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20020717 Summer Sky Objects (Ehrmin).doc
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From the Everett and Seattle Astronomical Societies, this is IT'S OVER
YOUR HEAD for the week beginning July 17th, a look at what's up in the sky
over Puget Sound.
The glory of the summer sky is highlighted by the Milky Way, a giant
spiral galaxy, which stretches from the northern horizon in Perseus, through
the cross-shaped constellation Cygnus and down to Sagittarius in the south.
The Milky Way is packed with riches such as star clusters, nebulae, double
stars and variable stars. The disk of the galaxy is a flattened, rotating system
which contains our Sun and other intermediate-to-young stars. The Sun sits
about 2/3 of the way from the center to the edge of the disk, about 25,000
light years. The Sun revolves around the center of the galaxy once every 250
million years. To see the Milky Way in all its beauty, you need to travel to a
location far away from the glow of city lights.
One of the highlights of the Milky Way is the Big Dipper, our stellar
signpost. The Big Dipper now lies in the northwest with its handle pointing
toward Arcturus. The Big Dipper is not a constellation but an asterism or
distinctive group of stars. It consists of 7 bright stars forming a dipper, a small
pot with a long handle. In England it is often called 'the plough'. Fugitive
slaves before the Civil War knew it as 'the drinking gourd', a signpost in the
sky pointing the way north to safety in Canada, where slavery was outlawed.
The Big Dipper and the north star, Polaris, are featured on the Alaskan state
flag.
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High overhead and the first star to appear after sunset is Vega, in Lyra
the Harp. Vega, along with Altair and Deneb, form the summer triangle, a
familiar asterism of three stars. Vega is also one of three brilliant stars that
divide the northern heavens into thirds, the others being Arcturus and Capella.
At magnitude zero, Vega is the sky's fifth brightest star. Epsilon Lyrae, one of
the most observed multiple star systems, is easy to find in the constellation of
Lyra. Epsilon Lyrae splits into two pairs of stars, Epsilon 1 and Epsilon 2.
Epsilon 1 is two stars of magnitude 4.7 and 6.2, physically connected and
taking about 1200 years to complete one orbit. Epsilon 2 consists of 5.1 and
5.5 magnitude stars. They are also physically connected and orbit once every
585 years.
Globular clusters are gravitationally bound concentrations of
approximately ten thousand to one million stars. They populate the halo or
bulge of the Milky Way. Our galaxy has about 200 globular clusters. Open or
galatic clusters are loose aggregations of dozens or hundreds of young stars.
They are generally not gravitationally bound and are believed to originate from
large cosmic gas and dust clouds. Most open clusters have only a short life as
stellar swarms. As they drift along their orbits, some of their members escape
the cluster due to velocity changes in mutual closer encounters and
encounters with field stars and interstellar clouds crossing their path.
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There are several types of nebulae. Emission nebulae are clouds of high
temperature gas. The atoms in the cloud are energized by ultraviolet light
from a nearby star and emit radiation as they fall back into lower energy
states. Emission nebulae are sites of recent and ongoing star formation. The
Orion nebulae, also know as M42, is a good example of an emission nebulae.
Dark nebulae are clouds of dust which are simply blocking the light from
whatever is behind. Reflection nebulae are clouds of dust which are reflecting
the light of a nearby star or stars. Planetary nebulae are shells of gas thrown
out by some stars near the end of their lives. Our Sun will probably produce a
planetary nebulae in about 5 billion years. They have nothing to do with
planets. The name came about because they often look like planets in small
telescopes.
Variable stars are stars that change in brightness. These brightness
changes can range from a few hundredths to as much as twenty magnitudes
over periods of a fraction of a second to years, depending on the type of
variable star. Stars change in brightness when they are very young, or when
they are very old. Over 30,000 variable stars are known and catalogued, and
many thousands more are suspected to be variable. There are a number of
reasons why variable stars change their brightness. Pulsating variables, for
example, swell and shrink due to internal forces. An eclipsing binary will dim
when it is eclipsed by a faint companion and then brightens when the
occulting star moves out of the way.
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For more on this week's topic, or to find out about local astronomy
groups and activities, set your browser to “www.itsoveryourhead.org”. Our
broadcast was written by Jim Ehrmin and engineered by Greg Donohue. Until
next time, this is Jim Ehrmin and ________ __________, wishing you clear
skies and good viewing.