Celestia Forums

For the longest time, I've been looking to create a texture for a White Dwarf. The first obstacle I had to overcome was time. I had no time to work on textures or anything Celestia related. That obstacle is now behind me, for now.
The next obstacle was, trying to figure out exactly what a White Dwarf would look like. I found plenty of references to electron degeneracy, the Chandrasekhar Limit, the types of white dwarfs (DA, DB, DO), their diameter, hydrogen or helium surfaces, even surface convection. What I didn't find was a detailed description of what one might look like up close.
I did find a number of artistic illustrations depicting White Dwarfs. One of the curiosities I noticed was that, in all of the more detailed pictures I came across, the White Dwarf was depicted as having a mottled surface.
For Example:
http://www.rkm.com.au/ASTRONOMY/White-Dwarf.html

http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/10/video/b

http://www2.warwick.ac.uk/fac/sci/physics/research/astro/postgraduate/whitedwarfs/

http://starryskies.com/articles/2004/02/diamond.html

http://www.virtualsciencefair.org/2002/wongj/public_html/whitedwarfs.html

The problem (again) with these images was that, I couldn't find any references explaining the mottled surfaces being depicted. Was it due to the fast moving degenerate electrons coupled with the high temperatures and gravitational fields? I couldn't find an explanation! So, I just went with the flow (so to speak). And here's what I came up with:


And an alternate side view:


I use it in my copy of Celestia in order to depict Sirius-B. Once I finished, I figured I would continue and do an A-Star texture for Sirius-A. This is how it looks thus far:

And a granulation close-up:


I haven't added any Star Spots (Sunspots) since my research is still inconclusive as to whether an A-Star (like Sirius-A) would actually have them. As far as adding the textures to these stars in Celestia, it was easily done by adding a texture parameter under each respective star inside the nearstars.stc file located in the Celestia/Data folder.

Problem is, nobody has a clue what they look like up close. Though to be honest, I think most are probably still going to be so bright that up close they'll just look like a saturated, blinding white star.

Some time ago, I created a whte dwarf texture for use in my educational activities. It too is mottled without sunspots. The main differences between the two images are the depth of mottling and the spectral color. I made mine lavender-hot, since a White Dwarf is so hot from the explosion that created it (the explosion of a Red giant and resulting planetary nebula) that it would be glowing very brightly in UV and near UV. I also have it spinning very fast, as would be the case. As to mottling, I assumed the density of a White Dwarf would be very high with a minimum of convection currents, so I toned down the mottling).

Here is a screenshot:



If you like it, I can post it as a texture on the Motherlode, with an accompanying ssc file.



Frank

Malenfant Wrote:

Problem is, nobody has a clue what they look like up close.

Yeah, I kinda figured that. I guess I just had to hear it from somebody else.

Frank Wrote:

If you like it, I can post it as a texture on the Motherlode, with an accompanying ssc file.

Absolutely, post it. I'd download a copy. And, thanks.

I have sent the White Dwarf to ML. Look for it in the stars section.

Enjoy

Frank

Chen,

as far as I know Sirius A probably will not have a granulated surface like the sun.

Main sequence stars like Sirius A and the Sun develop an outer convection zone only in the low-mass branch (say, up to 1.5 solar mass units).
An OCZ is turbulent "boiling" gas which transports the heat energy out of the star - the boilling bubbles seen from above are exactly what we call the granulation.
Thus, the sun is below that border and still has a thin OCZ - which is the granulation we see, Sirius A is above the border and probably has no OCZ thus no granulation. To be honest I don't know what it surfaces will look like -- same problem as for the white dwarf!

To the ones further interested in astrophysics, the presence of the OCZ in the sun has still more consequences. The rapid motion of the hot (electrical conducting) plasma also generates strong magnetic fields which are responsible for phenomena like sunspots, prominences and the beautiful corona of our sun. Lacking the rapid motion of the OCZ Sirius A probably won't have all that nice stuff.
Due to the presence of a convective core in main-sequence stars of Sirius' size and above, an overall magnetic field could be generated deep inside. At surface, it then would be mainly of dipole character (unlike our sun's with a lot of chaotic loops) but I have no idea what it may look like.
Giant sunspots at the poles? Some straight polar promonences? Take your artistic freedom of mind here...

~Diane.

