The range of distances from a star within which an Earth-like planet would be habitable.
Harvard Spectral Classification
A commonly used classification scheme for stars by which the temperature of a star is denoted by the assignment of a letter in the sequence OBAFGKM, running from hottest (O) to coolest (M)
The process by which stars fuse three helium nuclei into one carbon nucleus via the triple alpha process.
Hertzsprung-Russell (HR) diagram
A diagnostic diagram whereby the evolutionary state of a star may be discerned via a combination of stellar temperature and luminosity. Named after its originators Ejnar Hertzsprung and Henry Norris Russell.
A giant planet, with a mass close to or greater than that of Jupiter, and which orbits very close to its parent star.
Hubble deep field
A project with the Hubble Space Telescope to obtain a series of very deep exposures to search for distant galaxies at high redshift.
The observation that, for distant galaxies, the greater a galaxy’s distance, the greater the redshift of the radiation received from that galaxy.
The process by which stars on the main sequence release energy via the conversion of hydrogen into helium in their cores.
hydrogen shell burning
The process by which, in post-Main Sequence stars, hydrogen is converted into helium in a thin shell exterior to the core.
In a star, the state at which physical forces directed inwards are exactly balanced by those directed outwards.