Where is VY Canis Majoris?

[FaLLeN_SOuL]

can't find this star in celestia, but it is the biggest known star in our galaxy with probably 2100 solar radii. will it be added some day?



here is a link to the wikipedia article:
http://en.wikipedia.org/wiki/VY_Canis_Majoris[/img]

[t00fri]

What do you mean? Of course, VY CMa is in Celestia. But stars can have MANY different names. Celestia uses the HIPARCOS catalog (as you might know). Hence you must search for the HIP equivalent of VY CMa, which is

HIP 35793 (didn't you know this by heart? )

or it's HD number

HD 58061

or it's SAO number

SAO 173591


Content?

Bye Fridger

[ajtribick]

Maybe the designation "VY CMa" is not in starnames.txt.

Try finding it under the designation HIP 35793 (which I found by looking up the star on SIMBAD) - unfortunately I do not have an install of Celestia on the OS I am currently running to test whether this star is in the stars.dat file.

EDIT: hmmm... looks like I've crossposted with Fridger.

[BobHegwood]

I thought it was "Cannabis" Majoris... You got my attention there for a
minute.

[FaLLeN_SOuL]

What do you mean? Of course, VY CMa is in Celestia. But stars can have MANY different names. Celestia uses the HIPARCOS catalog (as you might know). Hence you must search for the HIP equivalent of VY CMa, which is

HIP 35793 (didn't you know this by heart? )

or it's HD number

HD 58061

or it's SAO number

SAO 173591


Content?

Bye Fridger

i can't find it under anyone of this names

[t00fri]

What do you mean? Of course, VY CMa is in Celestia. But stars can have MANY different names. Celestia uses the HIPARCOS catalog (as you might know). Hence you must search for the HIP equivalent of VY CMa, which is

HIP 35793 (didn't you know this by heart? )

or it's HD number

HD 58061

or it's SAO number

SAO 173591


Content?

Bye Fridger

i can't find it under anyone of this names

Then you obviously don't know how to use the professional database SIMBAD?

Go here:
http://simbad.u-strasbg.fr/simbad/
Make it one of your bookmarks!

click under Queries: by identifyer

Enter e.g.

VY CMa

Then you find that your favorite star has 32 other OFFICIAL names!

Among that list, you will also find the HIP,HD,SAO numbers.

You can of course also display a photo of your star, by clicking the Aladin previewer at the bottom. You can also get involved in professional analysis work if you activate the Aladin applet further to the right.

Bye Fridger

[ajtribick]

Fridger - I suspect that what our friend means by being unable to find it is not that he can't use SIMBAD, but is assuming that because the name "HIP 35793" does not show up in the list of objects in Celestia, it's not there.

FaLLeN_SOuL: in Celestia, just press ENTER, type in "HIP 35793" (without the quotes) then press ENTER again. Voil? , it's there.

Celestia does not in general show the HIP, HD or SAO designations in the list of objects, for the simple reason that there are far too many of them.

[Johaen]

i can't find it under anyone of this names

You have to type out the whole name, but it's there. Note that it's nowhere near as big in Celestia as the wiki article claims it to be. Its radius is "only" about .4 AU.

[Cham]

Note that it's nowhere near as big in Celestia as the wiki article claims it to be. Its radius is "only" about .4 AU.

This is one of the "bugs" I've described in my "bugs list", recently. Currently, Celestia is giving some **VERY WRONG** radius values to most red (cool) stars. Celestia should be updated on that front : it's way too approximate.

[ajtribick]

Note that it's nowhere near as big in Celestia as the wiki article claims it to be. Its radius is "only" about .4 AU.

This is one of the "bugs" I've described in my "bugs list", recently. Currently, Celestia is giving some **VERY WRONG** radius values to most red (cool) stars. Celestia should be updated on that front : it's way too approximate.

Any suggestions for how to go about correcting it? Do we even know what the cause of the problem is? Is it the bolometric correction being used, or is the problem more related to breakdown of the uniform blackbody approximation (e.g. lots of spectral lines, limb darkening).

[granthutchison]

The bolometric corrections used in the lookup table are pretty standard values: so our estimate of total energy output is likely to be reasonably accurate, assuming the spectral class is correct.
The problem comes with the black-body calculation that uses energy output and temperature to calculate surface area, and hence radius. Limb darkening, absorption lines and perhaps large spots all make the black-body assumption faulty for cool stars, and all of the above reduce the actually radiative efficiency of a star's surface, causing Celestia to make it misleadingly small.
I doubt if there is a scientific approach that could generate accurate radii for red giants and supergiants without using prohibitively complex calculations. The alternative, as far as I can see, is yet another lookup table, which lists a set of jigger factors determined by the difference between the naive black-body radii as calculated by Celestia and the real radii derived from proper calculations. (We'd need to make sure planet temperatures were calculated from the original bolometric correction, rather than using temperature and this new corrected radius. I can't remember what's actually used at present.)

But given Fridger's desire for purity in these matters, I do wonder if such a cludged approach would be acceptable to him. Since I now view myself as (at best) a maintainer rather than a developer, I certainly have no wish to push an approach that is unacceptable to others.

