Omega Centauri, 47Tuc, M2: which is the largest?

[mental4astro (Alexander)]

Hi all,

My recent sketch of 47Tuc has provoked a question about the three remnant core globular clusters that orbit the Milky Way - Which is the largest?

My question comes from the difference in appearance between Omega and 47Tuc. Angular size would suggest Omega is closer, yet I suspect that due to the fainter component stars of 47Tuc, 47Tuc may actually be a larger system.

Am I correct? And where would this place M2?

Cheers,

Alex.

[GrampianStars (Rob)]

Alex
here you go
http://www.astropix.com/HTML/L_STORY...T/GLOBULAR.HTM

[Robh (Rob)]

Hi Alex,

Are you implying anything by "remnant core". Omega Centauri is speculated (unproven) to be the remnant core of a dwarf galaxy. I'm not aware that 47 Tuc or M2 has been put in this category.

This info is from wikipedia, omitting error ranges ...
omega Centauri, distance 15800 ly, angular size 36', maybe 5000000 stars, diameter 172 ly.
47 Tucanae, distance 16700 ly, angular size 31', maybe 1000000 stars, diameter 120 ly.
M2, distance 37500 ly, angular size 16', maybe 150000 stars, diameter 175 ly.

In comparison, the Sagittarius globular M54 is 87400 ly away, angular size 12' and is thought to be 306 ly in diameter!

[mental4astro (Alexander)]

Rob (Grampian), thanks for the link. I'm actually after an assesment of their actual size rather than their apparent one.

Robh, thanks for clearing up a point. From some articles I've come across, the three GC's mentioned had been pooled together into that 'remnent core' category. I now see that there is still uncertainty to this being so. The reasons being given to all three being remnent cores is the popullation types of the component stars. Their compositions are much more complex with heavier elements being present than that of "normal" GCs, suggesting extragalactic origins.

Holy cow! Check out the size of M54!!! Now, dare I plonk M54 in that remnent core hole too!

[madbadgalaxyman (Robert)]

The problem with "diameters" of globular clusters is that there are different ways to measure this parameter, e.g. the angular radius or diameter containing half the total light, or the radius containing 90% of the total light.
(and the measured diameters for the same object can therefore be incompatible)

As far as how to find the actual physical size of a specific globular, from its angular diameter, the first thing we have to know in order to calculate the size is the actual distance of the globular.

This link to a (year 2010) catalog by Willam E Harris, a noted researcher, gives reliable distances and other properties for all of the Milky Way's globulars.

http://www.physics.mcmaster.ca/~harris/Databases

An interesting discussion, folks!

I might have a go at comparing the physical sizes of these globulars, and a few others.

[madbadgalaxyman (Robert)]

Oh, one more thing...

Incidentally, the concept that a globular star cluster could be what is left after the rest of a galaxy is tidally stripped is very physically plausible.

Many dwarf spheroidal galaxies and low-mass spiral galaxies contain an object near to the centre of the system that is very similar to a globular cluster.