How huge is the Milky Approach? It’s a straightforward query to ask, however a troublesome one to reply. Think about a single cell in your physique attempting to find out your whole mass, and also you get an thought of how troublesome it may be. Regardless of the challenges, a brand new examine has calculated an correct mass of our galaxy, and it’s smaller than we thought.
One solution to decide a galaxy’s mass is by what’s generally known as its rotation curve. Measure the velocity of stars in a galaxy versus their distance from the galactic middle. The velocity at which a star orbits is proportional to the quantity of mass inside its orbit, so from a galaxy’s rotation curve you possibly can map the operate of mass per radius and get a good suggestion of its whole mass. We’ve measured the rotation curves for a number of close by galaxies akin to Andromeda, so we all know the plenty of many galaxies fairly precisely.
However since we’re within the Milky Approach itself, we don’t have a terrific view of stars all through the galaxy. Towards the middle of the galaxy, there’s a lot fuel and mud we can’t even see stars on the far side. So as an alternative we measure the rotation curve utilizing impartial hydrogen, which emits faint mild with a wavelength of about 21 centimeters. This isn’t as correct as stellar measurements, however it has given us a tough thought of our galaxy’s mass. We’ve additionally regarded on the motions of the globular clusters that orbit within the halo of the Milky Approach. From these observations, our greatest estimate of the mass of the Milky Approach is a couple of trillion photo voltaic plenty, give or take.
This new examine relies on the third information launch of the Gaia spacecraft. It incorporates the positions of greater than 1.8 billion stars and the motions of greater than 1.5 billion stars. Whereas that is solely a fraction of the estimated 100-400 billion stars in our galaxy, it’s a massive sufficient quantity to calculate an correct rotation curve. Which is precisely what the workforce did. Their ensuing rotation curve is so exact, that the workforce might determine what’s generally known as the Keplerian decline. That is the outer area of the Milky Approach the place stellar speeds begin to drop off roughly in accordance with Kepler’s legal guidelines since nearly all the galaxy’s mass is nearer to the galactic middle.
The Keplerian decline permits the workforce to put a transparent higher restrict on the mass of the Milky Approach. What they discovered was shocking. The very best match to their information positioned the mass at about 200 billion photo voltaic plenty, which is a fifth of earlier estimates. Absolutely the higher mass restrict for the Milky Approach is 540 billion, that means that the Milky Approach is at the very least half as huge as we thought. Given the quantity of recognized common matter within the galaxy, this implies the Milky Approach has considerably much less darkish matter than we thought.
Reference: Jiao, Yongjun, et al. “Detection of the Keplerian decline in the Milky Way rotation curve.” arXiv preprint arXiv:2309.00048 (2023).
