Actually, there are bars at the center of most spiral-shaped galaxies.  But you won’t find any drunken Wookies or neon Budweiser signs, only vast sweeps of dust and stars.

A barred spiral galaxy

Galaxies are enormous, turning cities of stars. There may be a hundred billion galaxies in the universe, separated by vast stretches of mostly empty space. Galaxies come in several basic shapes; the most common are a simple elliptical or a spiral, like a pinwheel.

Our home galaxy, the Milky Way, is a collection of at least 100 billion stars (one of them the Sun), arrayed in a rotating spiral.  As Earth and its siblings planets orbit the Sun, the Sun traces an almost circular orbit around the galactic center.  Traveling along a spiral arm at about 490,000 mph, it takes the Sun some 220 million years to circle once around the Milky Way’s core.  (The last time the Sun was near its current position, dinosaurs roamed North America.)

Such distance are measured in light-years, the distance light can travel in a vacuum in one year (about 6 trillion miles).  Luckily, our solar system cruises through the galaxy’s outskirts, about 26,000 light-years from the core — where lurks a massive black hole.

At least two-thirds of all spiral galaxies have bar-shapes running through their centers, created by dust and millions of stars in very peculiar orbits.  Instead of traveling in near-circles, the stars are orbiting the galactic center in long, bar-shaped loops.

But it wasn’t always so.  Since light from distant galaxies can take billions of years to reach Earth, looking out into space is looking back into time.  And scientists say that the spiral galaxies of 7 billion years ago were much less likely to have bars.  Since the universe is about 13 billion years old, it seems like bars may be the signature of a mature spiral–a grown-up galaxy in the prime of its life.

How do bars form?  The enormous gravitational pull of the galaxy’s center keeps stars in orbit.  Computer models show that over time, as orbits are disturbed by stars’ gravitational attraction to other passing stars, circular orbits can become more elongated.  As orbits stretch out, stars travel toward and then away from the galactic center.  Gradually, over many millions of years, enough stars are locked into long, narrow orbits to form a visible bar across the galaxy.

But bars aren’t just a scenic galactic feature.  The orbiting stars’ gravity pulls more gas, the raw material for new stars, into the inner galaxy.  This may explain the ring of glowing gas around the center of many galaxies, studded with newborn suns.

On the less-cheery side, the oldest stars in a bar tend to follow the most elongated paths, carrying them closest to the galaxy’s center.  In our Milky Way, such bar-crawling old stars may fall into our galaxy’s black hole, disappearing with a burst of x-rays.

View a photo of a barred galaxy at http://apod.nasa.gov/apod/ap050112.html.  See an artist’s conception of the Milky Way at http://apod.nasa.gov/apod/ap050825.html.

If we could look closely at our ceilings, we’d see the crisscrossing paths of thousands of tiny footprints, left by flies, ladybugs, and other insects (as well as by spiders).  In fact, the problem for flies and other bugs may not be holding onto the ceiling, but breaking free from it.  Turns out, a fly strolling across a ceiling is a bit like a person walking across a field of wet mud.

How do flies walk upside down, apparently effortlessly? Being tiny certainly helps.  Very-low-mass animals like wall-walking insects and spiders feel less of a pull from gravity.  So it’s easier for a fly than a pig to stick to the ceiling (even Spiderpig needed Homer’s help).

On a rough surface, an insect can use its claws, rappelling up or across like a climber on a rock wall.  But many insects and spiders also rely on special leg or foot pads, often covered with bristly hairs, when they need to climb up surfaces.  Scientists once thought that the rough, bumpy hairs allowed flies to cling to tiny nooks and crannies on even smooth-looking surfaces, including ceilings.  A substance secreted by the hairs helped, adding a bit of adhesion.

But in 2006, scientists at the Max Planck Institute in Germany discovered that the substance secreted by the hairs on a fly’s feet is a sticky glue, tailor-made for striding confidently across the ceiling, upside-down.

The glue-y stuff oozing out of a fly’s footpad hairs is a mixture of oils and sugars.  Researchers say that all insects may secrete the glue, since all 300 wall-climbing insects studied at the Institute left a trail of tiny, sticky footprints on the wall.

