  During October 2019 and March 2020, the luminous red supergiant Betelgeuse
demonstrated an unusually deep minimum of its brightness. It became fainter by
more than one magnitude and this is the most significant dimming observed in
the recent decades. While the reason for the dimming is debated, pre-phase of
supernova explosion, obscuring dust, or changes in the photosphere of the star
were suggested scenarios. Here, we present spectroscopic studies of Betelgeuse
using high-resolution and high signal-to- noise ratio near-infrared spectra
obtained at Weihai Observatory on four epochs in 2020 covering the phases of
during and after dimming. We show that the dimming episode is caused by the
dropping of its effective temperature by at least 170 K on 2020 January 31,
that can be attributed to the emergence of a large dark spot on the surface of
the star.
