Milky Way’s new mass calculated

It's 1.5 trillion solar masses

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ESO VLT Laser taking Milky Way photo
Image of the night sky above Paranal on 21 July 2007, taken by ESO astronomer Yuri Beletsky. A wide band of stars and dust clouds, spanning more than 100 degrees on the sky, is seen. This is the Milky Way, the galaxy to which we belong. Three of the four 8.2-m telescopes forming ESO's VLT are seen, with a laser beaming out from Yepun, Unit Telescope number 4. The laser points directly at the Galactic Centre. (Credits: ESO/Y. Beletsky)

The new calculation of the measurement of the size and mass of the Milky Way is more accurate and the galaxy turned out to be more massive than thought earlier.

The galaxy has been calculated to have a mass of about 1.5 trillion Sun’s worth of mass (solar masses) within a radius of around 129,000 light years. This calculation exceeds over twice as much as previous estimates of 2016’s study in which it was estimated to have around 700 billion solar masses.

To accurately map the Milky Way in three dimensions, ESA’s Gaia Mission has been launched. This mission has given the most detailed map of our home galaxy ever made and has been refining our knowledge all over the shop.

A search team has been able to infer the galaxy’s size and mass based on the orbital motion of groups of stars called globular clusters, out in the galactic halo by combining Gaia data with those from Hubble Space Telescope observations.

Hubble Telescope
The Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying STS-125, HST Servicing Mission 4. Image Credit: Wikimedia/ Ruffnax (Crew of STS-125)

With the dark matter in play, the mass of the Milky Way can’t just be guessed based on what we can see. And the dark matter cannot be detected directly. But there is an assumption that something is out there, because of the orbital velocity of the outer region of the galaxy.

The matter orbits much faster than it should, based on the matter that can be detected – as though something, some undetectable mass, is creating extra gravity in the Universe.

It is important to infer its mass based on other methods because the dark matter can’t be observed directly. By starting with that outer-galaxy orbital velocity, astrophysicists can work backward to calculate the mass responsible, based on Kepler’s laws of orbital motion.

On the same subject, Gaia and Hubble are dedicated to working. It has been 10 years since they have been combined. And they have provided more accurate measurements of the orbital motion of globular clusters in the outer reaches of the Milky Way.

“The more massive a galaxy, the faster its clusters move under the pull of its gravity,” said astrophysicist Wyn Evans of the University of Cambridge in the 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 velocity, and consequently the galactic mass, can be calculated.”

On this basis, the team reached the 1.5 trillion solar masses figure. But the thing is that there are only about 200 billion stars in the galaxy. Sagittarius A*, the supermassive black hole at the galactic center, accounts for another 4 million solar masses. And there’s a bunch of dust and gas. But all that concludes around 90% of the mass meaning, there is the dark matter that is yet to be found out.

“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,” explained physicist Roeland van der Marel of the Space Telescope Science Institute in the US.

The Milky Way galaxy has been noted to be in an intermediate range according to the new measurements put it at a pretty healthy size and mass for its class, but the extra heft doesn’t even put us near the biggest galaxies – those are in the range of 30 trillion solar masses.

For many years, the Milky Way has been thought to the biggest galaxy in nearby intergalactic space was Andromeda, with the Milky Way coming in second.

But according to Andromeda’s new calculations last year, the Milky Way was put to be at around 800 billion solar masses which could mean that it is actually number one – and has been all along.
And so, rather than the other way around as we previously thought, it could mean that Andromeda gets subsumed into the Milky Way when the pair collide in 4.5 billion years.

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