Astronomers using the NASA Hubble Space Telescope for the first time accurately measured the distance to one of the oldest objects in the universe, a collection of stars born shortly after the Big Bang.
This new criterion of refined distance provides an independent estimate for the age of the universe. The new measurement will also help astronomers improve stellar evolution models. Star clusters are the key ingredient in stellar models because the stars in each cluster are the same distance, have the same age and have the same chemical composition. Therefore, they constitute a unique stellar population to study.
This star cluster, a globular cluster called NGC 6397, is one of the closest groups to Earth. The new measurement establishes the distance of the cluster to 7,800 light years away, with only a margin of error of 3 percent.
So far, astronomers have estimated distances to globular clusters in our galaxy by comparing the luminosities and colors of stars with theoretical models, and the luminosities and colors of similar stars in the solar neighborhood. But the accuracy of these estimates varies, with uncertainties ranging from 10 to 20 percent.
However, the new measurement uses direct trigonometry, the same method used by pollsters, and as old as ancient Greek science. Using a novel observation technique to measure extraordinarily small angles in the sky, the astronomers managed to stretch the Hubble stick out of the disk of our Milky Way.
The research team calculated the age of NGC 6397 at 13.4 billion years. "The globular clusters are so old that if their ages and distances deduced from the models disappear a little, they seem to be older than the age of the universe," said Tom Brown of the Space Telescope Science Institute (STScI) in Baltimore. , Maryland, Hubble study leader.
Exact 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 more faith in the application of that model to more distant stars," Brown said. "The nearby star clusters serve as anchors for the stellar models, so far, we only had precise distances to the much younger open clusters within our galaxy because they are closer to Earth."
In contrast, about 150 globular clusters orbit outside the comparatively younger crashed disk of our galaxy. These spherical, densely packed swarms of hundreds of thousands of stars are the first settlers of the Milky Way.
Hubble astronomers used trigonometric parallax to nail the distance of the cluster. This technique measures the small apparent change in the position of an object due to a change in an observer's point of view. Hubble measured the apparent and small oscillation of the stars due to the movement of the Earth around the Sun.
To obtain the precise distance to NGC 6397, Brown's team used an intelligent method developed by astronomers Adam Riess, winner of the award Nobel, and Stefano Casertano of 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 a precise expansion velocity of the universe.
With this technique, called "space exploration", Hubble's Wide Field Camera 3 measured the 40-star parallax NGC 6397, taking measurements every 6 months for 2 years. Then, the researchers combined the results to obtain the accurate distance measurement. "Because we are seeing a lot of stars, we can get a better measure than simply observing individual Cepheid variable stars," said team member Casertano.
The small wobbles of these cluster stars were only 1/100 of a pixel in the telescope's camera, measured to an accuracy of 1/3000 of a pixel. This is equivalent to measuring the size of a car tire on the moon with an accuracy of one inch.
Researchers say they could reach 1 percent accuracy if they combine the Hubble distance measurement of NGC 6397 with the next results. Obtained from the European Space Agency's Gaia space observatory, which measures the positions and distances of the stars with unprecedented precision. The data release for the second batch of stars in the survey is at the end of April. "When you reach 1 percent accuracy you will nail this distance measure forever," Brown said.