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21 votes
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When will the number of stars be a maximum?

TL; DR Somewhere between now and a few hundred billion years time. (For a co-moving volume) Now read on. If stellar remnants are included, then the answer is very far in the future indeed, if and ...
ProfRob's user avatar
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20 votes
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Can we violate law of physics in any other planet?

If there is a way to violate a law of nature, it will become a law of nature as soon as it is discovered, studied and formalized into a scientific theory. Therefore, many new discoveries in science do ...
tuomas's user avatar
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14 votes

Can we violate law of physics in any other planet?

Universality of physics As far as we know, laws of physics seem invariant both across space and time. It's not absolutely certain and taken for granted, there's lots of research probing to try and ...
Peteris's user avatar
  • 241
12 votes

What does this tweeted Astronomy Plot of the Week mean? What does it represent?

TLDR; Its a diagram showing all the physics and modeling choices that go into different models (the colored boxes in the middle) to compute an SED. Longer answer: First a SED is a spectral energy ...
Rob's user avatar
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12 votes
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analemma for a specified lat/long at a specific time of day?

If you're open to using a bit of Python, you can use the following script to generate an analemma for arbitrary times and locations. ...
Roy Smart's user avatar
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10 votes
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What good is the Einstein de-Sitter model?

The scale factor of the universe depends on the matter and energy density in a way that depends on which component is dominant. This is discussed in the question How does the Hubble parameter change ...
John Rennie's user avatar
  • 1,649
10 votes
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How do comets' nuclei stick together?

Van-der-Waals forces and gravitation are the main answer. The main property keeping together bodies smaller than a few $100 {\rm km}$ are their constituents surface forces between the grains (van-der-...
planetmaker's user avatar
  • 19.9k
10 votes

Was the Nobel prize for gravitational waves handed out based on a hand-drawn theoretical curve? Has the data been fit more rigorously now?

To understand the history of the figure in question it is important to understand some of the context. In the run up to the operation of advanced LIGO, it was widely expected that the first detections ...
TimRias's user avatar
  • 2,720
9 votes

analemma for a specified lat/long at a specific time of day?

In the desktop version of Stellarium, you can do this on the Ephemeris tab of the Astronomical calculations window. Set the beginning of the date range with the desired time of day, and make the time ...
Mike G's user avatar
  • 18.7k
7 votes

Can we violate law of physics in any other planet?

No. That's because we believe the laws of physics to be the same everywhere (this itself is a consequence of the law of conservation of momentum, via Noether's theorem). Therefore, if you can't ...
Allure's user avatar
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6 votes
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Why does the convective core in an intermediate- to high-mass star shrink?

Whether convection exists depends on whether the interior radiative temperature gradient reaches the adiabatic temperature gradient. The interior radiative temperature gradient is proportional to ...
ProfRob's user avatar
  • 155k
6 votes

How to model and subtract nebular emission lines from stellar spectrum

In general, you can't. If obtaining spectra in regions where there is expected to be a spatially varying background then you either need to do long-slit spectroscopy so that you have a good ...
ProfRob's user avatar
  • 155k
6 votes
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What does this tweeted Astronomy Plot of the Week mean? What does it represent?

SED fitting SED fitting is the practice of inferring physical properties of a stellar population (a galaxy, a cluster...) from measures of the Spectral Energy Distribution. The spectrum of a stellar ...
Prallax's user avatar
  • 4,471
5 votes
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What if you 'placed' the Chicxulub asteroid onto the Earth?

An asteroid resting on Earth would be a mountain. Or, for smaller asteroids, a pile of gravel. Mountains are limited in altitude by the strength of stone to resist compression: a too tall mountain ...
Anders Sandberg's user avatar
4 votes

Has stellar evolution ever been modeled analytically?

Analytic models have been applied to various phases of the evolution, though it would be impossible to apply a single model to all phases because the physics changes so much. Also, a star will often ...
Ken G's user avatar
  • 5,336
4 votes

Why does the precession of the earth affect other planets?

Why would precession affect the motion of the other planets? First things first: That's an unreferenced portion of a wikipedia article. That said, a perfectly spherical body acts exactly like a ...
David Hammen's user avatar
  • 34.1k
4 votes

Was the Nobel prize for gravitational waves handed out based on a hand-drawn theoretical curve? Has the data been fit more rigorously now?

