<P> According to recent studies, the Milky Way as well as the Andromeda Galaxy lie in what in the galaxy color--magnitude diagram is known as the "green valley", a region populated by galaxies in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star - formation activity in green valley galaxies is slowing as they run out of star - forming gas in the interstellar medium . In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short - term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy . In fact, measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies . </P> <P> Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way . The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long - lived radioactive elements such as thorium - 232 and uranium - 238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology . These yield values of about 12.5 ± 3 billion years for CS 31082 - 001 and 13.8 ± 4 billion years for BD + 17 ° 3248 . Once a white dwarf is formed, it begins to undergo radiative cooling and the surface temperature steadily drops . By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made . With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years . Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years . </P> <P> Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80 - billion - year age of the Universe . In 2007, a star in the galactic halo, HE 1523 - 0901, was estimated to be about 13.2 billion years old . As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way . This estimate was made using the UV - Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R - process . The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology . Another star, HD 140283, is 14.5 ± 0.7 billion years old . </P> <P> The age of stars in the galactic thin disk has also been estimated using nucleocosmochronology . Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago . These measurements suggest there was a hiatus of almost 5 billion years between the formation of the galactic halo and the thin disk . Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before . </P>

Most x-ray emission in the milky way galaxy comes from