From the March 2018 issue

Which supernova did Tycho observe in 1572?

Gene Scovell Los Osos, California
By | Published: March 29, 2018

The Chandra X-ray Observatory has imaged Tycho’s supernova in detail. An outer expanding shell of high-energy electrons associated with the initial shock wave is visible as blue. The debris left over from the explosion (red and green) glows due to heating from a rebounding, inward-moving shock wave.

The supernova reported by Danish astronomer Tycho Brahe (and many others, independently) occurred in the constellation Cassiopeia. Tycho noticed the “new” star on November 11, 1572, after which it brightened to about the magnitude of Venus (–4) and was visible during the day for about two weeks. The supernova then slowly faded until it was no longer visible with the naked eye in the night sky by March 1574. Keep in mind that the telescope was not invented until 1608, so follow-up observations at the time were impossible.

The supernova’s remnant was identified in the mid-20th century via telescope. Today, astronomers refer to it by several names, including B Cassiopeiae and SN 1572, but it also carries the informal name of Tycho’s supernova. The remnant is about 13,000 light-years away within the Milky Way. It is spherical and consists of an expanding cloud of debris, led by an outer shell of high-energy electrons associated with the initial shock wave from the blast. A second shock wave, called a rebound shock wave, travels inward and has lit up the debris.

Using the remnant’s spectrum and light reflected off nearby dust, astronomers have classified Tycho’s supernova as a type Ia event: the explosion of a white dwarf that tips over its allowed mass limit after accreting matter from a nearby companion. But recent work published in September in Nature Astronomy offers an alternative scenario, based on new observations of the remnant and its environment: The progenitor may have been the merger of two white dwarf stars, rather than a single white dwarf stealing mass from a stellar companion.

Alison Klesman 
Associate Editor