  Type Ia supernovae (SNe Ia) are thermonuclear explosions of white dwarf stars
in close binary systems. They play an important role as cosmological distance
indicators and have led to the discovery of the accelerated expansion of the
Universe. Among the most important unsolved questions are how the explosion
actually proceeds and whether accretion occurs from a companion or via the
merging of two white dwarfs. Tycho Brahe's supernova of 1572 (SN 1572) is
thought to be one of the best candidates for a SN Ia in the Milky Way. The
proximity of the SN 1572 remnant has allowed detailed studies, such as the
possible identification of the binary companion, and provides a unique
opportunity to test theories of the explosion mechanism and the nature of the
progenitor. The determination of the yet unknown exact spectroscopic type of SN
1572 is crucial to relate these results to the diverse population of SNe Ia.
Here we report an optical spectrum of Tycho Brahe's supernova near maximum
brightness, obtained from a scattered-light echo more than four centuries after
the direct light of the explosion swept past Earth. We find that SN 1572
belongs to the majority class of normal SNe Ia. The presence of a strong Ca II
IR feature at velocities exceeding 20,000 km/s, which is similar to the
previously observed polarized features in other SNe Ia, suggests asphericity in
SN 1572.
