Our sun makes up over 99.8% of the total mass of our solar system. But the sun is just one star, imagine a ball of around 1 million stars. You probably can’t, and for good reason, because such a ball’s size and mass would be beyond our visualising ability because we’ve seen nothing with our own eyes that could come even close to how big such a ball would be. Now, what if I told you that such a ball really existed and that two billion years ago, it was torn and stretched by our galaxy’s gravitational pull into what can only be considered a cosmic river?
Yes, there really does exist a river full of ancient stars, and it is 27000 light-years in length. For some comparison, the Earth’s longest river is the Nile, stretching 6650 km across Africa. This stellar river, known as the Phoenix Stream, is 38400000000000x the length of the Nile. Don’t look at me, I warned you.
When scientists decided to analyse the chemical makeup of the certain stars (known as red giants) inside the Phoenix Stream, they found them to be surprisingly low in metallicity (elements heavier than H and He), just 0.3-0.4% of our own Sun. This is of note because astronomers believe that star clusters such as the Phoenix Stream should not even be able to form and coalesce together without a minimum level of metallicity, called a ‘metallicity floor’.
So how does this cosmic river even exist?
Well, scientists believe that the only way the Milky Way could have reached its current size and mass would be through consuming densely-packed star clusters from the edges of other galaxies, or from interstellar space. After these clusters are consumed in their original form, all that remains are these stellar ‘rivers’ that make their way around the galaxy. So astronomers hypothesize that the original cluster of the current Phoenix Stream must have formed at a different time, at a different place, and under different conditions than the other star clusters we have observed, and it was snatched up by our galaxy and converted into a stellar river.
The search for others like Phoenix is on, and researchers hope sophisticated space observatories such as the European Space Agency’s Gaia spacecraft or the much-anticipated James Webb Space Telescope will be able to spur a surge in the discovery of similarly low-metal witnesses to our Milky Way’s volatile past.