M7: Ptolemy's Cluster

Over the last few weeks I've posted about a few star clusters. These are groups of stars you can see using an inexpensive telescope from the suburbs while you're under lockdown.

Here's another one. This cluster was named after its (supposed) discoverer, the ancient Greek astronomer Ptolemy. It's in all sorts of catalogues, including Messier's catalogue as M7 and the New General Catalogue as NGC 6475.

You don't actually need a telescope to see this one. A pair of binoculars will do, because not only is it made up of nice bright stars, it's also quite large. In fact, if you're in a dark sky area, you might see it with your naked eye.

The cluster itself is just outside Scorpio. This is a very recognisable constellation that's rising in the East in the evening at this time of year (July). I always think that it looks like a gigantic backwards question mark. I've attached a screenshot from Stellarium, but of course you can use your own: there are lots of similar programs available. The results will be the same, because the stars are the same.

Notice the end of the scorpion's tail? There's a line of stars. These are Lesath, Shaula, Mula and Girtab. You'll see them in the screenshot. There's another star outside the question mark in the map, called Fuyue. This slightly yellow star is your signpost.

Looking through your scope, start from a spot between Girtab and Mule, and find your way to Fuyue.

Then, make the same hop again, and you will find yourself at Ptolemy's Cluster. I've marked your hops with green arrows.

I've also attached a photo, along with circles that show roughly what you'll see using 10mm and 25mm eyepieces using 650mm and 910mm telescopes.

Just for fun, I have also included my photo taken from my back yard in Kew. the camera picks up more than the human eye can see, and the background of Ptolemy's cluster is a nice chunk of the Milky Way. If you want to see this with your eye, you'll need an 8" or 10" Dobsonian and a dark sky - but it's worth it!