The Sky Astronomy: Horizons 10th edition Michael Seeds Chapter 2 The Sky The Southern Cross I saw every night abeam. The sun every morning came up astern; every evening it went down ahead. I wished for no other compass to guide me, for these were true. - CAPTAIN JOSHUA SLOCUM Sailing Alone Around the World The Sky • The night sky is the rest of the universe as seen from our planet. – When you look up at the stars, you look out through a layer of air only a few hundred kilometers deep. – Beyond that, space is nearly empty, and the stars are scattered light years apart. The Sky • Here, you will begin your search for the natural laws that govern the universe— by trying to understand what the universe looks like. The Sky • As you read this chapter, keep in mind that you live on a planet. – Stars are scattered in the void all around you, most very distant and some closer. – Earth rotates on its axis once a day—and that makes the sky appear to revolve around you in a daily cycle. – Not only does the sun rise in the East and set in the West, but so do the stars. The Stars The Sky • On a dark night, far from city lights, you can see a few thousand stars in the sky. The Stars The Sky • As you begin your study of the sky, the first step is to organize what you see— by naming groups of stars and individual stars and by specifying the brightness of individual stars. – That will make the sky familiar territory, and you will be ready to explore further. Constellations The Sky • All around the world, ancient cultures celebrated heroes, gods, and mythical beasts by naming groups of stars after them—constellations. Constellations The Sky • You should not be surprised that the constellations do not look like the creatures they represent any more than Columbus, Ohio, looks like Christopher Columbus. – The constellations ‘celebrate’ the most important mythical figures in each culture. Constellations The Sky • The constellations named within Western culture originated in Mesopotamia over 5,000 years ago. • Other constellations were added by Babylonian, Egyptian, and Greek astronomers during the classical age. – Of these ancient constellations, 48 are still in use. Constellations The Sky • To the ancients, a constellation was a loose grouping of stars. – Many of the fainter stars were not included in any constellation. – Regions of the southern sky not visible to the ancient astronomers of northern latitudes were not identified with constellations. Constellations The Sky • Constellation boundaries, when they were defined at all, were only approximate. – So, a star like Alpheratz could be thought of as part of Pegasus or part of Andromeda. Constellations The Sky • In recent centuries, astronomers have added 40 modern constellations to fill gaps. Constellations The Sky • In 1928, the International Astronomical Union established 88 official constellations with clearly defined boundaries. – A constellation now represents not a group of stars but an area of the sky. – Any star within the region belongs to one and only one constellation. – Alpheratz belongs to Andromeda. Constellations The Sky • Apart from the 88 official constellations, the sky contains a number of less formally defined groupings called asterisms. – The Big Dipper is a well-known asterism that is part of the constellation Ursa Major (Great Bear). Constellations The Sky – Another asterism is the Great Square of Pegasus, which includes three stars from Pegasus plus Alpheratz from Andromeda. Constellations The Sky • Although you can refer to constellations and asterisms by name, most are made up of stars that are not physically associated with one another. – Some stars may be many times further 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 the Stars The Sky • In addition to naming groups of stars, ancient astronomers named the brighter stars, and modern astronomers still use many of those names. The Names of the Stars The Sky • The constellation names come from Greek versions translated into Latin, the language of science from the fall of Rome to the 19th century. The Names of the Stars The Sky • Most star names come from ancient Arabic, though have been altered much by the passing centuries. – The name of Betelgeuse, the bright red star in Orion, comes from the Arabic yad al-jawza, meaning ‘armpit of Jawza (Orion).’ – Names such as Sirius (the Scorched One) and Aldebaran (the Follower of the Pleiades) are beautiful additions to the mythology of the sky. The Names of the Stars The Sky • Naming individual stars is not very helpful. – You can see thousands of them. – Names do not help you locate stars in the sky or guess their brightness. The Names of the Stars The Sky • Another way to identify stars is to assign Greek letters to the bright stars in a constellation in the approximate order of brightness. – Thus, the brightest star is usually designated α (alpha), the second brightest β (beta), and so on. The Names of the Stars The Sky • For many constellations, the letters follow the order of brightness. • However, some constellations—by tradition, mistake, or the personal preferences of early chartmakers—are exceptions. The Names of the Stars The Sky • To identify a star by its Greek-letter designation, you give the Greek letter followed by the possessive (genitive) form of the constellation name. – The brightest star in the constellation Canis Major is α Canis Majoris. – This identifies both the star and the constellation and gives a clue to the relative brightness of the star. – Compare this with the ancient name for this star, Sirius, which tells you nothing about location or brightness. The Names of the Stars The Sky • This method of estimating a star’s brightness is only approximate. • To discuss the sky with precision, you must have an accurate way of referring to the brightness of stars. – For