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Early winter constellations

December 28, 2010
By Aileen O'Donoghue

The time for spending the evening in the chaise lounge is mostly gone, but it's worth dragging it out and piling on some warm blankets to spend some time feasting your eyes on the sky above. The outer arms of the Milky Way, our home galaxy of around 300 billion stars, arches high across the sky in the evenings of late November and early December. But also visible is the great spiral galaxy in Andromeda, our nearest major cosmic neighbor.

To find the neighbor, face south and use Jupiter to find the Great Square of Pegasus, as shown in the diagram. Three of the stars belong to the constellation of Pegasus, the winged horse whose neck stretches down to the southwest and whose nose is marked by the star Enif. The fourth star in the square, Alpheratz (AL-FEE'-RATZ, the horse's navel) in the northeast corner, actually belongs to the constellation Andromeda.

Though it represents a woman changed to a rock, the mnemonic stick figure is that of a cornucopia opening as it arcs up and away from Alpheratz.

Follow the lower arc to Mirach (MEE'-ROCK), the same brightness as Alpheratz but redder due to being 3.5 times cooler and about 40 times larger. From Mirach, find fainter Mu Andromedae, then even fainter Nu. Just northwest of Nu is a fuzzy

blur that is the combined fires of a trillion stars seen across two million years of intergalactic space. It is faint so you may need to use averted vision to see it.

This is a technique astronomers use for sighting faint objects. Look at Nu Andromedae, and sweep your eyes slowly to the northeast, around the position of the galaxy. The rods on the sides of your retina are more sensitive to dim objects, so you should be able to see it in your peripheral vision. It is the most distant object that can be seen from Earth by the unaided eye.

Binoculars should easily

bring it into view. This galaxy is so distant that when the light you see left it, our early ancestors were just adapting to life in the grasslands of Africa and developing the first

stone tools. And yet it is the closest major galaxy.

Closer to us, the constellations consisting of stars in our own galaxy provide a glittering display overhead. From bright Deneb, follow the Milky Way to Cepheus (CEF'-EE-US), the King, represented by a stick figure of a house as a child would

draw it.

Lurking in this constellation are two of the most impressive stars in our sky and the entire galaxy, Erakis and VV. Erakis, or Mu Cephei (CEF'-EE-EYE), is also

called the Garnet Star for its ruddy color. Its distance is thought to be about 2,400 light years, and yet astronomers have managed to measure its angular diameter to be about seven and a half times the radius of Earth's orbit. Yes, Earth's orbit! If the sun were replaced by Erakis, its outer layers would lie halfway between the orbits of Jupiter and Saturn. Mercury, Venus, Earth, Mars and Jupiter would all be inside the star.

VV Cephei is even more impressive with a radius 8.8 times Earth's orbit so that Saturn's orbit, would be at a slightly greater distance from that giant star as Earth is from the sun. But unlike the sun seen from Earth, VV Cephei would fill Saturn's sky and quickly destroy the planet with its intense radiation and stellar winds.

Past Cepheus is Cassiopeia, which my students refer to as "the cosmic W" and Perseus. This is a wonderful part of the sky to explore with binoculars and telescopes as it is filled with star clusters and nebulae. Between Cassiopeia and Perseus is the "double cluster" of eta and chi (KYE rhymes with eye) Persei (PER'-SEE-EYE). Though the H is the Greek letter Eta, astronomers refer to this

cluster as h and chi Persei, more rhythmically pleasing than eta and chi.

These can be seen with the unaided eye in a dark sky and are easily spotted as rich clusters with binoculars. Stars form from vast clouds of interstellar gas and dust, and thus

come into existence in clusters, sibling groups, known as open or galactic clusters. H and chi are two such clusters that formed about 13 million years ago. They are quite distant from us at 6,800 and 7,600 light years. This puts them in the Perseus

spiral arm of our galaxy, farther from the center than the Orion spur of the Sagittarius arm where the solar system resides. For comparison, the Alpha Persei cluster of stars surrounding Mirfak (MEER'-FAHCK, the elbow) is in our own arm, 580 light years away and 50 million years old. The closest open cluster is the Pleiades, also in the Sagittarius arm, 100 million years old and 450 light years away.

So lying back under blankets on a chaise lounge, a sweep of your eyes can take you from Pleiades and Alpha Persei clusters in our own spiral arm to h and chi Persei in the next spiral arm then across two million years of intergalactic space to

our galactic neighbor in Andromeda. From a back yard in the Adirondacks, you can cast your eyes and your mind across the universe.

 
 

 

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