The application of physical theory to celestial systems such as stars, planets, galaxies, supernovae, and black holes. Astrophysics proper is concerned with explaining phenomena more so than making observations, the latter falling under the purview of astronomy.

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What is the ultimate “matter reaching diameter” (radius) for a laser beam with information sent out today compared to the observable universe?

Diameter (radius) refers here to the distance the laser beam can travel to reach matter before Dark Energy will make this impossible. Hubble's constant implies an expanding universe in which ...
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2answers
34 views

Circular Orbit in a spherical potential

If we some point mass of mass $M$, in a spherical potential given by $$\Phi(R,z) = \frac{GM}{R},$$ then under what condition would we get a circular orbit, supposing our initial radius from a central ...
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33 views

Could heat detected from Jupiter's red spot explain the temperature of the Sun's corona?

This is an excerpt from BBC Infrared Telescope, published 27th July 2016 Jupiter's Great Red Spot - a hurricane three times bigger than Earth - is blasting the planet's upper atmosphere with heat, ...
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2answers
45 views

Can a binary star system create a stationary black hole?

Research over the last 200 years suggests that half or more of visible stars are part of multiple star systems. I apologise for the number of assumptions in my question but, because of the numbers of ...
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1answer
55 views

Why is there an upper bound for structure size in the universe?

The Hercules-Corona Borealis Great Wall is often said to be the largest structure in the universe, and to cross a certain upper bound for the size of structures. For instance in this article, a writer ...
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2answers
34 views

The difference between sub-Eddington and super-Eddington accretion disk models

I am studying accretion disks around active galactic nuclei (AGN). I have seen two sets of models, sub-Eddington and super-Eddington accretion disks. What do these terms refer to in this case? I ...
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1answer
50 views

How can core of Mercury be a molten liquid?

We all know that Mercury's surface temperature varies a lot from day to night, from 427 °C in a day dropping down to −173 °C at night. Mercury has a hot molten outer core inside it. How can it ...
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3answers
503 views

Can a planet form before the parent star ignites?

I'm unable to find an answer to my questions via my searches. This questions pertains to the timeline of a star system creation. My question: During the creation of a star system, can satellite ...
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27 views

Book on galaxy formation and theoretical astrophysics for physics graduate [closed]

I am looking for a graduate level book on galaxy formation and theoretical astrophysics and also numerical relativity for a graduate student who has completed quantum field theory and general ...
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4answers
245 views

Why are some stars very large (i.e., $r \geq 1000 \ R_{\odot}$) but not super massive?

Background While I was in graduate school, I put together some cartoon-like comparisons of multiple stars to show the order of magnitude differences in radii. At the time, VY Canis Majoris was the ...
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1answer
57 views

Can we estimate the size of a neutron star based on direct observation? [duplicate]

Are there any neutron stars near enough for astronomers to get a good size-estimate, based on direct observation? I know that there are many theories regarding what happens inside neutron stars, but I ...
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2answers
157 views

Have scientists ever experimentally proved the Andromeda paradox-like events? [closed]

We all know that a light year is of the order of 1016 metres and all the objects in the space are beyond a light year. So the Andromeda paradox should give considerable difference in time while ...
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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" ...
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1answer
37 views

Maximum “size” of a galaxy

I am aware of upper limits to the mass of burning stars and neutron stars, beyond which they degenerate into neutron stars and black holes respectively. And also, if I understand it right, there is a ...
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1answer
25 views

Effect of relativity on observers of two different inertial frames observing an event taking place very far away [duplicate]

I am new to relativity ,So please correct me If I am wrong. In the case I have mentioned, In a gravity free space, Let us assume a stationary observer and an observer moving with a velocity 'V' w.r.t ...
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1answer
669 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 ...
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25 views

When was the first Earth-like planet formed?

By Earth-like planet I mean the formation of a planet near a Star analog to our Sun, almost the same size as Earth and all the parts needed to form cells. If we can pinpoint this Earth-like planet ...
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1answer
68 views

Which theoretical models are there between quantum mechanics and cosmology? [closed]

I'm an enthusiast/hobbyist right now and I'm quite curious about the subject of understanding which scales come between the quantum scale (ab initio/first principles) and the macroscopic scale. After ...
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0answers
15 views

Stretch Factor for SNIa

How do I compute the stretch factor for a Supernova Ia ? Data I have : Redshift, Points on the light curve, Rise time (∆m=2), Fall time (∆m=2), Time from the peak day for an inflection point in the ...
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1answer
2k views

What is the theoretical lower mass limit for a gravitationally stable neutron star?

I ask here intentionally not for the size of the smallest possible observed size of neutron stars, which corresponds approximately to the well-known Chandrasekhar-limit for the upper limit of the ...
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0answers
137 views

View of the sky from inside a black hole

Consider an observer located at radius $r_o$ from a Schwarzschild black hole of radius $r_s$. The observer may be inside the event horizon ($r_o < r_s$). Suppose the observer receives a light ray ...
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0answers
6 views

Calculating photon number density inside a volume on whose surface the energy flux is known

I have the power spectrum for a galaxy of radius $R$, which we can approximate as a sphere at some distance $D$. Now, on earth we can measure the power spectrum which is given as a list of tuples, ( $...
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1answer
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Does the lunar orbital plane precess?

I read that the lunar orbit undergoes apsidal precession in which the major axis of the elliptic orbit shifts. Does the lunar orbital plane precess as well?
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1answer
600 views

What would the collision between a (large) solid planet and a gas giant be like?

Assuming a Jupiter-like planet and an Earth-like planet (Except, say... half the mass of Jupiter), what would happen when the two collide? For clarification: What would the actual collision be like? ...
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1answer
138 views

At what altitude would the air be too thin to carry a sound wave?

