All Questions
534 questions
7
votes
2
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3k
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Can the Sun's core be treated as an ideal gas?
I know that a gas behaves more like an ideal gas at higher temperature, and that is very well achieved in the Sun's core. But also low pressure is needed for a gas to behave like an ideal gas, and the ...
- 3,934
7
votes
1
answer
819
views
About the hump on galaxy rotation curves
The past days I have been studying the rotation curves of disk galaxies and I am currently trying to understand how we can extract information about the dark matter of a galaxy by looking its rotation ...
- 329
7
votes
1
answer
782
views
Are "typical" black holes rotating or static?
From my (somewhat limited) understanding of GR I know that there are two different kinds of solutions that produce a black hole, some that rotate and some that do not. What I can't figure out from my ...
- 1,569
6
votes
2
answers
2k
views
How is a blackbody spectrum formed in the Sun?
Sunlight can be treated as BB radiation. Why is it a continuous spectrum while the sun contains only a few elements and the radiation from the jumps between atomic levels are discrete? How does the ...
- 1,231
6
votes
1
answer
557
views
Does the bullet cluster prove that dark matter must consist of particles?
Some time ago the remnants of a collision between two galaxy clusters were (accidentally) discovered. These remnants are called the bullet cluster.
There are some theories around to explain the ...
6
votes
3
answers
25k
views
How does gravity on earth affect concentrated sound waves aimed on a horizontal plain? [closed]
Someone asked me this question and I don't think I gave him an adequate answer (I was trying to think of the extreme case - that of neutron stars)
- 3,662
6
votes
4
answers
1k
views
Could dark energy be the effect of gravity at great distances?
This may be a silly question, but is it possible that dark energy and gravity are related to each other?
Space-time is deformed everywhere in space by objects with mass. The more massive the object, ...
David Burhans
5
votes
4
answers
994
views
Are galactic stars spiraling inwards?
Are the stars in our galaxy spiraling inwards towards the center, or are they in a permanent orbit?
And if we are heading towards the center then what is the rate of this process?
I started ...
- 153
4
votes
1
answer
100
views
Observational evidence for superfluidity in neutron stars
What is the earliest evidence for the existence of superfluidity in neutron stars?
I'm about to present the subject in a seminar and I'm under the impression that at the moment there is no ...
- 1,447
4
votes
1
answer
444
views
Minimum size of a "water star"
So I had this weird idea the other day. We know that stars form from simple matter (dust, gasses, etc) forming a gravity well, attracting to each other, until the amount of mass and density in the ...
- 193
3
votes
2
answers
1k
views
What would Happen if a Primordial Black Hole, with 5-10x time the mass of Earth, were to fall into our Sun?
So lately i heard of this theory that planet 9 might be a Primordial Black Hole (PBH) with 5 - 10 times the mass of Earth. I was thinking to myself, what would happen, if such a PBH (if it even exists)...
- 41
3
votes
4
answers
3k
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How far has a 13.7 billion year old photon travelled
I've read that the size of the observable Universe is thought to be around ~46 billion light years, and that the light we see from the most distant galaxies were emitted ~13.7 billion years ago as a ...
- 2,525
2
votes
0
answers
320
views
Electric and magnetic field in a black hole
I have many questions about this topic:
Does the electric field of a charged black hole look like this? I mean how can it have an electric field if nothing can escape from a black hole, and what is ...
2
votes
2
answers
297
views
Would a giant satellite laser work as a weapon?
If someone built a satellite to float about in space, which used mirrors to focus and magnify energy from the sun directly into one direct spot (like in the James Bond film Die Another Day) would it ...
- 195
2
votes
1
answer
336
views
Discrepancy problem in lithium?
Why is there a discrepancy between the amount of lithium-7 predicted to be produced in Big Bang nucleosynthesis and the amount observed in very old stars?
2
votes
1
answer
3k
views
What is the pressure and density of a neutron star?
I am interested in finding pressure of neutron star! So. Please could any tell me how to choose central density for the inner and outer core of neutron Star. What numeric value should me in both core....
- 155
1
vote
3
answers
525
views
Effect of expansion of space on CMB
Is it true that the expansion of space time cause the CMB to become microwaves from a shorter wavelength. If it is has the amplitude been increased?
Seeing as the amplitude has decreased; why hasn't ...
