continued from Starlit Sky: 11. Leo
Let us take a small diversion from Leo and find an interesting constellation.
Modern astrology recognizes 88 constellations in the sky. The largest of them is the Hydra. It is called the Vasuki mandalam in the Indian Astrological System. It was recorded by the 2nd century astronomer Ptolemy too.
Hydra or the water snake, spans 100 degrees, its northern end is near Cancer and the southern tip ends near Libra.
Let us now locate it.
Locate the Regalus or Alpha Leonis and Algieba or Gamma Leonis of the Leo constellation and draw an imaginary line connecting the two and proceed in a southerly direction to about 2 and a half times their distance. You will reach a slightly orange coloured star of maganitude of about 2. That is the Alphard or otherwise called Alpha Hydrae. Alphard is about 177 light years away from the Earth.
A little above the Alphard, we can see a small cluster of stars that forms the head of the water snake. That is designated as the Sigma Hydrae (σ), also called Minaruja. It is quite a faint star with a magnitude of only about 4.5. In the Indian astrological system, it is called the Ashlesha star.
The Hydra constellation has many stars spread out like a snake across the sky. There are about 17 prominent stars in the constellation. Most of them are very faint. Hydra also contains some variable stars.
The Hydra is neighbors to many constellations in the sky, they are Antlia, Cancer, Canis Minor, Centaurus, Corvus, Crater, Leo, Libra, Lupus, Monoceros, Puppis, Pyxis, Sextans and Virgo.
Hydra is also home to many deep sky objects.
Hydra contains three Messier objects, namely M83, M68 and M48. M83 is called the SouthernPinwheel galaxy, M68 is a globular cluster and M48 is an open star cluster.
Variable stars: A variable star is one whose brightness seems to change when seen from Earth. The change in brightness can be because of the nature of the star (intrinsic) or because it is being eclipsed by another object (extrinsic). An intrinsic variable star’s luminosity changes because the star itself shrinks and swells periodically. An extrinsic variable star may be blocked by a companion star and hence its brightness seems reduced when seen from Earth.
Deep sky objects: Deep sky objects are those objects in the sky that are observed to be not stars or belonging to the Solar system (planets, meteors, comets, moons), namely clusters, nebulae and galaxies other than Milky Way. (Milky Way galaxy is where we live).
Messier objects: A French astronomer Charles Messier along with his assistant Pierre Mechain were hunting the sky for comets. He kept encountering deep sky objects that resembled comets but were not. He got frustrated and set about identifying these objects to avoid wasting time on them and published a list of these 103 objects in 1771. Later scientists added a few more to this list. These objects include globular clusters, open clusters, nebulae and other galaxies.
Globular cluster: A globular cluster is a collection of stars, that is spherical in structure that orbits a galactic core as a satellite. These clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.
to be continued…………….