Hunting the Hunter

One of the most striking sights in the winter sky is the star-studded figure of Orion the Hunter. After the Big Dipper, Orion’s hourglass shape is perhaps the best known star pattern in the night sky. Once learned, it is recognized by most people without difficulty.

Because it straddles the celestial equator, the constellation--or some part of it--can be seen from everywhere on the planet. The celestial equator is the imaginary line that represents where the plane of the Earth’s equator, were it extended out into space, would intersect with the sky.

Diagram by Dr. Guy Worthey

Orion’s visibility around the globe has given the distinctive star grouping a prominent place in the lore of most cultures. In Greek mythology, Orion was depicted as a tall, buff, and beautiful giant, as well as an exceptionally skilled hunter. He once cleared an entire island of wild beasts in order to win the hand of the island king’s daughter. He even became the constant hunting companion of Artemis, goddess of the hunt.

There are two commonly held versions of how Orion ended up in the sky. In one, Apollo, Artemis’s brother, grew jealous of all the time she was spending with her hunting buddy. He tricked Artemis into accidentally killing Orion. The grieving Artemis paid tribute to her friend by placing him among the stars. In another tale, Orion was killed by a sting from a poisonous scorpion. Zeus placed Orion in the sky at Artemis’s request and ensured that the Hunter (winter constellation) was never in the sky at the same time as the Scorpion (summer constellation). They chase each other around the celestial sphere for eternity.

Orion the Hunter, from an 18th century star atlas
Image courtesy of G.M. Caglieris

The name Orion is believed to mean “Light of Heaven.” Although it comes down to us via Greek myths, it originated with the Akkadians, who lived along the Euphrates River from 2350 to 2200 BCE, in what is present-day Iraq. Other Middle Eastern cultures knew the constellation by names meaning the “Giant” and the “Strong One.”

We can even spot this bold constellation from urban areas, so let’s go hunting, shall we?

1) You’ll need to look south, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you will be facing approximately south.

2) Wait at least one hour after sunset to begin observing, so your sky is good and dark. The waning (shrinking) crescent moon will not rise until the wee hours of the morning and so will not interfere with your observing. New Moon, when the moon does not appear at all in the night sky, occurs on Wednesday, February 6.

3) Look south, about halfway between the southern horizon and the zenith, the point directly above your head. Now look a bit to the left, toward the eastern horizon, and there stands the Hunter. Keep in mind that if you begin observing later than an hour after sunset, Orion will be farther to the west. Since the Earth is spinning toward the east, the stars appear to move from east to west throughout the night. In the continental U.S., Orion will be due south--and at its highest above the horizon--around 9:00 p.m., after which it will continue on toward the western horizon.

4) Look for the hourglass shape: four stars marking the corners of a quadrilateral, with a diagonal line of three evenly-spaced stars cinching the middle of the hourglass. The diagonal line of three stars is quite noticeable and is known as Orion’s Belt. This is one of several asterisms (recognizable star groups) in Orion that can help us imagine the Hunter as he has traditionally been depicted. The two end stars Alnitak (pronounced ALL nitt ahk) and Mintaka (pronounced minn TAH kuh) both mean belt in Arabic. The middle star Alnilam (pronounced ALL nill ahm) is from the Arabic for string of pearls.

5) Hanging vertically below Orion’s Belt is a chain of three stars that forms the asterism of Orion’s Sword. At least, they look like three stars to the naked eye. But in the night sky, things are not always as they appear. We’ll come back to Orion’s Sword for a closer look.

6) Let’s zoom out now and look at the stars that make up the quadrilateral. Orion is unique in the sky in the number of giant and supergiant stars the constellation claims. The star at the upper left is Betelgeuse (pronounced BAY tull juice) which means armpit. This amazing red supergiant is 60,000 times brighter than our Sun. Can you see the red color of this star? The hydrogen supply that powered its internal nuclear reactor has run out, so it has begun nuclear fusion of heavier elements. This process will eventually lead to its demise in a violent supernova explosion. When it explodes, from Earth it will appear as bright as a crescent moon and will be visible in the daytime.

7) The star at the lower right, diagonal from Betelgeuse, is the brightest star in Orion, Rigel (pronounced RYE jull), which means foot. Rigel is a blue supergiant with a much hotter surface temperature than Betelgeuse. Blue stars are typically extremely hot and short-lived. And Rigel--like Betelgeuse--is dying. The scientific jury is still out on whether or not Rigel will end its life in a supernova explosion.

