JWST keeps delivering amazing astronomical results. For example, for the first time it’s seen star clusters in a galaxy less than 500 million years after the birth of the Universe [link to journal paper]!

This takes a bit of explanation. While JWST can see very distant galaxies, those galaxies still have to be pretty bright, or else they’re too faint and small to see. However, a fun little bit of cosmic magic gives us a big hand: gravitational lensing.

Massive objects literally bends space, and the more massive an object is the more space gets warped. You yourself have mass and do this too, but only a teeny amount. Galaxies, however, are very large and extremely massive — some have trillions of times the mass of the Sun — so they bend space a lot. Even better, some galaxies exist as part of galaxy clusters, which combine the might of hundreds or even thousands of galaxies.

As light from an even more distant galaxy passes by that cluster, it follows the warped path as space is bent. This can distort the images we see of that galaxy, like looking at your reflection in a funhouse mirror. But there are other effects, too: we can see multiple images of the same object as light from it follows slightly different paths to Earth, and the light itself is magnified as well. That’s why we call this lensing.

That means tiny faint galaxies can be seen when otherwise they’d be invisible. In this case, the lensing cluster is called SPT-CL J0615−5746 and it’s about 7.7 billion light years from Earth (or, more technically, its light takes 7.7 billion years to get to Earth). Its immense mass has lensed a much more distant galaxy, and they find a good fit with its distance that means its light takes a whopping 13.3 billion years to reach Earth, which in turn means we see it when the Universe itself was only about 460 million years old! That’s only 3% of its current age, so we’re seeing this galaxy when it was an infant. 

The galaxy is greatly distorted, appearing as a long line, and it shows a series of small fuzzy clumps along it. Looking at models of what star clusters would look like at this age, the astronomers find a good fit with the clump colors and brightness if they are proto-globular clusters, collections of hundreds of thousands or millions of stars in a roughly spherical ball some dozens of light-years across. Our Milky Way has about 160 globulars known, though there could be fainter ones far enough away we haven’t found them yet. Some galaxies have thousands of these clusters.

Interestingly, these clusters are brighter than the galaxy itself! The light we see from them is hugely redshifted (by a factor of over 11), so the light we see from these clusters actually started off as ultraviolet light and the expansion of the Universe since then has stretched their wavelengths out to infrared, allowing JWST to see them.

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