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Is the James Webb Space Telescope finding the furthest, oldest, youngest or first galaxies? An astronomer explains.

sky full of stars and galaxies
The first publicly released science-quality image from NASA's James Webb Space Telescope, revealed on July 11, 2022, is the deepest infrared view of the universe to date. (Image credit: NASA, ESA, CSA, and STScI)

This article was originally published at The Conversation. (opens in new tab) The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Michael J. I. Brown (opens in new tab), Associate Professor in Astronomy, Monash University

We've now seen the first data from the James Webb Space Telescope (opens in new tab). It has observed the atmospheres of distant planets, groups of nearby galaxies, galaxy light bent by unseen dark matter, and clouds of gas and dust in stellar nurseries.

We have also seen headlines claiming that the James Webb Space Telescope has found "the oldest galaxies we have ever seen (opens in new tab)," but what does that mean?

I'm a professional astronomer who studies old galaxies (opens in new tab), and even I find this a little puzzling.

Gallery: James Webb Space Telescope's 1st photos

Looking far, looking back

One of the key science goals of Webb (opens in new tab) is to peer back in time and observe the early universe. Webb can do this because, like all telescopes, it is a time machine.

Light travels at 300,000 kilometers per second, so when we look at the moon we are seeing it as it was a second ago. As the planets of our solar system are millions or billions of kilometers away, we see them as they were minutes or hours ago.

Going further still, when we look at distant galaxies with telescopes we are often looking at light that has taken millions or billions of years to reach us. This means we are seeing these galaxies as they were millions or billions of years ago.

What has James Webb seen?

The James Webb Space Telescope is able to see more distant galaxies than other telescopes, including the Hubble Space Telescope.

Like Hubble it is above the glowing and turbulent atmosphere of the Earth. However, whereas Hubble has a 2.3 meter mirror for focusing light, Webb has a vast 6.5 meter mirror formed from 18 hexagonal segments. Finally, Webb is optimized to detect infrared light, which is what we observe from the most distant galaxies as the expansion of the universe has stretched ultraviolet and infrared light into the infrared.

James Webb has a vast segmented mirror that allows it to look into the distant past. (Image credit: NASA)

Among the first data obtained by Webb were infrared images looking towards a cluster of galaxies called SMACS 0723 (opens in new tab).

The light from SMACS 0723 has taken 4.6 billion years to reach us, so we are seeing it as it was 4.6 billion years ago. That's slightly older than the sun and the Earth, which only formed 4.56 billion years ago.

In recent weeks, galaxies far beyond SMACS 0723 have gained attention. Webb has detected a number of galaxies in the direction of SMACS 0723 and other regions (opens in new tab) that could be so distant their light has taken 13.5 billion years to reach us.

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I say "could" because more data will be needed to absolutely confirm their distances, but some of these galaxies are very compelling (opens in new tab) candidates (others less so (opens in new tab)).

As the light has taken 13.5 billion years to reach us, we are seeing these galaxies as they were 13.5 billion years ago. The universe itself is 13.8 billion years old, so we could be seeing galaxies as they were just a few hundred million years after the Big Bang.

Maisie’s Galaxy may be one of the most distant celestial objects yet observed. (Image credit: Steve Finkelstein/Twitter)

Young, old or early?

While these very distant galaxies have been advertised as the "oldest galaxies (opens in new tab)," I find this a little confusing. We are actually seeing these galaxies as they appeared when they were very young, perhaps a hundred million years old or so.

It is true that these galaxies will be old now, but our own Milky Way galaxy is very old now too. While our sun is 4.56 billion years old, many stars in our galaxy are 10 billion years old and some stars in the Milky Way are 13 billion years old (opens in new tab).

The galaxy we live in, the Milky Way, is billions of years old. (Image credit: Caroline Jones/Flickr)

Furthermore, the very distant galaxies Webb has spotted will look very different today. Galaxies grow by acquiring gas and dark matter, forming new stars and merging with other galaxies.

A small galaxy that was vigorously forming stars soon after the Big Bang may have ended up being the seed of a galaxy that today is very massive and stopped forming stars long ago. That small galaxy and its old stars could also have ended up being just part of a larger galaxy formed relatively recently by merging galaxies together.

A record set to fall

So should we call these most distant galaxies young or old? Perhaps neither.

James Webb is seeing the earliest galaxies yet observed — some of the first galaxies that formed soon after the Big Bang.

I have thrown in one last caveat — "yet observed." Webb has only just begun its mission (opens in new tab), and current analyses are based on data collected over hours.

With days' worth of data, Webb will push its view out to fainter and further objects, and see yet-more-distant galaxies. The record for the most distant and thus earliest observed galaxy will probably tumble a few times before the year is out.

This article is republished from The Conversation (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab).

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Michael J. I. Brown
Associate Professor in Astronomy, Monash University

I am an observational astronomer, studying how galaxies evolve over billions of years.

I was born and raised in Melbourne’s southeastern suburbs. My interest in astronomy began as a child, when the Voyager spacecraft visited the outer planets. I undertook my undergraduate and postgraduate studies at the University of Melbourne during the 1990s. For my PhD, I used (now antiquated) photographic plates to identify thousands of galaxies and measure their distribution in space.

In 2000 I joined the staff of the National Optical Astronomy Observatory, and started working on surveys of the distant Universe with large ground-based telescopes and satellites. In 2004 I was awarded Princeton University’s Henry Norris Russell Fellowship, and studied the growth of the most massive galaxies. Using thousands of galaxies in the constellation of Bootes, I found that the most massive galaxies have grown slowly over the past seven billion years, which is almost certainly due to mergers of galaxies.

Since 2007 I have been at Monash University’s School of Physics and Astronomy. I am measuring spectra of galaxies across the electromagnetic spectrum, which is useful for measuring the distances to galaxies, the luminosities of galaxies and how rapidly galaxies form stars. I am also using large astronomical surveys to measure how rapidly galaxies are growing, and how this growth compares to the growth of dark matter halos.