SkyWatching: Stars Of Early Summer

Early summer is an in-between time in the skies. The realm of the galaxies has moved off to the west, but the summer Milky Way has not yet arrived. This is the best time of year to observe globular clusters and double stars.

The centrepiece of the early summer constellations is Boötes, the herdsman, with the bright star Arcturus at his heart. Arcturus is easy to find by following the arc of the Big Dippers handle away from the ladle: it is the only bright star in this part of the sky. Alternately, if you live in the northern hemisphere, simply look straight overhead around 11 p.m.

Just after dark on a June evening, look overhead to see the constellations of early summer: Boötes, Corona Borealis, and Serpens Caput.  Credit: Starry Night software.

Although Boötes looks like it might be pronounced like booties, the diaeresis (double dot) over the second o gives you a clue: the two os are pronounced separately: Boh-OO-tes. Its stars form a distinctive kite shape, complete with tail.

Arcturus is the third brightest star in the night sky, after Sirius and Canopus. It is relatively close to us, only 37 light years distant. It is an orange giant star, slightly cooler than the sun, but quite a bit larger in diameter.

Boötes contains relatively few deep sky objects, but is rich in double and variable stars. Izar (Epsilon Boötis) is one of the finest double stars in the sky. With a separation of only 2.9 arc seconds, it requires at least 3 inches aperture, steady skies, and high magnification to see its duality; its stars are gold and greenish in colour. Alkalurops (Mu Boötis) is a much wider double at 2 arc minutes separation, but it is a challenge to see that one of its stars is itself a double.

Although not within Boötes itself, most amateur astronomers use the stars of Boötes to starhop to the Messier globular cluster Messier 3 in the dim constellation of Canes Venatici. M3 forms an almost perfect equilateral triangle with Arcturus and Rho Boötis. This is one of the finest globular clusters in the sky.

Just to the left (east) of Boötes is a small circlet of stars forming Corona Borealis, the northern crown. Look within the circle to see if you can see R Corona Borealis, a very unusual variable star. Some have called this an inverse nova. Most of the time it shines steadily with a brightness of about magnitude 7, just below naked eye visibility, but easily seen in binoculars. At long and irregular intervals, instead of brightening like a nova, it dims by about 6 magnitudes. This dimming is caused by occasional expulsion of a dark obscuring cloud of dust. Currently R is entering its dark phase, but keep watching, and it should soon reappear.

These three constellations contain many interesting objects to look at with binoculars or a small telescope. Credit: Starry Night software.

Below Corona Borealis is one of the most unusual constellations, or rather half constellations. Serpens represents a snake cut in half, each half held in one hand of Ophiuchus. This is the front half: Serpens Caput, or the head of the serpent. The other half, located quite a ways to the east, is Serpens Cauda, the tail of the Serpent. A triangle is supposed to represent the head of the spent, but I always see this and the two stars above as a large X.

The brightest star in Serpens bears the ugly name Unukalhai, which is Arabic for the serpents neck. Just above Unukalhai is Delta Serpentis, a fine pair of pale yellow stars in a telescope.

But the real prize in Serpens Caput is the globular cluster Messier 5, every bit as fine as Messier 3 to the northwest. Like all globular clusters, M5 responds well to aperture and magnification. Besides resolving the cluster into myriads of tiny stars, a large telescope will reveal chains of stars and clusters within the cluster.

Night Sky Tour: The Spring Sky

The spring is a wonderful time to observe. Learn what to look for in the spring night sky tour.

Your Celestial Guidepost

The Big Dipper is a great starting point to learn your way around the night sky; in spring it's conveniently overhead. The Big Dipper itself is not an official constellation but part of a larger constellation called Ursa Major (the Great Bear). Ancient cultures saw the Big Dipper's star pattern as a bear with a long tail. 

The orientation of the Big Dipper from mid-northern latitudes one hour after sunset.

The name originates from the dipper-shaped pattern formed by the seven main stars of the constellation, although observers with keen vision will see that Mizar, the second star from the end of the dipper's handle is, in fact, a double-star. Binoculars make it easier to split the pair.

