Chemical substance identity of benzene, until the correct structure was described by Friedrich August Kekulé. Likewise, the idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers. An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it
NASA Star and Exoplanet Database direct access to frequently accessed tables: NStED also serves photometric time-series data from surveys that aim to discover transiting exoplanets such as "Kepler" and the CoRoT. NStED provides access to over 500,000 light curves from space and ground-based exoplanet transit survey programs, including: The transit survey data are linked to an online periodogram service that can be used to search for periods in the transit survey data sets. A user can also upload their own time series data to search for periods in their own data. The planetary orbital parameters are linked to an online transit ephemeris prediction tool which