GeeKay

swansont - many thanks for your succinct answer to what has been a long-term headache for me. Much appreciated!

I am tying up the loose ends of a SF short story I wrote last year, featuring a neutron star. Right now, though, I'm stuck over a long-term problem concerning the star's luminosity. It's surface temperature is 8,200 K and has a diameter of 20 kms. My problem has consisted of trying (and failing) to calculate what the star's absolute magnitude would be. I'm afraid this is beyond my abilities to solve, and unfortunately I don't know anyone among my friends who has the mathematical nous to help me out here. So I would be extremely appreciative if someone here could put me out of my misery. Many Thanks!

What would be the theoretical maximum spin-rate of a small asteroid (25 - 100m diameter?) if it were, for example, composed of a single 'monolithic' chunk of iron? This question arose after reading up on the subject in a Wikipedia article. Listed in it is one such asteroid with a spin-rate of around 30 seconds. Could this be bettered, given the above parameters? Many thanks. https://en.wikipedia.org/wiki/List_of_fast_rotators_(minor_planets)

Yes, I understand now. In other words the only 'practical' way for an object to decelerate while passing through an intense gravitational field would depend on powered means, not inertial. Moreover, the fact that the incoming object is moving at 0.01c relative to a given mass would (I should imagine) rule out all variations of power assist - this insofar that the proper velocities of nearly all stars aren't remotely relativistic, that's to say within local frames. Many thanks for clearing up this matter for me.

I'm wondering if it's possible to use the gravitational field of a massive object to decelerate a spacecraft moving at high velocity. An example is this: if a spacecraft moving purely on inertia at one percent light-speed performed a flyby of a one sol mass neutron star (or black hole) at a given altitude, would the gravity field cause the spacecraft to swing round the star, slow it up or else deflect its trajectory in a meaningful way? According to one online calculator the escape velocity of a solar mass neutron star at a distance of 25,000 km equals 3,000 km/s or 0.01 c. At this distance the tidal pull exerted by the star's gravity would be minimal, therefore represent no hazard to the ship or its crew. That said, I'm curious to know what would be the outcome. PS. It's the escape velocity I'm interested in: not the star's proper motion as a means of affecting a change in delta-v. Many thanks.

Hi Yes, I'm looking forward to trying out these ideas once I get the offending discs back from a friend of mine whose been investigating the problem. I'll start with Phil for All's solution - this being a likely cause, given my tendency to fumble around with the player remote when watching films in subdued lighting. Failing that, I'll attempt to change the movie's audio setup in the discs' formating options (thanks, Memammal) and see what happens. Again, thanks for all the helpful advice - I've learnt a great deal from it

Certainly seems to be. One question, though: how come the the discs and/or the player have started playing up like this? I say this because they always played okay in the past, using the existing player. It's only now I've started having this problem here. Thanks for getting back to me, by the way.

I'm afraid I don't - the only Blu-ray player I have is the one I use for the TV (both my PCs are DVD-only). However, a friend of mine reports that the discs play fine on his own Blu-ray player. So it seems the source of the problem lies with my player. Yet it has no troubles playing my other Blu-ray films. That's what's bugging me, perhaps more than anything else.

A strange conundrum: nowadays whenever I try to play my Blu-ray edition of 'The Lord of the Rings', I get plenty of video, but no audio. That's to say there's audio throughout the preamble (adverts etc) that comes before the main feature starts. But as soon as the actual cinematic part of the disc begins it goes completely silent. The same problem exists on all three discs - though, interestingly enough, it doesn't occur on any of the special features discs bundled with the boxed edition. They come out loud and clear! No other Blu-ray film or DVD of mine has this weird and wonderful audio issue. They play just fine. It occurs only on the three LOTRs discs - and then it's just the cinematic sections. I haven't loaned the box set to anyone. The problem seems to have manifested entirely by itself. NB. I must confess to having hawked this problem around various online help forums, and so far I've received zilch responses. So this is my last port of call. As for myself, things have reached the point whereby I simply want to know what the cause is, even if nothing can be done about it. There has to be a rational scientific/technological explanation, one that ultimately makes sense. Otherwise I might as well start believing in fairies. . . perhaps even in the existence of Middle Earth? Many thanks.