Sometimes it's easier to see the subtleties of star color when you can compare nearby stars of different hue. Take this opportunity to contrast the stark blue-white color of Rigel with the smoldering red-orange of Betelgeuse.

8) The star at the upper right of the quad is blue giant Bellatrix (pronounced BELL uh trix). Bellatrix is Latin for female warrior, which may be a reference to the Arabic name and a mysterious female figure lost to history. This star is sometimes called the Amazon Star. The star at the lower left of the quad is Saiph (pronounced SAFE), Arabic for sword. Saiph is a blue supergiant, hotter even than Rigel and probably destined for death by supernova.

9) Are you up for a stargazing challenge? If you are observing from a dark enough location, you can try to spot the dim asterisms of Orion’s Head and Orion’s Shield. Centered above a line connecting Betelgeuse and Bellatrix is a three-star group anchored by the star Meissa (pronounced MAY suh), Arabic for the shining one. This marks the position of Orion’s head. To the right (west) of Bellatrix is a curve of stars that marks Orion’s shield. In artistic depictions of Orion, this object is seen variously as a hide-covered shield or a lion’s pelt.

I hope you enjoyed this naked-eye tour of the magnificent Orion. Gather up your binoculars, opera glasses, telescopes, and spyglasses, because next week, we’ll take an up-close look at Orion’s Sword, wherein lies one of the most spectacular objects in the night sky. Until then, happy star trails to you.

House of Cepheus

Poor King Cepheus. All he wanted was to go quietly about the business of ruling his ancient kingdom of Ethiopia from his comfortably appointed palace. Instead he found himself saddled with a dangerously vain wife, Queen Cassiopeia, and a sea monster hell-bent on ravaging his coastline. To make matters worse, he was forced to offer his daughter Andromeda in sacrifice to the bloodthirsty sea creature. However, the right-place-right-time intervention of the hero Perseus saved Andromeda from a gruesome death--and gave the relieved Cepheus an excellent prospect for a son-in-law.

King Cepheus, in an 18th century star atlas
Image courtesy of G.M. Caglieris

In the night sky, Cepheus (pronounced SEE fee yuss) keeps his mythological family close: Cassiopeia, Andromeda, and Perseus are also immortalized as constellations and the four star groups are known collectively as “the Royal Family.” Cepheus and Cassiopeia are circumpolar constellations. As explained in a previous post, circumpolar constellations appear to circle the North Celestial Pole, which is conveniently marked for us by Polaris, the North Star.

Since it is a somewhat dim constellation, Cepheus is best observed when it‘s high in the sky. It will be dipping down closer to the horizon in spring and summer, so let’s close out January by paying homage to the king.

1) You’ll need to know where north is. If you don’t have a compass, take note of where the sun sets, which is approximately west. Stand with your left shoulder to the west, and you will be facing approximately north.

2) Wait until one hour after sunset to begin observing, so the sky is good and dark. The waning (shrinking) moon won’t rise until about an hour and a half later on Thursday the 24th, and then later each night as the week progresses. This late-rising moon will give you plenty of time to observe dim Cepheus in a dark sky.

3) Face north and tilt your head all the way back until you are looking at the zenith, the point in the sky that is directly above your head. Then slowly lower your head about one-quarter of the way down toward the northern horizon and then turn it slightly to the west. You should spot a sprawling “M” pattern in the sky. This is the Lazy W asterism (prominent star pattern) of the constellation Cassiopeia, but it will be upside down because it is in mid-revolution around the North Star.

4) If you can find the queen, you can find the king. The open arms of the Lazy W (the bottom of the M, in the current orientation) point toward Cepheus. The defining asterism that helps us locate Cepheus is called The House. It looks like a child’s drawing of a house: a square-like shape with a triangular pointed roof. Since the Lazy W will be upside down when you see it, The House will be under the M, on its side, with the peak of the roof pointing to the ‘right,’ relative to the M. Can you see the house shape? If you’re having trouble, avoid all white light for about 20 minutes to ensure that your eyes are dark adapted for optimum night vision. This will aid you in spotting dim stars.

