Density Shells of Stars

[Techpriest93]

I have a question: Density of the material of a star increases as you travel further inward, does it not?

Then, how far inwards can you travel before you hit the density of water(1g/cm³)? Rock(2.7g/cm³)?

Say you had a spaceship and had to do an "helio" breaking maneuver because you were coming in hot from another star and didn't yet have the tech to hold the fuel for a more conventional breaking maneuver.

At what percentage of the star's radius would you hit 1g/cm³? 3g/cm³?

Is their a standard equation that I missed that I plug in the star's mass and radius to get a density at a certain altitude?

I am wondering for stars like:

-Brown Dwarfs

-Orange ZAMS

-Yellow ZAMS

-White ZAMS

-Blue ZAMS

-Blue Giants

-Yellow Giants

-Blue Supergiants

-Yellow Giants

-Red Giants

-Yellow Supergiants

-Blue Hypergiants

-Yellow Hypergiants

-Red Supergiants

-Red Hypergiants

(I think I listed them all in order of standard radius)

Hopefully there is an easy equation for this, if not, maybe just for the average/typical star in each category. Thanks!

[LordFerret]

Might help...

https://www.physicsforums.com/threads/average-density-of-a-star.281229/

http://www.maths.qmul.ac.uk/~svv/MTH725U/Lecture4.htm

[Techpriest93]

Might help...

https://www.physicsforums.com/threads/average-density-of-a-star.281229/

http://www.maths.qmul.ac.uk/~svv/MTH725U/Lecture4.htm

Well, the first one was average density, which doesn't tell me squat (except that degenerate matter would make better bullets than depleted uranium, but good luck with that) but the second link might be useful. I will read over it tomorrow and see if I can build an equation where I can plug in the density of 1 g/cm3 and 3 g/cm3 and see at what % of radius that it occurs.

I assume it is way denser at the core and fluffy at the surface, am I not correct?

[Brotoro]

There is no simple equation for this, but there are stellar models for all of those types of stars that could be looked up. I believe the Sun gets to 1 gram per cc at a little over 50% of its radius, and gets to 150 grams pet cubic centimeter at its center.

[YNM]

Actually, you can't really measure the density inside stars, nor to predict it. All calculations you see (mostly about temperature and pressure), needs a way to define the density, either constant or in a relation to the radius (other way is possible too). Another way to do it would be by using polytrope, but the hydrostatic equilibrium equation (this one in the second link) would become pretty messy perhaps ? Most stars are simply modeled with an inverse relation between radius and density, which fairly fit the available observations.

The only way to know this properly would be via stellar seismology, which currently only doable for Sun.

Edited July 22, 2015 by YNM

[Hcube]

The only way to know this properly would be via stellar seismology, which currently only doable for Sun.

How does it work ?

[LordFerret]

Actually, you can't really measure the density inside stars, nor to predict it. All calculations you see (mostly about temperature and pressure), needs a way to define the density, either constant or in a relation to the radius (other way is possible too). Another way to do it would be by using polytrope, but the hydrostatic equilibrium equation (this one in the second link) would become pretty messy perhaps ? Most stars are simply modeled with an inverse relation between radius and density, which fairly fit the available observations.

The only way to know this properly would be via stellar seismology, which currently only doable for Sun.

There was another paper I ran across which was a bit more specific than the first two links I posted, but it seems the focus in example is on white dwarf stars. Maybe more information can be gleaned from it?

http://articles.adsabs.harvard.edu//full/1972ApJ...175..417N/0000417.000.html

[YNM]

There was another paper I ran across which was a bit more specific than the first two links I posted, but it seems the focus in example is on white dwarf stars. Maybe more information can be gleaned from it?

http://articles.adsabs.harvard.edu//full/1972ApJ...175..417N/0000417.000.html

Degenerate stars are far easier for that they don't differentiate, and are fully radiative (hence polytropes works easier for them, pressure directly links to density neglecting temperature fully). A standard star, on the other hand, most likely will be convective, or both convective and radiative in different sites - even late age star (like AGB, RG, or Wolf-Rayet) convection extends to the outer space, creating huge mass loss. An example is corona, where pressure and density is very low but the temperature is very high, not to mention a convective layer would have different density gradient (and temperature, also pressure gradient) than radiative layers. Then fusion in cores help to keep adequate pressure against collapse, which means additional pressure than temperature-induced ones.