This issue is a bit complex, and at its core revolves around the claim of a Copenhagen group led by Andrew Jackson that LIGO data was not handled correctly, a mislabeled plot, and some small (but ...
Justin T's user avatar
  • 3,404
4 votes
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How did astronomers "measure a large (>40 degrees) misalignment between the black hole spin and the orbital spin" of MAXI J1820+70?

See Figure 3 from https://arxiv.org/pdf/2109.07511.pdf: In this picture, the North and East directions are in the plane of the sky, and the $\hat{o}$ direction is in the direction of the observer. In ...
Alwin's user avatar
  • 336
4 votes
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Using cosmological model GADGET - 4

Whether there is a tool that allows to define ICs of your choice, mainly depends on what is your choice. If you want to start from cosmological initial conditions, there are various flags in the code ...
Prallax's user avatar
  • 4,471
4 votes
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How do various astrophysical models deal with so many orders of magnitude in scale? Are there completely different models for different scales?

An attempt at a partial answer, or perhaps "one possible answer": If I understand correctly, the problem you describe appears in numerical simulations where you have to choose between ...
pela's user avatar
  • 38.9k
3 votes

Angles in Ptolemaic Model

You misunderstood Ptolemy’s model: The epicycle doesn’t “roll on” the deferent; rather, the epicycle’s center is on deferent. Mind you, it’s only a minor detail in this situation. In the diagram you ...
Pierre Paquette's user avatar
3 votes

DAMIT how do they get 3D shape and rotational trajectory of a tumbling asteroid from photometry?

Your question consists of two sub questions: How do they get the 3d shape of an asteroid? How do they get the rotational trajectory of an asteroid? A paper on The representation of asteroid shapes: ...
DialFrost's user avatar
  • 2,115
3 votes

What good is the Einstein de-Sitter model?

EdS universe represents a matter-dominated universe. Since the universe evolving towards lambda dominated and recently it was matter dominated, its a good approximation to use the EdS model to ...
seVenVo1d's user avatar
  • 576
3 votes

Can we violate law of physics in any other planet?

Well, it's possible that physics as we know them are somewhat different elsewhere, or on different scales (both microscopic and macroscopic). So if we manipulate our environs in particularly extreme ...
Peter - Reinstate Monica's user avatar
3 votes
Accepted

Dynamical modelling of galaxies with counter-rotating components using the Schwarzschild method

Convert the 3D luminosity distribution to mass using a bunch of mass particles. Well, you probably could do that, but I suspect it would be inaccurate and extremely time-consuming. Most Schwarzschild ...
Peter Erwin's user avatar
  • 17.3k
3 votes
Accepted

Estimate prevalence of ʻOumuamua-style interstellar obects inside Jupiter orbit now that C/2019 Q4 (Borisov) is found?

The enhancement of a cross-section due to gravitational focusing is given by $$ \sigma_{\rm eff} = \pi a_J^2 \left(1 + \frac{2GM_{\odot}}{a_J\ v^2}\right),$$ where $a_J$ is the semi-major axis of ...
ProfRob's user avatar
  • 155k
3 votes
Accepted

What is the lyman-werner background?

The Lyman-Werner background (LWB) is the part of the "meta-galactic" background radiation consisting of photons that are able to photo-dissociate molecular hydrogen ($H_\mathrm{2}$), but not ionize ...
pela's user avatar
  • 38.9k
3 votes

Physical Meaning of the Principal Tidal Fourier Modes and Their Integers

In that paper, we actually applied the term "principal" to the semidiurnal mode $\omega_{2200}$. When the planet is not synchronised (i.e., when it is not showing the same side to the ...
Michael_1812's user avatar
  • 1,494
3 votes
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What does "RV model including a GP with a quasi-periodic covariance structure" mean?

What does "RV model including a GP with a quasi-periodic covariance structure" mean? RV stands for radial velocity, which is an often used quantity in exoplanet science - it is a usual ...
Daddy Kropotkin's user avatar
3 votes

Measure the deprojected distance between two points?

@Greg Miller's point is that the rotation angle doesn't count. The only measurement you care about is the inclination. It makes a point's arc distance from the major axis of the apparent ellipse cos(...
stretch's user avatar
  • 1,797

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