5) Let’s look at the three brightest stars in Cepheus. They lie in a line, from the peak of the house to its base, along the ‘right’ side of the house (if it were oriented normally, with floor down and roof up). The star at the peak of the roof is Errai (pronounced uh RYE) from the Arabic for shepherd. A Jupiter-sized planet has been discovered orbiting Errai. The next star in the line is Alfirk (pronounced all FURRK), Arabic for flock. Alfirk is a pulsating blue giant. The ancient Arabs thought the stars in the vicinity of the North Pole represented a shepherd and his flock of sheep, which explains why some of the Arabic star names in Cepheus are pastoral in nature. The third star in the line, at the base of the house, is the brightest star in Cepheus, Alderamin. Alderamin (pronounced ull-derr-ah-MEEN) is from the Arabic for right forearm. This white dwarf star is spinning furiously--125 times faster than our Sun spins!

6) If you draw an imaginary line from Alderamin to Alfirk and continue past Alfirk about twice more the distance between the two stars, you will come to a star that is a bit brighter than Alderamin. This is Polaris, the North Star.

In about 5500 years, Alderamin will be our pole star, our North Star, replacing Polaris. That’s right, Polaris has a limited engagement. This change will occur because, as planet Earth spins on its axis, it wobbles slightly like a top. This means that, over time, our axis inscribes a small circle and points at different locations in the sky. If we’re lucky--as we are now with Polaris--that circle intersects with various stars that we Earthlings can use as North Stars, convenient visual and navigational markers for north.

7) At the base of The House of Cepheus, midway between Alderamin and the other (nameless) base star and also slightly below them, is a crown jewel of a star. The Garnet Star is one of the reddest stars in the sky and certainly one of the reddest stars that can be seen naked eye. Can you spot it? You may not see any color while looking at it naked eye. However, if you gaze at a magnified view of this star through binoculars or a telescope, its distinctive color will be revealed. It can vary from deep red to copper, depending on atmospheric conditions and the quality of your sky.

The Garnet Star is a red supergiant, discovered by Sir William Herschel, the famed British astronomer who also discovered the planet Uranus. The Garnet Star is so immense that if it were dropped in our solar system in the Sun’s position, it would extend beyond the orbit of Jupiter. Its great size pretty much ensures it will go out with a bang in the most cataclysmic of cosmic events: a supernova explosion. Enjoy the pretty bauble, while it lasts.

Walking With Galileo

Reader Steve Welch asks, “What can be done with the department store telescope that a well-meaning but clueless parent or grandparent (who didn't read your blog) gave someone for Christmas? I think the kids who get those scopes should be helped.” Excellent question, Steve. My diatribe against department store telescopes notwithstanding, the reality is that thousands of small, cheap refractors are given as first telescopes. They can, in fact, be the instruments that spark many a life-long interest in observational astronomy.

If you’ve received one of these, you have a great advantage: you now have a tool to take you beyond the naked-eye stargazing that you’re probably already doing. And I must confess you also have a great advantage over me. In my science-bereft youth, I never saw a telescope and never even knew that a regular person was allowed to own a telescope. Telescopes were big, fancy, complicated pieces of equipment restricted for use by professional scientists in observatories. Or so I thought.

So, where in the Milky Way shall we go? Your new spyglass is probably best suited for exploring our galactic ‘hood, better known as the solar system. You can use it to examine the Moon and the planets. You can even go beyond the solar system and explore the band of the Milky Way. And you’ll be in great company as you do so, because you’ll be walking with Galileo.

We’ve all heard of Galileo Galilei, the 17th century Italian astronomer and “father of modern astronomy” who butted heads with the Catholic Church. Every stargazer who strives to see more through binoculars or telescopes, every amateur telescope maker who grinds glass, every artist who sketches sunspots or moon craters at the eyepiece--they all walk in Galileo’s footsteps. He was the big kahuna who got this star party started.

Galileo Facing the Roman Inquisition - painting by Cristiano Banti

Just one year after the invention of the refractor telescope, the enterprising do-it-yourselfer Galileo turned one of his own improved instruments skyward. He was the first person to use a telescope to make a set of astronomical observations. He quickly made a series of remarkable discoveries, and science and humankind would never be the same.

