| Subject | Date Asked |
|
| Follow-up solution | 11/5/2009 |
Q: Okay, I think I have worked this out, but can you tell me if it’s right? From the link you gave in ...
A: Congrats! You have it correct! The key to the solution was in seeing that, after you obtain the ...
|
| Binary star problem | 11/3/2009 |
Q: Can you help with this problem? A visual binary has a period of 100 years, and the semi-major axis ...
A: This looks like a fairly basic HW problem, so I will just get you oriented to solving it yourself. ...
|
| Relative and Absolute Time | 11/3/2009 |
Q: I have no training in astrophysics and very little knowledge about it. However, I am an interested ...
A: Okay, you have really two separate issues and I don't want to get them confused - so will stick with ...
|
| astrophysics/ black hole theory | 10/31/2009 |
Q: I am really in need of help on a theory I am coming up with. I have found most of my answers that ...
A: There are several problems with your theory. Let's look at it from the aspect of stellar ...
|
| cmbr | 8/6/2009 |
Q: sir, what is CMBR?
A: "CMBR" refers to the *cosmic microwave background radiation*. That is the 2.7 K temperature "relic ...
|
| boundry of universe | 8/4/2009 |
Q: sir, what does latest research guide on the subject i mentioned above? is there any boundry of ...
A: There is no firm answer yet to whether the universe has a "boundary". This is because more ...
|
| Escape velocity | 5/25/2009 |
Q: I've always been told Earth's escape velocity is 25,000 mph, but never understood WHY? If a space ...
A: Well, there is a non-math explanation but: a) it won't eliminate fully your 3,000 mph belief (that ...
|
| Escape velocity | 5/24/2009 |
Q: I've always been told Earth's escape velocity is 25,000 mph, but never understood WHY? If a space ...
A: You probably have never understood escape velocity - the concept, because it was always presented to ...
|
| Atom formation | 5/13/2009 |
Q: I was browsing through the WMAP website and found this time line of our universe. ...
A: You are correct that the universe had to cool down, a lot actually, in order for atoms to be able to ...
|
| Solar plasma instabilities | 5/10/2009 |
Q: I am interested in the sort of plasma instabilities that occur in solar conditions (say before solar ...
A: Yes, there are a number of instabilities associated with solar plasma, mostly defined in terms of ...
|
| Hall term-Hall MHD | 4/6/2009 |
Q: Can you elaborate on the meaning and significance of the "Hall term"? Also when is it important and ...
A: The "Hall term" arises in one electro-dynamic formulation of Ohm's law often used in solar work, and ...
|
| Theory of relativity | 4/2/2009 |
Q: Relativity says that an observer can be moving in any way and but it will always observe the same ...
A: Theories, once created or proposed - and which stand the test of time, don't get registered in ...
|
| Theory of relativity | 3/31/2009 |
Q: Relativity says that an observer can be moving in any way and but it will always observe the same ...
A: The "relative velocity formula" is actually an equation for the addition of velocities in a ...
|
| Theory of relativity | 3/30/2009 |
Q: Relativity says that an observer can be moving in any way and but it will always observe the same ...
A: Yes, and you can work it out using the relative velocity formula: thus, u = (v1 + v2) / [1 + ...
|
| Number of galaxies | 3/23/2009 |
Q: Stahl; My brother in law and I were in a discussion about time travel. I had heard (though I don't ...
A: Let's order the answers in the order of your questions: i)I think the university you may be ...
|
| expanding universe | 3/18/2009 |
Q: Philip, If the universe is expanding faster than the speed of light, how does any light get back to ...
A: Alas I don't think I can be much more help because getting into metric space and intervals based on ...
|
| expanding universe | 3/18/2009 |
Q: Philip, If the universe is expanding faster than the speed of light, how does any light get back to ...
A: No, the expansion of space to which I referred was the "metric expansion of space" which is quite ...
|
| expanding universe | 3/8/2009 |
Q: Philip, If the universe is expanding faster than the speed of light, how does any light get back to ...
