Chapter 2 =//= The Stars
The night sky is the rest of the universe as seen from our planet. When we look up at the stars, we look out through a layer of air only a few hundred kilometers deep. Beyond that, space is nearly empty, with the stars scattered light-years apart. We begin our search for the natural laws that govern the universe by trying to understand what the universe looks like.
As you read this chapter, keep in mind that we live on a planet. The stars are scattered into the void all around us, most very distant and some closer. Our planet rotates on its axis once a day, so from our viewpoint the sky appears to rotate around us each day. Not only does the sun rise in the east and set in the west, but so also do the stars. That apparent daily motion is caused by the rotation of our planet.
On a dark night far from city lights, we can see a few thousand stars in the sky. Like ancient astronomers, we will try to organize what we see by naming groups of stars and individual stars and by specifying the brightness of individual stars.
-CONSTELLATIONS-
All around the world, ancient cultures gave names to groups of stars--constellations--to honor gods, heroes, and animals. Of course, each culture created its own unique set of constellations. The constellations we are familiar with in Western culture originated in Mesopotamia over 5,000 years ago, with other constellations added by Babylonian, Egyptian, and Greek astronomers during the classical age. Of these ancient constellations, 48 are still in use today.
To the ancients, a constellation was a loose grouping of stars that symbolized a certain figure, and constellation boundaries were not precisely defined. Many of the fainter stars were not included in any constellation, and regions of the southern sky not visible to the ancient astronomers of the northern latitudes were not divided into constellations. In recent centuries, astronomers added 40 modern constellations to fill the gaps, and in 1928 the International Astronomical Union established 88 official constellations with clearly defined boundaries in the sky. Thus a constellation now represents not a group of stars, but an area of the sky, and any star within the region belongs to one and only one constellation.
In addition to the 88 official constellations, the sky contains a number of less formally defined groupings called asterisms. The Big Dipper, for example, is a well-known asterism, but it is only part of the larger constellation Ursa Major (the Great Bear).
Although we name constellations and asterisms, keep in mind that they are made up of stars that are usually not physically associated with one another. Some stars may be many times farther away than others and moving through space in different directions. The only thing they have in common is that they lie in approximately the same direction from Earth.
-THE NAMES OF STARS-
In addition to naming groups of stars, ancient astronomers named the brighter stars, and modern astronomers still use many of those names. Whereas the names of the constellations are in Latin, the common language of science in Renaissance Europe, most star names derive from ancient Arabic, though much altered by the passing centuries. The name of Betelgeuse, the bright red star in Orion, for example, comes from the Arabic yad al-jawza, meaning "hand of Jawza [Orion]." Aldebaran, the bright red eye of Taurus the bull, comes from the Arabic aldabar an, meaning "the follower [of the Pleiades]."
Naming individual stars is not very helpful because we can see thousands of them, and names do not help us locate stars in the sky. Another way to identify stars is to assign Greek letters to the bright stars in a constellation in approximate order of brightness. Thus the brightest star is usually designated α (alpha), the second brightest ß (beta), and so on. For many constellations, the letters follow the order of brightness, but some constellations, by tradition, mistake, or the personal preferences of early chartmakers, are exceptions.
To identify a star by its Greek letter designation, we give the Greek letter followed by the genitive (possessive) form of the constellation name, such as α Canis Majoris. This both identifies the star and the constellation and gives us a clue to the relative brightness of the star. Compare this with the ancient name for this star, Sirius, which tells us nothing about location or brightness.
This method of identifying a star's brightness is only approximate. In order to discuss the sky with precision, we must have an accurate way of referring to the brightness of stars, and for that we must consult one of the first great astronomers.
-THE BRIGHTNESS OF STARS-
Hipparchus, a Greek astronomer who lived about 2100 years ago, divided the stars into six classes. The brightest were first-class stars, and those slightly fainter were second-class stars. Continuing down to the faintest stars he could see, the sixth-class stars, he recorded his classifications in a great star catalogue that became a basic reference in ancient astronomy. His method, slightly modified, is still in use today.
In spite of its value, Hipparchus's method may seem a bit confusing. First, when early astronomers translated the catalogue into Latin, they used the word magnitudo, meaning "size." In English, this became magnitude, even though it refers to the brightness of the stars and not to their size. Thus the magnitude scale is the astronomer's brightness scale.
The second source of confusion is that the fainter the star, the larger the magnitude number. For example, 6th-magnitude stars are fainter than 1st-magnitude stars. This may seem backwards at first, but think of it as Hipparchus did. The brightest stars are first-class stars, and the fainter stars are second- and third-class, and so on.
