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What's Up Hydra


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What’s Up HYDRA
By Steve Coe

This month’s constellation contains the reddest star in the sky, so I would like to take a minute to chat about red stars. It turns out that many of the observers of years past dearly loved to look for stars with prominent color. This chase will quickly make you see that the perception of color involves a lot of different variables. The interaction of the eye and brain that gets us to see color is not a straightforward chemical and physical process. Far from it! Call several folks over to your telescope while one of these tinted stars is in the field and listen to the descriptions that arise.

The invention and widespread use of the photometer and spectroscope started to create some knowledge of the physics of what is going on with these red stars. Some are stars like our Sun at the end of its life. The “red giant” phase does indeed redden the starlight from these old stars. The reddest stars are “carbon stars” that have literally carbon soot in their outer layers. This dust cloud acts like the atmosphere of the earth when the setting sun is turned red by the dust and gas in the air.

So, let’s look at some interesting deep sky objects in Hydra and save the red star for last.

M 48 is a big open cluster that is “underneath” the Head of Hydra from mid northern latitudes. On a dark night I can see it with the naked eye. Using 8X42 binoculars I see 12 stars in a pretty compressed cluster at this low power. The middle of the cluster is much brighter. Moving up to the 4" (100mm) f/6 refractor will bring out 48 stars, honest! With the 21mm Stratus eyepiece it is bright, large, somewhat compressed, pretty rich, well detached. You can’t miss it at right ascension 08 hours 13.7 minutes and declination -05 degrees 45 minutes.

In my old 13 inch Newtonian at 100X there are few faint members of this cluster, so it seems that the larger aperture has resolved it all. I counted 76 stars in M 48 at 100X and it almost filled the 30 arc minute field of view. It is bright, very large, pretty rich, little compressed and shows stars from 9th to 13th magnitude. There are several nice curved chains of stars within the cluster and one of them includes a very nice orange star of about 9th magnitude.


M 48 with a 300mm lens. Yes, I shot it on film, you remember film? 20 min at f/5.6.

NGC 2610 is a small, but pretty high surface brightness planetary nebula. It is somewhat overlooked at 08 33.4 and -16 09. In the Nexstar 11 SCT and a 22mm Panoptic eyepiece this planetary is just detected as a faint, pretty small non-stellar blob with an irregular figure. Moving up to the 14mm Ultra Wide Angle eyepiece makes this object much easier to see. It is pretty faint, pretty small and comet shaped with a star of about 12th magnitude at the tip. Averted vision makes it larger in size. Both the UHC and OIII filters are little help, both increase the contrast of the nebula, but make the star much more difficult to see. In my opinion, the view is better without a filter.


NGC 2610 image from the NGC/IC Project.

NGC 2935 is pretty bright, elongated 1.8X1, and brighter in the middle at 200X in the Nexstar 11. Averted vision makes this galaxy much larger. It is representative of the myriad on galaxies within the borders of Hydra. This long constellation moves away from the Milky Way and then “underneath” Leo, so the majority of objects involved within its borders are galaxies. Look for it at 09 36.7 and -21 08.

NGC 2935 image from the NGC/IC Project.

NGC 3242 is one of the premier planetary nebulae in the sky. It is called the Ghost of Jupiter because it is about the same size as the disk of the planet Jupiter and it is also somewhat oblate, or elongated in shape. NGC 3242 is located at 10 hours 24.8 minutes of Right Ascension and -18 degrees and 39 minutes of Declination. With a Nexstar 11 SCT at 200X it is bright, pretty small and elongated 1.2X1. 320X provides a good view of a famous object. At high power there is plenty of internal detail, the “CBS eye” effect is easy and obvious. What I mean is that on a good night at high power this planetary looks like the logo of the CBS company. The disk of this planetary shows a light aqua color and the central star is seen about 20% of the time. There are two bright spots, other than the central star, within the disk of NGC 3242. The UHC filter makes them more obvious, but with the filter the central star is difficult to see.

At the Texas Star Party some years ago I was observing with Jeannie Clark, of Amateur Astronomy Magazine. Using her 20 inch f/5 Newtonian and a 7mm eyepiece, this is a striking object with lots of internal detail. The disk is a neon green color that surrounds the central star, which is blue-white. There is a small dark circular area around the central star. The CBS eye effect is obvious. Averted vision shows a faint outer extension of the disk that doubles the size of this nebula.

NGC 3242 image with 20” R-C by Adam Block/NOAO/AURA/NSF

M 68 is a very nice globular cluster that does not get observed very often, it is at 12 39.5 -26 45. At 100X in my old Dobsonian, a 17.5 inch, it was seen as bright, pretty large, rich and compressed. It was seen in the finder scope. The stars of this cluster were resolved at all powers. The fact that there are stars in curved streamers that spray out from one side of the cluster caused me to call it "kidney-shaped". There are about 50 stars resolved at 165X. Many stars are at the limit of 17.5", averted vision makes it grow. At 300X the core has about 10 stars resolved and the central area is very grainy. The entire cluster looks like a spinning garden sprinkler, with extremely faint stars seen as a set of curved chains that exit the main body and swirl around.

M 68 image from NGC/IC Project.

OK, here we are at the reddest star known, V Hydrae. It’s B-V index is 5.5! For comparison, Mu Cephei (Herschel’s Garnet Star) is 2.3. Brian Skiff says that “it normally ranges between 7-9 magnitude in 17 months. About every 18 years dust condenses around the star causing it to dim to as faint as mag 12. The most recent dimming occurred in the mid 1990's, since then V HYA has recovered to nearly its usual brightness.” This is from an article in May 1998 Sky and Telescope Magazine.

In every telescope I have owned, V Hydrae is a beautiful red or deep orange star with a fainter bluish companion. A.J. Crayon, my observing buddy, and I have followed it for decades and it is always at its reddest when it is faintest. Don’t miss this beauty at 10 51.6 and -21 18.







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