Appearance of stars in the midst of a globular cluster?

[potoole]

To an entity situated on a planet, for instance, that lies within a globular cluster, how bright would the stars of the cluster appear? Would the stars appear to be close together? Would the additive light of all those stars be quite bright? Would the stars still appear as points of light? Would the stars be light years, light days, light minutes apart? Could a planetary system be possible within such a cluster?

Thank you
Patrick

[chornedsnorkack]

To an entity situated on a planet, for instance, that lies within a globular cluster, how bright would the stars of the cluster appear? Would the stars appear to be close together? Would the additive light of all those stars be quite bright? Would the stars still appear as points of light? Would the stars be light years, light days, light minutes apart? Could a planetary system be possible within such a cluster?

Thank you
Patrick

Globular clusters differ - as do positions inside them.

Globular clusters being old (excluding for now young globular clusters like Tarantula, not found in Milky Way unless Westerlund 1 qualifies), they do not contain any supergiants, these having completed fusion. Absolutely brightest stars in globular clusters would be the stars currently passing through red giant stage - or Type I supernovae when these happen.

1 light day is 180 au. 1 light minute is 1/8 AU. Both are common values for distances between binary stars.

How do frequencies of multiple stars at various periods inside globular clusters compare to frequencies of multiple stars of similar periods on field?

Regarding visibility of discs - since massive stars cannot exist in globular clusters, the biggest should be late M class red giants, like mirids. Mira is 300...400 solar radii.

Venus, at 1 arc minute across, is hard to tell apart from a point. Sun would be 1 arc minute across at Neptune, 30 AU, but since it is rather more blinding than Venus, it would not be smart to try to see whether it is spot. Perhaps at 10...15 AU could its non-spot nature be spotted? Then a mirid could be told apart from a spot at 3000...6000 AU.

Are mirid variables common in globular clusters?

Apart from the dim FSR1767, M4 is the closest globular cluster, at 7200 lightyears. Its brightness is variously quoted as +5,6 and +7,1. When it is +5,6, it is a naked eye object in Scorpio, near Antares. Yet it is dim and loose for a globular cluster - bright only because near us

It extends as much as 36 arc minutes across - bigger than moon disc. However, much of the 36 minutes is dim edges.

How does the view through a cluster along a narrow beam/field of view compare against a view from inside the cluster?

We have the opportunity to compare the view from inside the disc of a spiral galaxy (Milky Way) with the view from outside near the plane of another (Andromeda Nebula).

Andromeda Nebula is 190 minutes long and has magnitude +3,44.

Magnify Andromeda Nebula 32 times. It will then span 100 degrees - and have total magnitude -4.

Now magnify M4 160 times - comparable to viewing it at 45 light years, but not equivalent due to perspective. It would span 96 degrees in the eyepiece (remember, 100 and 110 degrees TeleVue eyepieces exist), making sure that you have the adequate light collecting power (at least 1000 mm aperture, better 1100 mm). It would be brightened 11 magnitudes - being -5,4. Brighter than Andromeda - but since unlike Andromeda it is circular, its average luminosity would be only slightly brighter.

But does anyone know the exact distribution of the luminosity inside M4?

At 160 magnification, the bar, 2,5´ long, would be magnified to 7 degrees, and the brightest stars (+10,8) would be -0,2. "Half-mass" radius is quoted as 3,65´. How is that known? How much of the mass of M4 (quoted as 67 000 solar) is known to be borne by black holes, how much by neutron stars, how much by G dwarfs, how much by K dwarfs, how much by M dwarfs and how much by brown dwarfs?

A planet is known to exist in M4 - a circumbinary at 23 actually. So much for globular clusters disturbing planetary orbits.

[neilzero]

Likely chorn is correct. There are a few normal planets, but rather rare; perhaps many "planets" brown dwarfs and lesser bodies following random paths between the stars and compact stars. Possibly moon light is average brightness, which means cold is normal except for close approaches several times per million minutes. Neil

[Romanus]

For a planet in the middle of a globular cluster, the sky would be unbelievably spectacular. Robert Burnham, Jr. gives us a good idea in his entry on Messier 13:

[The sky] would be filled with uncountable numbers of blazing stars which would dwarf our own Sirius and Canopus to insignficance. Many thousands of stars ranging in brilliance from Venus to the full moon would be continually visible, so that there would be no real night at all on a planet in a globular cluster...[Their view beyond the cluster] would be completely blocked by the brilliance of their own skies. (p. 987)

Then again, their density and very low metallicity make them unfavorable environments for any planet, certainly any habitable ones. The following link has much more relevant information:

http://www.iac.es/gabinete/iacnotici...8/xplaneta.htm

[chornedsnorkack]

For a planet in the middle of a globular cluster, the sky would be unbelievably spectacular. Robert Burnham, Jr. gives us a good idea in his entry on Messier 13:

[The sky] would be filled with uncountable numbers of blazing stars which would dwarf our own Sirius and Canopus to insignficance. Many thousands of stars ranging in brilliance from Venus to the full moon would be continually visible

M13 is estimated at 600 000 solar masses. But this is indirect estimate.

What can be directly checked is the number of stars by magnitude, and their distribution across the viewing direction.

M13 is guessed to be 8000 parsecs away, and said to be 23 arc minutes diametre.

The single brightest star, variable V11, is +11,95.

At 10 pc from the centre (distance modulus +14,5), the observer would be inside the cluster (4 arc minutes away, while the edge is 11,5 arc minutes).

At that distance, V11 would be still just -2,5. Yes, brighter than Sirius - but is it really "dwarfing Sirius to insignificance" at just 1 magnitude brighter? It would be dimmer than Venus and about as bright as Jupiter or Mars.

Say you go closer... V11 would be -4,5 at 4 pc. But exactly how many stars does M13 have between magnitudes 12 and 15? Seeing that the very brightest is +11,95, the stars in the range above +13 should be easy to count. How many are there? Facts?

Their distribution is, of course, unknown. For while we see all stars in the centre of M13, we have no way of knowing which they are. What we see is cylinder through the centre. Which contains all stars within the cylinder´s radius of the centre, but also all stars within the cylinder in the outskirts of the cluster in front of the centre, and all stars within the cylinder in the outskirts of the cluster behind the centre.

Note that M13 is old. About as old as the total age of Sun.

Meaning that only stars as massive and bright as Sun, and smaller, are left on main sequence.

A Sunlike star in M13 would be +19 for us.
And thus, for an observer at 4 pc, still +2,5.

At 4 pc, all bright stars would be giants or nearby dwarfs - most of the main sequence in the cluster would be either dim stars or invisible.

The whole M13, at +5,8, is 300 times brighter than V11.

How many individual stars in M13 are brighter than main sequence turnoff at +19 or so? How many brightest stars carry half the total light?

The entire M3, at +5,8, fits in a cylinder with diametre 23 minutes. What is the diametre of the cylinder from which half of that M13 light comes?(The sphere where half light comes from, if it could somehow be identified, would obviously have a bigger diametre.)

Oh, and to get V11 as bright as Moon, you need to approach to 0,1 pc. Possible, obviously. But how many of the other stars are so near? Unknown, because the distances are unknown. But, say, draw a cylinder of 2,5 arc second radius around V11. How many stars brighter than +20 are there?

[potoole]

Thank you, all.

Patrick