April 19, 2008

Hydra

The constellation Hydra sprawls across an angular distance of about ninety degrees in the Southern sky. The head, near the northern end, is bordered by Sextans, Leo, Cancer, Canis Minor and Monoceros (all constellations I've talked about previously). The tail at the southern end is sandwiched between Centaurus, Libra and Virgo. Corvus and Crater are found on top of the body, while Antlia and Pyxis are just below the body. In mythology, I've already talked about the story of the water snake, Corvus, Crater and the god Apollo, but another legend has Hydra as the serpent brother of Draco the dragon. In this legend, the Hydra is the beast killed by Heracles as part of his twelve labors.

Rather than doing stars and then deep sky objects, I'll start at the northern (head) end of Draco and work my way backwards towards the tail. The northernmost object of interest in Hydra is the open galactic cluster Messier 48. You can find this by first finding Canis Minor. Follow the line from Gomeisa to Procyon, and then go about three times that distance along that line toward the South, and you will see M 48. This cluster is about 300 million years old and contains a few yellow giant stars that have already evolved off the main sequence. Most of the rest of the 80 or so starts are A-type main sequence stars resembling Sirius. This cluster is about 1500 light years distant and roughly 20 light years in diameter subtending an angle roughly equal to the Pleiades cluster in Taurus.

The head of Hydra can be found by following a line from Regulus in Leo about 2/3 of the way to Procyon in Canis Minor. About a degree or two south of that imaginary point in the dark sky, you will find the dim circle of stars that makes up the head. You can also find it by proceeding about 13 degrees due south from the Praesepe cluster (M44) in Cancer. A dim orange giant star about 350 light years away marks the nose of the serpent, Sigma Hydrae. I've seen several names for this star, all of which come from the Arabic for "nose of the serpent": Minaruja, Minchir, Michar and al Minchar al Shuja.

One of the other bright stars that makes up the circle of the head is Epsilon Hydrae, a noted multiple star system. To the naked eye, this system is about 3rd magnitude, but it is due to the combination of light from two fainter stars, a yellow giant and a blue subgiant orbiting each other at about the same distance as our Sun and Saturn on average (it is a very eccentric orbit). About 4.5 arcseconds away is another much more tightly spaced binary system in orbit around the two larger stars, and this quartet (likened to Mizar and Alcor in Ursa Major) is itself orbited by another faint companion star about 19 arcseconds away.

Continuing now from the head down into the body region of the constellation Hydra, we next run into Alpha Hydrae, Alphard, by far the brightest star in this dim constellation. Alphard translates to "the solitary one", and it truly is alone in the sky without many bright stars around. To find 2nd magnitude Alphard, start from Algeiba in the sickle of Leo and proceed South through Regulus. Follow that line about three times as far as the separation between these two stars (about 24 degrees in total), and you'll get to Alphard. This star is about 175 light years away and is an orange giant star, similar to Aldebaran in Taurus.

About 12 degrees Southwest along the body of Hydra, you can find a very nice planetary nebula known as the Ghost of Jupiter Nebula. This is also known as NGC 3242. Through a small telescope, it bears a resemblance to Jupiter, being a slightly larger angular size and apparently with some banding, just a different color scheme. This nebula is 1400 light years away, a distance estimated by observing the angular expansion rate along with Doppler shift estimates of the shell speed moving toward us.

Moving about 8 degrees ENE from the Ghost of Jupiter, we run into Nu Hydrae, also known as Sherasiph, which translates to "rib" of the snake. This is another orange giant similar to Alphard and at about the same distance, but it is about 5 times fainter thanks to interstellar gas and dust along our line of sight to the star. Moving further down the body, if you draw a line connecting Epsilon Hydrae through Alphard and go that same distance along this line, you will find a galaxy cluster known as the Hydra Cluster.

