A loose group of stars that was known for over 180 years but never
before studied in detail has been discovered to be an important new tool
in the quest to understand the evolution of stars like the Sun, and in
the search for planets like Earth.
The cluster, known as Ruprecht 147 or NGC 6774, was first discovered in 1830 by British astronomer John
Herschel. Jaroslav Ruprecht rediscovered it in the 1960s and thus got
its current name.
When searching for planets with an Earth-like mass and an orbit that
allows liquid water to exist on the surface, astronomers often search
around stars the mass of the Sun and smaller. Jason T. Wright, an assistant professor of astronomy and astrophysics at
Penn State University, who conceived and initiated the research said, “The Ruprecht 147 cluster is very unusual and very important
astrophysically because it is close to Earth and its stars are closer to
the Sun’s age than those in all the other nearby clusters.”
Wright’s team has shown that Ruprecht 147 is 800 to 1,000 light-years
from Earth, which is so close that it is bright enough to be seen with
binoculars in late-summertime skies in the constellation Sagittarius. Although it appears to be relatively large on the sky, the cluster
can be difficult to spot because it is not very compact and it is
located in the densest, brightest region between Earth and the center of
the Milky Way galaxy.
To study the Ruprecht 147 cluster, which is much larger on the sky
than most objects astronomers study, Wright’s team had to use some
specialized, wide-field cameras -- including those on the MMT telescope
in Arizona and the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii
-- in order to get its many stars within the frame of view.
Wright’s team’s work has proven, for the first time, that the
Ruprecht 147 cluster is only a bit younger than the Sun on the
astronomical time scale. The stars in Ruprecht 147 are about 2.5-billion
years old, or about half the age of the Sun, and about the age the Sun
was when the first multicellular life emerged on Earth.
The team’s initial observations also have measured the distance to
Ruprecht 147, as well as the directions and velocities of its stars to
verify that they are moving together through space in three dimensions,
both across the sky and in the same angle away from Earth. These
observations confirm that these stars are members of a true cluster, not
just a random pattern on the sky. Wright’s team already has identified
100 stars as members of the cluster, and is working to find more.
