Comments on: Globsmacked

By: amphiox

amphiox — Sun, 29 Jul 2012 11:24:22 +0000

It was my understanding that planetary nebulae and supernova remnants don’t last very long on astronomical time scales, before the gases making them up disperse so much that they are no longer recognizable. So one would expect them to be quite rare in things as old as globular clusters?

Supernova remnants especially so, since, the stars originally in the cluster big enough to produce supernovae would surely have all exploded billions of years ago.

By: Regner Trampedach

Regner Trampedach — Sun, 29 Jul 2012 00:48:41 +0000

TMB @ 21 and Artor @ 18: Most of the stars are on highly elliptical orbits, randomly oriented in space, giving the spherical distribution of stars. The orbits going almost radially towards the core are called plunging orbits. So each star in its orbit, feels the total gravitational potential from all the other stars. There are enough stars to make this potential smooth and very close to constant in time. In such a situation the stellar orbits would all be stable, in that each star cannot change its total energy (kinetic + potential energy).
You need close encounters between stars, where they can feel each others gravity, more than they can the total cluster field. But just two stars colliding (in physics, objects don’t need to touch to have undergone a collision – as long as their mutual forces [gravity or electric] change their course, they are said to have collided) won’t change much. That is because the stars can’t exchange angular momentum, L, since they are spherical (L = vector{r}dotvector{v}= the dot-product of the stars position relative to the center of the cluster, with its velocity – in circular orbit it is just radius of orbit times orbital velocity. L is a conserved quantity, just like energy).
In a collision between one star and a binary system, on the other hand, angular momentum can be exchanged between the orbit of the binary and the cluster orbit of the intruder – sometimes resulting in a closer (harder) binary system and the intruder in a higher energy orbit – sometimes thrown out of the cluster. In this way a cluster can slowly evaporate, while its core gets denser.
As the cluster orbits the galaxy (highly elliptical randomly oriented – just like the stars in the cluster. That’s why the Milkyway’s halo of globular clusters is spherical like the clusters themselves) it will most likely pass through the plane of the galaxy (where gravity is large and changes sign), stripping the most loosely bound stars by tidal forces (different gravity felt on opposite sides of the cluster).
BigBob @ 15: “Collisions like that are so rare they’re non-existent in the volume of space our Sun occupies”
The BA was right here. Our neck of the Milkyway has a density of stars that is so low that collisions are exceedingly rare – you’d be better of getting a lottery ticket than waiting for two stars to collide the next billion years… Space is big!
Why did you think the BA meant “…space our galaxy occupies.”?

I hope that answered some questions – and sorry it got so long…
Cheers,
Regner

By: TMB

TMB — Sat, 28 Jul 2012 22:41:26 +0000

@Artor: In general, the random velocities of the stars are enough for them to maintain the shape… you could imagine that you take a rotating disk galaxy and tilt the orbit of each star by a random amount. Each star is then still on a stable orbit, but instead of a flat disk you have a spherically-symmetric cluster. (that’s not exactly what happens – the stars aren’t typically on circular orbits – but it gives you kind of an idea).

By: Jon Hanford

Jon Hanford — Sat, 28 Jul 2012 17:06:21 +0000

@7 jackd:

“I get that there aren’t any large gas or dust clouds in clusters, but are there planetary nebulae and supernova shells, just too faint to see at this distance?”

There are four planetary nebulae known to be associated with globular clusters in our galaxy -

Pease 1 in M 15: http://messier.seds.org/more/m015_h2.html

GJJC 1 in M 22: http://messier.seds.org/more/m022_pn.html

JaFu 1 in Palomar 6 &
JaFu 2 in NGC 6441: http://www.blackskies.org/JaFu_challenge.htm

(more info here: http://messier.seds.org/xtra/supp/gc_pn.html )

I don’t know of any supernova remnants (aside from pulsars) associated with globular clusters, but as MTU notes above, they along with their massive progenitors are likely long gone.

By: James Evans

James Evans — Sat, 28 Jul 2012 16:14:36 +0000

…because they’re amongst the most ancient metal-poorest stars.

Once we perfect ion drives, or plasma thrusters, or wormholes, or whatever, we’ll have to send Sepultura there for the M5 Globular Cluster Tour. That should raise the level of m/etal in the galactic neighborhood sufficiently.

By: Artor

Artor — Sat, 28 Jul 2012 15:17:54 +0000

I’ve always wondered about the gravitational dynamics in a globular cluster. It’s easy to see how a spiral galaxy can maintain a stable configuration for billions & billions of years, but how does a glob stay globular without collapsing into a common center? Or it it doing just that, hence the prevalence of “blue stragglers.” For that matter, how does an elliptical galaxy keep it’s figure?

By: Messier Tidy Upper

Messier Tidy Upper — Sat, 28 Jul 2012 15:15:19 +0000

D’oh! Wrong thread. Thought we were talking about this one :

http://blogs.discovermagazine.com/badastronomy/2012/07/26/coathook-to-the-stars/

That & my overtiredness explains it. Sorry.

By: Messier Tidy Upper

Messier Tidy Upper — Sat, 28 Jul 2012 14:35:21 +0000

@ ^ BigBob : I reckon so! Otherwise we’re in an improbable amount of trouble!

Incidentally, does anyone else think Anser et vulpeculae – “the Fox and the Goose” – is the closest thing among the 88 constellations to a pub name?

@13. Jess Tauber : “Perhaps, then, these globulars might be good hideouts, holes in the wall, as it were, for aliens who have gotten on the bad side of some of their neighbors?”

Hmm .. yeah perhaps. If they can find or make themselves a world to hide in there!

@10. Jess Tauber : Now where did that second comment of yours suddenly come from interrupting my ^ symbol dangnabbit! Sigh.

Oh well, guess it all make sense kinda anyhow.

As for your suggestion there, well it sounds like a Stephen Baxter / Isaac Asimov / Brinford type idea. Dunno if you plan on writing it but you could well do. Neat thought.

By: BigBob

BigBob — Sat, 28 Jul 2012 12:10:57 +0000

“Collisions like that are so rare they’re non-existent in the volume of space our Sun occupies”

I wonder if you meant space our galaxy occupies?

Bob

By: Jess Tauber

Jess Tauber — Sat, 28 Jul 2012 11:22:54 +0000

Perhaps, then, these globulars might be good hideouts, holes in the wall, as it were, for aliens who have gotten on the bad side of some of their neighbors. If it were me, though, and I had the kind of advanced tech needed for such an endeavor, I’d probably just give my species a thorough biomolecular and morphological makeover so nobody recognized us. A good memory wipe as well and you’re home free.