Terms in this set (14)

Canis Major, the Great Dog
Canis Major loyally follows its mythical master, Orion, across the southern skies of winter.

The brightest star in Canis Major also is the brightest in the entire night sky — brilliant Sirius, which is just 8.6 light-years away. That's only twice as far as our closest stellar neighbor. Because it is the brightest star of the Great Dog, Sirius is known as the Dog Star. Its first appearance in the dawn sky in August heralds the "dog days" of summer.

Sirius is actually a double star. The star that we see as Sirius is about 20 times brighter than the Sun. It is also hotter and more massive than the Sun. Its companion is known as Sirius B, and is nicknamed The Pup. It is a white dwarf -- the hot, dense core of a star that was once like Sirius itself. At the end of its life, the star blew its outer layers into space, leaving only the core. Sirius B is as massive as the Sun but less than one percent as bright. It shines simply because it's still hot.

Canis Major's second-brightest star is Adhara. It is well below Sirius, in one of the dog's legs. It is about 425 light-years away. It emits about a thousand times more visible light than Sirius does, so if it were moved to the same distance as Sirius, it would look about 200 times brighter than Sirius does. Because the surface of Adhara is extremely hot, the star emits more ultraviolet energy than visible light. If our eyes were tuned to the ultraviolet, Adhara would outshine every other star in the night sky.

One interesting sight in Canis Major is a star cluster known as M41, which is about 2,300 light-years away. It's visible to the unaided eye as a faint smudge of light below Sirius. Binoculars or a small telescope reveal a swarm of several dozen stars.
Cassiopeia, the Queen
The mythological queen Cassiopeia floats overhead in fall and winter.
The best time to see her is in late fall, when she stands high in the northeastern sky during the evening hours. Cassiopeia looks like a flattened "W" against the frothy background of our home galaxy, the Milky Way.

Gamma Cassiopeia is the star at the middle of the W. If you add up all of its energy -- not just the light that is visible to the eye -- it shines about 40,000 times brighter than the Sun. That's because Gamma Cas is about 15 times as massive as the Sun and hundreds of times wider. Such a monster produces a lot of energy through the nuclear reactions in its core. That energy works its way to the surface and shines into space as visible light, ultraviolet, and other forms of electromagnetic energy. Eventually, the star will no longer be able to produce energy in its core, causing the core to collapse and its outer layers to explode as a supernova. For a few days or weeks, it will outshine everything in the night sky except the Moon.

Another of the constellation's stars, Rho Cassiopeia, is also destined to explode as a supernova. It is about 10,000 light-years away, making it one of the most distant stars that is visible to the unaided eye.

Rho Cas, which is to the right of Cassiopeia's W, is far bigger and more massive than the Sun and at least a half-million times brighter. It may be just about as bright as a star can get without blowing itself apart. Actually, Rho Cas is in the process of doing just that. Three times in the last six decades, it has blasted shells of gas into space. The most recent expelled 20 times more material than is contained in all of the planets and moons of our solar system combined. The star could be building up to another eruption soon.

Cassiopeia has produced two supernovae in recent centuries. One of those, known today as Cassiopeia A, exploded about three centuries ago as seen from Earth. It blasted its outer layers into space at a few percent of the speed of light. This material continues to expand into space as a giant shell. It contains lots of oxygen and other elements that will disperse into the galaxy. These elements may someday be incorporated into new stars, planets, or even living organisms.

In addition to dead stars and stars that are about to die, Cassiopeia is also home to one of the largest collections of young stars in the galaxy. A few years ago, astronomers discovered a tight cluster of several very young, hot, massive stars there. The stars are similar to the brightest stars of the Orion Nebula, which is the largest stellar nursery in our region of the galaxy. Today, the stars in the Cassiopeia nebula are hidden inside a dense cloud of dust. But over the next hundred thousand years or so, the radiation and winds from these stars will eat through the dust, allowing future skywatchers to see the many other stars that have been born there.
Leo, the Lion

The zodiacal constellation Leo, the lion, is one of a handful of constellations that really does look like its namesake. It consists of two patterns of stars that the brain puts together to make a lion. A backward question mark represents the head and mane, and a triangle of stars to the lower left forms the lion's hindquarters and tail. Look for Leo high in the south in April and May.

Leo's brightest star is blue-white Regulus, one of the brightest stars in the night sky. It is about 79 light-years away. Regulus rises almost due east, with the body of the lion following it into the sky over the next couple of hours. Once Regulus climbs into the sky, look to its left — toward the north — for the backwards question mark, known as the Sickle, that outlines his head and mane.

About two hours later, look low in the east for Leo's tail — a white star named Denebola, which comes from an Arabic name that, appropriately enough, means "tail of the lion."

