Nav: Home

Hubble makes the first precise distance measurement to an ancient globular star cluster

April 04, 2018

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.

NASA's Hubble Portal:

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514 /

Tom Brown
Space Telescope Science Institute, Baltimore, Maryland

NASA/Goddard Space Flight Center

Related Hubble Space Telescope Articles:

New way to weigh a white dwarf: Use Hubble Space Telescope
For the first time, astronomers have used a novel method to determine the mass of a type of star known as a 'white dwarf' -- the shrunken corpse of a dead star that used to be like our sun.
NASA's James Webb space telescope completes acoustic and vibration tests
At NASA's Goddard Space Flight Center in Greenbelt, Maryland the James Webb Space Telescope team completed the acoustic and vibration portions of environmental testing on the telescope.
Probing seven worlds with NASA's James Webb Space Telescope
With the discovery of seven earth-sized planets around the TRAPPIST-1 star 40 light years away, astronomers are looking to the upcoming James Webb Space Telescope to help us find out if any of these planets could possibly support life.
NASA restarts rigorous vibration testing on the James Webb Space Telescope
Testing on the James Webb Space Telescope successfully resumed last week at NASA's Goddard Space Flight Center, in Greenbelt, Md.
Robot would assemble modular telescope -- in space
A new concept in space telescope design uses a modular structure and an assembly robot to build an extremely large telescope in space, performing tasks in which astronaut fatigue would be a problem.
Science instruments of NASA's James Webb Space Telescope successfully installed
With surgical precision, two dozen engineers and technicians successfully installed the package of science instruments of the James Webb Space Telescope into the telescope structure.
James Webb Space Telescope's golden mirror unveiled
NASA engineers recently unveiled the giant golden mirror of NASA's James Webb Space Telescope as part of the integration and testing of the infrared telescope.
Earth-space telescope system produces hot surprise
Combining an orbiting radio telescope with telescopes on Earth made a system capable of the highest resolution of any observation ever made in astronomy.
NASA marks major milestones for the James Webb Space Telescope
NASA's James Webb Space Telescope just got a little closer to launch with the completion of cryogenic testing on its science cameras and spectrographs and the installation of the final flight mirrors.
NASA's James Webb Space Telescope secondary mirror installed
The sole secondary mirror that will fly aboard NASA's James Webb Space Telescope was installed onto the telescope at NASA's Goddard Space Flight Center in Greenbelt, Maryland, on March 3, 2016.

Related Hubble Space Telescope Reading:

Hubble's Universe: Greatest Discoveries and Latest Images
by Terence Dickinson (Author)

The Hubble Cosmos: 25 Years of New Vistas in Space
by David H. Devorkin (Author), Robert W. Smith (Author), Robert P. Kirshner (Foreword)

NASA Hubble Space Telescope - 1990 onwards (including all upgrades): An insight into the history, development, collaboration, construction and role of ... space telescope (Owners' Workshop Manual)
by David Baker (Author)

The Hubble Space Telescope: From Concept to Success (Springer Praxis Books)
by David J. J. Shayler (Author), David M. Harland (Contributor)

Expanding Universe: Photographs from the Hubble Space Telescope
by Owen Edwards (Contributor), Zoltan Levay (Contributor)

Hubble Space Calendar - Hubble Space Telescope Calendar - Calendars 2018 - 2019 Wall Calendars - Spaced Out 16 Month Wall Calendar by Avonside
by MegaCalendars (Author)

Hubble: Imaging Space and Time
by David H. Devorkin (Author), Robert Smith (Author)

Space: Views from the Hubble Telescope 2019 Wall Calendar
by Pomegranate Communications Inc. (Author), Scientific American (Illustrator)

Hubble Telescope, The (Blastoff! Readers: Exploring Space) (Exploring Space: Blastoff! Readers, Level 3)
by Derek Zobel (Author)

Space, Stars, and the Beginning of Time: What the Hubble Telescope Saw
by Elaine Scott (Author)

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Approaching With Kindness
We often forget to say the words "thank you." But can those two words change how you — and those around you — look at the world? This hour, TED speakers on the power of gratitude and appreciation. Guests include author AJ Jacobs, author and former baseball player Mike Robbins, Dr. Laura Trice, Professor of Management Christine Porath, and former Danish politician Özlem Cekic.
Now Playing: Science for the People

#509 Anisogamy: The Beginning of Male and Female
This week we discuss how the sperm and egg came to be, and how a difference of reproductive interest has led to sexual conflict in bed bugs. We'll be speaking with Dr. Geoff Parker, an evolutionary biologist credited with developing a theory to explain the evolution of two sexes, about anisogamy, sexual reproduction through the fusion of two different gametes: the egg and the sperm. Then we'll speak with Dr. Roberto Pereira, research scientist in urban entomology at the University of Florida, about traumatic insemination in bed bugs.