Grant

[t00fri]

The bolometric corrections used in the lookup table are pretty standard values: so our estimate of total energy output is likely to be reasonably accurate, assuming the spectral class is correct.
The problem comes with the black-body calculation that uses energy output and temperature to calculate surface area, and hence radius. Limb darkening, absorption lines and perhaps large spots all make the black-body assumption faulty for cool stars, and all of the above reduce the actually radiative efficiency of a star's surface, causing Celestia to make it misleadingly small.
I doubt if there is a scientific approach that could generate accurate radii for red giants and supergiants without using prohibitively complex calculations. The alternative, as far as I can see, is yet another lookup table, which lists a set of jigger factors determined by the difference between the naive black-body radii as calculated by Celestia and the real radii derived from proper calculations. (We'd need to make sure planet temperatures were calculated from the original bolometric correction, rather than using temperature and this new corrected radius. I can't remember what's actually used at present.)

I entirely agree...

But given Fridger's desire for purity in these matters, I do wonder if such a cludged approach would be acceptable to him.
...
Grant

No. That's also why I did not answer, so far.

Bye Fridger

[granthutchison]

<duplicate post>

[granthutchison]

There's your answer, guys.
Adieu.

Grant

[ElChristou]

There's your answer, guys.
Adieu.

Grant

Heu... Grant, sorry to ask but... I'm not sure but your last message seems to me... ironic? do you confirm? if yes, could you explain why? (Sorry to ask, but if yes, I don't understand at all )

(In fact it's also the "Adieu" that disturb me a bit because it is used when someone go forever (death or trip without return)... -> irony + "Adieu" = you quit the community?)

[granthutchison]

ElChristou, I quit the community some time ago, and said my goodbyes at that time. As I explained then, I got to a point at which I could make no particularly useful contribution that was of interest to me, and I had other things I could be doing. (I do feel a responsibility to maintain certain data files in the background, which is why I refer to myself as a "maintainer" rather than a "developer" these days.)

The above tweak to star depiction would have been a small development contribution I could have made (a brief guest appearance on the developer list, as it were) but I knew Fridger well enough to guess that such a development would not be welcome. Fridger confirmed that, and so I now drop back into my previous obscurity.
Hence: adieu.

Grant

[Cham]

Whatever what Fridger thinks, that "purity" concept is a thorn in the foot (it's also recalling me the "purity of the race" idea). It's preventing the development of some very interesting features, even if they are approximate. Currently, Celestia is TOTALLY unreliable about its stars. In many cases, it gives completely wrong and absurd radii, so the user can't trust the rest of the database because of these **important** cases (most cool red stars, dwarfs and giants). Celestia is losing credibility, because of this (many of my students were asking embarrassing questions, recently, about the size of stars in Celestia. I'm losing time to explain what is going on in the software). Celestia should AT THE VERY LEAST warn the user that the displayed radius is unreliable.

At the very least, I suggest that Celestia adds the comment "?", next to the radius of all the cool stars.

[chris]

Code: Select all

The problem comes with the black-body calculation that uses energy output and temperature to calculate surface area, and hence radius. Limb darkening, absorption lines and perhaps large spots all make the black-body assumption faulty for cool stars, and all of the above reduce the actually radiative efficiency of a star's surface, causing Celestia to make it misleadingly small.
I doubt if there is a scientific approach that could generate accurate radii for red giants and supergiants without using prohibitively complex calculations. The alternative, as far as I can see, is yet another lookup table, which lists a set of jigger factors determined by the difference between the naive black-body radii as calculated by Celestia and the real radii derived from proper calculations.


Kludgy or not, I'd vote for including such a table of jigger factors into Celestia. The sources of the discrepancy are understood, but there doesnt seem to exist any published table of corrections for cool stars. Given that, it seems fair to resort to a jigger factor in the mean time. It's certainly better than reporting grossly incorrect radii. I haven't the time now to create the table, but I could write code to make use of it in the star radius calculation.

--Chris

[t00fri]

Whatever what Fridger thinks, that "purity" concept is a thorn in the foot (it's also recalling me the "purity of the race" idea). It's preventing the development of some very interesting features, even if they are approximate. Currently, Celestia is TOTALLY unreliable about its stars. In many cases, it gives completely wrong and absurd radii, so the user can't trust the rest of the database because of these **important** cases (most cool red stars, dwarfs and giants). Celestia is losing credibility, because of this (many of my students were asking embarrassing questions, recently, about the size of stars in Celestia. I'm losing time to explain what is going on in the software). Celestia should AT THE VERY LEAST warn the user that the displayed radius is unreliable.

At the very least, I suggest that Celestia adds the comment "?", next to the radius of all the cool stars.

Celestia should ONLY give radii based on solid work as long as there is no mark about inherent uncertainties.

I always found it misleading that Celestia prints star radii out, based on a very primitive estimate, as if it was reading out a sophisticated data base!

At least the output should be of the type

Black-body estimate: R = xxxx

Then every expert knows that the result will be highly uncertain, yet these values may serve as simple 0th-order estimates in absense of better data.

Bye Fridger

PS: The word "purity" approach in the context of Celestia was Grant's invention quite some time ago, NOT mine. . I also find this word highly misleading.

[Cham]

Celestia should say something like "estimated radius", or some kind of symbol ("?", or something else), just to warn the user about the unsafe value.