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What “weighs” the Milky Way?

Formulated thus, the question is wrong: even a student of physics in the first year of high school knows that weight is the force of a unit (the attraction of a gravitational field of a mass), while for measuring one or astronomical objects is more correct to speak of outright mass.

And it is this measure – the mass of the Milky Way – a long time to be the subject of heated debate among astrophysicists. A new method developed by a group of researchers at McMaster University, in Ontario, Canada, could put a full stop to the controversy. According to their study, presented at a conference of the Canadian Astronomical Society of Winnipeg, our galaxy would contain a mass equal to that of 700 billion suns: less than previously thought. Than expected, however, the Milky Way contains a slightly larger amount of dark matter, the mysterious, invisible substance that pervades the universe according to physicists, wrapping like a cloud even our galaxy.

Determine the actual mass of the Milky Way may help scientists put it in a more accurate cosmological context. It appears that the evolution of a galaxy – how many stars are born and how often, which take the form, how long they last before they die – both related to its overall mass. In the past studies on the mass of the Milky Way they have given the disparate estimates: from the equivalent of 1.000 billion just to “only” 100 billion. In all of these measures it is included both visible matter – cosmic dust, planets, satellites, stars and dwarf galaxies that orbit around the main one – is the halo of dark matter. But the latter can be detected only through its gravitational effect on other objects, and it is particularly difficult to measure.

I have devised a method of studying the movements and speed of 89 globular clusters of stars very ancient groups that exist in various parts of the galaxy. I chose them because they are scattered at different distances, are relatively large and well-defined, and compared to single stars is easier to monitor their movements over time. Globular clusters orbit the center of the galaxy, and on them the effects of the gravitational field of dark matter are predictable. Adding this estimate to the notes of objects visible masses, I created a template determining precisely the mass of the entire galaxy at about 700 billion suns, siding so in the camp of those who see it as “light”.

According to the model, some of the objects that orbit the outskirts of the galaxy, such as some gaseous globular clusters and dwarf galaxies some widespread, are not actually related to its degree and then their mass should not be calculated from the total of the Milky Way. Since the visible mass of stars and planets of the galaxy is estimated at about 60 billion suns, and gas and dust are between one and three percent of the rest, I suggest that as many as 88 percent of the mass of the Milky Way composed of dark matter. The visible part of the Milky Way – that huge strip of stars and dust that we see across the night sky – is magnificent and extensive, but the idea that what we see is no more than a fifth of what there really inspires me to continue research to understand what we’re losing.


To open the video click on the image, good view from your Alessandro Brizzi.