All Questions
2,901 questions
7
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Why is the Sun’s emission spectrum shaped like this?
The Sun emits more at $450-600 \,\rm{nm}$ than a black body of the same effective temperature would, it also emits far less UV.
I’ve heard this is due, in part, to the fact that the Sun doesn’t have a ...
- 3,001
0
votes
1
answer
130
views
How would a Supermassive-black-hole-less galaxy behave?
The Supermassive black hole in the center of the galaxy only contains a tiny fraction of the mass of our Galaxy, so it has minimal direct effect on the orbits of most stars. However I have sort of one ...
- 3,001
0
votes
1
answer
48
views
Effect of Sun temperature on the thermosphere
Correct me if I’m wrong here.
The thermosphere is hot due to its absorption of moderately high energy UV radiation. (<200nm)
Cooler stars emit fewer high energy photons. So if the Earth orbited an ...
- 3,001
0
votes
0
answers
22
views
Why doesn't the lack of electromagnetic repulsion between dark matter particles result in the formation of black holes? [duplicate]
From what I've researched dark matter isn't subject to electromagnetic forces, which I'm assuming implies that there is a lack of electromagnetic repulsion between dark matter particles.
If there's ...
- 1
4
votes
1
answer
274
views
Why is the least massive neutron star less massive than the most massive white dwarf?
Martinez et al. (2015) gives an example of a neutron star with mass as low as $1.17M_\odot$ (solar masses).
There is also the recent discovery of this candidate neutron star which is apparently only $...
- 3,001
0
votes
2
answers
120
views
What’s the lightest you could make a "star" if you made it out of different materials?
What’s the lightest you could make a "star" if you made it out of different materials?
How large would the "star" be?
For example according to here https://astronomy.stackexchange....
- 3,001
2
votes
1
answer
185
views
Stopping galaxies' rotation...?
I have been told that galaxies will never stop rotating because conservation of angular momentum
But, there are planets inside of it can travel through dense nebulae and bodies of gas that would cause ...
- 2,878
0
votes
1
answer
38
views
Do only gas planets have discs or even rocky planets may have them orbiting around?
Do only gas planets have discs or even rocky planets may have them orbiting around? If rocky planets are great in volume is then a higher chance for an orbiting disc to exist in the planetary orbit?
- 2,275
0
votes
1
answer
78
views
What's the nearest star that could go Supernova in the near future?
https://en.wikipedia.org/wiki/IK_Pegasi B is the nearest supernova candidate, but that white dwarf that's part of the system won't go supernova for around 2 billion years.
What's the nearest star that ...
- 3,001
7
votes
2
answers
200
views
Why are black holes sometimes formed without supernovae?
I've heard that very massive stars can sometimes collapse into black holes without creating supernovae.
How does this happen?
(I suspect it's something to do with the relative lack of Urca process ...
- 3,001
2
votes
2
answers
269
views
Likelihood of MACHOs being the best candidate for dark matter
Massive compact halo objects ("MACHOs") include a wide variety of hardly detectable bodies such as brown / white / black dwarfs and black holes, to name a few. If we take into account the ...
- 1,880
2
votes
2
answers
759
views
Making sense of the Jeans mass
The Jeans mass, given by $M_J=\sqrt{\left(\frac{-5k_BT}{Gm}\right)^3\cdot\left(\frac{3}{4\pi\rho}\right)}$, is the threshold mass a dust cloud must have in order to begin gravitationally collapsing ...
- 1,880
1
vote
1
answer
64
views
Could rogue planets with cold nucleus have winds or water currents/waves due to the planet's rotation?
Are there any types of wind or waves caused and produced only and exclusively by a planet's rotation? Not influenced by the planet's rotation, but produced solely by it?
In the case of waves, are ...
- 2,878
4
votes
2
answers
391
views
Underestimation of peak wavelength by Wien's law for stellar spectra
I plotted some simulated stellar emission spectrums from PHOENIX which also each came with their respective effective temperatures. With the effective temperatures, I used Wien's law to estimate the ...
- 143
0
votes
1
answer
39
views
Difference between star formation rate and star formation history
When we speak about galaxies evolution, what is the difference between the star formation rate and the star formation history?
0
votes
2
answers
114
views
Time of collapse of stellar dust cloud
A stellar gas cloud collapses onto itself once it reaches Jean's mass, and the time it takes for said cloud to collapse is given by: $t_{coll}=\sqrt{\frac{3\pi}{32G\rho_0}}$, where $\rho_0$ is the ...
- 1,880
3
votes
1
answer
128
views
How many type II supernovae are there per stellar mass formed?
I have been searching for the number of type II (core-collapse) supernovae per unit of stellar mass formed.
