For years, scientists have been trying to find a way to most accurately measure our home galaxy, the Milky Way and it seems that the astronomers have finally succeeded. The mass of the galaxy is now being measured using NASA’s Hubble Space Telescope and the Gaia satellite of European Space Agency.
The galaxy which is home to our solar system weighs about 1.5 trillion solar masses as per the latest measurements. To recall, one solar mass is equal to the mass of our Sun.
It must be noted that only a small percentage of this is credited to the roughly 200 billion stars in our Milky Way and is inclusive of a 4-million-solar-mass supermassive black hole situated right at the centre of it.
A great degree of the rest of that mass is concentrated in dark matter, which is an unseen and mystifying substance that mimics a scaffolding throughout the universe and retains the stars in their galaxies.
In early years, research that goes back several decades made use of a range of observational techniques which gave estimates for Milky Way’s mass – differing between 500 billion to 3 trillion solar masses. The now improved measurement is close to the middle of the said range.
“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,” Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore, Maryland told NASA.
“Not knowing the precise mass of the Milky Way presents a problem for a lot of cosmological questions.”
The new mass measurement puts Milky Way on the heavier side in comparison to other galaxies in the vast universe.
The lightest galaxies can be about a billion solar masses, whereas the heaviest can be 30 trillion, or 30,000 times bigger.
Our galaxy’s mass of 1.5 trillion solar masses is quite normal for a galaxy having a brightness such as ours.
Astronomers made use of Hubble and Gaia to estimate the three-dimensional movement of globular star clusters — which are isolated spherical islands that contain hundreds of thousands of stars each and orbit the centre of Milky Way.
While it’s not visible to us, dark matter happens to be the dominant form of matter in our universe, and it can weigh through its influence on visible objects such as the globular clusters.
The bigger a galaxy, the faster its globular clusters will move under the pressure of gravity.
The oldest measurements have been with the line of sight to globular clusters, so astronomers have the knowledge of the speed at which a globular cluster is approaching or retracting from our Earth.
Nevertheless, Gaia and Hubble keep track of the sideways motion of the globular clusters, through which a more reliable speed – and therefore gravitational acceleration – can be measured.
Both Gaia and Hubble observations are complementary. Hubble having a smaller field of view, can calculate fainter stars and hence is able to reach more distant clusters.
Gaia was specially built to generate an accurate three-dimensional map of astronomical objects scattered throughout the Milky Way and record their motions. It made estimating all-sky measurements which include many globular clusters.
The new study enhanced Gaia measurements for 34 globular clusters out to 65,000 light-years, while Hubble measurements of 12 clusters out to 130,000 light-years which were procured from images taken through a span of a decade.
When both Hubble and Gaia measurements are gathered as anchor points, say like pins on a map, astronomers are able to calculate the distribution of the Milky Way’s mass out to about 1 million light-years from our Earth.
“We know from cosmological simulations what the distribution of mass in the galaxies should look like, so we can calculate how accurate this extrapolation is for the Milky Way,” NASA quoted Laura Watkins of the European Southern Observatory in Garching, Germany, lead author of the combined Hubble and Gaia study, to be published in The Astrophysical Journal.
These measurements based on the accurate calculations of globular cluster motion from Hubble and Gaia made it possible for the researchers to calculate the mass of the entire Milky Way.
Globular clusters, the earliest homesteaders of our galaxy, carry the oldest known stars, which date back to a few hundred million years post the big bang, the monumental event which created the universe as we know it.
They formed before the building of the Milky Way’s spiral disk, where our solar system and Sun are located.
“Because of their great distances, globular star clusters are some of the best tracers astronomers have to measure the mass of the vast envelope of dark matter surrounding our galaxy far beyond the spiral disk of stars,” Tony Sohn of STScI, who led the Hubble measurements told NASA.
The international team of astronomers in this research comprise of Roeland van der Marel (Space Telescope Science Institute, and Johns Hopkins University Center for Astrophysical Sciences, Baltimore, Maryland), Laura Watkins (European Southern Observatory, Garching, Germany), N. Wyn Evans (University of Cambridge, Cambridge, United Kingdom) and Sangmo Tony Sohn (Space Telescope Science Institute, Baltimore, Maryland).
The Hubble Space Telescope is an assignment of international cooperation that brought together NASA and ESA (European Space Agency).
The Goddard Space Flight Center of NASA situated in Greenbelt, Maryland, controls the telescope.
The Space Telescope Science Institute (STScI) located in Baltimore, Maryland, holds Hubble science operations.
STScI is managed by the Association of Universities for Research in Astronomy in Washington, D.C. for NASA.