Astronomers have discovered a mysterious stream of ancient stars on the distant edges of the galaxy – a strange star race so unlike any we’ve seen before, they may well be the last of their kind.
This unusual collection of stars, called the ‘Phoenix current’, after the constellation Phoenix in which they are visible, is what is known as a stellar current – an elongated chain of stars that used to exist in a spherical shape, known as a globular cluster.
Such clusters can be torn apart by the gravitational forces of a galaxy, in which case their globular shape is deformed, spreading out into a phantasmagoric caravan of stars, destined to distant orbit a distant galactic nucleus.
(James Josephides / Swinburne Astronomy Productions / S5 Collaboration)
Above: Artist’s impression of the stellar current enveloping the Milky Way.
Neither star currents nor globular clusters are new to science, but there is something about the Phoenix current that is. Their chemistry is unlike any globular group we’ve ever seen, almost as if it didn’t belong here.
“We can trace the lineage of stars by measuring the different types of chemical elements we detect in them, just as we can trace a person’s connection to their ancestors through their DNA,” explains astronomer Kyler Kuehn of the Lowell Observatory. in Arizona.
“It’s almost like finding someone with DNA who doesn’t match anyone else, dead or alive.”
There are about 150 known globular clusters in the Milky Way galaxy, all of which exist in what is called the galactic halo, a faint spherical structure that envelops the relatively flat galactic disk, where most of the stars in a galaxy congregate.
However, on the fringes of the halo, there are still many stars gathered within globular clusters. Each group can contain hundreds of thousands of stars, and group observations in the Milky Way galaxy have shown that all groups demonstrate a certain consistency in their stellar chemistry: the stars in the groups are enriched with ‘heavier’ chemical elements than they are more massive than hydrogen and helium.
After the Big Bang, the theory holds that all gas in the Universe was made up of hydrogen or helium, which in turn formed the first stars in the Universe. Other elements, such as oxygen, carbon, and magnesium, were only made possible much later through the fusion mechanisms of later generations of stars.
The chemical legacy of those later fusion mechanisms is all around us, as a certain proportion of heavier elements has been observed in all the globular clients of our galaxy. That is, until now.
The Phoenix current does not obey this chemical threshold, called the metallic floor, which shows less heavy elements in its stars than we think was theoretically possible for such a celestial structure.
“This sequence comes from a group that, to our understanding, should not have existed,” explains astronomer Daniel Zucker of Macquarie University in Australia.
Or at least, it shouldn’t exist nowIt could be another way of saying it.
Observations of the Phoenix Stream by an international team of researchers as part of the South Star Stream Spectroscopic Study Collaboration have revealed that its “abundance of metals is substantially below the empirical metallicity floor,” the authors explain in his new study.
Until now, metallicity flooring was a useful way to classify a scientific constant seen in all current globular clusters. It still is, as it happens, but the Phoenix current is not a current globular cluster.
The team thinks it may be a single survivor: a celestial relic from a bygone era in the early Universe, when stars made their light in different ways.
“One possible explanation is that the Phoenix Current represents the latest of its kind, the remnant of a population of globular clusters that was born in radically different environments than what we see today,” says astronomer Ting Li of Carnegie Observatories in Pasadena.
Many questions remain, of course. If the Phoenix stream is a remnant of a relic from the early Universe, is it the only one? Are there others, too, hidden in the vastness of the galactic halo?
“In astronomy, when we find a new type of object, it suggests that there are more of them,” says astronomer Jeffrey Simpson of the University of New South Wales (UNSW) in Australia.
If other old travelers are still on the road, we don’t have to find them forever. Like globular clusters, star currents are not immortal things. Once they spread out into a chain of stars, it is only a matter of time before they separate and spread across the galaxy.
“Who knows how many relics like the Phoenix Stream could be hidden in the Milky Way halo?” asks the German astronomer JM Diederik Kruijssen, from the University of Heidelberg, who was not involved in the study but wrote a comment about it.
“Now that the first one has been found, the hunt is on.”
The findings are reported in Nature.
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