The stars of the Big Dipper serve as a handy guide to other stars and constellations. The two stars that form the front part of the dipper's bowl point straight to Polaris, the North Star, and because Polaris marks the location of the celestial north pole, the other stars in the sky seem to turn counterclockwise around it. Find Polaris and you know which way is north!

The Little Dipper and Polaris as seen from mid-northern latitudes one hour after sunset.

Polaris itself marks the end of the handle of another dipper—the Little Dipper in Ursa Minor (the Little Bear). Wrapping around Polaris is the constellation Draco. The stars that form this constellation are associated with dragons in the mythologies of many different cultures. 

Earth's north pole has wandered over the millennia and so has pointed to different stars in different eras. Right now it points to Polaris, but one of Draco's stars, Thuban, was the North Star when the pyramids were built.

Arc to Arcturus...

The handle of the Big Dipper points to two of spring's brightest stars—Arcturus and Spica.

The Little Dipper and the star Arcturus as seen from mid-northern latitudes one hour after sunset.

Arcturus is the Alpha (meaning the brightest) star of the constellation Bootes (the Herdsman). Follow the arc of the handle of the Big Dipper until you come to a bright orange star. This is Arcturus, forming the point of a pattern of stars resembling a kite. 

Arcturus is a giant star, twice as massive and 215 times as bright as the sun. It takes 37 years for the light of Arcturus to reach us, so when we gaze at it, we're seeing the star as it looked 37 years ago.

Ancient astromers had measured the position of Arcturus for nearly 2,000 years, which gave Edmond Halley enough data, in 1718, to discover that it was slowly moving against the background stars of its constellation. Before this discovery of proper motion, the stars were thought to be permanently fixed in the sky. Today we know that all stars move, but Arcturus moves much faster than most—about the width of the full moon every 800 years.

...and Speed on to Spica

If you keep following the arc of the handle of the Big Dipper past Arcturus, you'll encounter another bright star, Spica. Keep them straight by remembering this phrase: "Arc to Arcturus and speed on to Spica."

Polaris to Arcturus and on to Spica as seen from mid-northern latitudes one hour after sunset.

Spica resides in the constellation Virgo (the Virgin), a large zodiacal constellation that over time has represented almost every major female deity.

The Prominent Constellation of Spring

The most prominent constellation in the cool evenings of spring is Leo (the Lion).

Leo as seen from mid-northern latitudes one hour after sunset.

To find Polaris, you used the front of the bowl. To find Leo, you need to draw a line through the stars at the back of the bowl—away from Polaris. Stretch out your arm to its full length and measure about three fist-widths from Phecda—the star at the bottom of the bowl. This should put you at a point within the constellation of Leo.

Leo is a constellation of the Zodiac whose main stars have been included in many different mythologies. Stars depicting the mane and head of Leo form a pattern of stars resembling a sickle or a backward question-mark.The brightest star in Leo is Regulus, marking the end of the handle of the sickle or the dot in the question-mark. Regulus, Arcturus, and Spica are the three brightest stars of spring.

The Leonids meteor shower radiates from this constellation around mid-November.

A Beehive of Stars

To the side of Leo is the fainter constellation of Cancer (the Crab).

The Beehive Cluster in Cancer as seen from mid-northern latitudes one hour after sunset.

M44, the Beehive Cluster.

Inside the boundaries of Cancer is a group of stars neatly tucked together into a beehive shape. The Beehive Cluster (also known as Messier 44) was first described by Galileo, but it has been known since antiquity. 

The beehive is easily visible to the unaided eye as a faint round patch of light. Through binoculars, it resembles a swarm of bees.

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The Ten Brightest Stars In The Sky

From our corner of the galaxy, these stars are the most brilliant signposts in the heavens and can be enjoyed even from the light-polluted hearts of major cities.


All stars shine but none do it like Sirius, the brightest star in the night sky. Aptly named, Sirius comes from the Greek word Seirius, meaning, "searing" or "scorching." Blazing at magnitude -1.42, it's twice as bright as any star in our sky besides the Sun.

