Ultraviolet astronomy must be carried out from space. Gamma-ray telescopes simply point in a certain direction and count photons received. Because the atmosphere is opaque at these short wavelengths, both types of telescopes must be placed in space. Radio and other nonoptical telescopes are essential to studies of the universe because they allow astronomers to probe regions of space that are completely opaque to visible light and to study the many objects that emit little or no optical radiation at all. The distances to the nearest stars can be measured by trigonometric parallax.
Stars have real motion through space as well as apparent motion as Earth orbits the Sun.
A star's proper motion ., its true motion across the sky, is a measure of the star's velocity perpendicular to our line of sight. Only a few stars are large enough and close enough that their radii can be measured directly. The absolute brightness of a star is equivalent to its luminosity. The apparent brightness of a star is the rate at which energy from the star reaches unit area of a detector. Apparent brightness falls off as the inverse square of the distance.