On the outskirts of the Milky Way, tiny stellar systems hide, so faint that even powerful telescopes barely spot them. Until recently, astronomers knew almost nothing about these objects, dubbed ultra-faint compact satellites. Now, a team of researchers has for the first time measured their velocities and chemical compositions using spectrographs — instruments that split starlight into colors, much as a prism breaks sunlight into a rainbow.
It turns out that the stars inside these systems move more slowly than those in larger dwarf galaxies. This suggests they likely lack dense clumps of dark matter — the invisible substance that holds stars together. At the same time, the chemical compositions of the systems are remarkably diverse: the iron content in the stars varies by a factor of 300. Some systems contain so few heavy elements that it calls into question their origin as ordinary star clusters.
Perhaps these tiny objects are ancient "building blocks" left over from the era when large galaxies assembled billions of years ago. Each such system is like a frozen drop of time, recording the history of the early Universe. By studying them, astronomers are essentially reading the first chapters of the cosmic chronicle, written long before the Sun and Earth existed.
Some of these systems are so faint that their combined light rivals that of a single bright star, even though they may contain hundreds of stars.
In Isaac Asimov's novel "The Gods Themselves," unusual stellar systems are described that turn out to be quite different from what they seem, just like these mysterious satellites.