Galileo was just getting warmed up when he discovered:
- the surface of the Moon is pockmarked with craters. Before Galileo, everyone thought the Moon was smooth, the way bodies in a ‘perfect’ universe should be.
- planets are discs, not points of light like the stars. Before Galileo, planets were thought to be a special type of star: “wandering stars” that moved against the backdrop of the fixed stars.
- the Milky Way is a horde of stars too numerous and faint to be resolved with the naked eye. Before Galileo trained his telescope on it, the long arm of the Milky Way that stretches across the sky could only be seen as a hazy band of light, so its nature was not known.

What makes these and a host of other discoveries truly amazing is that Galileo did it with what we would consider a substandard instrument. Galileo’s telescopes were long, skinny affairs, around four feet long and around two inches in diameter. Only the center of the two-inch glass objective (the optical lens) was usable, due to the primitive glass-grinding techniques of the day. Looking through his telescope would have been a bit like looking through a drinking straw: the field of view was quite small, about a quarter of the Full Moon. It is believed he only ever achieved 20x magnification with the telescopes he made. Additionally, the glass of Galileo’s time was green, due to a high iron content; this was probably a bit like looking through the bottom of an old-time glass Coca Cola bottle. To add yet another challenge to early telescopic viewing, the glass was full of little bubbles.

One of Galileo's telescopes
in the Institute and Museum of the History of Science, Florence, Italy

Sadly, Galileo ended his days confined to his house, because of his conflict with the Church. Moreover, he was nearly blind, probably due to glaucoma (an eye disorder that damages the optic nerve).

This week, let’s send our thanks back across the centuries to the great observer, in the only way that is fitting: by pointing our little refractors at the night sky. No matter how cheap or rinky-dink your telescope is, remember it’s giving you a better view than our pioneering pal Galileo had.


1) The moon will be a great target this week, as it grows towards Full Moon on January 22. It will be already visible in the sky at sunset every evening until January 23, when it will rise about an hour and a half after sunset. Even in my $20 drugstore refractor, the Moon is a marvel of ridges, dark lava seas, and craters of every size.



2) High in the eastern sky at sunset, the red planet Mars shines bright between the two horn stars of Taurus, east of the Pleiades. A view through your little refractor with your highest power eyepiece (lowest mm number) will clearly show the contrast between the disc shape of the planet and the pinpoint lights of the background stars. On Saturday the 19th, the Moon and Mars will dog each other across the sky, a mere sliver apart.

The Winter Milky Way, marked with gray band

3) If you are observing in a dark-sky location, you should be able to spot the gossamer band of the winter Milky Way stretching overhead from horizon to horizon. It's easiest to spot before the Moon rises or after it sets. The band's orientation across the sky and relative to the horizons will vary as the night progresses, but it will eventually move toward the western horizon, as the Earth spins toward the east and another morning. Slowly scan the length of the hazy band of light with your telescope to see thousands of otherwise invisible stars come within your reach. It is no less magic now than it was in the 17th century.

Crayon portrait of Galileo by Leoni

Morning Star

Venus imaged in violet light (NASA)

This one’s for all the morning people. I’m talking about you early birds who are up before the Sun. You need to take a moment this week, step outside with your bracing cup of joe, and greet Venus, the Morning Star.

The convention of referring to planet Venus as both the “Morning Star” and the “Evening Star” began long ago. The ancients, confused by what they observed, thought Venus was two separate bodies: two “wandering stars,” as planets were originally known. Whether we see Venus in the morning before sunrise or in the evening after sunset depends on where it is in its orbit around the Sun-- to the ‘right’ (west) or the ‘left’ (east) of the Sun from our earthly perspective.

Venus is the second planet from the Sun, orbiting between Mercury (closest to the Sun) and Earth (third rock). Venus is only slightly smaller than Earth. Like Earth, Venus is a terrestrial planet, that is, it is composed primarily of rock and metal. And that’s pretty much where the similarity ends.

Lava flows on Venus (NASA)

Thick clouds of sulfuric acid blanket Venus, and its atmosphere is predominantly carbon dioxide. Its barren landscape is arid and covered with old lava flows. Air pressure at the surface is extremely high, comparable to the pressure we would experience ocean diving at 3,000 feet. This dense atmosphere coupled with the cloud blanket raise the planet’s surface temperature high enough to melt lead. An unsuspecting interplanetary tourist would be asphyxiated, crushed, and cooked in one fell swoop.

