A star is reborn: how Hubble astronomers saw the earliest light
Earendel – “morning star” in Old English – is among the first stars to exist in our universe, born less than one billion years after the Big Bang. And the Hubble space telescope has just performed the remarkable feat of detecting light from it.
Mostly, the telescope gives us images of nearby galaxies in intricate detail, but those of distant galaxies are very murky indeed. Astronomer Brian Welch and his team, from Johns Hopkins University in Baltimore, discovered the star while hunting for hints of the earliest galaxies. These galaxies are very hard to see, and the team chose to examine a selection of images from the Hubble looking for clues.
Astronomers aren’t newcomers when it comes to observing ancient light. Just this week it revealed incredible pictures of a galaxy as it was, half a billion years before Earendel. Although this surpasses what we expected Hubble could do, it isn’t as remarkable a feat as resolving a single star, and when you see the shapeless smudge of that early galaxy, it brings it home how special Earendel is.
In detective fiction, the sleuth uses a magnifying glass to study evidence left at the crime scene, moving the lens to magnify the clue. We can’t launch ever more gigantic glass lenses into space but, luckily, nature provides us with an alternative, much more powerful method.
Massive clusters of galaxies provide such a gravitational pull that the light from background stars is bent around the cluster, just like light bends in a magnifying glass. This effect – “gravitational lensing” – is used in astronomy to see objects too faint or too far away to see otherwise. We cannot just go about moving galaxies to magnify where we like, though; we must follow where the universe leads us.
In one snapshot, Welch and his team saw a faraway galaxy that had been magnified and distorted. Nothing new there. But within that distorted galaxy, there was an unexpected bright smudge. One star in the galaxy aligned with the lens so precisely that its image has been enhanced a thousand times over, making it seem big and bright. And the colour of the light from Earendel indicates we are seeing ancient light.
Light has different properties depending on its energy: the electromagnetic spectrum stretches from low energy, long wavelength radio waves, through the infra-red, a rainbow of optical light, and up to the high energy, short wavelength X-rays. Starlight loses energy on its journey towards us, sliding down the spectrum, getting redder as it goes. Earendel’s light is very red indeed, suggesting that the light has travelled huge distances over the better part of 13 billion years, placing it in the era of the first stars. Observations of early galaxies are rare, and visuals of individual stars within this era have been nonexistent until now.
Earendel is not of the oldest generation of stars, but it’s tantalisingly close. Studying this era is like uncovering the early evolution of humankind, but on
a galactic scale. Our ancestors are like us, but there are important differences to explore. So it is with the universe now, compared to the universe then. We need to go as far back to the Big Bang as we can to fill in the gaps.
The observation of Earendel is record-breaking. It did not just improve upon the previous record, it smashed it to smithereens, and it might be a record that is here to stay. The light from Earendel is so faint that, had it not been so perfectly aligned with the cosmic lens, we wouldn’t have seen it at all. There is no guarantee that we will chance upon another cluster with a similarly serendipitous alignment and stars any further away might be too faint to see. In common with other major scientific advances, years of hard work, expertise and speculation have been helped over the finish line by a generous helping of good luck.
As with all evidence testing our powers of observation, there is a possibility of mistaken identity. While light from a distant, blue star will have lost energy so it appears redder, we could just be looking at something much closer that is red to begin with. The chances of a random, reddish star lining up with that old, distorted galaxy is small, but not impossible, so the team will use infrared data from the brandnew James Webb space telescope to rule out the suspect.
Next in the line-up is a black hole, which can look like a star if the surrounding matter spiralling inwards is magnified in a certain way. This time it is X-ray observations that will help us decide for sure. Combining different wavelengths of light is not new to astronomy; it is not new to everyday life either. Imagine a doctor diagnosing a broken arm in person. They’ll examine your arm in a well-lit room, but how do they know for sure it’s broken? How bad a break is it? Will you ever play tennis again? You’d hope they would at least send you for an X-ray. To distinguish what we are seeing in space, we need to use different light wavelengths in a similarly exhaustive way and interpret our observations as expertly as a doctor does.
Earendel looks to be the farthest star we have ever detected, and possibly will ever detect. A chance alignment of celestial objects, seen from an incomprehensibly vast distance, has provided exciting evidence of the universe in its infancy. We might be firmly on the trail of the faintest stars in the universe, but we’re going to need more than a magnifying glass to be sure. We’re going to have to think on an entirely different wavelength.
• Dr Emma Chapman is the author of First Light: Switching on Stars at the Dawn of Time(Bloomsbury Sigma), out now in paperback, £10.99. To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply