Free Astronomy Magazine May-June 2022

9 MAY-JUNE 2022 ASTRO PUBLISHING is not decisive in determining whether or not Earendel is a Popu- lation III star. In fact, Hubble gives its best in ultraviolet and visible light, while the bright peak of Earendel is in infrared. But Welch’s team has already gotten some observation time with the new Webb Infrared Space Telescope to investigate the starlight towards the end of this year. One of the instru- ments that will be used is the NIR- Spec (Near-Infrared Spectrograph), whose data will be decisive for defining the nature and chemical composition of Earendel. A concluding reflection from Welch: “Earendel existed so long ago that it may not have had all the same raw materials as the stars around us today. Studying Earendel will be a window in an era of the Universe that we are unfamiliar with, but that led to everything we do know. It’s like we’ve been reading a really interesting book, but we started with the second chapter, and now we will have a chance to see how it all got started.” L ensed star variabil- ity across observa- tions reveals that Earendel has re- mained consistently bright across 3.5 years of HST imaging. Panels a-d show WFC3/IR images of the lensed star (cir- cled in green) across four epochs. Panels a and b show epochs 1 and 2 respectively (2016-06-07 and 2016-07-17), taken as part of RELICS. Panels c and d show follow- up Hubble imaging taken in epochs 3 and 4 respectively (2019- 11-04 and 2019-11- 27). [Welch et al.] fore, the two virtual im- ages of the star are at least unresolved, if not perfectly superimposed (in a certain sense, the principle of this phe- nomenon recalls that of the Hartmann mask in photography). Thanks to the favorable gravitational lensing, the intensification of Earendel’s light reaches very high values: from at least 1,000 times to some tens of thousands of times. This wide range depends on the fact that near the critical curve (whose position can only be deduced ap- proximately) the inten- sity of the light varies dizzyingly. Even if Earendel’s light intensification factor were in the lower part of the considered range, it would still be a gigantic star (or al- ternatively a multiple system). Before characterizing the star as such, Welch’s team wanted to rule out that Earendel might be a star cluster, perhaps similar to the one that appears to be doubled along the Sunrise Arc, just before and just after the image of the star. The re- searchers ruled out this possibility by determining that the maximum ra- dius of the patch of light correspon- ding to Earendel is fewer than 0.36 parsecs, so at most just over a light year. No known star cluster is that small: the tiniest has a radius of about 0.7 parsecs (2.3 light years). By considering Earendel as a single star, the team calculated from its brightness an initial mass of be- tween 40 and 500 solar masses, therefore typical of an OB spec- tral class star, with a maximum surface temper- ature of 60,000 K. Although these stars shine with a decidedly blue light in the contemporary universe, Earen- del’s light is so red-shifted by the expansion of the universe that the Hubble data T his Space Sparks episode reveals how the NASA/ESA Hub- ble Space Telescope has established an extraordinary new benchmark. [Bethany Downer, Nico Bartmann, ESA, NASA] !