Modern astronomers have made a major improvement in Hipparchus's magnitude system by measuring stellar brightness with sensitive instruments. For example, instead of merely saying that θ (theta) Leonis is a 3rd-magnitude star, they can say specifically that its magnitude is 3.34.
If we measure the brightness of all of the stars in Hipparchus's first brightness class, we discover that some are brighter than 1.0. For example, Vega (α Lyrae) is so bright that its magnitude, 0.04, is almost zero. A few are so bright the magnitude scale must extend into negative numbers. On this scale, Sirius, the brightest star in the sky, has a magnitude of -1.47. If we use a telescope to search for very faint stars, we can find stars much fainter than the limit for the unaided eye. Thus the magnitude system has been extended to numbers larger than 6th magnitude to include fainter stars.
These magnitudes are known as apparent visual magnitudes. They refer to how bright the stars look and do not compensate for their distance from Earth. A star that emits a million times more light than the sun might appear very faint if it were very far away, and a star that is much fainter than the sun might look bright if it were nearby. In the chapter The Properties Of Stars, we will develop a magnitude scale that takes distance into account and tells us how bright the stars really are. Apparent visual magnitude only tells us how bright they appear.
Hipparchus used apparent visual magnitudes to describe how bright a star looks, but brightness is subjective. How bright a star looks depends on such things as the physiology of the eye and the psychology of perception. To be accurate, we should use the more precise term intensity--a measure of the light energy from a star that hits 1 square meter in 1 second. If we compare the intensity of the light coming from two stars, then we can be precise in describing their relative brightness (!!By the Numbers 2-1!!). Modern astronomers use the magnitude system to make precise calculations about the intensity of starlight, but can still use the same scale to make comparisons with all of the observations of stellar brightness made since the time of Hipparchus over 2,000 years ago.
-THE CELESTIAL SPHERE-
Ancient astronomers thought of the sky as a great, hollow, crystalline sphere surrounding Earth. The stars, they imagined, were attached to the inside of the sphere like thumbtacks stuck in the ceiling. The sphere rotated once a day, carrying the sun, moon, planets, and stars from east to west.
We know now that the sky is not a great, hollow, crystalline sphere. The stars are scattered through space at different distances, and it isn't the sky that rotates once a day, but Earth that turns on its axis. Although we know that the crystalline sphere is not real, it is convenient as a model of the sky. As long as we keep the true nature of the sky in mind, we can use the model to analyze the appearance and motions of the sky.
We will call this model of the sky (!!Window on Science 2-1!!) the celestial sphere, an imaginary hollow sphere of very large radius surrounding Earth and to which the stars seem to be attached (!!Draw model and refer to it!!). We must use a very large radius for our celestial sphere so that no part of Earth is significantly closer to a given star than any other part. Then it does not matter where on Earth we live; the sky always looks like a great sphere centered on our position.
If we watch the sky for a few hours, we can see movement. As the rotation of Earth carries us eastward, the sun moves across the sky and sets in the west. As it gets dark, we can see the stars, and in a few hours it becomes obvious that the rotation of Earth is making the sky rotate westward. As some constellations set in the west, others rise in the east.
-Angles on the Sky-
One way we might use the celestial sphere is to describe the location of objects. Just as we might tell a friend that we own a house 40 km north of Los Angeles, we might want to tell our friend to look for the moon a specific distance north of a certain star. We can't measure distances in the sky in kilometers, however. Rather, we need to measure angles on the celestial sphere.
Astronomers often use angles to describe distance across the sky. They might say, for instance, that the angular distance between the moon and the bright star Spica is 8°, meaning that if we point one arm at the moon and the other arm at Spica, the angle between our arms is 8°. We know the star is hundreds of light-years away, and we know that the moon is much closer, so the true distance between them is immense. But if we imagine them painted on the celestial sphere, we can think of their separation as an "angle on the sky." Thus we can discuss the angular separation of two objects even when we don't know their true distance from each other.
We measure angles in degrees, minutes of arc, and seconds of arc. There are 360° in a circle and 90° in a right angle. Each degree is divided into 60 minutes of arc (sometimes abbreviated 60'). If you view a 25 cent piece face-on from the length of a football field, it has an angular diameter of about 1 minute of arc. Each minute of arc is divided into 60 seconds of arc (abbreviated 60"). If you view the ball in the tip of a ballpoint pen from the length of a football field, it has an angular diameter of about 1 second of arc.
We can establish some angles on the sky that will be helpful in estimating other angles. The sun and moon are each about 0.5° in diameter. The pointer stars of the Big Dipper are about 5° apart, and the bowl of the Big Dipper is about 30° from the north celestial pole.
While it is sometimes convenient to locate things with respect to a bright star, we need to establish some reference points and lines on the sky. They will provide the basis for a precise way of locating objects.
