In the cold depths of space, interactions between gas and dust can produce a range of complex molecules. Everything from water and alcohol to the basic building blocks of life. While much of this chemistry occurs in cold molecular clouds, molecules can also be produced in the warm regions surrounding a young star, in what is known as a hot molecular core. A few hot molecular cores have been observed in our galaxy, but recently one was discovered in the Large Magellanic Cloud. Somewhat surprisingly, the molecular composition of this extragalactic hot molecular core is strikingly different from the ones seen in our galaxy. 

The young star in question is known as ST11, and it was recently observed by the ALMA radio telescope array. ALMA is particularly suited to observe complex molecules in space, because it is able to view them at the kind of sub-millimeter wavelengths these molecules often emit. ALMA was not only able to observe the warm dust around ST11, but also molecules such as formaldehyde and nitrous oxide. The region is lacking other molecules such as methanol, which are common in cold molecular clouds. Since the formation of different molecules depends not only on the available elements but also temperature and cloud density, the study of such hot molecular cores allows us to better understand complex molecular interactions in space, which can give us clues about how complex chemistry and even life formed on Earth.

One key difference between the Large Magellanic Cloud (LMC) and our own Milky Way galaxy is that the LMC has a much lower metallicity. This means it has a lower fraction of heavier elements such as silicon, oxygen and iron. These elements are common building blocks of interstellar dust, which is why the LMC has a lower fraction of such dust. A great deal of complex chemistry can happen on the surface of dust grains as it interacts with surrounding gases, and a lower metallicity means such interactions are less common. We see this in the types of molecules forming around ST11. Given the density of ST11’s hot molecular core, the levels of molecules such as formaldehyde and isocyanic acid are a tenth to a thousandth of levels seen in the Milky Way. This demonstrates the central role of dust surface chemistry to the formation of these molecules.

It’s a powerful insight into just how young stars can seed the cosmos with some of the complex molecules we see around us.

Paper: Takashi Shimonishi, et al. The detection of a hot molecular core in the Large Magellanic Cloud with ALMA. The Astrophysical Journal, Volume 827, Number 1 (2016)

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Our atmosphere is about 78% nitrogen and 21% oxygen, with traces of other things like water and carbon dioxide. It’s an odd mix compared to the atmospheres of other planets. Jupiter and Saturn are dominated by hydrogen and helium, The thick atmosphere of Venus is about 96% carbon dioxide, and only 3% nitrogen, which is about the same ratio as the thin atmosphere of Mars. So why is our atmosphere so dominated by nitrogen? 

It wasn’t always this way. Like most planets Earth’s earliest atmosphere was dominated by hydrogen and helium. These two elements are by far the most abundant in the Universe. About 92% of the atoms created by the big bang were hydrogen, and most of the rest were helium. All the other elements on the periodic table are formed through astrophysical processes such as nuclear fusion in the heart of a star. To this day they make up only a small trace of cosmic elements. When planets initially form, their composition is mostly hydrogen and helium. Some of the hydrogen bonds with other elements, but most of it remains free hydrogen. Both hydrogen and helium are light elements, so they will tend to evaporate into space over time. A large planet such as Jupiter has enough gravity to hold on to most of its hydrogen and helium, which is why these elements dominate the atmospheres of gas giants. But the gravity of Earth isn’t strong enough, so Earth’s early atmosphere of helium and free hydrogen evaporated into space.

The relative abundances of elements on Earth. Credit: Gordon B. Haxel, Sara Boore, and Susan Mayfield from USGS

Of the remaining elements, carbon, nitrogen, and oxygen are the most abundant. This is due to the fact that the main fusion reaction in large stars is the CNO cycle, which produces these elements as a by-product. These react easily with other elements, and produce gasses like water (H₂O), carbon dioxide (CO₂), and ammonia (NH₃). Young Earth was much more geologically active than it is today, and volcanic activity released large quantities of these gases, and over time they came to dominate Earth’s atmosphere.

So why are the atmospheres of Venus and Mars dominated by CO₂, while Earth’s is not? It all comes down to water. Earth’s vulcanism driven atmosphere was likely dominated by carbon dioxide like Venus and Mars, but Earth also has vast oceans of liquid water. Carbon dioxide dissolves easily in water, so our oceans absorbed much of the atmospheric CO₂, leaving an atmosphere dominated by ammonia. It turns out that ammonia is unstable in Earth’s atmosphere. When struck by ultraviolet light from the Sun, it breaks apart into nitrogen and hydrogen. The liberated hydrogen evaporated into space, leaving nitrogen behind. Venus’ atmosphere likely followed a similar process, but without vast oceans to pull CO₂ out of its atmosphere. While Venus’ atmosphere is mostly carbon dioxide, it is much thicker than Earth’s, and contains four times the nitrogen.

