Messier 15 – Wide Field

I posted a much closer shot of the Messier 15 (M15) globular cluster earlier. I took it with a large reflector. But this is a wider angle I shot with the Takahashi refractor. It exposes a whole lot of things that were hidden in the original because of the wide angle.

M15, in the Pegasus constellation, is far south of the galactic plane. So it generally offers a relatively clear shot of deep space unobscured by gas and dust in and around the plane. But this image shows a considerable amount of nebulosity. I’ve not found any authoritative studies on the clouds the images shows, so I’m left to guess at what we’re seeing here. The gray and brown material may be dark molecular cloud within the confines of our galaxy, or it may be integrated-flux nebula on the outskirts of the galaxy—or a combination of both.

Integrated flux nebulas (IFNs) are the result of extremely diffuse gas and dust at or beyond the perimeter of the galaxy reflecting the light of all the stars in the Milky Way. IFNs are very faint and are extremely hard to capture with a telescope, often requiring dozens or even hundreds of hours of exposure time to unravel. The red material is another story. That’s almost certainly ionized hydrogen gas, which I would surmise is inside the normal reach of the Milky Way.

In any case, it makes for a dramatic scene. As for M15, it is one of the oldest known globular clusters orbiting our Milky Way galaxy. And it was the first globular cluster (there are still only four) to have a planetary nebula—a gaseous cloud surrounding a dying star—discovered in its midst. It’s location is marked above in the overlay (which you can view by clicking the eye icon) as PK065-27.1, but it’s also known as Pease 1.

M15’s core contains a rare intermediate-mass black hole with a mass approximately four times the mass of our sun. I has a novel stellar population with a large concentration of Cepheid variable stars at around 112, and it also contains nine pulsars as well as a binary neutron-star system.

Globular clusters are ancient collections of stars that are gravitationally bound together in a spheroidal shape. Some are almost as old as the universe itself. They can contain anywhere from tens of thousands of stars to millions of stars. The stars they house tend to be very old as well, because star formation inside the cluster has largely ceased. The formation of globular clusters is not well understood. Current research leans toward the idea that they formed from very dense molecular clouds in the early universe. Some larger globular clusters may once have been dwarf galaxies whose larger star populations were stripped away from the core by larger galaxies.

M15 is the densest globular cluster astronomers have yet found. It has undergone a process called core collapse. This phenomenon occurs when so many stars are packed in such a small space that they start attracting each other gravitationally toward the center of the cluster. Of the 157 globular clusters known to orbit the Milky Way, 21 of them have undergone a core collapse.

M15 Facts

Distance: 35,700 light years

Number of stars: 100,000+

Diameter: 180 light years

Age: 13.7 billion years