At a distance of just 36 light-years, Denebola is a fairly close neighbor. Even so, the fact that we can see it at all means that Denebola is quite bright -- more than 10 times brighter than the Sun. Denebola's brightness is a product of its mass. The star is about twice as heavy as the Sun, so it "fuses" the hydrogen in its core at a much faster rate. That makes the star's surface thousands of degrees hotter than the Sun's. The combination of high temperature and large size makes Denebola outshine the Sun and all but a few percent of all the stars in the Milky Way galaxy.

Denebola is surrounded by a disk of dust grains. It may be supplied with fresh material by collisions between small planet-building bodies. So far, no one has seen any evidence of planets around the star, although they can't be ruled out, either. Astronomers continue to hunt for companions to this bright stellar neighbor.

Leo also contains quite a few bright galaxies. Leading the list are three galaxies that together form the Leo Triplet [1]: M65, M66, and NGC 3628.

NGC 3628 is the most interesting of the three. Like our own Milky Way, it's a spiral -- a pinwheel that spans at least a hundred thousand light-years. We see it edge-on, so it looks like a streak of light with lanes of dark dust running down the middle. Gravitational encounters with the other galaxies have triggered the birth of perhaps millions of new stars near the galaxy's center. They have pulled out a tail of gas that spans a quarter-million light-years. The tail contains enough gas to make half a billion stars as massive as the Sun. In fact, it has given birth to millions of stars in several large clumps.
Orion, the Hunter
Orion is one of the most beautiful of all constellations, and one of the easiest to find. It looks like a large rectangle high in winter's south-southeastern sky.
Two of the brightest stars in the evening sky lie at opposite corners of the rectangle: bright orange-red Betelgeuse at the northeastern corner and even brighter Rigel at the southwest.

Betelgeuse is is at least 300 times the Sun's diameter, and perhaps much more. It puts out about 100,000 times more energy than the Sun does. And when it dies, it will create a fireball that will briefly outshine billions of normal stars.

Betelgeuse is a red supergiant — the largest class of stars. It's probably close to 20 times as massive as the Sun. The gravity of such a star squeezes its core tightly, heating it to billions of degrees. Such a stellar blast furnace consumes its original hydrogen fuel in a hurry, "fusing" the atoms together to make helium and producing incredible amounts of energy in the process. It then fuses the helium to make heavier elements — carbon and oxygen at first, and eventually all the way up to iron. When that happens, the star no longer produces energy in the core. Without the reactions in its core to push outward, gravity quickly causes the core to collapse, forming a neutron star. A massive explosion rips through the star's outer layers, blasting them into space at a few percent of the speed of light — a titanic blast known as a supernova. We can't be sure when that will happen to Betelgeuse, but it's probably soon on the astronomical time scale: just about anytime in the next hundred thousand years.

Rigel is a blue supergiant. Like Betelgeuse, it is much bigger and heavier than the Sun. It's surface is thousands of degrees hotter, however, so it shines blue-white. That high temperature means that Rigel also pumps out a lot of ultraviolet energy, which produces sunburn and other problems. When you add up the ultraviolet, visible light, and other wavelengths, Rigel shines tens of thousands of times brighter than the Sun. In fact, depending on Rigel's exact distance, it could be up to 100,000 times brighter than the Sun.

With so much energy streaming its way, a planet would need to be billions of miles away from Rigel to be a safe abode for life, and it would need a thick ozone layer to screen out the ultraviolet. Even then, such a planet wouldn't be a good long-term home. In a few million years, Rigel, too, is likely to blast itself to bits as a supernova. The energy and shock wave would make quick work of life on any world around it.

Near the center of Orion's prominent rectangle, look for a short diagonal line of three stars that forms Orion's Belt. And extending south from the belt, you'll see another, fainter line of stars that forms Orion's Sword.
One of the objects in the sword isn't a star at all. It's M42, the Orion Nebula, which is a cloud of gas and dust that's like a giant fluorescent bulb.

M42 is part of a giant complex of clouds of interstellar gas and dust. Pockets of this material are collapsing to give birth to new stars. Hubble Space Telescope images reveal about 3,000 stars in the Orion Nebula alone, some of which could be as little as 10,000 years old.

The visible nebula is a bubble of turbulent gas that is energized by the Trapezium, a cluster of hot, bright stars at its center. Ultraviolet energy from the stars strips electrons from atoms in the nebula. When the electrons link up with new atoms, the atoms emit light.

The process of starbirth continues today, with several hundred dense blobs of gas and dust collapsing to make new stars. In addition, planetary systems appear to be taking shape around many of the newborn stars. Not all of them will survive the birth process, though, because radiation from the cluster's hot young stars is eroding the planet-making materials around many of the stars.
Scorpius, the Scorpion
Three bright stars form the "head" of Scorpius, the celestial scorpion, while its tail curves away below it in the southern sky of summer.
The brightest star in Scorpius is Antares, which is in the middle of the scorpion's curving body. This brilliant red star is one of the behemoths of our stellar neighborhood. If you placed it at the center of our own solar system, it would swallow Mercury, Venus, Earth, and Mars, and almost reach Jupiter.

