September 2009

Great Globulars

Globular clusters generally look great even in washed-out skies. Many globulars—especially the Messier globulars—are bright enough to be relatively easy to find even in heavy light pollution. And when you use high magnification to try to resolve the globular, you also improve the contrast by darkening the background. In washed-out skies it will certainly be harder to resolve some of the globulars, and you won’t see the fainter stars so you will loose a bit of star density. But any of the major globular clusters still look spectacular. When I have guests at the telescope, I first turn the telescope to any visible planets. But the very next thing I turn the telescope to is a globular cluster: I can count on it to generate plenty of “wows” from my audience.

Now there are a lot of globular clusters, so you might wonder which ones look best in an urban environment. Everyone has their favorites, but here are mine, all proven performers observing from washed-out skies…


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)…