A: The key here is that the contents of the cosmos itself (e.g. galaxies and galaxy clusters) are NOT ...
|
| Hall current in solar flares | 3/6/2009 |
Q: Can the Hall current exist in solar conditions? Can it trigger solar flare? If so, how? Thanks.
A: Yes the Hall current has been proposed to account for some solar flares. The first mention in ...
|
| Supermassive black holes | 3/6/2009 |
Q: Philip, My understanding of current theory is that supermassive black holes stop "feeding" because ...
A: Be aware that most of what you read about exotic astrophysical objects is not even on the level of ...
|
| Proton-Proton Solar Fusion | 3/6/2009 |
Q: Two protons combine then one turns into a neutron by the release of a positron, electron-neutrino ...
A: The difference you are referring to is what is called "the binding energy" of the particular nucleon ...
|
| Speed of light relative | 3/5/2009 |
Q: I understand some basic physics, and I understand the math behind it, but one question puzzles me ...
A: Actually, the two objects of which you speak would not even achieve c as their maximum, but only ...
|
| The powers of a black hole | 3/4/2009 |
Q: I have heard that black holes have the power to bend and rip space and time. Is this possible? ...
A: In actual fact, if one studies a space-time diagram (e.g. Kerr-type) for a black hole. it is not ...
|
| production of energy in stars | 3/3/2009 |
Q: sir...could you really think that only fusion reactions are responsible for burning of star...rather ...
A: Yes, because it is ONLY nuclear fusion reactions that can provide the energy budgets, detailed ...
|
| Flare time scales | 2/26/2009 |
Q: What is the difference between the different flare time scales: the conductive, radiative, and ...
A: The three time scales to which you refer (for LOOP flares of some length, L) can be given in ...
|
| Earth flares | 2/15/2009 |
Q: Dr Stahl, thank you for your patients in considering my controversial thinking: ...
A: Even if you can't provide a model, you should still be able to at least hypothesize based on ...
|
| SID flares | 2/14/2009 |
Q: I recently downloaded a paper you wrote on "SID flares" and am still confused as to what causes ...
A: Verey interesting that you were able to find and DL my paper. Anyway, let's get to your questions in ...
|
| Earth flares | 2/13/2009 |
Q: Dr Stahl, thank you for your patients in considering my controversial thinking: ...
A: Homologous flares, including those that are engendered via CMEs or coronal mass ejections, have been ...
|
| velocity, centripetal acceleration, gravitational acceleration | 2/1/2009 |
Q: This is home work. The question is what is the speed of a spacecraft in circular orbit at an ...
A: When doing any sort of physics problem (or astrophysics!) the first priority is to write down all ...
|
| Earth flares | 1/26/2009 |
Q: I've been reading the answers to the questions about Earth "flares" and find them intriguing. Isn't ...
A: Technically speaking one need not have magnetic reconnection to qualify for a flare. However, one ...
|
| infinity | 1/22/2009 |
Q: I was reading the latest Scientific American about naked singularities, and as far as I can ...
A: First, I am not about to get into any debate over the meaning of “infinite” or “infinity”. I think ...
|
| Surface temperature in gray atmosphere | 1/7/2009 |
Q: In a previous answer to a question some time ago, dealing with gray atmospheres of stars, you wrote ...
A: Okay, the key points - as I noted- were to assume: a) radiative equilibrium, and b) "LTE" or local ...
|
| Polytropic gas spheres | 12/30/2008 |
Q: Can you tell me what are polytropic gas spheres and how they factor into stellar evolution? Thanks!
A: Polytropic gas spheres are basically mathematical entities used for modelling of actual stars. As ...
|
| pop-science | 12/8/2008 |
Q: Philip A. Stahl, I appreciate the time you took to explain why you declined to answer my previous ...
A: These are all excellent questions. Unfortunately, it isn't always easy to discriminate between "pop ...
|
| Speed of cosmic rays | 12/8/2008 |
Q: I understand that cosmic rays (let's say from the sun) are mainly protons and small atomic nuclei - ...
