Free Astronomy Magazine September-October 2020

SEPTEMBER-OCTOBER 2020 R adio images of Antares with ALMA and the VLA. ALMA observed Antares close to its surface in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. In the VLA image a huge wind is visible on the right, ejected from Antares and lit up by its smaller but hotter companion star Antares B. [ALMA (ESO/NAOJ/NRAO), E. O’Gorman; NRAO/AUI/NSF, S. Dagnello] motion in a pot of boiling water. Not much is known about chromos- pheres, and this is the first time that this region has been detected in radio waves. Thanks to ALMA and the VLA, the scientists discovered that the star’s chromosphere extends out to 2.5 times the star’s radius (our Sun’s chromosphere is only 1/200th of its radius). They also found that the temperature of the chromosphere is lower than previous optical and ul- traviolet observations have sug- gested. The temperature peaks at 3,500 degrees Celsius (6,400 degrees Fahrenheit), after which it gradually decreases. As a comparison, the Sun’s chromosphere reaches temperatures of almost 20,000 degrees Celsius. “We found that the chromosphere is ‘lukewarm’ rather than hot, in stellar temperatures,” said O’Gorman. “The difference can be explained because our radio measurements are a sensi- tive thermometer for most of the gas and plasma in the star’s atmos- phere, whereas past optical and ul- traviolet observations were only sen- sitive to very hot gas and plasma.” “We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmos- phere,” said co-author Keiichi Oh- naka of the Universidad Católica del Norte in Chile who previously ob- served Antares’ atmosphere in in- frared light. “Imagine that their atmospheres are a painting made out of many dots of different colors, representing different tempera- tures. Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared tel- escopes can see, and hot dots that UV telescopes see. At the moment we can’t observe these dots individ- ually, but we want to try that in fu- ture studies.” In the ALMA and VLA data, as- tronomers for the first time saw a clear distinction between the chro- mosphere and the region where winds start to form. In the VLA im- age, a huge wind is visible, ejected from Antares and lit up by its smaller but hotter companion star Antares B. “When I was a student, I dreamt of having data like this,” said co-author Graham Harper of the University of Colorado, Boulder. “Knowing the actual sizes and temperatures of the atmospheric zones gives us a clue of how these huge winds start to form and how much mass is being ejected.” “Our innate understanding of the night sky is that stars are just points of light. The fact we can map the at- mospheres of these supergiant stars in detail, is a true testament to tech- nological advances in interferome- try. These tour de force observations bring the universe close, right into our own backyard,” said Chris Carilli of the National Radio Astronomy Observatory, who was involved in the first observations of Betelgeuse at multiple radio wavelengths with the VLA in 1998. !