Free Astronomy Magazine May-June 2019

49 MAY-JUNE 2019 SPACE CHRONICLES The group used Gaia’s sec- ond data release as a basis for their study. Gaia was designed to create a pre- cise three-dimensional map of astronomical ob- jects throughout the Milky Way and to track their motions. Its second data release includes mea- surements of globular clusters as far as 65,000 light-years from Earth. “Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy,” said Tony Sohn (Space Telescope Sci- ence Institute, USA), who led the Hubble measure- ments. The team combined these data with Hubble’s unpar- alleled sensitivity and ob- servational legacy. Observations from Hubble allowed faint and distant globular clusters, as far as 130,000 light-years from Earth, to be added to the study. T his globu- lar clus- ter, NGC 4147, seen with the NASA/ESA Hubble Space Telescope, was one of many which were used by astronomers to measure the total mass of the Milky Way. NGC 4147 is lo- cated about 60,000 light- years from Earth in the northern con- stellation of Coma Berenices (the Berenice’s hair). [ESA/Hubble & NASA, T. Sohn et al.] T his video highlights the new estimate of the mass of our home galaxy the Milky Way by Hubble & Gaia. [NASA/ESA] As Hubble has been observing some of these objects for a decade, it was possible to accurately track the ve- locities of these clusters as well. “We were lucky to have such a great combination of data,” ex- plained Roeland P. van der Marel (Space Telescope Science Institute, USA). “By combining Gaia’s meas- urements of 34 globular clusters with measurements of 12 more dis- tant clusters from Hubble, we could pin down the Milky Way’s mass in a way that would be impossible with- out these two space telescopes.” Until now, not knowing the precise mass of the Milky Way has pre- sented a problem for attempts to answer a lot of cosmological ques- tions. The dark matter content of a galaxy and its distribution are intrin- sically linked to the formation and growth of structures in the Universe. Accurately determining the mass for the Milky Way gives us a clearer un- derstanding of where our galaxy sits in a cosmological context. !