Where does the galaxy start and where does it end?

Our own galaxy The Milky Way is a so-called rod spiral galaxy. It consists of about 100-400 billion stars. Much of them surround the supermassive black hole in the center as a disc-like formation. This disc is about 200,000 light-years in diameter.

It is conceivable that the galaxy begins at one edge of the disk and ends at the other edge 200,000 light-years away. On the other hand, the black hole in the center is surrounded by a so-called central bulge that extends to either side of the flat disc. In addition, there are spherical constellations around our galaxy that extend far beyond the plane of the disk.

Measured in this way, the galaxy would start from the farthest spherical constellation and end at another spherical constellation on the opposite side of the disk.

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In addition to stars and gas, the Milky Way has a vast amount of dark matter, as do all other galaxies. In fact, there is much more dark matter in galaxies than visible matter, about 90 percent of the mass of the galaxy.

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All the visible matter in the galaxy, i.e. the disk, the central bulge, and the spherical constellations are located inside the halo of dark matter. The halo extends up to a million light-years from the center of the Milky Way.

Inside the dark halo are all the companion galaxies orbiting the Milky Way, such as the small and large clouds of Magellan. Determining the start and end points of our galaxy is therefore impossible from this point of view.

Defendant: Elina Lindfors

Academy Researcher, Finnish ESO Center and Docent, University of Turku.

Published in Science 5/2022.

Source: Tiede by www.tiede.fi.

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