Questions tagged [density]

Questions on the ratio of an object's mass with its volume.

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Besides Luna, are there any models of how moons become spherical and round?

Question Given the theorized exceptional formation of Luna, (Giant-impact Hyphothesis, Wikipedia), are there any similar animated models of how the other, much smaller, moons in our solar system ...
2 votes
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What is the sharpest stellar number density variation known?

In the solar neighborhood the stellar number density is about 0.14 stars per cubic parsec. This changes with a length scale of 220-450 pc as we move up or below the Milky Way disc, and 2.6 kpc ...
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If the ratio of actual total mass to critical mass (Ω) in the universe is only about 0.3, why do cosmologists believe it is flat, as far as we see?

What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell? Despite the critical Friedmann density being less than one, plus the existence of ...
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1 vote
2 answers
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Does the density of a black hole decrease after releasing a large amount of Hawking radiation?

Is is true that the smaller a 'regular', stellar-mass black hole is, the denser it is inside of its event horizon? After all, if you look up (or calculate) the Schwarzschild radii of the Sun and the ...
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2 votes
1 answer
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What do "overdense" and "underdense" mean in cosmology?

“Overdense" and "underdense" often appear in various documents to refer to high density and low density respectively. And I know that "overdensity" is defined as $$ \delta(x)=\...
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1 vote
1 answer
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Energy conservation of the universe

I have tried to derive an equation for the total energy of the universe. I have found that, $$E(t)= \delta\dot a(t)^2a(t)\Omega(t)$$ Where $\delta$ is just a positive constant, a(t) is the scale ...
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Aren't denser bodies more likely to collapse into hydrostatic equilibrium?

It is said that at small sizes, icy planets are more likely to be in hydrostatic equilibrium than terrestrial rocky ones. But why, as a matter of fact? Shouldn't denser bodies be more likely to ...
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Looking for analytical expression of Cosmic Variance - Poisson distribution?

I have an expression of Matter Angular power spectrum which can be computed numerically by a simple rectangular integration method (see below). I make appear in this expression the spectroscopic ...
1 vote
4 answers
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If a black hole has the same mass as me, then why dont i suck everything at my center of mass?

So, a black hole is very dense that you can get really close to its center of mass that it has a strong pull, well, if i have the mass of a black hole that has a really small event horizon, why doesnt ...
6 votes
2 answers
211 views

Are any supergiants translucent?

Are any supergiants translucent? Some have volumes thousands of times more than the Sun's while having maybe twenty times the mass of the Sun which makes them sound rather diffuse. If there was a very ...
6 votes
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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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Did the density wave theory for spiral arms take account of this phenomenon?

I came across this link. In it the author speculates that in the outer region of a galaxy, the spiral arms overtake the stars, and vice versa for the inner region. That means there must be a middle ...
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Is is possible for a main sequence star to have 0.77 solar masses yet only be 0.54 solar radii?

I know ordinarily a star's mass and radius tend to be proportional, so a 0.5 solar masses star tends to be about 0.5 solar radii and have a density of 4.82 g/cm³, 3.42 or about three and a half times ...
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2 answers
284 views

What increases or decreases the mass and density of a fixed radius star?

Assuming a relatively even proportion of mass and radius, a 0.25 solar masses and radii star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density. Keeping the radius of 0.25 ...
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What factors influence a star's temperature and density?

Assuming a relatively even proportion of mass and radius, a 0.25 solar mass and radius star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density. However, there is obviously a ...
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Are red dwarfs really 30-100 times our Sun's density?

This article by K. E. Edgeworth claims red dwarfs tend to be around 30-100 times our Sun's density. However, this seems a bit high. With smaller stars, mass tends to be proportional to radius but ...
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1 answer
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How do pressure, temperature, and gravity affect a smaller body than Earth with the same bulk composition?

