Astronomers have hauled the Milky Way on to a cosmic scale of sorts, and found that our galaxy has as much mass as 1.5tn suns, give or take a few. The measurement, the most accurate yet, covers all the stars and planets, dust and gas, and the supermassive black hole that sits at the centre. It alone comes in at 4m times more massive than the sun. “This is absolutely everything there is in the Milky Way,” said Laura Watkins, an astronomer at the European Southern Observatory headquarters at Garching in Germany. “Knowing its total mass will help us understand the galaxy better.”
Scientists have sought to weigh the Milky Way for decades. Previous estimates have ranged wildly from 500bn to 3tn suns. The latest estimate is in the middle of that range, and translates into about 3,000 trillion trillion trillion tonnes. To weigh the galaxy, Watkins and her colleagues pooled data from Nasa’s Hubble space telescope and the European Space Agency’s Gaia satellite, an observatory that is recording the positions of stars to produce a 3D map of the Milky Way. They used the telescopes to measure the movement of 46 “globular clusters,” distinct groups of hundreds of thousands of stars that orbit the galaxy’s centre. By analysing the movement of the clusters, they were able to calculate the gravitational pull exerted on them, and the mass that was generating the attractive forces. From there they extrapolated outward to estimate the mass of the Milky Way to nearly 1m light years from Earth. As well as calculating its mass, the scientists worked out the size of the galaxy, arriving at a radius of 129,000 light years.
With a mass of 1.5tn suns, the Milky Way ranks among the beefier galaxies in the universe. The lightest are about 1bn times more massive than the sun, and the heaviest about 30tn times heavier. Based on the collective brightness of its stars, the Milky Way’s mass is fairly normal, according to a report due to be published in The Astrophysical Journal. Despite hosting about 200bn stars, the collective heft of the alien worlds makes up only 4% of the Milky Way’s mass. The planets that circle them account for a smaller fraction still. About 85% of the mass is dark matter, the elusive substance that clumps around galaxies, unseen apart from its gravitational pull. “One reason it’s important to measure the Milky Way is that we live here, it’s the closest galaxy we have,” Watkins said. “A lot of the time, we try to understand the universe by putting it in context of the Milky Way.”
Credit: Ian Sample for The Guardian, 7 March 2019.