Living in a Globular Cluster

[McC]

Specifically, what would life be like on a planet in FSR 1767?

FSR 1767 was recently discovered and is thought to contain approx. 10,000 stars (or, 10% of the stars in M4, formerly believed to be the closest GC). It is also a mere 6.5 light years across, which suggests a mean stellar density of ~69 stars per cubic light year (assuming a spherical volume).

Alternately, what would life be like on a planet in M4?

M4 is about 7,200 light years away, and about 55 light years across. If this is correct, it's comprised of about 100,000 stars, for a mean stellar density of 1.1 stars per cubic light year (assuming a sphere).

Specifically, would such a high concentration of stars inundate planets with too much radiation to support humanoid life, or is that not a concern?

[Sea Skimmer]

Higher forms of life might never evolve, because the higher density of stars will increase the frequency with which a planet is hit by the radiation from supernovas, which has something of a sterilizing effect. However I can’t think the day to day radiation levels would be too much for at least some life to exist.

[Turin]

I'd think (although obviously I haven't done the numbers on it), that the effects of gravity in that densely "populated" of an area would be more of an issue for life. Nascent planets could be tugged apart before they ever form. Or even if they do form, a nearby pass from one of the other 68 stars in the same CLY could send in a rain of comets or pull it out of orbit of its primary.

[GrandMasterTerwynn]

Higher forms of life might never evolve, because the higher density of stars will increase the frequency with which a planet is hit by the radiation from supernovas, which has something of a sterilizing effect. However I can’t think the day to day radiation levels would be too much for at least some life to exist.

The other problem with life in a globular cluster is that globular clusters tend to be comprised of very old stars formed early in the history of the Milky Way. As a result, the concentration of heavy elements in globular clusters may not be enough to support the formation of terrestrial planets. And the stellar densities may discourage significant planet formation for nearly all but the outermost members of the globular.

[Junghalli]

Assuming a habitable planet did exist there the big difference you'd notice would be what the night sky would look like. It would be much more bright and crowded than the night sky of Earth. That globular cluster has an average stellar density something like a hundred times that of Earth's region of the galaxy, so the night sky would have a lot more stars. The planet would have seriously awesome night skies.

Other than that the planet's environment should not be significantly effected on a day-to-day basis. A hundred times the density of stars as in our neigborhood should still be far enough apart for those stars to be stars, not suns, from the perspective of the planet. I admit I haven't done the actual math on that, but light years and parsecs are still huge volumes of spaces. Proxima Centauri is 13,000 AU out from Alpha Centauri A/B, but that translates into them being only .14 light years apart. Others have already discussed the long-term effects.

The high stellar density would also make an interstellar civilization without FTL much more feasible. Although the cluster would probably have a relative paucity of habitable worlds comparing to our neighborhood, for reasons already mentioned.

[Junghalli]

I'd think (although obviously I haven't done the numbers on it), that the effects of gravity in that densely "populated" of an area would be more of an issue for life. Nascent planets could be tugged apart before they ever form.

I doubt the stars would be anywhere near that close. To inhibit the formation of Earth another star would have had to orbit around Jupiter's distance from the sun. 70 stars per cubic parsec probably gives you average distances more on the scale of hundreds to thousands of AU. Stars randomly passing close to each other over the long term is a more likely problem.

[Turin]

I'd think (although obviously I haven't done the numbers on it), that the effects of gravity in that densely "populated" of an area would be more of an issue for life. Nascent planets could be tugged apart before they ever form.

I doubt the stars would be anywhere near that close. To inhibit the formation of Earth another star would have had to orbit around Jupiter's distance from the sun. 70 stars per cubic parsec probably gives you average distances more on the scale of hundreds to thousands of AU. Stars randomly passing close to each other over the long term is a more likely problem.

First, it's a cubic light year, not a cubic parsec, which is a good bit smaller volume. Second, it's not like planetary formation happens overnight. "Stars randomly passing close" over a period of even a million years is easily long enough to cause all kinds of disruption.

