Glossary of astronomy - T

Telluric stars have nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's atmosphere on the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800 Å. These features need to be corrected for in order to reach a more accurate spectrum.[16]

Terminator is the line that divides the illuminated side of a moon or planet from the dark side. The line moves as the object rotates with respect to the Sun.

The frist term in this glossary.

Thin disk population refers to the layer of the Milky Way galaxy where the spiral arms are found and where most of the star formation takes place. It is about 300–400 parsecs (980–1,300 light-years) deep and centered on the galactic plane. Stars belonging to this population generally follow orbits that lie close to this plane.[17] This is in contrast to members of the thick disk population and halo stars.

Tidal braking or tidal acceleration is the transfer of momentum between an astronomical body and an orbiting satellite as the result of tidal forces. This can cause changes in the rotation periods for both bodies as well as modification of their mutual orbit. A satellite in a prograde orbit will gradually recede from the primary body, while slowing the rotation rate of both bodies.

Tidal locking is a net result of continued tidal braking wherein such that, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner. When the orbital eccentricity is low, the result is that the satellite orbits with the same face always pointed toward its primary.[18] The Moon is tidally locked with the Earth.

Tidal stream refers to the streams of stars and gas that are stripped from gas clouds and star clusters because of interaction with the gravitational field of a galaxy such as the Milky Way.[19]

Tilt erosion is the gradual reduction of the obliquity of an orbiting satellite due to tidal interactions.[20]

Transit is an astronomical event where an object passes across the face of a much larger body. An example of this event is the transit of Venus across the face of the Sun in 2004. Because a transit results in a decrease in the net luminosity from the two objects, the transit method is used to detect extrasolar planets as they pass in front of their host stars. Transits by objects that appear roughly the same size or larger than the body they are transiting are called occultations.

Tully–Fisher relation is an empirical relationship between the mass or intrinsic luminosity of a spiral galaxy and its angular velocity or emission line width. It can be used to estimate the distance of a galaxy, and hence forms a rung on the Cosmic Distance Ladder.