Monday, October 15, 2007

The Hero in Central Park

Perseus the Hero and NELM.

Rey's Perseus with less markup.
Perseus constellation with no markup.

Perseus is among my favorite constellations. The Hero.

He serves as a reminder of when I started and conjures emotions of my first discovery - Alpha Persei Association! From left field of a Heckscher diamond, I looked above the buildings lining Fifth Avenue out into the eastern sky. Off the beaten path in Central Park, this was a relatively dark sky spot. No lighting fixtures in my immediate area. They lit the walk that surrounded the baseball diamonds but I was well in.

On that seemingly black background sky Aries, Taurus, Cassiopeia, Perseus, and Andromeda twinkled before me. Yet I could see nothing. The sky was brand new to me and black is an absolute term not to be used in this circumstance. With persistence I returned to the field with Fujinon 7x50 binoculars and a Tele Vue Ranger, sometimes with Chet Raymo's An Intimate Look at the Night Sky or Peterson's Field Guide Stars and Planets, learning to identify the constellations.

How surprised was I when I "discovered" a waterfall of stars below Mirfak, the alpha star of Perseus. I thumbed through the books I had on hand to find no catalog number. The Pleiades, Andromeda Galaxy, and the Double Cluster were grabbing the headlines while this bright splash was waiting for announcement.

I am sure we all have similar stories.

Turns out this is well known as the Alpha Persei Association. It is cataloged as Melotte 20 and described as a stellar stream about 600 light years distant.

How apt is that name: stellar stream? When describing the nature of open clusters to pedestrtians I often relate how leaves fall from the same tree into a stream. Over time the leaves part in their own direction yet retain the same proper motion of its siblings. Stars born from the same molecular cloud.

The Hero pulls up the late Fall and early Winter sky and provides a means to judge our naked eye limiting magnitude (NELM). Comfortable, cool temps that I prefer over summer's heat and humidity. This photo above is pretty close to what we see at TotL on transparent nights. All the stars labeled in this 8 second exposure captures the stars we use to measure NELM. The first star I look for is the magnitude 5.2 star, HIP14043, near the Steeple (labeled as "Cap"). Charlie, Kin and I count off the stars of the Fish Hook. The magnitude 5.4 star is difficult but doable on the clearest of nights. In the Alpha Persei Association we can count 4, rarely 5 stars, between Mirfak and delta Per. Even though we don't get a hint (never had a hint to date) of the Milky Way, we can resolve stars as dim as mag 5.4.

As John Bortle notes in his article, The Bortle Dark-Sky Scale, naked eye limiting magnitude is not a reliable descriptor of a site's transparency.

Amateur astronomers usually judge their skies by noting the magnitude of the
faintest star visible to the naked eye. However, naked-eye limiting magnitude is
a poor criterion. It depends too much on a person's visual acuity (sharpness of
eyesight), as well as on the time and effort expended to see the faintest
possible stars. One person's "5.5-magnitude sky" is another's "6.3-magnitude

Quoted from Sky&Telescope site

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