A: Glad to hear this isn't a HW question, since it fits the profile of many problems I used to give in ...
|
| Advance of perihelion in relativity | 12/6/2008 |
Q: I have been reading the book, Principle of Relativity and in particular one paper by Einstein to do ...
A: This is not difficult but does require paying extremely close attention to the units used, as well ...
|
| space travel | 12/2/2008 |
Q: iam wondering if you could answer these questions for me.the first one is this.how fast is ...
A: The question is totally meaningless because you have NO units to go with that number! What is it: ...
|
| why sky is dark at night ? | 11/17/2008 |
Q: I am not a physicist or something but still interested to know things which are not clear. Learned ...
A: Olber's paradox is generally rendered with vastly more complexity than it needs to be, using ...
|
| Big tube from earth... | 10/20/2008 |
Q: What would happen if you took a big tube and put on end inside the earth's atmosphere and the other ...
A: Not likely at all! The air molecules of our atmosphere, remember, are held (more or less) in place ...
|
| Gravitational pull | 9/17/2008 |
Q: Im trying to figure out the gravitational attraction between Jupiter and the Sun in units of earth ...
A: First things first - I have to ask if this is a homework question. If it is, the most I can do is ...
|
| plasma | 9/6/2008 |
Q: Philip, In the dicta of an article I read recently it listed the various baryonic matter we know of ...
A: I think you are confusing two different domains of plasma. You are thinking of "hot plasma" such as ...
|
| more info | 9/5/2008 |
Q: I recently sent you a question about a rotating pulsar. The whole question is: The Crab Pulsar, ...
A: Actually, that value is: 5.3 x 10^8 m/s^2 Not m/s! Remember this is an acceleration, and actually ...
|
| more info | 9/4/2008 |
Q: I recently sent you a question about a rotating pulsar. The whole question is: The Crab Pulsar, ...
A: Okay, the only things we need to know relevant to the question you asked is: the radius, and the ...
|
| Logical Answer | 9/4/2008 |
Q: While doing a physics problem involving finding the rotational acceleration of a pulsar, I first ...
A: I worked out the maximal rotational velocity for the x-ray pulsar Centaurus X-3 using the standard ...
|
| Modern physics | 8/21/2008 |
Q: Whats the reason for so much high gravity in black hole that light can not escap from black hole.and ...
A: The reason for the extremely high gravity arises from the incredible high density of the matter that ...
|
| speed of light | 7/31/2008 |
Q: I was wondering, did Einstein say NOTHING could go faster than the speed of light (particles etc. ...
A: In his paper, 'Does the Inertia of a Body Depend on its Energy Content? (in Annalen der Physik, Vol. ...
|
| White holes -- my favorite astronomical object | 7/3/2008 |
Q: What about white holes makes their existence impossible? A white hole is the opposite of a black ...
A: The 2nd law of thermodynamics or "entropy law" states that the tendency of a closed thermodynamic ...
|
| White holes -- my favorite astronomical object | 7/2/2008 |
Q: What about white holes makes their existence impossible? A white hole is the opposite of a black ...
A: Your question introduces the critical notion of the difference between what is theoretically ...
|
| Binary star problem | 6/4/2008 |
Q: Can you help me with this problem? Find the mass of the stars Sirius A and Sirius B which form a ...
A: Well, what I will do is set this problem up so you can work it out. First, note that for a “true ...
|
| Fermi acceleration | 4/22/2008 |
Q: My questions seems to have gone to James Gort by mistake. I was really looking for your help here. ...
A: Jani First, let's revisit the plasma mirror machine problem you posed last year. As you recall we ...
|
| Double layers | 3/19/2008 |
Q: Can you elaborate a bit about double layers and what role they might play in solar flares? I have ...
