Comet tails

[ajtribick]

1.5.0pre5

How is the question of whether comet tails get drawn decided? For example, consider the following:

Code: Select all

"Comet" "PSR 1257+12" {
   Class "comet"
   Radius 10
   Texture "asteroid.*"
   EllipticalOrbit {
      SemiMajorAxis 2
      Eccentricity 0.95
      Period 100 # not consistent with semimajor axis
   }
}

The comet gets up to a calculated temperature of over 2400 K yet a tail never gets drawn.

Also, the tip of comets never seems to disappear, e.g. in this screenshot of Halley at aphelion:

[symaski62]

1.5.0pre5

How is the question of whether comet tails get drawn decided? For example, consider the following:

Code: Select all

Comet" "PSR 1257+12" {
   Class "comet"
   Radius 10
   Texture "asteroid.*"
   EllipticalOrbit {
      SemiMajorAxis 2
      Eccentricity 0.95
      Period 100 # not consistent with semimajor axis
   }
}

The comet gets up to a calculated temperature of over 2400 K yet a tail never gets drawn.

Also, the tip of comets never seems to disappear, e.g. in this screenshot of Halley at aphelion:

http://tribick.users.btopenworld.com/te ... astron.jpg

humm

celestia 1.5.0 pre 5

cel://Follow/PSR%201257+12:Comet/1999-1 ... 2048&ver=2

[ajtribick]

symaski62: don't know what you are trying to point out here, but that cel URL illustrates my point - only the "coma" is being drawn when a comet at that temperature in solar orbit would have the full tail drawn.

[BobHegwood]

I had thought that the tails were drawn or not drawn based upon
the comet's distance from the star. Don't remember where it is, but
there is a thread on this site describing - in detail - how the tails are
handled. Have a search there mate.

Edit: See the thread HERE.

[t00fri]

Chaos,

I have done most of the respective coding and the underlying
theoretical discussion of tail display -- including multiple star
systems-- already for version for our Celestia-FT-1.x
versions which has been implemented into 1.4.0 later.

Here is a reference thread

http://www.celestiaproject.net/forum/viewtopic ... 53&start=0

There is also a Maple workshet about all this.

The code is such that the coma does never vanish. This has
to be improved. The vanishing of the tail is triggered as
function of distance from the relevant stars, not temperature.
This might also be changed eventually.


Bye Fridger

[ajtribick]

Ok, coma disappearance isn't implemented.

Something odd seems to be going on here with PSR B1257+12...

Celestia reports the following parameters for the pulsar:
r=10 km
T=5,000,000 K
L=0.00000000136 times solar

Now as I understand it the displayed value corresponds to visual luminosity, but we probably want the bolometric luminosity.

By calculation, we have:

L=4?€r????T^4

which works out as 116 times solar -- a discrepancy of 11 orders of magnitude!!!

Now we take the description in the thread:

For our solar system, the threshold irradiance where the tail starts fading, is (empirically) known to be
Irr_crit =L_sol/x0_sol^2, with x0_sol ~ 5 AU and L_sol being the known solar luminosity.

It is now very sensible to assume that comet tails start vanishing at a universal threshold irradiance
Irr_crit that we know already from our solar system.

Hence for any other system (with 1 sun), we immediately obtain the scaling law for the fading start x0

+++++++++++++++
x0 = x0_sol sqrt(L/L_sol).
+++++++++++++++

This gives x0_pulsar=x0_sol*sqrt(116)=54 AU!

However the tail is not being drawn!

Does the comet tails code use the bolometric luminosity (as used in the temperature calculation) or the displayed (visual) luminosity?

If it is using visual luminosity, we get x0_pulsar=x0_sol*sqrt(0.00000000136)=0.00018 AU

Lo and behold, when the comet goes inside this radius, the tail gets drawn!

So it looks like the visual luminosity is being used for the comet tail calculation! Shouldn't the code really be using the bolometric luminosity though?

[ajtribick]

I've submitted this one to the bug tracker.

[symaski62]

symaski62: don't know what you are trying to point out here, but that cel URL illustrates my point - only the "coma" is being drawn when a comet at that temperature in solar orbit would have the full tail drawn.