Left: The globular cluster tore and flowed around the Milky Way. Right: The red giants that helped astronomers understand the cluster.
James Josephides / Swinburne Astronomy
At some point in the distant past, a clot of stars hit the Milky Way, and the galaxy’s enormous gravitational effects tore them apart. Gravity molded the clot into a “big piece of spaghetti”, full of stars, that constantly flows around our galaxy.
New research, looking at this stream of spaghetti-like stars known as Phoenix, has shown that their origins are very unusual.
The study, published in the journal Nature on Wednesday, is part of a project to study stellar currents, such as Phoenix, in the Milky Way and is known as the “Spectroscopic Study of the Stellar Current.” The survey allowed the research team to focus on Phoenix, discovered in 2016 during the Dark Energy Survey that scanned the skies between 2013 and 2019.
Phoenix was once a nice, clean ball of stars, held together by the forces of gravity … until it got too close to the Milky Way and tore.
“Phoenix is a long, thin stream. It is 27,000 light-years long, but only 150 light-years wide,” says Geraint Lewis, an astronomer at the University of Sydney and one of the study’s authors. “So it is a great piece of spaghetti, which is a definitive sign that it has been totally shattered by the galaxy.”
The current, Lewis explains, comes from a globular cluster: giant ball-shaped collections of 100,000 to a few million stars orbiting the Milky Way in a region of space known as the “stellar halo.” The Milky Way is home to around 150 of these groups, and astronomers know them well.
But the globular cluster that was shattered billions of years ago to form the Phoenix Current is peculiar.
“This flow comes from a group that, to our understanding, should not have existed,” said Daniel Zucker, an astronomer at Macquarie University, Australia, in a press release.
The team examined the spectra of a dozen bright red giant stars contained in Phoenix to understand their chemistry and figure out what elements make them up. Most globular clusters that scientists know of contain heavier elements than hydrogen and helium, which increases their “metallicity.” In fact, astronomers studying groups believe that they have a “metallic floor” and that groups cannot form below this point.
But Phoenix consists of low-metallicity stars, which means that the cluster from which it originated extends below the metallicity floor.
“Its chemical enrichment is far below that of all the other globular clusters that exist,” says Lewis.
That suggests an ancient origin for the globular cluster, when the universe was quite young, and points to the type of environment in which the globular cluster could have formed, one that no longer exists. We have captured Phoenix towards the end of its life, knocked down by the gravity of the Milky Way in the thin stream of spaghetti we see today.
“If we could have looked at the Milky Way halo billions of years ago, we would have seen that there would be more objects like Phoenix,” explains Lewis. “Phoenix is the last of its kind.”
What fate awaits the stream? Eventually, it will disappear. The ever-present effects of gravity will begin to disperse the high concentration of stars and fade into the stellar halo. But, Lewis says, it takes a few hundred million to billion years to make another orbit of the Milky Way. So Phoenix will be circling for a while yet, and then, after another orbit or two, it will be gone.
If we had looked only a billion years later, we would never have known that it existed at all.