A: If you go back to two questions you asked last year, to do with magnetic mirrors in plasmas, you ...
|
| inductance and solar flares | 3/13/2008 |
Q: I have read that solar flare eruptions can be compared to disruptions of inductive electrical ...
A: First, there is no way that the inductance in a flare region can be measured. What is more accurate ...
|
| Transfer equation for brightness temperature | 3/10/2008 |
Q: What would be the transfer equation for the brightness temperature of radiation in the vicinity of a ...
A: The transfer equation for the brightness temperature of radiation in the vicinity of a flare is ...
|
| Normalized Stokes parameters and coherency matrix | 2/28/2008 |
Q: Help! Can you show me how to do at least one of these problems? Three radio waves from different ...
A: Very good! You have solved part (iii) correctly. Just bear in mind that the coherency matrix (also ...
|
| Normalized Stokes parameters and coherency matrix | 2/27/2008 |
Q: Help! Can you show me how to do at least one of these problems? Three radio waves from different ...
A: Well, I am not going to work any of those problems for you, but will set up another example and full ...
|
| radial & transverse star velocities | 2/23/2008 |
Q: Can you please help with this problem? Barnard's star has a proper motion of 10.31"/ yr. and a ...
A: This is a fairly straightforward problem, so I will just point you in the right direction to get you ...
|
| Received spectral power | 2/20/2008 |
Q: I am hoping this is an astrophysics question, you can help me solve, though it is to do with radio ...
A: The normalized Stokes parameters are given as: S[s_i] = S x (s0) (s1) (s2) (s3) where ‘S’ is ...
|
| Equation of motion in a force-field | 2/17/2008 |
Q: Help! Can you give advice on solving this? Show that: m(dv_z/ dt) = -mu (@B/@z) where @ are ...
A: You certainly have a lot of plasma physics problems these days, don't you? The key point to note in ...
|
| Magnetic field problem | 2/15/2008 |
Q: Can you give advice on solving this problem? Determine the magnetic field of a cylindrically ...
A: This problem is actually relatively straightforward. The first thing to note is the force-free ...
|
| Ordiinary and extraordinary waves | 2/13/2008 |
Q: What is the difference between the X- and O-modes of plasma waves? Btw, I never did receive a notice ...
A: Using the graph below, the “O-mode” occurs for the specific case in which the perturbed E –field is ...
|
| Gravitational Shift in Multi-Star Systems | 2/12/2008 |
Q: I was wondering what would happen to a multi-star system if one star went supernova or for some ...
A: Assuming some binary star system, a supernova would herald some form of immensely dense collapsar - ...
|
| cut-off frequency v. resonant frequency | 2/11/2008 |
Q: For parallel propagation in an EM field, how can one *quantitatively* distinguish the cut-off plasma ...
A: In your parallel propagation (3D) case, we assume: w >> W _i (e.g. plasma electron frequency much ...
|
| Number of atoms | 1/31/2008 |
Q: How do scientists come up with a number of atoms in the universe of 10 to the 80th power? ANSWER: ...
A: The estimate of the currently observed cosmological density is a highly complex one and beyond the ...
|
| Number of atoms | 1/30/2008 |
Q: How do scientists come up with a number of atoms in the universe of 10 to the 80th power?
A: More technically, it is the number of *particles* in the *observable* universe (since one ...
|
| Plasma problem | 1/18/2008 |
Q: Help! I am fit to be distracted with this problem. Can you help? A proton moves in a uniform ...
A: Jani I am not going to solve this for you - but rather provide you with what I regard as ample ...
|
| Research | 12/26/2007 |
Q: I want to pursue research in the field of Astronomy and Astrophysics. I have done my M.Sc. in ...
A: Actually - study time isn't much of an issue since the GRE is first and foremost an *aptitude* test. ...
|
| Research | 12/24/2007 |
Q: I want to pursue research in the field of Astronomy and Astrophysics. I have done my M.Sc. in ...
