vendredi 19 septembre 2008

Hydra 5

Double stars in Hydra:
Beta Hydrae is a pair of nearly equal stars (4.5, 5) at PA 8º and a separation of 0.9".
Epsilon Hydrae is a multiple binary; four stars can be seen and another has been calculated to exist.
Components A and B form a rapid binary with a period of 15.05 years; its orbit is nearly circular. Presently (late February 1996) the companion star has a PA of 166º and separation 0.26".
Component C is much easier to resolve, with a period of 990 years. At present it can be found at a PA of 298.5º and separation 2.7".
Chi1 Hydrae is a binary of two similar stars (5.8, 5.8) with an even more rapid orbit. Its period of 7.4 years means an exceedingly difficult binary to resolve. If you've a large enough telescope, you'll find the companion at these values in late February 1996: PA 31º and separation 0.046".
Sigma 1474 is a fixed binary forming a wonderful triple. AB: 6.8, 7.9; 24º and separation 70"; C: 6.9, 23º, 76" separation.
To find the binary, locate nu Hydrae then move one degree northwest. (Just north half a degree is the nearly attractive Sigma 1473.)

Variable stars in Hydra:
R Hydrae ranges from a visual magnitude of a fairly bright 3.5 to a faint 10.9 every 388.87 days. However this star has shown a decrease in its period. (In 1920 it had a 404 day period.) In 2000, based on its current period of 388 days, the maximum should be attained during the first week of August.
This star is one of the earliest variables to be catalogued, having been discovered in 1704. Only Mira (omicron Ceti), beta Persei, and chi Cygni predate R Hydrae.
Burnham gives a finder's chart. Note that the brightest star in the region, SS Hydrae, is also a variable, so don't base R Hydrae's visual magnitude on this star.
T Hydrae has a shorter period, 298.7 days, and a range of 6.7-13.5. To find T Hydrae, star hop westward from Alphard. First, due west of alpha Hydrae you find 24 Hydrae, then 20 and 19 (which is the brighter of the two). Now just about the same distance that you've taken to get to 19 Hydrae you'll find T Hydrae, due south of 17 Hydrae one and a half degrees.
This star, 17 Hydrae, is the brightest star in the region, and a visual binary as well (7, 7; 359º, 4.3"). The star is useful to determine the visual magnitude of T Hydrae to the south.
In 2000 T Hydrae should attain its maximum brightness in mid October.

Deep Sky Objects in Hydra:
Hydra has three Messier objects: M48, M68, and M83.
M48 (NGC 2548). Messier actually gave the wrong location for this star cluster, putting it four degrees north of the current position. But by his description this seems to be the right object.
Not terribly spectacular, this is a group of fifty stars, the brightest of which is about 8.8 visual magnitude. The cluster is thought to be about 1700 light years away, and is easily seen in binoculars or small telescope.
M68 (NGC 4590) is a much richer globular cluster of over a hundred thousand stars, resolved in medium-sized telescopes.
The cluster lies in a desolate part of the sky. Locate gamma Hydrae then move west to beta Corvi. Now drop down three degrees to the brightest star in this region, a fifth-magnitude star (this is the binary B230: 5.5, 12; 170 degrees, separation 1.3"). M68 is about a half degree to the northeast.
M83 (NGC 5236) is a spiral galaxy sitting on the Hydra-Centaurus border farther to the east, nearly twenty degrees south of Spica (alpha Virginis).
While Burnham says this is considered one of the brightest galaxies with a visual magnitude of about 8, other references give it only a 10. And since it is very diffuse, it may be difficult for those living in northern latitudes.
NGC 5694 is an extremely compact globular star cluster, thought to be in the region of 100,000 light years away.
The cluster sits just east of the mid-way point between pi Hydrae and sigma Librae, at the border with Hydra. From pi Hydrae move east until you encounter a group of five magnitude stars lined up roughly north-south. These are 54, 55, 56, and 57 Hydrae. NGC 5694 lies one degree west of 57 Hydrae.
NGC 3242 clearly deserves to be a Messier object. Small but bright, at a visual magnitude of 8.6, this planetary nebula is often called The Ghost of Jupiter because of its slight resemblance (?) to that planet. Also at times called The Eye Nebula, perhaps a closer description.
The nebula gives off a soft bluish colour, sometimes described as a "glow", that is clearly visible even in small scopes. The central star may be difficult to resolve: this is an 11.4m star, a blue dwarf considered to be as hot as 60,000 kelvin.
The nebula is one of the easiest to find. Just locate mu Hydrae then move south two degrees.
Trying to resolve the inner ring could prove difficult. Large telescopes should show the object as resembling an eye, with the central star the pupil. The greenish-blue colour adds to this intriguing sight.

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