Venusian cloud cover imaged in ultraviolet (NASA)

But from a safe distance, Venus is an ornament for our sky, beautiful and stunningly bright. I’ve seen punch-drunk amateur astronomers, after a long night of observing, nearly drop equipment in shock when Venus rises, wondering aloud what on Earth that blinding object is! I’ve also heard many an amateur grumble that Venus rising ruins their night vision--in the same way they’d grumble about moonrise destroying their dark adaptation. In fact, after the Sun and the Moon, Venus is the brightest object in our sky.

Why is Venus so darn bright? For one thing, it is the closest planet to Earth, so its apparent size (the amount of sky it covers from our perspective on Earth) is significant. For another thing, its thick cloud cover is extremely reflective, so most of the sunlight it receives bounces back.

Got your steaming cup of joe? Let’s go!

1) Venus will rise in the southeast. If you know about where the Sun rises, watch that spot and you will see Venus rise. If you’re completely flummoxed by cardinal directions, don’t worry. When Venus rises, it will be the brightest object in the sky by far. You can’t mistake it for anything else. It will shine with a blue-white color, unless you have atmospheric murk near the horizon, in which case it may scintillate in a number of colors, like a twinkling star or a UFO.

2) In the continental United States, Venus will rise between 4:30 and 6:00 a.m. on the morning of Friday, January 11, depending on your location. The farther north and west you are in the U.S., the later the rise time. The curve of the Earth hides a slice of southern sky from you folks in the north, and hides a slice of eastern sky from you folks in the west. Therefore, Venus needs to be a little bit higher in the morning sky before she peeks over your southeastern horizon.

3) Each night after the 11th, Venus will rise a few minutes later than the night before. Wait a minute, that’s not how the stars behave! Let’s take a closer look at this Venusian phenomenon.

If you were to track the time that a particular star rises over the eastern horizon--over a series of nights--you would notice that each night it rises a few minutes earlier than the previous night. This makes sense, if you think about how the Earth travels around the Sun in its year-long voyage. Each night, the Earth is a little farther along in its 365-day orbit around the Sun. As a result, our view of the night sky is ever so slightly different--shifted toward our east, the direction in which we’re traveling. Therefore, a star that breached the eastern horizon at a particular time the night before would, at the same time the following night, already have risen. The Earth’s position is constantly changing relative to the panorama of space.

But why, then, is Venus currently rising a few minutes later each night? Aha! Because Venus herself is orbiting the Sun, moving in the same direction as the Earth travels. Venus is outpacing the Earth, moving about 15% faster in a smaller orbit. The result of all this fast living is that Venus is now starting the quarter-leg of her journey when she swings back behind the Sun and, from our vantage point, appears to be getting closer to the Sun. So after we Earthlings rotate for 24 hours and find ourselves again at the same minute we saw Venus rise the night before, she can’t be seen yet. She has moved a little ‘closer’ to the edge of the Sun, is rising over our horizon a little nearer to the time of sunrise, and will eventually disappear behind the Sun for a while.

The mapped surface of Venus (NASA)

4) After you greet the bright goddess in the southeastern sky, turn around and bid farewell to red-faced Mars, hanging low above the northwestern horizon. Now, if talk of the ecliptic still makes you apoplectic, this is a golden opportunity to get some clarity (and serenity) on the concept. With two bright planets in the sky at opposite horizons, you can begin to visualize how the band of zodiacal constellations stretches across the sky. Imagine a curved line connecting the two planets, with the line curving slightly down toward the southwestern horizon. This line that you are imagining approximates the ecliptic, the path that the Sun appears to take against the backdrop of the stars, from our point of view as we circle around the Sun. And since we know from last week’s post that the zodiac is a daisy chain of constellations that, along with the Moon and the planets, appears to hug the ecliptic, voila! You can see that having a couple planets in the night sky can really help with sky orientation.

5) You know, you’ve been such a great audience, I’m going to throw in a bonus planet. With your left shoulder pointed toward brilliant Venus and your right shoulder pointed toward ruddy Mars, you’ll be facing southwest. About halfway between the zenith (the point directly above your head) and the southwestern horizon, you will see a bright, golden-colored “star.” This is the ringed planet Saturn, currently spending a little quality time in the zodiacal constellation of Leo the Lion and audaciously close to the ecliptic.