A related question When does an aerobraking space craft create a sonic boom? has spawned a couple of answers, but so far no compelling answers. It is a common belief that in space there is no sound,...
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2answers
157 views

Formation of supermassive black holes

Scientists have found very bright source of light which they call quasar and the are found to be supermassive black holes. So these black holes are so massive that they cannot be formed by a supernova....
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4answers
5k views

Artificial planetary magnetic field

I wonder how difficult it is to create an artificial planetary magnetic field with generators? What power they would need? The question is inspired by thinking about possible colonization of Jupiter'...
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0answers
19 views

What is meant by Hydrogen dilution in Plasma?

I was going through the paper of methanol masers and it says that for a strong maser the geometrical dilution factor of HII emission should have low values. What is the meaning geometrical dilution ...
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1answer
63 views

How Long Did It Take For The Sun To Form?

I realise this may be a difficult question to answer because, AFAIK, we don't have an accurate estimate of the size of the protostellar cloud, or whether our sun formed from a subsection of a much ...
2
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3answers
99 views

Size and density of neutron stars

Most of the books which I looked at give approximately 10 km as the radius of a neutron star. Just yesterday I looked at a book by Dave Goldberg titled The Universe In the Rearview Mirror (2013) which ...
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0answers
35 views

Books for someone interested in physics/space [duplicate]

As an avid reader of more realistic SF and someone who has always been interested in physics and astrophysics, I was wondering if someone could suggest both some introductory books on the subject, and ...
4
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2answers
115 views

Determining Mass of Spectroscopic Binaries

I know that the mass of a binary star system is given by Kepler's Law: $$\mathrm{m_1 + m_2 = \frac{4 \pi^2 r^3}{GT^2}}$$ Further we know that: $$\frac{r_2}{r_1} = \frac{v_2}{v_1} = \frac{m_1}{m_2}$$ ...
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3answers
959 views

Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
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6answers
2k views

Why isn't dark matter just matter?

There's more gravitational force in our galaxy (and others) than can be explained by counting stars. So why not lots of dark planetery systems (ie without stars) ? Why must we assume some undiscovered ...
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3answers
7k views

Why don't galaxies orbit each other?

Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered ...
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1answer
38 views

Formula for cosmic variance

I was reading this page: Sample and Cosmic Variance. The section states that The multipoles $C_\ell$ can be related to the expected value of the spherical harmonic coefficients by $$ \Bigg\...
3
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1answer
94 views

Angular momentum in an accretion disk

I need to plot the time evolution of the total angular momentum in an accretion disc. This confuses me because I thought this should be constant, since angular momentum has to be conserved? I'm given ...
11
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1answer
428 views

Why is the spectrum of old stellar populations characterized by broad lines?

I'm taking a course in astrophysics and my teacher said that old stellar populations have broad lines whereas young populations have narrow emission lines. My first thought was to consider then case ...
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5answers
3k views

Can elements heavier than Iron be present in a star's core?

My understanding is that elements heavier than Iron and Nickel are not formed in a star but, can heavy elements such as lead and others be present/found in a star's core ? I ask because the following ...
16
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1answer
671 views

Do supernovae produce an appreciable amount of lithium?

David Z's answer to this question got me wondering - is any appreciable amount of lithium produced as the result of a supernova explosion, either by fusion (which seems unlikely to me, but I don't ...
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1answer
86 views

Is cosmological constant really constant?

As the Universe expands, the dark energy in it also increases. I heard that the cosmological constant $\Lambda$ represents dark energy, so that constant must change as time passes, right? Correct me ...
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1answer
37 views

What is exactly the “progenitor bias”?

I am taking a course in astrophysics and my teacher mentioned different biases that are present when taking a sample of galaxies: the progenitor bias and the Malmquist bias. I understand very well the ...
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5answers
4k views

Why is the Sun called an “average star”?

This is a statement (presumably in mass, longevity, energy output) many people that I've met have heard in school, and it is known in pop culture. However, according to Wikipedia, about 75% of the ...
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1answer
54 views

What is the most commonly used density model for globular clusters?

It is possible to model a globular cluster using a number of different density models: Plummer model King model Isothermal sphere . . . They all have advantages and disadvantages, depending on ...
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3answers
89 views

Measuring Rotation of Sun

If the sun had a uniform surface (i.e., if there were no sunspots to look at), is there a practical way to measure its rotation? In other words, if some external force flipped the sun's spin suddenly,...
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1answer
43 views

Is it possible that cosmic ray particles and neutrinos account for a significant portion of dark matter?

Sounds a bit naive but I read somewhere that neutrinos were thought to account for dark matter to an extent and I think Zwicki came up with the idea before cosmic rays were announced (not sure though)....
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3answers
2k views

Is the speed of sound almost as high as the speed of light in neutron stars?

Have you ever wondered about the elastic properties of neutron stars? Such stars, being immensely dense, in which neutrons are bound together by the strong nuclear force on top of the strong gravity ...
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1answer
293 views

Relation between isophotal radius and virial radius in spiral galaxies?

Is there any (proposed) relation between the $B$-band isophotal radius of a spiral galaxy and its virial radius ($R_{200}$)? If you know of such a relation, please post a reference paper.
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1answer
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Need help with determining the total mass using the NFW profile

My review assignment has a question that asks us to use the Navarro–Frenk–White (NFW) profile to find total mass in the galaxy using $$\rho(R)=\frac{\rho_0}{1+\frac{R}{R_c}}$$ then taking a triple ...
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3answers
83 views

Can black holes grow via accretion of dark matter particles?

I'm assuming that the answer to the question in the title is a resounding yes. Since Baryonic matter and dark matter interact via gravitational forces. If this is the case how is information not lost ...