CHESHIRE STAR WATCHby Jim Maynard
Star date: 05:11:97
The spring sky, often the
sparcest for astronomical targets, offers a few fine objects to view in
this week's morning sky. The chain of nebulae we will be exploring this
week can be found in the south, around 4:30 a.m., an hour before sunrise.
For this, we will need to measure distances across the sky.
One way of estimating degrees of arc in the sky is to hold a fist at arms length. The distance across the back of your hand is about 10 degrees.
One of the beautiful treats this week is a cluster of nebulae almost due south in the pre-dawn sky. Most are not visible to the naked eye, but a telescope or binoculars suddenly make these bright clouds of gas jump out at the waiting observer. They can be found starting low, just above the horizon to the south-southwest. They extend for 25 degrees, ending almost due south. They can be seen on the map above as squares beginning southwest of the constellation Scorpius (the scorpion), and heading northeast above the constellation Sagittarius (which looks like a teapot, and is low in the southeast sky). The nebula nearest the horizon is known as NGC 6231. These objects are all shown on the map above as small squares. (NGC stands for New General Catalog, one of the lists astronomers use to label nebulae, which are cloud-like objects.)
NGC 6231 is an open cluster, which means it is a collection of a few dozen thousand stars, held to each other by gravity. It is clearly visible to the naked eye, shining at magnitude 2.6. It appears to us, however, only about 1/4 degrees across.
Four degrees to the northwest lies another open cluster, known as NGC 6281 (the names of astronomical bodies are often practical, but rarely poetic). This is a magnitude 5.4 nebula (nearly 16 times dimmer than NGC 6231), but is an entire degree across. That is nearly twice the angular diameter of the full moon.
Eight degrees to the northwest of that open cluster lies the magnitude
5.5 open cluster NGC 6383. This spans an even greater portion of sky than
NGC 6281, as it arcs across 1.3 degrees of the night starfield.
Eleven degrees further, in the same directions brings you to the Trifid nebula, M20 (also known, of course, as NGC 6514). It can be found directly above the spout of the teapot of Sagittarius. This nebula is half of a degree across, and remains one of the most photographed objects in the night sky. The Trifid nebula shines at magnitude 6.3. Very near this nebula is the large (from our vantage point) Lagoon nebula M8 (NGC 6613). If you can see both object& in the-same field, the larger one is the lagoon nebula.
Eleven more degrees in a straight line from the last object brings you to a trio of heavenly bodies known as M18, an open cluster 1/6 degree across that shine at magnitude 6.9.
NV 6605 is the next nebula to the northwest and is another open cluster, with a magnitude of 6.
The final treat in the sky is just slightly further to the northwest and is known as the Eagle nebula, M16 (NGC 6611). It is a gaseous nebula with dust, shining at magnitude 6. A dark cloud in "front' of the nebula resembles an eagle, either just landing or just taking off. The nebula as a whole takes up 1/2 degree of sky (about the same as the sun or moon), but the eagle is a challenge to find. See if you can.
Do
you see that bright light in the east-southeast sky? That's the planet Jupiter,
due to sneak back into our evening sky by late spring.
Clear skies and good viewing.
Jim Maynard is the head of the astronomy department at Earth Treasures and has been an amateur astronomer for more than 20 years. He is an astronomy student at Keene State College and leads star parties at Wheelock Park in Keene. If you have any questions about astronomy or stargazing, call him at 357-7192.
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