Dark
Pronunciation:
dahrk
Definition:
The absence of light or illumination; the opposite of light.
Sample Sentence:
The room was so dark that I couldn’t see anything in front of me.
Dark Matter
Pronunciation:
dahrk MAT-er
Definition:
Hypothetical matter that is believed to make up a significant portion of the universe’s total mass, but does not emit, absorb or reflect light.
Sample Sentence:
Astronomers study the gravitational effects of dark matter to better understand its nature and distribution in the universe.
Credit: Image by KiDS survey
Debris Disk
Pronunciation:
duh-BREE disk
Definition:
A disk-shaped region of dust and debris that surrounds a star, usually found in newly formed or mature star systems.
Sample Sentence:
The presence of a debris disk around a star is often an indication of recent or ongoing planet formation.
Credit: Image by ALMA (ESO/NAOJ/NRAO)/L. Matrà/M. A. MacGregor
Doppler Effect
Pronunciation:
DOP-ler ih-fekt
Definition:
The change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source, creating a perceived change in pitch or color.
Sample Sentence:
The Doppler effect is responsible for the change in pitch of an ambulance siren as it passes by a stationary observer.
Drake Equation
Pronunciation:
drayk ih-KWAY-zhun
Definition:
An equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy based on a set of factors such as the rate of star formation, the number of habitable planets, and the likelihood of life developing on those planets.
Sample Sentence:
The Drake equation remains a valuable tool in the search for extraterrestrial intelligence, despite uncertainties in some of the factors that it includes.
The Drake equation is a mathematical formula that estimates the number of detectable extraterrestrial civilizations in the Milky Way galaxy. The equation has several variables, which are multiplied together to calculate an estimate of the number of advanced, communicating civilizations in the galaxy. These variables include R*, the rate of star formation in the galaxy; fp, the fraction of stars with planets; ne, the number of planets per star that are capable of supporting life; fl, the fraction of planets where life actually develops; fi, the fraction of life-bearing planets where intelligent life develops; and L, the length of time during which such civilizations release detectable signals into space. While the Drake equation is not intended to provide a precise number of civilizations, it provides a framework for thinking about the factors that may influence the emergence of intelligent life in the universe.