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Light brings us information about the planets, moons, and comets in our solar system; the stars, star clusters, and nebulae in our galaxy; and the objects beyond.

In ancient times, folks didn’t think about the physics and chemistry of the stars; they absorbed and passed down folk tales and myths: the Great Bear, the Demon star, the Man in the Moon, the dragon eating the Sun during a solar eclipse, and more. The tales varied from culture to culture. But many people did discover the patterns of the stars. In Polynesia, skilled navigators rowed across hundreds of miles of open ocean with no landmarks in view and no compass. They sailed by the stars, the Sun, and their knowledge of prevailing winds and currents.

Gazing at the light from a star, the ancients noted its brightness, position in the sky, and color. This information helps people distinguish one sky object from another, and the ancients (and now people today) got to know them like old friends. Some basics of recognizing and describing what you see in the sky are

❯❯ Distinguishing stars from planets

❯❯ Identifying constellations, individual stars, and other sky objects by name

❯❯ Observing brightness (given as magnitudes)

❯❯ Understanding the concept of a light-year

❯❯ Charting sky position (measured in special units called RA and Dec)

They wondered as they wandered: Understanding planets versus stars

The term planet comes from the ancient Greek word planetes, meaning “wanderer.” The Greeks (and other ancient people) noticed that five spots of light moved across the pattern of stars in the sky. Some moved steadily ahead; others occasionally looped back on their own paths. Nobody knew why. And these spots of light didn’t twinkle like the stars did; no one understood that difference, either. Every culture had a name for those five spots of light – what we now call planets. Their English names are Mercury, Venus, Mars, Jupiter, and Saturn. These celestial bodies aren’t wandering through the stars; they orbit around the Sun, our solar system’s central star.

Today astronomers know that planets can be smaller or bigger than Earth, but they all are much smaller than the Sun. The planets in our solar system are so close to Earth that they have perceptible disks – at least, when viewed through a telescope – so we can see their shapes and sizes. The stars are so far away from Earth that even if you view them through a powerful telescope, they show up only as points of light. (For more about the planets in the solar system, flip to Part 2. I cover the planets of stars beyond the Sun in Part 4.)

If you see a Great Bear, start worrying: Naming stars and constellations

I used to tell planetarium audiences who craned their necks to look at stars projected above them, “If you can’t see a Great Bear up there, don’t worry. Maybe those who do see a Great Bear should worry.”

Ancient astronomers divided the sky into imaginary figures, such as Ursa Major (Latin for “Great Bear”); Cygnus, the Swan; Andromeda, the Chained Lady; and Perseus, the Hero. The ancients identified each figure with a pattern of stars. The truth is, to most people, Andromeda doesn’t look much like a chained lady at all – or anything else, for that matter (see Figure 1-2).

© John Wiley & Sons, Inc.

FIGURE 1-2: Andromeda is also known as the Chained Lady.

Today astronomers have divided the sky into 88 constellations, which contain all the stars you can see. The International Astronomical Union, which governs the science, set boundaries for the constellations so astronomers can agree on which star is in which constellation. Previously, sky maps drawn by different astronomers often disagreed. Now when you read that the Tarantula Nebula is in Dorado (see Chapter 12), you know that, to see this nebula, you must seek it in the Southern Hemisphere constellation Dorado, the Goldfish.

The largest constellation is Hydra, the Water Snake. The smallest is Crux, the Cross, which most people call the Southern Cross. You can see a Northern Cross, too, but you can’t find it in a list of constellations; it’s an asterism within Cygnus, the Swan. Although astronomers generally agree on the names of the constellations, they don’t have a consensus on what each name means. For example, some astronomers call Dorado the Swordfish, but I’d like to skewer that name. One constellation, Serpens, the Serpent, is broken into two sections that aren’t connected. The two sections, located on either side of Ophiuchus, the Serpent Bearer, are Serpens Caput (the Serpent’s Head) and Serpens Cauda (the Serpent’s Tail).

The individual stars in a constellation often have no relation to each other except for their proximity in the sky as visible from Earth. In space, the stars that make up a constellation may be completely unrelated to one another, with some located relatively near Earth and others located at much greater distances in space. But they make a simple pattern for observers on Earth to enjoy.

As a rule, the brighter stars in a constellation were assigned a Greek letter, either by the ancient Greeks or by astronomers of later civilizations. In each constellation, the brightest star was labeled alpha, the first letter of the Greek alphabet. The next brightest star was beta, the second Greek letter, and so on down to omega, the final letter of the 24-character Greek alphabet. (The astronomers used only lowercase Greek letters, so you see them written as α, β, … ω.)