-Reference Marks On The Sky-
Just as we use the earth's poles and equator as reference marks on the earth, we can use corresponding reference marks on the sky. The celestial poles and celestial equator are defined by the earth's rotation.
If we watch the night sky for a few hours, we can see the stars moving westward. In the northern sky, they appear to revolve around a point called the north celestial pole, the point on the sky directly above the earth's North Pole. The south celestial pole is the corresponding point directly above the earth's South Pole. If the earth's axis could be extended out to the celestial sphere, it would touch at the celestial poles.
Another important reference mark on the celestial sphere is the celestial equator, an imaginary line around the sky directly above the earth's equator. The celestial equator divides the sky into two equal hemispheres and is everywhere 90° from the celestial poles.
The orientation of the celestial poles and equator with respect to our horizon depends on our latitude. To be precise, the angular distance from the horizon up to the north celestial pole equals the latitude of the observer. For example, if we stood in the ice and snow of the earth's North Pole, our latitude would be 90° N, and the north celestial pole would be directly overhead. If we walked southward, our latitude would decrease and the north celestial pole would sink closer to the northern horizon. When we finally stood on the earth's equator, our latitude would be 0°, and the north celestial pole would lie on our northern horizon. This relationship, by the way, makes it simple for navigators to find their latitude by measuring the angle between the northern horizon and the north celestial pole.
The star Polaris happens to lie very near the north celestial pole and thus hardly moves as the earth rotates eastward. At any time of night, in any season of the year, Polaris stands above the northern horizon and is consequently known as the North Star.
The rotation of the earth also defines our system of directions. The horizon is the circle that surrounds us, marking the meeting of the sky and earth; and the point on the horizon directly below the north celestial pole is the north point. South is exactly opposite the north point. The east and west points, halfway between north and south, mark the meeting of the celestial equator and the horizon. Finally, the point on the sky directly overhead, is called the zenith.
We see most of the constellations rise and set as the earth rotates, but some stars near the celestial poles never reach the horizon. Those near the north celestial pole make up the north circumpolar constellations, and they never set as seen from a latitude typical of the United States. At higher latitudes, more constellations are north circumpolar, and at lower latitudes, there are fewer. There are also south circumpolar constellations around the south celestial pole that never rise above the southern horizon as seen from middle northern latitudes.
The technical terms that describe the sky give us a powerful vocabulary that allows us to analyze the motion of the sky. Thus, our goal is not just to name the part of the sky but also to understand how the sky moves through the day and through the year. In fact, the celestial poles and equator are the basis for a system of precise celestial coordinates much like the system of latitude and longitude on the earth. But we must beware. These critical reference marks on the sky are moving, and that can tell us something new about the motion of the earth.
-PRECESSION-
Over 2,000 years ago, Hipparchus compared a few of his star positions with those made nearly two centuries before and realized that the celestial poles and equator were slowly moving across the sky. Later astronomers understood that this motion is caused by the toplike motion of the earth.
If you have ever played with a gyroscope, you have seen how the spinning mass resists any change in the direction of its axis of rotation. The more massive the gyroscope and the more rapidly it spins, the more difficult it is to change the direction of its axis of rotation. But you may recall that the axis of even the most rapidly spinning gyroscope does not remain absolutely fixed. A spinning gyroscope wobbles in a conical motion called precession. The gyroscope precesses because of the interaction of its weight and its rotation. The earth's gravity pulling on the gyroscope (its weight) tends to make the gyroscope tip over, and this combines with its rapid rotation to make its axis sweep around in a conical motion about a vertical line.
The earth behaves like a giant gyroscope. Its large mass and rapid rotation keep its axis of rotation pointing near the star Polaris. If the earth were a perfect sphere, its axis of rotation would remain fixed, but the earth, because of its rotation, has a bulge around its middle. The moon's gravity pulls slightly more on the near side of the bulge than on the far side and thus twists the earth's axis of rotation, tending to set it upright in its orbit. The sun, though farther away than the moon, has much more mass, and it too twists the earth's axis of rotation. The combination of these forces and the earth's rotation causes the earth's axis to precess in a conical motion, taking about 26,000 years for one cycle.
Because the celestial poles and the equator are defined by the earth's rotation, precession changes these reference marks. We see no change at all from night to night or year to year, but precise measurements reveal the precessional motion of the poles and the equator.
Over centuries, precession has dramatic effects. For example, it makes the celestial poles move across the sky. Egyptian records show that 4,800 years ago the north celestial pole was near the star Thuban (α Draconis). The pole is now approaching Polaris and will be closest to it about AD 2100. In about 12,000 years, the pole will have moved away from Polaris and will be within 5° of Vega (α Lyrae).