Medusa,
Maybe you can help me here. I have found many references referring to the existence of surface convection, not only on A-Type stars but, also on White Dwarfs. For example:
"The next part was on surface convection in A type stars, which possess two distinct convection zones (H/HeI + HeII). The simulations show that the buffer zone between convection zones is fully mixed and the upper boundary of photosphere is dynamic (see Freytag & Steffen 2004). The surface convection cells of A type stars are larger than in the Sun. The simulations can be used to calculate synthetic spectra from A type stars, which are presented in a poster by Oleg Kochukhov (No. 1 in Terrace I)."
REFERENCE:http://astronomy2006.blogspot.com/2006/08/convection-in-astrophysics-session.html

And for the White Dwarf:
"White-dwarf atmospheres have surface gravities some four orders of magnitude greater than solar-type stars, and their granulation structure is predicted to be correspondingly smaller and more energetic. Granular features are expected to exist on scales down to hundreds of meters, while velocities of tens of km/s imply characteristic timescales of perhaps 10 milliseconds, well in the range of high-speed astrophysics. Even if spectral lines would prove difficult to accurately observe or interpret, granulation on white dwarfs could be studied by high-speed photometry with large telescopes, observing the stellar microvariability in response to the evolution of [the finite number of] granular features."
REFERENCE:http://www.astro.lu.se/~dainis/HTML/GRANUL.html

These are merely one example each of many references I've come across in researching surface granulation on specific star types. There was some discrepancy as to the presence of surface granulation on A-Type stars hotter than (not sure of the exact temperature 11000K - 12000K --maybe hotter) but, even this particular reference (not currently at my disposal) was sketchy in it's presentation.
If you could point me toward some solid references, I'd really appreciate it since there seems to be different ideas/beliefs on these issues depending on the Scientist/Astrophysicist who is presenting it.

Chen,

these references are new to me, but the argumentation looks sane from a scientific view. It seems even if Sirius A has no true deep convection zone, there is at least a very thin portion at the boundary "star interior vs. atmosphere" which is in motion. The author you cited tells about "photosheric convection", which would be exactly that.
Such detail views are usually hidden when dealing with stellar evolution codes. That's what I have done on an amateur scale; I run EVCODE, which is a stellar evolution simulation code used for educational purposes I have fitted to Linux for my purposes (EVCODE courtesy of University of Victoria, B.C., Canada). Very thin atmospheric sheets are usually just "boundary condition" in codes like this, they are fitted to give good convergence of the model in the interior (which is the main goal of the code).
One of the authors you cited also pointed out that 1D modeling isn't sufficient to show surface granulation. EVCODE like most older stellar evolution codes *is* 1D. Maybe thats the point why only recent work of detail 3D simulation show it in A type stars.

(If you're further interested in stellar interior physics I give two refs here:
1 - R. Kippenhahn: Stellar Structure and Evolution, Springer A&A lib series 1999
2 - M Stix: The Sun - an Introduction, Springer A&A lib series, 2nd ed. 2004)

Dealing with photospheric convection, there is only one question left for me. The photosphere of a star is the layer where the gas becomes transparent. Would the photosperic granulation cells be fully visible or just a cloud shadow above a uniform background? Maybe that's not important for it would look nearly the same for your surface texture, but I'm curious about...
so, from this point of view your granulation now looks at least satisfying to me...

~Diane.[/i]

fsgregs wrote:I have sent the White Dwarf to ML. Look for it in the stars section.

Well, you will get it here.
Later on I will add it to the catalog.

Adirondack

Medusa,
I understand the concept of Photosperic, Chromospheric, etc., admittedly at some great cost. Their references to "surface convection cells" and "granulation", at least to me, seem to refer to actual, physical properties (viewable through a telescope). I certainly would feel silly creating a star texture that was not commensurate with current theory, i.e., I always endeavor to maintain realism.
My problem is that, I don't always understand the abstracts or papers I'm studying that explain the physical properties of the star I'm researching. Astronomy was always a great love of mine but, most of what I learned was well over 25 years ago. It's not like riding a bike where, if you lay it down for a while, you can usually get back on and ride again later on.
So much has changed or been completely thrown out the window. My math (which is crucial for Astronomical calculations) also went out the window many years ago. I was usually at the top of my class in astronomy (not astrophysics) but, even the Planets are different now. Trying to understand the new concepts and theories has been an uphill battle. Even the references contained in a specific abstract are new to me and I must research each of those to even get a basic understanding of the point they're trying to make. Frustration is often the order of the day.

Well, thanks for your input. If you find any additional references, post them here. I don't want to get too lengthy here since we're beginning to tread into material meant for the P & A section.