The Other Seven Sisters

The winter constellation Taurus the Bull is believed to be one of the first constellations recognized by our early ancestors. Its notable star pattern has been seen as a bull since ancient times.

The Winter Sky with Taurus and Orion

In a number of early cultures, Taurus--not Aries--was considered the first sign of the zodiac and therefore marked the beginning of the zodiacal year. The zodiac is the band formed by the twelve constellations that straddle the ecliptic, as seen from Earth. The ecliptic is the imaginary line that represents the path the Sun takes across the sky. It is marked in the image above with the gold line. Because the Earth, the Moon, and the planets all lie in roughly the same platter-shaped orbital plane, the Moon and the planets appear to stick close to the ecliptic path. This is why we see the Moon and the planets moving through the zodiacal constellations over time.

The path of the ecliptic through the zodiacal constellations
Diagram from Spacetech’s Orrery

Taurus is easy to locate because of the distinctive “V” pattern that outlines the bull’s face. This V is composed of the orange giant star Aldebaran (pronounced ahl DEBB er ahn) and the star cluster The Hyades (pronounced HIGH uh deez). This V-shaped asterism, or recognizable star grouping, is very near the most famous object in Taurus, The Pleiades (pronounced PLEE uh deez) or Seven Sisters star cluster. The image below shows a typical naked-eye view of the two clusters, the V asterism, and the two stars marking the tips of the bull‘s horns. The straight lines have been drawn in for reference.

© T. Credner & S. Kohle, AlltheSky.com

The Pleiades may have the star power, but this week we’re going to focus on the other seven sisters. In mythology, the Hyades, like the Pleiades, were the daughters of Atlas. Atlas, you may recall, was the beefy dude who was assigned to hold up the sky after his tactical error of siding in battle against the god Zeus. Also seven in number, the Hyades were half-sisters to the Pleiades, by a different mother.

Atlas holding up the celestial sphere
Image by Colin Gregory Palmer

The word Hyades comes from the Greek for “to rain,” an ancient reference to the cluster’s appearance during the rainy season. This seasonal apparition must have fueled the superstitious belief that the cluster actually caused storms to occur on land and at sea. It was said that Zeus placed the Hyades in the heavens to assuage their grief at the death of their brother, who was killed by a wild boar.

Aldebaran is the brightest star in Taurus, and it marks the position of the bull‘s eye. Its name is from the Arabic for “follower,” because it follows the Pleiades across the sky.

The beautiful, sparkling Hyades help take the sting out of a cold winter night, so let’s go take a look:

1) Plan to wait at least one hour after sunset to begin stargazing.

2) Face east. If you’re not sure of your cardinal directions, use a compass, or note where the sun sets and put your back to that spot. You will then be facing east.

3) About midway between the zenith, the spot directly above your head, and the eastern horizon, you will find the V asterism of Taurus. It will be slightly below the Pleiades (also known as M45), that prominent fuzzy-looking star cluster that looks like a little bunch of grapes or a mini-ladle. Bright planet Mars, gleaming coppery-red, is now just outside of the two stars that mark the tips of the bull’s horns, slightly toward the northeast.

© T. Credner & S. Kohle, AlltheSky.com

4) After you’ve located the V, pick out Aldebaran. It will be unmistakable as the brightest star in the V and the most reddish-colored.

5) The Hyades is a wonderful binocular object. If you have binoculars or even opera glasses, train them on the point of the V and explore the area. Although you can only see a handful naked-eye, there are more than 250 stars in this cluster. Binoculars are a great way to peer into its depths. Can you spot differences in the size, brightness, and color of the cluster’s members?

© T. Credner & S. Kohle, AlltheSky.com

6) Like all the stars that we can see in the night sky, the Hyades cluster is in the Milky Way, our home galaxy. About 150 light years away, it is notable as the nearest open cluster to us.

7) At only 60 light years away, Aldebaran is more than twice as close to us as the Hyades cluster. It is not physically associated with the cluster. It is merely a happy accident that it glows there, giving the bull’s face its most distinguishing feature.

8) As you spy upon the lesser-known but still lovely sisters and their multitude of attendants, keep in mind that this cluster--like all star clusters--is three dimensional. Its members are spread over a volume of space around 80 light years in diameter.