#core collapse

M68 is currently 33,000 light years from us, but orbits in an eccentric path that takes it out to 100,000 light years from the core, and back in again.

The centre shows signs of core collapse and rotation, this is when the mass at the centre begins to pull in the mass around it. Likely a collection of black holes that have fallen towards the centre and now are pulling more and more in. If left to it's own devices it will merge and create a intermediate black hole (if not already) and then later a supermassive blackhole, like galaxy cores.

It is thought to have been captured from a dwarf galaxy sometime in the Milky Ways distant past.

Messier 69 is metal rich for a globular cluster, meaning it has stars that are made up of material that has been through a few generation of stars before it.

It's relatively close to the galactic bulge at just 5,200 ly and 28,700 from Earth.

Like M68, M70 is another globular cluster undergoing core collapse, making the central region more dense and packed with the larger stars, while the smaller ones move out towards the edges.

From SpaceTelescope.Org Picture of the Week; April 9, 2012:

Tight and Bright

In this image, the NASA/ESA Hubble Space Telescope has captured the brilliance of the compact center of Messier 70, a globular cluster. Quarters are always tight in globular clusters, where the mutual hold of gravity binds together hundreds of thousands of stars in a small region of space. Having this many shining stars piled on top of one another from our perspective makes globular clusters a popular target for amateur skywatchers and scientists alike. Messier 70 offers a special case because it has undergone what is known as a core collapse. In these clusters, even more stars squeeze into the object's core than on average, such that the brightness of the cluster increases steadily towards its center.

The legions of stars in a globular cluster orbit about a shared center of gravity. Some stars maintain relatively circular orbits, while others loop out into the cluster's fringes. As the stars interact with each other over time, lighter stars tend to pick up speed and migrate out toward the cluster's edges, while the heavier stars slow and congregate in orbits toward the center. This huddling effect produces the denser, brighter centers characteristic of core-collapsed clusters. About a fifth of the more than 150 globular clusters in the Milky Way have undergone a core collapse.

Although many globular clusters call the galaxy's edges home, Messier 70 orbits close to the Milky Way's center, around 30 000 light-years away from the Solar System. It is remarkable that Messier 70 has held together so well, given the strong gravitational pull of the Milky Way's hub.

Messier 70 is only about 68 light-years in diameter and can be seen, albeit very faintly, with binoculars in dark skies in the constellation of Sagittarius (The Archer). French astronomer Charles Messier documented the object in 1780 as the seventieth entry in his famous astronomical catalog.

This picture was obtained with the Wide Field Camera of Hubble’s Advanced Camera for Surveys. The field of view is around 3.3 by 3.3 arcminutes.

Messier seems to have a thing for Globular clusters, at least at the start of his catalogue, although to be fair to him, his comments on these objects explains this somewhat.

This Messier described M12 as "A nebula without stars!"

It's easy to forget in a time of Hubble and JWST that Messier was dealing with instruments not that much better than a good pair of binoculars, and to him the object looked like a giant fuzzy patch, with no individual stars showing.

Messier 12 is odd in that it has a surprisingly low amount of the longest lived low mass stars (red dwarfs, K and G type stars), and with an age of 13.8 billion years old (pretty much the age of the universe itself), it's clearly passed through the milky way so many times, many of the lower mass stars have been ejected or found themselves pulled away, and likely now still live on in the central bulge.

M13 - The Great Hercules Cluster

The brightest of the stars in this cluster are the red giants, one in particular V1554 Hercules has an apparent mag 11.95 from 23,000 light years.

Interestingly, there's some blue stars here, but they are not young hot B or O type stars you find in Open Clusters, they are what are known as blue stragglers. Stars are so compressed in the heart of globular clusters that they do regularly merge, and when that happens, a blue straggler can be the product.

M14

Some 30,000 light years away from us in the constellation of Ophiuchus, it's 100 light years across and in total, 400,000 times the mass of our star, 10 times less than the supermassive black hole Sag A*.

M15 - The Great Pegasus Cluster

One of the most densely packed globular clusters known, so much so, the core (central region) has begin to contract in what is known as core collapse, where the mass of stars is so intense, it begins to pull in mass from around it, and most likely forming a intermediate black-hole, although none has ever been confirmed as such.

It is also the only globular cluster to ever be found to have a planetary nebula, Pease 1

When you're interest extends to astronomy, you quickly realise that in almost every area, we have learned so much, yet are still so far off from certainties.

One area this very much extends to is core collapse. We know a fair amount about the process, we know gravity eventually win's over as fusion in the core falters through increased content of Iron, but in all our attempts to actually model it, it's clear we're missing part of the puzzle.

above, a computer simulation of the inside of a star as it collapses

The problem is, as the core collapses, and gravity crushes down, the rest of the star falls inwards too, and a shockwave moves outwards. We known part of the process that moves a star from main sequence to red giant is that the shockwave pulls in enough fusible material for the star to continue on. In order for a supernova to really occur (or at least, so the theory goes), the shockwave needs to push out beyond the star's atmosphere, but in almost all models, it never does.

The conclusion is, we're clearly missing a bit of the puzzle, and one of the possible causes is that in the final collapse, the shockwave has much more energy than we're assuming, and neutrino's may hold the key to explaining it.

The last nearby supernova was in 1987(although not in the Milky Way itself, in the LMC), when only a handful of neutrino detectors existed, and they were able to capture a few that coincided with sn1987a, but not nearly enough. Today we have many more, but our Milky Way seems reluctant to share a nearby supernova with us, being long overdue for it.

It's quite possible that we actually have had several since the last in the Milky Way (SN1604), but they occurred within what Edwin Hubble coined the zone of avoidance, effectively behind the galaxy centre, meaning we may have missed it due to the dust and gas obscuring our view of it, from this side of the galaxy.

Still, eventually it will happen, and our ability to capture these neutrino's may point to the missing part of our understanding.

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