Ask Ethan: What are the “first stars” in the Universe?

In our physical Universe, there’s always an order in which things happen. The Sun, Earth, and the rest of our Solar System all formed at one particular moment in time: around 4.5 billion years ago, right here in our own Milky Way. When we look at our Sun in detail, however, we find that it contains a large percentage of heavy elements: about 1-2% of the Sun is composed of elements that could only have been forged in previous generations of stars. Our Universe, however, is an impressive 13.8 billion years old: fully three times as old as the Sun. If we could rewind the clock back closer to the initiation of the hot Big Bang, we would find that stars existed for most of that time, but were more pristine, less evolved, and contained fewer heavier elements.

At some point, in this imaginary time-running-backwards scenario, we would encounter something remarkable: the very first stars of all to form in cosmic history. If we were to go back earlier than that, we would find no stars at all, just neutral atoms, and before that, even more primitive states of matter. But what would finding the first stars actually mean? That’s what John Caponas wants to know, writing in to request:

“I don’t understand what ‘first stars’ are. If the Universe came into existence spontaneously, then wouldn’t ALL stars be the same age no matter where they are positioned??”

It’s true that the Big Bang arose spontaneously, and it’s also true that time is relative. However, no, all stars aren’t the same age, and that’s something we know very well. Here’s the story that compels us to continue embarking on the quest for the very first stars of all.

The visible light spectrum of the Sun, which helps us understand not only its temperature and ionization, but the abundances of the elements present. The long, thick lines are hydrogen and helium, but every other line is from a heavy element that must have been created in a previous-generation star, rather than the hot Big Bang.

The first star we ever discovered was our Sun. Not “discovered” in terms of it being the first star ever seen by humans and our hominid ancestors (although that’s probably true), but discovered in the sense that we learned what it was made of. That story goes all the way back to the pioneering work of Cecilia Payne, who back in 1925 became the first to understand the relationship between:

• a star’s spectrum, which includes absorption lines owing to the presence and abundance of various elements,
• a star’s temperature, which reflects the temperature of the outermost layers of a star’s photosphere,
• and ionization, which details how many electrons are bound versus kicked-off of the various species of atom present within that star’s outer layers.

Her work applied to all stars, but included the Sun as a main focus, as its light was the easiest to spectroscopically observe. Although about 98% of the Sun, by mass, was composed of hydrogen (at about 70%) and helium (at around 28%), the other 2% was remarkably important, as it was composed of a wide variety of elements found throughout the periodic table. These heavy elements, a mere afterthought in determining what the stars are made of, turn out to be of remarkable cosmic importance.

The most current, up-to-date image showing the primary origin of each of the elements that occur naturally on the periodic table. Neutron star mergers, white dwarf collisions, and core-collapse supernovae may allow us to climb even higher than this table shows. The Big Bang gives us almost all of the hydrogen and helium in the Universe, and almost none of everything else combined. Most elements, in some form or another, are forged in stars.

To an astronomer, any element heavier than helium has perhaps the most dubious cosmic misnomer of all: we call it a “metal.” Sure, the next elements up, lithium and beryllium, are indeed metals, but they’re followed by non-metals like boron, carbon, nitrogen, oxygen, fluorine, neon, and so on. It seems silly, at first glance, to group everything that isn’t hydrogen or helium into the same category — and to name that category “metals,” of all things — but there’s a good reason for that. When we do this same spectroscopic analysis of other stars, we find that there are huge variations in metallicity. Specifically, it looks like where a star is located, within the Milky Way, plays a major role in determining this abundance of metals, or metallicity, of a star will be.

Why is that?

Our Sun, with a relatively high heavy element content for a star, is known as a Population I star: the first type of stellar population ever discovered. Population I stars are rich in heavy elements, with high metallicities, which is strong evidence that they formed at late cosmic times. The reason we say they formed at late cosmic times is because of how elements are made. Whereas the lightest elements and their isotopes, including:

• hydrogen,
• deuterium,
• helium-3,
• helium-4,
• and lithium-7,

were largely created during the early stages of the hot Big Bang — during the period of Big Bang nucleosynthesis in the first few minutes of cosmic history — all of the other elements can only form in any substantial amounts once stars have been born.

The anatomy of a very massive star throughout its life, culminating in a type II (core-collapse) supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive stars achieve a core-collapse supernova the fastest, typically resulting in the creation of black holes, while the less massive ones take longer, and create only neutron stars.

Because of our ability to perform spectroscopy on light — whether that’s light coming from individual stars or star clusters, or the cumulative light coming from an entire galaxy — we can measure things like:

• how strong is the presence of oxygen,
• how strong is the presence of iron,
• and how strong are they compared to one another, as well as to hydrogen.