This isn't too hard to find with a telescope since the brightest galaxy in the group is about 5th magnitude. Two faint red stars bracket the group. They have magnitudes of about 4.5 and 6. The cluster, also known as Abell 1060, is about 140 million light years from Earth (which makes that 5th magnitude giant elliptical galaxy near the cluster core, NGC 3311, remarkably bright). The highlight of this little cluster is definitely NGC 3314, a pair of spiral galaxies that overlap each other along our line of sight. You can clearly see through the foreground galaxy and make out the shape of the background galaxy, and the nuclei of these two galaxies overlap almost exactly.

Following a line from Epsilon to Alphard took us to the Hydra Cluster last time. From there, continue on to the ESE about another 12 degrees, and this line will lead you to the spiral galaxy NGC 3621, about 22 million light years distant. This is one of those bright galaxies at a kind of intermediate distance that makes it very important in the distance ladder. It is close enough for us to make out some of the brightest stars, and by comparing those with the brightest stars in our own galaxy, we can work out the distance.

At the same time, when supernova explosions go off in galaxies like this, knowing the distance helps us work out the intrinsic luminosity of these supernovae. That way, when supernovae go off and far more distant galaxies, we will know how far away they are. Further along the tail of the snake, about 25 degrees Southwest of the bright star Spica (which, if you remember, can be found by following the arc of the Big Dipper's handle through Arcturus), is the globular cluster known as Messier 68 (or NGC 4590).

Over 100,000 stars crowd this cluster's densely packed core, including a number of variable stars (which are useful for establishing a distance to the cluster). The cluster is about 100 light years in diameter and likely formed at about the same time as the Milky Way. It's elliptical orbit about the galactic center is currently bringing it back in toward the galaxy, and it is currently about 30 degrees above the galactic center in galactic coordinates. That means if we are looking at the Milky Way edge on and flat with respect to the horizon, this cluster would be about 30 degrees above the direction of the galactic center.

About nine degrees ENE from here or 12 degrees due South from Spica is the star Gamma Hydrae, also known as Dhanab al Shuja, the tail of the serpent, a yellow giant star about 130 light years away. About three degrees due East of Dhanab al Shuja, you can find the interesting variable star known as R Hydrae. This red supergiant is known as a Mira-type variable, oscillating with a period of just over a year, during which time its intrinsic luminosity changes by a factor of nearly 500, from an easily visible 3rd magnitude star into a faint 10th magnitude star.

Burnham notes that the oscillating period of this star has shortened dramatically in the past 200 years, from about 500 days to 389 days, probably indicating a major change ongoing in the star's internal structure. The star is about 300 light years from Earth, and as it moves through the interstellar medium, the strong stellar wind from this star is blowing off a thick cloud of gas and dust. The outflow from the star forms a bow shock wave out in front of the star, imaged here by the Spitzer Space Telescope.

About six degrees South and maybe one degree East from R Hydrae is the Southern Pinwheel, a very nice symmetric face-on spiral galaxy also known as Messier 83. the southernmost galaxy in the Messier Catalog. Seen here in X-rays, this galaxy is very active with star formation, a process that tends to result in lots of high energy, short wavelength emitted light. This galaxy is about 15 million light years away and is one of the brightest and closest spiral galaxies to us, though still about seven times further than the great spiral in Andromeda.

The final object I want to talk about is a tough one to find, the globular cluster NGC 5694, located about 25 degrees directly Southeast from Spica (in the constellation Virgo). At 10th magnitude, this cluster is tough to pick out. It is about three times further away than the other globular we looked at earlier, M 68, at a distance of about 110,000 light years. In fact, it is so far away and moving so quickly that most Astronomers doubt it is even bound to the Milky Way galaxy at all. According to Burnham, it is moving at about 270 km/sec with respect to the galactic center, and at that distance, the escape velocity (even including the dark matter) for our galaxy is less than 200.

Has this cluster been a part of our galaxy the whole time and only recently ejected by a close encounter with a passing dwarf galaxy, such as the Canis Major dwarf that is being devoured by our galactic disk or one of the Magellanic Clouds? Or is this cluster truly an intergalactic wanderer, just passing through? Only detailed studies of the cluster's composition will tell us for sure, and this cluster is so remote, it will be a difficult task.

Posted by Observer at April 19, 2008 08:31 PM
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