Globular clusters... the eternal companions of galaxies, so old that they've witnessed the birth of the galaxy they're bound to. They can contain hundreds of thousands of stars in a spherical area with a diameter usually less than 30 lightyears. Or in other words, the stellar density's hundreds to even a thousand times greater than in our part of the galaxy. If you'd stare at the sky in a globular cluster, it would be filled with stars brighter than our own Moon!
Such a place would be most unfavourable for life because the tidal forces that all these close-by stars generate would disturb any planetary orbit too much, let alone the fierce radiation that they'd cause. A strange quality of globulars is that their stars, in spite of being among the oldest stars in the universe, often appear very blue and hot. Usually old stars tend to cool down and become orangy-red. The reason for this is that in a globular cluster the stars are so close to one another that they're able to strip each other's atmosphere, exposing the extremely hot core. Now imagine what they could do to a miserable planet's atmosphere!
Due to their huge density globular clusters are able to resist the galaxy's gravitational pull because stars within a few hundreds of lightyears' distance are influenced more by the gravity of the globular cluster than by the galaxy. Therefore they remain very compact and will continue to do so.
But there are always exceptions.
Next time you're planning to observe the imposing globular M3, half-way between Bootes and Canes Venatici, point your telescope slightly more towards the northeast. There you'll find this much less known globular, called NGC5466. You'll immediately notice that it's not perfectly spherical like most of its peers, but highly irregular in shape. Stars seem to be scattered all over the place and the thing that struck me was that the stars appeared to form chains in more or less the same direction. After having done some research, my observation turned out to be correct. This particular globular cluster's losing the battle against our Milky Way and is being ripped to pieces as we speak! Its stars are being spread into a large stream between Bootes and the Big Dipper (invisible to amateur telescopes because too faint and distant), just like if you'd be smearing out a blob of paint with a brush. In a few hundreds of thousands of years this globular cluster will be no more...