But only 60 billion are detectable. Everything else consists of mysterious dark matter.
By A. J. Oliveira
Dec 13, 2019, 7:53 PM
There is no difficulty in weighing things like a bunch of bananas or our own body: just position the object on a scale. But how do you find out the mass of the Milky Way? Determining the mass of celestial bodies is often a very complicated task for astronomers, especially in the case of our galaxy, where the perspective we have is skewed – after all, we are embedded in it.
An international team of researchers used a sophisticated method to get a new and more accurate estimate of the Milky Way mass. Its size had already been well defined by previous research, which today speaks of a diameter of 256,000 light years. The recent study determined that its mass is around 890 billion times the mass of the sun.
That gives about 1.7 trillion pounds (one trillion equals number 10 followed by 42 zeros).
Since there is no cosmic-sized scale – and scales only work on the Earth's surface, where they are calibrated for gravity – astronomers had to use very complex models that took several factors into account. All to better visualize the look of our galaxy and the behavior of the stars and gases it contains – which are largely invisible to us.
The first step was to understand how these materials move, which scientists call the rotation curve. The Milky Way is not like a carousel, where everything rotates in a fixed and orderly manner. Some objects run faster, others slower. Therefore, the research approach, submitted to the JCAP journal with manuscript already on arXiv, was to characterize this locomotion to reveal the distance between objects and, finally, the mass.
This is possible because rotation is related to the gravitational forces to which objects are subjected – and from gravity it is possible to calculate mass. Each star and gas molecule in the galaxy is in a constant tug of war between the gravitational pull of the central black hole and the push of centrifugal force produced by the speed of rotation, which attempts to expel objects into the intergalactic environment.
There is a balance, a balance between these two forces – as with a child who holds on to a spinner so as not to fall backwards.
This is why most bodies are neither engulfed in the black hole nor thrown out into the universe. And it was precisely by calculating this “swing state” that the team was able to figure out the mass of each object in the galaxy. It would be just adding them to get the final result, were it not for the infamous dark matter. This mysterious and invisible substance is estimated to be 93% of the Milky Way mass.
Since dark matter does not interact with light, but has mass and its gravity can be clearly observed, it had to be included in the bill. With this, in addition to completing the full weight of the Milky Way, the researchers were also able to estimate the mass of dark matter it contains: 830 billion times the mass of the sun. That is: only 60 billion is something we know. Everything else remains shrouded in mystery.