Those measurements, among other similar ones, teach us what the metallicity of the matter in the region from which that light emerges. And even when we look at the stars in our own backyard, right here in the Milky Way, we find that they’re not all like the Sun.

In fact, the stars we’re most familiar with fall into two categories.

1. There are stars like our Sun: Population I stars, with comparable metallicities to what we find in our own star. Sure, there are variations, but the Sun is a relatively high metallicity star, and so are many of the stars we find nearby. In particular, these tend to be stars that form in the galactic plane, near the center of the galaxy, and appear to be relatively young, in that most of them have stellar ages of only a few billion years, at most.
2. And there are stars that are much poorer in heavy elements: Population II stars, because they were the second population of stars ever discovered. Often containing fewer than 10%, and sometimes even less than 1% of the heavy elements that the Sun possesses, these lower metallicity stars are found in isolation, in the galaxy’s outer halo, and in very old globular clusters.

The globular cluster Messier 69 is highly unusual for being both incredibly old, with indications that it formed at just 5% the Universe’s present age (around 13 billion years ago), but also having a very high metal content, at 22% the metallicity of our Sun. The brighter stars are in the red giant phase, just now running out of their core fuel, while a few blue stars are the result of the mergers of initially lower-mass stars: blue stragglers.

This is important to note. Stars come in different ages, and we can typically estimate the ages of stars that exist in clusters — both open star clusters and globular clusters — by looking at their color-magnitude diagram. When you observe a star, you can tell what its color is simply by looking at the light that comes in at different wavelengths, and you can tell what its magnitude is simply by observing its brightness. Most stars fall along a snake-like curve known as the main sequence:

• when stars are born, they have a color and magnitude determined by their initial mass,
• with the lowest-mass stars being dimmer and redder, and the higher mass stars being brighter and bluer,
• and the highest mass stars run through their life cycles the fastest, causing them to evolve off of the main sequence.

Therefore, when you look at the color-magnitude diagram for a population of stars (e.g., a star cluster) that was born all at once, you can determine how old the star cluster is by looking at which stars are still alive and on the main sequence, versus where they’ve evolved off of the main sequence. While many populations of stars found in open clusters, which are common in the plane of the Milky Way, are relatively young (millions to up to about 4 billion years) and consist of Population I stars, the stars found in globular clusters are often 11-13 billion years old, and are composed mostly of Population II stars.

When stars first form, the color-magnitude diagram (with brightness on the y-axis and color on the x-axis) looks like a curved line, from lower-right to upper-left. As the stars age, the brightest, bluest, most massive ones evolve off of this curve first. Identifying the point at which this “turn-off” occurs enables astronomers to determine the ages of the stellar populations within them, with only the youngest stellar populations containing the most massive stars.

In other words, even if we just look at our own galaxy, we find that there are stars within it that formed all throughout cosmic history: from just a few hundred million years after the Big Bang all the way up to the present day, 13.8 billion years after the Big Bang began.

But not every galaxy that we can look at has that opportunity. Remember what the Big Bang is: an event where the Universe, everywhere all at once, became

• hot,
• dense,
• rapidly expanding,
• and nearly-uniformly filled with particles, antiparticles, and radiation.

This event occurred 13.8 billion years ago. In all the time since, the Universe has been expanding and cooling, and also gravitating. Light, produced in the Big Bang as well as by all the stars that have formed since, travels at the speed of light through this expanding Universe. Here on Earth, 13.8 billion years after that event, we can only see the light that’s arriving now. For objects within our galaxy, that journey only takes up to a few tens of thousands of years, so we’re only looking a little bit into the past when we see open star clusters and globular clusters nearby.

But when we look far out into the distant Universe, at galaxies that are many billions of light-years away, we’re looking billions of years back in time. At the farthest recesses of what we can observe, we’re able to see galaxies that existed way back when: mere hundreds of millions of years after the Big Bang, back to when the Universe was just ~2% of its current age.

This figure shows the NIRCam (top) and NIRSpec (bottom) data for now-confirmed galaxy MoM-z14: the most distant galaxy known to date as of May 2025. Completely invisible at wavelengths of 1.5 microns and below, its light is stretched by the expansion of the Universe. Emission features of various ionized atoms can be seen in the spectrum, below, as well as the significant and strong Lyman break feature.

And yet, even at these enormous distances and extremely early times, the populations of stars that exist within these galaxies still display substantial signatures of heavy elements: carbon, nitrogen, oxygen, iron and more. These elements could not have been made in the Big Bang, alone, implying that previous generations of stars had already formed, lived, died, and ejected their pollutants — i.e., heavy elements forged within them — back into their interstellar mediums before the stars we’re seeing in them, today, came into existence. Therefore, as early as these stellar populations we’re seeing are in cosmic history, overall, they’re still not the very first stars.

