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56 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, ...
Tosic's user avatar
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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 ...
David Hammen's user avatar
44 votes
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Does the Earth constantly lose mass?

You are wrong that "to keep any object in circular motion requires energy" In a circular orbit, the force of gravity is always perpendicular to the motion of the moon, so no work is done by ...
James K's user avatar
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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 ...
Steve Linton's user avatar
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36 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 ...
ProfRob's user avatar
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34 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 ...
David Hammen's user avatar
30 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 ...
Jean-Marie Prival's user avatar
23 votes

From an outsider's perspective, how can a black hole grow if nothing ever crosses the event horizon?

First off, there's no real time dilation effect. In coordinate systems that cover the event horizon (Kruskal-Szekeres, Eddington-Finkelstein, Gullstrand-Painlevé, Kerr-Schild), objects fall through it ...
benrg's user avatar
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21 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 ...
Mark's user avatar
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21 votes
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What is going on with this (sub?) brown dwarf WISEPA J174124.26+255319.5?

I had a look at the original source of the measurements quoted on the Wikipedia page (Zhang et al. 2021) and the problem is simply systematic errors in the models that are being used to infer the ...
ProfRob's user avatar
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20 votes
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How are the masses of solitary stars weighed?

There are a number of ways, none of them is as precise as measuring binary motion. The mass-luminosity relationship, calibrated using stars in binary systems with measured masses, works quite well for ...
ProfRob's user avatar
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19 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 ...
pela's user avatar
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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 ...
Aryan Bansal's user avatar
  • 1,095
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 ...
James K's user avatar
  • 122k
12 votes

From an outsider's perspective, how can a black hole grow if nothing ever crosses the event horizon?

Due to time dilation, an outside observer never sees a falling object actually cross the event horizon. This is correct, but perhaps not quite in the way that you visualize it. It's trivially true, ...
Ilmari Karonen's user avatar
11 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 ...
HDE 226868's user avatar
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11 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 ...
ProfRob's user avatar
  • 152k
11 votes

How are the masses of solitary stars weighed?

With difficulty. For stars on the main sequence, there is a mass-luminosity relationship Sourced from (Astronomy by Openstax and Department of Physics and Astronomy at Douglas College, Chapter 18.2 ...
James K's user avatar
  • 122k
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, ...
Peter Erwin's user avatar
  • 16.7k
10 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 ...
SpaceCore's user avatar
  • 3,033
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 ...
HDE 226868's user avatar
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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 ...
ProfRob's user avatar
  • 152k
9 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 ...
ProfRob's user avatar
  • 152k
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 ...
Schwern's user avatar
  • 514
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 ...
pela's user avatar
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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 ...
zephyr's user avatar
  • 15k
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 ...
Mark Olson's user avatar
  • 7,650
8 votes
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What if a primordial black hole went through the Sun?

The answer is that there is a limited rate at which matter that can be crammed into a black hole of this size. That rate is small enough that the black hole will traverse through the Sun hardly ...
ProfRob's user avatar
  • 152k
7 votes

Why are black holes that massive?

The mass of a object does not increase when it collapses into a black hole. So a supermassive black hole must have started off quite small, and then grown. The formation and growth of supermassive ...
James K's user avatar
  • 122k
7 votes
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Would gravitational waves be subject to external gravitational perturbations?

Firstly, gravitational waves (GWs) are not an echo - we measure the direct signal. The process you describe here is known as gravitational lensing, the deviation of (usually) light rays due to ...
AstroFloyd's user avatar
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