that, you must consult two of the first great astronomers. The Brightness of Stars The Sky • Astronomers measure the brightness of stars using the magnitude scale—a system that first appeared in the writings of the ancient astronomer Claudius Ptolemy about 140 AD. – The system may have originated earlier than Ptolemy. – Most astronomers attribute it to the Greek astronomer Hipparchus (190-120 BC). The Brightness of Stars The Sky • The ancient astronomers divided the stars into six classes. – The brightest were called first-magnitude stars and those that were fainter, second-magnitude. – The scale continued downward to sixth-magnitude stars—the faintest visible to the human eye. – Thus, the larger the magnitude number, the fainter the star. – This makes sense if you think of the bright stars as first-class stars and the faintest stars visible as sixth-class stars. The Brightness of Stars The Sky • Hipparchus is believed to have compiled the first star catalog, and he may have used the magnitude system in that catalog. • Almost 300 years later, Ptolemy used the magnitude system in his own catalog. • Successive generations of astronomers have continued to use the system. The Brightness of Stars The Sky • Modern astronomers can measure the light of stars to high precision. – So, instead of saying that the star known by the charming name Chort (Theta Leonis) is third magnitude, they can say its magnitude is 3.34. The Brightness of Stars The Sky • If you measure magnitudes, you would discover that some stars are brighter than 1.0. – Vega (α Lyrae) is so bright that its magnitude, 0.04, is almost zero. The Brightness of Stars The Sky • A few are so bright that the magnitude scale must extend into negative numbers. – Sirius, the brightest star in the sky, has a magnitude of -1.47. The Brightness of Stars The Sky • The faintest stars you can see with your unaided eyes are about sixth magnitude. • If you use a telescope, you will see stars much fainter. – Thus, the scale has also been extended to include numbers larger than sixth magnitude to include fainter stars. The Brightness of Stars The Sky • These numbers are known as apparent visual magnitudes (mv). • They describe how the stars look to human eyes observing from Earth. – Although some stars emit large amounts of infrared or ultraviolet light, humans can’t see it, and it is not included in the apparent visual magnitude. – The subscript ‘v’ stands for ‘visual’ and reminds you that you are including only light you can see. The Brightness of Stars The Sky • Another problem is the distance to the stars. – Very distant stars look fainter and nearby stars look brighter. – Apparent visual magnitude ignores the effect of distance and informs you only how bright the star looks as seen from Earth. The Brightness of Stars The Sky • Your interpretation of brightness is quite subjective—depending on both the physiology of human eyes and the psychology of perception. • To be accurate, you should refer to intensity—a measure of the light energy from a star that hits one square meter in one second. The Brightness of Stars The Sky • A simple relationship connects apparent visual magnitudes and the intensity of starlight. – Thus, modern astronomers can measure the brightness of stars to high precision, while still making comparisons to observations of apparent visual magnitude that go back to the time of Hipparchus. The Brightness of Stars The Sky • It’s time for you to review the preceding section on constellations, star names, and magnitudes. • How you review is critical when you study a science. – Memorizing facts won’t help you much. – Organizing your understanding into scientific arguments will line the facts up in a meaningful way. The Brightness of Stars The Sky • The review tool that follows—Building Scientific Arguments—will help you review an important concept from the section. – You should use the same process to review each concept from the beginning of the section to the end. Building Scientific Arguments The Sky • Nonastronomers sometimes complain that the magnitude scale is awkward. – Why would they think it is awkward? – How did it get that way? Building Scientific Arguments The Sky • One of the main characteristics of a scientific argument is that it is carefully organized. • In this case, you can identify two things that might make the magnitude scale seem awkward. Building Scientific Arguments The Sky • First, it is backward—the bigger the magnitude number, the fainter the star. – That arose because ancient astronomers were not measuring the brightness of stars but rather classifying them. – First-class stars would be brighter than second-class stars. Building Scientific Arguments The Sky • The second awkward feature of the scale is its mathematical relation to intensity. – If two stars differ by one magnitude, one is about 2.5 times brighter than the other. – However, if they differ by two magnitudes, one is 2.5 x 2.5 times brighter. Building Scientific Arguments The Sky • Now, extend your scientific argument. – If the magnitude scale is so awkward, why do you suppose astronomers have used it for over two millennia? The Sky and Its Motion The Sky • The sky seems to be a great blue dome in the daytime and a sparkling ceiling at night. – Learning to look at the sky requires that you begin thousands of years ago. The Sky and Its Motion The Sky • Ancient astronomers believed the sky was a great sphere surrounding Earth, with the stars stuck on the inside like thumbtacks in a ceiling. The Sky and Its Motion The Sky • Modern astronomers know that the stars are scattered through space at different distances. – However, it is still convenient to think of the sky as a great starry sphere enclosing Earth. The Celestial Sphere The Sky • As you study the sky, notice three important points. • One, the sky