So Sirius, the brightest star in the night sky – in Canis Major, the Great Dog – is called Alpha Canis Majoris. (Astronomers add a suffix here or there to put star names in the Latin genitive case; scientists have always liked Latin.) Table 1-1 shows a list of the Greek alphabet, in order, with the names of the letters and their corresponding symbols.

TABLE 1-1 The Greek Alphabet

When you look at a star atlas, you discover that the individual stars in a constellation aren’t marked α Canis Majoris, β Canis Majoris, and so on. Usually, the creator of the atlas marks the area of the whole constellation as Canis Major and labels the individual stars α, β, and so on. When you read about a star in a list of objects to observe, say, in an astronomy magazine (see Chapter 2), you probably won’t see it listed in the style of Alpha Canis Majoris or even α Canis Majoris. Instead, to save space, the magazine prints it as α CMa; CMa is the three-letter abbreviation for Canis Majoris (and also the abbreviation for Canis Major). I give the abbreviation for each of the constellations in Table 1-2.

TABLE 1-2 The Constellations and Their Brightest Stars

Astronomers didn’t coin special names such as Sirius for every star in Canis Major, so they named them with Greek letters or other symbols. In fact, some constellations don’t have a single named star. (Don’t fall for those advertisements that offer to name a star for a fee. The International Astronomical Union doesn’t recognize purchased star names.) In other constellations, astronomers assigned Greek letters, but they could see more stars than the 24 Greek letters. Therefore, astronomers gave some stars Arabic numbers or letters from the Roman alphabet, or numbers in professional catalogues. So you see star names such as 61 Cygni, b Vulpeculae, HR 1516, and more. You may even run across the star names RU Lupi and YY Sex. (I’m not making this up.) But as with any other star, you can recognize them by their positions in the sky (as tabulated in star lists), their brightness, their color, or other properties, if not their names.

When you look at the constellations today, you see many exceptions to the rule that the Greek-letter star names correspond to the respective brightness of the stars in a constellation. The exceptions exist because

❯❯ The letter names were based on inaccurate naked-eye observations of brightness.

❯❯ Over the years, star atlas authors changed constellation boundaries, moving some stars from one constellation into another that included previously named stars.

❯❯ Some astronomers mapped out small and Southern Hemisphere constellations long after the Greek period, and they didn’t always follow the lettering practice.

❯❯ The brightness of some stars has changed over the centuries since the ancient Greeks charted them.

A good (or bad) example is the constellation Vulpecula, the Fox, in which only one of the stars (alpha) has a Greek letter.

Because alpha isn’t always the brightest star in a constellation, astronomers needed another term to describe that exalted status, and lucida is the word (from the Latin word lucidus, meaning “bright” or “shining”). The lucida of Canis Major is Sirius, the alpha star, but the lucida of Orion, the Hunter, is Rigel, which is Beta Orionis. The lucida of Leo Minor, the Little Lion (a particularly inconspicuous constellation), is 46 Leo Minoris.

Table 1-2 lists the 88 constellations, the brightest star in each, and the magnitude of that star. Magnitude is a measure of a star’s brightness. (I talk about magnitudes in the later section “The smaller, the brighter: Getting to the root of magnitudes.”) When the lucida of a constellation is the alpha star and has a name, I list only the name. For example, in Auriga, the Charioteer, the brightest star (Alpha Aurigae) is Capella. But when the lucida isn’t an alpha, I give its Greek letter or other designation in parentheses. For example, the lucida of Cancer, the Crab, is Al Tarf, which is Beta Cancri.

If you’re a long-time Astronomy For Dummies reader (possessing at least one of the three previous editions of the book as well as this edition), you may notice some changes in Tab1e 1-2. In 2016, the International Astronomical Union issued a list of official names for bright stars. Seven stars in Table 1-2 were affected, with minor changes in spelling or a whole new name. In one case, a star was named after its constellation: Alpha Pavonis, in Pavo the Peacock, was itself named Peacock.

Identifying stars would be much easier if they had little name tags that you could see through your telescope. If you have a smartphone, you can download an app to identify the stars for you. Just download a sky map or planetarium app (such as Sky Safari, Star Walk, or Google Sky Map) and face the phone toward the sky. The app generates a map of the constellations in the general direction your phone is facing. With some apps, when you touch the image of a star, its name appears. (I describe more astronomy apps in Chapter 2; for the full scoop on stars, check out Chapter 11.)