I gather that Mars regularly experiences aurorae, this despite the planet's lack of a global magnetic field. It's also stated that these aurorae occur in the ultraviolet wavebands. I assume from this that these displays are invisible to the human eye. This being so, I'm intrigued to come across artistic interpretations on the net that suggest how "auroras will look on certain parts of Mars" (to quote the text accompanying one such image). Does this mean that aurorae are actually visible, after all? Or am I missing something here?

Ah, yes, I understand now. Thanks for pointing this out to me. I had intended to use the image of an incoming comet and a rogue minor planet (of Ceres mass) passing through the solar system as an analogy for the expansiveness of sub-atomic space. But I can see now that this is no analogy at all. PS. Those cloud chamber shots are astonishing!

Yes, I should have used 'concentration' rather than 'density'. I stand corrected. So I guess then that if 1 cm2 of skin contains (in round figures) a quadrillion atoms, this means that there's an excess of around 15,000 more atoms than those neutrinos passing through this same area per second. But as Janus points out, a given 'atom' consists mostly of space, which explains why the strike rate is so low. (At some point I'm going to sit down and calculate exactly what this means at a sub-atomic level). Still, I'm intrigued as to why there appears to be such a glaring contrast between the strike rates of neutrinos and (say) cosmic rays. Yes, I surmise that neutrinos are chargeless and nearly massless particles, while cosmic rays, which being composed mostly of high-energy protons, are both enormously more more massive and carry a charge. Yet if atoms are comprised largely of emptiness, I'm left wondering why cosmic ray nuclei are many times more damaging in a biological context. Or am I missing out on something here?

I understand that around 65 billion neutrinos pass through every square centimetre of human skin (or any other skin for that matter) per second. I also gather that a human being can expect to be struck by an individual neutrino perhaps once or twice in a lifetime. These statistics, if true, are remarkable on several counts. Nevertheless, in order to get an handle on what this continual neutrino blitztkrieg really means at an atomic level, I've been trying to find out what the average density of atoms there are in a given square (not cubic) centimetre of human skin - but no luck so far. Does anyone have even an approximate answer to this admittedly obscure question? Many thanks.

Thanks for unsticking my brain - and to ajb for supplying the correct formula. Just one other point - well, two actually: I assume that kilometers and kilos respectively should be used as a metric when applying the above formula to a given celestial body. The second point is rather more complicated: how does one use the formula's solution as a means of calculating a celestial's body's focal sphere? As examples, a minimum of 550 AU is cited for the Sun, 15,300 AU for the Earth, and a focal sphere of just 10 million km for Sirius B, but I don't know how these figures are derived. (I'm afraid I only have an antique GCSE in Maths to assist me here). Many thanks. http://www.centauri-dreams.org/?p=35498 Correction: I meant 'focal distance' not 'focal sphere.'

I came upon this statement about gravity lensing the other day. It reads: "The ratio of a planet's radius squared to its mass lets us calculate the distance a spacecraft must reach to take advantage of gravity lensing." It's the first part of this statement that I can't understand - "The ratio of a planet's radius" especially. While I know the meaning of each individual word, I simply cannot string them together in any meaningful sense. Neither can Google apparently. Any suggestions?

Yes, I think I've got that. I guess then there's no reason why the calendars of both planets should coincide in any particular way.

For me at least, space ends a fingertip's thickness beyond the grey walls of my imagination. Perhaps I should learn to stretch my mind a bit, or perhaps not? Hmm.

Having stumbled upon Wikipedia's article about the Darian Calender intended for use on Mars, I'm interested to know how this calendar corresponds with our own one here on Earth. More precisely, if the first day of the Martian year begins on Sol 1 in the month of Sagittarius (warning: this itself is very much a stab in the dark) does this coincide, temporally speaking, with our first day of the new year - due allowances made for the smallish, if variable time-lag existing between Earth and Mars? Or am I barking up the wrong planet? Many thanks https://en.wikipedia.org/wiki/Darian_calendar

Yes, I dream of being under inky skies. . . or even clearer skies, come to that. I intend to get away from the perennially cloud-bound UK to enjoy an extended spell of stargazing in Southern Spain later this year. I also hope to visit Australia before I shuffle off this mortal coil and explore a full third of the southern heavens I've never seen before - and all this by means of a day's drive out from Alice Springs? Meanwhile, Texas? Now there's a thought. . . Re. the issue about how astrophotography affects a scope's light-gathering potential: the link below offers some useful info here. http://www.madawaskahighlandsobservatory.com/Astronomers-WFT.html

