We ran this question in December 2023 and couldn’t believe no one got it right. So let’s try this again.
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FROM THE APRIL-JUNE ISSUE: We asked you to identify the cosmic feature pictured at right and explain how such features figure into research related to the expanding universe. We didn’t receive any responses, so we asked astronomer Frank Summers from the Space Telescope Science Institute in Baltimore to weigh in:
“The object is a red giant star observed by the Hubble Space Telescope. Red giants are extremely big and extremely luminous, such that the brightest can be observed in other galaxies. Importantly, the brightest red giants have the same absolute luminosity. [Note: absolute luminosity measures brightness as would be seen from a standard distance, while apparent luminosity measures brightness as seen from Earth.] Astronomers can study a galaxy’s stars to measure the apparent luminosity for the “tip of the red giant branch” (i.e., the maximum red giant brightness). By comparing the apparent luminosity to the known absolute luminosity, they determine a distance to the galaxy. Plotting those galaxy distances versus galaxy redshifts (measured using spectra) is the Hubble Diagram. The slope of a linear fit to that distance vs redshift plot is the Hubble parameter, which measures the expansion rate of the universe.”