What do I spy? Spotting the Messier Catalog and other sky objects

Naming stars was easy enough for astronomers. But what about all those other objects in the sky – galaxies, nebulae, star clusters, and the like (which I cover in Part 3)? Charles Messier (1730–1817), a French astronomer, created a numbered list of about 100 fuzzy sky objects. His list is known as the Messier Catalog, and now when you hear the Andromeda Galaxy called by its scientific name, M31, you know that it stands for number 31 in the Catalog. Today 110 objects make up the standard Messier Catalog.

You can find pictures and a complete list of the Messier objects at The Messier Catalog website of Students for the Exploration and Development of Space at messier.seds.org. And you can find out how to earn a certificate for viewing Messier objects from the Astronomical League Messier Program website at www.astroleague.org/al/obsclubs/messier/mess.html.

Experienced amateur astronomers often engage in Messier marathons, in which each person tries to observe every object in the Messier Catalog during a single long night. But in a marathon, you don’t have time to enjoy an individual nebula, star cluster, or galaxy. My advice is to take it slow and savor their individual visual delights. A wonderful book on the Messier objects, which includes hints on how to observe each object, is Stephen James O’Meara’s Deep-Sky Companions: The Messier Objects, 2nd Edition (Cambridge University Press).

Since Messier’s time, astronomers have confirmed the existence of thousands of other deep sky objects, the term amateurs use for star clusters, nebulae, and galaxies to distinguish them from stars and planets. Because Messier didn’t list them, astronomers refer to these objects by their numbers as given in other catalogues. You can find many of these objects listed in viewing guides and sky maps by their NGC (New General Catalogue) and IC (Index Catalogue) numbers. For example, the bright double cluster in Perseus, the Hero, consists of NGC 869 and NGC 884.

The smaller, the brighter: Getting to the root of magnitudes

A star map, constellation drawing, or list of stars always indicates each star’s magnitude. The magnitudes represent the brightness of the stars. One of the ancient Greeks, Hipparchos (also spelled Hipparchus, but he wrote it in Greek), divided all the stars he could see into six classes. He called the brightest stars magnitude 1 or 1st magnitude, the next brightest bunch the 2nd magnitude stars, and on down to the dimmest ones, which were 6th magnitude.

Notice that, contrary to most common measurement scales and units, the brighter the star, the smaller the magnitude. The Greeks weren’t perfect, however; even Hipparchos had an Achilles’ heel: He didn’t leave room in his system for the very brightest stars, when accurately measured.

So today we recognize a few stars with a zero magnitude or a negative magnitude. Sirius, for example, is magnitude –1.5. And the brightest planet, Venus, is sometimes magnitude –4 (the exact value differs, depending on the distance Venus is from Earth at the time and its direction with respect to the Sun).

Another omission: Hipparchos didn’t have a magnitude class for stars that were too dim to be seen with the naked eye. This didn’t seem like an oversight at the time because nobody knew about these stars before the invention of the telescope. But today astronomers know that billions of stars exist beyond our naked-eye view. Their magnitudes are larger numbers: 7 or 8 for stars easily seen through binoculars, and 10 or 11 for stars easily seen through a good, small telescope. The magnitudes reach as high (and as dim) as 21 for the faintest stars in the Palomar Observatory Sky Survey and about 31 for the faintest objects imaged with the Hubble Space Telescope.

BY THE NUMBERS: THE MATHEMATICS OF BRIGHTNESS

The 1st magnitude stars are about 100 times brighter than the 6th magnitude stars. In particular, the 1st magnitude stars are about 2.512 times brighter than the 2nd magnitude stars, which are about 2.512 times brighter than the 3rd magnitude stars, and so on. (At the 6th magnitude, you get up into some big numbers: 1st magnitude stars are about 100 times brighter.) You mathematicians out there recognize this as a geometric progression. Each magnitude is the 5th root of 100 (meaning that when you multiply a number by itself four times – for example, – the result is 100). If you doubt my word and do this calculation on your own, you get a slightly different answer because I left off some decimal places.

Thus, you can calculate how faint a star is – compared to some other star – from its magnitude. If two stars are 5 magnitudes apart (such as the 1st magnitude star and the 6th magnitude star), they differ by a factor of 2.512⁵ (2.512 to the fifth power), and a good pocket calculator shows you that one star is 100 times brighter. If two stars are 6 magnitudes apart, one is about 250 times brighter than the other. And if you want to compare, say, a 1st magnitude star with an 11th magnitude star, you compute a 2.512¹⁰ difference in brightness, meaning a factor of 100², or 10,000.

