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54 votes
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Moon's unusual gravity

As you said, the mass of the Moon is 1.2 percent that of the Earth. Now, if you mean the gravitational acceleration at the surface, it is calculated like this $G\frac{M}{R^2}$, where $M$ is the mass, ...
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52 votes
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Is the sun's volume shrinking currently?

Is the sun shrinking currently? It's the other way around: The Sun is slowly growing hotter and thus is slowly expanding. The Sun accumulates ever more helium in its core as it ages. This growing ...
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42 votes
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Why are most discovered exoplanets heavier than Earth?

There are a number of methods of detecting exoplanets, but all of them favour detection of larger planets over smaller ones, albeit for slightly different definitions of large: Radial velocity ...
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35 votes
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Why don't (or can't) stars be more than 325 or so times the mass of the sun? What limits their size?

There is basically an upper limit to the mass of a star because their luminosity is so great that the radiation pressure prevents the accretion of further mass. However, the upper limit depends on the ...
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32 votes
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How was the mass of Venus determined?

How was the mass of Venus measured for the first time? In the mid 19th century, Urbain Le Verrier's predicted of the existence of a then unknown planet beyond the orbit of Uranus. He even predicted ...
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25 votes
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How do we get to know the total mass of an atmosphere?

There is a simple$^*$ way to know the total mass of the atmosphere: measuring the pressure it exerts on the surface, which necessarily integrate all of the atmosphere above ground level. If you take ...
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19 votes

Is the sun's volume shrinking currently?

Sun's mass is almost $M_\odot = 1.9*10^{30}$ kg. The mass-loss scale you are talking about is around $\delta M \approx 10^{10}$ kg/s. I'm guessing this mass-loss is due to both stellar winds and ...
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18 votes
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Can dark matter be found in the shape of planets, galaxies etc.?

Planets and stars, no. Globular clusters and galaxies, yes. Small scales To condense into such relatively compact objects as planets, stars, and even the more diffuse star-forming clouds, particles ...
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18 votes

How was the mass of Venus determined?

The mass of Venus was determined by weighing the Earth, or more precisely, by determining the ratio of the density of the Earth to the density of Schiehallion, and assuming Schiehallion to be typical ...
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  • 2,958
14 votes

Is Jupiter made entirely out of gas?

Comet Shoemaker–Levy 9 crashed into Jupiter a few years back. As well as these molecules, emission from heavy atoms such as iron, magnesium and silicon was detected, with abundances consistent ...
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14 votes
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Size and mass of comet ATLAS (C/2019 Y4)

The confusion comes from the difference between the nucleus and the coma. The nucleus is a small icy body, only a few km across. The coma is the cloud of gas and dust released from the nucleus as it ...
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13 votes

Is Jupiter made entirely out of gas?

It doesn't matter if the body is made of gas, rocks, liquid or plasma, the four states of matter all have mass. So, as we know, mass create a gravitational field, and the more mass the stronger the ...
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13 votes
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What is the exact mass of the Sun?

The mass of the Sun is determined from Kepler's laws: $$\frac{4\pi^2\times(1\,\mathrm{AU})^3}{G\times(1\,\mathrm{year})^2}$$ Each term in this component contributes to both the value of the solar ...
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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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11 votes
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How small would you have to crush an object for it to become a black hole?

Well, I wasn't going to answer but the other two answers are wrong, or at least incomplete. If you wish to make a black hole from a stellar-sized object, then there is no need to compress it to as ...
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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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10 votes

How do we get to know the total mass of an atmosphere?

Suppose the atmosphere has a density that decays exponentially with height. e.g. $$ \rho = \rho_0 \exp[-h/h_0]\ ,$$ where $\rho_0$ is the density at some surface and $h_0$ is a characteristic height ...
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  • 115k
9 votes

Is Jupiter made entirely out of gas?

I concur with everyone else here (of course) that the gravity at the "surface" of Jupiter is entirely determined by the mass contained within that surface. The composition makes no difference. ...
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  • 115k
9 votes

Are planets moving away from the sun?

I asked a somewhat similar question but just about the Earth. Here and in my question there's some links that you might be interested in. Jupiter for example is thought to have moved closer to the ...
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9 votes
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Is there an upper limit to the mass of terrestrial planets?

I'll shamelessly reference an answer I wrote on Worldbuilding to an almost identical question. Lammer et al. (2014) suggested that "super-Earths" with masses of $2$-$5M_\oplus$1 could retain massive ...
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9 votes
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Stellar mass limits for Neutron Star and Black Holes

A succinct summary of supernova types is given in the following image based on Heger et al. (2003): Image courtesy of Wikipedia user Fulvio 314 under the Creative Commons Attribution-Share Alike 3.0 ...
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9 votes
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Mass of black holes compared to parent star

There is no general consensus on this. Different evolutionary models give different results. The factors (in addition to the initial mass of the star) that effect the final black hole mass would be ...
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  • 115k
9 votes

Why isn't Eris considered a planet despite being the body of dominant mass?

You are correct that the IAU definition of "clearing the orbit" has the problem of being not explicitly quantified. And a complete clearing was obviously never the intention behind the definition. I ...
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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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8 votes
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Does the mass of the observable universe ever change?

Yes, the mass of the observable Universe is always increasing. Matter Even if you're only referring the "ordinary" matter (such as stars, gas, and bicycles) and dark matter, the mass of the ...
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8 votes
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Why do certain massive stars leave no remnants?

The gap appears because of pair instability supernovae. In short, as one looks at such massive stellar cores at increasing temperatures, an ever-larger fraction of the photons are sufficiently ...
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8 votes
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How do I create a galaxy stellar mass function?

Your approach is completely correct, just note three things: Logarithmic distribution First, since the distribution of masses is logarithmic in nature (as is most other things), be sure to bin them ...
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8 votes
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How is the total mass of all the Solar System's small asteroids and meteoroids estimated?

Your question body is different from your question title and it seems you really want to ask what you did in the question body so I'll address that. Short Answer: The simple power law which applies ...
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8 votes
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Relation between molecular cloud mass and galactic mass

The mass of an average galaxy appears to be totally dominated by dark matter, so your calculation would not give the galaxy mass. Even if all you wanted was the baryonic (non dark matter) mass then ...
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  • 115k
8 votes

Why the Sun-Jupiter barycentre is outside Sun?

There are three factors that all play into this: The Jupiter/Sun mass ratio The Jupiter/Sun distance The Sun's radius The barycenter of any pair of orbiting masses lies on the line connecting their ...
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