Messier 15 - A View from the Arid Lands

Imagine a world where the sky blazes with the radiance of a hundred thousand suns. That in any direction you ever cared to look you saw more stars crowded together than anywhere else in the Milky Way – anywhere, perhaps, than that of our own galaxy's hidden heart.

Imagine a world of midnight brightness, a world without shadow, where the love of warmth would burn everything to ash. This would be the world of Messier 15, a primordial remnant forming from the birth of the Milky Way. The densest known star cluster in our galaxy, cranking out a vast array of exotic stellar objects from pulsars, variables, binaries, red giants, white dwarfs and neutron stars, planetary nebula, supernova and maybe even a black hole.

Messier 15 Sketch

As the story continues, Messier 15 is about 12-billion-years old, some 33,600 light years away and hurtling towards Earth at around 385,000 km an hour. It has a diameter of about 175 light years and an absolute magnitude of roughly 360,000 times that of our sun.

The cluster has been identified with an overall spectral type of F – typically characterized by weak hydrogen but strong metal emissions - suggesting that the vast majority of its stars have evolved away from main sequence and – depending on their mass – will one day explode into supernovae or collapse into planetary nebulae, leaving the remnant of neutron or white dwarf stars respectively.

Images from the Hubble Telescope show that Messier 15’s super dense core has undergone a process of core-collapse driven by the intense gravitational pull of thousands of stars converging on a small volume of space rather like cosmic bees swarming to their hive. Photos of the 22 light year diameter core, reveal a world of over 30,000 distinct stars and it is still unknown whether this core is packed to such an extent by mutual gravitational interaction of the stars or whether it now houses an object resembling that of our galaxy’s nucleus.

At this core, neutron stars and white dwarfs outnumber main-sequence and red giant stars by a ratio of about 100 to 1 and theories suggest that either the core has stopped collapsing and stars near the center have settled into uneasy cosmic dances, both attracted to each other by gravity and repelled by close encounters that slingshot them through space, or that at some stage in M 15’s ancient history there has already occurred a massive pileup of stars forming a black hole several thousand times more massive than that of our sun.

Artist's impression of Black Hole at the heart of a Globular Cluster

Some 150 light years away from the intense brilliance of its central core, towards the cluster’s halo, the Milky Way’s own tidal forces and tidal shocks are stripping away and destroying any low-mass stars remaining. Messier 15 is an unforgiving world and it is probable that its current star population is a mere fraction of what it once was.

At the very limit of naked eye visibility with its apparent visual brightness of magnitude 6.2, as shown in the sketch above, M 15 appeared as a small, rounded and mottled nebula in the 4” refractor, with at best subtle indication of some of its brightest stars or star-areas. The compact core remained unresolved but with attentive inspection hints of star streams could be seen radiating out from its core in all but its most westerly direction.

Equipment used for the observation was a Tal 100rs, an 18mm X-Cel LX mounted on an AZ 4. The sketch was drawn the first weekend of November, a cold night using a 2B pencil for the stars and a blending stub for the globular cluster.