Metal-poor globular cluster forces astronomers to rethink theories - EarthSky

The discovery of the most metal-poor globular cluster recorded to date has forced scientists to rethink how both galaxies and globular clusters form.

An international team of scientists made an unusual discovery when they used extra observing time at Keck Observatory in Hawaii to take a quick look at a massive globular cluster in the galaxy next door, the Andromeda galaxy.

Observations revealed it to be the most metal-poor globular cluster observed to date; that is, it lacks the “heavier” or more complex elements made inside stars as they move through their life cycles.

The newly discovered metal-poor globular cluster was hanging out in plain view in M31, aka the Andromeda galaxy, the large spiral galaxy next door to our Milky Way.

According to the new study, the massive globular cluster RBC EXT8 hosts stars containing 800 times fewer metals – fewer heavier elements – than can be found in our own sun.

It weighs roughly a million times more than our sun, suggesting it is home to roughly a million stars, and – like its sibling clusters in the Andromeda galaxy’s halo – it’s assumed to have formed early in the history of the galaxy.

This is surprising because such pristine gas was thought to be in building blocks too small to form such massive star clusters.

So the most metal-poor stars and star clusters [those consisting mainly of hydrogen and helium] should have formed in the smallest protogalaxies.

To form a cluster as metal-poor as RBC EXT8 you would need a very small protogalaxy, too small to form such a massive globular cluster according to current theories for galaxy formation.

Depending on who you ask, some of the most metal-poor globular clusters in the Milky Way are Messier 15 and ESO 280-SC06, with about 200 to 300 times less metals – iron – than our sun.

Those explanations may have to be reconsidered; we now have to try and understand how it is possible that such a metal-poor globular cluster could form.

Spectra – that is, starlight split into its component colors – used by scientists in the study of RBC EXT8 to understand deficiencies in this globular cluster’s magnesium and iron as compared to another globular cluster, Messier 15.

The majority of what astronomers call the alpha-elements – silicon, calcium, titanium – present were deficient in the cluster to a factor of roughly 400, a number Larsen said could be expected based on observations of other globular clusters, but magnesium – also an alpha-element – told an entirely different story.

This is quite puzzling because magnesium belongs to a class of elements – the alpha elements – that are usually less deficient than iron in globular clusters.

According to a statement from astronomers, the strange journey that led to noticing the metal-poor globular cluster RBC EXT8 started with an unexpected hour of extra observing time on the Keck telescope in October 2019.

Spectra taken with the HIRES spectrometer at the Keck telescope split the makeup of RBC EXT8 into a rainbow, with absorption lines – dark regions – unveiling which elements, and how much of them, had been absorbed into the atmospheres of stars within the cluster.

Bottom Line: The discovery of the most metal-poor globular cluster recorded to date has forced scientists to rethink how both galaxies and globular clusters form