In my earlier story on 3C 273 [3], 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 [4], 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 Quasars

Because I covered 3C 273 [3] in detail in an earlier story, the rest of this article focuses on the other three quasars: Mrk 421, Mrk 501, and OJ 287. For completeness and reference I am including 3C 273 in the tables below, but refer to my earlier article for more information about 3C 273 [3]. Here is the basic data on the quasars:

For each of these, I have prepared finder charts using the AAVSO Variable Star Plotter [5]. I generated these plots in the standard AAVSO "A", "B", "C", "D", and "E" scales, with north up and east on the left. You can use the AAVSO Variable Star Plotter to generate any other scales or orientations you prefer. I've also labeled some additional stars to help you correlate these finder charts to each other and to stars found in standard star atlases (such as the Pocket Sky Atlas).

Dealing with the variability

Because these three quasars are blazars, they can vary in brightness a magnitude or more on short time-scales. Before trying to observe any of them, I recommend you check their current brightness. You can do this easily using the AAVSO Light Curve Generator [26]. Just type in the name (Mrk 421, Mrk 501, and OJ 287 are all recognized) and hit the "Plot Data" button (you can adjust the settings, but the defaults work fine). You'll see a light curve that shows you the current brightness of the quasar. For example, you'll see that OJ 287 has currently (Apr 2010) faded to nearly magnitude 15, although it was as bright as magnitude 14.0 in December 2009. That is also impressive: this implies that the tremendous energy OJ 287 is radiating comes from an area only a couple of light-months across.

Markarian 421 (Mrk 421)

This is a BL Lacertae object and is one of the closest blazars to earth. Like all blazars, it is so luminous because a super-massive black hole at the center of a giant elliptical galaxy is blasting a superluminal relativistic jet nearly directly towards Earth. The rapid variability is a side-effect of us "looking down" the throat of this jet. While Mrk 421 fluctuates rapidly in brightness, it mostly stays brighter than magnitude 13.2, often getting up around magnitude 12.8. Mrk 421 is also probably the easiest of the four quasars to find, because it is adjacent to a 6th magnitude star. (Photo by Aimo Sillanpaa with the Nordic Optical Telescope.)

Markarian 501 (Mrk 501)

This is another BL Lacertae object. Because we are not looking down the jet as tightly as Mrk 421, Mrk 501 is not nearly as variable, staying pretty consistently between magnitude 13.6 and 13.9. I think that Mrk 501 is the hardest of the four to locate because there are no near by brighter stars, and you have several stars of similar magnitude to Mrk 501 in the immediate vicinity. Using my 10 inch reflector from my front yard under washed-out skies, I find both the quasar and the nearby stars to be close to the edge of visibility, making it hard to get a good fix on them and confirm I'm looking at the right object.

OJ 287

This remarkable object has two super-massive black holes [27] in a binary system (actually, a super-massive black hole of 100 million solar masses and a super-duper-massive black hole of 17-18 billion solar masses--six times larger [28] than the next largest black hole known). A recent research paper [29] used this system as a test of general relativity, showing that its orbit is deteriorating at a rate consistent with energy being radiated away in gravitational waves. OJ 287 is also the faintest of the four, varying between magnitude 14.0 and 15.0. At the upper end of this range you can see OJ 287 in a 10 inch (250mm) telescope from urban environments. At OJ 287's faintest, you'll probably need at least a 14 inch (350mm) telescope to see it under washed-out skies. In dark skies, a 10 inch telescope is enough to show OJ 287 throughout its range. OJ 287 is not too hard to locate, because the only nearby stars of similar brightness are a pair of stars just east of it. It's pretty easy to distinguish the pair from OJ 287 itself. (Diagram courtesy of Tuorla Observatory.)