Three Leaps of the Gazelle

Between two of the boldest naked-eye star patterns in the spring sky lies a third more demure pattern that’s one of my personal favorites. It’s quite easy to spot when you know where to look, so let’s go stargazing, shall we?

1) About an hour after your local sunset time, face west. 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— close enough for our purposes.

2) Tilt your head back and look at the zenith, the point in the sky that’s directly overhead. A little to the north (right), you should spy the distinctive seven-star pattern of the Big Dipper. The Big Dipper is an asterism, a recognizable star pattern, and it lies in the constellation of Ursa Major the Big Bear.

Star maps created with Your Sky

3) Now look a little south (left) and west from the zenith. Look for the Sickle, the curved asterism that marks the head of Leo the Lion. The Sickle looks a bit like its namesake, the old-fashioned farm implement, or like a backwards question mark. Spot it? Great.

4) Look in the space between the curved top of the Sickle and the bottom of the bowl of the Dipper for three pairs of stars widely spaced from one another. Each pair is around the same distance from the other pairs, and the spacing between the two stars of each pair is around the same. In addition, the stars are all around the same magnitude of brightness. This similarity of pattern helps the little pairs to stand out from the stars around them.

These form the asterism known as Three Leaps of the Gazelle. The three star pairs all lie within the constellation of Ursa Major the Big Bear, and they mark three of the bear’s paws.

Ancient Arabic star lore relates that the gazelle was startled by the lash of the lion’s tail when it sprang from “the Pond,” what we know as the Coma Star Cluster in the constellation Coma Berenices (Berenice's Hair). You can spot the Pond naked eye from a dark site; it looks like a large, bright patch just off Leo's tail.

5) Starting at the Pond, the first set of hoofprints you come to are the stars Alula Australis and Alula Borealis. Alula (uh-LOO-luh) is from the Arabic for first leap; Australis and Borealis are Latin for southern and northern, referring to the respective position of each star.

The middle set of hoofprints are the stars Tania Australis and Tania Borealis. Tania (TAH-nih-yuh) is from the Arabic for second leap.

The final set of hoofprints are the ones farthest from the Pond. The northernmost star is Talitha (TAH-lith-uh), from the Arabic for third leap. Its companion has no traditional name, so we know it simply as Kappa, its star catalog designation.

Can you imagine the swift gazelle leaving behind those three pairs of watery hoofprints as it leapt between twin perils of lion and bear?

Astronomy Essential: The Milky Way galaxy belongs to a galaxy group.

The Milky Way, our home galaxy, is a member of a group of galaxies called the Local Group. It contains around 30 galaxies. The two that are closest to the Milky Way are called the Large Magellanic Cloud and the Small Magellanic Cloud. They are much smaller than the Milky Way and can be seen with the naked eye from the Southern Hemisphere.

Another well-known member of our Local Group is the Andromeda Galaxy, which can be seen naked eye from the Northern Hemisphere. The Andromeda Galaxy is the closest large galaxy to our Milky Way; it lies around two and a half million light years from Earth.

Galaxies throughout the universe tend to cluster into groups. The members of these galaxy groups interact gravitationally and sometimes collide.

The Big Dipper Revisited

A couple weeks ago, we located that big drinking vessel in the sky, the Big Dipper. Then we used it to find Polaris, the North Star. This week, let’s take a closer look at the stars of the Big Dipper, both as a group and as individuals.

Most skywatchers know that the famous Big Dipper asterism (recognizable star pattern) is composed of seven stars. What you may not know is that the central five stars are related--what we could call a family group. Collectively they’re known as the Ursa Major Cluster: “Ursa Major” after the constellation that contains the Dipper, Ursa Major the Great Bear, and “Cluster” because they constitute a star cluster, a group of stars that formed around the same time in the same nebula or gas cloud. Astronomers have been able to determine that these five stars are a cluster because of their similar distance from us and the similar direction of their motion through space. All five are around 80 light years from Earth. A light year is the distance light travels in one Earth year, nearly six trillion miles.

Ursa Major Cluster

Chart created with Your Sky

If the Ursa Major Cluster is a family group, then the Big Dipper is an extended family. Let’s get to know each member a little better.

1) If you’re unfamiliar with the location and circular motion of the Big Dipper, read my earlier post.

2) The Summer Solstice on Friday, June 20, is the longest day of the year and, consequently, the shortest night. But even the shortest night gives us plenty of quality time with the Dipper. The bright Moon won’t rise until around two hours after sunset on Friday night and a bit later each subsequent night of our observing week.

Stars of the Big Dipper

Chart created with Your Sky

3) Moving from left to right along the right-side-up Dipper, we’ll start with the three stars of the handle and finish with the four stars of the bowl. The end star of the handle is Alkaid (pronounced AL-kayd). Alkaid, from the Arabic for leader, is a hot, blue-white star six times the size of our Sun.

4) The middle ‘star’ of the handle is probably the best known, because it is a naked-eye double star referred to as the Horse and Rider. The brighter star of the pair is Mizar, and its dimmer companion is Alcor. Mizar and Alcor are reported to have been used by the Romans and the Arabs as a vision test, in the days before they had eye charts with the big “E” on top. Now test your vision: can you see both stars? They are quite close together, and the light from dim Alcor blends together with Mizar’s when you look quickly. If you’re having trouble seeing the double, try using averted vision by looking at a point in space right next to Mizar. Sometimes not looking directly at the main star will help the companion star pop into view. This occurs because our peripheral vision is better than our straight-ahead vision.

Mizar (pronounced MY zahr) is from the Arabic for groin (of the bear). With a telescope, you can split bright Mizar, that is, observe that it is two stars close together. But you won’t have enough magnification to see that each of those two stars is also a double star. That’s right, Mizar is really a system of four stars--two orbiting pairs! Alcor (pronounced AL core) is from the Arabic for black horse. Although Alcor and Mizar both belong to the Ursa Major Cluster and Alcor appears to be incredibly close to Mizar, astronomers believe that Alcor is a bit too distant from Mizar to be in orbit around it.

So, now you can amaze your friends and family when you tell them that what looks like one star in the middle of the Dipper handle is really five stars.

5) The third star in the handle is Alioth (pronounced AL ee ahth), also from the Arabic for black horse. The luminous, white Alioth is the brightest star in the Big Dipper, as well as the entire Ursa Major constellation.

6) Megrez, the top left star of the bowl, is from the Arabic for base of the tail (of the bear). Megrez (pronounced MEG rezz) is the faintest star in the Big Dipper. This white star is about twice the size of our Sun.

7) Phecda (pronounced FEK duh) is the bottom left star of the bowl. Phecda, from the Arabic for thigh (of the bear), is spinning very fast--over 80 times faster than our Sun. It is believed to have a spinning disk of gas surrounding it.

8) Merak, the bottom right star of the bowl, is from the Arabic for flank (of the bear). Merak (pronounced MERR ahk) is one of the two pointer stars that point the way to the North Star. This white star is surrounded by a disk of heated dust, and it may be the most likely candidate of the Dipper stars to have a planetary system.

9) Our final bowl star, Dubhe (pronounced DOO bee), is from the Arabic for--amazingly enough--bear. Dubhe is the second brightest star in the Dipper, just a tad dimmer than Alioth. At around 124 light years from Earth, it’s the most distant Dipper star. In marked contrast to the white spectrum of the other Dipper stars, Dubhe is an orange giant. Can you see the difference in color between the two pointer stars, orange Dubhe and white Merak?

Dubhe is a rebel like Alkaid, moving in a different direction from the five stars of the Ursa Major Cluster. The renegade movements of Dubhe and Alkaid will ultimately distort the Big Dipper’s familiar shape beyond recognition, perhaps in 200,000 years or so. In the way that we now wonder, scratching our heads, how the ancients ever conjured a bear from the stars of Ursa Major, future generations may wonder what possessed us to see a water dipper emerge from a ragtag string of stars.

Follow the Drinking Gourd

Two weeks ago, we used the most famous star pattern in the night sky to locate an exceptional deep-sky object that lies outside our Milky Way galaxy. This week, let’s return to our galactic ‘hood and take a closer look at the notable star pattern we call the Big Dipper.

Known to every child in the Northern Hemisphere, the Big Dipper may be the most ancient of all star patterns recognized and recorded by humans. The Big Dipper is not a constellation, as many believe. The Big Dipper is the central asterism (recognizable star pattern) in the constellation Ursa Major, the Great Bear.

Ursa Major is a circumpolar constellation. In the Northern Hemisphere, a circumpolar constellation is one that circles the North Celestial Pole, the imaginary fixed point in the sky that the Earth‘s axis would intersect, were it extended from the North Pole into space. That imaginary point just happens to be extremely close to the star Polaris, which is why we call Polaris the North Star. We can use the Big Dipper to locate the North Star and geographic north; more on that in a minute.

Diagram of celestial poles by Dr. Guy Worthey

For skywatchers in latitudes above 35 degrees north, circumpolar constellations never set; they are always above the horizon as they endlessly circle the North Star counterclockwise, making one revolution per day. But although we commonly say they are "circling," constellations like Ursa Major are not really moving around the Pole. Their apparent motion--or the way they appear to move in our sky--is actually caused by our rotation, as planet Earth reliably spins 24/7 on its axis.

The Big Dipper’s proximity to the North Star has made it a favorite of travelers and navigators throughout history--a reliable guide in the night sky when plotting one’s course across land, water, or air. Let’s get oriented, shall we?

1) You’ll need to face north, 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 left shoulder to the west, and you’ll be facing approximately north.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. A waxing (growing) crescent moon will be in the western sky after sunset through the weekend. First Quarter Moon, when the Moon’s face is half illuminated, occurs on Tuesday, June 10. The earlier in this observing week you look at the Dipper, the less interference you’ll have from moonlight. Or, if you’re a night owl, you can simply wait until the Moon sets.

3) Facing north, look for a distinctive seven-star asterism that looks like a giant, long-handled saucepan. It will be due north to a little west of north and somewhere between the zenith, the point directly above your head, and the northern horizon. The saucepan will be oriented upside-down. Found it? Great. This is the famous Big Dipper (water dipper), or as the French call it, “La Casserole” (saucepan).

Chart created with Your Sky

4) All seven stars of the Dipper are bright enough to be seen even in urban areas that are somewhat light polluted. In a future post, we’ll take a closer look at the individual stars.

5) Make a point of looking at the Big Dipper early in the evening and noting its orientation. Then look at it again before you turn in for the evening, or if you get up before sunrise, look at it again before dawn lights the sky. Do you see how its orientation has changed, how it has rotated counterclockwise and is approaching right-side-up? It’s exciting when you first realize how the stars above the North Pole will 'move' in a predictable, circular motion every night of the year, simply because we live on a spinning globe.

They are all ‘moving’ around the North Star. You can locate Polaris using the two stars on the right side of the right-side-up Dipper’s bowl. These two stars are called the pointer stars, because they point the way to Polaris.

Chart created with Your Sky

6) Draw an imaginary line connecting the two pointer stars. Then extend that line about five times its length, above the right-side-up Dipper. The first fairly bright star you come to is Polaris; it’s slightly dimmer than the top pointer star. Beginning stargazers are often surprised to learn that the North Star isn’t one of the brightest stars in the sky. Its importance to us is its location, not its magnitude of brightness. Now that you’ve found Polaris, face it square and you will be facing geographic north, also known as true north.

Our ability to recognize the Big Dipper enables us to reliably find north at night--without a compass, a map, location familiarity, or knowledge of where the sun set earlier. This is a celestial landmark that might even spell the difference between life and death, or captivity and freedom. According to American folklore, runaway slaves traveling under cover of night oriented to the Big Dipper and followed it north to freedom. They called it the “Drinking Gourd,” a reference to the hollowed-out gourd used in rural areas as a water dipper.

A hauntingly beautiful folk song of uncertain origin called “Follow the Drinking Gourd” may have been used by the Underground Railroad to provide coded directions in its lyrics, guiding the slaves to freedom. Click here to play an MP3 of “Follow the Drinking Gourd,” courtesy of Roger McGuinn’s Folk Den.

When the sun comes back and the first quail calls,
Follow the Drinking Gourd.
For the old man's waiting for to carry you to freedom,
If you follow the Drinking Gourd.

Reward notice for runaway slaves Harriet Tubman and her brothers