How Come?

How come there are bars in some galaxies?  Or is this an illusion?  asks George W. Bowman.

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.

How come flies don’t fall off the ceiling?  asks reader J. Jones.

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.)