Sirius resides in the constellation Canis Major, the Big Dog, and is commonly called the Dog Star. In ancient Greece the dawn rising of Sirius marked the hottest part of summer—the season's "dog days."

Sirius no longer marks the hottest part of summer, because it now rises later in the year. This happens because the Earth has been wobbling slowly around its axis in a 25,800-year cycle. This wobble—called precession—is caused by the gravitational attraction of the Moon on Earth's equatorial bulge, and it gradually changes the locations of stars on the celestial sphere

The best time to see Sirius is probably in winter (for northern-hemisphere observers), because it rises fairly early in the evening. To find the Dog Star, use the constellation Orion as a guide. Follow the three belt stars 20 degrees southeast to the brightest star in the sky. Your fist at arm's length covers about 10 degrees of sky, so it's about two fist-widths down.

Sirius, the red giant star Betelgeuse, and Procyon in Canis Minor form a popular asterism known as the Winter Triangle.

Sirius is 23 times as luminous as the Sun, and about twice the mass and diameter. At a mere 8.5 light-years away from Earth, Sirius seems so bright in part because it's the fifth-closest star to the Sun.

The brilliance of Sirius illuminates not only our night skies, but also our understanding. While observing it in 1718, Edmond Halley (of comet fame) discovered that stars move in relation to one another—a principle now known as proper motion. 

This Hubble Space Telescope image shows Sirius A, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B.

In 1844, the German astronomer Friedrich Bessel observed that Sirius had a wobble, as if it were being tugged by a companion star. And in 1862, Alvan Clark solved this mystery (while testing his new 18.5-inch lens, the largest refracting telescope in the world at that time). Clark discovered that Sirius was not one star but two.

This proved to be the first discovery in what became a whole class of stars: the compact stellar remnant or white dwarf. These are stars that, once depleting all their hydrogen, collapse to a very dense core. Astronomers have calculated that Sirius's companion—dubbed Sirius B—contains the mass of the Sun in a package as small as the Earth. 

Sixteen milliliters of matter from Sirius B (that is, about one cubic inch of the stuff) would weigh 2000 kilograms on Earth.

At magnitude 8.5, it is one four-hundredth as luminous as the Sun. The brighter and larger companion is now known as Sirius A.


Canopus resides in the constellation Carina, the Keel. Carina is one of three modern-day constellations that once formed the ancient constellation of Argo Navis, named for the ship Jason and the Argonauts sailed in to search for the Golden Fleece. Two other constellations form the sail (Vela) and the stern (Puppis). 

In modern odysseys, spacecraft like Voyager 2 used the light from Canopus to orient themselves in the sea of space.

Canopus is a true powerhouse. Its brilliance is due more to its great luminosity than its proximity. This number two on our list of stars has 14,800 times the intrinsic luminosity of the Sun! But at 316 light-years away, it's more than 37 times as far from us as the number one star, Sirius.

With a magnitude of minus-0.72, Canopus is easy to find in the night sky, though it is only visible at latitudes south of 37 degrees north. 

To catch a glimpse of it from middle-latitude or southern locations in the United States, look for a bright star low on the southern horizon during the winter months. Canopus is 36 degrees below the brightest star in the sky, Sirius. The further south you are, the better your view will be.

Canopus is a yellow-white F super giant—a star with a temperature from 5,500 to 7,800 degrees Celsius (10,000 to 14,000 degrees Fahrenheit)—that has stopped hydrogen fusion and is now converting its core helium into carbon. This process has led to its current size, 65 times that of the Sun. If we were to replace our Sun with Canopus, it would nearly envelop Mercury. 

Canopus will eventually become one of the largest white dwarfs in the galaxy and might just be massive enough to fuse its carbon, turning into a rare neon-oxygen white dwarf. These are rare because most white dwarfs have carbon-oxygen cores, but a massive star like Canopus can begin to burn its carbon into neon and oxygen as it evolves into a small, dense, and cooler object.

Canopus lost its place in the celestial hierarchy for a short time in the 1800s when the star Eta Carinae underwent a massive outburst, surpassing Canopus in brightness and briefly becoming the second-brightest star in the sky. And Eta Carinae may yet outdo even Sirius, the brightest. It is fated to become a supernova, perhaps very soon in cosmic time-terms: within a few hundred thousand years.

