Ask Ethan: How Many Generations Of Stars Formed Before Our Sun?
“[My professor] and I were discussing the status of our Sun. I was leaning towards the Sun being a third generation star and she was thinking the Sun was second. […] Any thoughts? And how might this be resolved with [future] technology?”
How many generations of stars formed before our Sun did? Surprisingly, this is not a questions astronomers know the answer to, nor is it one that we can envision coming up with a definitive answer for. In the modern Universe, we don’t have access to history or pre-history of the Solar System; all we can see is what remains today. The rest of it is up to us, the scientific community, to reconstruct.
But we’re pretty incredible at what we can say based on what we do know. The Sun cannot be a first-generation, pristine star, since none of those have ever been even discovered. It couldn’t be a second-generation star, because those are ubiquitous and have very different properties than our own Sun. But where are we then? And what do we know, and what remains mysterious?
It’s a great question with a remarkable set of facts behind what we know so far, all for your enjoyment on this week’s Ask Ethan!
What Was It Like When The Universe Made The Very First Galaxies?
“The first galaxies required a large number of steps to happen first: they needed stars and star clusters to form, and they needed for gravity to bring these star clusters together into larger clumps. But once you make them, they are now the largest structures, and can continue to grow, attracting not only star clusters and gas, but additional small galaxies. The cosmic web has taken its first major step up, and will continue to grow further, and more complex, over the hundreds of millions and billions of years to follow.”
For millions upon millions of years, there were no stars in the Universe. As the first one finally formed, the star clusters that birthed them became the largest structures in the Universe. Yet these were too small and limited to be considered galaxies. For that, we need more than one massive star cluster in the same place. We need for them to merge, triggering a starburst and creating a larger, more luminous object. It takes much longer for that to happen than to merely form stars, and the Universe was a very different place by then. The Big Bang may have started everything off uniformly and without anything more than the seeds of structure, but gravity, and time, are awfully powerful tools.
Come learn what the Universe was like when we made the very first galaxies. It’s a story you won’t soon forget!
What Was It Like When The Universe Made Its Second Generation Of Stars?
“The very first stars live only an extremely short time, owing to their high masses and large luminosities and rates-of-fusion. When they die, the space around them becomes polluted with the fruits of their lives: heavy elements. These heavy elements enable the second generation of stars to form, but they now form differently. The heavy elements radiate heat away, giving rise to a less massive, more diverse generation of stars, some of which survive even to the present day.
When the James Webb Space Telescope begins operations, it may yet reveal a population of these first stars, likely to be found alongside polluted, second-generation stars. But once these second-generation stars begin to form, they make something else possible: the first galaxies. And that, in just a few years, is likely where the James Webb Space Telescope will truly shine.”
The first stars in the Universe, as astronomers define them, are stars made out of pristine materials left over from the Big Bang: almost exclusively hydrogen and helium. Because of this, their options are limited. They’re all very massive, they have no rocky planets around them, they live a short time, and they almost all die in a supernova. That’s not a life-friendly environment! But all of that changes with the second generation of stars, which forms just a few million years after the first. Some of these may even survive in the Milky Way to the present day… and we might have even found them already.
Come learn what the Universe was like when it form the second generation of stars, and why this is so important in our own cosmic story!
What Was It Like When The First Stars Died?
“It’s theorized that this is the origin of the seeds of the supermassive black holes that occupy the centers of galaxies today: the deaths of the most massive stars, which create black holes hundreds or thousands of times the mass of the Sun. Over time, mergers and gravitational growth will lead to the most massive black holes known in the Universe, black holes that are millions or even billions of times the mass of the Sun by today.
It took perhaps 100 million years to form the very first stars in the Universe, but just another million or two after that for the most massive among them to die, creating black holes and spreading heavy, processed elements into the interstellar medium. As time goes on, the Universe, at long last, will begin to resemble what we actually see today.”
Our Universe, shortly after the Big Bang, proceeded in a number of momentous steps. The first atomic nuclei formed just minutes after the Big Bang, while neutral atoms took hundreds of thousands of years. It took another 50 to 100 million years for the very first star to be created, but only, perhaps, a million or two years for the most massive among the first stars to die. They may have been short-lived, but the first stars were truly spectacular, and their deaths set up the first steps in a changing Universe that would take us from a pristine set of materials to, eventually, the Universe as we know it today.
Take a major step in the cosmic journey of how we got to today by looking at what it was like when the first stars died!
What Was It Like When The First Stars Began Illuminating The Universe?
“After the Big Bang, the Universe was dark for millions upon millions of years; after the glow of the Big Bang fades away, there’s nothing that human eyes could see. But when the first wave of star formation happens, growing in a cosmic crescendo across the visible Universe, starlight struggles to get out. The fog of neutral atoms permeating all of space absorbs most of it, but gets ionized in the process. Some of this reionized matter will become neutral again, emitting light when it does, including the 21-cm line over timescales of ~10 million years.
But it takes far more than the very first stars to truly turn on the lights in the Universe. For that, we need more than just the first stars; we need them to live, burn through their fuel, die, and give rise to so much more. The first stars aren’t the end; they’re the beginning of the cosmic story that gives rise to us.”