A: Your query presents an intriguing dilemma, and alas, one for which I am unable to offer much help. ...
|
| Jupiter | 11/27/2007 |
Q: Seeing the size of jupiter and it being a gas giant is it possible for it to someday become a sun
A: While Jupiter does, indeed, seem to have ample mass to become a star, the fact is that its total ...
|
| Astrophysics Career | 11/25/2007 |
Q: I wish to be an astrophysicist. Is it related to daily life? How? ANSWER: Hello, First, it is well ...
A: You are correct that most governments will not support astrophysics unless: a) they have a large ...
|
| Astrophysics Career | 11/24/2007 |
Q: I wish to be an astrophysicist. Is it related to daily life? How?
A: First, it is well to recognize that astrophysics is not one uniform discipline but rather a general ...
|
| Velocity of EMR ! | 11/22/2007 |
Q: I wanted to know that why there is variation in velocity of light when it enter from one medium to ...
A: Okay, yes there IS a variation, when light transits from one medium to another. For example, there ...
|
| Loss cone instability and flares | 11/19/2007 |
Q: Under what conditions might a loss-cone instability lead to a solar flare?
A: First, let's go back and review a bit about what a loss cone is and then loss-cone instability. We ...
|
| global warming | 11/18/2007 |
Q: Has anyone studied the possibility that the weight we are taking off of the earth everyday by ...
A: NO one to my knowledge has studied this at all, but it is hard to see why they would. While the ...
|
| Deviation of emr under infulence of gravity | 11/18/2007 |
Q: Greetings!! can you tell me what was the actual deviation of emr passing by the star, in the famous ...
A: In the reference I have ('Gravitation and Spacetime', 2nd ed. by Hans O'Hanion and Remo Ruffini) the ...
|
| Mass of Photon is not equal to ZERO ! | 11/18/2007 |
Q: Greetings .!!! Ive derived the equation for da mass of photon .. just wnted to make sure that is it ...
A: While you are correct that the mass of the photon is non-zero - you're own estimate of it is ...
|
| big bang | 11/14/2007 |
Q: Firstly I approach this question as one who has little knowledge and appalling maths, but a general ...
A: What I think you are referring to is the inflationary aspect of early cosmic expansion. Note that ...
|
| Stars and Elements in the Universe | 11/10/2007 |
Q: How is energy released in stars and how does this effect the composition of elements in the ...
A: Energy is released in stars by the process of nuclear fusion which can take different forms, with ...
|
| Galactic Alignment | 11/1/2007 |
Q: I've heard the talk of the sun aligning with our galactic center this decade, a rare event. It's ...
A: The link you provided had some fascinating information for sure, e.g. clusters of craters of similar ...
|
| Galactic Alignment | 11/1/2007 |
Q: I've heard the talk of the sun aligning with our galactic center this decade, a rare event. It's ...
A: Joe I also seem to recall that an apocalyptic "disaster" was predicted 25 years ago when Jupiter ...
|
| The big rip? | 10/16/2007 |
Q: The theory of the big rip explains that all matter in the universe is moving away from each other, ...
A: The possibility of recollapse of the universe actually hinges on a very critical quantity which we ...
|
| life | 10/10/2007 |
Q: What got you interested in astrophysics?
A: Michelle I think I first became interested in astrophysics not long after I built my first ...
|
| Mean molecular weights of stellar gases | 10/8/2007 |
Q: How do I show that: i) the mean molecular weight of a pure hydrogen gas in a star is 1/2? ii) the ...
A: and thanks for a genuine astrophysics question! In obtaining the mean molecular density we assume ...
|
| forces of nature | 8/10/2007 |
Q: Is there a relationship between the forces of nature that exist on an astronomical level and those ...
A: The only force we know of that exists on the "astronomical scale" is gravitation. Currently, there ...
|
| Gravity in space | 8/9/2007 |
Q: First who came up with this theory and since moons and planets for that matter dont often rotate on ...
A: I think, from your answer - you are really confusing an actual theory (General Relativity) which ...
|
| a career in research in the field of astrophysics | 8/8/2007 |
Q: but when i get out and have my college money i want to study astrophysics just for the fact i would ...
