Hydra the Water Snake

Tycho Brahe first named it Cor Hydrae (the Hydra's Heart). This 2nd magnitude giant, K3 II-III orange star is reminiscent of Antares in Scorpius, also known as the heart of the scorpion. The estimated age for Alphard is 420 million years. Having moved away from the main sequence, Alphard has ballooned to around 100 times the diameter of our Sun.

Analysis of it’s spectrum hint at an unseen white dwarf companion, mass transfer from the dwarf star contributing to Alphard’s slight increase in barium. Alphard lies at 177±8 light years.

Epsilon is an interesting multiple star of combined magnitude 3.3; the two easy components are of yellow and blue hues. The primary star is a close double system, first noticed by Schiaparelli in 1888. However, don't expect to see the secondary star as it is too close to be resolved. The blue companion is a dF7 type star measured by Otto Struve in 1830 as having a separation of 3.2" from the primary star in PA 196°. It has closed to 2.7” and is also a spectroscopic binary. A 4th star lies 19.3" away, being a k-type dwarf star of magnitude 12.4. There are two wider companions of 10th magnitude each, lying at separations of 336" and 424". The epsilon system lies around 135 light years distant from our Sun.

V Hydrae is a stunning double system of red and blue stars easily split in a 20cm mirror at 100x. This intense red star is what is known as a carbon star, the brightest of its type in the sky and it varies in brightness with 2 distinct cycles. The first changes by around 1.5 magnitude over a period of about 530 days whilst the second cycle brings a noticeable drop off in brightness of 5 magnitudes to around 12th magnitude over a period of 6500 days. At the end of its life, V Hydrae is shedding much of its gaseous material and this large drop off in brightness is caused by dust and gas condensing around the star. As with other famous carbon stars, V Hydrae’s colour is best observed at minimum brightness. One not to be missed.

An interesting variable star is R Hydrae, an M-type giant similar to Mira in Cetus and lying around 2000 light years distant. This jewel was the 3rd long-period variable star to be identified, credited to Maraldi in 1704. Hevelius probably saw it in 1662 but never recognised it for its variability. The red colour of R Hydrae needs to be observed near minimum (10.9 mag.), the colour falling away noticeably as it approaches maximum at magnitude 3.2. The period is 389.6 days and the brightness ranges from 3rd to 11th magnitude. R Hydrae is a double star, first measured by G.W. Hough in 1891. The 12th magnitude dwarf companion lies 21" away.

Struve 1474 is a lovely triple system of magnitude 7.5 & 7.8 stars separated by 6.7” with the third companion of magnitude 6.7 to the northeast. Nearby to the south by 0.5° lies the nice binary Struve 1473.

M48 (NGC 2548) is an open cluster catalogued by Charles Messier in 1771. However, Messier made an error of 4° in position for this object and it was known for some years as one of Messier's missing objects. There is no doubt which object Messier meant because of his description of it. This large cluster spans an area the size of the full moon. Burnham describes it as:

"triangular in shape dominated by a "central chain-like grouping of 10 or so 10th and 11th magnitude stars; several dozen fainter members increase the total population to about 50 stars down to the 13th magnitude".

Three yellow stars reside here, the rest being A-type main sequence stars, the brightest of which shines at magnitude 8.2. With a diameter around 30’, it should present itself to the naked eye under dark skies. Binoculars of the 10x50 variety will show around a dozen stars.

Two globular clusters reside in Hydra, the best being M68 (NGC 4590). Messier found this ball of over 100,000 suns in 1780, though it must have appeared to him as a hazy glow. It is partially resolved in a telescope of 15cm and shows as a very rich group, brightening towards the centre with a strong central condensation and several chains of stars.

NGC 5694 is the other globular, first seen by Sir William Herschel in 1784. It is one of the more remote clusters, with its brightest stars glowing dimly at 16.5 magnitude and seems to have a velocity in excess of the escape velocity of the Galaxy. If this is so, it may eventually escape from the Galaxy's gravitational hold. To find NGC 5694, look 1.5° southwest of the group 54,55,56 and 57 Hydrae. This asterism of above stars was shown on maps of the early 19th century as Noctua or ‘Night Owl’ but unfortunately it has been lost from current maps.