The adhesive is strong enough to keep each foot planted on the ceiling, fly standing still.  Walking, however, isn’t trouble-free.  Although the journey across the top of a room may look effortless from our perspective, it’s a struggle for the fly.  The researchers found that flies use at least four different techniques to get a foot unstuck and moving again.

Watching slow-motion tapes of each foot detachment, scientists found that a fly sometimes pushed a foot away from himself, popping the footpad off the surface like a freed suction cup.   Flies also twisted their footpads until they loosened from the wall, or jerked them quickly like a yanked-off band-aid.  Flies also used the handy, built-in claws on their feet to pull a footpad off the ceiling, like a person tugging off a boot.

According to the scientists, the techniques that involved peeling the pad off the ceiling or wall work best, because they require less energy.

Using four of six legs as they crawled across the ceiling also helped the flies make their gravity-defying journeys.  (On the ground, scientists say, flies often use just three legs at a time to move around:  two legs on one side and the middle leg on the other, forming a stable triangle, alternating sides with each step.)

No one’s sure where the myth started, but it has legs (er, wings):    Bumblebee flight is impossible.  According to the principles of aerodynamics, the story went, a big, fuzzy bumblebee, powered only by tiny wings, shouldn’t leave the ground.  A French book from the 1930s, for example, cited calculations that “proved” insects in general shouldn’t be able to fly.  But using mechanically-driven fixed aircraft wings to model the flight of bumblebees and other insects just didn’t work.

The mysteries of insect flight are still being unraveled, but today’s scientists say insects fly “in a sea of vortices,” using the swirling eddies of air created by their beating wings to stay aloft.

Bumblebees beat their wings up to 200 times a second, faster than the nerve impulses to their muscles can fire.  This works because bumblebee wing muscles don’t contract and expand with each electrical signal.  Instead, they continuously vibrate, like a repeatedly plucked guitar string.

When a bumblebee rests, its body temperature drops (or rises) to that of its surroundings. But according to entomologists, the temperature of a bumblebee’s wing muscles must be a toasty 86 degrees F. to lift the bee into the air.  So to fly to the nearest flower cafeteria, a bumblebee must dramatically raise its temperature in all but the hottest weather.

How?  Basking in the sun isn’t usually enough, so the bee will shiver her way to a higher temperature.  By rhythmically contracting and releasing her abdominal muscles (faster and faster as her temperature rises), a bee generates enough warmth to take flight.  The warmer the air temperature, the quicker the lift-off.  On a chilly, 42-degree day, a bumblebee must pump her thorax muscles for a tiring 15 minutes to reach 86 F.

Once in flight, a bumblebee’s muscle temperature remains in a range of about 86 to 111 F.  All of this activity expends enormous amounts of energy, provided by the sugars in flower nectar.  Not surprisingly, scientists have found that bumblebees are especially attracted to warm flowers, floral rooms whose color and shape keeps them heated in the sun.

According to Oxford University research published this month, bumblebee flight really is different from that of dragonflies and other insects.  Using a smoke-filled wind tunnel and cameras snapping 2,000 images a second, researchers found that bumblebee flight is surprisingly inefficient.  For example, a bumblebee’s left and right wings flap independently of each other.  And instead of using the varying air pressure at its wingtips to provide some lift, bumblebees use “brute force” to fly and hover.  Bumblebees, say scientists, are the “tanker trucks” of the flying insect world, using incredible amounts of energy to lumber (charmingly) through the sky.

In fact, bumblebees manage to fly even in places where human beings find it difficult to breathe.  On Earth’s highest peak, Mount Everest, bumblebees have been sighted at more than 18,000 feet up.  Although there’s much less air pressure for tiny wings to beat against, bumblebees power on, hot to the touch even in the frigid cold.

It begins to pour.  You have no umbrella, or printed newspaper.  (The internet, alas, won’t keep your head dry.)  Never mind singing in the rain.  The question is, To run or not to run?

Assuming you’re healthy enough for a mad dash to that building across the street (or the shelter house in a public park), is it worth the energy?  Can you really slip between the drops?  Or will sprinting through a downpour actually make you wetter?

At first glance, the answer appears obvious:  Running means less time spent in the rain, and less-wet clothing.  But the walking advocates have arguments that seem to make sense, too.  If you run through falling raindrops, they point out, you’ll catch more raindrops on the front of your body — chest, stomach, fronts of legs.  If you walk, most of the drops will fall on your head.  Since the front of your body has more surface area than the top of your head and shoulders, you’ll get more water-logged if you run into the wall of rain.

