Greater Attractor?
32. Moving galaxy

I don't know, marcus. That would be an interesting thing to find out.
33. Moving galaxy

I believe the peculiar motion of our galaxy is due to a number of things. Firstly there's the motion of our galaxy within the Local Group, then there's the motion of our Local Group towards the centre of the Virgo supercluster (of which the Local Group is an outlying member), and then there's...
34. Moving galaxy

You can reference it to the cosmic expansion itself (i.e. the Hubble flow). One can use the cosmic microwave background radiation (CMBR)for this purpose. If we were at rest with respect to the Hubble flow, then the CMBR would look isotropic on the large scale (after subtracting out the...
35. Moving galaxy

Well, we don't really know, but the physics and the mathematical models behind it don't require another universe into which our universe is expanding.
36. Moving galaxy

Well, there's the expansion of the universe, and there's the peculiar motion due to gravitational interaction with nearby galaxies.
37. Astronomy Q&amp;amp;A Game

Oops, I didn't see this last page, Marcus. OK, Labguy is right, and can ask the next question.
38. Astronomy Q&amp;amp;A Game

OK, my question is the following: what is meant by the term relaxation time, as applied to agglomerations of stars, e.g. open clusters, globular clusters, galaxies?
39. Astronomy Q&amp;amp;A Game

Here are my guesses. 1. One of the stars has decreased in brightness over the years. 2. Light pollution was much less significant back then.
40. Nagging question about inflation of universe

So you're saying that the solar system has expanded by roughly a factor of two since it formed (7mm * 365 * 4.65 billion ~ 12 billion km)? Actually, it would be more, given that Hubble's constant is closer to 70km/s/Mpc (although it's complicated by the fact that Hubble's constant actually...
41. Astronomy Q&amp;amp;A Game

[zz)] Hey chroot! CHROOT! Sorry to wake you up, but it's your turn to ask a question.
42. Astronomy Q&amp;amp;A Game

The Hubble time is not necessarily the age of the universe. Indeed, it's unlikely to be so. But it's close. The age of the universe, at least in the most successful model, depends on the mass/energy density, curvature and size of the cosmological constant. The actual formula for the age of the...
43. New Mars Photos

Suckers like me are born every minute.
44. Likelyhood of Extraterrestrial Intelligence

Why? Why should interstellar travel be inevitable? Why should colonisation be inevitable?
45. Astronomy Q&amp;amp;A Game

labguy's answer is more or less correct. Mostly, astronomers speak of a bimodality in the metallicity distribution of globular clusters in our Galaxy (metallicity tends to scale roughly linearly (*) with colour, so the two terms tend to be interchangeably used). There appears to be two...
46. Astronomy Q&amp;amp;A Game

OK, my turn. The globular cluster system of the Milky Way (and indeed, of many galaxies) is comprised of two distinct populations. What is the main observational parameter which distinguishes these two populations? The answer isn't size or luminosity, but that should give you an idea of what I...
47. Astronomy Q&amp;amp;A Game

A supernova. Just a guess, though.
48. Astronomy Q&amp;amp;A Game

Boötes, I'm just correcting LURCH's spelling.
49. Astronomy Q&amp;amp;A Game

No, it's bogdan's question. Go ahead, bogdan, ask any astronomy question you like.
50. Likelyhood of Extraterrestrial Intelligence

I think you're confusing wisdom with intelligence. Beethoven's Ninth Symphony was clearly created by an intelligent mind. A life form which did nothing of note except create equivalents of Beethoven's Ninth Symphony would be intelligent, but not very wise. Don't get me wrong, though. I'm...