We know our Milky Way galaxy is big, but how big has been something of an open question. In the past, estimates of the total mass ranged from 500 billion to 3 trillion solar masses. It was difficult to pin down because of the unmeasurable dark matter making up the majority of the galaxy’s mass. Researchers from the European Southern Observatory (ESO) have managed to accurately measure the galaxy’s mass including all the dark matter. They say the Milky Way weighs in at a hefty 1.5 trillion solar masses.
The problem scientists have always encountered when trying to measure the galaxy is that dark matter could account for as much as 85 percent of all matter. We can’t detect dark matter with any known mechanism, as it doesn’t emit or absorb energy on the electromagnetic spectrum. But we know it’s there because of the gravitational effects on surrounding matter. For example, calculations show that galaxies would fly apart instead of rotating without all the hidden mass of dark matter.
The ESO team came up with a clever workaround to measure the Milky Way’s mass without directly observing dark matter. They employed NASA’s Hubble Space Telescope and Europe’s Gaia spacecraft to study the movement of globular clusters orbiting the Milky Way. These clumps of stars are part of the Milky Way in some ways, but they also orbit independently. The team looked at 46 globular clusters — 34 of them with Gaia and 12 with Hubble. The most distant was about 129,000 light years away from Earth.
The globular star cluster 47 Tucanae photographed by the Hubble Space Telescope.
The velocity of the clusters has a direct relationship with the mass of the Milky Way (more mass makes clusters orbit faster), thus the team arrived at a total mass of 1.5 trillion suns. Technically, that’s the total mass present within 129,000 light-years of the galactic center. The disk of the Milky Way is about 100,000 light years across, but it’s smack in the middle of the range predicted in past studies.
Having an accurate measurement of the galaxy’s mass is necessary to better our understanding of the universe. The Milky Way is just one galaxy, but it’s the one we can study best. The distribution and effects of dark matter relate to the formation of structures in the universe like the Milky Way, but also distant galaxies stretching back to the dawn of the time. These objects should work much like our own galaxy, so knowing its mass is an essential piece of the puzzle.
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