Some have even done mathematical simulations that say you’ll be wetter if you run.  With rain falling straight down from the sky, and no wind, they argue, you’ll scoop up water through the rain field as you dash forward.  But actual experiments with real human beings have found otherwise.

According to physicist Jearl Walker, of Cleveland State University, running keeps you drier because you spend less time being pelted with water.  If there’s no wind, or the wind is blowing toward you, Walker says, you can minimize the number of drops your body encounters by leaning forward, while running as fast as you can.  With a wind at your back, he advises matching your speed to that of the horizontally blowing drops.  Moving with the rain, he says, you’ll avoid both front and back splatters; most drops will strike your head.

And in a 1997 report in the British journal Weather, two climate researchers in Asheville, NC also found that running trumps walking.  On a rainy day, Trevor Wallis and Thomas Peterson suited up in identical sweats, wearing trash bags underneath so that water wouldn’t soak through.  One ran and the other walked through the pouring rain over a 100-meter (328-ft.) course, weighing their track suits before and after their wet dash/stroll.  What they found:  The clothes of Wallis, who ran, were a full 40 percent drier.

If you’re already soaked, or the nearest shelter is very far away, it probably doesn’t matter whether you walk or jog.  To find out if it’s really worth breaking into a run in the rain, physicist Doug Craigen has devised a handy calculator.  You’ll need to enter your height, take some body measurements, and guess at the wind speed of your own rainy day.  Find out how wet you’ll get at   http://www.dctech.com/physics/features/0600.php.

Did you ever pour a packet of cocoa mix into a cup of hot water…and notice that the pitch of your spoon striking the cup seemed to rise or lower as the mix dissolved?  If so, you’re familiar with the Hot Chocolate Effect.  The strangely musical effect can also occur with instant coffee, or when we add powdered creamer to coffee or tea, spoon sugar into a hot drink, or even drop a scoop of ice cream into a mug of root beer.

To listen for the hot chocolate effect, you’ll need a metal or wooden spoon; use either end.  (You can also use your knuckle, and rap against the outside of the cup.)  After filling the cup with hot water or milk, quickly tap the bottom or side of the cup before adding cocoa.  That way, you’ll tune into the mug’s powder-free sound.

Now add the cocoa, and keep tapping.  You should hear the pitch of the tap first drop, and then begin to rise.  How come?

Physicist Frank Crawford explained why in a 1982 article in the American Journal of Physics.  Crawford dubbed it The Hot Chocolate Effect, and since then, researchers have expanded on his original explanation.  (Read about engineer Kevin Kilty’s experiments with “the cheap instant coffee effect” at www.kilty.com/coffee.htm.)

Scientists say that when you add cocoa to your cup, some of the air dissolved in the hot water gloms onto the powder grains.  The result is tiny bubbles, clinging to the powder.  In other words, foam.  The cloud of foam reduces the speed of sound through the liquid.  Meanwhile, the frequency at which sound resonates inside the mug depends on the sound wave’s speed.  The lower the speed, the lower the frequency.

So when you first add cocoa powder, sounds will actually decrease in frequency.  Since the frequency determines a sound’s pitch, your tapping spoon will sound up to an octave lower than it did in plain hot water.  (Think low thunk.)

But as the bubbles float to the surface and pop, the sound traveling through the cocoa quickly speeds up.  The frequency of the sound begins to rise, and along with it, the pitch.  And so we hear higher and higher notes with each new, now-tinkly tap.  Astonishingly, the pitch can rise up to three octaves, or about the vocal range of a well-trained singer.

(Watch a video of the effect using instant coffee at www.youtube.com/watch?v=JCVaOzlOUfY.)

Experiments to try yourself:  Does the composition (glass, ceramic, plastic) or thickness of the cup make a difference in the changing pitch?  What about a taller or shorter cup or glass?  How about a cup with or without a handle?  (Try tapping on the cup’s bottom, then on or near the handle.)  Does using different liquids (water, skim milk, whole milk) alter the sound?  Finally, what about adding whipped cream or marshmallows?  At the very least, it’s all a good excuse to make a second (or third) cup of cocoa on a cold winter’s day.

