Unanswered Questions
263 questions with no upvoted or accepted answers
12
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220
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Estimating the tangential and cross component of the galaxy's shear using Gnomonic projection
I would like to know how I can estimate the tangential and cross component of the galaxy's shear using Gnomonic projection
of the right ascension and declination the galaxies relate to the center of ...
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10
votes
0
answers
214
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How well conserved is etendue in extreme gravitational lensing scenarios?
This excellent answer to Could dark matter exist in the Universe in the form of sufficiently dense objects? includes the following image and description:
Light from the background galaxy circles a ...
- 30.7k
8
votes
1
answer
187
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What is the history of the average pressure, density, and temperature of the matter in the universe over time?
This question is inspired by this more specific question where Cerelic wanted to know if conditions were suitable for liquid water to exist during an epoch when the characteristic temperature of the ...
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7
votes
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256
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Get the expression of probed volume between 2 redshifts
1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume.
2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
CommunityBot
- 1
7
votes
0
answers
535
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How do I calculate the Hill radius of a star in a binary system?
Is there an easy way to calculate the Hill radius of a star in a binary system at different orbital radii where both stars are of the same mass and in circular orbit around one another’s centre of ...
- 30.7k
6
votes
0
answers
130
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Kepler ellipse orbit fit issues
I am helping my high schooler with a research paper that uses PyAstronomy to fit the orbit of stars around Sag A. It isn't working because the angles (parameters Omega, omega, and i) seem to have a ...
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6
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0
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260
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Did Aliens make Oumuamua?
Dr. Avi Loeb, the distinguished astrophysicist and chair of Harvard's Astronomy Program, just published a book called Extraterrestrial: The First Sign of Intelligent Life Beyond Earth.
In the book, Dr....
- 17.3k
6
votes
0
answers
101
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Callisto's orbit: circular vs JPL is periodic 4 degree error, why?
I am approximating the azimuthal angle of Callisto from Jupiter or the Jupiter+moons barycenter using a constant angular speed circular orbit. This differs from the JPL ephemerides by only about 4 ...
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6
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0
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265
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RA/Dec of a meteor shower's radiant point based on its associated comet's orbit; simple set of equations? "Classic" early reference to cite?
Below this answer to Why are Delta Aquariids “for the southern hemisphere” while the Perseids are “for the north”? I wrote the comment:
+1 To make this complete, ...
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5
votes
0
answers
95
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Are dust-dust collisions necessary to explain Kordylewski clouds at Earth-Moon L4/5? Aren't the cross-sections, number density timescales too small?
The recent question Can dark matter accumulate at Lagrange points? mentions Kordylewski clouds and that article begins:
Kordylewski clouds are large concentrations of dust that exist at the L4 and L5 ...
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5
votes
0
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203
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Different techniques for calculation of celestial bodies dynamics with different time-scales
Differential equations describing the dynamics of celestial bodies (rotation, nutation, precession and other effects) can be solved numerically, which makes it possible to study their general non-...
5
votes
0
answers
70
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Do all the accretion disks around a compact object (black hole or neutron star) emit astrophysical jet outflow?
Do all the accretion disks around a compact object (black hole or neutron star) emit astrophysical jet outflow? I mean is it mandatory for a jet to be emitted from an accretion disk?
If so, then why?
...
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5
votes
0
answers
176
views
Why does the axis tilt (obliquity) of Earth change over time?
I have recently learned that the polar axis of Earth moves in a complex fashion. There are both precession and nutation. I have learned that Earth's obliquity varies between 22.1 and 24.5 degrees on a ...
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5
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0
answers
119
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Reasons for extremely high number density of galaxies at low redshift
I am currently analyzing the SDSS data especially in the extremely low redshift area, in order to measure the cosmological statistics. What I mean by extremely low is z<0.1. One peculiarity that I ...
- 301
5
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0
answers
128
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Explain as simply as possible how the Sunyaev-Zeldovich (SZ) effect is used to estimate the Hubble constant
The Sunyaev–Zeldovich effect (SZ effect) is useful in determining the Hubble constant because it is independent of the cosmic distance ladder. This effect occurs when CMB (cosmic microwave background) ...
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