The only way to know this properly would be via stellar seismology, which currently only doable for Sun.

How does it work ?

Much like how geologist know the Earth have mantles with Mohorovicic discontinuity, and two iron cores - by looking up seismic activites. In sun this happens out of flares - I'm no expert though, so I can't tell the details.

Edited July 22, 2015 by YNM

[Techpriest93]

Degenerate stars are far easier for that they don't differentiate, and are fully radiative (hence polytropes works easier for them, pressure directly links to density neglecting temperature fully). A standard star, on the other hand, most likely will be convective, or both convective and radiative in different sites - even late age star (like AGB, RG, or Wolf-Rayet) convection extends to the outer space, creating huge mass loss. An example is corona, where pressure and density is very low but the temperature is very high, not to mention a convective layer would have different density gradient (and temperature, also pressure gradient) than radiative layers. Then fusion in cores help to keep adequate pressure against collapse, which means additional pressure than temperature-induced ones.

Much like how geologist know the Earth have mantles with Mohorovicic discontinuity, and two iron cores - by looking up seismic activites. In sun this happens out of flares - I'm no expert though, so I can't tell the details.

I know a white dwarfs density, I learned that in Astronomy 205.

But I am trying to find out where the mass is of a star, and that is my problem! I find it hard to believe that we cant estimate the density zones of our own star then apply that to different types of stars as a theoretical model, or very rough estimates.

If, say, 50% of the star's mass was located around the core, then the remaining 50% would be in the rest of the 98% of the volume of the star, and would diminish as it went outwards.

Then I could make some rough estimates.

But say you have a ship from science fiction that has thermal shielding and radiation shielding, or is a drone, would it be able to explore the "atmosphere" of the sun, and how far down could it "dive" before it hit any meaningful densities that would impair movement?

[YNM]

If you plugged polytropic relation into the hydrostatic equilibrium, replacing dP/dr with something that contains ÃÂ, it's doable I guess. I'll try it next day... you can also get away by using linearly increasing density (to the center), which is called standard solar model. Good luck in your searches.

For the ship thing (guess this is an incarnation of what-if xkcd's Tungsten Countertop), you can get Stokes law and plug in values consistent with a site some radius from Sun's center.

[Techpriest93]

Heh, I don't know why I just didn't google the Sun, instead of looking around for stellar density models and the interior structures of stars.

Using the information that I provided below, I can safely say that the sun's core is quite larger than I expected. XD I guess I thought of it as this little thing violently reacting and fusing, but it turns out to be more of a calm sphere. It is also a no-go zone, at 150g/cm3 in the core (162.2 at its peak) and accounts for 50% of the stars mass.

I will now make an assumption here, and assume that all cores are 50% of the star's mass, for extrapolation to other stars.

So, thats at 25-30% of the solar radius. The next 40-45% is the radiative zone, with a density gradient of 20 to 0.2. Could someone help me with density gradiants at 5% intervals? I have a lot of other research and writing to do, (and you would get lots of brownie points). I assume it is logarithmic, and the density dissipates rapidly, but I have been wrong before (like how I didn't realize that the sun is more like a compost heap rather than a fusion bomb).

Anyways, the densities that I am looking for are in this area. It is somewhere between 30% and 70% of the solar radii that you can find the density of plasma comparable to density of things on earth, like our atmosphere, then our water, and then our rocks.

I will make another assumption and assume that the solar radius of the convection zone is not consistent among other star types. Unless it is logrithmic... Like, Giants and Supergiants probably don't have massive radiative zones.

