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90 votes
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Is there a star over my head?

Summary There's a 1 in 500 billion chance you're standing under a star outside the Milky Way, a 1 in 3.3 billion chance you're standing under a Milky Way star, and a 1 in 184 thousand chance you're ...
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  • 828
45 votes

Why is the Sun's density less than the inner planets?

The sun isn't the same density all the way through. According to MSFC's solar interior page, the core density at the centre of the sun is a whopping 150,000 kg/m$^3$. Surrounding it the radiative ...
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37 votes
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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?

The Pauli Exclusion Principle forbids two indistinguishable fermions occupying the same quantum state. It does not prevent them getting arbitrarily close together so long as they have very different ...
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  • 118k
29 votes

Why is the Sun's density less than the inner planets?

Fusion inside of a star affects the sun's density (which does not happen with a planet). It produces an outward pressure that balances against the attraction of gravity, thereby reducing the density ...
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  • 6,104
23 votes

Are there any planets or moons denser than Earth?

I feel it's a cheap answer but heavy Jupiters can get much denser than Earth because planets with Jupiter's mass stop adding size as they add more mass. A planet with Jupiter's size and 10-12 times ...
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21 votes

Why is the Sun's density less than the inner planets?

The density of matter depends not only on its composition, but also on temperature and pressure. It's not meaningful to say that substance A is denser than substance B without specifying the ...
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19 votes
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What is the most dense object in the universe?

Let us define this as the largest observable density of a stable object, in order to exclude black holes which may have a very large (infinite) density at their centers or objects collapsing towards ...
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  • 118k
15 votes

Is there a star over my head?

In short: no one knows for sure, but currently it looks that the probability is 1. Longer: On our current understanding, the Universe is probably infinite in space. This depends on the recent WMAP ...
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  • 3,102
13 votes

Why is the Sun's density less than the inner planets?

I'd say the most important answer is because the volume of stars is counted differently than for (inner) planets.For the former, most of the gas surrounding the dense core is counted. The latter don't ...
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  • 239
11 votes

What is the minimum mass required so that objects become spherical due to its own gravity?

Icy objects, such as most in the Kuiper belt can reach an equilibrium if they are about 400km across, whereas the rocky asteroid Pallas, at 572km clearly has an irregular, non-spherical shape. All ...
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10 votes
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Size of Saturn's ring material

The vast majority of the particles in Saturn's rings are small, on the order of $\sim10^{-1}$ m or lower. The columnar number density, according to data from Voyager 1 and Earth-based observations, ...
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10 votes
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Are white dwarf stars supported by proton degeneracy as well?

Straightforwardly no. For a start there are almost no free protons inside a white dwarf. They are all safely locked away in the nuclei of carbon and oxygen nuclei (which are bosonic). There are a few ...
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10 votes

Can "rogue" supermassive black holes be made this way?

I assume you're asking about central supermassive black holes (SMBHs, one per galaxy), not stellar-mass black holes. The answer is yes, but what actually happens is the two SMBHs have to merge first, ...
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9 votes
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Are there any planets or moons denser than Earth?

From the Wikipedia page on Chthonian planet: Transit-timing variation measurements indicate for example that Kepler-52b, Kepler-52c and Kepler-57b have maximum-masses between 30 and 100 times the ...
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  • 801
9 votes
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Are red dwarfs really 30-100 times our Sun's density?

Red dwarfs, depending on your definition, can range from 2.5 to 150 times more dense than the Sun. What is the cause of this discrepancy? They give no calculations, so I can only guess. The article ...
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  • 328
8 votes

Why isn't the Sun hollow?

As the surface of the earth is solid the crust, Correct. underneath the crust the mantle which is liquid, No the mantle is also solid, although more plastic than the crust. There is a liquid ...
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8 votes

Is there a star over my head?

Does "overhead" mean over the center of your head, or over some part of your head? If we assume the latter, it changes the problem! I don't want to recapitulate all MichaelS's lovely work above, so I'...
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  • 370
7 votes

Are white dwarf stars supported by proton degeneracy as well?

Proton degeneracy is not important, because its effect is much smaller -- much like nuclear particles in theory also are dictated by gravity, but the electromagnetic and nuclear forces are dominating, ...
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7 votes
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Why is the core of a gas giant supported by electron degeneracy pressure instead of nuclear fusion?

The test to see whether degeneracy pressure is going to be significant is to compare $kT$ with the Fermi energy $E_F$ The Fermi energy is the energy level up to which all energy states would be ...
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  • 118k
7 votes
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Maximum and minimum gas giant & ice giant densities

Here is a plot I generated in 5 minutes at the site exoplanets.org To construct this I took planets discovered by the transit method and which had a $M \sin i$ measured using radial velocities. I ...
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  • 118k
7 votes
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Is the Sun homogeneous?

No, the sun and all other stars do not have the same composition and density within them. The composition varies with depth. Most stars overall composition reflects the interstellar medium from which ...
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7 votes
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Planets classification by density

There is no 1:1 mapping between density and composition/structure. You have to look at detailed planetary models. For example, some hot Jupiters are extremely dense ($\geq 10$ g/cm$^3$) but they are ...
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  • 118k
5 votes
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How do star densities work?

The mean density of the star is really only defined by the formula $\bar\rho=M/V=3M/4\pi R^3$. The radius of a star is a generally a very complicated function of a star's other properties. When we ...
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  • 2,692
5 votes
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Unbelievably high masses/ densities in NASA website data

Starting from the index you mentioned, I clicked through the links for some individual planets, which in turn link to discovery papers or other relevant observations. For planets around Kepler-23, -24,...
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  • 34.1k
5 votes

Is the Sun homogeneous?

No it does not have the same composition everywhere. In the core hydrogen is fused into helium, so the fraction of hydrogen (denoted by $X$, between 0 and 1) decreases while the fraction of helium ($Y$...
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  • 3,321
5 votes
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what determines the density of atmospheres

Most simply, the density is determined by the number of molecules, the surface area of the planet, the temperature, and the gravity. But it sounds like what you really care about would be called the "...
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5 votes
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Black hole density

We can understand gravity as following a set of mathematical equations called "General Relativity" which were discovered by Einstein (and others) around the start of the 20th century. The same ...
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5 votes

Black hole density

It's simply because you can "get closer" to it, that's all. No special sauce. You know how gravity is pretty weak far away, and gets stronger close by? The closer you get to the Sun - more ...
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5 votes

Can a planet exist with the same mass as the earth, but have a different diameter?

Yes, a planet can exist with the same mass as Earth, but have a different diameter - if it has a different density. The lower the density, the larger will be the diameter. In terms of an exact formula,...
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5 votes
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Factors included in the average density of planets

Whether of not you include the mass of the atmosphere doesn't matter much. For instance, the mass of Earth's atmosphere is roughly $10^{-6}$ of Earth's mass, corresponding to the uncertainty in the ...
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