Thursday, March 5, 2009

Perseus the Hero

The constellation Perseus is— along with Andromeda, Cassiopeia, and Cepheus— a member of the star groupings known collectively as the Royal Family. The four royals featured prominently in Greek mythology and have been immortalized in the heavens above.

Perseus, Cassiopeia, and Cepheus all lie along the band of the Winter Milky Way, and if you’re observing from a dark site, you can use this knowledge to aid you in locating our hero.

1) Wait at least one hour after sunset to begin observing, so your sky is good and dark.

2) Face north. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your left shoulder to the west, and you’ll be facing approximately north.

Star maps created with Your Sky

3) First locate the Lazy W asterism (recognizable star pattern) of Cassiopeia, west of the meridian. It will be inverted, so it will look more like an “M.” Below it, toward the northern horizon is the House asterism of Cepheus. If you can see the glowing band of the Winter Milky Way stretching up from the northern horizon behind Cepheus and Cassiopeia, follow it along to the next group of bright stars. This is Perseus.

4) If you can’t see the Milky Way, draw an imaginary line between the middle star of the Cassiopeia “M” and the star to its right, or south. Continue extending the line south, and the next bright group of stars you come to is the “backbone” of the constellation Perseus, the asterism known as the Segment.

This curved string of six naked-eye stars is quite prominent, and if you were to extend the curve beyond the last star, you would come to blazing Capella, the brightest star in Auriga the Charioteer.

5) The brightest star in the Segment— and in Perseus— is Mirfak (MURR-fahk). Mirfak is Arabic for the elbow of the Pleiades. I can’t help but imagine the daughters of Atlas breaking with decorum and giving Perseus a sharp elbow to the ribs.

Mirfak is a yellow-white supergiant star, and the brightest member of a young open cluster of stars. An open cluster is a loose group of stars that formed around the same time in the same nebula (cloud of gas and dust). You can see the brighter stars of the cluster surrounding Mirfak with your naked eye, but binoculars or a telescope will reveal a gorgeous collection of stars.

6) The second brightest star in Perseus is the notorious variable star Algol, aka the Demon Star. Algol (AL-goll) is Arabic for the demon’s head. Algol is variable because it is a double star, and its orbiting companion star eclipses it like clockwork every 2.8 days. This makes Algol’s brightness dip suddenly and dramatically.

Perhaps this eerie “winking out” effect is what led a number of cultures to associate this star with death, violence, and danger. The ancient Greeks considered Algol the decapitated head of Medusa, being brandished by her slayer, Perseus. Medusa was the snake-haired creature whose frightful appearance turned anyone who looked upon her to stone.

Look for Algol southwest of Mirfak.

7) At the end of a string of five stars that meanders south from Mirfak is the star Atik. Atik (AH-tick) is Arabic for collarbone of the Pleiades. Atik is a blue-white giant star that is dimmed considerably to our view by interstellar dust in the Milky Way.

8) Go back up the five-star string two places to locate the only other star in Perseus with a traditional name: Menkib. Menkib (MENN-kibb) is Arabic for shoulder of the Pleiades. These references to the Pleiades in the star names of Perseus recall a larger Arabic constellation that encompassed the Pleiades and certain stars in Perseus and Cassiopeia.

Menkib is a blue giant star, 40 times more massive than our Sun. Menkib is destined to end its life in a cataclysmic explosion known as a supernova.

Astronomy Essential: Most of the mass of the solar system is the Sun.

To be more specific, the Sun contains 99.8 percent of the mass (quantity of matter) of the solar system. That’s right, the planets, moons, asteroids, comets, and interplanetary dust represent just two-tenths of a percent of the mass of the entire solar system!

To help you picture the scale involved, imagine a bowling ball. This is the Sun. Now imagine a single peppercorn placed next to the bowling ball. This is the Earth.

The Sun rules. Any questions?


Carolyn624 said...

Hi Karen, Is it correct that there are five bright stars in Perseus. The others being lesser stars?

Karen Keese said...

Carolyn, thanks for your question. Yes, I think that is a good characterization--to say that there are five bright stars in Perseus. The five stars with the greatest apparent magnitude, that is, the brightness as seen from Earth, are: Mirfak, Algol, Atik, Epsilon, and Gamma (in order from brightest to dimmest). These are the stars that really pop out when you view Perseus with the naked eye.

But I would be hesitant about characterizing the other stars as "lesser." Some of the stars that don't appear so bright are giants and supergiants, really gigantic stars! Their surface temperature and their distance can both have an impact on how bright they appear to us Earth-bound observers.