- 6,987
1
vote
3
answers
549
views
Mean Gravitational 'Potential' Energy in Space
Imagine an arbitrary point in space. It is within the gravitational 'potential' of every mass (although billions of ly away) in the entire universe.
Since every mass adds a tiny fraction, what is ...
- 78
0
votes
0
answers
80
views
Graviational Lensing Data
Can anyone provide me with (or point me to) real life gravitational lensing data that I can plug in to the first equation on this page (the standard lensing equation): http://en.wikipedia.org/wiki/...
- 2,525
-1
votes
1
answer
134
views
Do neutron stars fuse elements?
I have read these questions:
In reality, they fuse the hydrogen nuclei (the cores of the atom) together into helium nuclei.
https://solarsystem.nasa.gov/genesismission/gm2/mission/pdf/NeutronStars....
- 29.3k
63
votes
3
answers
9k
views
Do solar systems typically spin in the same direction as their galaxy?
Is the net angular momentum vector of our solar system pointing in roughly the same direction as the Milky Way galaxy's net angular momentum vector? If yes or no, is that common for most stars in the ...
- 6,065
58
votes
5
answers
8k
views
Why did the gamma ray burst from GW170817 lag two seconds behind the gravitational wave?
The ABC, reporting on the announcement of gravitational wave GW170817, explained that for the first time we could identify the precise source of a gravitational wave because we also observed the event ...
- 956
33
votes
3
answers
3k
views
Can the Sun capture dark matter gravitationally?
I think my title sums it up. Given that we think the dark matter is pseudo-spherically distributed and orbits in the Galactic potential with everything else, then I assume that its speed with respect ...
- 137k
33
votes
3
answers
4k
views
Why are there three bright spots in the first picture of Sagittarius A*?
Why are there 3 distinct bright spots? The picture of the black hole in M87 had half bright and half dark, which I believe is a result of the different relative velocities of particles orbiting it, (...
- 6,797
30
votes
2
answers
6k
views
Why would a black hole explode?
It is common in popular science culture to assume that Hawking radiation causes black holes to vaporize. And, in the end, the black hole would explode. I also remember it being mentioned in A Brief ...
- 5,114
22
votes
1
answer
3k
views
Why are there no known white dwarfs between 1.35 to 1.44 solar masses?
The Chandrasekhar limit for white dwarfs is 1.44 Solar masses, however the heaveist known white dwarf is only 1.35 solar masses. https://earthsky.org/space/smallest-most-massive-white-dwarf/
What's ...
- 3,001
20
votes
2
answers
5k
views
What happens to the neighboring star of a type Ia supernova?
Supernovae of type "Ia" are those without helium present, but with evidence of silicon present in the spectrum. The most accepted theory is that this type of supernova is the result of mass accretion ...
- 1,526
19
votes
4
answers
3k
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Can the Sun / Earth have a dark matter core?
If dark matter interacts with ordinary matter at all, it should most likely occur where ordinary matter is densest. Hence we have papers about neutron stars possibly containing dark matter cores (...
- 22.1k
17
votes
1
answer
1k
views
Thermodynamically possible to hide a Dyson sphere?
You build a Dyson sphere around a star to capture all its energy. The outer surface of the Dyson sphere still radiates heat at much higher temperature than the cold space background, so you're easy to ...
- 645
17
votes
4
answers
1k
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What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko?
What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko?
The following high resolution picture from ESA's Rosetta mission shows the dunes:
At a distance of ...
16
votes
3
answers
2k
views
What is the probability that a star of a given spectral type will have planets?
There is a lot of new data from the various extrasolar planet projects including NASA's Kepler mission on extra-solar planets. Based on our current data what is the probability that a star of each of ...
- 6,467
15
votes
5
answers
5k
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Mining a neutron star -- what do you get?
In science fiction you get neutronium which supposedly makes darn good armor--which of course has absolutely nothing to do with reality.
You hook up your Acme star spinner (pay no attention to the ...
- 1,769
15
votes
1
answer
1k
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Introduction to neutron star physics
I enjoy thinking about theoretical astrophysics because I want to understand black holes. Given that no one understands black holes, I like to ponder the nearest thing to a black hole: a neutron star! ...
