Applying this method to gas in the Milky Way or to old stars is less precise, however, due to assumptions about the primordial abundances of various isotopes. These calculations pointed to a universe between 12 and 15 billion years old, with a large uncertainty of plus or minus 3 to 4 billion years.
Alternatively, astronomers measured the ages of white-dwarf stars, the shrunken remnants of stars that are as heavy as the Sun but only as large as Earth. By finding the faintest, and thus oldest, white dwarfs, astronomers estimated how long they have been cooling.
Comprehensive attempts at cataloging white dwarfs and measuring their ages yielded about 10 billion years for the age of the Milky Way’s disk. The galaxy’s disk formed about 2 billion years after the Big Bang, yielding an age of the universe of about 12 billion years.
Measuring the ages of ancient star clusters offers yet another avenue for exploring the age of the universe. By looking at the most luminous stars in a globular cluster, astronomers can determine an upper limit for the cluster’s age. They look at the brightest stars on the so-called main sequence — the primary track on a plot of stellar brightnesses versus temperatures.
Such studies of numerous globulars, based on distance measurements provided by the European Space Agency’s Hipparcos and Gaia missions, suggested an age for many of the oldest stars of around 13 billion years. And astronomers think the age of globulars gives a pretty good indication for the age of the universe. That’s because globulars contain hardly any elements heavier than hydrogen and helium, and so had to be among the first objects to form.
Any discrepancies narrowed significantly with the release of WMAP data, before essentially disappearing when researchers announced Planck’s latest findings in 2015. By carefully examining the microwave background radiation, astronomers have pinned down the universe’s age to 13.8 billion years, accurate to better than 1 percent. The results pretty much ended the debate, but what a debate it was.