Perceiving the Imperceptible
The Orbits of Stars and the Search for Dark Matter
Look up at the sky on a clear night, and you can see a million stars — distant, colossal giants embedded in spinning galaxies, many of them hosting planetary collections not unlike our own solar system. As a species, we have been staring up at these stars for thousands of years, drawing our gods among the constellations and trusting their positioning to guide us home at sea. It’s hard for us to understand, then, just how insignificant they are in the vastness of space: normal matter — planets, stars, and galaxies — make up only fifteen percent of matter in the universe.
The remaining 85 percent is composed of an invisible substance known as dark matter. It is currently impossible to directly observe or measure and can only be detected because its presence has a significant impact on the gravitational forces within galaxies. The only way its properties can be studied is by observing the systems it impacts. A recent study in the journal Nature Astronomy, published by astronomers from the University of Groningen in the Netherlands, showed that this was possible by measuring the proper motion of stars in a galaxy called Sculptor. As dwarf spheroidal galaxies like Sculptor are thought to be some of the most dark matter dominant systems in the universe, the movement of their stars can tell scientists a lot about the distribution and nature of the dark matter around them.
Proper motion refers to a star’s changing position in our sky, and it is incredibly difficult to measure. Even with the best equipment, it takes years to detect any movement from our perspective. The data used in this study was collected by the Hubble Space Telescope and the Gaia space observatory, some 12 years apart (2002 and 2014–15, respectively). The precision of the measurements allowed the researchers to track the motion of a star across the sky even when its distance was comparable to “less than the size of a pinhead on the Moon as seen from Earth”, the study’s lead author Davide Massari told Phys.org in November. This amount of movement, although not even remotely visible to the naked eye, was enough for the team to calculate exact three-dimensional motion for 15 stars. This marked an important turning point in the study of proper motion: it is the first time this kind of movement has been measured with such great accuracy on stars outside of the Milky Way galaxy. The results, which suggest that some standard models for dark matter and the movement of stars may be invalid, will help scientists studying even more distant galaxies in the future as they attempt to unravel the enigma of dark matter.