The faintest object visible with the Hubble Space Telescope is about 25 magnitudes fainter than the faintest star you can see with the naked eye (assuming normal vision and viewing skills – some experts and a certain number of liars and braggarts say that they can see 7th magnitude stars). Speaking of dim stars, 25 magnitudes are five times 5 magnitudes, which corresponds to a brightness difference of a factor of 100⁵. So the Hubble can see , or 10 billion times fainter than the human eye. Astronomers expect nothing less from a billion-dollar telescope. At least it didn’t cost $10 billion.

You can get a good telescope for well under $1,000, and you can view the billion-dollar Hubble’s best photos on the Internet for free at hubblesite.org.

Looking back on light-years

The distances to the stars and other objects beyond the planets of our solar system are measured in light-years. As a measurement of actual length, a light-year is about 5.9 trillion miles long.

People confuse a light-year with a length of time because the term contains the word year. But a light-year is really a distance measurement – the length that light travels, zipping through space at 186,000 miles per second, over the course of a year.

When you view an object in space, you see it as it appeared when the light left the object. Consider these examples:

❯❯ When astronomers spot an explosion on the Sun, we don’t see it in real time; the light from the explosion takes about 8 minutes to get to Earth.

❯❯ The nearest star beyond the Sun, Proxima Centauri, is about 4 light-years away. Astronomers can’t see Proxima as it is now – only as it was four years ago.

❯❯ Look up at the Andromeda Galaxy, the most distant object that you can readily see with the unaided eye, on a clear, dark night in the fall. The light your eye receives left that galaxy about 2.5 million years ago. If there was a big change in Andromeda tomorrow, we wouldn’t know that it happened for more than 2 million years. (See Chapter 12 for hints on viewing the Andromeda Galaxy and other prominent galaxies.)

Here’s the bottom line:

❯❯ When you look out into space, you’re looking back in time.

❯❯ Astronomers don’t have a way to know exactly what an object out in space looks like right now.

When you look at some big, bright stars in a faraway galaxy, you must entertain the possibility that those particular stars don’t even exist anymore. As I explain in Chapter 11, some massive stars live for only 10 million or 20 million years. If you see them in a galaxy that is 50 million light-years away, you’re looking at lame duck stars. They aren’t shining in that galaxy anymore; they’re dead.

If astronomers send a flash of light toward one of the most distant galaxies found with Hubble and other major telescopes, the light would take billions of years to arrive. Astronomers, however, calculate that the Sun will swell up and destroy all life on Earth a mere 5 billion or 6 billion years from now, so the light would be a futile advertisement of our civilization’s existence, a flash in the celestial pan.

HEY, YOU! NO, NO, I MEAN AU

Earth is about 93 million miles from the Sun, or 1 astronomical unit (AU). The distances between objects in the solar system are usually given in AU. Its plural is also AU. (Don’t confuse AU with “Hey, you!”)

In public announcements, press releases, and popular books, astronomers state how far the stars and galaxies that they study are “from Earth.” But among themselves and in technical journals, they always give the distances from the Sun, the center of our solar system. This discrepancy rarely matters because astronomers can’t measure the distances of the stars precisely enough for 1 AU more or less to make a difference, but they do it this way for consistency.

Keep on moving: Figuring the positions of the stars

Astronomers used to call stars “fixed stars,” to distinguish them from the wandering planets. But in fact, stars are in constant motion as well, both real and apparent. The whole sky rotates overhead because Earth is turning. The stars rise and set, like the Sun and the Moon, but they stay in formation. The stars that make up the Great Bear don’t swing over to the Little Dog or Aquarius, the Water Bearer. Different constellations rise at different times and on different dates, as seen from different places around the globe.

Actually, the stars in Ursa Major (and every other constellation) do move with respect to one another – and at breathtaking speeds, measured in hundreds of miles per second. But those stars are so far away that scientists need precise measurements over considerable intervals of time to detect their motions across the sky. So 20,000 years from now, the stars in Ursa Major will form a different pattern in the sky. (Maybe they will even look like a Great Bear.)

In the meantime, astronomers have measured the positions of millions of stars, and many of them are tabulated in catalogs and marked on star maps. The positions are listed in a system called right ascension and declination – known to all astronomers, amateur and pro, as RA and Dec:

❯❯ The RA is the position of a star measured in the east–west direction on the sky (like longitude, the position of a place on Earth measured east or west of the prime meridian at Greenwich, England).

❯❯ The Dec is the position of the star measured in the north–south direction, like the latitude of a city, which is measured north or south of the equator.