It is my understanding that a star must have an initial mass of at least 8 times and no more ...
- 31
4
votes
1
answer
301
views
Why the Fermi's energies of the proton, neutron and electron are related in this way in a neutron star?
I'm referring to this answer made by ProfRob about why neutrons are stable against beta decay in neutron stars.
I've partially understood the answer: when the Fermi's momentum of the electron $p_f = (...
- 45
1
vote
0
answers
66
views
Brans-Dicke formalism: Validity for curve of scale factor vs cosmic time
Within the framework of the Brans-Dicke formalism, after having run a MCMC sampler and, once the best-fits have been found, I inject them into an ODE system resulting from the modified Friedmann ...
- 45
1
vote
0
answers
42
views
What state structure is related to each interval $z$ in Hyperscaling violating black holes?
Hyperscaling violating black holes are defined by parameters such as $z$ and $θ$. My main question is: what kind of state structure corresponds to each value of $z$, or what does it signify and relate ...
- 75
1
vote
0
answers
46
views
Does strongly gravitating object travel along geodesic of a background field？ [closed]
That test particles travel along the geodesic is assumed in the context of GR. But does it apply to strongly gravitating object, such as black hole in an expanding universe, binary neutron star, etc.? ...
- 631
1
vote
1
answer
192
views
Numerical Integration for White Dwarf Model [closed]
I'm creating a numerical integration model of a white dwarf. So I've started with calculating electron pressure vs number density across a wide range of values.
Using the equations of state in ...
- 11
0
votes
0
answers
26
views
Proportion of Oxygen in a typical white dwarf
What's the proportion of Oxygen in a typical white dwarf relative to the proportion of Carbon?
- 3,001
0
votes
0
answers
54
views
How does according to modern physics collision between two massive black holes create new space or new spacetime?
Now (hypothesis) -- And the Physics of Time
In this page the author talks about (find in page) (in the context of LIGO Observatories-->) "In their most powerful event, reported just this past ...
0
votes
0
answers
88
views
Relationship with double summing of $a_{\ell m}$
I would like to convince myself of the following relationship in an astrophysical context:
\begin{aligned}
& \sum_{m}\sum_{m^{\prime}}\left\langle a_{\ell m} a_{\ell m}^* a_{\ell m^{\prime}} a_{\...
- 45
0
votes
1
answer
55
views
Common choice in FLRW between dimensionless of scale factor (and coordinates of r lenght dimension) or the contrary
I have an old cosmology book in which the scale factor is expressed in the Roberston-Walker metric as:
$\mathrm{d} s^2=c^2 \mathrm{~d} t^2-R(t)^2 \mathrm{~d} l^2$
with: $\mathrm{d} l^2=\dfrac{\mathrm{...
- 45
1
vote
1
answer
108
views
Is Earth's magnetic moment vector oriented parallel or antiparallel to the sun's magnetic moment?
Just curious how these two magnets are spatially related to?
And in case there is no any special preferred particular orientation, what is anyway their spatial orientation relation (relative angles)? ...
- 4,376
1
vote
1
answer
189
views
Would a black hole instantly form when a neutron star slips below the phantom event horizon?
So lets say we have a neutron star that is just few inches away from the phantom horizon and only needs 500 kg before collapsing. So lets say hypothetically that a ship that is designed to survive the ...
- 1,554
0
votes
1
answer
37
views
SETI: Are there holes in the EM spectrum that are quiet enough to communicate at decently large distances?
So this is a variant of this other question.
I know stars are big, they radiate a lotta energy, they have spectral lines. But how dense are these spectral lines and is there a noise floor at ...
- 1,733
0
votes
2
answers
110
views
Could Dark Matter have negative pressure?
Is it possible for dark matter to have negative (but negligible) pressure? How small should it be to fit with observations? Dark matter pressure is actually known and measured?
- 751
1
vote
0
answers
36
views
What percentage of Mars' surface would be under the water if we dropped all of the water from moon Enceladus on it? [closed]
Enceladus, one of the Saturn's moons that is known for water geysers, is estimated to contain a body of water as big as a ball $220 ~\mathrm{km}$ in radius. That's about $\sim 4.46 \times 10^{7} ~\...
- 342
0
votes
0
answers
36
views
Can we have rocky planets the size of gas giants and stars made up of rocky/solid materials?
Why are terrestrial planets like the Earth so much smaller in size than the gas giants? Why can't they be the larger ones?
Why are there no stars made up of something solid(/rocky)?(To generate energy,...
- 229
0
votes
0
answers
37
views
(Uncertainty propagation) Should variance of log radiated energy be scale invariant?
I need to propagate the error from the measure of the bolometric fluence $ S_\text{bolo}$ to the $\log_{10} $ of the isotropic-equivalent radiated energy $E_\text{iso}$ which is related to $S_\text{...
