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Since before the dawn of history, our ancestors have gazed skyward in awe and wonder. At first, the universe seemed cold, inhospitable, and filled with danger. Was the sky populated by gods of the Moon, the Sun, and planets who ruled mortals and could destroy or spare Earth with a mere wave of their hands? Did our very lives depend on their whimsy? As time evolved, curiosity grew about the exact nature of the Sun, the Moon, and other sky objects. The ancient Babylonians, Assyrians, and Greeks were among the first civilizations to study the sky in an attempt to understand how these objects influenced the course of human events. Their studies gave birth to the pseudoscience of astrology but more importantly also laid the basic foundation for the science of astronomy. Indeed, many of the names for stars and star patterns, called constellations, that we still use today trace their origin back to this early epoch.
Perhaps the greatest study of the universe by one of our ancient ancestors was performed by the Greek astronomer Claudius Ptolemaeus, or Ptolemy for short. While living in Egypt in the second century A.D., Ptolemy devised a scheme that predicted the movements of the Sun, the Moon, and planets in our sky with amazingaccuracy. Ptolemy's geocentric system, which placed Earth at the center of the universe, remained the dominant model for more than a thousand years, until the European Renaissance in the sixteenth and seventeenth centuries.
Today, we know the true order of the universe. No longer populated by fearsome gods and goddesses, our universe plays host to stars and galaxies, planets and moons, and many other wonders that beckon us to stare skyward with the same awe and wonder felt by the first astronomers. We know that Earth is not at the center of the universe, and in fact that the universe really has no center at all. Instead, the universe is populated by millions, if not billions, of galaxies (Figure 1.1), each a huge system of stars. With rare exception, all galaxies are seemingly racing away from one another, motions induced by the Big Bang, which created the universe some 15 billion years ago. Some are independently traveling through the universe, while others travel in groups and clusters.
Each galaxy, including our own Milky Way, is made up of millions, if not billions, of individual stars. Many stars exist alone, while others formed in pairs, trios, or larger groupings. The largest star groupings are referred to as star clusters (Figure 1.2). Depending on the type, a cluster may hold anywhere from a dozen to half a million stars!
Interspersed throughout many galaxies are large clouds of gas and dust called nebulae (Figure 1.3). Some nebulae may be thought of as stellar nurseries, marking regions where new stars are forming. Other nebulae are stellar corpses, all that remain of once powerful suns.
Closer to home, our star, the Sun, serves as the focal point of a collection of comparatively small celestial bodies that we collectively call the solar system. In addition to the Sun, the solar system includes the nine planets-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto-as well as many moons, asteroids, comets, and meteoroids.
All of these objects are waiting for you in the sky. But gazing skyward, trying to identify one from another or simply knowing where to look, can prove to be a daunting task for someone brand-new to the universe! Where is everything? Which star is which? Are there any planets out tonight? I just bought a new telescope; what should I look at first? And how do I use it?
All good questions. Where do you find the answers? I hope you find them here! Think of this book as your passport to places that few of the world's population are even aware exist. You and I are heading off to the farthest depths of space.
The Sky: An Overview
At first glance, the sky appears to be a mishmash of points of light, seemingly scattered at random and impossible to fathom. But the sky isn't as chaotic as it seems at first. It has a clear order that just takes a little time to understand. Let's begin with some basics.
Watching the stars move silently overhead at night, we can easily see how Ptolemy and other ancient skywatchers came away with the idea that everything circled Earth. Instead, we know that this effect is caused by Earth rotating on its axis, an imaginary line that passes through the center of Earth, with the North Pole at one end and the South Pole at the other. This motion, called daily or diurnal motion (Figure 1.4), causes the Sun, the Moon, planets, and stars to appear as though they rise in the east, move across the sky, and set in the west. Daily motion also opens our sky window toward different stars at different times of the night.
In addition to rotating on its axis, Earth also revolves around the Sun, taking one year to complete the trip. We call this annual motion. It takes Earth 365 days 5 hours 49 minutes to complete a revolution of the Sun, at an average speed of 66,500 miles (107,000 kilometers) per hour or 18.46 miles (29.73 kilometers) each second!
