Celestia Forums

I searched the Internet for largest known galaxy, and its IC 1101.
Wikipedia puts its diameter at 5-6 million lightyears.
In Celestia, its radius is 1.832e+05 ly, which would only be a diameter of 366,400 ly.
It seems that there's a little discrepancy here.
So which is wrong?

TranslightDefender wrote:I searched the Internet for largest known galaxy, and its IC 1101.
Wikipedia puts its diameter at 5-6 million lightyears.
In Celestia, its radius is 1.832e+05 ly, which would only be a diameter of 366,400 ly.
It seems that there's a little discrepancy here.
So which is wrong?

The linear diameter in ly of a galaxy is measured by means of its angular size in radian and it's distance in ly:

angular size =diameter/distance


Both the reliable RC3 galaxy catalog and the SIMBAD world database give a similar value for the angular size ~ 1.2 - 1.4 arcmins. However, the distance of IC 1101 is largely unknown! The paper that is referred to in the IC 1101 Wikipedia article (that you presumably used) is not a standard accepted scientific reference. Correspondingly, there is no distance value of IC1101 in the NED1d catalog on galaxy distances. As a rough estimate of the distance I therefore used Hubble's law for Celestia's data base (galaxies.dsc). The resulting diameter and distance are consistent with the observed angular size.

If you want a much larger diameter, the distance must be also much larger, in order to match the reasonably well known angular size. In any case there are very large uncertainties involved. Since IC 1101 is the central object in a galaxy cluster, the distance from Hubble's law (and the spectroscopically determined redshift z) might receive large corrections due to so-called peculiar (i.e. non-radial) velocity components induced by other gravitating cluster members!

Fridger

I thought it might be something like that, that the distance isn't known very well.

It seems that the Google results say a diameter of 5-6 million lightyears frequently.
A blog, answers.com, and Wikipedia say that its diameter is ~5 million ly, yet at the same time say its 1 billion ly away, which is the distance Celestia has it at.

TranslightDefender wrote:I thought it might be something like that, that the distance isn't known very well.

It seems that the Google results say a diameter of 5-6 million lightyears frequently.
A blog, answers.com, and Wikipedia say that its diameter is ~5 million ly, yet at the same time say its 1 billion ly away, which is the distance Celestia has it at.

I am only using scientific resources, i.e those that professional astronomers consult. Numbers are worth nothing in this context as long as you don't know about their uncertainties as well! The fact that the NED1d data base for galaxy distances doesn't have an entry for IC 1101 means that there is NO serious distance value available, where the uncertainty can be sensibly estimated.

Once more: the ratio of diameter/distance is relatively well known.

When you read 5-6 million ly as diameter, you always need to consult the original scientific source NOT some obscure webpage that copied such numbers from somewhere. The essential question is whether there are several independent scientific sources for a distance leading to 5 million ly diameter by means of the reliable angular size of 1.2-1.4 arcmins! The only alternative method of extracting a IC 1101 distance is via Hubble's law (that I used). Possibly some of your resources also used this method whence the agreement.

Fridger

The Celestian size is wrong. IC 1101 is 5.8 million light years.

Actually, sadly for a software that is made to be education and 100% accurate, Celestia gets most sizes (especially that of stars) wrong.

Joey P. wrote:The Celestian size is wrong. IC 1101 is 5.8 million light years.

Actually, sadly for a software that is made to be education and 100% accurate, Celestia gets most sizes (especially that of stars) wrong.

Well, you're right on that one. While I don't know a lot of examples, I do notice Antares as having a bigger radius than Betelgeuse, whereas in reality, Antares is slightly smaller than Betelgeuse. Also without putting the Largest Stars database, the size of Mu Cephi is at an impossible 3900 radi! So far, not a single star has reached that radi, and most likely the Laws of Physics would prevent stars from reaching those sizes.

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For Reference:

I know about Mu Cep. Many Italian astronomy websites have relied on Celestia and have accepted the 3,900 solar radii figure. I even saw a Brazilian Portuguese YouTube video that made Mu Cephei that big, and VY Canis Majoris at 7,720 sr!

The largest possible star size (not including Quasi-stars) is 2,800 solar radii.
Quasi-stars are 35,000 solar radii.

Joey P. wrote:I know about Mu Cep. Many Italian astronomy websites have relied on Celestia and have accepted the 3,900 solar radii figure. I even saw a Brazilian Portuguese YouTube video that made Mu Cephei that big, and VY Canis Majoris at 7,720 sr!

The largest possible star size (not including Quasi-stars) is 2,800 solar radii.
Quasi-stars are 35,000 solar radii.

If I remember, the 2800 solar Radii was considered as an estimate for a certain star (forgot the name), and it was the highest estimate. The lowest estimate was around 1400 I think.

As for Quasi-Stars, none of them exist today but it would be interesting for a modder to add some Quasi-stars for reference.

I used an .stc to modify HIP 102276 (which is severely overestimated at 5,000 solar radii) into a Quasi-star.
The star that had 2,800 for maximum (actually 2,850) is KY Cygni.

Also, the largest galaxy is now J1420-0545 at 25 million light years wide.

Joey P. wrote:Also, the largest galaxy is now J1420-0545 at 25 million light years wide.

Those are amazing discoveries indeed. But I know there are more of them out there, because the farther we see, the more common they will appear

Lafuente_Astronomy wrote:Well, you're right on that one. While I don't know a lot of examples, I do notice Antares as having a bigger radius than Betelgeuse, whereas in reality, Antares is slightly smaller than Betelgeuse. Also without putting the Largest Stars database, the size of Mu Cephi is at an impossible 3900 radi! So far, not a single star has reached that radi, and most likely the Laws of Physics would prevent stars from reaching those sizes.

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For Reference:

Well we all remember in 2006 when we thought VY Canis Majoris was around that radius or over, and almost right afterward another calculation put it anywhere from 1800 to 2100 solar radii, though it wasn't til we approached 2012 and got a much more accurate calculation for its radius which's 1420 solar radii with a margin of error of only +/- 120 solar radii.
Though in late 2013, Matsuura and others put forward a competing method of finding its radius within the envelope, putting the value at a vague 2069 solar radii.

The latest sources are still years old but the most up to date.
https://arxiv.org/abs/astro-ph/9811280
https://arxiv.org/abs/1203.5194
https://ui.adsabs.harvard.edu/abs/2013A%26A...559A..93A/abstract
https://arxiv.org/abs/1310.2947

Eric Nelson, please don't necropost. You've already been warned about this. (Yes, I realize the thread was already necro'd by Joey P. in 2019, but that doesn't mean you should do it again.)

Edit: Thread locked.