Download 秋 夕(七言絕句) 作者:杜牧 銀燭秋光冷畫屏,輕羅小扇撲流螢。 天階夜色

秋
夕(七言絕句) 作者:杜牧
銀燭秋光冷畫屏,輕羅小扇撲流螢。
天階夜色涼如水,臥看牽牛織女星。
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.
The legend of Lyra tells the story of Orpheus, who was given a harp by the god Apollo. Orpheus' music
was sweeter than that of any other mortal man. It could soothe any savage, bring joy to the heart of the
weary. It was even said that rivers changed course to stay near its beauty.
Orpheus married the lovely maiden Eurydice. But after their wedding, as she walked with her
bridesmaids, she was bitten by a snake and died.
Orpheus was so stricken with grief that he journeyed to the underworld to win her return. His music not
only gained him entry to Hades, it caused Pluto, the god of the underworld, to soften his heart and grant
Orpheus' wish. But there was one condition: Eurydice would follow Orpheus, who could not look back
until both had gained the upper world.
Despite his anxiety, Orpheus followed his instructions -- until he reached the surface. Before Eurydice
could take the final steps into the light, he turned to gaze upon her. She vanished, with only one word
to her love: Farewell.
Crazed with grief, Orpheus wandered the hills of Greece until he was murdered. The Muses buried
him, and Apollo placed his magical harp in the sky -- as Lyra.
Lyra's leading light, Vega (織女星), is only about 25 light-years away, making it one of our closest
neighbors. It shines pure white, which tells us that its surface is thousands of degrees hotter than the
surface of the Sun.
Models of how stars age indicate that, like the Sun, Vega is about halfway through its "normal"
lifetime -- the time when it "fuses" the hydrogen in its core to make helium. But stellar lifetimes aren't
all the same. Vega is probably around one-tenth the age of the Sun, so it has only a few hundred
million years to go, versus billions (十億) of years for the Sun.
The difference is the mass of the two stars. Vega is more than twice as "heavy" as the Sun, so it burns
through its hydrogen much faster than the Sun does. So Vega will finish its hydrogen, puff up to
become a red giant, blow its outer layers off into space, and fade into obscurity as a white dwarf -- all
while the Sun continues to shine much as it does today.
Aquila, the Eagle
Aquila glides on outstretched wings through the glowing band of the Milky Way. Look for it high in
the south in late summer.
The brightest star in Aquila is a white star about 17 light-years from Earth called Altair (牽牛星), the
Arabic word for eagle. Altair is the southern point of a pattern of three bright stars called the Summer
Triangle. Deneb, in the constellation Cygnus, forms the triangle's northeastern point. Vega, in Lyra, the
harp, is in the northwest. Altair is nice and bright and easy to find right up to the beginning of winter.
Altair is about twice as massive as the Sun, so it will live only about two billion years to the Sun's 10
billion. Despite its size, Altair appears to turn on its axis once every 10 hours, versus about four weeks
for the Sun. One of the effects of Altair's high-speed rotation is that its gas is forced outward at the
equator, giving the star a flattened appearance -- it is about 14 percent wider through the equator than
through the poles. If Altair spun about twice as fast as it does now, it would fly apart.
Astronomers refer to Aquila's two "tail" stars as Zeta and Epsilon Aquilae. Together, they also have an
older Arabic name: Deneb al Okab, which means "the eagle's tail." The northern star in the tail is
actually a system of three or more stars. Only one of them is bright enough to see with the unaided eye.
It's a type of star known as a red giant. That means it's in the final stages of life. Its core is getting
hotter and denser, while its outer layers are puffing out and getting cooler. That's the same process that
awaits the Sun in several billion years. The southern star is a multiple-star system, too. Its brightest star
is fairly early in life. It's bigger, heavier, and hotter than the Sun, and it spins close to 200 times faster
than the Sun. That means the star bulges out at the equator, so it looks like a squashed beachball.
The Pioneer 11 spacecraft, which was launched in 1973, is heading toward one of the eagle's stars,
known as Lambda Aquilae, which is 125 light-years away. Pioneer will pass the star in about four
million years. Although the spacecraft has already expired, it carries a message from home: a small
plaque with information about the craft and its makers -- a greeting to the galaxy from the people who
made Pioneer 11.
Cygnus, the Swan
The brightest stars of Cygnus form a cross, so the swan is also known as the Northern Cross. Find it
soaring high overhead during late summer evenings.
The constellation's brightest star is Deneb (天津四) — an Arabic word that means "the tail." Deneb —
the tail of the swan — marks the top of the cross. The swan's outstretched wings form the horizontal
bar of the cross, while the head of the swan — a double star called Albireo — is the bottom of the
cross.
Although it lies about 1,500 light-years from Earth, Deneb shines brightly in our night sky because it's
a white supergiant — a star that's much larger, hotter, and brighter than the Sun. Deneb is the
northeastern point of a star pattern called the Summer Triangle.
If you use binoculars to scan the area between the two bright stars that define the swan's eastern wing,
you'll see the remnant of a supernova — a faint, incomplete ring of light called the Cygnus Loop.
