Welcome to the Night Sky Map for July 2021! This month, we talk about the three “stars of summer,” which make up the Summer Triangle: Vega, Altair, and Deneb! Plus, learn why looks can be deceiving…
Just click here or on the image below to open the printable map—then bring outside!
The Summer Triangle: How Looks Can Be Deceiving
Year after year, century after century, star gazers have celebrated the return of the Summer Triangle. At this time every year, the distinctive three-cornered pattern formed by the bright stars Vega, Altair, and Deneb, is prominent in the southern sky.
Meet the Stars of Summer
The three stars of the Summer Triangle appear similar in brightness. Vega in the constellation Lyra the Lyre is the brightest of the trio and the 5th brightest of all stars. In Carl Sagan’s novel “Contact,” Vega is the source of the first message ever received from an alien civilization. The 1997 movie version features actress Jodie Foster’s quest for the senders of the Vega message. Back in the real world, we’ve yet to hear anything from the possible inhabitants of the Vega system, but researchers are listening to Vega and thousands of other stars every day, just in case.
Altair, in Aquila the Eagle, is another Hollywood star. In the 1956 film “Forbidden Planet,” the fourth planet in the Altair system (Altair IV) is home to the relics of an ancient alien civilization and to an eccentric Earth scientist and his beautiful daughter (Walter Pidgeon and Anne Francis). Altair is the 2nd brightest member of the Summer Triangle and 13th brightest star of all. We don’t know if Altair is surrounded by any planets, so Altair IV may or may not exist.
Number three in the Summer Triangle and 20th brightest star is Deneb, which marks the tail of Cygnus the Swan. Alas, Deneb has never starred in a major motion picture, but it has other claims to fame. Whereas Vega and Altair are relatively close to us in astronomical terms—25 and 17 light-years respectively—Deneb is much farther away, an estimated 2,600 light-years from Earth. A light-year is the distance light travels in one year—a big, BIG number!
The three stars of the Summer Triangle appear to be about equal in brightness. Put another way, their “apparent brightness” is roughly the same. But looks can be deceiving! We know that Deneb is more than 100 times farther from us than Vega or Altair, yet it appears nearly as bright. How can this be? The only way Deneb can both be farther away yet appear equal in brightness is if its actual (or “intrinsic brightness”) is much greater than the others. In fact, Deneb is one of the most luminous of all stars—an astounding 200,000 times brighter than our Sun! The light we see from Deneb left the star about the time the Pyramids of Egypt were being built.
Using the Summer Triangle
Once you’ve spotted the Summer Triangle, you can use it to find other sights. The largest and most prominent asterism (unofficial star pattern) associated with the Triangle is the Northern Cross, comprised of the brightest stars in Cygnus. Smaller and less prominent, but quite striking, is the exquisite little Parallelogram which hangs just below blazing Vega in Lyra. Sagitta the Arrow is a dim but delightful constellation that sits at the upper left of Altair. It’s one of the smallest constellations, and it really does look like a little arrow! Just below Sagitta and similar in size is tiny Delphinus the Dolphin.
Elsewhere on this month’s map, you’ll find other asterisms of note, including the constellation Cepheus, shaped like a house, and Cassiopeia, which looks like a big “W.” And if your horizon is free of trees and houses, you may glimpse the Great Square, the body of Pegasus the Flying Horse.
Click here or on map below to enlarge (PDF).
Sky map produced using Chris Marriott’s Skymap Pro
Note: How to Read the Sky Map
Our monthly sky map does not show the entire sky which would be almost impossible. Instead, the map focuses on a particular region of the sky each month where something interesting is happening. The legend on the map always tells you which direction you should facing, based on midnight viewing. For example, if the map legend says “Looking Southeast,” you should face southeast when using the map.
The map is accurate for any location at a so-called “mid northern” latitude. That includes anywhere in the 48 U.S. states, southern Canada, central and southern Europe, central Asia, and Japan. If you are located substantially north of these areas, objects on our map will appear lower in your sky, and some objects near the horizon will not be visible at all. If you are substantially south of these areas, everything on our map will appear higher in your sky.
The items labeled in green on the sky map are known as asterisms. These are distinctive star patterns that lie within constellations. When getting your bearings under the stars, it’s often easiest to spot an asterism and use it as a guide to finding the parent constellation.
The numbers along the white “Your Horizon” curve at the bottom of the map are compass points, shown on degrees. As you turn your head from side to side, you will be looking in the compass direction indicated by those numbers. The horizon line is curved in order to preserve the geometry of objects in the sky. If we made the horizon line straight, the geometry of objects in the sky would be distorted.
See more monthly highlights of the night sky in the July Sky Watch.