- 299
1
vote
2
answers
367
views
The center of the Schwarzschild black hole
My question is why, when expressing the Schwarzschild solution in isotropic coordinate system, the coordinate system is considered valid only outside the event horizon.
For simplicity, we assume that ...
- 19
0
votes
1
answer
82
views
Is there a limit to the radiative efficiency for mass converted to luminosity around a black hole?
The luminosity generated by accretion onto a black hole is given by $L = \epsilon \dot{m} c^2$, where $\dot{m}$ is the mass accretion rate and $\epsilon$ is the radiative efficiency. I read that the ...
0
votes
1
answer
77
views
Derivation of the Eddington limit
The Eddington limit can be shown as:
Momentum of a photon: $ p = E/c = h\nu / c $
Force due to this radiation is change in momentum. Supposedly then,
$$ F_{rad} = \int^\inf_0 \frac{L_\nu}{h\nu}\frac{h\...
0
votes
2
answers
93
views
Visibility on an alien world [closed]
I asked this over on Worldbuilding, and I was told it'd be best to try here instead:
First, I should probably mention that I'm really bad at mathematics.
I'm working on a scientifically plausible ...
- 109
0
votes
2
answers
102
views
Are electrons bounded or unbounded to nuclei when they are degenerate?
I have a question about degenerate electrons in white dwarfs.
So, as far as I know, when the gas contained in stars is compressed so much, the electrons start to fill the lowest energy level and then, ...
- 45
0
votes
2
answers
45
views
Age of the universe due to expansion
"If gravity working on matter is the only force at work on large scales, then the attactive force of gravity will act to slow the expansion. In this case, the universe was expanding more rapidly ...
0
votes
1
answer
36
views
Thermal models of gravitational radiation?
How is the background noise of gravitational waves modeled? Is it a thermal model, giving a stochastic distribution of the curvature tensor (field-strength tensor) in ambient space? That is, every ...
- 241
0
votes
1
answer
42
views
Where can you find the velocity dispersions of globular clusters?
I'm trying to use the Virial theorem to determine the mass of the M5 cluster and I need the velocity dispersion to calculate this. Does anyone know where I can find a table with these velocity ...
1
vote
1
answer
59
views
Do Solar Eruptions happen in more than one rotational "layer" in Sun's Differential Rotation? [closed]
Do Solar Eruption occupy more than one rotational layers?
Is it possible to confirm this just for most outer layers?
Does the number of layers occupied by those eruptions when they happen tell us ...
- 11
0
votes
1
answer
69
views
Finding velocity field components from electric and magnetic field components in ideal MHD
How can I determine the components of the velocity field ($V_x, V_y, V_z$) using the components of the electric field $(E_x, E_y, E_z)$ and magnetic field $(B_x, B_y, B_z)$, assuming the ideal ...
- 500
9
votes
2
answers
1k
views
Why don't less massive stars explode in form of supernovas?
I'm a high school student with a question about supernovas and the life cycle of stars. I understand that supernovas occur in massive stars when they run out of fuel and collapse, resulting in a ...
- 647
0
votes
0
answers
29
views
The role of gas pressure in the life cycle of stars
As a high school student, I am curious about the significance of gas pressure in the life cycle of a star. While I have read about the radiation pressure and the pressure caused by the energy released ...
- 647
0
votes
0
answers
566
views
Finding the (idealised) minimum radius of a neutron star using SEMF
If one considers the Semi Empirical Formula (SEMF) in nuclear physics containing the Volume, Surface, Coulomb energy and Asymmetry Energy terms for the binding energy of a nuclide, then by replacing ...
1
vote
1
answer
250
views
Do all galaxies in our observable universe have more or less the same age?
What is the current consensus about the age variation of the existing galaxies in our observable universe?
Not to be confused with the age of very distant galaxies as observed today by our telescopes ...
- 4,376
3
votes
1
answer
163
views
Color temperature or effective temperature?
I am a high school student interested in astronomy and physics, and I am trying to understand the difference between color temperature and effective temperature of a star. As I have searched this ...
- 647
-1
votes
2
answers
74
views
Question about the energy of the gravitational waves produced by massive astrophysical objects merger
My doubt lies in the fact that gravitational waves are produced even before the merger so I kindly ask an expert to make me a clear picture of the source of energy of grav. waves before the merger and ...
- 2,275
2
votes
2
answers
192
views
How do we know the chemical composition of the crust of neutron stars?
Although neutron stars are mostly made of neutronium, the pressure at the surface is not very high which allows regular atomic matters to exist. Emission spectrum can reveal the chemical composition ...
- 1,537