A: Actually - an almost identical question to yours was asked last year, and the answer I gave is evry ...
|
| Brier P-Score for Flare Prediction | 5/16/2007 |
Q: Can you explain a bit about the Brier P-score for predicting flares and how it would actually work?
A: The Brier P-Score is a method of statistical flare forecast evaluation developed in 1950 as a ...
|
| brightness temperature of a supernovae remnant | 5/8/2007 |
Q: I'm having a little trouble with a problem that I hope you can point me in the right direction with ...
A: I think all you really need to solve your problem is a nudge in the right direction with respect to ...
|
| Magnetic helicity | 5/5/2007 |
Q: What is magnetic helicity and how can we recognize it on the Sun?
A: Before going on to magnetic helicity, it is useful to get a general idea of helicity – for example ...
|
| Virial Theorem & Total stellar energy | 4/9/2007 |
Q: I've read that the virial theorem can be used to show the total energy of a star is negative and ...
A: The virial theorem is very important in stellar astrophysics. According to the virial theorem: 2K ...
|
| Static corona? | 4/6/2007 |
Q: I have read that the assumption of a static corona (no gas expanding into space as solar wind) can ...
A: Of course, a static corona seems on first blush to be reasonable but that is why it is necessary to ...
|
| Problem | 4/5/2007 |
Q: I have been trying to work out this past exam problem but with no success. Can you provide any hints ...
A: This problem actually isn't too difficult to work, and is largely a variant on the previous problem ...
|
| Plasma mirror problem | 4/4/2007 |
Q: Can you help with ideas to solve this problem? Consider a plasma mirror machine of length 2L with ...
A: Jani ----------------!-------------- ----------------0-------------- B = (L) B = (-L) ...
|
| Two stream instability | 4/3/2007 |
Q: Can you tell me under what conditions the two stream instability occurs for plasmas? I understand ...
A: Two stream instability is a very common phenomenon in plasmas. It’s usually induced by an energetic ...
|
| gravity | 4/1/2007 |
Q: how far from the sun's core would you need to be to get 1.1 to 0.9 times Earth gravity? is this out ...
A: One can obtain the surface gravity of the Sun (at the photosphere - needed as a reference level) by ...
|
| Alfven waves | 2/10/2007 |
Q: Can you explain to me how Alfven waves arise? Please use any necessary math to show properties of ...
A: Alfven waves are the most important waves propagating in the solar atmosphere, as well as the ...
|
| Magnetic Reynolds number | 12/26/2006 |
Q: Can you tell me how exactly the magnetic Reynolds number is obtained and what its values is for ...
A: The magnetic Reynolds number is a (dimensionless) measure of the degree of coupling between the ...
|
| Maxwell equations applied to solar fields | 12/17/2006 |
Q: Can you tell me how the Maxwell equations for electromagnetic fields have to be modified for solar ...
A: Maxwell’s equations in differential form (much easier to write than in integral form) for free space ...
|
| Big Bang theory | 12/3/2006 |
Q: Is not the Big Bank Theory more applicable on a galactic scale than a universe wide scale?
A: No - if the Big Bang is applicable to anything, it is the cosmos as a whole. First, The 2.7 k ...
|
| Astrophysics problems | 11/28/2006 |
Q: Can you give assistance in solving the following? Thanks! 1) The lines of the Balmer series crowd ...
A: Here are suggested strategies, hints for solving the problems: Problem 1 The Balmer lines are ...
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| Gray atmosphere and distance modulus | 11/21/2006 |
Q: In my studies I have come across "gray atmosphere" to with stellar radiation. How do you explain ...
A: Modeling stellar atmospheres is a very complex undertaking that often requires we make basic ...
|
| hydrogen absorption lines | 11/15/2006 |
Q: How are the hydrogen absorption lines formed in a star like the Sun?
A: Differences in stellar spectra are actually due mostly to the widely varying temperatures in the ...
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