Night Sky September 2005—Our galactic home

—CHARLIE CHRISTMANN
Summer is the best time to see the Milky Way. It winds its way across the sky looking like a fuzzy ribbon. You will need to get away from the city lights to see much of the detail in our home galaxy.

The Sun is just one of several hundreds of billion stars that are thought to be in our galactic home. Looking at the structure of the galaxy, astronomers believe that it consists of a relatively flat disc about one hundred thousand light-years across, with a central bulge, lying in the direction of the constellation Sagittarius (The Archer), and has six arms spiraling out from the bulge. Our sun lies about twenty-six thousand light-years from the center and halfway to the edge of the spiral arms.

Unfortunately for observers, the Sun and the Earth are embedded in the spiraling arms of the galaxy. From our perspective, the center and far side of the galaxy are hidden from us by thick swaths of dust. Visible light just can not penetrate the veil. If you use binoculars to examine the Milky Way, you will see large expanses of dark dust hiding whatever resides behind it.

However, in August 2003 NASA launched the Spitzer Space Telescope. Spitzer looks in the infrared part of the spectrum, which will penetrate the intergalactic dust clouds.

During its two-and-a-half-year mission, Spitzer will obtain images and spectra by detecting the infrared energy, or heat, radiated by objects in space between wavelengths of three and 180 microns (a micron is one-millionth of a meter). Space is the best place to be to look in the infrared, because Earth's atmosphere blocks most of the infrared radiation. Telescopes on the ground can not see this part of the spectrum.

Using the Spitzer Telescope, astronomers from the University of Wisconsin-Madison looked at thirty million stars in the plane of the galaxy, trying to build a detailed map of the inner regions of the Milky Way. The task is like trying to describe the boundaries of a forest from deep inside the woods. The data collected shows that the Milky Way has a long central bar feature that distinguishes it from the more pedestrian spiral galaxies. The bar seems to consist of relatively old and red stars, spanning twenty-seven thousand light years through the center of the galaxy, and is oriented at about a forty-five-degree angle relative to a line joining the sun and the center of the galaxy.

Astronomers have also detected several monsters at the heart of our galaxy. The Galactic Center harbors a number of potential gamma-ray sources, including a supermassive black hole, remnants of supernova explosions, and possibly an accumulation of exotic “dark matter” particles. Any planet orbiting a star too near the center of the galaxy would be fried by the massive amounts of radiation in the region. Other hazards include a swarm of ten thousand or more black holes that may be orbiting the Milky Way's central supermassive black hole, according to new results from NASA's orbiting Chandra X-ray Observatory. These relatively small stellar-mass black holes, along with neutron stars, appear to have migrated into the Galactic Center over the course of several billion years. Anything getting to close to a black hole is destined for oblivion. Even the smallest black holes are so massive and compact that nothing, not even light, can escape.

The black holes and neutron stars in the central cluster are expected to gradually be swallowed by the supermassive black hole at a rate of about one every million years. At this rate, about ten thousand black holes and neutron stars would have been captured in a few billion years, adding about three percent to the mass of the central supermassive black hole, which is currently estimated to contain the mass of 3.7 million suns.

So, next time you have the opportunity to look at the Milky Way, be happy that we are a safe twenty-six thousand light-years away from the center.

Venus SHINING Bright

Picture this: you're in a car riding along a country road at night. The sky is clear; the stars are twinkling. The silhouettes of moonlit trees glide by the side window. Flash! A blue-white light beams through a gap in the forest. Flash! It happens again. And again, and again. It's following you.

In the movies, this is when the spaceship lands. A door opens. Eerie-green lights flood the roadside. Something alien steps out and you have a Close Encounter. You know it is definitely time to dial 911! But you can relax. It's only Venus, the second planet from the Sun.

Venus is the brightest of all planets. It makes Sirius, the brightest star in the night sky, look feeble. This month, Venus is shining at a bright -3.7 magnitude, bright enough that it can actually cast faint shadows. Only the Sun and Moon outshine it. At sundown, Venus materializes close to the western horizon, where it can beam through trees and make you think you're being chased by something from outer space. No wonder so many people call 911 to report a UFO when they see it.

Why is Venus so bright? It's a cloud-shrouded world only slightly smaller than Earth. Those clouds reflect almost all the sunlight that hits them. Venus's clouds hide the planet's surface. Even the biggest telescopes on Earth can't see what lies below. But if you have a small telescope or binoculars, take a look at Venus anyway. There is something to see: Venus looks like a fat gray banana.

Just like the Moon, Venus has phases. It can be full, gibbous, half, or crescent. These phases occur for the same reason that Moon phases do: geometry. One side of Venus is sunlit (the "day side"). The other side is dark (the "night side"). As Venus orbits the Sun it turns one side, then the other, toward Earth. Now, we can see only see a part of the day side, hence the crescent.
In one way, Moon phases and Venus phases differ. The Moon is bright when it's full and dim when it's a crescent. Venus is just the opposite. It reaches greatest brilliancy at crescent phase. A full Venus, on the other hand, is dim because it is on the far side of the Sun from us.

While you're looking at Venus this week, here is something to think about. Venus is no garden spot. The planet's surface is hot enough to melt lead. Venus' atmosphere, ninety times heavier than Earth's, is almost pure carbon dioxide. The thick blanketing clouds are made of sulfuric acid. Robot spaceships sent to Venus have landed, but they never last long. Russia's Venera 13 lander operated for 127 minutes—the all-time record—before being overwhelmed by the acid, the heat, and the crushing pressure of Venus' atmosphere.

While Venus may be pretty to look at, no one will ever live on its surface.

THE PLANETS AND THE MOON

• Mercury stays close to the Sun this month and will not be visible. Mercury will reach superior conjunction, directly opposite the Sun from Earth, on September 18.
• Venus slowly sinks lower in the evening sky, setting around 9:00 p.m. Look for Venus, Jupiter, and Spica to cluster in the evening sky on September 1.
• Mars is a morning star, rising about 5:00 a.m., and resides due south at dawn. The waxing gibbous Moon passes Mars on the morning of September 22. A waxing crescent Moon again passes Mars on the morning of September 28.
• Jupiter, too, descends into the evening twilight and will disappear from the evening sky. The Moon joins Jupiter and Venus in the western sky the evening of the September 7.
• Saturn rises about two hours ahead of the Sun. Look for the crescent Moon and Saturn on the eastern pre-dawn sky on September 1.
• The Moon is new on September 3 and full on September 17.
• Fall begins at 4:32 p.m. MDT on September 22.