Bad Astronomy. Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax'

two objects, one of them half the size of the other, but both smaller than the seeing on a given evening. Because of seeing, they will both get blurred out to the same size, and we cannot tell which object is larger. This puts a lower limit on how small an object we can observe and still accurately measure its size. Anything smaller than this lower limit will be blurred out, making it look bigger.

Even worse, objects that are close together will get blurred together by seeing, and we cannot distinguish them. This really puts the brakes on how small an object we can detect.

There are actually several ways to work around seeing. One way is to work over it. If you launch a telescope up over the atmosphere it won’t be affected by seeing at all. That’s the basic reason the Hubble Space Telescope was put into orbit in 1990. Without an atmosphere between it and the objects it studies, it has a better view than telescopes on the ground (for more, see chapter 22, “Hubble Trouble”). Hubble is not limited by seeing, and can usually resolve objects much better than its land-based brethren. The problem is that launching a telescope is very expensive, and can make a space telescope cost ten times as much as one built on the ground.

Another way around seeing is to take a lot of really short exposures of an object. If the exposure is fast enough, it freezes the image of the star before the turbulent air can blur it. It’s like taking a fast exposure of a moving object. A one-second exposure of a race car is hopelessly blurred, but one taken at ¹/_10,000 of a second will be clean and clear. A very fast exposure will show a clear image of the star, but the position of the star’s image will jump around from exposure to exposure as the light bends. Astronomers can take hundreds or thousands of very short exposures of a star and then add the separate images together electronically, yielding detail that is impossible with longer exposures. This technique was used to get the first resolved image of a star other than the Sun. The red giant star Antares was the target, and the image, though blurry, was definitely resolved and not just a point of light.

The big disadvantage of this technique is that it only works for bright objects. A faint one won’t show up in the short exposure times necessary. This severely limits the available targets and therefore the usefulness of the process.

There is a third technique that shows amazing promise. If the observer can actually measure just how the atmosphere is distorting a star’s image, then the shape of the telescope mirror itself can be warped to compensate for it. This technique is called adaptive optics, or AO for short, because the optical system of the telescope can adapt to changes in the seeing. It’s done by small pistons attached to rods, called actuators, located behind the telescope mirror. In some cases the rods push on the mirror, changing its shape, distorting the mirror just enough to correct for seeing changes. Another way is to use a collection of hexagonal mirrors that fit together like kitchen tiles, each with its own actuator. Little mirrors are much easier and less expensive to make than big ones, so many of the world’s largest telescopes are designed this way.

A close binary star pair may look like a blob of light when seen without adaptive optics (a), but is separated easily once the adaptive optics of the CFH telescope is switched on (b). Further image processing using computers can make the observation even better (c). The stars are separated by only about 0.3 arcseconds, or the apparent size of a quarter seen from a distance of almost 15 kilometers.

(Image courtesy Canada France Hawaii Telescope Corporation, © 1996.)

The results are nothing less than incredible. The pictures above are from the Canada-France-Hawaii 3.6- meter telescope outfitted with AO. The image on the left is a picture of a binary star taken with the AO turned off. All we see is an elongated blur. But in the image on the right the AO is turned on, correcting for the seeing, and the two individual stars snap into focus.

The European Southern Observatory has several telescopes in Chile outfitted with adaptive optics. One is the Very Large Telescope, or VLT for short. The name isn’t exactly poetic, but it does describe the huge, 8-meter, hexagonally segmented mirror pretty accurately. There are actually four such ’scopes, and with adaptive optics their images rival Hubble’s. One of the only disadvantages of adaptive optics is the narrow field of view; only a small area of the sky can be seen in each exposure. As the technology improves, though, so will the area, and eventually these telescopes will routinely use AO for much larger chunks of the sky.

The next time you’re out on a clear night and the stars dance their dance, you can remember how even the simplest things like the twinkle of a star can have complicated origins, and how difficult it can sometimes be to work around them.

Or, you could just watch the stars twinkle. That’s okay, too.

10. Star Light, Star White: Stars of Many Colors

On a clear night, one of my favorite activities is to haul out my telescope and simply look at stuff in the sky. Usually, I have the ’scope set up in my yard, somewhere out of the way of trees, streetlights, and other obstacles. Still, a neighbor always manages to see me and drops by to take a peek.

The last time this happened, my neighbor brought her two school-age kids. They were being home-schooled and needed a science credit. She figured a night outside with a telescope would count.

After we looked at the Moon, Saturn, Jupiter, and a few other showpieces, the kids wanted to see a star through the telescope. I cautioned them that the stars would just look like points of light, and not disks. No ordinary telescope can magnify images that much. Then I turned the ’scope to Vega, one of the brightest stars in the sky. Without saying anything else, I let them take a look.

The gasps of delight were wonderful. “It’s like a gem!” one of them breathed. “I can’t believe how blue it is!”

I expected that reaction. My neighbor’s daughter looked away from the ’scope and I pointed out Vega to her in the sky. She looked at it for a moment, and then said, “I didn’t know stars really had color. I thought they were all white.”

I expected that, too. I hear it a lot. Despite this common belief, stars do have color, and some are quite beautifully hued. Most look white because of our eyes; the fault lies with us and not the stars.

Amazingly, immense objects like stars emit colors because of the tiniest things of all: atoms.

Stars are basically giant balls of gas. Near the center, the immense pressure of the outer layers squeezes the atoms of gas together. When you squeeze something, it gets hot. The pressure is so high in the centers of stars like the Sun that the temperature can reach millions of degrees. At temperatures this high, the nuclei of atoms — their centers, composed of positively charged protons and neutral neutrons — smash into each other and stick together in a process called nuclear fusion. This process releases energy in the form of very energetic light called gamma rays.

Light acts like a messenger, transferring energy from one place to another; in this sense, light and energy are the same thing. The gamma rays don’t get far before getting absorbed by another nucleus. They are promptly re-emitted, move out again, and get reabsorbed. This process happens over and over, countless trillions of times, and the energy of fusion in the center of the star works its way out to the surface.

When a gamma ray smacks into a subatomic particle, the particle increases its energy. In other words, it gets hot. Near the core the temperature can be millions of degrees, but the temperature drops with distance from the core. Eventually, near the surface of the star, the temperature is a comparatively chilly few thousand degrees Celsius (compare that to room temperature here on Earth, which is about 22 degrees Celsius).

This temperature is still more than enough to strip electrons from their parent atoms. All these particles near the Sun’s surface are zipping around, bumping into each other, absorbing and emitting energy in the form of light. For a long time, it was a major problem in physics to figure out just how the Sun emitted this light. Around the year 1900, Max Planck, a German physicist, imagined that the particles in the Sun were like little oscillators, little