One of the best galaxies in the sky and certainly in Hydra is M83 (NGC 5236), commonly called the ‘Southern Pinwheel Galaxy’. Lacaille first saw this superb face-on barred spiral galaxy in 1752. At 8th magnitude, this large round glow with a very bright nucleus and two spiral arms can be seen well in as little as a 15cm telescope. The spiral arms appear as a reversed letter S and a 3rd spiral arm can be seen with larger scopes. Binoculars will show its hazy round glow under a dark sky.

Lying at a distance of 15 million light years, it is one of the closest galaxies to our Galaxy. The total luminosity is about 5 billion times that of our Sun. The remarkable thing about this galaxy is its frequency of supernovae, 6 appearing within 60 years where the average for most galaxies is 1 every 100 years or so. The first supernova discovered was in 1923 when it shone as a 13th magnitude glow. Then followed supernovae in 1945, 1950, 1957 and 1968. J.C. Bennett, a South African amateur astronomer, discovered the latter. It shone at 12th magnitude photographic, nearly central in the nucleus, equal to the combined light of 100 million Suns. The latest supernova was in 1983 so this galaxy is well worth monitoring around the clock.

M83 is at the centre of one pair of a galaxy group, often referred to as the Centaurus A/M83 Group with Centaurus A in Centaurus at the hub of the second group. Both groups are physically close to one another and seem associated.

There are other galaxies worth looking at, though none as impressive as M83. NGC 2835 is a spiral of high surface brightness available to smaller apertures. NGC 3109 is an irregular galaxy of cigar-shape and will need a 20cm telescope while NGC 3124 is a barred spiral with a bright nucleus. NGC 3145 appears as a hazy glow with an oval nucleus visible in a 15cm or larger telescope.

NGC 3621 lies in a barren field and can be difficult to find. This galaxy should be a nice target in medium to larger apertures, the large glow showing an elongated central region with care.

The highlight of this area is NGC 3242, an exceptional planetary nebula with an extremely high surface brightness. With a diameter similar to Jupiter, it is commonly called the ‘Ghost of Jupiter’ after a description by Admiral Smythe.

Sir William Herschel first found it on Feb. 7th, 1785. Employing his newly constructed 18.7” f/12 telescope, he described it as:

“a beautiful, very brilliant globe of light; a little hazy on the edges, but the haziness goes off very suddenly, so as not to exceed the 20th part of the diameter, which I suppose to be from 30 to 40 arcseconds. It is round, or perhaps a very little elliptical, and all over of a uniform brightness: I suppose the intensity of its light to be equal to that of a star of the 9th magnitude.”

Small telescopes will show a pale greeny-blue football shaped glow, located about 1.8° south of mu Hydrae. Larger apertures give a stunning view, with an intense greeny-blue colour rarely seen elsewhere and internal structure within the nebula when high power is employed. The edges are distinctly fuzzy and not well defined. The 11.4 magnitude blue central dwarf star can also be seen.

Another planetary nebula worth looking for, though needing darker skies is NGC 2610. Visually, it appears as a faint, round, pale grey disk approximately 40" in diameter surrounded by a delightful field. A 12th magnitude star is superimposed on the northeastern edge and a bright orange star sits 3' away to the northwest. A 15cm telescope should show this nebula and a 20cm at high power might show mottling with averted vision.

Finally, for those lovers of the feline variety, the 18th century French astronomer Joseph de Lalande included a now lost constellation in his third edition of Astronomie. Named Felis, it nestled between the stars mu, lambda and upsilon in Hydra and alpha and theta in Antlia. As Lalande explained:

"I am very fond of cats. I will let this figure scratch on the chart. The starry sky has worried me quite enough in my life, so that now I can have my joke with it."

Clear skies and good hunting.

~CosMos

Credits:

Image: Hourglass Nebula copyright NASA.