If you’ve ever heard the doubled sound of your own footsteps in a long, empty hallway, you’re familiar with echoes.  Like an undeliverable letter, an echo is sound returned to sender.

Echoes are sound waves that bounce back at us from a hard surface.  When you shout into a cave, you often hear your own voice, a split second later.  That’s because the hard stone walls reflect sound waves back, like light reflecting from a mirror.  Instead of seeing yourself, as in a mirror, you hear yourself.  Making an echo chamber a kind of ear mirror.

For the clearest echoes, a sound-reflecting surface should be flat, smooth, and perpendicular to the ground.  Sound travels through sea-level air at about 1,100 feet per second.  Stand too close to the echo-making surface, and the sound you make will shoot back too quickly, overlapping your original words or clap or musical note.

Then there are stranger echoes.  Clap your hands near a wooden staircase, and you may hear a drawn-out clap in response, its frequency dropping over time.  Scientists call it the “picket-fence effect,” since a long picket fence can produce the same odd echo.

How does it work?  Physicist Jearl Walker, of Cleveland State University, suggests picturing a staircase from the side.  When the sound waves from your clapping hands strike the stairs, waves reflect from the risers of the steps.  But since each step is set behind the next, pulses from higher steps take a fraction of a second longer to return to you than those from lower steps.

Your ears perceive the widening gap between the “stepped” sound pulses as a gradually dropping tone.  So what you hear, over less than a second, is a single, prolonged, almost musical echo.  (Something similar happens with a picket fence; think of the fence as a vertical staircase.)

A famous example of the effect can be heard at the ancient Mayan Temple of Kukulkan in Mexico.  Clap your hands at the bottom of the pyramid’s 92 stone steps, and you’ll hear a chirping echo.  To acoustics expert David Lubman, the chirps sounded eerily like those of the quetzal, a brilliantly colored bird whose 2-ft.-long tail feathers once adorned Mayan helmets.

Named after the serpent god Kukulkan, the temple displays a kind of homage to the god around the time of the spring and fall equinoxes.  During those days, a shaft of sunlight creeps along the side of the pyramid.  Triangles of light and shadow appear, stretching from a giant stone serpent’s head at the bottom to the top of the pyramid, creating a serpentine body.

In one Mayan stone carving, the god Kukulkan is accompanied by a huge quetzal. Lubman suggests that the stairs’ chirping echoes add a kind of sound track to the temple, the voice of the bird thought to be the messenger of the gods.

For more on the Mayan pyramid echoes, including a recording of the Quetzal bird’s chirps and the echoing steps, visit www.ocasa.org/MayanPyramid.htm.  View a sonogram comparing the two at www.ocasa.org/MayanPyramid2.htm.

In August, it seems like we’re surrounded by paper. Before the start of a new school year, stores aisles are blocked by stacks of notebooks, binder paper, construction paper, and Post-it notes. Once school starts, there’s more paper: forms to fill out, exams to take, reading lists to take home.

But while most of the paper around us was made from wood, the paper we pay for it with wasn’t. Whether it’s a one-dollar bill or a hundred-dollar bill, U.S. paper currency is actually 75 percent cotton and 25 percent linen. So the folded-up money in our wallets is made from fibers usually spun into fabrics. Which explains why you can throw your laundry in the washer, dry it, and pull a still-usable ten out of your jeans pocket. (A tiny care label on each bill might help.)

But making paper currency out of cotton and linen isn’t odd. Some 2,300 years ago, people in Egypt wrote on pressed strips of a swamp plant called papyrus (and where we got the word “paper.”) Others painted words on stretched, dried animal skins (”parchment”). In the beginning, people scratched symbols on stone, a tradition we continue today on buildings and monuments. Writing was also etched onto hammered-out sheets of copper and brass; today, we engrave plaques and rings.

Until the mid-1800s, most paper was made from cloth rags, beaten until they fell apart. The cloth fibers soaked in a vat of water, spread into a thin layer, and drained on screens.

But when people turned to making paper from wood, a new industry took off. The basic process: Trees are sawed down and cut into logs. The logs are moved into a huge, turning drum, which shaves off their bark. Then the bare logs enter a chipper, and are whittled into small pieces.

