Free Astronomy Magazine May-June 2019

48 MAY-JUNE 2019 SPACE CHRONICLES servations made with the NASA/ESA Hubble Space Telescope, astron- omers have found that the Milky Way weighs in at about 1.5 trillion solar masses within a radius of 129,000 light-years from the galactic centre. Previous estimates of the mass of the Milky Way ranged from 500 billion to 3 trillion times the mass of the Sun. This huge uncer- tainty arose primarily from the dif- ferent methods used for measuring the distribution of dark matter — which makes up about 90% of the mass of the galaxy. “We just can’t detect dark matter directly,” ex- plains Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis. “That’s what leads to the present uncertainty in the Milky Way’s mass — you can’t measure ac- curately what you can’t see!” Given the elusive nature of the dark matter, the team had to use a clever method to weigh the Milky Way, which relied on measuring the ve- locities of globular clusters — dense star clusters that orbit the spiral disc of the galaxy at great distances. “The more massive a galaxy, the faster its clusters move under the pull of its gravity” explains N. Wyn Evans (University of Cambridge, UK). “Most previous measurements have found the speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight. However, we were able to also measure the sideways motion of the clusters, from which the total veloc- ity, and consequently the galactic mass, can be calculated.” by NASA/ESA Hubble & Gaia accurately weigh the Milky Way T he mass of the Milky Way is one of the most fundamental measurements astronomers can make about our galactic home. However, despite decades of intense effort, even the best available esti- mates of the Milky Way’s mass dis- agree wildly. Now, by combining new data from the European Space Agency (ESA) Gaia mission with ob- T his artist’s impression shows a computer generated model of the Milky Way and the accurate positions of the globular clusters used in this study surrounding it. Scientists used the measured velocities of these 44 globular clusters to determine the total mass of the Milky Way, our cosmic home. [ESA/Hubble, NASA, L. Calçada]