Astro Glossary

Gaia

A European space telescope that is using a technique known as astrometry to provide a precise three-dimensional map of more than one billion stars in the Milky Way galaxy. It was launched December 19, 2013, from the Kourou launch center in French Guiana. It will measure the distances to stars and plot their motions across the sky, as well as providing basic information about the stars themselves. It also is expected to discover thousands of exoplanets, as well as comets and asteroids in the solar system.

Galaxy

A vast “island universe” of stars, gas, and dust. The universe may contain 100 billion or more galaxies. Galactic size and structure range from dwarfs and irregulars with no discernible structure and only a few million stars to giant spirals and ellipticals that span 100,000 light-years or more and contain hundreds of billions of stars. Galaxies clump together in web-like structures. Despite its great luminosity, most of a galaxy’s mass consists of dark matter, which produces no detectable energy.

Galaxy Cluster

A grouping of galaxies. The Milky Way belongs to a small cluster known as the Local Group, but other clusters can contain hundreds or thousands of galaxies. Clusters of galaxies, in turn, can group together to form superclusters.

Galaxy Formation and Mergers

Galileo Galilei

An Italian scientist who lived from 1564 to 1642. Galileo was the first astronomer to use the newly invented telescope to make astronomical observations. He discovered the four largest moons of Jupiter, craters and mountains on the Moon, and sunspots, and was the first to see that the Milky Way consists of a myriad of individual stars. Galileo supported the Copernican system, which held that the Sun, not Earth, was the center of the universe. As a result, he was charged with heresy by the Inquisition, and was forced to recant. He lived out his final years under house arrest.

Galileo to Jupiter

Galileo was the first spacecraft to enter orbit around an outer planet. It arrived at Jupiter in December 1995. It dropped a probe into Jupiter’s turbulent atmosphere, then observed Jupiter and its moons for seven more years. Its mission ended when it was intentionally crashed into Jupiter. Galileo’s success was limited by a failed radio antenna that reduced the amount of data it could transmit to Earth.

Gamma-Ray Astronomy

The study of the most energetic wavelength, or frequency, of electromagnetic radiation. Gamma-rays have wavelengths smaller than 10^-11 meters and frequencies greater than 10^20 Hz. They can be quite harmful to life because they are strong enough to ionize atoms and thus destroy cells. Fortunately, Earth’s atmosphere shields us from most astronomical gamma-ray radiation.

Gamma-Ray Bursts

The most powerful explosions in the modern-day universe, they most likely are caused by the explosion of a supergiant star and the accompanying collapse of its core to form a black hole. The explosion’s gamma rays are funneled into narrow beams from the star’s poles. From Earth, we see a gamma-ray burst only when such a beam sweeps across our planet. A typical gamma-ray burst produces more energy in a few hours than the Sun will produce in its entire lifetime.

Gemini Project

Designed as a bridge between Mercury and Apollo, Project Gemini primarily tested equipment and mission procedures and trained astronauts and ground crews for future Apollo missions to the Moon.

Gemini, the Twins

A constellation of the zodiac, visible high overhead in mid-winter.