# Tag Info

### How to calculate the mean molecular weight of the Sun

For your second question, it depends upon whether you are talking about composition by number, or composition by mass (the numbers you give are roughly the composition by mass). If you want to get ...
Accepted

### Are larger stars rounder?

In terms of mean angular velocity, the distribution of rotation rates among main sequence stars is well known. Allen (1963) compiled data on mass, radius, and equatorial velocity, which was then ...
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### What is the time/size/rigidity ratio for a space object to become mostly round?

Based on observable data, I assume that there is a relationship between the size, rigidity and the passage of time; that will result in all objects subject to only their own gravitational ...

### Why 225088 Gonggong is not a dwarf Planet yet?

As the comments already say, an object being a dwarf planet is a matter of convention. If the IAU says it's a dwarf planet, it's a dwarf planet. Otherwise, it's not. The requirements you are listing ...
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### Aren't denser bodies more likely to collapse into hydrostatic equilibrium?

No. Hydrostatic equilibrium for celestial bodies occurs when the internal gravitational forces overcome the rigidity of the materials of the body. So, a dense rigid body may not be in hydrostatic ...
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### Using a differential force in the derivation of hydrostatic equilibrium in a star

The whole exercise illustrates the usefulness of vectors in addressing problems. $$\vec{F_g} = -\frac{GM(r)\delta m}{r^2}\ \hat{r}$$ \vec{F_P} = -\left(\frac{dP}{dr}\right) \delta r \delta A\ \hat{...

### Virial Shock Heating

Virial shocks appear when gas falls in toward a galaxy, and the shocked gas stays hot so the pressure in the post-shock gas can support the shock and keep it away from the galactic disk. The ...
1 vote
Accepted

### What does it mean for cold clouds to be in pressure equilibrium with a diffuse hot medium?

Pressure equilibrium means that the pressure of the hot component and the pressure of the cold component are equal at their interface, so that neither expands nor contracts relative to the other. The ...
1 vote

### Why 225088 Gonggong is not a dwarf Planet yet?

The IAU recognized three dwarf planets as part of its declaration of a planet definition in 2006: Ceres, Pluto and Eris. In 2008, the IAU named Makemake and Haumea under dwarf planet naming rules ...
1 vote
Accepted

### Formulae for gravitatitional equilibrium

Mass = $\text{atomic mass} \times \text{number of atoms} = 4.4400513610370694 \times 10^{30} \mathrm{\ kg} \approx 2.3M_☉$ Herein lies the problem: your units and exponents are messed up. The mass of ...
1 vote

### Is there a mechanism that makes small moons more rounded than comets?

Seems to me that moons are regularly subjected to tidal forces that comets, asteroids and the like are not. Repeated stress, such as rocks in a rock tumbler, or two circles of glass on a mirror maker'...
1 vote

### Is there a mechanism that makes small moons more rounded than comets?

I can only speculate but here goes nothing. 67P/Churyumov–Gerasimenko apparently came from the Kuiper Belt. There are a lot of KBO (Kuiper Belt Objects) in that region, so it is not unreasonable to ...
1 vote

### What happens to the Gas Pressure when working out the Eddington Luminosity?

The Eddington luminosity is defined in this way - but I guess that is not the answer you are looking for! However, if we ask what isotropic luminosity is required in order to balance the inward ...
1 vote

### Can objects enter hydrostatic equillibrium through processes other than the influence of gravity?

Yes, a droplet of water would do that. However, at the planetary scale, the contribution of superficial tension is negligible. At that scale, gravity dominates all other forces. In fact, you could ...
1 vote

### Can objects enter hydrostatic equillibrium through processes other than the influence of gravity?

Video: Space Station Astronauts Grow a Water Bubble in Space Surface tension tends to draw the water into a nice sphere.
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### Can objects enter hydrostatic equillibrium through processes other than the influence of gravity?

Strictly speaking (as far as I know), hydrostatic equilibrium applies whenever a fluid balances external body forces with the pressure gradient. From Wikipedia: In continuum mechanics, a fluid is ...
1 vote

### How can astronomers determine the difference between "hydrostatic equilibrium" and "just happens to be spherical"?

I think you're asking, "If we know an object's shape, can we determine if it is in in hydrostatic equilibrium?" If so, one might wonder if astronomers classify basketballs or ball bearings as being in ...
1 vote

### What's an order-of-magnitude main sequence star look like?

To get an order of magnitude estimate you can just use the total mass $M$ and luminosity $L$ of the star and an assumption of your fusion process. Main sequence stars fuse Hydrogen in to Helium ...

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