The convection zone, where crazy scary stuff happens, reaches to the "surface" and has 2% of the mass in 66% of the total volume. I believe the density is like 1/6000 that of earth's atmosphere at sea level. Probably not good for a "solar sailer" that has stubby wings and relies on "atmosphere" for lift. Unless their are really strong updrafts in this "atmosphere".

I wonder if a hardened probe could use this zone as an atmospheric breaking maneuver if it was shot from our solar system to another with no other way to slow down. Or would it need to briefly dip into the radiative zone to get any meaningful "air" resistance. Would it be called "Heliobreaking", like how you call impacts lithobreaking when you accidentally hit the ground at orbital transfer speeds?

I will make another assumption and assume that this zone increases on the age of the star. Like how a red giant would have a huge upper zone that is mostly puffy. If our sun expanded to envelop earth, would the earth still remain when the sun collapses because the density and drag would not overpower the solar wind and pressure coming from the core? Of course, the earth that would remain would be barren and scorched, but would it still be orbiting? (like, not spiraled into the center. It may have migrated inwards, but i dont think it would be destroyed)

How are my assumptions? What have I gotten wrong?

"The core of the Sun extends from the center to about 20–25% of the solar radius.^[61] It has a density of up to 150 g/cm^3[62][63](about 150 times the density of water) and a temperature of close to 15.7 million kelvin (K)."

"The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated within 24% of the Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely."

"Theoretical models of the Sun's interior indicate a power density of approximately 276.5 W/m³,^[68] a value that more nearly approximates reptile metabolism than a thermonuclear bomb.^[e] Peak power production in the Sun has been compared to the volumetric heat generated in an active compost heap.^{[citation needed]} The tremendous power output of the Sun is not due to its high power per volume, but instead due to its large size."

"The density drops a hundredfold (from 20 g/cm³ to only 0.2 g/cm³) from 0.25 solar radii to the top of the radiative zone."

"At the photosphere, the temperature has dropped to 5,700 K and the density to only 0.2 g/m³ (about 1/6,000th the density of air at sea level)"

- - - Updated - - -

huh, link me the full equation and I will try it.

Thanks for the read on the XKCD article, it reminded me of a few things that I forgot. Like sputtering.

This scenario of course is science fiction, so our counter-top probe has a high tech plasma/magnetic shield from TV, or repulsors for a steady decent for the science of the journey.

What is this stokes law? I probably had to learn it in class, but I don't remember what equation peoples names link to.

Well, I am off to find out the structure of red and brown dwarfs and see how radically different their density and composition is compared to our sun. I wonder if it is more uniform or still rapidly decreases from the center.

[Skyler4856]

If one is traveling at interstellar speeds, is it really the best idea to attempt to aerobreak?

Solar sails seem like a cool concept that could apply here

Actually, you can't really measure the density inside stars, nor to predict it. All calculations you see (mostly about temperature and pressure), needs a way to define the density, either constant or in a relation to the radius (other way is possible too). Another way to do it would be by using polytrope, but the hydrostatic equilibrium equation (this one in the second link) would become pretty messy perhaps ? Most stars are simply modeled with an inverse relation between radius and density, which fairly fit the available observations.

The only way to know this properly would be via stellar seismology, which currently only doable for Sun.

???

Could one not simply predict how hydrogen compacts, then use the surrounding mass of the star to dictate the degree to which hydrogen contracts?

[YNM]

???

Could one not simply predict how hydrogen compacts, then use the surrounding mass of the star to dictate the degree to which hydrogen contracts?

You don't even know how the mass is distributed first hand, which causes changes in density and pressure. You don't even know, again, where fusion stops, where temperature gradient is changing more rapidly. And remember, a hot convective water can be pretty violent and violates what you predict (uh, giving up a huge bubble, for example). A good way to tell this would be to look for seismic waves (p-waves actually, s-waves are nonexistant - just that the waves are quite uniform, matter actually moves with them) at the surface, which tells you what happened inside and what have they passed. Hence our knowledge for Sun interior is more advanced than, say, Vega.