If stars appear as points, why are some stars big and some small in different images?

If stars appear as mere points, as we’re always told, why are some stars big and some small in every image I’ve ever seen?

Photography does strange things to stars. In fact, the sky on photographs looks rather different from the sky we see visually (S&T: June 2004, page 128).

A star in the sky is indeed a point, but a star image in a camera is not. It has a bright core and a dimmer fringe, due to diffraction, scattered light, and optical imperfections.

The longer you expose, the more light you pump into the core. But the image receptor (whether film or a CCD chip) quickly becomes 100 percent, or “fully,” exposed; additional light beyond that is lost to overexposure. At the same time, however, the fringe increasingly comes into visibility. And light also gets scattered sideways inside the film or CCD array itself.

As a result, astronomical photos present several illusions that every astronomer should know:

· If you enlarged the photo so that each star image was the size of your living-room floor, the actual star causing the image would be roughly the size of a pinhead in its middle. And that’s only for stars nearby!

· In the densest starclouds toward the center of the Milky Way, where photos seem to show stars packed so closely they almost touch, black space actually covers about a trillion times as much sky area as stars do. The pinheads are far apart.

· The sizes of binary-star components in a photo have nothing to do with how the binary would look if you were there. The brighter star is often the smaller one physically.

· Photos make bright and faint stars look too much alike (since much of a bright star’s light is lost by overexposure). This is why constellations and other bright patterns tend to disappear in sky photos. To circumvent this, constellation photographers sometimes put a special diffusing filter over the lens to turn bright stars into especially big blobs.

— Alan MacRobert