$\begin{array}{ll}& \text{As astronomers gaze into the depths of space, they}\\ & \text{do so with unease: They don’t know precisely what}\\ & \text{the universe is made of.}\\ & \text{Surprisingly, no one knows the stars’ exact}\\ 5& \text{chemical composition: how many carbon, nitrogen}\\ & \text{and oxygen atoms they have relative to hydrogen, the}\\ & \text{most common element.}\\ & \text{These numbers are crucial, because they affect}\\ & \text{how stars live and die, what types of planets form and}\\ 10& \text{even how readily life might arise on other worlds.}\\ & \text{Twenty years ago, astronomers expressed}\\ & \text{confidence in the numbers they had been working}\\ & \text{with. Now, not so much. The problem lies not in the}\\ & \text{far corners of the cosmos, but much closer to home.}\\ 15& \text{Astonishingly, scientists don’t know exactly what the}\\ & \text{sun is made of. As a result, they don’t know what the}\\ & \text{other stars are made of, either.}\\ & \text{“The sun is a fundamental yardstick,” says Martin}\\ & \text{Asplund, an astrophysicist at the Max Planck}\\ 20& \text{Institute for Astrophysics, in Germany. “When we}\\ & \text{determine the abundance of a certain element in a}\\ & \text{star or a galaxy or a gas cloud anywhere in the}\\ & \text{universe, we use the sun as a reference point.”}\\ & \text{The sun’s location in the Milky Way also makes it}\\ 25& \text{a good representative of the entire galaxy. Most stars}\\ & \text{reside in giant galaxies like the Milky Way, which}\\ & \text{makes the sun a touchstone for the entire cosmos.}\\ & \text{For nearly a century, astronomers have judged}\\ & \text{stars normal or not by seeing whether their chemical}\\ & \text{compositions match the sun’s. Most stars near us do;}\\ & \text{some don’t.}\end{array}$

Scientific American. 1 July 2020. Adaptado.

Conforme o texto, um critério tradicionalmente utilizado por astrônomos para avaliar estrelas envolve