Alpha Centauri

Alpha Centauri (or Rigel Kentaurus, as it is also known) is actually a system of three stars gravitationally bound together. The two main stars are Alpha Centauri A and Alpha Centauri B. The tiniest star in the system is Alpha Centauri C, a red dwarf. 

The Alpha Centauri system is a special one. At an average distance from us of 4.3 light-years, these stars are our nearest known stellar neighbors. 

A comparison of the sizes and colors of the stars in the Alpha Centauri system with the Sun. 

Centauri A and B are remarkably Sun-like, with Centauri A a near twin of the Sun (both are yellow G stars). In comparison to the Sun, Alpha Centauri A is 1.5 times as luminous and shines at magnitude -0.01 while Alpha Centauri B is half as luminous and shines at magnitude 1.3.

Alpha Centauri C is one seven-thousandth as bright and shines at eleventh magnitude. 

Of the three stars, the smallest is the closest to the Sun, 4.22 light-years away. Because of its proximity, it is known as Proxima Centauri.

When night falls and the skies are clear in summer, the Alpha Centauri system shines at a magnitude of minus-0.27, low in the southern sky. You can find it at the foot of the Centaur in the constellation Centaurus. 

Because of its position in the sky, the Alpha Centauri system is not easily visible in much of the northern hemisphere. An observer must be at latitudes south of 28 degrees north (or roughly from Naples, Florida and locations further south) to see the closest stellar system to us. 

The two brighter components of the system make a wonderful double star to observe in a small telescope.

Naked-eye Alpha Centauri appears so bright because it is so close. This also means that it has a large proper motion—the drifting of stars relative to each other due to their actual movements in space. In another 4,000 years, Alpha Centauri will have moved near enough to Beta Centauri for the two to form an apparent double star.


Arcturus is the brightest star in the northern celestial hemisphere. (The first three stars on this list are actually in the southern celestial sphere, though seasonally they are visible from the northern hemisphere of Earth).

Known as the Bear Watcher, Arcturus follows Ursa Major, the Great Bear, around the north celestial pole. The name itself derives from the Greek word arktos, meaning bear.

Arcturus is an orange giant, twice as massive and 215 times as bright as the Sun. It takes 37 years for the light of Arcturus to reach us, so when we gaze upon it, we are seeing the star as it looked 37 years ago. It glows at magnitude -0.04 in our night sky.

variable star, Arcturus is in the last stages of life. 

During its internal struggle between gravity and pressure, Arcturus has swelled to 25 times the Sun's diameter. 

Eventually the outer envelope of Arcturus will peel away as a planetary nebula, similar to the famed Ring Nebula (M57) in Lyra. The star left behind will be a white dwarf.

Arcturus is the alpha (meaning brightest) star of the springtime constellation Bootes, the Herdsman. You can find it by using the Big Dipper as your celestial guidepost. Follow the arc of the handle until you come to a bright orange star. This is Arcturus, forming the point of a pattern of stars resembling a kite. 

In the spring, if you keep following the arc, you'll encounter another bright star, Spica. (Keep it straight by remembering the phrase: "Arc to Arcturus, speed on to Spica.")

In the 1930s, astronomers were busy measuring the distance to nearby stars and determined—incorrectly, it turned out—that Arcturus was 40 light-years from Earth. During the 1933 World's Fair in Chicago, the light from Arcturus was collected with new photocell technology and used to activate a series of switches. Light believed to have originated at the time of the previous Chicago World's Fair 40 years earlier was used to illuminate and officially open the fair in 1933.

The science of astronomy progresses, and we now know that Arcturus is only 37 light-years away.


The name Vega comes from the Arabic word for "swooping eagle" or "vulture." Vega is the luminary of Lyra, the Harp, a small but prominent constellation that is home to the Ring Nebula (M57) and the star Epsilon Lyrae. 

The ring is a luminous shell of gas resembling a smoke ring or a doughnut that was ejected from an old star. Epsilon Lyrae appears to the naked eye as a double star, but through a small telescope you can see that each of the two individual stars is itself a double! Epsilon Lyrae is popularly known as the "double double."