I think I may have phrased the question wrongly. When I wrote 'time-lapse' photography, I really meant (for want of a better term) keeping the camera 'shutter' open for a extended periods of time. This question was prompted in part from hearing about professional astronomers, such as those at the Keck, deploying this means to rack up the magnitudes - say from (approx) +22 mag to around the +29/30 mark? Also, I can't help but notice that photos of dim DSOs reveal an awful lot more (i.e. fainter) detail than what one would expect to see through the eyepiece of a telescope of the same aperture. The reason why I'm asking this question is because I'm thinking of trying my hand at some astrophotography. It would mean buying another mount, however, plus a slow-motion drive. Plus there's the added issue of my scope's aperture, which were I ever to encounter a truly dark sky (sigh) would give me a limiting factor of around +13 mag, this assuming very good seeing conditions, of course. This is okay for me, but I do wonder if the added expense will be worth it if I only replicate via the camera what I see through the tube? I guess my real question is this: if a star's brightness is measured in magnitudes - and I accept that's pretty much cast in stone as far as aperture is concerned - where do all the fainter stars, or that added structure in a galaxy, globular cluster etc, come from as a result of (say) an extended exposure taken by a camera? Same telescope, but more photons, I should have thought. So what does this mean exactly?

I apologise for this somewhat obvious question, but does a telescope's limiting magnitude increase when using time-lapse photography - this compared to real-time viewing?

Yes, our reach appears to be growing exponentially longer, but at the expense of our grasp? That question, though, more properly belongs to another thread. . . There is something else, though. . . one aspect about monopoles that has long intrigued me. I understand that they could (in theory?) speed up the processes inside a fusion reactor by serving as a catalyst. But how this could be achieved in a nuts and bolts sense I still find baffling. In other words, just how - albeit it in layman's terms - can a magnetic monopole catalyse the fusing of hydrogen atoms? Or am I missing out on something important here? Again, many thanks.

Thanks again for the imput. Yes, I get what you say about the Higgs particle. Having read "The Particle at the End of the Universe' by Sean Carroll, I can quite understand the adventures the researchers had tracking down that pesky boson. Whether magnetic monopoles - should they exist at all - involve the same kind of protracted hunt, time will only tell, I guess.

Thanks, imatfaal. I guess the real point I am trying to make has less to do with whether magnetic monopoles exist or not. All I can hope for here is that scientists will eventually discover them, or failing that, make a fundamental discovery in a related field that ends all further speculation about their existence: end of question. Instead, assuming for the sake of this argument that they do exist, my main point of interest here is centred on their properties, especially how these properties may conceivably apply to their potential uses as a future power source. This is the sort of information I am seeking. . . conjectures, if you will. That seems a reasonable question to ask.

Hi, I have, albeit reluctantly, almost come round to accepting what appears to be a fairly widespread view in the scientific community: that monopoles do not exist, at least according to the current models we have about the universe, post Big Bang. This is to say that the likes of Ampere, Gauss and Maxwell have thus far proved correct, over that of Curie and Dirac. Nonetheless, part of me has always wondered if this will be the full story on the subject of monopoles. I'm also extremely wary about scientists making negative pronouncements before all the the evidence has been examined - or even discovered. Clarke's First Law ("When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong") springs to mind here. The great Lord Kelvin's comment about X-rays being a hoax is a prime example of this ingrained attitude in action. On the other hand we are no longer saddled with obsolete theories harping on about phlogiston or the aether. So will monopole theories seem no less quaintly wrong-headed in another hundred years' time? That was my take on the situation until quite recently, that is to say until I read 'The Eerie Silence' by Professor Paul Davies. He discusses monopoles in some detail in the chapter 'Evidence for a Galactic Diaspora', and although he too hedges his speculations about their existence, what does emerge is a tantalising glimpse into what might be round the next dozen or so corners, were monopoles finally proven to exist. In particular, the potential any such monopole technology would offer humanity - for good or for ill - almost seems to rival the fabled attributes of antimatter as a power source. Could this possibly be true? Or in interpreting the data, am I misreading myself back into the world of Leonardo's helicopters? I have since then scoured the internet for more information of the subject of monopoles as a (potential) power source. Unfortunately, if not altogether surprisingly, the results so far have been meagre in the extreme. I am therefore interested to know what the thoughts are by other members of this forum, and whether anyone is able to throw some fresh light on this subject. Many thanks.