Astronomers usually list RA in units of hours, minutes, and seconds, like time. We list Dec in degrees, minutes, and seconds of arc. Ninety degrees make up a right angle, 60 minutes of arc make up a degree, and 60 seconds of arc equal a minute of arc. A minute or second of arc is also often called an “arc minute” or an “arc second,” respectively.

DIGGING DEEPER INTO RA AND DEC

A star at RA 2h00m00s is 2 hours east of a star at RA 0h00m00s, regardless of their declinations. RA increases from west to east, starting from RA 0h00m00s, which corresponds to a line in the sky (actually half a circle, centered on the center of Earth) from the North Celestial Pole to the South Celestial Pole. The first star may be at Dec 30° North, and the second star may be at Dec 15° 25’12” South, but they’re still 2 hours apart in the east–west direction (and 45° 25’12” apart in the north–south direction). The North and South Celestial Poles are the points in the sky – due north and due south – around which the whole sky seems to turn, with the stars all rising and setting.

Note the following details about the units of RA and Dec:

● An hour of RA equals an arc of 15 degrees on the equator in the sky. Twenty-four hours of RA span the sky, and , or a complete circle around the sky. A minute of RA, called a minute of time, is a measure of angle on the sky that makes up of an hour of RA. So you take , or . A second of RA, or a second of time, is 60 times smaller than a minute of time.

● Dec is measured in degrees, like the degrees in a circle, and in minutes and seconds of arc. A whole degree is about twice the apparent or angular size of the full Moon. Each degree is divided into 60 minutes of arc. The Sun and the full Moon are both about 32 minutes of arc (32’) wide, as seen on the sky, although, in reality, the Sun is much larger than the Moon. Each minute of arc is divided into 60 seconds of arc (60”). When you look through a backyard telescope at high magnification, turbulence in the air blurs the image of a star. Under good conditions (low turbulence), the image should measure about 1” or 2” across. That’s 1 or 2 arc seconds, not 1 or 2 inches.

A few simple rules may help you remember how RA and Dec work and how to read a star map (see Figure 1-3):

❯❯ The North Celestial Pole (NCP) is the place to which the axis of Earth points in the north direction. If you stand at the geographic North Pole, the NCP is right overhead. (If you stand there, say “Hi” to Santa for me, but beware: You may be on thin ice because there’s no land at the geographic North Pole.)

❯❯ The South Celestial Pole (SCP) is the place to which the axis of Earth points in the south direction. If you stand at the geographic South Pole, the SCP is right overhead. I hope you dressed warmly: You’re in Antarctica!

❯❯ The imaginary lines of equal RA run through the NCP and SCP as semicircles centered on the center of Earth. They may be imaginary, but they appear marked on most sky maps to help people find the stars at particular RAs.

❯❯ The imaginary lines of equal Dec, like the line in the sky that marks Dec of 30° North, pass overhead at the corresponding geographic latitudes. So if you stand in New York City, latitude 41° North, the point overhead is always at Dec 41° North, although its RA changes constantly as Earth turns. These imaginary lines appear on star maps, too, as declination circles.

© John Wiley & Sons, Inc.

FIGURE 1-3: Decoding the celestial sphere to find directions in space.

Suppose you want to find the NCP as visible from your backyard. Face due north and look at an altitude of x degrees, where x is your geographic latitude. I’m assuming that you live in North America, Europe, or somewhere in the Northern Hemisphere. If you live in the Southern Hemisphere, you can’t see the NCP. You can, however, look for the SCP. Look for the spot due south whose altitude in the sky, measured in degrees above the horizon, is equal to your geographic latitude.

In almost every astronomy book, the symbol ″ means seconds of arc, not inches. But at every university, a student in Astronomy 101 writes on a lab report, “The image of the star was about 1 inch in diameter.” Understanding beats memorizing every day, but not everyone understands.

Here’s the good news: If you just want to spot the constellations and the planets, you don’t have to know how to use RA and Dec. Just consult a star map drawn for the current week or month (you can find these on the website of Sky & Telescope or one of the other magazines that I mention in Chapter 2, in the magazines themselves, or using a desktop planetarium program for your home computer or a planetarium app for your smartphone or tablet; I recommend programs, websites, and apps in Chapter 2 as well). But if you want to understand how star catalogs and maps work and how to zero in on faint galaxies with your telescope, understanding the system helps.

And if you purchase one of those snazzy and surprisingly affordable telescopes with computer control (see Chapter 3), you can punch in the RA and Dec of a recently discovered comet, and the scope points right at it. (A little table called an ephemeris comes with every announcement of a new comet. It gives the predicted RA and Dec of the comet on successive nights as it sweeps across the sky.)