Washed-out Astronomy

As I discussed in an earlier story, globular clusters hold up well when viewed from washed-out skies. In that story I highlighted the biggest and brightest globulars. Today I want to discuss a globular cluster that is neither big nor bright, but is nevertheless very interesting: NGC 2419, the so-called Intergalactic Wanderer. It got its name because it is so far from the galactic center (300,000 light years, farther than the Magellanic Clouds and well beyond the “halo” containing most globular clusters) that it was long believed to wandering through space independently. We now know that NGC 2419 is indeed gravitationally bound to our galaxy, taking about 3 billion years to complete an orbit. (Photo courtesy of Mike Richmann)

NGC 2419 is a large, intrinsically bright globular cluster, comparable to Omega Centauri. But its great distance makes NGC 2419 rather faint and small, at magnitude 10.4 and less than 4 arc min across. It is somewhat challenging to see in urban skies, harder than its official magnitude suggests. In my experience you’ll need at least a 6 inch (150 mm) aperture and great transparency to see NGC 2419 under light polluted skies. If the transparency isn’t very good or if NGC 2419 isn’t close to zenith, it can be hard to see even in my 10 inch Dob. But seeing this unusual globular cluster is worth the effort and here is some information for you if want to take up the challenge.

In Taurus just over the border from Perseus is NGC 1514, a planetary nebula with an important role in history. William Herschel, based on his experience resolving globular clusters, believed that all nebulae were clusters of stars that were too faint or too remote to resolve. But when he discovered NGC 1514 in 1790, Herschel encountered a solitary star “surrounded with a faintly luminous atmosphere”. This object forced him to revise his thinking and conclude that “Our judgement I may venture to say, will be, that the nebulosity about the star is not of a starry nature”.

Although officially listed at magnitude 9.4, NGC 1514’s relatively low surface brightness and its bright central star make it a rather challenging object to see from an urban environment. Even though it isn’t the most beautiful planetary, it is worth tracking down simply because of historical interest. Here is some information to help you find NGC 1514...

(Photo courtesy of Chris Cole)

M35 and M38 are two beautiful open clusters that are well positioned for observing during the winter months. M35 is located in western Gemini, just a few degrees north and west of η Geminorum. It is a big cluster spread across an area nearly the size of the full moon. At magnitude 5.3 overall, and with nearly a hundred stars of 7^th through 11^th magnitude, M35 is an easy target even in heavily light polluted skies. Smaller and fainter at magnitude 7.5 overall, M38 is also a beautiful object. It consists of about a hundred stars, with the brightest ones forming a very distinctive letter “π” that is clearly visible even with small telescopes (you can see the “π” upside down in the photo to the right).

While these clusters are quite beautiful and easily observed from washed-out skies, there is something that sets them apart from the other many bright open clusters visible in the winter skies: both of these have fainter clusters within the same field of view. The combination of two clusters in one field of view makes them a particularly interesting sight. Because these fainter clusters are at the edge of visibility for urban observers, they make great challenge objects when observing in washed-out skies. I have more information for those of you wanting to take up this challenge....

Photos of M38  (upper, with NGC 1907 at the bottom edge) and M35 (lower, with NGC 2158 at the bottom-right edge) courtesy of Land of Oz Observatory.

A good star atlas is essential for finding your way around the night sky and locating interesting objects. This is especially true in washed-out skies, where you may not see enough stars to easily recognize all the constellations. But which star atlas is right for you? In this first of a series of atlas reviews, I’ll look at Sky & Telescope’s Pocket Sky Atlas. This is a beautifully printed atlas designed specifically for use in the field. Its spiral binding and compact format make it easy to use at the telescope. It’s also a good match for washed-out astronomers to use for star-hopping because its magnitude limit for stars corresponds well to what you can see using finder scopes and binoculars in light-polluted skies, and the deep sky objects included cover just about anything you can hope to see with a telescope from urban areas. On the other hand, if you are looking for something to help you learn the constellations and generally find your way around the night sky, the Pocket Sky Atlas may be the wrong tool: the charts are too narrow and too detailed for that. To make effective use of this atlas you should already be able to locate the major constellations. Allow me to explain further...

In the early 1900s astronomers were stunned to discover that some stars had masses comparable to the Sun's but packaged into earth-sized volumes. The first such stars discovered were spectral type A (white) and thus became known as White Dwarfs. The most famous white dwarf is Sirius B, the companion to Sirius. Most white dwarfs don’t make good targets for washed-out astronomers because they are faint, inconspicuous stars or are hard to see companions to bright stars such as Sirius. But there is one white dwarf that is easy: 40 Eridani B.

40 Eridani (also known as ο² Eridani or Keid) is a relatively bright 4^th magnitude star. Even in urban areas, it is easy to find by star-hopping from Rigel going via β Eri, μ Eri, and ν Eri. I recently did it from my light polluted front yard using a 50mm finder. Even looking directly over my neighbor’s very bright holiday lights it was an easy hop. 40 Eridani is a triple star system. The primary has magnitude 4.4. Component B—the white dwarf—has magnitude 9.5 and is widely separated from A (83”, PA 105°) making it easy to see in even small telescopes. (Image courtesy of David Darling)

As a special prize, the third component C is a red dwarf flare star of magnitude 11.2. The B-C separation is much tighter at about 8” but it isn’t hard to split except for the faintness of the C component. You’ll probably need at least a 4 inch (100mm) aperature to see C in washed-out skies, but it’s rather easy with 6 inches (150mm) or more. In my 10 inch (250mm) Dob, it looks pretty much like the photo above.

So don’t miss the chance to “bag” a white dwarf. If you want to learn more about white dwarfs, read the excellent Wikipedia article. Now 40 Eridani is interesting for several other reasons, including a connection to Star Trek....