Next, mechanical grinders or strong chemicals or both break the chips down into individual wood fibers. The yellowish or brown mush of wood fibers is called “pulp.” (Look closely at a brown paper bag, and you’ll see the individual strands.) If the pulp is destined to be white paper, it’s bleached.

The wood fibers are washed to get rid of chemicals, then beaten and rubbed. Finally, the fibers may be treated with a water-resisting material like rosin. That way, ink won’t soak through your notebook paper. If the paper will come in colors, dye is added now.

Then the fibers, mixed into water, flow across a moving wire screen, collecting into a wet, draining web. Presses flatten the pulp into a sheet, which is pulled over huge drums and dried. The dry paper, up to 30 feet wide, is rolled out and cut to size.

Many papers are made from used, recycled paper, saving trees and reducing waste. Besides wood, cotton, and linen, modern paper is also made from bamboo, straw, hemp, and even sugar cane waste. For more on papermaking using wood, visit www.tappi.org/paperu/all_about_paper/paperMade.htm.

Have you ever seen a long-legged insect skitter across a pond? Water striders can stroll on water, thanks mainly to surface tension. Surface tension, a property of liquids that makes the surface act like an elastic skin or membrane, also causes water to bead up on wax paper. That same tension keeps water hanging in a big drop on the end of a faucet as if it’s held in a tiny, see-through balloon.

Surface tension also allows a metal sewing needle to rest on the surface of a bowl of water, even though it’s too dense to “float.”

What causes surface tension? Imagine a bowl full of water. Water molecules under the surface are strongly attracted to their fellow water molecules. Each molecule of water, or H2O, has two hydrogen atoms and one oxygen atom. Because of how the molecules are constructed, the hydrogen end has a positive electrical charge, while the oxygen end has a negative charge. Since opposites attract, water molecules tend to pull together, the hydrogen atoms in one molecule attracted to the oxygen in another.

This ever-shifting electrical attraction known as hydrogen bonding holds the molecules together in a loose liquid mass. But at the surface, water molecules aren’t equally attracted to gas molecules in the air above them. Their main attraction is “downward and inward,” to other water molecules in the bowl, and to their fellow H2O molecules on the surface. The effect of this attraction is to create a tight surface, like a thin, rubbery skin on the water.

(Different liquids have “skins” that are stronger or weaker. Alcohol, for example, has a weaker surface tension than water. But liquid mercury’s surface tension is six times stronger, which is why a broken mercury thermometer spills out those gray (and very toxic) little balls.)

So if we gently place a lightweight sewing needle lengthwise on the surface of a bowl of water (using tweezers helps), the needle should float. (It also helps if the needle is a little oily, or water-repellant.) Look closely, and you’ll see a little indentation where the needle rests, as if the water’s surface were a sheet of rubber. As long as the needle isn’t too heavy, the forces exerted along its length by surface tension will trump the downward-pulling force of gravity.

But put a few drops of liquid detergent in the water, and you’ve changed the balance of force. Detergents (and soaps) are surfactants, or “surface-active agents.” Surfactants turn down the surface tension of water. They actually make water wetter, by disrupting and weakening the attraction between water molecules. The result: Gravity wins; your needle sinks like a stone.

But you can also sink your needle in plain water simply by pushing it under the springy surface. Or just turn it on end, piercing the surface “skin.” Finally, try gradually raising the temperature of the water. As water molecules gain energy and pull apart, surface tension will decrease, and the needle will fall.

Have you avoided wearing a hat on a sub-zero day, afraid of the bad-hair aftermath? After scuffing across the carpet in your slippers, have you approached doorknobs warily? Do you pull masses of stuck-together socks out of the dryer? Does rustling your blanket at night produce tiny, crackling sparks?

The culprit, of course, is static electricity, the same phenomenon behind brilliant flashes of lightning in July. Static electricity is a build-up of negative or positive charges.

Most atoms in a material are electrically balanced, or neutral. The positively charged protons at the center (nucleus) are balanced by an equal number of negatively charged electrons whizzing around them. Losing one or more of its negative electrons will make an atom positively charged. Gaining an extra electron or two will make an atom negatively charged.

When you rub a balloon against your head, electrons transfer from atoms in your hair to atoms on the surface of the balloon. Since electrons have a negative charge, the extra electrons have built up a negative charge on the balloon. Meanwhile, in losing electrons, your hair has built up a positive charge.