Vega is a hydrogen-burning dwarf star, 54 times as luminous and 1.5 times as massive as the Sun. At 25 light-years away, it is relatively close to us, shining with a magnitude of 0.03 in the night sky. 

In 1984, a disk of cool gas surrounding Vega was discovered—the first of its kind—extending 70 AU from the star, roughly the distance from our Sun to the edge of the Kuiper Belt. This discovery's important because a similar disk is theorized to have played an integral role in planet development within our own solar system. 

Astronomers also found a "hole" in the Vega disk, indicating the possibility that planets might have already coalesced and formed around the star. This led the astronomer and author Carl Sagan to choose Vega as the source of advanced alien radio transmissions in Contact, his first science-fiction novel. (In real life, no such transmissions have ever been detected.)

Together with the bright stars Altair and Deneb, Vega forms the popular Summer Triangle asterism that announces the beginning of summer in the northern hemisphere. The asterism crosses the hazy band of the Milky Way, which is split in two near Deneb by a large dust cloud called the Cygnus Rift. 

This area of the sky is ideal for sweeping with binoculars of any size in dark-sky conditions.

Vega was the first star to be photographed, on the night of July 16, 1850, by the photographer J.A. Whipple. With the daguerreotype camera used at the time, he made an exposure of 100 seconds using a 15-inch refractor telescope at Harvard University. Fainter stars (those of second magnitude and dimmer) would not have registered at all using the technology of the time.

Vega used to be the North Star, but 12,000 years of Earth's precession has altered its place in the celestial sphere. In another 14,000 years, Vega will be the North Star again.


Capella is the primary star in the constellation Auriga (the Charioteer), and the brightest star near to the north celestial pole. 

Capella is actually a fascinating star system of four stars: two similar class-G yellow-giant stars and a pair of much fainter red-dwarf stars. The brighter yellow giant, known as Aa, is 80 times as luminous and nearly three times as massive as the Sun. The fainter yellow giant, known as Ab, is 50 times as luminous as the Sun and two-and-a-half times as massive. The combined luminosity of the two stars is the equivalent of about 130 Suns.

The Capella system is 42 light-years away, its light reaching us with a magnitude of 0.08. 

It is highest in the winter months and circumpolar (meaning it never sets) at latitudes higher than 44 degrees north (or roughly north of Toronto, Canada).

To locate it, follow the two top stars that form the pan of the Big Dipper across the sky. Capella is the brighter star in the irregular pentagon formed by the stars in the constellation Auriga.

South of Capella is a small triangle of stars known as the Kids. One of the most ancient legends had Auriga as a goat herder and patron of shepherds. The brilliant golden yellow Capella was known as the "She-Goat Star." The nearby triangle of fainter stars represents her three kids.

Both yellow giants are dying, and will eventually become a pair of white-dwarf stars.


On the western heel of Orion, the Hunter, rests brilliant Rigel. In myth, Rigel marks the spot where Scorpio, the Scorpion, stung Orion after a brief but fierce battle. Its Arabic name means the Foot.

Rigel is a multiple-star system. The brighter component, Rigel A, is a blue supergiant that shines a remarkable 40,000 times stronger than the Sun! Although it's 775 light-years distant, its light shines bright in our evening skies, at magnitude 0.12.

Rigel resides in the most impressive of the winter constellations, mighty Orion. After the Big Dipper, it's the most-recognized and easiest-to-identify constellation. It helps that the shape made by Orion's stars closely matches the shape of a human hunter: three bright stars are lined up together to form a belt, the other four stars surrounding the belt compose shoulders and legs.

Telescope observers should be able to resolve Rigel's companion, a fairly bright seventh-magnitude star. But the jewel in Orion is the Great Orion Nebula (M42), a vast stellar nursery where new stars are still being born. It can be found six moon-widths south of the belt stars.

A heavy star of 17 solar masses, Rigel is likely to go out with a supernova-sized bang one day. Or it might become a rare oxygen-neon white dwarf.


