A newfound object in the constellation Cetus may be the youngest globular cluster in the galaxy, says an astronomer in Chile. If so, it’s akin to a 30-year-old living in a retirement home.
Globulars are tight-packed clusters that typically boast hundreds of thousands of ancient stars. Most of the Milky Way’s 150 globular clusters belong to the halo, which surrounds the disk. The halo formed when the galaxy did, some 13 billion years ago, so astronomers observe its stars and clusters to probe the Milky Way’s distant past.
In 2002, Alan Whiting at Cerro Tololo Inter-American Observatory in Chile and his colleagues were searching for nearby galaxies in the southern sky. They spotted a sparse cluster of blue stars a few degrees west of Mira, a famous red star whose light varies.
Now, Giovanni Carraro of the University of Chile in Santiago has observed the cluster, named Whiting 1, with the 1-meter telescope at Cerro Tololo. In December, he measured the stars’ magnitudes and colors, then plotted them on a Hertzsprung-Russell (H-R) diagram — a graph comparing stellar brightness with stellar color. By matching the cluster’s diagram with models of how stars evolve, he estimated its distance, age, and complement of elements heavier than helium, which astronomers call metallicity.
Carraro says the cluster resides in the outer halo, far below the galaxy’s disk. The cluster is 150,000 light-years from the Sun and 160,000 light-years from the galactic center. Furthermore, the cluster lies 130,000 light-years south of the galactic plane. For comparison, the Sun is in the disk, only 27,000 light-years from the galactic center.
The location in the outer halo suggests Whiting 1 is a globular cluster. The outer halo houses several other sparse globulars, such as Palomar 1 in Cepheus and Terzan 7 in Sagittarius. In contrast, the Milky Way’s open star clusters, most of which are young, make their home in or near the galaxy’s disk.
The cluster is 1′ (1/60 of a degree) across. At Whiting 1’s distance, this works out to a diameter of about 45 light-years.
Carraro determined that Whiting 1’s stars possess only 6 percent of the Sun’s amount of iron. Such a low amount of iron is another sign of a globular cluster, not an open cluster.
Surprisingly, however, Carraro finds that the cluster is young — only 5 billion years old. That’s nearly the same age as the Sun, and much younger than the typical 13-billion-year-old globular. “[W]e here tentatively propose that Whiting 1 is the Milky Way’s youngest globular cluster so far known, although a clear explanation for its formation and evolution remains very challenging,” Carraro writes in a paper to appear in Astrophysical Journal Letters.
“I think this is a very interesting object,” says William Harris of McMaster University in Hamilton, Canada, who maintains an on-line database of all the Milky Way’s globulars. “But I wouldn’t stake my life on an age of 5 gigayears. It’s hard to tell the difference between a 5-gigayear, 8-gigayear, and 10-gigayear cluster for such a sparse object as this unless you’ve got very high-quality photometry.”
Unfortunately, because Whiting 1 is so sparse and distant, Harris says a telescope larger than the 1-meter one Carraro used is needed to pin down the cluster’s age. Harris also hopes astronomers measure its velocity, which might reveal whether it’s falling toward the center of the galaxy.
In the next month, Carraro plans to analyze observations obtained last week with the 3.5-meter WIYN telescope atop Kitt Peak in Arizona. The new observations should yield an improved H-R diagram and age estimate for the cluster.
Whiting 1 may have come from another galaxy. “If it is as young as 5 gigayears and it’s that far out in the halo,” says Harris, “I think the guess would probably be that it formed in a very-low-luminosity dwarf galaxy. You’d have to look for the traces of its parent dwarf around it.” Several Milky Way globular clusters, such as M54 in Sagittarius, also came from other galaxies.
Whiting 1 is the first globular cluster ever found in Cetus. Although Cetus is the fourth largest constellation, it has no other globulars. That’s because most globular clusters reside closer to the galactic center than does the Sun, whereas most objects in Cetus reside farther away.