The Pod Bay

2:07 AM

Can anyone answer a question?

2:27 AM

Say an object has a polytrope of 0.5. How can I relate its pressure to its density?

*polytropic index

2:42 AM

@SirCumference I do not even know what a polytropic index is.

@Phiteros Simply put, a polytropic index is a number you can put in an equation of state, and it'll relate the density and pressure of an object.

Polytrope

In astrophysics, a polytrope refers to a solution of the Lane–Emden equation in which the pressure depends upon the density in the form P = K ρ ( n + 1 ) / n , {\displaystyle P=K\rho ^{(n+1)/n},} where P is pressure, ρ is density and K is a constant of proportionality. The constant n is known as the polytropic index and is sometimes linked to the polytropic exponent γ ...

See that equation? Presumably if I put the Sun's polytropic index ($n = 3$) in it, I'd find a function that relates the Sun's density with its pressure

Problem is, I have no idea what to put for $K$

2:57 AM

So it sounds like you've answered your original question, but just need the constant K?

@Phiteros Yep. My original question was basically asking "how can I complete this equation?"

No idea what K is.

@SirCumference And that Wikipedia page doesn't really specify. Unfortunately, I have not taken a fluids mechanics class and do not have any textbooks that would adequately explain things. Have you tried looking up stuff about the Lane-Emden equation?

Wikipedia isn't helpful in the slightest.

@Phiteros Yeah, but it's far too complicated.

I just need a simple way to relate density and pressure.

Fluid mechanics are all differential equations, so I don't htink there will be a simple answer.

*think

@Phiteros Well yeah. And this is becoming a huge pain.

3:07 AM

@SirCumference What do you need it for?

@Phiteros A paper I'm reading.

@SirCumference About the density of the sun?

@Phiteros That would be a good enough approximation

Technically I'm reading about quasi-stars but the polytropic index is the same as that of the Sun

user218912

@Phiteros how can you be a grad student in astrophysics and not know fluid mechanics?

user218912

k bye

3:25 AM

@SirCumference No, I was asking what the paper was about.

@bloo b/c I'm a first year and my undergraduate program didn't have fluid mechanics.

0celo7

@Phiteros Polytropes should have been covered in a first course on GR.

Jesus...

It's not a big deal.

3:54 AM

@0celo7 In general relativity? I didn't do astrophysics in undergrad, just physics with an astronomy concentration. I'm literally about 2 months into my grad studies.

3 hours later…

7:05 AM

@OrganicMarble did you receive the shockabsorber bounty? I just accepted the answer but I don't really understand how that works.

7:17 AM

Anybody: I need to do some tidying up - is there an easy way to get a list of my own unanswered questions?

14 hours later…

9:30 PM

@uhoh Well, you can just look at your questions on your profile and look for the ones without answers

1 hour later…

10:42 PM

I'm looking for an easy way to get a list of my unanswered questions. Do you know of one?

11:01 PM

@uhoh Sorry, I don't