Mike Moss's weather blog post on the "plethora of optical phenomena" recently seen over Huntsville, Ala., reminded me a bit of a similar but space-based optical phenomenon.

It's not crystals, which are bending light in images from the Hubble Space Telescope and other powerful telescopes. It's gravity.

Mass in a galaxy can bend and magnify light coming from behind, making normally hidden quasars and other galaxies detectable from Earth.

This gravitational lensing effect can make a single galaxy appear two or more times around the galaxy, black hole or other massive object.

One of the most spectacular examples is Einstein's Cross.

This galaxy in the constellation Pegasus lies 400 million light years away. The quasar that it lenses is over 7 billion light years beyond that. When viewed from Earth, the quasar appears as four distinct points of light, labeled A-D by astronomers, bent around the center of mass in that galaxy.

When discovered, gravitational lensing was seen as a curiosity that allowed observers a distorted view of what lies behind galaxies.

Recent decades have realized its value as a research tool.

Gravitation lensing also provides a way to "see" things that cannot normally be seen. The phenomenon can reveal black holes, neutron stars and anything else in the night sky that doesn't produce or reflect light of its own. Even dark matter. Streaky smears of light coming from behind galaxies are being revealed as signs of dark matter.

Measuring the distribution of the ghost images helps researchers map the distribution of mass within a galaxy. Repeated measurements over many years provide clues not only about changes in these galaxies but also the shape of the universe and the rate of expansion.