They were formerly characterized by the presence of hydrogen spectral lines, which their counterparts do not exhibit.  
The empirical discovery of the Phillips relation allowed for the use of these astronomical phenomena as standard candles by correlating their intrinsic luminosity to the rate at which they grow fainter over time.  They were formerly characterized by the presence of hydrogen spectral lines, which their counterparts do not exhibit.  FTP, name these objects, the result of thermonuclear runaway converting an entire white dwarf into iron, the most famous example of which might be 1987A.
The empirical discovery of the Phillips relation allowed for the use of these astronomical phenomena as standard candles by correlating their intrinsic luminosity to the rate at which they grow fainter over time.  
Type Ia Supernovae (prompt on "Supernova")