What are the Largest stars in Celestia ?

TNK · Post #1by **TNK** » 18.05.2003, 03:27

Hi all !

I am working on an astronomy tutorial using Celestia (something graphic that should "visually" grab the kids.
I wonder, could someone tell me what the biggest (in rsun) stars are in celestia ? The biggest I've found is Tet. Musca (698 rsuns). Is it the largest ?

Tnx.

TNK

ElPelado · Post #2by **ElPelado** » 18.05.2003, 08:37

in my celestia, Tet. Musca has only 19.72 Rsun, not 698.
and i know that Antares(alf sco) is 449.59 Rsun.
so someone has an error on tet musca

Guest · Post #3by **Guest** » 18.05.2003, 12:58

What version of Celestia (& what add-ons) do you have ?
The largest star cannot be Antares. The Garnet star for exemple has (and is in my celestia) to be much larger (my Antares has BTW 260 Rsuns).
Hmmm...that's strange indeed. I wonder how reliable those datas are...
Where do those datas come from ? (I've read it comes from some star library like the Hipparcos...we indeed have those HIP numbers).

ElPelado · Post #4by **ElPelado** » 18.05.2003, 13:06

i have the 1.3.0 versio nof celestia. and my add-ons doesnt change those stars. so i dont know what to say.

Post #5by **granthutchison** » 18.05.2003, 13:15

The radius is calculated from the temperature appropriate for the spectral class, and the apparent magnitude - Celestia works out what surface are the star should have in order to appear so bright. So it will give wrong answers if the distance is in error.
Celestia 1.3.0 also underestimates the radius of very blue and very red stars, because it uses the visual rather than bolometric magnitude. A bolometric correction will be introduced in the next version, and that should give better results.

Grant

jrobert · Post #6by **jrobert** » 19.05.2003, 05:11

I found HIP 500053 which appears to be over 700 Rsun...

cel://Follow/HIP%20500053/2003-05-19T03:57:47.42251?x=AAAAInxYWfyteX3R/////w&y=AAAAyqVqyaBMsTGY/////w&z=AAAA2kP7QlzIzT7M/v///w&ow=0.932974&ox=0.227594&oy=-0.073990&oz=0.268859&select=HIP%20500053&fov=45.000000&ts=1.000000&rf=38807&lm=0

ElPelado · Post #7by **ElPelado** » 19.05.2003, 09:51

well well well, it seems that i have a problem on my stars.
you said that it has only 700 Rsu, well in my celestia, it has a little more than that: 2420 RSun!!!!!
i can not even get to the point where the earth should be(1 AU from teh star)

Post #8by **granthutchison** » 21.05.2003, 12:59

jrobert wrote:I found HIP 500053 which appears to be over 700 Rsun...

You must have an add-on of some kind - that's a non-Hipparcos catalogue number, and Celestia's stars.dat reaches maximum Hip number at EPS Tuc: 118322.

Grant

jrobert · Post #9by **jrobert** » 23.05.2003, 03:00

Yeah, I have Rallison's NGC2237 Nebula add-on and also the v2.1C star cagalog by someone (I forgot who)... :oops:

but that is by far the largest star I found yet.

praesepe · Post #10by **praesepe** » 23.05.2003, 03:29

jrobert wrote:Yeah, I have Rallison's NGC2237 Nebula add-on and also the v2.1C star cagalog by someone (I forgot who)... but that is by far the largest star I found yet.

This is why you have HIP stars up to 500053 number, the NGC2237 (Rassilon's Rosseta nebula add-on) uses a fictional star cluster generated by his globular cluster generator, so all stars there are just that, fictional :wink:

jrobert · Post #11by **jrobert** » 29.05.2003, 20:42

I found a star that IS in the default distribution that comes with Celestia. This star is HD 184283 and according to the data provided, is 871.92 Rsun.

jrobert · Post #12by **jrobert** » 18.06.2003, 17:37

In 1.3.1 pre 4, HD 184283 is now 10194.45 Rsun!!

(And that's how it is without any add-on star databases) That can't be right, can it? I figure any star that big would begin to collapse in on itself almost immeadiately.

Post #13by **granthutchison** » 18.06.2003, 17:41

Sounds like this star is a victim of a parallax error - Celestia calculates the intrinsic luminosity from the apparent magnitude and the parallax. If the measured parallax is too low (implying a large distance), this gives a false impression that the star is very luminous, and therefore very large.

Grant

jamarsa · Post #14by **jamarsa** » 18.06.2003, 17:57

In 1.3.1 pre 4, HD 184283 is now 10194.45 Rsun!! icon_eek.gif (And that's how it is without any add-on star databases) That can't be right, can it? I figure any star that big would begin to collapse in on itself almost immeadiately

In my celestia (1.3.0), is listed as 6032.22 Rsun. Too big to be credible...

I have found 72 Leo too, (by trial and error in the Celestial Catalog), which is 1423.78 Rsun.

My favorite is Deneb, 228.09 Rsun. I guess there are larger stars out there, though.

Evil Dr Ganymede · Post #15by **Evil Dr Ganymede** » 18.06.2003, 18:02

I've found a lot of sources on the net that claim Antares is supposed to be one of the biggest stars around, with a radius of between 4 and 5 AU, which is between 862 and 1077 solar radii.

For it to be 5 AU in diameter, the apparent magnitude in Celestia has to be about -1.5. Is the way that Celestia figures out the radius from the luminosity accurate, or just an approximation?

Post #16by **granthutchison** » 18.06.2003, 18:26

There are a couple of approximations involved. First, there's a conversion from absolute visual magnitude to total energy output - that's based on the temperature of the star for its given spectral class, and is just a reverse application of the idea of luminous efficacy we discussed on another thread. It assumes that the star behaves like a black body radiator, which is a good approximation for all but the very coolest stars, which begin to have enough molecules in their atmospheres to produce a significant density of absorption lines.
Then Celestia works out the number of watts per square metre a black body of the star's assumed temperature would radiate. Knowing the total energy output and the number of watts per square metre allows the calculation of the necessary surface area to produce the observed energy output, and hence a radius.
The problem with red giants and supergiants is, I think, that this surface area calculation effectively assumes that the star has a uniformly radiating disc - looking like the Sun, with a hard cut-off to the visible photosphere.
But the red giants are so rarified in their outer envelopes that they'd (I guess) appear as a bright central glow that just gradually fades off into dimness. Celestia is therefore fooled into underestimating the radius for a given luminosity in this class of stars.

Of course, if the database provides the wrong distance, all accuracy goes out the window, and I think this is probably where the implausibly large stars come from - they've been assigned too low a parallax.

Grant

pint · Post #17by **pint** » 18.06.2003, 21:58

is that true? red giants seem to be brighter in the middle than at the perimeter? if so, they should look very interesting! i want to see a red giant!!!

Post #18by **granthutchison** » 19.06.2003, 00:22

pint wrote:is that true? red giants seem to be brighter in the middle than at the perimeter?

Well, the gas at the outer limit of their photospheres is several orders of magnitude more rarified than air. Even the Sun shows a marginal reduction in brightness at the edges, if you project its image on to a white screen - so my guess is that a red giant would be a huge ball of light that just faded to black at the margins.
I'm prepared to be convinced otherwise if anyone has a counterargument, but it does seem to be internally consistent with the problem Celestia has in underestimating the radii of these stars.

Grant