Earth has a density of about ${\rm 5.513 ~g/cm^3}$ with one Earth radius and one $g$. Venus, a very similar planet to Earth and likely with about the same bulk composition, has a density of about ${\...
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1 answer
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Demo to get Matter Power Spectrum in cosmology

I would like please to demonstrate the expression of Power spectrum in Cosmology : First, I have the relative contrast: $$\delta_{i}(\vec{x}, z) \equiv \rho_{i}(\vec{x}, z) / \bar{\rho}_{i}(z)-1\quad(...
2 votes
1 answer
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How is the density parameter estimated via cosmic microwave background anisotropies?

In 2003 and again in 2018, the density parameter was estimated based on measurements of the cosmic microwave background anisotropies. Can someone please explain how that works? Why do the anisotropies ...
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3 votes
1 answer
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Historical estimates of the density parameter

From the reference in this answer I learned that our current estimate for the density parameter (i.e., the density of the universe divided by the critical density, which determines the shape of the ...
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1 vote
1 answer
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density of dark matter halo

I was surfing the net for knowing the actual density of dark matter halo for a particular Mh(Mass of the halo) and finally, I got two equations: for pseudo-isothermal halo and by NFW-profile So, ...
2 votes
2 answers
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How to calculate the orbital period and the density of a planetary body?

I'm a student studying in 9th grade, I am making this fictional habitable planet named "Darwin B" for a planet making competition. It orbits a sun-like star at a distance of 1.15 AU or 172 ...
2 votes
2 answers
279 views

Can a high enough density ignite a gas giant into a brown dwarf?

I wonder whether the nuclear fusion of brown dwarfs and more massive stars really depends on their mass only or if it could also start nuclear fusion if it is dense enough but not as massive as brown ...
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Density of a sunspot compared to the surrounding photosphere?

Is the density of a sunspot different by more than a factor of ten compared to the area surrounding the sunspot?
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1 answer
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At the Big Bang, when everything was close together, why did it not "collide", violating Planck length or Pauli Exclusion Principle?

How could so much matter, or "all" in fact, have been concentrated in a smaller universe without being in the actual same place? Why did this not result in undercutting the Planck Length or ...
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5 votes
1 answer
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Interpretation and understanding of the relation for the photometric redshift in a given bin

In the context of photometric probe of surveys (like LSST), I need to understand the relation I have to use for photometric bins. Considering $p_{ph}(z_p|z)$ the probability to measure a photometric ...
13 votes
3 answers
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Are there any planets or moons denser than Earth?

Earth has the highest density out of all planets, planetoids and moons of our planetary system, and also has a higher density than the Sun. Do we know any exoplanets or moons denser than Earth?
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Factors included in the average density of planets

When the average density for a planet is stated does it incorporate the mass of the atmosphere, when a planet has an atmosphere, or is the mass of the atmosphere ignored?
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Is the atmosphere of neutron star solid, liquid or gaseous state? [duplicate]

Neutron star has a thin layer of atmosphere consist mostly of hydrogen and helium and is about less than a metre thick on average, but what wasn't stated in many articles I read is the state of matter ...
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2 votes
1 answer
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If interstellar medium was dense plasma and light slowed down, would things appear to move slower far away?

Voyager 2 recently left the solar system and registered an increase in plasma density. If light moved slower in this dense plasma, would it appear like events in other solar systems played out slower ...
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1 answer
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What is the least dense exoplanet?

An exoplanet with density 0.31 grams per cubic centimeter has been found. Is this the least dense exoplanet we know of?
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Can a planet exist with the same mass as the earth, but have a different diameter?

Can there be a planet with the same mass, and a similar composition, as the earth, but with a larger or smaller diameter, and how could that theoretically occur? I've tried to see if there is any ...
53 votes
5 answers
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Is there a star over my head?

