Astronomers have discovered a mysterious stream of ancient stars to the far reaches of the galaxy: a strange stellar rock, so unlike any we’ve seen before, they may be the last of their kind.
This unusual collection of stars, called the Phoenix Stream, after the Phoenix constellation in which they are visible, is what is known as a stellar stream: an elongated chain of stars that existed in a spherical shape, known as a globular cluster.
Such clusters can be broken by the gravitational forces of the galaxy, and in this case their spherical shape becomes curved, extending into a ghostly caravan of stars destined for the distant orbit of a distant galactic nucleus.
Above: the artist’s impression of a stellar stream revolving around the Milky Way.
Neither stellar streams nor globular clusters are new to science, but there is something about the Phoenix stream. Its chemistry is different from any globular cluster we have ever seen, almost as it does not belong here.
“We can trace the origins of stars by measuring the different types of chemical elements we find in them, just as we can trace human connections to ancestors through their DNA,” explains astronomer Kyler Kuen of the Lowell Observatory in Arizona.
“It’s almost like finding someone with DNA that doesn’t match any other person, living or dead.”
There are about 150 known globular clusters in the Milky Way, all of which exist in what is called a galactic halo, a gloomy spherical structure that envelops a relatively flat galactic disk where most of the galaxy’s stars otherwise gather.
However, within the halo there are still many stars gathered inside the globular clusters. Each cluster can contain hundreds of thousands of stars, and observations of clusters in the Milky Way have shown that all clusters show a certain sequence in their stellar chemistry: the stars in the clusters are enriched with “heavier” chemical elements that are more massive than hydrogen and helium.
After the Big Bang, the theory is that all the gas in the universe consisted of either hydrogen or helium, which in turn formed the first stars in the universe. Other elements, such as oxygen, carbon and magnesium, became possible only later due to the mechanisms of synthesis of subsequent generations of stars.
The chemical legacy of these later synthesis mechanisms is all around us, as a certain proportion of heavier elements are observed in all globular clients of our galaxy. That is still.
This chemical threshold, called the metallic floor, is not subject to the Phoenix flux, which exhibits less heavy elements in its stars than we thought were theoretically possible for such a celestial structure.
“This flow comes from a cluster that we don’t think should have existed,” explains astronomer Daniel Zucker of Macquarie University in Australia.
Or at least it should not exist now, there may be another way to state this.
Observations of the Phoenix Stream, conducted by an international team of researchers in collaboration with a spectroscopic study of the South Star Stream, have shown that its “metal sufficiency is well below the empirical level of metallicity,” the authors explain in their new study.
Until now, the floor of metallicity has been a useful way of classifying the scientific constant observed in all modern spherical clusters. It still is, as it used to be – but the Phoenix stream is not a modern globular cluster.
The team believes it may be the only survivor: a celestial relic of a bygone age in the early universe, when stars made their light differently.
“One possible explanation is that the Phoenix Stream is the last of its kind, a remnant of a population of globular clusters born in radically different environments than what we see today,” said astronomer Ting Lee of Carnegie Observatory in Pasadena.
Of course, many questions remain. If the Phoenix Stream is a remnant of a relic from the early universe, is it the only one? Are there others hidden in the expanses of the galactic halo?
“In astronomy, when we find a new kind of object, it means that there are more of them,” said astronomer Jeffrey Simpson of the University of New South Wales (UNSW) in Australia.
If other old travelers are still on the trail, we will not find them forever. Like globular clusters, stellar streams are not immortal. After they stretch out to a series of stars, there is only time before they can scatter and scatter across the galaxy.
“Who knows how many relics like the Phoenix Stream can be hidden in the halo of the Milky Way?” wonders German astronomer JM Diderik Kruissen from the University of Heidelberg, who did not participate in the study, but wrote a comment to it.
“Now that the first one has been found, the hunt continues.”
The results of the work are reported in Nature.