9.7 Key Terms
Absolute distances (of the planets): how far from the Sun one planet is compared to another. 9.2
Apparent brightness: the amount of a star’s energy that reaches a given area (say, one square meter) each second here on Earth. 9.4
Baseline: the distance between the two different vantage points in a triangulation setup. 9.2
Brown dwarfs: objects with masses intermediate between stars and planets that do not become hot enough for hydrogen fusion to take place. 9.6
Deuterium fusion: a process that brown dwarfs are capable of completing which involves fusing deuterium (a rare form of hydrogen with one proton and one neutron in its nucleus) and which happens at a lower temperature than the fusion of hydrogen. 9.6
Doppler effect: the very high-resolution stellar spectroscopy technique that lets astronomers measure a star’s radial velocity. 9.5
Exoplanets: planets of other nearby stars. 9.5
Globular clusters are groups of tens to hundreds of thousands of stars held tightly together by gravity. 9.1
Luminosity: the total amount of energy at all wavelengths that a star emits per second. 9.4
Mini-Neptune: a planet that is intermediate between the largest terrestrial planet in our solar system (Earth) and the smallest jovian planet (Neptune); generally, mini-Neptunes have sizes between 2.8 and 4 times Earth’s size. 9.5
Open clusters are groups of up to a few thousand stars that are loosely held together by gravity. 9.1
Parallax: the apparent change in direction of the remote object due to a change in vantage point of the observer. 9.2
Parsec (pc): a unit of distance in astronomy, equal to 3.26 light-years; at a distance of 1 parsec, a star has a parallax of 1 arcsecond. 9.2
Radial velocity: the speed of a star, toward us or away from us, relative to the observer. 9.5
Relative distances: distances in light-seconds or metres or other standard units of length. 9.2
Star clusters: larger groups of hundreds or thousands of stars. 9.1
Star: an object that during some part of its lifetime derives 100% of its energy from the same process that makes the Sun shine — the fusion of hydrogen nuclei (protons) into helium. 9.6
Star systems: stars that are grouped tightly together. 9.1
Super Earth: a planet larger than Earth, generally between 1.4 and 2.8 times the size of our planet. 9.5
Transit: when one astronomical object moves in front of another. 9.5
Triangulation: a technique that allows us to measure distances to inaccessible objects by getting the angle to the object from two different vantage points where we can calculate the properties of the triangle they make and thus the distance to the object. 9.2
