Thursday, October 2, 2008


Although autumn is beginning to nip the air and sweaters are starting to replace tank tops, you can still hold on to that endless-summer feeling a little longer. If you look up after sunset, you’ll find the star pattern known as the Summer Triangle overhead. Like the Winter Hexagon, the Summer Triangle is a seasonal asterism (recognizable star pattern) that spans more than one constellation.

Even in somewhat light-polluted areas, you should be able to spot the three bright stars that define the Triangle, but this asterism is worth a trip to the outskirts of town to view it under dark skies. The Triangle has naked-eye and binocular treasures to plunder, if you’re willing to go the extra mile.

Once you master the Summer Triangle, you will know:
- five constellations
- the brightest stars in three of those constellations
- which star is used as the standard for star brightness
- the direction in which our solar system is traveling
- another prominent asterism
- a notable double star
- a naked-eye star cluster
- a binocular star cluster

This is a great deal. In fact, just think of this as a celestial end-of-summer closeout sale. Hurry hurry hurry!

1) Wait at least one hour after sunset to begin observing, so that twilight’s finished and your sky’s good and dark.

2) Check the time of moonset before you observe. The waxing (growing) Moon will be in the western sky at sunset and will set a bit later than the night before, each night this week. The Moon will also be a bit brighter each night, as the illuminated part of its face increases. Your optimum observing window— for least Moon interference and best placement of the Triangle in the sky after moonset— ends on Sunday night, October 5.

3) Face south. 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’ll be facing approximately south.

Star maps created with Your Sky

4) Tilt your head all the way back until you're looking at the zenith, the point in the sky that’s directly overhead. Now look for a large triangle formed by three bright stars. It will straddle the zenith or be slightly south of it, depending on your latitude. The sides of the triangle will be two to three fist-widths long. A fist-width is defined here as your fist held at arm’s length against the sky and measured across the knuckles.

5) The stars that mark the points of the triangle are Vega (VAY-guh) in the constellation Lyra the Harp (LYE-ruh), Altair (AHL-tair) in the constellation Aquila the Eagle (uh-QUILL-uh), and Deneb (DENN-ebb) in the constellation Cygnus the Swan (SIGG-nuss).

Vega is Arabic for swooping vulture, Altair is Arabic for flying eagle, and Deneb is Arabic for hen’s tail. Let’s see, Aquila is an eagle, and Cygnus is a swan. Yes, there’s an ornithological motif here, with its roots entwined in the ancient star lore of several cultures.

6) Be careful to identify the correct triangle. It’s possible to be distracted by Rasalhague (RAH-sahl-hayg), the brightest star in neighboring Ophiuchus (oh-fee-YOU-kuss) the Serpent Handler, and to combine it with Vega and Altair. However, Rasalhague is farther west than the three Triangle stars, and it’s not as bright as Deneb.

Another way to ensure you have the correct triangle, with Deneb, is to look for the asterism called the Northern Cross. Deneb is at the top of the cross, and the bottom of the cross terminates inside the Triangle, nearly equidistant from Vega and Altair.

7) Now that you’re sure you’ve found the Triangle, let’s take a closer look at the point stars.

Vega, a white dwarf, is the brightest star in Lyra the Harp and the fifth brightest star in the night sky. Apparent magnitude, how bright a star appears to us from Earth, is one of the ways in which stars are measured, categorized, and compared. Vega is a zero magnitude star, and it’s the standard of brightness to which all other stars are compared. Dimmer stars have a magnitude expressed with a positive number, while stars brighter than Vega have a magnitude expressed with a negative number. It’s just the opposite of how you’d think it would be.

A light year is the distance light travels in one Earth year, nearly six trillion miles. Vega lies 25 light years from Earth, and it just happens to lie close to the solar apex. The solar apex is the point in the night sky toward which our solar system appears to be heading, as it travels around the center of the Milky Way galaxy in a 225-million-year-long orbit.

Altair, also a white dwarf, is the brightest star in Aquila the Eagle. Altair is flanked evenly by two dimmer stars, Alshain (all-SHANE) and Tarazed (TAH-ruh-zedd). The names of both stars are from a Persian phrase meaning scale beam, that is, the balance bar from which a set of scales hangs. This straight line of three stars is easy to pick out in the night sky. In old star atlases, Altair marked the heart of the Eagle. Altair is about 16 light years from Earth, the closest of the three Triangle stars.