Procyon resides in the small constellation of Canis Minor, the Little Dog. The constellation symbolizes the smaller of Orion's two hunting dogs (the other is, of course, Canis Major). 

The word procyon is Greek for "before the dog," for in the northern hemisphere, Procyon announces the rise of Sirius, the Dog Star.

Procyon is a yellow-white, main-sequence star, twice the size and seven times as luminous as the Sun. Like Alpha Centauri, it appears so bright because at 11.4 light-years, it is relatively close.

Procyon is an example of a main sequence subgiant star, one that is starting to die as it converts its remaining core hydrogen into helium. Procyon is currently twice the diameter of the Sun, one of the largest stars within 20 light-years.

Canis Major can be found fairly easily east of Orion during northern-hemisphere winter. Procyon, along with Sirius and Betelgeuse, form the Winter Triangle asterism.

Procyon is orbited by a white-dwarf companion detected visually in 1896 by John M. Schaeberle. The fainter companion's existence was first noted in 1840, however, by Arthur von Auswers, who observed irregularities in Procyon's proper motion that were best explained by a massive and dim companion. 

At just one-third the size of Earth, the companion dubbed Procyon B has the equivalent of 60 percent of the Sun's mass. The brighter component is now known as Procyon A.


Achernar is derived from the Arabic phrase meaning "the end of the river," an appropriate name for a star that marks the southernmost flow of the constellation Eridanus, the River.

Achernar is the hottest star on this list. Its temperature has been measured to be between 13,000 and 19,000 degrees Celsius (24,700 and 33,700 Fahrenheit). Its luminosity ranges from 2,900 to 5,400 times that of the Sun. Shining at magnitude 0.45, its light takes 144 years to reach your eye. 

Achernar is more or less tied with Betelgeuse (number ten on this list) for brightness. However, Achernar is generally listed as the ninth-brightest star in the sky because Betelgeuse is a variable whose magnitude can drop to less than 1.2, as was the case in 1927 and 1941.

For northern-hemisphere observers, Achernar rises in the southeast during the winter months and is visible only from latitudes south of 32 degrees north; those further north only see a portion of the constellation. 

(For Star Trek fans, the constellation of Eridanus is also home to Epsilon Eridanus, the star around which Mr. Spock's imaginary home planet of Vulcan supposedly revolves!)

Achernar is a massive class-B star containing up to eight solar masses. It is currently burning its hydrogen into helium and will eventually evolve into a white dwarf star.


Don't let Betelgeuse's ranking as the tenth-brightest star in the sky fool you. Its distance—430 light-years—hides the true scale of this supergiant. With a whopping luminosity of 55,000 suns, Betelgeuse still shines bright in our skies at a magnitude of 0.5.

Betelgeuse (pronounced "beetle juice" by most astronomers) derives its name from an Arabic phrase meaning "the armpit of the central one." 

Image from ESO's Very Large Telescope showing the stellar disk.

The star marks the eastern shoulder of mighty Orion, the Hunter. Another name for Betelgeuse is Alpha Orionis, indicating that it's the brightest star in the winter constellation of Orion. But Rigel (Beta Orionis) is actually brighter. This misclassification probably happened because Betelgeuse is a variable star (a star that changes brightness over time) and it might have been brighter than Rigel when Johannes Bayer originally categorized it. 

Betelgeuse is an M1 red supergiant, 650 times the diameter and about 15 times the mass of the Sun. If Betelgeuse were to replace the Sun, all the planets out to the orbit of Mars would be engulfed! 

Observe Betelgeuse and you are witnessing a star approaching the end of its long life. Its huge mass suggests that it might fuse elements all the way to iron. If so, it will blow up as a supernova that would be as bright as a crescent moon, as seen from Earth. A dense neutron starwould be left behind. The other possibility is that it might evolve into a rare neon-oxygen dwarf. 

Betelgeuse was the first star to have its surface directly imaged, a feat accomplished in 1996 with the Hubble Space Telescope.

Perhaps a much more advanced orbiting telescope will be watching someday when Betelgeuse goes supernova, an event which will certainly make it the brightest star in Earth's skies—if only for a few months.

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Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram