April 2010

Sky Atlas 2000.0

Sky Atlas 2000.0 CoverThis review of Sky Atlas 2000.0 is the second in a series of atlas reviews to help you decide which star atlas is right for you. My first review covered S&T’s Pocket Sky Atlas. Sky Atlas 2000.0 is a large-scale atlas beautifully printed in a large-page format. It has stars down to magnitude 8.5 and deep sky objects down to around magnitude 13. Sky Atlas 2000.0 comes in three editions: Desk (black stars on white background), Field (white stars on black background), and Deluxe (color-coded deep-sky objects on white background), and each edition comes in a paper version and a laminated version. All versions include close-up charts of crowded areas such as the Virgo cluster, as well as a transparent plastic coordinate-grid overlay for determining positions accurately.

Regardless of the edition, this is clearly a star atlas for serious star-hopping which will help you find objects well beyond the Messier catalog. It is a fantastic atlas, but for observers in urban (and even sub-urban) areas, Sky Atlas 2000.0 is probably overkill: most of the deep-sky objects won’t be visible in washed-out skies, even with relatively large amateur telescopes. Although having more stars plotted compared to Pocket Star Atlas or Norton’s Star Atlas can be an advantage for star hopping if you have a 50mm or better finder scope, I haven’t found the difference significant when star-hopping in light-polluted skies. However, if you regularly go to a dark-sky site in addition to observing from urban areas, Sky Atlas 2000.0 makes sense as an atlas that will serve you well in both locations....


A Fist Full of Quasars

In my earlier story on 3C 273, I promised to cover some other quasars that are within reach of urban astronomers. In fact, there are four quasars brighter than about magnitude 14. While this may sound pretty faint, because quasars are star-like, you can use high magnification to darken the background and get enough contrast to see them, even in washed-out skies. Under favorable conditions you should be able to see all four of them with a 10 inch (250mm) telescope from your front yard. (NASA artist's impression of a quasar.)

The brightest of the four at magnitude 12.8 is 3C 273. The next brightest is Mrk 421 in Ursa Major at magnitude 13.3. Both of these are within reach of a 6 inch (150mm) telescope, and easily visible with an 8 inch (200mm) telescope, even in urban skies. The other two are more challenging: Mrk 501 in Hercules at magnitude 13.9 and OJ 287 in Cancer at magnitude 14.2. Three of these, Mrk 421, Mrk 501, and OJ 287, are not only quasars, but blazars, which means they are highly variable in brightness over relatively short time periods. This is particularly true for Mrk 421 and OJ 287.

Quasars are cool objects in and of themselves. But there are other reasons that make these fascinating objects. One is the tremendous distances involved: 3C 273 is 2.5 billion light years away and OJ 287 is over 3.5 billion light years away. They are also powered by massive black holes: Mrk 421 has a black hole of about a billion solar masses; OJ 287 has a binary pair of black holes, the primary weighing in at 18 billion solar masses—the largest black hole known.

Now finding specific 13th and 14th magnitude "stars" can be a challenge, so if you are interested I have a full set of finder charts for all of these....


The Intergalactic Wanderer

NGC 2419 photo courtesy of Mike RichmannAs 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.