Faraway blue star cluster shines in Hubble Space Telescope photo

Hubble Space Telescope photo of the globular cluster NGC 2031, located in the constellation Mensa in the Large Magellanic Cloud.
Hubble Space Telescope photo of the globular cluster NGC 2031, located in the constellation Mensa in the Large Magellanic Cloud. (Image credit: NASA, ESA, and L. Bianchi (The Johns Hopkins University); Processing: Gladys Kober (NASA/Catholic University of America))

The Hubble Space Telescope affords astronomers great looks at stars within the Milky Way, but it's sensitive enough to also study objects outside our home galaxy. 

And Hubble's views of distant places allow astronomers to ask a variety of interesting questions, such as, Why are so many stars in NGC 2031 shining in blue?

Stars can come in an array of shapes, sizes, ages and colors. But in the case of NGC 2031, the context doesn't quite match the colors. NGC 2031 is an object known as a globular cluster, which is known to have older stars. However, blue stars usually shine brightly and die young.

Related: The best Hubble Space Telescope photos of all time

In an image description that accompanies a glittering Hubble shot of NGC 2031, NASA officials offer information about what astronomers think so far. One possible explanation they provide is that other stars outside the globular cluster are distorting their appearance. 

NGC 2031 is located outside the Milky Way, in a much smaller satellite galaxy called the Large Magellanic Cloud. The "cloud" part of the name alludes to the object's smudge-like appearance in the southern sky. 

According to NASA, the globular cluster is located within a dense part of the Large Magellanic Cloud. "Its location in this crowded area results in 'stellar contamination,' a phenomenon where the atmospheres and surface features of nearby stars affect the measurements of objects under study," according to the space agency.

Another possibility is that older stars are combining to form new objects, known as "blue stragglers." This could happen because stars of a globular cluster are huddled very close to one another, bound to each other by their mutual gravity.

NGC 2031 also houses another stellar oddity. According to NASA, the globular cluster is home to about 14 known Cepheid variable stars. Thanks to their periodic dimming and brightening, astronomers can use Cepheids as distance beacons. Using measurements from Cepheid variable stars, scientists estimate the distance of NGC 2031 to be about 150,000 light-years from Earth.

And then, the visual beauty of the blue stars in this scene is a bonus treat on top of all the science.

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Doris Elin Urrutia
Contributing Writer

Doris is a science journalist and Space.com contributor. She received a B.A. in Sociology and Communications at Fordham University in New York City. Her first work was published in collaboration with London Mining Network, where her love of science writing was born. Her passion for astronomy started as a kid when she helped her sister build a model solar system in the Bronx. She got her first shot at astronomy writing as a Space.com editorial intern and continues to write about all things cosmic for the website. Doris has also written about microscopic plant life for Scientific American’s website and about whale calls for their print magazine. She has also written about ancient humans for Inverse, with stories ranging from how to recreate Pompeii’s cuisine to how to map the Polynesian expansion through genomics. She currently shares her home with two rabbits. Follow her on twitter at @salazar_elin.

  • rod
    Blue stars are commonly associated with young stars. Here is an interesting report on star formation and molecular clouds.

    Molecular clouds extend their lives by constantly reassembling themselves, say astronomers, https://phys.org/news/2023-01-molecular-clouds-constantly-reassembling-astronomers.html
    ref - Clouds of Theseus: long-lived molecular clouds are composed of short-lived H2 molecules, https://arxiv.org/abs/2301.10251, 24-Jan-2023.

    My observation. The arxiv.org report indicates host molecular cloud lifetimes range 1-90 Myr so the upper limit or max age is about 90 Myr for a GMC type gas cloud. Other reports I have in my home database suggested perhaps 30 Myr. Consider the age of the Universe using Hubble time, 13.8 Gyr and the age of the Milky Way galaxy with globular clusters dated some 12 Gyr or older. Star formation to proceed in our galaxy would require just how many GMC and molecular clouds to be recreated with max lifetimes now about 90 Myr? I enjoy views of M42 in Orion using my telescopes. How many M42s evolved in the galaxy since its origin and then lasted perhaps 90 Myr, creating new stars?

    Anytime we see blue stars commonly considered as youth, just how many were regenerated in the Milky Way or another galaxy over Gyr time spans?
  • rod
    My observation. This NASA reports indicates NGC 2031 is 140 Myr and with many blue stars, a problem for stellar evolution modeling to explain. Hubble Beholds Brilliant Blue Star Cluster, https://www.nasa.gov/image-feature/goddard/2022/hubble-beholds-brilliant-blue-star-cluster, 07-Dec-2022. "...The NGC 2031 cluster lives in an extremely dense and starry region of the LMC. Its location in this crowded area results in “stellar contamination,” a phenomenon where the atmospheres and surface features of nearby stars affect the measurements of objects under study. Stellar contamination is one theory that could explain observations of bright blue stars in the cluster center. Stars like these typically burn very hot and have short lifespans, but globular clusters are known for housing only ancient stars. Another theory is that these bright blue stars are in fact blue stragglers, a type of star that forms later than its neighbors, enabling astronomers to observe them in older globular clusters such as NGC 2031. Blue stragglers are thought to form from the merging of two old, red stars, resulting in a star with greater mass and therefore bluer color – a theory developed with Hubble’s help from imaging another globular cluster, 47 Tucanae. NGC 2031 is estimated to be 140 million years old and has a mass more than 3,000 times that of our Sun. Astronomers studied this cluster using Hubble’s ultraviolet capabilities."

    Finding blue stars in clusters that should be old is a problem and various methods are used to explain age dating issues like this, efforts to reconcile different ages. Molecular clouds and lifetimes of 90 Myr runs into this too.