We like to think of the Universe evolving as a story that follows a particular order: first we had the Big Bang, then things expanded and cooled, then gravitation pulled things into clumps, we formed stars, they lived and died, and now here we are. But in reality, things are messier than that! The very first stars didn’t immediately spread light throughout the Universe, but instead had a cosmic ocean of neutral atoms to contend with: one that they weren’t energetic enough or numerous enough to break through. The first stars in the Universe fought a battle against the clumping, neutral, atomic-based matter that surrounded them… and lost.
Come get the valiant but ultimately unsuccessful story of the first stars in the Universe, and learn why “letting there be light” didn’t illuminate the Universe!
First Stars Formed No Later Than 250 Million Years After The Big Bang, With Direct Proof
“We see MACS1149-JD1 as it was 530 million years after the Big Bang, while inside, it has a special signature: oxygen. Oxygen is only produced by previous generations of stars, indicating that this galaxy is already old.
MACS1149-JD1 was imaged with microwave (ALMA), infrared (Spitzer), and optical (Hubble) data combined.
The results indicate that stars existed nearly 300 million years before our observations.”
One of the great quests of astronomers today is to measure and locate the very first stars in the Universe. As far back as Hubble can see, to when the Universe was just 3-5% its current age, the Universe is still full of galaxies, even though they’re smaller and bluer than the ones we have today. But within these galaxies, we can also find evidence that the stars in there aren’t the very first ones; they contain evidence for prior generations of stars in their spectral signatures. From the second-most distant galaxy ever discovered, itself just 530 million years after the Big Bang, we see evolved stars. They indicate that the very first ones formed no later than 250 million years after the Big Bang.
The James Webb Space Telescope will be able to see that far! In less than 3 years, we’ll peer beyond where we’ve ever seen before. And there will no doubt be something breathtaking to look at.
Astronomers Confirm Second Most-Distant Galaxy Ever, And Its Stars Are Already Old
“Scientists have just confirmed the second most distant galaxy of all: MACS1149-JD1, whose light comes from when the Universe was 530 million years old: less than 4% of its present age. But what’s remarkable is that we’ve been able to detect oxygen in there, marking the first time we’ve seen this heavy element so far back. From the observations we’ve made, we can conclude this galaxy is at least 250 million years old, pushing the direct evidence for the first stars back further than ever.”
When it comes to the most distant galaxies of all, our current set of cutting-edge telescopes simply won’t get us there. The end of the cosmic dark ages and the dawn of the first cosmic starlight is a mystery that will remain until at least 2020: when the James Webb Space Telescope launches. Using the power of a multitude of observatories, we’ve managed to find a gravitationally lensed galaxy whose light comes to us from over 13 billion years ago. But unlike previous galaxies discovered near that distance, we’ve detected oxygen in this one, allowing us to get a precise measurement and to estimate its age.
For the first time, we have evidence from galaxies, directly, that the Universe’s first stars formed no later than 250 million years after the Big Bang. Here’s how we know.
Astronomers’ Hopes For Pristine Stars Dashed: They’re Polluted After All
“In 2011, we found the first evidence for unpolluted, pristine gas, but it hadn’t yet collapsed to form stars.
But even bigger news came in 2015, when the galaxy COSMOS Redshift 7 (CR7) was discovered.
From 13 billion years ago, helium lines were observed, without any carbon or oxygen lines.
The hope was that CR7 contained stars made of hydrogen and helium alone.”
In 2015, the galaxy COSMOS Redshift 7 (CR7) was discovered. Its distance is so great that its light is arriving after a 13 billion year journey, meaning it comes from when the Universe was only 6% its current age. Earlier observations, taken with Hubble, showed the presence of hydrogen and helium, but not of heavier elements like carbon, nitrogen or oxygen. Could this be the first example of Population III stars: the first stars made from the pristine material left over from the Big Bang? For years, astronomers thought it just might be, but new observations in the radio from ALMA have crushed those dreams. Looking for the long-wavelength signature of singly-ionized carbon, they’ve found it everywhere, surrounding all the individual, independent components of galaxy CR7.
It may be extremely metal-poor, but it isn’t truly pristine. Come get the full story in images, video, and no more than 200 words. Talk less, smile more.
Earliest Evidence For Stars Smashes Hubble’s Record And Points To Dark Matter
“And most importantly, this is a glimpse into what it’s like to push back the frontiers of science. The first evidence for anything new is almost always indirect, weak, and difficult to interpret. But these unexplained signals have the power to explain what we don’t yet fully understand: how the Universe came to be the way it is today. For the first time, the Universe has given us an observational clue of where and when and what to look for. It’s up to us to take the next step.”
Earlier today, a new study was released in Nature, showcasing the earliest evidence of stars in cosmic history. The previous record was held by Hubble, which had spotted a galaxy from when the Universe was just 400 million years old: 3% of its current age. Now, indirect measurements of starlight, through the technique of radio astronomy looking for a particular hydrogen transition, has shown us evidence for a tremendous population of stars from when the Universe was between 180 and 260 million years old. This could be, truly, the first stars and galaxies of all, and it’s occurring in exactly the range that the James Webb Space Telescope will be sensitive to. Moreover, the gas that we’re observing shows signs of being far cooler than we’ve anticipated, meaning that something strange is going on, and one leading candidate is that dark matter is interacting with and cooling the gas!
This is the earliest evidence for stars in the entire Universe, and it’s just smashed Hubble’s previous record. Come get the full story right now!