Everywhere we see stars today, including within every galaxy we know of (except, perhaps, one), all of the stars are only Population I and Population II stars; they all have some amount of heavy elements present within them. And yet, in order to form stars that have heavy elements in them, something extraordinary must have happened at some point in the even more distant past: stars must have formed from a cloud of material that was left over, untouched, from the Big Bang itself. These stars would have been pristine, with no pollutants from prior generations of stars. It is these types of stars, known as Population III stars, that make up what we call the first stars.

The first stars and galaxies in the Universe will be surrounded by neutral atoms of (mostly) hydrogen gas, which absorbs the starlight. Without metals to cool them down or radiate energy away, only large-mass clumps in the heaviest-mass regions can form stars. The very first star will likely form at 50-to-100 million years of age, based on our best theories of structure formation and our best observations of the Universe to date, which corresponds to a redshift of between 30-and-50.

But it’s important to note that we’ve never found a population of these first stars at all, despite claims to the contrary. Now that we’re here in the JWST era, based on what we’ve seen when we push it to its most extreme limits, we have to face the truth that even this new telescope, with all it’s capable of accomplishing, might be out of its depths if we’re asking it to find the first stars ever formed.

And that shouldn’t come as a surprise. When stars form today, they form from a contracting cloud of gas that fragments into clumps. Those clumps then gravitationally contract further and further, shedding energy by radiating it away out into the distant Universe. The ability to shed energy, or cool (particularly in its outer layers), is critical to the formation of a star. For the stars that form in the Universe today, it’s this cooling — dominated by heavy elements — that sets the average mass of a star that forms. Today, with the heavy elements we have, the average star has a mass of only 40% the mass of the Sun, with the most massive stars coming in at couple of hundred solar masses.

But hydrogen and helium are the lightest elements of all, and that’s what 99.99999% of the Universe was made of after the hot Big Bang, before any stars had ever formed. When we run simulations for the formation of stars under those conditions, we find that much larger clumps arise.

An artist’s conception of what a region within the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. But the conversion of matter into energy does something else: it causes an increase in radiation pressure, which fights against gravitation. Surrounding the star-forming region is darkness, as neutral atoms effectively absorb that emitted starlight, while the emitted ultraviolet starlight works to ionize that matter from the inside out.

The average “first star” would have had a mass that was 25 times as great as the average star that forms today: something like ten times the mass of the Sun. The most massive first stars may have been many hundreds or even thousands of times the mass of our Sun. We don’t know when they would’ve formed, but our simulations indicate that you’d have to peer back into the first 200 million years of cosmic history to have a chance to spot the first stars, with the recognition that some locations will form the first stars earlier than others, while others won’t form their first stars until significantly later. Gravitation takes time, and only when a region has gravitated sufficiently will enough mass gather in one location to form stars at all.

These first stars, with such large masses, will likely only live for a very short time — perhaps one or two million years, only — before dying in stellar cataclysms and enriching the interstellar medium around them with heavy elements. Once you have an interstellar medium that possesses heavy elements, including abundant elements like carbon, oxygen, and iron, the stars that form from those clouds of material will no longer be pristine, Population III stars; they’ll be later generation, Population II stars. (When enough generations live-and-die, you’ll make Population I stars.) We’ll have to get lucky to find a set of Population III stars in isolation, that isn’t confounded by having later-generation, more polluted stars alongside them.

An artist’s conception of what the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. The neutral atoms surrounding it get ionized, and get blown off, quenching (or ending) star formation and growth in that region. These stars will be short lived, with fascinating and important consequences.

This might lead to a fascinating follow-up question: if that’s what the first stars are and the conditions under which they form, then how far back in time (or space) is the very first star — i.e., the earliest example of the first stars — likely to be? This is something that we have to purely rely on simulations for, and there’s a tremendous range of uncertainty here. Conservatively, simulations predict that the first stars should generally begin forming 50-to-100 million years after the Big Bang, which corresponds to redshifts of between z = 30 and z =50. But some higher-resolution simulations predict even earlier formation times for the first stars of all.

If the first star of all formed 36 million years after the Big Bang, that would imply a redshift of z = 60, something that could only be observed with either a far-infrared or a radio telescope. The most extreme formation scenario I’ve seen would have the first star form a mere 25 million years after the Big Bang, with a redshift of z = 77. Considering that JWST has been remarkable in pushing the frontier of cosmic distance from the Hubble era (at z = 11, or a time of ~400 million years) to its modern record of z =14.4, with an age of ~282 million years after the Big Bang, looking back to the first 100 million years of cosmic history is certainly asking for another great leap. Still, the promise of finding the first stars of all is a scientific goal we should never stop chasing until we get there. After all, nothing less than the story of how the Universe came to be the way it is today is at stake.

Send in your Ask Ethan questions to startswithabang at gmail dot com!