Astronomers using the NASA/ESA Hubble Space Telescope have for the first time precisely measured the distance to one of the oldest objects in the Universe — a globular cluster called NGC 6397, one of the closest such clusters to Earth. The new measurement sets NGC 6397’s distance at 7,800 light-years away, with just a 3% margin of error, and even provides an independent estimate for the age of the Universe.
This image shows the nearby globular cluster NGC 6397, located at a distance of 7,800 light-years in the southern constellation Ara. Image credit: NASA / ESA / T. Brown & S. Casertano, STScI / J. Anderson, STScI.
When you want to know the size of a room, you use a measuring tape to calculate its dimensions. But you can’t use a tape measure to cover the inconceivably vast distances in space.
And, until now, astronomers did not have an equally precise method to accurately measure distances to some of the oldest objects in the Universe — ancient groupings of stars outside the disk of our Milky Way Galaxy called globular clusters.
Estimated distances to Milky Way’s globular clusters were achieved by comparing the brightness and colors of stars to theoretical models and observations of local stars. But the accuracy of these estimates varies, with uncertainties hovering between 10% and 20%.
Using Hubble, Dr. Tom Brown of the Space Telescope Science Institute and co-authors were able to use straightforward trigonometry — the same method used by surveyors, and as old as classical Greek science — to precisely measure the distance to NGC 6397, one of the closest globular clusters to Earth.
They calculated NGC 6397 is 13.4 billion years old and so formed not long after the Big Bang.
“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,” Dr. Brown said.
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,” Dr. Brown noted.
“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 Milky Way’s comparatively younger starry disk.
These spherical, densely packed swarms of hundreds of thousands of stars are the first homesteaders of our Galaxy.
The 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, Dr. Brown and his colleagues employed an innovative method 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 (WFC3) gauged the parallax of 40 NGC 6397 cluster stars, making measurements every 6 months for 2 years.
The team 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,” said co-author Dr. Stefano Casertano, from Johns Hopkins University and the Space Telescope Science Institute.
The tiny wobbles of these cluster stars were only 1/100th of a pixel on Hubble’s WFC3 camera, measured to a precision of 1/3000th of a pixel.
“We could reach an accuracy of 1% if they combine the Hubble distance measurement of NGC 6397 with the upcoming results obtained from ESA’s Gaia space observatory, which is measuring the positions and distances of stars with unprecedented precision,” the scientists said.
“The data release for the second batch of stars in the survey is in April 2018.”
“Getting to 1% accuracy will nail this distance measurement forever,” Dr. Brown said.
The team’s findings are published in the March 20 issue of the Astrophysical Journal Letters (arXiv.org preprint).
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T.M. Brown et al. 2018. A High-precision Trigonometric Parallax to an Ancient Metal-poor Globular Cluster. ApJL 856, L6; doi: 10.3847/2041-8213/aab55a
