Black holes

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....


Black Hole Hunting

Cygnus X-1 is almost certainly a black hole. It was discovered in 1964 as a strong X-ray source, and has ever since been the object of intense study. Cygnus X-1 turns out to be too compact to be any known kind of object besides a black hole. It has a mass of about 8.7 solar masses (which exceeds the theoretical maximum mass of a neutron star of about 3 solar masses), but based on how quickly its x-ray intensity fluctuates, Cygnus X-1 has to be less than about 60 km wide.  Assuming Cygnus X-1 is a black hole, its event horizon is currently estimated to have a radius of about 26 km.

Most interestingly, Cygnus X-1 orbits the blue supergiant star HDE 226868 at a separation of about 0.2 AU. Cygnus X-1 has distorted HDE 226868 into a tear-drop shape and is eating it away (although whether X-1 is actively stripping away 226868’s outer layers or simply sucking up 226868’s solar wind is unclear; the edge of material gravitationally bound to 226868 is close to the star’s surface). For washed-out astronomers, the most interesting aspect is that HDE 226868 is a 9th magnitude star, making it an easy target for even small telescopes in urban environments. And while you can’t actually see Cygnus X-1 itself, it’s still pretty cool to be looking at a star that’s being eaten alive by a black hole. (ESA/Hubble illustration)

 Here’s how to find HDE 226868 (and Cygnus X-1)…



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