The water snake is the largest constellation in the sky, so it's not surprising that it contains a number of deep-sky objects. Although its stars and deep-sky objects tend to be dim and distant, the development of modern telescopes has revealed them as never before.

Planetary nebulae
Planetary nebulae are created when a dying sunlike star loses its outer layers. They can be various possible shapes and colors, and some seemed even to have a disk like a planet when seen in 18th century telescopes. This is why William Herschel first described them as “planetary”, and the name has stuck.

The best known planetary nebula in Hydra is NGC 3242, which Herschel discovered in 1785. He saw it as a blue-green disk. Its nickname is Ghost of Jupiter because it's round, and appeared to be about the same size as Jupiter in a telescope. In fact, the nebula is about two light years across. (A light year is close to six trillion miles or 9.5 trillion kilometers.) It looks small because it's 1400 light years away.

Star clusters
Star clusters are groups of stars formed from the same nebula and held together by their mutual gravity. The two basic types are globular clusters and open clusters.

Globular clusters
The French astronomer Charles Messier (1730-1817) discovered an object that he numbered 68 in his catalog. M68 (NGC 4590) turns out to be a beautiful globular cluster about 33,000 light years away, though Messier didn't distinguish it from other nebulous objects. It took William Herschel (1738-1822) to see that M68 and some other nebular objects were round and made of stars. He called them globular clusters in his 1789 catalog.

Globular clusters have so many stars that their mass pulls them tightly together into a roughly spherical shape. They also contain some of the oldest stars in the Galaxy. While open cluster M48 is about 300 million years old, the globular cluster M68 is over eleven billion years old.

In this diagram of the Milky Way you can see a large spherical halo of stars around the central bulge. This is where the globular clusters orbit. About 90% of the globular clusters are seen in a hemisphere which is centered on Sagittarius, the constellation that shows the direction of the Galactic Center. However, M68 is something of an oddity, because it's in the opposite direction to the Galactic Center.

In 1784 Herschel discovered an object that's over 100,000 light years away. It's now known as NGC 5694. Herschel didn't recognize it as a globular cluster, and it was nearly a century and a half later that Clyde Tombaugh (the discoverer of Pluto) realized what it was. It's been nicknamed Tombaugh's Globular Cluster and is one of the oldest known clusters in the Milky Way. At nearly 12 billion years old, it's well over twice the age of our Sun.

Open clusters
An open cluster has many fewer stars than a globular cluster, so its stars are more loosely held together and the cluster tends to break up over time. The open cluster M48 (NGC 2548), which is 1500 light years away, has around eighty stars, but is a conspicuous object. In good conditions it's even visible to the unaided eye. Charles Messier discovered it and listed it in his catalog of nebulae. However he made a mistake in calculating its position, so the published position was wrong, making it impossible to find from the catalog. In 1783 Caroline Herschel – William's sister – independently rediscovered it.

Galaxies
Just on Hydra's border with Centaurus is the dramatic and beautiful M83, also known as the Southern Pinwheel Galaxy. It's a grand design spiral galaxy, which is a galaxy with notable and well-defined spiral arms. The great 18th century French astronomer Nicolas-Louis de Lacaille discovered it. M83 is noted for its supernovae, the enormous explosions that occur at the death of massive stars. Although the last supernova observed in the Milky Way was in 1680, six have been seen in M83 since 1945.

Galaxy clusters
Most galaxies that we know of aren't isolated, but are likely to be members of a group or cluster. A cluster is bigger than a group, but both are collections of gravitationally-bound galaxies. The Milky Way is a member of the Local Group of about fifty galaxies. Groups and clusters may themselves be part of even bigger groups called superclusters.

The galaxy Hydra A looks very bright in radio telescopes. It's in the center of a large galaxy cluster named the Hydra A Cluster that's 840 million light years away. The cluster also has a vast cloud of hot gas that extends several million light years from its center. And when we say hot, this is very hot indeed – even its cooler inner region has a temperature of 35 million degrees.

There is also a Hydra Cluster (Abell 1060) 158 million light years away and containing 157 galaxies. (The two brightest objects in the picture are foreground stars, not part of the cluster.) The Hydra Cluster spans some ten million light years and notably has a high proportion of dark matter. Dark matter is a strange kind of matter that we can detect by its gravitational effect, but cannot see. It explains why the cluster has fewer galaxies than expected from its mass. Nonetheless its three largest galaxies have diameters of 150,000 light years, which is much bigger than the Milky Way. [Photo: Angus Lau]

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