Thursday, August 20, 2009

The Center of the Galaxy

Of the many naked-eye, binocular, and telescopic treasures to be found in the summer constellation Sagittarius the Archer, none fires the imagination like the one we can’t actually see.

Just off the spout of the Archer’s central asterism, the Teapot, lies the spot that marks the direction of the center of the Milky Way, our home galaxy. An asterism is a recognizable star pattern, and the Teapot is a prime example of why amateur astronomers use asterisms to navigate the sky, not ancient, obscure constellation patterns.

I defy anyone to spot a centaur sporting a bow and arrow in the jumble of stars that is Sagittarius. But happily, the brighter stars in that region of the sky collectively resemble a teapot, an object with which we are perhaps a bit more familiar. The softly glowing billows of the Summer Milky Way intersect with the Teapot in a way that suggests steam vigorously issuing from the spout.

We can’t see the center of our galaxy in visible light because of the obscuring dust that lies between us and the galactic core. However, astronomers use cameras that operate in the infrared and radio wavelengths of light in order to “see” past that dust.

Composite image of the center of the Milky Way, in visible and infrared wavelengths
Credits: NASA, ESA, Q.D. Wang (University of Massachusetts, Amherst), Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech)

1) About an hour after your local sunset time, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Try to observe from a dark site, away from urban light pollution, so you can enjoy the picturesque “steam” rising from the Teapot.

2) Look for the teapot shape low over the southern horizon and a little bit northeast of the Scorpion’s stinger. The Teapot is oriented with its curving handle on the eastern (left) side and the spout pointing toward the Scorpion on the right.

Star map created with Your Sky

3) The galactic center lies in the glowing band of the Milky Way, our edgewise view of our platter-shaped galaxy. As shown on the star map, the Teapot’s spout points the way.

Since our eyes aren’t up to the task of penetrating the thick dust separating us from our quarry, we’ll just have to use our imagination. We can imagine the massive cluster of stars that congregate at the bustling galactic core. We can imagine the star-strewn pinwheel-shaped arms of our spiral galaxy radiating out from that core. And if we squint, perhaps we can even imagine the supermassive black hole that lurks at the heart of our home galaxy, a reassuringly distant 26,000 light years away.

Astronomy Essential: Visible light is just a small portion of the electromagnetic spectrum.

Light is a form of energy that travels in different wavelengths. Collectively, these different wavelengths of light are called the electromagnetic spectrum. Separately, we know them as gamma rays, X-rays, ultraviolet or UV, visible light, infrared, microwave, and radio. Gamma rays have the shortest wavelength and radio has the longest wavelength, up to several miles in length.

Notice that visible light, what we humans can see, represents just one wavelength on the electromagnetic spectrum. That is why astronomers use cameras with special detectors to capture data from the other wavelengths— the ones that remain hidden to our eyes.

Can you imagine a species that could see in all wavelengths of light?!