Weighing the Milky Way

Weighing the Milky Way

Using data from NASA's Hubble Space Telescope and the European Space Agency's Gaia satellite, astronomers have determined the most accurate measurement of its mass: Our vast galaxy clocks in at 1.5 trillion solar masses. The new finding of 1.5 trillion solar masses puts the Milky Way among the universe's larger galaxies.

The new 1.5 trillion solar mass estimate fits right in with previous research on the subject that put the mass of our galaxy at 500 billion to 3 trillion solar masses. According to scientists, huge uncertainty in those estimates arose due to the use of different methods for measuring the distribution of dark matter within the galaxy.

"We just can't detect dark matter directly", said Laura Watkins, a researcher working at the European Southern Observatory in 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 accurately what you can't see!' This is exactly what an worldwide team of astronomers have done and instead of looking for something that has escaped all detection so far, they measured the velocities of globular clusters to work out the mass of the Milky Way.

"The more massive a galaxy, the faster its clusters move under the pull of its gravity" co-author N. Wyn Evans of the University of Cambridge says in another press release.

Scientists have previously been able to measure the speed at which a globular cluster approaches or recedes from Earth along our line of sight but this information alone is quite limiting.

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This artist's impression shows a computer generated model of the Milky Way Galaxy and the accurate positions of the globular clusters used in this study surrounding it. Watkins et al used the measured velocities of these 44 globular clusters to determine the total mass of the Milky Way. "With this method, we can measure the total mass of everything inside the Milky Way (the dark matter, gas, stars, planets, black holes), all added together".

Gaia mission aims to create a 3D map of Milky Way and to provide details about its composition and formation.

Estimating the mass of the Milky Way is tricky because most of the matter it contains is invisible. Image credit: NASA / ESA / Hubble / L. Calçada. When data from Hubble and Gaia was combined, it allowed the researchers to estimate the distribution of mass in the Milky Way out to 1 million light-years from Earth. It's hard to see it all at once, buried as we are within one of its spiral arms. That means there's a pretty large margin of error in the estimate, meaning the true mass of the Milky Way may be somewhere between 0.79 and 2.29 trillion solar masses-but the current estimate is a good start. The space telescope uses that to its advantage by measuring fainter stars and distant clusters. "By combining Gaia's measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way's mass in a way that would be impossible without these two space telescopes".

The April release of data included data on globular clusters as far away as 65,000 light years from Earth. "A lot of the time, we try to understand the universe by putting it in context of the Milky Way". Since Hubble has been observing some of these objects for ten years, it was also possible to accurately track the velocities of these clusters as well.

'We were lucky to have such a great combination of data, ' explained Roeland van der Marel of the Space Telescope Science Institute in the US.

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