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We examine a century of radial velocity, visual magnitude, and astrometric observations of the nearest red supergiant, Betelgeuse, in order to reexamine the century-old assertion that Betelgeuse might be a spectroscopic binary. These data reveal Betelgeuse varying stochastically over years and decades due to its boiling, convective envelope, periodically with a 5.78 yr long secondary period (LSP), and quasiperiodically from pulsations with periods of several hundred days. We show that the LSP is consistent between astrometric and radial velocity data sets, and argue that it indicates a low-mass companion to Betelgeuse, less than a solar mass, orbiting in a 2110 day period at a separation of just over twice Betelgeuse’s radius. The companion star would be nearly 20 times less massive and a million times fainter than Betelgeuse, with similar effective temperature, effectively hiding it in plain sight near one of the best-studied stars in the night sky. The astrometric data favor an edge-on binary with orbital plane aligned with Betelgeuse’s measured spin axis. Tidal spin–orbit interaction drains angular momentum from the orbit and spins up Betelgeuse, explaining the spin–orbit alignment and Betelgeuse’s anomalously rapid spin. In the future, the orbit will decay until the companion is swallowed by Betelgeuse in the next 10,000 yr.

We describe recent spectroscopic observations of red giant stars made by the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope, which has provided spatially resolved observations of the warm chromospheric winds that predominate for early K to mid-M giants. The H i Ly α lines of a set of 11 red giants observed with the STIS/E140M echelle grating are first analyzed to ascertain wind H i column densities and total wind mass-loss rates. The M giants have estimated mass-loss rates of $\dot{M}=(14\mbox{--}86)\times {10}^{-11}$ M _⊙ yr ^−1 , while the K giants with detected wind absorption have weaker winds with $\dot{M}=(1.5\mbox{--}2.8)\times {10}^{-11}$ M _⊙ yr ^−1 . We use long-slit spectra of H i Ly α for two particular red giants, α Tau (K5 III) and γ Cru (M3.5 III), to study the spatial extent of the Ly α emission. From these data we estimate limits for the extent of detectable emission, which are r = 193 R _* for γ Cru and r = 44 R _* for α Tau. The cross-dispersion emission profiles in the STIS echelle spectra of the larger sample of red giants also show evidence for spatial resolution, not only for H i Ly α but for other lines with visible wind absorption, such as Fe ii , Mg ii , Mg i , O i , and C ii . We characterize the nature of these spatial signatures. The spatial extent is far more apparent for the M giants than for the K giants, consistent with the stronger winds found for the M giants from the Ly α analysis.