Period 6 element devices though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favor of alcohol-filled, galinstan-filled, digital, or thermistor-based instruments. It remains in use in scientific research applications and in amalgam material for dental restoration. It is used in lighting: electricity passed through mercury vapor in a phosphor tube produces short-wave ultraviolet light which then causes the phosphor to fluoresce, making visible light. Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray other metal resembles tin but discolors when exposed to air. The
SN 2003gd SN 2003gd, and which is believed to be the supernova's progenitor star. This progenitor star was a red supergiant, consistent with the expectations of existing single-star stellar evolution models. It is the first progenitor of a normal type II-P supernova to have ever been detected. SN 2003gd SN 2003gd was a type II-P supernova occurring in the spiral galaxy Messier 74 in the constellation Pisces. SN 2003gd was discovered on 12 June 2003 by Robert Evans, using a 0.31m reflector, and its discovery was confirmed on 13 June 2003 by R. H. McNaught using the 1.0m telescope at the Siding