[Turin]

Just to follow up quickly on that last post, a very quick back-of-the-envelope calculation shows an average distance of ~7500 AU between 70 stars in a 1 LY diameter sphere.

Plugging and chugging those numbers into the formula for force of gravity shows that a sun-mass star at that average distance has roughly 4 orders of magnitude less gravitational pull than Jupiter (vs a hypothetical fixed mass at the same distance as Sol is from Jupiter). But it would only need to pass within 75 AU of to produce similar gravitational forces as Jupiter -- a little less than twice the distance to Pluto.

[Junghalli]

First, it's a cubic light year, not a cubic parsec, which is a good bit smaller volume. Second, it's not like planetary formation happens overnight. "Stars randomly passing close" over a period of even a million years is easily long enough to cause all kinds of disruption.

A light year is 63,240 AU, so even if you arranged the 70 stars into a straight necklace they'd still be seperated by 900 AU. By way of comparison, to disrupt the formation of Neptune another star would have to be within around 150 AU of the sun, and to disrupt the formation of Earth you'd need one seperated by around 5 AU (Jupiter's orbital distance). So as far as normal formation processes goes it shouldn't be an issue.

I really don't know how to calculate how frequent collisions would be though.

[Turin]

A light year is 63,240 AU, so even if you arranged the 70 stars into a straight necklace they'd still be seperated by 900 AU. By way of comparison, to disrupt the formation of Neptune another star would have to be within around 150 AU of the sun, and to disrupt the formation of Earth you'd need one seperated by around 5 AU (Jupiter's orbital distance). So as far as normal formation processes goes it shouldn't be an issue.

I've double-checked my numbers below, so the idea of influence on formation is a good bit more far-fetched than I thought. It also occurs to me that my assumption about "Jupiter's gravity" is probably nonsense; a jupiter-like force will influence planetary formation heavily but not stop it from happening (this should have been obvious in retrospect). But I'm curious as to where you're getting your figures for the distances required to disrupt planetary formation -- we don't need collisions, just enough near-misses over the couple million years it would take for planets to form. Like you, however, I'm not sure how to calculate that.

Just for posterity's sake, my calcs:

sphere of 1 cu LY = 2.53e14 cubic AU / 70 stars = 3.61e12 cubic AU each star
We can assume on average each star sits in a grid with spacing of 15340 AU
This is twice as much as my original back-of-the-envelope calculation above.

Mass of sun-like star = 1.98892e30 kg
Mass of Jupiter = 1.8987e27 kg
Orbit of Jupiter = ~5.2 AU = 7.78e10 m
Fg = (GMaMb)/d^2
Because we're only interested in relative forces vs an unknown second mass, we'll let GMa = a constant C in the calculations below.

1. Relative force of G for Jupiter at 5.2AU from unknown mass
Fg = (GMaMb)/d^2
Fg = (C * 1.8997e27 kg) / (7.78e10 m)^2
Fg = 313852 * C

2. Relative force of G for sun-like star at 15340 AU from unknown mass
Fg = (GMaMb)/d^2
Fg = (C * 1.98892e30 kg) / (2.29e15 m)^2
Fg = .037926813 * C

So at the average distance we have 7 orders of magnitude difference.

3. Distance to star to have same relative force of G as Jupiter, from unknown mass
Fg = (GMaMb)/d^2
(313852 * C) = (C * 1.98892e30 kg) / d^2
d = 2.5173651e12 m = 16AU
A little less than the orbit of Uranus

[Junghalli]

But I'm curious as to where you're getting your figures for the distances required to disrupt planetary formation

Planets in a binary system must orbit within around .19 times the periastron distance between the component stars (if they don't orbit both stars).

