Perseus the Hero

Perseus the Hero
Perseus was a first-class Greek hero – demi-god, monster-slayer, founder of Mycenae and ancestor of the Persians. His greatest tales are told in the sky by fiveconstellations: Perseus, Andromeda, Cassiopeia, Cepheus, and Cetus.

The Story
King Acrisius wanted a son. It was bad news when an oracle told him that not only would he have no son, but in addition, his grandson would kill him. This was also bad news for his only daughter Danaë, for he imprisoned her in a securely-guarded dungeon.

But Danaë was a beauty, and Acrisius hadn’t reckoned on the wily god Zeus who always had an eye for a pretty face. Zeus entered the prison as a golden shower through a tiny opening. Nine months and a bit later there was a baby named Perseus. The king didn’t dare kill his daughter and the child outright, but stuck them in a chest and set it adrift on the sea.

Danaë and Perseus were rescued and sheltered by Dictys, whose brother Polydectes was king of the island of Seriphos. Over time Polydectes became infatuated with the lovely Danaë, and wanted to marry her, but minus Perseus. He tricked Perseus into a quest to bring back the head of Medusa the Gorgon. Merely to see her face turned you into stone, so it didn’t seem as though Perseus would be back.

But young Perseus had a god for a parent, and other gods lent him a shiny shield, a sharp sword, an invisibility hat, a special bag and winged sandals. Using the shield as a mirror, Perseus crept up on the sleeping Medusa. One blow of the sword removed her head, which he tucked safely in the bag.

On his way home he spotted a beautiful girl chained to a rock. This was Andromeda awaiting death at the jaws of the monster Cetus. An oracle had told her parents, King Cepheus and Queen Cassiopeia, that it was the only way to save the kingdom from the monster’s ravages. Perseus did a deal with them to kill the monster in exchange for marrying Andromeda.

Perseus duly killed Cetus and got the girl. They married and had nine children, and founded an impressive dynasty. But what about the oracle of Acrisius? All unawares, Acrisius had attended some athletic games in which Perseus was taking part. Perseus’s discus throw went slightly awry and hit Acrisius, killing him instantly.

When Perseus died he was placed in the heavens. His constellation contains many fascinating objects.

Perseus’s brightest star Alpha Persei is nearly first magnitude. The star, also known as Mirfak or Algenib, is a supergiant 5000 times brighter than the Sun, but it's a long distance away. Zeta Persei (Atik) and Xi Persei (Menkhib) are brighter than Alpha Persei, but even farther away.

The second brightest star is Beta Persei, a variable star representing Medusa’s head. It’s probably the best-known star in the constellation. The traditional name Algol is a short form of a name meaning "demon’s head" and it’s commonly known as the "demon star". In the nineteenth century, American astronomer Edward Pickering realized that it wasn’t a single star, but an eclipsing variable. Two stars in our line of sight orbit each other. One is much brighter than the other, so the amount of light we see varies. In fact, we now know that it’s a triple system, but the third star never blocks the others.

Deep-sky objects
Perseus contains a variety of nebulae. Having lived in California for many years, I quite like the California Nebula (NGC 1499), even though it’s so faint you can only see its shape in a long-exposure photo. It’s an emission nebula, glowing red from hydrogen gas energized by nearby Xi Persei.

The Little Dumbbell Nebula (Messier 76) has a central area with two lobes which make it look a bit like an exercise dumbbell. It’s a planetary nebula, formed from the outer layers of a dying star.

NGC 1333 is young nebula, only about a million years old. (That’s young in astronomy!) It’s a reflection nebula, which means that it shines by reflecting the light of a nearby star. It’s also a very active site of star formation.

There are a number of clusters and galaxies in Perseus, but the most impressive structure is the Perseus Cluster (Abell 426). It’s a cluster of hundreds of galaxies some 250 million light years from Earth, and one of the most massive structures in the known Universe. One galaxy, NGC 1275, dominates the cluster. It contains an active black hole that makes the cluster the brightest one in the sky in X-rays. In 2003 scientists in Cambridge, England brought fame to the black hole for its singing.

A black hole doesn’t suck things in as you might drink a milkshake through a straw. Its strong gravity pulls material into a ringlike structure called an accretion disk. Some of the disk material is then thrown out at nearly light speed in jets at the poles. Using the Chandra X-Ray Telescope, the Cambridge team found that the jets hit the superhot cluster gas to form bubbles, making ripples like gigantic sound waves. You can't hear them, but can see them in X-Ray light.

The lowest sound the human ear can hear has a frequency of 20 waves a second. The Perseus black hole has a frequency of one wave in 10 million years. That would make for a long concert.

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