Star cluster with similarities to sun’s composition offers clues on Milky Way’s evolution
BLOOMINGTON, Ind. — New observations of the richly populated
star cluster NGC 2420 taken by the refurbished WIYN 0.9-meter
telescope at Kitt Peak National Observatory suggest that the cluster
contains a multitude of clues about the history and evolution of the
Milky Way.
Gathered as part of a long-term project called the WIYN Open
Cluster Study, the new photometric (brightness) data of NGC 2420
indicate that the cluster lies 35 percent farther from the center of
the Milky Way galaxy than does the sun. But in contrast to the sun,
which resides within the main disk of the galaxy about 25,000
light-years from the center, this cluster lies about 3,000
light-years above the galactic disk.
One surprising finding presented today (June 3) in Albuquerque,
N.M., at the meeting of the American Astronomical Society is that the
cluster’s average composition seems to be very similar to the
sun’s.
"It is unclear how a cluster that is as rich in chemical
elements as the sun is can have such a location and motion around the
galaxy. Such clusters usually lie right in the galactic disk,"
said lead author Emily Freeland of Indiana University. Indiana is one
of the major partners in the two Wisconsin-Indiana-Yale-National
Optical Astronomy Observatory (WIYN) telescopes on Kitt Peak in
Arizona.
One possibility is that NGC 242tuted form in the disk of the
Milky Way, but then got kicked out by a close encounter with a
massive object such as a giant molecular cloud. However, it is not
readily apparent that such an encounter could throw NGC 2420 so far
above the galactic disk.
Another possibility is that NGC 2420 originally belonged to
another small galaxy that was later absorbed by the Milky Way as our
galaxy grew. "One problem with this idea is that small galaxies
of this type are generally believed to be less rich in chemical
elements than the sun. How could this little galaxy generate such a
high abundance of heavy elements?" said Constantine Deliyannis,
a WIYN Open Cluster Study team member from Indiana University and
Freeland’s research adviser.
The combination of NGC 2420’s composition and its relatively
rapid motion remains puzzling, Deliyannis added. Solving this puzzle
may ultimately yield clues about how our Milky Way galaxy was put
together and how it evolved.
The Open Cluster Study team determined the cluster to be roughly
1.7 billion years old, or about one-third of the age of the sun.
Although this is only 15 percent of the age of the Milky Way, most
star clusters do not survive this long because they get torn apart by
gravitational tidal forces from the galactic disk.
NGC 2420 consists of approximately 1,000 stars packed into a
roughly spherical volume of space just 30 light-years in diameter.
Because all of the stars in a star cluster have similar ages and
initial compositions, star clusters can be used as a laboratory to
study many different areas of astronomy.
For example, stars of different initial masses evolve at
different rates. The new data on NGC 2420 delineate stars in several
distinct stages of life: a very clear main sequence (stars that are
converting hydrogen into helium in their cores, like the sun);
subgiant and giant branches (stars that have run out of core
hydrogen); a red clump (giant stars converting helium into carbon in
their cores); and blue stragglers (somewhat mysterious stars which
may represent a past merger of two stars into a single star).
There is also evidence for a significant population of binary
stars in NGC 2420, with a surprisingly large fraction of these
binaries being twins of roughly equal mass. Other star clusters don’t
seem to have nearly as many twins, Deliyannis said. This interesting
result, together with studies of the general distribution of binary
mass fractions, will teach us about the environments in which stars
and star clusters form.
The new data on NGC 2420 contain very precise information about
the surface temperatures of the stars that were observed, which is
crucial for accurate interpretation of spectra of these same stars
taken at the larger 3.5-meter WIYN telescope nearby on Kitt Peak.
Together, data from both WIYN telescopes may ultimately reveal
whether stellar rotation has caused the interior of these stars to
mix over time and, if so, to what extent, Deliyannis said. This
distribution of internal composition has implications for
understanding the chemical enrichment history of our galaxy, and for
testing Big Bang nucleosynthesis.
Other co-authors of display poster 9.01 in the Southwest Exhibit
Hall of the AAS meeting are Aaron Steinhauer (Indiana University) and
A. Sarajedini (University of Florida).
An image of NGC 2420 taken by the WIYN 0.9-meter telescope and
the full text of this release are available at http://www.noao.edu/outreach/press/pr02/pr0204.html.
The WIYN consortium assumed operation of the historic 0.9-meter
telescope on Kitt Peak in fall 2001 to enable the telescope to serve
as a complement to the existing 3.5-meter WIYN telescope on Kitt
Peak, and to offer a facility for hands-on learning and research by
students at the three partner universities.
For example, Freeland did this research as part of her
undergraduate honors thesis at Indiana University, and she will
remain associated with WIYN through pending graduate study in
astronomy at the University of Wisconsin-Madison.
More information about the transfer of the 0.9-meter telescope
to the WIYN consortium is available at http://www.noao.edu/outreach/press/pr01/pr0107.html.
Located southwest of Tucson, Ariz., Kitt Peak National
Observatory is part of the National Optical Astronomy Observatory,
which is operated by the Association of Universities for Research in
Astronomy under a cooperative agreement with the National Science
Foundation.
Media Contacts:
Constantine DeliyannisIU Astronomy Department
812-856-5197
con@astro.indiana.edu
Douglas IsbellNOAO
520-318-8214
disbell@noao.edu
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