Even with it’s vast oceans, Earth’s atmosphere would likely have been dominated by carbon dioxide were it not for the appearance of life. Early cyanobacteria used sunlight and the carbon dioxide dissolved in Earth’s oceans to produce energy, and released oxygen as a by-product. Early oxygen bonded with iron to form a layer of rust, but eventually began to build up in Earth’s atmosphere. As carbon dioxide was broken down by cyanobacteria, more CO₂ could be dissolved into the ocean. This gave rise to our modern atmosphere dominated by nitrogen and oxygen.

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The first elements to appear in the Universe were hydrogen and helium, created soon after the big bang. Other elements on the periodic table are produced through nuclear interactions within stars. Lighter elements such as carbon, nitrogen and oxygen are formed through nuclear fusion in a star’s core, but heavier elements such as gold are formed through catastrophic events such as a supernova explosion or the collision of neutron stars. It’s known as r-process nucleosynthesis (due to the rapid neutron interactions) and is still a bit of a mystery. 

We can distinguish r-process elements not only by their presence in stars, gas and dust, but also by their relative abundances. The r-process abundances are distinctly different from other nucleosynthesis methods such as the s-process (slow neutron) that occurs in the late stage fusion of large stars. So we know that heavier elements can are produced through r-process events, but one of the big debates has been over which type of events create the most heavy elements.

There’s basically been two schools of thought. One is that core-collapse supernova are the main factor. These are fairly common on a cosmic scale, but the amount of heavy elements released in a particular supernova is relatively low. In this model a galaxy would be seeded with a low but steady flow of heavy elements. The other idea is that stellar collisions create most heavy elements. The collision of two neutron stars, for example, is fairly rare, but the amount of heavy elements released from such an explosion would be quite high. In this model heavy elements are seeded into a galaxy in bursts every now and then. The challenge is to determine which model is right.

Recently astronomers found evidence that the collision model seems to be the right one. They looked at the abundance of elements in a dwarf galaxy known as Reticulum II. They found that the 9 brightest stars in this galaxy have heavy element abundances 100 to 1,000 times greater than seen in other similar galaxies. This would imply that the abundance of r-process elements was unusually high during their formation, which is what you would expect if they are produced at high quantities in rare events. It seems clear, then that stellar collisions play a major role in the production of heavy elements.

Since gold is one of those heavy elements, you could say that the collision of neutron stars causes a galaxy to enter a gilded age.

Paper: Alexander P. Ji, et al. R-process enrichment from a single event in an ancient dwarf galaxy. Nature 531, 610–613 (2016) doi:10.1038/nature17425

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Tholins are a broad class of complex organic molecules. They are typically formed when ultraviolet light strikes simple organic molecules such as methane. They were first categorized by Carl Sagan when reproducing the Miller-Urey experiment. While tholins don’t exist naturally on Earth since our high-oxygen atmosphere tends to prevent them from forming, they are very common on the cold worlds of the outer solar system.

The formation of tholins on Titan. Credit: NASA-JPL

Tholins recently hit the news during the flyby of Pluto. The reddish brown coloring of the dwarf planet is likely caused by tholins on its surface. The molecules also give Titan’s atmosphere its brown coloring. They are suspended in the Saturnian moon’s atmosphere like smog. We’ve found tholins on comets and asteroids, and there’s even some evidence of tholins on planets orbiting other stars.

Tholins may have been common on Earth before the rise of free oxygen in our atmosphere about 2.3 billion years ago. There is some speculation that tholins may have played a role in the appearance of life on Earth. We know, for example, that many soil bacteria can feed off tholins as a source of carbon. Since tholins absorb ultraviolet light, they could also have played a role in protecting young Earth from UV rays that can tend to harm fragile living organisms.

Paper: M. Köhler, et al. Complex Organic Materials in the HR 4796A Disk? The Astrophysical Journal 686 L95 (2008)

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There are buckyballs in space. More formally known as fullerenes, these carbon molecules such as C₆₀ have long been known to exist in space due to the complex chemistry that can occur between gas and dust in interstellar clouds. But we now know fullerenes can help explain one of the nagging mysteries of astronomy known as diffuse interstellar bands.

Diffuse interstellar bands are absorption features seen in stellar spectra. While many absorption lines can be identified with particular atoms or molecules, DIBs don’t have a clear identification. But two bands in the infrared (at 958 and 963 nanometers) were thought to be caused by C₆₀. Now a paper in Nature confirms this hypothesis. In this work, the team looked at the spectra of buckyballs when cooled to about 6 Kelvin (a common temperature for interstellar clouds). They observed absorption bands 963.27 ± 0.1 and 957.75 ± 0.1 nanometers, in good agreement with the interstellar bands.