Say I'm standing up straight, and I draw a straight line from my core through the top of my head (perpendicular to the ground). What is the probability that that line intersects with a star? EDIT: I'...
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4 votes
0 answers
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Cosmology : Formula for the bias of galaxies

first I have posted originally on the physics stackexchange but I have not had answers about my question, so I try my luck on this specific forum (if forums leaders want to delete the original post on ...
2 votes
1 answer
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How to find inverse steradian from $\text{arcmin}^{-2}$ for density of galaxy

I am using a code on EUCLID future mission. The original author of this code has set a value for the density of galaxy equal to : ng = 354543085.80106884 I ...
3 votes
4 answers
1k views

Black hole density

How does more compression relate to a stronger gravitational pull. Like, when we say that a black hole is a tiny space that has 20-30 suns compressed in it, how does this increase its density and ...
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1 answer
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what determines the density of atmospheres

What factors determine the atmospheric density on various planets and moons? Some larger bodies have less dense atmospheres, while some smaller ones have a more dense atmosphere.
2 votes
1 answer
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From original burst, fraction of stellar mass still surviving on Main sequence

Suppose that all stars in this galaxy were born in a single major-merger burst event about 10 Gyr ago. From this original burst, I want to compute the fraction of stellar mass still surviving as stars ...
1 vote
1 answer
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number density of Dark Matter Halos Versus number density of Galaxies

Can we count only one dark matter halo for each Galaxy ? Indeed, I am trying to estimate the total density of galaxies in given volume and I have often seen on physics exchange and other forums the ...
3 votes
1 answer
773 views

Why the forbidden lines of [OIII],[NII] and [SII],[OII] are sensitive to electron temperature and electron density, respectively?

I am trying to understand the topic of "Metallicity estimates", either HII regions or planetary nebula. For this latter, the electron temperature (and density) must be known. I am studying the ...
1 vote
1 answer
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Are there any known asteroids with average density similar to that of Earth's?

In this answer I mention that for very low orbits around spherical bodies, the period tends to scale only as the inverse square root of the density, and not the diameter. For a low orbits where the ...
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8 votes
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Can "rogue" supermassive black holes be made this way?

Could two galaxies (one big and one small)intersect at a velocity to allow the smaller black hole to escape but not the galaxy around it?
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Why isn't the Sun hollow? [closed]

[Edited to remove incorrect geological assumptions] The sun is self-rotating, i.e. it spins on its axis. When something spins, there's an apparent force away from the centre, and matter gets flung ...
4 votes
2 answers
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Does Flat Universe mean even distribution?

I've read through all the other related answers to "flatness" questions but I need a bit more clarification. I understand that a triangle in a 2D universe would not equal 180 degrees in both a closed ...
6 votes
1 answer
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Does the sun blow a bubble into the interstellar medium?

I once read that the solar wind is more or less blowing a "bubble" into the interstellar medium. However, the numbers I found while trying to verify this don't give me a clear picture yet: The ...
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3 votes
2 answers
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Question about $3p/c^2$ in Friedmann equation

The Friedmann Equation: $$\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}$$ I want to know, where does the $3p/c^2$ come from. I know it is something ...
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1 answer
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Approximating density of hydrogen in [observable] universe

Let universe be completely made from hydrogen. And also we have redshift $z= 6$. with Hubble constant $H_{0} = 2.1941747572815535\times 10^{-18}\:\mathrm{s}^{-1}$. We also know that density of the ...
1 vote
1 answer
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Finding equation for energy density of matter

I'm trying to get an equation for the energy density of matter of the universe $\rho(t)$, assuming the number of particles is conserved and rest mass energy is much greater than kinetic energy. $\rho(...
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3 votes
1 answer
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Equating critical energy density to matter density

I'm asked first to calculate the critical energy density $\rho_{crit}$ of the universe given certain constants. The value I got was $5500\,\mathrm{MeV}\,\mathrm{m}^{-3}$. Then I'm asked to calculate ...
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Does our current understanding of the mass of black hole only allow for it to be located at the singularity? [duplicate]

Does the distribution of mass or density of mass inside the event horizon matter for a black hole to exist as we currently theorize? Could the mass inside the event horizon be distributed in any way ...
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