Globulars in M31

Subject classification: this is an astronomy resource.

M31, the Great Andomeda Galaxy

The first recorded observation of this object as a fuzzy patch was by the Persian astronomer, Abd al-Rahman al-Sufi. The nature of this "little cloud" has been the source of speculations throughout history. The answer was partially revealed shortly after the invention of the telescope which allowed Simon Marius to study it in 1612.

As a nebulousity it was cataloged in the 19th century and many other similar 'spiral nebulas' were discovered.

Photography institued a new era in the exploration of such objects and in 1917 Heber Curtis noticed that nova from the distant object were 10 magnitudes fainter making the object 500,000 light-years away. He hypothesized that such objects were "island universes" and were large copies of the stars, dust and gas that we called the milky way.

This remained controversial for a full decade, until Edwin Hubble, using the large palomar telescope, resolved some of its variable stars. As the timing of changes in the variable star was conclusively related to its brightness, he was able to conclusively show that M31, was indeed far beyond this galaxy. We now know it to be 2.2 million light-years away.

A globular cluster

A globular cluster is a very old and tightly bound group of stars. A globular cluster may have some million stars and therefore it shines very brightly. But on the other hand, globular clusters are found in the Halo around a galaxy and thus are much further away than many objects seen in the telescopes (galaxies being an obvious exception). The milky way galaxy has 158 known globular clusters, however many galaxies have far more. The Andromeda galaxy is suspected to have 500.

Observing globulars in M31

It is amazing to see any star groups 2 million light-years away, yet nevertheless globular clusters are a definite (but demanding observing target for a 10" reflecting telescope). At this distance, all sense of it being a cluster is lost and it shines as one of M31 brighter stars. While some maps show many hundreds, many of which can be imaged with modest equiptment, the limit for a 10" scope is somewhere around 6-12.

The following sketch was carefully made of all field stars I saw during a clear night with a 10" dobsonian. Two globulars are shown, a third was clearly missed (having a brightness similar to other observed field stars)

Please feel free to attach any sketches of images which capture M31 globulars.