Stars

Urban Orion

Washed-out Astronomer — Mon, 20 Feb 2012 15:26:57 +0000

Orion is not only the best known winter constellation, but it is also a great urban constellation. Thanks to its distinctive arrangement of rather bright stars, Orion shines through in heavily washed-out skies, even when few other constellations can recognized. It is also the home of the Great Orion Nebula (M42) big, bright, and easily visible even in washed-out skies. However, M42 is only the most famous of many interesting astronomical objects in Orion. And best of all for urban astronomers, quite a few of Orion's objects—like the constellation itself—can be enjoyed in light-polluted skies. Here is a run down...

Hunting the Great White Dwarf

Washed-out Astronomer — Wed, 30 Dec 2009 17:03:54 +0000

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....

Kruger 60

Washed-out Astronomer — Sat, 03 Oct 2009 22:09:58 +0000

Kruger 60 is a particularly interesting binary star system in Cepheus. With a short period of only 44.7 years, you can easily see Kruger 60’s PA change about 8 degrees per year. There’s even a convenient nearby reference star that makes the change in PA obvious. Only 13 light years away, Kruger 60 is also one of Earth’s nearest neighbors. Both components are low-mass red dwarfs, but with only 0.18 solar masses, Kruger 60 B is one of the lowest mass stars known. Finally, Kruger 60 B is also a flare star, irregularly doubling in brightness for periods lasting about 5 to 10 minutes. When it flares, it can match or exceed Kruger 60 A in brightness.

Kruger 60, however, is a challenging target for observers in light polluted environments. If you want to take on this challenge, I have more information for you…

Black Hole Hunting

Washed-out Astronomer — Wed, 23 Sep 2009 02:04:41 +0000

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 9^th 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)…