October 17, 2007


Ok, time for another constellation. Today, I want to talk about the constellation Pegasus, which is very prominent (nearly overhead toward the SE) during evenings in October. The constellation is featured in this month's Hubble Site review of the sky, and their take on it as a baseball diamond is actually pretty amusing.

Pegasus is most easily found by looking for the Great Square asterism (sometimes referred to as the bowl of the Autumn Dipper). The square consists of the three brightest stars in Pegasus plus the brightest star in Andromeda. Third brightest (but still called alpha due to inaccuracy in the original rankings) is Markab (saddle) in the SW corner of the square. This star is a little over 100 light years away (for reference, the nearest star is just over 4 light years away, so this is still kind of in the neighborhood) and very massive with a luminosity of 100 times that of our Sun.

Beta Pegasi is called Scheat (foreleg), a red giant that is slowly pulsating as many large stars do. Scheat appaers to have two faint companions when viewed through a telescope and was once thought to be a binary pair with at least one of them, but the objects do not share the same proper motion through the sky over the years and so are likely unrelated. Gamma is Algenib (wing), another very intrinsically bright star a few hundred light years away.

As Burnham tells it, the most common legend is that Pegasus was born from the blood of the medusa after it was slain by Perseus. He was never the steed of Perseus and wasn't present at the rescue of Andromeda by Perseus, according to the original myths. Instead, he was the steed of the Greek hero Bellerophon, who had many adventures with Pegasus. Once, when Bellerophon tried to fly Pegasus up to Mount Olympus, Pegasus threw him off, kililng him.

Among the notable deep sky objects in Pegasus are NGC 1 and 2, the first two objects in Dreyer's New General Catalogue of Nebulae and Clusters of Stars. They are ordered in sequence by increasing Right Ascension, and these two have the smallest, being closest longitudinally to the Vernal Equinox. Think of Right Ascension like Celestial Longitude. The prime meridian runs through the North and South Celestial Poles and passes through the position of the Sun on the day of the vernal equinox, just like the prime meridian on Earth passes through Greenwich, England, and we measure celestial longitude (right ascension) starting at zero from that line of reference.

Also present is NGC 7331, a galaxy very similar to our own Milky Way. Just half a degree away toward the SSW of this beautiful object is Stephan's Quintet. This is an apparent cluster of galaxies. I say apparent because four of the members are roughly the same, but the fifth has a hugely different velocity, yet they all seem connected by tidal tails, so there is either a coincidence or something very odd dynamically going on with this system. Perhaps it is a cluster in the process of being disrupted.

Another very nice object is the globular cluster M15, only about 35,000 light years away (that's close for a globular), visible with binoculars from a dark location. It is very dense in the center, which makes it easy to see, the third brightest globular cluster in the sky. A good target for those just starting out with amateur astronomy since you can probably spot it through your telescope's finder scope and just center it more easily than most deep sky objects.

The first extrasolar planet discovered with the Doppler wobble technique was found orbiting 51 Pegasi, an otherwise unremarkable looking star. This sun-like star's unseen companion has a minimum mass of about half that of Jupiter, and its orbital distance is amazingly close (only about 5 million miles, which is much closer than Mercury's distance to our Sun). As we have discovered more and more systems like this one, we've had to rethink our ideas about how planets form out of disks of gas and dust (a story for another time).

There are also a couple of interesting binaries here. First, is U Pegasi (no link, this is all from Burnham), a pair of sun-like stars orbiting each other at a distance of a mere 1.2 million miles. They are likely so close that they share a common envelope or are at least exchanging mass periodically, which would account for the strange variations in the orbital period of this system. Who knows how the tremendous tidal forces these objects must exert on one another are affecting the interior structure and changing the evolutionary path?

There is also IK Pegasi, which is a bright A-class star with a massive white dwarf companion. When the A-star (similar to Altair in mass and size but further away and so much dimmer) ultimately expands to become a red giant, it will begin dumping mass onto the white dwarf, which means the white dwarf may collapse and explode if its Chandrasekhar Limit is overcome by the constant additions of mass. It is probable that the A-star was originally more sunlike but grew by adding mass dumped onto it from what is now the white dwarf companion back when the companion evolved off the main sequence.

Right now, this system is about 150 light years away from the Earth, but by the time it may eventually go supernova, it will have migrated much further away thanks to the flow of traffic in the galactic disk near the Sun.

Posted by Observer at October 17, 2007 07:46 AM

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Sometimes you trigger the even geekier side of me:

In the constellation of cygnus
There lurks a mysterious, invisible force
The black hole
Of cygnus x-1

Six stars of the northern cross
In mourning for their sisters loss
In a final flash of glory
Nevermore to grace the night....

To telescopic eye
The star that would not die

All who dare
To cross her course
Are swallowed by
A fearsome force

Through the void
To be destroyed
Or is there something more?
Atomized ---- at the core
Or through the astral door ----
To soar....

I set a course just east of lyra
And northwest of pegasus
Flew into the light of deneb
Sailed across the milky way

On my ship, the rocinante
Wheeling through the galaxies,
Headed for the heart of cygnus
Headlong into mystery

The x-ray is her siren song
My ship cannot resist her long
Nearer to my deadly goal
Until the black hole ----
Gains control....

Spinning, whirling,
Still descending
Like a spiral sea,

Sound and fury
Drowns my heart
Every nerve
Is torn apart....

Posted by: Humbaba on October 17, 2007 11:41 AM