Angular size of stars in Hubble photos

[msmith]

We see apparent size of stars in the beautiful Hubble photos. Are these because bright objects burn bigger areas on the image? Can the photos actually record a size of such distant things?

[Chris Peterson]

We see apparent size of stars in the beautiful Hubble photos. Are these because bright objects burn bigger areas on the image? Can the photos actually record a size of such distant things?

The HST cannot resolve stars, possibly outside a handful of large, nearby ones. Its aperture (as with all telescopes) causes diffraction, which turns star images into broadened spots at the focal plane. Although the stars are actually all the same size on the image (what is called full width at half maximum, or FWHM), brighter ones stick up higher out of the noise, and so appear to have a larger diameter.

BTW, your eye does the same thing when looking at stars, either unaided or with a telescope- bright ones appear larger.