Sagittarius, the Archer
Sagittarius, the archer, slides low across the southern sky of summer. Sagittarius is a centaur — a
mythological half-man, half-horse — who has drawn his bow. His arrow is pointing at Antares, the
bright red heart of Scorpius, the scorpion. The archer is avenging Orion, who was slain by the
scorpion's sting.
The constellation originated in Sumeria, then was picked up by the Greeks. But they appear to have
missed something in the translation. The original story may have described a satyr, which was part man
and part goat. One of these creatures was credited with inventing archery, which was why he was
depicted in the sky with a bow and arrow. The Greeks kept the bow and arrow, but changed the
creature.
To most modern eyes, though, the brightest stars of Sagittarius appear to outline not a mythological
creature but a teapot. Steam pours from the spout — the hazy starclouds of the Milky Way.
The center of the Milky Way galaxy is immersed in that steam, about 27,000 light-years away.
Although the galaxy's core is packed with stars, interstellar dust between the core and Earth absorbs
most of its light, rendering it invisible except to instruments that study wavelengths of light that
penetrate the dust.
One part of the classical picture that is fairly easy to see is the bow. The star that represents the top of
the bow is at the top of the teapot, with the bottom of the bow at the bottom right of the teapot. A star
that is about halfway between them represents the middle of the bow. The stars are named Kaus
Borealis, Kaus Media, and Kaus Australis, which is a combination of Arabic and Latin that means the
northern, middle, and southern bow.
Kaus Australis -- the southern end of the bow -- is the brightest star in the constellation. It is a stellar
giant -- a star that has puffed up as it nears the end of its life. It's many times larger than the Sun, and
almost 400 times brighter. That makes it quite easy to see even though it's about 145 light-years away.
The bow's other stars are giants, too. The one in the middle is actually the most impressive of the lot -it's bigger and brighter than the others, but farther away.
Trifid Nebula
For telescopic viewers, one of Sagittarius' most popular features is a glowing cloud of gas and dust
known as M20, the Trifid Nebula. Lanes of dark dust across its face give it a three-part appearance. A
young star at the center of the nebula, which lights up much of the material around it, is triggering the
formation of more than a hundred other stars.
Sagittarius also contains several globular clusters, which are tightly packed collections of hundreds of
thousands of stars.
Pegasus, the Flying Horse
Pegasus is a large pattern of stars marked by a great square — four bright stars that form the body of
the winged horse.
Pegasus is a character from Mediterranean mythology, although he's best known for a couple of myths
that he wasn't part of -- those of Perseus and Hercules.
In classical Greek mythology, Perseus killed the snake-headed Medusa to save the princess Andromeda
from a nasty sea monster. A few drops of Medusa's blood fell into the sea. When it mixed with the
foam, it gave birth to Pegasus, who later played a part in the story of another hero, Bellerophon. In
modern times, though, Pegasus has become a more integral part of the Perseus story. In recent tellings,
when Perseus shows up to save Andromeda, he is riding Pegasus. And the tale of the flying horse
became even more convoluted with Disney's animated version of Hercules, in which Herc and Pegasus
are pals.
The brightest star in the Great Square — Alpheratz — isn't in the constellation Pegasus. It's in
Andromeda, which is just northeast of Pegasus. It is part of the ancient sky picture that we know as
Pegasus. But when astronomers drew the official boundaries for the constellations, this star was placed
just across the line in Andromeda.
The brightest star in the constellation is called Markab, which means "the saddle." It's at the
southwestern corner of the Great Square.
At another corner is Beta Pegasi. "Beta" means it should be the second-brightest star in Pegasus, and
sometimes it is. The star pulses in and out like a beating heart. When it pulses out, its surface cools, so
it gets fainter. When it squeezes back in it gets hotter, so it glows brighter, making it Pegasus's
brightest star.
Another star in Pegasus, II Pegasi, produced a monster flare a few years ago -- a powerful explosion on
its surface that produced a torrent of energy and charged particles. The flare was 100 million times
more powerful than a typical flare on the Sun. If Earth orbited II Pegasi, such a monster would
obliterate the ozone layer and alter the climate, and probably kill much of the planet's life. The star's
magnetic field may be energized by a nearby companion star, which causes II Pegasi to spin on its axis
once every seven days (versus about 29 days for the Sun). This fast rotation may keep the star's internal
layers in high-speed motion, generating a powerful magnetic field that produces flares and other
storms.
Ursa Major, the Great Bear
Ursa Major — the great bear — is always above the horizon in the northern latitudes, but the best time
to see it is in the spring when its high above the northeastern horizon. Ursa Major is best known as the
home of the Big Dipper.
Of all the star patterns in the sky, the Big Dipper is the most universally recognized. The dipper's seven
bright stars form a portion of the great bear. It's hard to see the rest of the bear, especially from lightpolluted cities.
After you locate the dipper, look at the two stars that mark the outer edge of its bowl. Now connect
these two stars, then extend the line above the dipper's bowl. Polaris, the north star, lies along this line,
about five times the distance between the two pointers. No matter where the Big Dipper is in our sky,
those two stars always point to Polaris.
Virgo
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 Cluster
While 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 may eventually join it.
The largest member of the cluster is M87, which spans one million light-years and contains a trillion
(兆) 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 -- a pair of stars known as Wolf 424
-- is just four light-years farther. Both stars are classified as red dwarfs. 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.