astrojargon | Word of the Day

13 December 2008 00:11

bodies of ice, gas, dust, and rocks orbiting the sun
as comets near the sun, material boils off and comets develop a coma, a gaseous ball, and a tail which stretches out behind the comet
the word comet comes from Greek; Aristotle called them komētēs or "stars with hair"
comets are described by their orbital period, those with periods of 200 years are less are considered short-period, and greater than 200 years are considered long-period
some comets only swing by the sun once before being tossed out of the solar system like unruly drunks
many comets are visible only through telescopes, about one a year is visible to the naked eye
comets used to be described by the year of their appearance, then they were named for their discoverer, today they are named for two or three independent discoverers and given a coded designation by the IAU
sometimes comets brighten suddenly - as happened with Comet Holmes in 2007 pictured below

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8 December 2008 12:33

a group of stars that create a recognizable pattern
asterisms are not the same as the 88 standard constellations - some make up a part of the constellation, while some are made up of stars from multiple constellations
the Big Dipper is the most well-known asterism - it is part of the constellation Ursa Major
the Summer Triangle of Altair, Deneb, and Vega is made up of the brightest stars in three different constellations - Aquila, Cygnus, and Lyra
the Coathanger (also known as Ronchi's Cluster) in the constellation Vulpecula is a popular and very striking asterism when viewed in binoculars or a small telescope
while the 88 official constellations are the product of the International Astronomy Union, no organization is responsible for cataloging asterisms
author Phil Harrington started a list of asterisms in his book Touring the Universe through Binoculars that is now called the STAR list (small telescope asterism roster)

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5 December 2008 16:55

a large spherical group of stars bound together by gravity containing as few as ten thousand or as many as a million stars
also known as globular star clusters, or sometimes just globs
contrast with open clusters (aka, galactic clusters)
all galaxies above a certain size are believed to be orbited by associated clusters
the term was first used by William Herschel
in general, clusters appear to contain stars of roughly the same age, and they appear to be some of the oldest stars known
the Milky Way is estimated to have as many as 200 associated globular cluster, fewer than 160 have been discovered - the rest may not be visible from Earth
a visualization of the Milky Way's globular clusters can be found here (scroll to the bottom)
a few globular clusters are visible to the naked eye; Omega Centauri, for example, is a globular cluster masquerading as a star

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