I've encountered the 1/5 the distance between the components figure several times in discussion of planets around binary stars, I think I got the exact figure from this paper:

http://www.soest.hawaii.edu/GG/FACULTY/ ... gy2005.pdf

[Turin]

But I'm curious as to where you're getting your figures for the distances required to disrupt planetary formation

Planets in a binary system must orbit within around .19 times the periastron distance between the component stars (if they don't orbit both stars).

Your article doesn't support such a precise answer. Page 7-8 are the relevant sections:

About two-thirds of stars are members of multiple systems. Companion stars would influence planet formation in the same manner as giant planets, but at yet larger distances. Whitmire et al. (1998) studied the conditions under which perturbations by a companion star would prevent
runaway accretion of planetesimals in the HZ. They derived criteria based on binary semimajor axis (a), eccentricity (e), and stellar masses. For example, habitable planets will form in a 2 solar mass system with an eccentricity of 0.5 only if a > 32 AU.

Here they're referring to the earlier section in which gas giants can "affect the formation of terrestrial planets within the HZ by accelerating planetesimals to crossing speeds where destruction rather than aggregation occurs, inhibiting embryo formation..." They continue, however, to indicate that gas giants outside of 4AU appear to increase the formation of "large" (Earth-like) bodies in the HZ. So we should see similar effects with binaries at distances > 32AU.

That being said, the article goes on to discuss the work of Benest from 1988 on, where he concludes that "stable planetary orbits exist at radii of roughly half the binary’s periastron separation," although you'll notice that the experiment Benest calculated is a very simple system with no other bodies (i.e. a very hypothetical one). That distance is going to vary significantly depending on the masses of the two stars and if there are other objects involved.

It appears that the existence of other large bodies (if at a sufficient distance) can improve the chances of Earth-size worlds in stable orbits in the habitable zone by creating stochastic perturbations in the protoplanetary disc. But to say that "it happens at N.02 distance in binary systems" is overstretching, at least from what you've provided here.

[Junghalli]

My bad, I think it was actually this paper. It occurred to me that I posted the wrong one as I was buying dinner.

http://arxiv.org/PS_cache/astro-ph/pdf/ ... 2706v1.pdf

[Turin]

Thank you, interesting read. For anyone reading in who doesn't want to slog through a 27 page PDF:

An important result shown by figure 7 is the existence of a trend between the binary perihelion distance and the location of the outermost terrestrial planet. Figure 8 shows this for a set of different simulations. The top panel in this figure represents the semimajor axis of the outermost terrestrial planet, aout , as a function of the binary eccentricity, eb . The bottom panel shows the ratio of this quantity to the perihelion distance of the binary, qb . As shown here, simulations with no giant planet (shown in black) follow a clear trend: Terrestrial planets only form interior to roughly 0.19 times the binary perihelion distance. This has also been noted by Quintana et al. (2007, see their figure 9) in their simulations of terrestrial planet formation in close binary star systems. The smallest binary perihelion that allows terrestrial planets to form outside the inner edge of the habitable zone (0.9 AU) in these systems is simply 0.9/0.19 = 4.7 AU, comparable to the estimate by Quintana et al. (2007). In binaries with no giant planets, Sun-like primaries with companions with perihelion distances smaller than approximately 5 AU are therefore not good candidates for habitable planet formation. It is, however, important to note that, because the stellar luminosity, and therefore the location of the habitable zone, are sensitive to stellar mass (Kasting, Whitmire & Reynolds 1993; Raymond et al. 2007), the minimum binary separation necessary to ensure habitable planet formation will vary significantly with the mass of the primary star.

In simulations with giant planets, figure 8 indicates that terrestrial planets form closer-in. The ratio aout /qb in these systems varies between approximately 0.06 and 0.13, depending on the orbital separation of the two stars. The accretion process in such systems is more complicated since the giant planet’s eccentricity and its ability to transfer angular momentum
are largely regulated by the binary companion.

Note the bolded sections, of course, and that the first paragraph refers only to the trend found in binary systems without a gas giant outside the HZ.