Annual motion also opens our sky window onto different stars and constellations at different seasons of the year. Those stars and constellations seen on winter evenings, for instance, are called the winter stars, and are completely different from those visible on summer evenings. During the summer and winter months, the night side of Earth is aimed toward different portions of the plane of our galaxy, where the gentle rifts of the Milky Way stretch across the sky to give spectacular views of rich star clouds and subtle nebulae. The spring and fall skies open away from the obscuring dust clouds of our galaxy to reveal a universe that is full of other distant galaxies. Figure 1.5 shows Earth in its annual trek around the Sun as compared with the seasonal constellations.
Finally, Earth also wobbles like a toy top in a motion called precession. If you've spun a top, you know that its axis of rotation is usually perpendicular to the surface on which it is spinning. But give it a slight nudge and the top will begin to tip over. Still spinning, the top attempts to right itself, in the process causing its axis of rotation to trace out a circle. The combined effect of Earth slightly bulging at the equator, combined with gravitational nudges from the Sun and the Moon, has nudged Earth enough to set it into a slow wobble. Precession is barely noticeable compared to our planet's rotation and revolution, taking some 26,000 years for Earth to complete just one twist. While this shift cannot be detected with the naked eye over the brief course of a human lifetime, the change is obvious across the span of human history. Right now, the Earth's North Celestial Pole is aimed almost directly at the star Polaris, also known as the North Star. But back in the time of ancient Egypt, the pharaohs saw the night sky turning about the star Thuban in our constellation Draco the Dragon. In 13,000 years, the star Vega in the constellation Lyra will be closest to the North Celestial Pole.
When astronomers talk about how bright something appears in the sky, they are referring to that object's magnitude. The magnitude system dates back over 2,000 years to the Greek astronomer Hipparchus, who was first to survey the night sky and devise a system for categorizing stars according to their brightness. His method was quite simple. The brightest stars visible to the eye were labeled 1st magnitude, while the faintest stars were 6th magnitude. The remaining stars fell somewhere in between. In the Hipparchus magnitude system, the larger the magnitude number, the fainter the star. The magnitude system in place today is far more precise but still strongly reminiscent of Hipparchus's. We still use the basic 1st-through-6th magnitude designations, but we now specify that a 1-magnitude jump (say from 1st to 2nd, or 2nd to 3rd) corresponds to a change in brightness of 2.5 times. Therefore a 1st-magnitude star is 2.5 times brighter than a 2nd-magnitude star, while a 2nd-magnitude star is 2.512 times brighter than a 3rd-magnitude star. By this method, a 1st-magnitude star is about 6.3 times brighter than a 3rd-magnitude star (2.512 x 2.512 = 6.310), about 15.8 times brighter than a 4th-magnitude star (2.512 x 2.512 x 2.512 = 15.85), about 40 times brighter than a 5th-magnitude star (2.512 x 2.512 x 2.512 x 2.512 = 39.81), and so on. A 5-magnitude jump, say from 1st to 6th magnitude, equals a change in brightness of exactly 100 times.
Astronomers of the nineteenth century refined and expanded Hipparchus's magnitude system to include the very brightest and very faintest celestial objects, so the scale doesn't stop at 1st or 6th magnitude. A zero-magnitude object is 2.512 times brighter than a 1st-magnitude object, while negative-value objects are brighter still. The Sun, for instance, is magnitude -26. At the same time, stars that are too faint to be seen with the naked eye have magnitude values greater than 6th magnitude. As you can see from Figure 1.6, binoculars reveal stars to about 9th magnitude, a 6-inch (15-cm) telescope to 13th magnitude, and larger telescopes deeper still. The Hubble Space Telescope can record stars as faint as 30th magnitude, about 19 billion times fainter than the Sun!
Magnitudes only refer to how bright objects appear in our sky. Just because a star looks bright in the sky doesn't mean that it is big, of course. For example, the Sun is the brightest object in our sky, but pull back an appreciable distance and it quickly disappears into the crowd. The Sun is only bright because it is so close. A star's apparent brightness, or magnitude, depends on two factors: its intrinsic luminosity and distance. To express a star's true brightness, astronomers use the term luminosity. The luminosity scale uses the Sun as its baseline value of 1.0. Stars with luminosities greater than 1.0 are intrinsically brighter than the Sun, while those less than 1.0 are dimmer. Sirius, the brightest star in the sky, has a luminosity of 24, meaning that it is 24 times more luminous than the Sun, while the star Deneb in Cygnus has a luminosity of 24,000. So why does Sirius look brighter in our sky than Deneb? Measurements show that Sirius lies only 9 light-years away, while Deneb is more than 3,200 light-years from us.