So that’s two DIBs identified, and a few hundred more with origins yet to be discovered.

Paper: E. K. Campbell, et al. Laboratory confirmation of C₆₀⁺ as the carrier of two diffuse interstellar bands. Nature 523, 322–323 (2015)

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This week in Nature it was announced that complex organic molecules were observed in a young planetary system. This isn’t the first time such molecules have been observed in space, nor is this result particularly surprising, but the result does confirm some of our suspicions about the chemistry of the early solar system.

The team looked at a star known as MWC 480. It is about a million years old, and has a protoplanetary disk surrounding it. By analyzing the spectra from this star they found evidence of methyl cyanide and hydrogen cyanide in the region equivalent to the Kuiper belt of our solar system. This means that as the star’s planetary system forms, these organics will become locked up in cometary bodies.

We’ve long known that cyanide molecules can exist in comets. In fact, one of the first detections of an organic molecule in space was the discovery of cyanogen in Halley’s comet in 1910. What this new work shows is that the comets of other planetary systems are likely to have similar organics.

One of the reasons cyanides in space are interesting is that they are the types of molecules necessary to build amino acids and eventually proteins. We know that these “building blocks” of life have been found in meteors, and there is speculation that these kinds of molecules could have helped jump start the formation of life on Earth. Since they appear common to other planetary systems as well, they might help jumpstart life across the cosmos.

At this point we can’t be sure, but it does appear that the conditions of the one planetary system where we know life exists are not particularly unusual.

Paper: Karin I. Öberg, et al. The comet-like composition of a protoplanetary disk as revealed by complex cyanides. Nature 520, 198–201 (2015)

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Much of the modeling of astrophysical systems focuses on dynamical behavior. That is, how stars, planets and interstellar clouds move under the forces of gravity. While such dynamical modelling can prove useful for studying the motion of galaxies, they are not quite as good at modeling the evolution of galaxies. That’s because galaxies evolve over time not only due to the motion of stars within them, but they also evolve chemically as old stars die and new stars form. The chemical makeup of new stars depends where and when they form.

One way you can look at the chemical makeup of a galaxy is to examine the metallicity of stars. The metallicity of a star is a measure of how much a star has of elements other than hydrogen and helium. Usually this is done as a ratio of iron to hydrogen, or oxygen to iron, for example.

In our own galaxy, stars with high metallicity tend to be near the central region and galactic plane, while lower metallicity stars tend to lie further from the galactic plane. This variation in metallicity is due to the fact that more stars lie near the galactic center and plane, therefore more stars die in that region, which frees up more metal (elements other than hydrogen and helium) to be used in new stars.

While it is fairly straightforward to model the broad trends of the chemical composition of stars with a galaxy, it would be nice to have better models. Enter a new attempt at a model integrating the dynamical motion of stars with their chemical evolution. The first results were presented in Astronomy and Astrophysics (paywalled, but the arxiv is here). In this paper the authors look at modelling the Milky Way.

To model the chemical evolution of the Milky Way, the team had to account for the fact that stars have the metallicity of their surroundings at the time and location of their formation, but also that stars tend to drift outward over time. So metallicity is a product of both star formation and stellar drift.

You can see the results in the figure here (from the paper). In the top row shows the stellar formation rate over time and you can see that star formation peaks at about 3 billion years and gradually tapers off. In the middle row shows a more detailed history, plotting the stellar formation rate over time at different distances from galactic center. The bottom row shows the metallicity in terms of the iron-hydrogen ratio and oxygen-iron ratio over at different times as a function of distance from the center.

The results are a reasonable match to the Milky Way, and this allows the authors to do a bit of detective work regarding the real Milky Way and our Sun in particular. Given the metallicity of our Sun, and its current location about 26,000 light years from galactic center, the authors calculated that our Sun likely formed at a location 18,000 light years from the center, and then gradually drifted out to its present location.

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One of the exciting aspects of astrophysics (and any area of science really) is how odd discoveries lead to new understanding. As a case in point, consider our understanding of the gas and dust in our galaxy. Gas and dust is often seen as an annoyance because it gets in our way. Observational astronomy would be much easier if it weren’t for all the gas and dust. Sure, gas clouds will sometimes collapse to form new stars, but beyond that it surely wasn’t doing anything interesting.

It was long thought that the gas and dust in the galaxy was largely inert. Occasionally a couple of atoms might collide and bond, but if by some chance a molecule larger than two atoms were to form it would quickly be broken apart by the ultraviolet light from nearby stars. Besides, deep space is really cold (about 3K) so there isn’t enough heat to drive any real chemistry.

As radio astronomy developed, however, we starting observing interstellar clouds containing larger molecules such as water, methane and alcohols. More recently infrared observations have observed hydrocarbon gases. It would seem that interstellar clouds contained far more than simple diatomic molecules.

