IMAGE GALLERY

GALAXIES A galaxy is a large island of tens or even hundreds of billions of stars, with a diameter of a few thousand to hundreds of thousands of light years. Astronomers estimate that there are roughly 100 billion galaxies in the visible universe. The word galaxy comes from a Greek word (galakt) for "milk." The reference is to the "Milky Way" (the milky river that arcs across a dark, moonless sky at night, and comprises the center of our own galaxy). As ever larger telescopes were constructed in the centuries following Galileo's crude instruments, astronomers detected an increasing number of small oval and circular smudges in the sky. These were called "nebulae," from the Latin word for cloud. The exact nature of these nebulae was not known until the 1920s when it was conclusively shown that the vast majority of them are galaxies separated from us by enormous distances. Our own Milky Way galaxy is a member of the "Local Group" of some 60 neighboring galaxies that include M31, M33, and NGC253 (all shown in this gallery). Edwin Hubble, one of the foremost astronomers of the 20th century, made the following observation, "We know our immediate neighborhood rather intimately. With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial."

OPEN STAR CLUSTERS An open star cluster is a group of a few dozen to a few thousand stars that are gravitationally linked. Most clusters share a common origin in a large cloud of hydrogen gas. Some are still embedded in the gas cloud that birthed them, which is illuminated by its offspring through the process of fluorescence. Examples in this gallery include the Pleiades (M45) in Taurus, and NGC2244 in Monoceros. Some open clusters are visible to the naked eye, including the Pleiades and the so-called Beehive Cluster in the constellation Cancer. Because of their common origin, most stars in an open cluster are roughly the same age and are of equal distance from Earth. However, they may differ greatly in size, temperature, and color. Hundreds of open star clusters in our Milky Way galaxy can be seen in amateur telescopes.

GLOBULAR STAR CLUSTERS A globular star cluster is a very compact group of several thousand stars. The larger globular clusters may have nearly one million stars. Astronomers have counted 150 of these clusters in our galaxy. With a diameter of up to one hundred or so light years, the density of stars within the core of such a cluster may approach 100 stars per cubic light year, meaning that stars are separated by only one-tenth of a light year (roughly 80 times the diameter of our Solar System). Most of the globular clusters in our galaxy are located in Sagittarius, Ophiuchus, and Scutum, which is in the direction of the galactic center.

PLANETARY NEBULAE A planetary nebula represents the explosive discharge of much of the contents of a star toward the end of its burning cycle. A remnant of the star remains, which is often called the "central star" (often visible in the center of the nebula). The central star can be seen in some of the images in this gallery. The nebula itself is a small, but expanding, cloud of gas that is illuminated by the high energy of the central star. These objects are called "planetary" nebulae because many of them resemble the planet Uranus (blue disks).

NEBULAE The word nebula is the Latin word for cloud. Nebulae (the plural variant) are clouds of gas or dust. There are a number of different kinds of nebulae, including the following: (1) Planetary nebulae (summarized above). (2) Diffuse nebulae. These are enormous clouds of hydrogen gas and dust. Some (emission nebulae) are illuminated by hot stars, while others (reflection nebulae) reflect the light of emission nebulae and stars. (3) Dark nebulae. These are large clouds of dust that appear "dark" because there is no nearby source of illumination. A good example can be seen in M11 (in the open star clusters gallery). (4) Supernova remnants. The best example is M1, the Crab Nebula. These are the remnants of stars that exploded in a supernova eruption ages ago.

DOUBLE STARS Most of the stars in the universe are double or multiple stars. A double star is a pair of stars that orbit about a common center of gravity (like the Earth and Moon). The best example is Mizar, the second star from the end of the "Big Dipper" handle. On some clear night, see if you can split this star into its two components. Few if any of us can do so today, but it is interesting to note that splitting Mizar was a test of visual acuity used by the Roman legions two thousand years ago. Some star systems are triples, and some consist of 4 or 5 stars. Some doubles display dramatic color combinations. A good example is Gamma Andromedae in this gallery. Many stars "appear" to be doubles, because of the chance alignment of their lines of sight. These are not true doubles because they do not orbit about a common center of gravity. In fact, one star may be several light years closer or farther to us than the other. Here's another fascinating fact about double stars. Over 400 years ago, astronomer Johannes Kepler proposed his famous Three Laws of Planetary Motion. As modified by Isaac Newton, the Third Law states that the mass of any two orbiting bodies can be determined by the cube of the semimajor axis (the greatest separation of the two bodies) divided by the square of their orbital period. Take a look at the double stars in this gallery. If I take photographs of the same double stars ten years from now, you will see definite movement. You then can easily calculate their orbital period. Computing the semimajor axis of the components of any double star system can also be easily done. By dividing the cube of the semimajor axis by the square of the orbital period you end up with the combined mass of the two stars. So, with an equation derived centuries ago we can determine the combined mass of double stars many trillions of miles away!

SOLAR SYSTEM The Solar System is the collection of matter in the vicinity of our Sun. The most notable components are the Sun, planets (and their satellites), asteroids, and comets.