appears to rotate westward around Earth each day. – That is a consequence of the eastward rotation of Earth. – That produces day and night. The Celestial Sphere The Sky • Two, astronomers measure distances across the sky as angles and express them as degrees, minutes, and seconds. The Celestial Sphere The Sky • Three, what you can see of the sky depends on where you are on Earth. – If you lived in Australia, you would see many constellations and asterisms invisible from North America, but you would never see the Big Dipper. – Alpha Centauri is in the southern sky and isn’t visible from most of the United States. – You could just glimpse it above the southern horizon if you were in Miami, but you could see it easily from Australia. The Celestial Sphere The Sky • This is a good time to eliminate a couple of common misconceptions. • Lots of people—without thinking about it much—assume the stars are not in the sky during the daytime. – The stars are there day and night. – They are just invisible during the day because the sky is lit up by sunlight. The Celestial Sphere The Sky • Also, many people insist that Polaris is the brightest star in the sky. – You can see that Polaris is important because of its location, not because of its brightness. The Celestial Sphere The Sky • The celestial sphere is an example of a scientific model—a common feature of scientific thought. – Notice that a scientific model does not have to be true to be useful. Precession The Sky • In addition to the daily motion of the sky, Earth’s rotation adds a second motion to the sky that can be detected only over centuries. – Over 2,000 years ago, Hipparchus compared a few of his star positions with those made nearly two centuries earlier 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 Earth. Precession The Sky • If you have ever played with a gyroscope or top, you have seen how the spinning mass resists any change in the direction of its axis of rotation. – The more massive the top and the more rapidly it spins, the more difficult it is to change the direction of its axis of rotation. Precession The Sky • However, you probably recall that the axis of even the most rapidly spinning top sweeps around in a conical motion. – That is, the axis of the top pivots, so the axis sweeps out the surface of a cone. – The weight of the top tends to make it tip. – This combines with its rapid rotation to make its axis sweep around in a conical motion called precession. Precession The Sky • Earth spins like a giant top, but it does not spin upright in its orbit—it is tipped 23.5° from vertical. – Earth’s large mass and rapid rotation keep its axis of rotation pointed toward a spot near Polaris. – The axis would not wander if Earth were a perfect sphere. Precession The Sky • However, Earth, due to its rotation, has a slight bulge around its middle. • The gravity of the sun and of the moon pull on this bulge, tending to twist Earth upright in its orbit. Precession The Sky • The combination of these forces and Earth’s rotation causes Earth’s axis to precess in a conical motion, taking about 26,000 years for one cycle. Precession The Sky • As the celestial poles and equator are defined by Earth’s rotational axis, precession moves these reference marks. – You notice no change at all from night to night or year to year. – Nevertheless, precise measurements reveal the precessional motion of the celestial poles and equator. Precession The Sky • Over centuries, precession has dramatic effects. – 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 in about 2100. – In about 12,000 years, the pole will have moved to within 5° of Vega (α Lyrae). – Someday, Vega will be a very impressive north star. Precession The Sky • The figure shows the path followed by the north celestial pole. Precession The Sky • As you study astronomy, notice the special terms used to describe such things as precession and the celestial sphere. – You need to know those terms. • However, science is about understanding nature, not about naming its parts. – Science is more than just vocabulary. Building Scientific Arguments The Sky • Does everyone see the same circumpolar constellations? – Here, you must use your imagination and build your argument with great care. – You can use the celestial sphere as a convenient model of the sky. Building Scientific Arguments The Sky • A circumpolar constellation is one that does not set or rise. Building Scientific Arguments The Sky • Which constellations are circumpolar depends on your latitude. – If you live on Earth’s equator, you see all the constellations rising and setting and there are no circumpolar constellations at all. Building Scientific Arguments The Sky – If you live at Earth’s North Pole, all the constellations north of the celestial equator never set and all the constellations south of the celestial equator never rise. – In that case, every constellation is circumpolar. Building Scientific Arguments The Sky – At intermediate latitudes, the circumpolar regions are caps on the sky whose angular radius equals the latitude of the observer. – If you live in Iceland, the caps are very large and, if you live in Egypt, near the equator, the caps are much smaller. Building Scientific Arguments The Sky – For people in Canada, Ursa Major is circumpolar, but people in Mexico see most of this constellation slip below the horizon. – From much of the United States, some of the stars of Ursa Major set and some do not. – In contrast, Orion rises and sets as seen from nearly everywhere on Earth. – Explorers at Earth’s poles, however, never see Orion rise or set. Building Scientific Arguments The Sky • Now, use the argument you have just built. – How would you improve the definition of a circumpolar constellation to clarify the status of Ursa Major? – Would your definition help in the case of Orion?