CAPE CANAVERAL, Fla.—NASA’s new space telescope opened its huge, gold-plated, flower-shaped mirror Saturday, the final step in the observatory’s dramatic unfurling. The last portion of the 21-foot mirror swung into place at flight controllers’ command, completing the unfolding of the James Webb Space Telescope. “I’m emotional about it. What an amazing milestone. We see that beautiful pattern out there in the sky now,” said Thomas Zurbuchen, NASA’s science missions chief. More powerful than the Hubble Space Telescope, the $10 billion Webb will scan the cosmos for light streaming from the first stars and galaxies formed 13.7 billion years ago. To accomplish this, NASA had to outfit Webb with the biggest and most sensitive mirror ever launched—its “golden eye,” as scientists call it. Webb is so big that it had to be folded orgami-style to fit in the rocket that soared from South America two weeks ago. The riskiest operation occurred …