15
votes
1
answer
808
views
Near-Earth supernova
There are 51 stars within 17 light years of the Earth (source). If one of these stars was to become a supernova, how would they effect the Earth?
I have read the Wikipedia article Near-Earth ...
Lynda
14
votes
1
answer
2k
views
If matter and antimatter repel, how do we know other galaxies aren't made of antimatter? [duplicate]
Wikipedia's article on antimatter says this:
There is considerable speculation as to why the observable universe is composed almost entirely of ordinary matter, as opposed to a more even mixture of ...
- 267
14
votes
5
answers
12k
views
The final death of a black hole
What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and ...
- 897
14
votes
3
answers
937
views
How would cold neutrinos get trapped by stars?
Continuing on from the cool physics Q&A'd on the threads Where are all the slow neutrinos?, Is it possible that all "spontaneous nuclear decay" is actually "slow neutrino" ...
- 135k
13
votes
2
answers
2k
views
If you hover just above the event horizon of a black hole, would you see the future of the universe?
Let's do this thought experiment. You have an insanely powerful rocket and it can accelerate to 0.999999c. Now you fly to a supermassive black hole and hover just above its event horizon, where the ...
- 131
13
votes
2
answers
50k
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Differences between astronomy, astrophysics and cosmology? [closed]
What is the main difference between Astronomy, Astrophysics, and Cosmology?
I have the impression that astronomy is a subject that runs parallel to physics but it is outside the physics field. This ...
- 163
12
votes
3
answers
528
views
If neutrinos travel faster than light, how much lead time would we have over detecting supernovas?
In light of the recent story that neutrinos travel faster than photons, I realize the news about this is sensationalistic and many tests still remain, but let's ASSUME neutrinos are eventually proven ...
Matias Nino
11
votes
2
answers
2k
views
Plasma and Stars
I have read that most stars are made mostly of plasma.
My questions in this statement are:
Are there stars not made of plasma?
In what percentage stars are made of plasma?
- 495
11
votes
2
answers
2k
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Storing a Planet-sized Chunk of Metal Inside a Star
Would it be physically possible to "store" a planet-size or larger sum of metal, say gold or platinum, inside a star by letting it fall to the core?
Would it be possible to detect which stars had ...
- 365
10
votes
1
answer
848
views
What really supports neutron stars?
I have read this question (to Andrew's answer, in the comments):
What supports neutron stars is the repulsion provided by the strong nuclear force between closely-packed neutrons. The central ...
- 29.3k
10
votes
0
answers
101
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What allows the modified Urca process to work at lower density than direct Urca in neutron star cooling? [duplicate]
Possible Duplicate:
What allows the modified Urca process to work at lower density than direct Urca in neutron star cooling?
This comes from an unanswered question over at physics.se:
The ...
- 5,248
10
votes
1
answer
9k
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Solving the Tolman–Oppenheimer–Volkoff (TOV) equation
The Pressure of a static spherical object (say star), which has the Schwarzchild metric outside it, satisfies the following differential equation called the TOV equation.
$$\frac{\mbox{d}P}{\mbox{d}r}...
user7757
10
votes
2
answers
3k
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Is it known what causes the "knee" in the observed Cosmic Ray spectrum?
I've seen many versions of the figure shown below -- the famous Swordy plot. They tend to explicitly point out two features in the CR spectrum, the knee and the ankle. I know that the source of UHECR'...
- 1,383
9
votes
3
answers
779
views
Gross "temperature" of a globular cluster
Globular clusters can be very large, which means we can do statistics about the stars in them. And that means we can try matching their star-as-particle potential/kinetic energy distribution against ...
- 193
9
votes
3
answers
321
views
What's dark matter and who discovered it?
I have heard about dark matter that's called the Master Of The Universe. What's this and is the dark matter the reason galaxies exist?
Azzurrio
9
votes
1
answer
582
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Upper Mass Limit of Quark Stars
While there is no confirmation that quark stars exist, is there any theoretical limit analogous to (but different from) the Tolman–Oppenheimer–Volkoff limit for neutron stars?
In other words, what is ...
- 16.3k
8
votes
5
answers
2k
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Two orbiting planets in perpendicular planes
Inspired by this question. Can a 3 body problem, starting with two planets orbiting a larger one (so massive it may be taken to stand still) in perpendicular planes, be stable?
Is there known an ...
- 5,301