Star Sizes and Distances
The sizes of stars, planets, and other objects are referred to in two different ways. One is a measure of their actual size. The Moon, for instance, measures 2,159 miles across (3,476 kilometers), while the Sun is 864,400 miles (1,392,000 kilometers) in diameter, and so on.
Apparent size, or how large something appears in the sky, is expressed in angular degrees, which can be further divided into arc-minutes and arc-seconds. Figure 1.7 offers an example. The Andromeda Galaxy apparently spans 5º in length in photographs. This is considerably more than the Moon or Sun, each of which measures half a degree, or 30 arc-minutes (abbreviated 30'). Sky objects that are even smaller than 1 arc-minute are measured in arc-seconds. There are 60 arc-minutes (60') in 1 degree and 60 arc-seconds in 1 arc-minute.
Values in miles and kilometers quickly become too cumbersome to use when talking about the actual distances to the stars, so astronomers refer to distances in light-years. One light-year is equal to the distance that a beam of light would travel in space in one Earth year, more than 5.87 trillion miles (9.45 trillion kilometers), or 186,000 miles per second (300,000 kilometers per second)! The Andromeda Galaxy, usually regarded as the most distant object visible to the naked eye, lies about 2.9 million light-years away. That means the light we would see from it tonight, traveling at 186,000 miles every second, took 2.9 million years to get here! That translates to a distance of 17 billion miles (27.4 billion kilometers)!
Astronomers use very precise tools to measure apparent sizes of and distances between objects in the sky with great accuracy, but you and I were born with a handy tool that we can use to approximate those same values. It's your hand! Take a look at Figure 1.8. It turns out that the ratio of the size of the human hand to the length of the arm is proportional for everyone, regardless of age or gender. For instance, at arm's length, your fist covers 10º. Your middle three fingers extended as in a Scout salute cover 5º of sky, the same span as the pointer stars at the end of the bowl of the Big Dipper. The span between your pinky finger and forefinger equals 15º, while the span between your thumb and pinky equals 25º. (Note that some people can stretch their hands more than others, which may throw this last measurement off a bit. To find out your hand span, hold it up against the Big Dipper. If your finger and thumb can cover its length fully, your span is 25º; a little less, and it's probably closer to 20º.)
When you get to the later chapters that discuss each seasonal sky, remember this "handy" method for finding distances between stars and constellations. It makes getting around the sky much easier.
If you were to give me directions to your home, you might take me from a major highway or thoroughfare to a secondary road, and finally to your street and house number. That is pretty much how most amateur astronomers find objects in the sky. They begin at a major constellation, travel to a particular star in the pattern, then follow fainter stars to the target itself. This technique, called star hopping, is discussed later in this chapter.
While these methods work fine out in the field, they can be cumbersome when trying to list either an earthly location or celestial object in a data catalog. Instead, geographers have divided up Earth into a north-south and eastwest coordinate system called latitude and longitude, respectively. Astronomers have similarly divided up the sky into a north-south, east-west coordinate system. Rather than use longitude and latitude, celestial coordinates refer to right ascension and declination.
Let's look at declination first. Just as latitude is the measure of angular distance north or south of the Earth's equator, declination (abbreviated Dec.) specifies the angular distance north or south of the celestial equator. The celestial equator is simply the projection of Earth's equator up into the sky. If we were positioned at 0º latitude on Earth (the equator), we would see 0º declination pass directly through the zenith, while 90º north declination (the North Celestial Pole) would be overhead from the Earth's North Pole.
Excerpted from Star Watch by Philip S. Harrington Excerpted by permission.
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1. Your Passport to the Stars.
2. The Moon
3. The Planets and Asteroids.
4. The Sun.
5. The Deep Sky.
6. Spring Sky Windows.
7. Summer Sky Windows.
8. Autumn Sky Windows.
9. Winter Sky Windows.
Appendix A. The Constellations.
Appendix B. Planets 2003-2015.
Appendix C. The Messier Catalog Plus.
Bibliography and References.
Posted July 22, 2014
Posted July 22, 2014