Since electrical opposites attract, your hair clings to the balloon as you move it away from your head. But since like charges repel each other, your positively-charged strands separate from one another, producing that Bride-of-Frankenstein look.

Sparks sometimes fly when atoms rebalance themselves. When you rub your shoes on the carpet and then touch a metal doorknob, electrons make a tiny leap, with a spark and a zap.

In a list of materials that easily lose electrons, human skin and hair are near the top. So are glass, nylon, and wool. Materials that easily gain electrons include PVC plastic, silicon, and Teflon. While everyday static electricity is annoying, it’s also useful. Electrical attraction is what makes plastic wrap cling. And inside a Xerox machine, the attraction between a charged plastic drum and toner powder helps produce printed copies.

On the down side, static electricity can be dangerous at the gas pump. Gasoline vapors can ignite from sparks produced by sliding in and out of your car seat while the gas is pumping. Can’t avoid re-entering your car? Gas producers recommend touching metal on your vehicle, away from the pump, with your bare hand, to discharge the electron build-up.

Static electricity can even be hazardous on the Moon, where there aren’t any gas pumps in sight. For a few days each month, the Moon sweeps through the Earth’s magnetic field. During the pass, energetic electrons caught in the field strike the lunar surface, charging the moon dust. Much like static-y hair, charged moon dust can actually levitate. Scientists say that when astronauts next visit the Moon, projected to occur in 2020, dust could clog equipment, and sparks could damage delicate electronics.

For more on static electricity, visit www.sciencemadesimple.com/static.html. Rub a balloon on your head and make a soda can race across the room?  See how at www.exploratorium.edu/science_explorer/roller.html.

When the sun sets, the light streaming through our western windows turns a deep gold, and bright oranges and reds streak the sky. But when the sun rises, the light that first floods through our east-facing windows may be a beautiful blue, often followed by a delicate, rosy pink as the sun makes it appearance. In fact, we can sometimes tell by the sky colors whether a photo has captured a sunrise or a sunset.

The colors come courtesy of the Earth’s atmosphere, which is thickest nearer the ground. As white sunlight zips into the atmosphere, it is invisibly full of color — red, orange, yellow, green, blue, indigo, violet. Teased out by tiny particles in the atmosphere, it’s these colors that we see splashed across the sky at sunrise and sunset.

As sunlight encounters the atmosphere’s gas molecules (such as oxygen and nitrogen), it is broken up into its colors, and then scattered every which way. Longer-wavelength light, like red, is scattered less. Sixteen times as much shorter-wavelength light — blues and violets — is scattered out from gas molecules, flooding the sky.  And we see blue.

At sunrise and sunset, with the Sun near the horizon and its rays traveling nearly parallel to the ground, sunlight must pass through a thick blanket of air before it reaches our eyes. As even more of the blue end of the spectrum is scattered out of the light beam and into the sky, we see the Sun’s face dim and redden. Clouds lit by the altered sunlight turn shades of red and orange and pink. The more blues that are deflected from the beam, the deeper red the sky around the setting or rising sun will appear.

So why do sunrises often display a sky in delicate shades of pink, while sunsets blaze crimson? Scientists say that the air light must travel through is usually thicker at sunset than at sunrise. Dust and pollutants are near their peak at sunset, after a day’s activity by human beings — digging in the ground, constructing buildings, operating machinery and driving cars.

In addition, the air at sunset is usually much warmer than the air at sunrise, since the Earth cools overnight, radiating daytime’s heat into space. And hotter air is more turbulent than cooler air.

So at sunset, we are looking through the thickest layer of gas molecules, dust, and other pollutants, such as soot. More blues are scattered out of incoming beams of sunlight, and we see red, orange, and deep yellow light. But at dawn, the lower atmosphere is calmer, cooler, and cleaner. Daytime’s dust has settled overnight. Air molecules still scatter blues out of the streaming sunlight. But since more blue is left in the beam, the rising sun itself appears less red, and the dawn sky wears a more delicate tint of blue-pinks, pale yellows, and peach.

Your sunrise may vary; sunrises over the sea may look different than sunrises over land. And summer smog, or the lofted dust from a volcano, can make a sunrise as vivid as a sunset.