Hubble’s View of Dazzling Globular Star Cluster NGC 6397
Credit: NASA, ESA, and T. Brown and S. Casertano (STScI)
Credit: NASA, ESA, and T. Brown and S. Casertano (STScI)
Astronomers using NASA’s Hubble Space Telescope have for the first time precisely measured the distance to one of the oldest objects in the universe, a collection of stars born shortly after the big bang.
This new, refined distance yardstick provides an independent estimate
for the age of the universe. The new measurement also will help
astronomers improve models of stellar evolution. Star clusters are the
key ingredient in stellar models because the stars in each grouping are
at the same distance, have the same age, and have the same chemical
composition. They therefore constitute a single stellar population to
study.
This stellar assembly, a globular star cluster called NGC 6397, is
one of the closest such clusters to Earth. The new measurement sets the
cluster’s distance at 7,800 light-years away, with just a 3 percent
margin of error.
Until now, astronomers have estimated the distances to our galaxy’s
globular clusters by comparing the luminosities and colors of stars to
theoretical models, and to the luminosities and colors of similar stars
in the solar neighborhood. But the accuracy of these estimates varies,
with uncertainties hovering between 10 percent and 20 percent.
However, the new measurement uses straightforward trigonometry, the
same method used by surveyors, and as old as ancient Greek science.
Using a novel observational technique to measure extraordinarily tiny
angles on the sky, astronomers managed to stretch Hubble’s yardstick
outside of the disk of our Milky Way galaxy.
The research team calculated NGC 6397’s age at 13.4 billion years
old. “The globular clusters are so old that if their ages and distances
deduced from models are off by a little bit, they seem to be older than
the age of the universe,” said Tom Brown of the Space Telescope Science
Institute (STScI) in Baltimore, Maryland, leader of the Hubble study.
Accurate distances to globular clusters are used as references in
stellar models to study the characteristics of young and old stellar
populations. “Any model that agrees with the measurements gives you more
faith in applying that model to more distant stars,” Brown said. “The
nearby star clusters serve as anchors for the stellar models. Until now,
we only had accurate distances to the much younger open clusters inside
our galaxy because they are closer to Earth.”
By contrast, about 150 globular clusters orbit outside of our
galaxy’s comparatively younger starry disk. These spherical, densely
packed swarms of hundreds of thousands of stars are the first
homesteaders of the Milky Way.
The Hubble astronomers used trigonometric parallax to nail down the
cluster’s distance. This technique measures the tiny, apparent shift of
an object’s position due to a change in an observer’s point of view.
Hubble measured the apparent tiny wobble of the cluster stars due to
Earth’s motion around the Sun.
To obtain the precise distance to NGC 6397, Brown’s team employed a
clever method developed by astronomers Adam Riess, a Nobel laureate, and
Stefano Casertano of the STScI and Johns Hopkins University, also in
Baltimore, to accurately measure distances to pulsating stars called
Cepheid variables. These pulsating stars serve as reliable distance
markers for astronomers to calculate an accurate expansion rate of the
universe.
With this technique, called “spatial scanning,” Hubble’s Wide Field
Camera 3 gauged the parallax of 40 NGC 6397 cluster stars, making
measurements every 6 months for 2 years. The researchers then combined
the results to obtain the precise distance measurement. “Because we are
looking at a bunch of stars, we can get a better measurement than simply
looking at individual Cepheid variable stars,” team member Casertano
said.
The tiny wobbles of these cluster stars were only 1/100th of a pixel
on the telescope’s camera, measured to a precision of 1/3000th of a
pixel. This is the equivalent to measuring the size of an automobile
tire on the moon to a precision of one inch.
The researchers say they could reach an accuracy of 1 percent if they
combine the Hubble distance measurement of NGC 6397 with the upcoming
results obtained from the European Space Agency’s Gaia space
observatory, which is measuring the positions and distances of stars
with unprecedented precision. The data release for the second batch of
stars in the survey is in late April. “Getting to 1 percent accuracy
will nail this distance measurement forever,” Brown said.
The team’s results appeared in the March 20, 2018, issue of The Astrophysical Journal Letters.
The research team consists of T. Brown, S. Casertano, and D. Soderblom (STScI); J. Strader (MSU); A. Riess and J. Kalirai (STScI, JHU); D. VandenBerg (UVic); and R. Salinas (Gemini).
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy, in Washington, D.C.
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Contact
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Tom Brown
Space Telescope Science Institute, Baltimore, Maryland
410-338-4902
tbrown@stsci.edu
Source: HubbleSite/News
