For several weeks in mid-2020, NASA’s Solar and Terrestrial Relations Observatory, or ‘STEREO,’ had the best view of the solar system’s star Betelgeuse, whose extreme fading over time has intrigued scientific experts. STEREO’s calculations revealed more unexpected dimming by Betelgeuse, further adding to the mysteries around this star’s recent behavior.
Now Closer to the Sun
In late spring 2020, Betelgeuse appeared closer to the Sun in the sky because of Earth’s position in space. The STEREO spacecraft is now about 70 degrees away from Earth — meaning that in the middle of summer, STEREO was in roughly the same position that Earth was in around mid-April and could consequently see the stars that appeared in Earth’s night sky during April.
Scientists took advantage of this unprecedented orbital position to keep tabs on Betelgeuse while the star was invisible to Earth-bound observatories. During this time between late June and early August, STEREO examined Betelgeuse for five days, rotating the spacecraft for approximately two hours each time to set Betelgeuse in view of the STEREO’s Heliospheric Imager. This imager is an apparatus used to obtain images of the Sun’s outflowing matter, the solar wind, as it passes over the spacecraft and towards Earth. The crew reduced the instrument’s exposure time to account for Betelgeuse’s relative brightness corresponded to the solar wind. The instrument’s broad range of view covers about 70 degrees of sky, enabling scientists to calibrate their measurements using steady stars in the night sky over several weeks.
Betelgeuse is Dimming Once Again
STEREO’s measurements showed that Betelgeuse is dimming once again — an unexpected development shortly after its last dim time. Betelgeuse typically goes through brightness cycles that last around 420 days, with the previous minimum in February 2020, meaning this dimming is happening surprisingly early. The science crew reported these observations through The Astronomer’s Telegram on July 28, 2020. This is an intriguing event that scientists will study with additional Earth-orbiting and ground-based observatories when Betelgeuse returns to the night sky.