Antares is at least 15 times as massive as the Sun. The pressure in the centers of such stars makes them extremely hot. The heat allows chemical elements to fuse together to make heavier ones. So by the end of its life, Antares will have built up an iron core surrounded by layers of oxygen, carbon, silicon, and other elements. And at the end of its life, Antares will explode as a supernova. The extreme heat of the blast will produce elements that are even heavier than iron — elements like lead, uranium, and gold.

The explosion will hurl these elements outward at a few percent of the speed of light. Eventually, some of the debris may ram into clouds of cold gas and dust, squeezing them and causing them to collapse and give birth to new stars and planets. These newborn objects will be seeded with the elements created by the supernova.

Because Scorpius lies along the path of the Milky Way, it is speckled with bright star clusters and nebulae, including the clusters M6 and M7, which are above the two stars that mark the scorpion's stinger.

M7 is the brighter of the two. It is almost 1,000 light-years away, and its several hundred stars all are about 200 million to 300 million years old. Several of its stars are nearing the ends of their lives, so they have puffed up to gigantic proportions, helping M7 stand out.

M6 is farther than M7, so it doesn't look as big or bright, so you need dark skies to see it without optical aid. Through binoculars or a telescope, the cluster's colorful stars form a pattern that resembles the outline of a butterfly.

Virgo is the second-largest constellation -- so big that it takes about four hours for the whole thing to clear the eastern horizon. To the unaided eye, though, that huge area is basically a void, because only one bright star resides within its borders.
In mythology, Virgo was associated with the harvest. It represented a harvest goddess or the daughter of a goddess. The constellation's brightest star, Spica, was a stalk of wheat held in her hand. Virgo was associated with the harvest because the Sun passed across the constellation during late summer or early autumn, when farmers were reaping the crops they had planted months earlier.

Virgo is known as a spring constellation because, although you can see some or all of its stars on most nights of the year, the stars put on their best display in the evening sky during spring.

Although blue-white Spica is Virgo's only bright star, it is the 16th-brightest star in the night sky. It actually consists of two stars, both of which are much hotter, brighter, and heavier than the Sun. But they are separated by only about 10 million miles, so from Spica's distance of 260 light-years, it is impossible to see them as individual stars.

Because the stars are big, heavy, and close together, they exert such a strong gravitational tug on each other that each star causes the other to bulge outward. Seen in profile, the system would look like two eggs with the narrow ends pointed at each other.

In the next few million years, the heavier star will near the end of its "normal" lifetime, so it will puff up to many times its current size. As it swells, some of its gas will begin to dump onto the surface of the other star. And as it gets even bigger, its outer layers will engulf its partner, pulling the two stellar cores closer together. No one knows exactly how the scenario will play out after that. The stars may merge to form a single star, or the larger star may explode before that can happen, blasting itself to bits and perhaps sending its companion careening through the galaxy like a stellar bullet.

The second-brightest star in Virgo, Gamma Virginis, is also a binary system. Unlike Spica, its stars are so far apart that binoculars easily reveal them as individual stars. They are about 40 light-years from Earth. The stars are both white, which means their surfaces are a little hotter than the surface of the Sun. They're both brighter than the Sun, too.

Gamma Virginis has been known by several other names over the centuries. One of the most enduring is Porrima, for the goddess of prophecy. In ancient Babylon, it was called Star of the Hero, and in China, it was the High Minister of State.
A few galaxies of the Virgo ClusterWhile Spica is the most prominent object in Virgo, the most interesting may be the Virgo Cluster -- a collection of several thousand galaxies. It is centered about 60 million light-years away.

The cluster forms the largest structure in our region of the universe. The galaxies are bound to each other by their gravity, so they move through space together. The cluster exerts a strong tug on our own galaxy, the Milky Way, and the small band of galaxies that it's bound to, the Local Group. The Local Group is being pulled toward the Virgo Cluster, and eventually may join it.

The largest member of the cluster is M87 [1], which spans one million light-years and contains one trillion stars or more. It's a type of galaxy known as a giant elliptical. It looks like a fat, fuzzy football. Its core is inhabited by one of the largest black holes yet discovered, a monster about 6.6 billion times the mass of the Sun.

At the other end of the distance scale, Virgo also is home to two of the closest star systems. But the stars are so puny that they are not visible to the unaided eye. One system, Ross 128, is less than 11 light-years away; only 10 known star systems are closer. The other, which a pair of stars known as Wolf 424, is just four light-years farther. Both stars are classified as red dwarfs [2]. They are much less massive than the Sun, so they're only about one ten-thousandth as bright. If any of these stars took the Sun's place, daytime on Earth would be only a few times brighter than a night with a full Moon, and Earth would be an iceball, with no chance for life as we know it.