Deneb, a white supergiant 200 times the size of our Sun, is the brightest star in Cygnus the Swan. Deneb marks the tail of the Swan, and as we learned earlier, the top of the Northern Cross.

In reality, spectacular Deneb is considerably brighter than either Vega or Altair. It appears to us the dimmest of the three because of its great distance. It’s 2600 light years away— that’s 2600 times six trillion miles. It may be forgiven if its light is a bit weak after that long journey to our eyes.

8) In addition to Lyra, Aquila, and Cygnus, the Triangle also encompasses parts of two minor constellations: Vulpecula the Fox (vull-PECK-you-luh) and Sagitta the Arrow (suh-JEE-tuh). The map above shows the modern boundaries of these obscure old constellations. Don’t worry about identifying the specific stars of these constellations inside the Triangle; it’s enough to know that they’re there.

When the Fox was introduced to star atlases in the 17th century, he was depicted carrying in his jaws a goose. Yet another species for our celestial birdwatching list!

9) Earlier we identified the Northern Cross asterism. The star at the bottom of the Cross, inside the Triangle, is the showy double star Albireo (al-BEER-ee-yoh). Albireo marks the beak of the Swan. Now that you know the location of the beak and tail of the Swan, you can extend the crosspiece of the Northern Cross on either side to visualize the Swan’s outstretched wings.

Albireo is commonly known as the Cub Scout Star. To discover why, you’ll need to examine it through binoculars, 10x50s or larger. Use your binoculars on a tripod if you have one. If not, steady your arms by propping them on a fencepost or other stable object. With sharp focus, you should see a bright gold star with a dimmer blue star nearly touching it. Blue and gold: the Cub Scout colors.

Albireo is a visual double, that is, two stars that appear to be close together but in fact aren’t in proximity and aren’t interacting. Space is three dimensional, and that depth creates many pleasing— albeit deceptive— star pairings, as seen from our earthly vantage point.

10) Now draw an imaginary line between Altair in Aquila and Vega in Lyra. About a third of the way from Altair to Vega is a naked-eye object called Brocchi’s Cluster. Can you see the sparkly patch of light? The object is named for the amateur astronomer who mapped it in the 1920s, although it’s been known since antiquity. Despite its name, it probably isn’t a true star cluster, since its stars don't seem to be related. Rather it’s an asterism, a chance arrangement of stars forming a recognizable pattern.

That pattern is revealed when you view the object through binoculars, as is its other name: the Coathanger. Magnified, the patch is transformed into a whimsical star grouping that resembles a coat hanger. A foxy trick, perhaps, but then the Coathanger does lie in the constellation of Vulpecula the Fox.

The Coathanger
© T. Credner & S. Kohle,

11) Speaking of foxes, let’s find our final target, one of my favorite star clusters: the charming Foxhead Cluster in Cygnus. You’ll need good-sized binoculars for this one; I’d recommend 10x50s or larger. Draw an imaginary line from Vega in Lyra to Sadr in Cygnus. Sadr (SAH-durr) is the star at the center of the Northern Cross. Train your binoculars on a spot that’s a little more than halfway from Vega to Sadr. Look for a triangle-shaped clutch of stars with two bright stars flanking it. The triangular cluster is the Foxhead, and the longer you look at it, the more it begins to resemble the pointed snout and ears of a fox. An overactive imagination can be a great advantage in this endeavor.

The Foxhead Cluster
Image from Sloan Digital Sky Survey

With all those birds flitting around the Triangle, I imagine this wily fox is just biding his time.

12) You hopefully have noticed the long, bright “cloud” of the Summer Milky Way running through the Triangle. The glowing band is a sort of celestial runway for Cygnus the Swan, who appears to be preparing to fly south for the winter.

The Summer Milky Way is our edge-on view of the star-packed arms of our platter-shaped galaxy. In the summer, due to Earth’s location in its orbit around the Sun, we are looking inward, towards the galactic core.

How exciting that we can see, with our naked eyes, some of the structure of that vast star system to which our lucky old Sun— and we— belong.