Windows in the Sky

When children are in the Observatory Dome before we have the telescope up and running I usually keep a running patter up. One of my standard questions is "What do you have to be able to do if your house only has one window?" Various answers come back from my young friends including the correct one "Move the window where you need it!". And then I rotate the Dome until the shutter window is aligned with the first target of the evening.

When astronomers talk about windows, it isn't about a dome shutter or the operating system in their PC but opening in the atmosphere which allows some part of electromagnetic spectrum to pass. We think of air as transparent, but in much of the spectrum, air is as opaque as a brick wall. Light passes easily. So does a good deal of the radio frequencies, but far infrared, far ultraviolet, x-ray and gamma rays are filtered out. In general this is a good thing. A sunburn from near visible ultraviolet radiation is miserable, but a sunburn from solar x-rays would cause cancer rates to soar. This doesn't mean that astronomers wouldn't like to look at these parts of the electromagnetic spectrum.

One way to safely look at these frequencies is to place the telescope in orbit. Far above the atmosphere, a fleet of telescopes look at x-rays, gamma rays, near and far ultraviolet, visible light and the infrared. Each of these parts of the electromagnetic spectrum faces its own problems. The high frequencies x-rays and gamma rays simply refuse to be focused by mirrors in the normal fashion. The low frequencies of microwaves and radio require huge dishes. Much of the ultraviolet and the infrared respond like visible light. Mirrors focus them well, but infrared has one very disconcerting property. Infrared radiation is emitted by everything. In the infrared, you shine as brightly as a hundred watt bulb.

If we lined our telescopes with bright electric bulbs we wouldn't have a chance to see anything but the sun and the moon. In the infrared, the telescope itself shines as brightly as if the walls were made of fluorescent tubes. When we place an infrared telescope in orbit, things only get worse. The sun heats the telescope mercilessly causing it to shine brilliantly in the infrared. The only solution is to chill the telescope's critical parts until they no longer radiate in the infrared. If the crucial parts can be chilled to a few degrees absolute, the only frequencies emitted are in the microwave part of the spectrum. This means that infrared telescopes are refrigerators before they are optical instruments.

Opening a new window has never failed to advance our understanding of the universe. When the IRAS infrared telescope was built, we expected that it would be able to see through the dusty center of our galaxy (dust doesn't block infrared radiation very much), and we thought it would allow us to look into the nebulae where new stars are forming. It has done both admirably. However surprises were still in store for us. No one expected that infrared telescopes would tell us much about normal main sequence stars. We knew they emitted huge amounts of infrared radiation in the form of heat, but we thought that we basically knew what they would look like in infrared - simple points of light. However a few stars looked like a lamp surrounded by millions of faint sparkling points too small to be resolved individually but brilliant in the aggregate. Even Vega, one of the nearest and very brightest stars is enclosed in one of these sparkling globes of particles. This was a huge surprise because Vega is a standard calibration star, chosen as the standard because it was though to be so regular and normal.

We are fairly sure that what we are seeing is Vega's cloud of comets (the Oort Cloud) reradiating Vega's hot visible and ultraviolet radiation as infrared radiation. If our telescope in Charlestown were able to see effectively in the infrared it would see Vega as a disk rather than a point of light.

Leslie Coleman
Leslie Coleman
Entry Date:
Dec 1, 2002
Published Under:
Leslie Coleman's Columns
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