How heavy is the Milky Way galaxy? It may seem impossible to get a sense of it from our tiny little planet, but astronomers have their ways. Now a team has used data from Hubble and the Gaia satellite to make the most accurate measurement yet of the mass of the galaxy by studying the motions of star clusters orbiting the Milky Way.
Previous estimates of the galaxy's mass give a pretty wide range of possibilities. The Milky Way has been thought to tip the scales at anywhere between 500 billion solar masses – where, as you'd probably guess, one solar mass equals that of the Sun – right up to 3 trillion solar masses.
The new figure falls smack-bang in the middle of that range – about 1.5 trillion solar masses. In the grand scheme of things, that makes the Milky Way kind of an upper-middle mass galaxy. The lightest are as slim as one billion solar masses, while the heaviest can top out at a whopping 30 trillion.
Interestingly, the vast majority of the Milky Way's mass is invisible. The hundreds of billions of stars in the galaxy only account for a few percent, and even the supermassive black hole at the center is just a drop in the ocean at 4 million solar masses.
The rest of it is tied up in the ever-mysterious dark matter. Although it's the most plentiful type of matter in the universe, it's effectively invisible because it doesn't interact with light and only barely with regular matter. So how do we know it's there at all? Through its gravitational influence on the stuff we can see.
And that's just how the astronomers managed to measure the Milky Way's mass. The heavier a galaxy is, the faster objects orbit it, so scientists can track the speed of such objects and work backwards to calculate the mass. In this case, the objects in question are globular clusters – ancient bunches of stars that orbit the Milky Way.
The team used data from Hubble and Gaia to measure how fast dozens of these clusters were moving across the sky. The two space telescopes have different advantages that balance each other out nicely: Hubble can only look at a relatively small area at once, but can see further away. Gaia meanwhile is designed to scan a much larger swathe of sky but does so more superficially.
The researchers examined Gaia measurements of 34 globular clusters as far as 65,000 light-years away, along with Hubble data on 12 clusters up to 130,000 light-years away. By combining these into anchor points, the team was able to estimate the distribution of the Milky Way's mass, extending out to almost 1 million light-years from Earth.
While it might seem like a useless fact to know that the Milky Way weighs about 1.5 trillion Suns, plenty of important measurements rely on it – measurements that improve our understanding of our own galaxy and others, as well as the universe as a whole.
"We want to know the mass of the Milky Way more accurately so that we can put it into a cosmological context and compare it to simulations of galaxies in the evolving universe," says Roeland van der Marel, an author of the study. "Not knowing the precise mass of the Milky Way presents a problem for a lot of cosmological questions."
The research was published in The Astrophysical Journal.
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