Who was Stephan and why is a compact galaxy group named after him?
Édouard Jean-Marie Stephan (1837-1923) was Director of the Marseilles Observatory and was working on measuring the exact positions of as many deep sky objects as he could to use them as a fixed background reference system in order to determine the proper motions of stars within our Milky Way galaxy.
Edouard Stephan.jpg
In the process Stephan discovered 139 deep sky objects using the world’s first large silver-on-glass mirror (made by Leon Foucault) that was the heart of the 78.7cm (31 inch) Marseilles telescope shown below. In many respects, this is the first modern reflecting telescope.
31inch Marsellies telescope.jpg
Stephan came across his quintet of galaxies on September 23, 1876 – the first compact galaxy group ever discovered - but at the time he saw only four of the five as he didn’t resolve the close pair of NGC 7318A and B, perhaps due to the light pollution from nearby Marseilles.
Even though he went on to discover several more compact galaxy groups, only this first one is named after him.
We now know that four of the five galaxies are physically interacting with each other, and a quick look at the NOAO photo below will show you which one isn’t – the light blue galaxy NGC 7320 is in the foreground and is a random alignment only 40 million light years away. The yellow-ish interacting galaxies of Stephan’s Quintet are all around 290 million years away and illustrate red shift at a glance.
Annotated NAOA photo of SptehansQuintet.jpg
The galaxies range from 13.1 to 14.3. Physically associated with the main group but 17 arc seconds to the east is small NGC 7320C at magnitude 16.7.
The main five galaxies of the quintet all have NGC numbers and each one might be detected in an 8 inch scope under ideally dark and steady skies, and their combined glow would likely be visible with a 6 inch under the same conditions. With a 16 inch or larger scope you can use high powers to separate the individual galaxies for a view much like that shown in my sketch. My sketch was made with my 28 inch scope under dark but not great skies (21.47 Sky Quality Meter reading) using 408x. It’s similar to views I’ve had with my old 20 inch under better conditions.
SQ_28inch.jpg SQ_28inch_invert.jpg
I could see the main five galaxies quite well but it took averted vision to see NGC 7320C at the far left of the sketch. Under truly dark skies it can pop out, but it’s quite faint and most observer's shouldn't expect to see it in anything smaller than a 16 inch.
Conveniently, Stephan’s Quintet is easy to find because it’s so close to the magnitude 9.4 galaxy NGC 7331 in northern Pegasus. Find 7331, move your scope less than a degree to the southwest, and you’re there.
GIVE IT A GO AND LET US KNOW!
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