First Image of the Surface of a Star with the Hubble Space Telescope^*

A direct image of the surface of a star, Betelgeuse (=α Ori; M2 Iab), has been obtained with the Faint Object Camera on the Hubble Space Telescope. Images in two ~300 Å-wide bands centered at 2550 and 2800 Å cover ~10 resolution elements on the stellar disk. The ultraviolet diameter of Betelgeuse of 125 ± 5 mas at 2500 Å (corresponding to 108 ± 4 mas, FWHM) is a factor of 2.2 larger than the optical diameter, thus indicating a substantially extended chromosphere in this supergiant. A single bright, unresolved area is present in the southwest quadrant of the disk (P.A. = 235°) in both images, with a peak amplitude of 1.3-1.8 as compared to the surrounding disk and a temperature differential in excess of 200 K. This feature may be the result of magnetic activity, atmospheric convection, or global pulsations and shock structures that heat the chromosphere. Spatially resolved spectroscopy of the Mg II λλ2795, 2802 doublet with the Goddard High Resolution Spectrograph suggests complicated dynamics, with outflowing material in the chromosphere indicated by the Mg II emission.