Since these complex molecules are located in large gas and dust clouds, they are protected from the ultraviolet radiation that would break them apart. But that didn’t explain how they formed in the first place. Even given the fact that molecular clouds are somewhat warmer (about 10K) and denser, the random collisions of gas molecules aren’t enough to create the range and quantity of complex molecules we observe. Something else must be driving their creation.

What we now know is that molecular clouds have a catalytic converter. The catalytic converter in your car consists of some type of catalyst (typically platinum) applied thinly over a large surface area. Having lots of surface area is key, because your catalytic converter relies on surface physics. Basically when chemical reactions happen on a surface they are constrained a thin layer on the top of the surface. This means you can get chemical reactions that are faster and more complex. This surface chemistry is necessary for your catalytic converter to be effective.

In interstellar clouds the surfaces of dust particles act as a catalytic converter. Sometimes atoms and molecules collide with a dust particle and get trapped on its surface. As more molecules get trapped they can interact in complex ways, which can create the complex molecules we see.
So we now know that interstellar clouds are far from inert. A complex chemical dance occurs between their dusts and gases, and we are only beginning to understand it.

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If you live in the United States, you will likely take in an evening of fireworks. While you are enjoying them safely, you’ll notice that fireworks come in a variety of colors. The different colors are due to various metallic salts that are used in the fireworks. For example, reds can be created with strontium or lithium salts, orange with calcium, green with barium, blue with copper, and so on. A wide variety of colors can be produced by mixing these compounds as well.

The reason these salts give color to fireworks is because of their emission spectra. When these metallic compounds are superheated they emit light. Because of their molecular structure, they only emit light at particular wavelengths (colors). Particular compounds are chosen because the wavelengths they emit are in a particular color range. If the brightest wavelengths are green, then the firework will look green, for example.

The emission spectra of each atomic and molecular compound has a unique pattern. It is like a fingerprint that uniquely identifies the type of material. So as you watch the evening fireworks you can identify the type of salt compounds by their color. See a blue firework and it is likely copper. See a red one and it is likely strontium.

With the naked eye you can only identify broad types of compounds, and since some colors have multiple possibilities you can’t be exactly sure which type is used. However if you were to analyze the emission spectra of the fireworks, then you could identify the exact compounds being used. This is a bit tricky to do precisely, but if you have a pair of “fireworks glasses” or “rainbow glasses” to watch the fireworks, then you can observe some of the spectra. You can also check out some emission spectra here.

So what does this have to do with astronomy? When you identify compounds by the color of fireworks, you are using the same technique astronomers use to identify elements in distant stars. The unique fingerprint of emission spectra is the same everywhere in the universe, so when astronomers measure a spectra in a distant star they know what particular elements and molecules exist in that star.

So if you watch fireworks under a starry sky tonight, try your hand at identifying compounds. And take a moment to ponder their connection to the stars.

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There’s an old joke that the astronomer’s periodic table consists of three elements: hydrogen, helium, and metal. It’s a nice joke, but when you understand how little of matter in the universe is “metal”, you can understand why astronomers focus on hydrogen and helium.

You can see this in the figure, which gives the relative abundance of different elements by area, the larger the square, the more of that particular element there is. As you can see, most of the elements of the periodic table are too small to even appear. The few others that do show up are dwarfed by the hydrogen and helium squares.

Where the joke is accurate is in the fact that astronomers often refer to all the elements beyond helium as metals. This is is why we use terminology as the metallicity of a star. Since the “metals” of a star includes everything not hydrogen and helium, one way to define the metallicity of a star is simply as the fraction of a star’s mass which is not hydrogen or helium. For the Sun, this number is Z = 0.02, which means that about 2% of the Sun’s mass is “metal”.

Another way to express the metallicity of a star is by its ratio of Iron to Helium, known as [Fe/He]. This is given on a logarithmic scale relative to the ratio of our Sun. So the [Fe/He] of our Sun is zero. Stars with lower metallicity will have negative [Fe/He] values, and ones with higher metallicity have positive values.

Stars are often categorized by their metallicity. For example, Population I stars have an [Fe/He] of at least -1, meaning they have 10% of the Sun’s iron ratio or more. Population II stars have an [Fe/He] of less than -1. There is a third category, known as Population III. These would be the first stars of the universe, with essentially no “metals” in them. We have yet to observe this type of star.

Population I stars tend to be younger than Population II stars. Population I stars also tend to be located in the spiral arms of our galaxy, while Population II stars tend to be in the outer (halo) region of our galaxy. This makes sense, because one would expect Population II stars to either be older, and therefore formed when there was less metal around, or simply formed in areas where there is less dust (which tends to be high in “metal”).

The metallicity of a star does have certain consequences. For example, higher metallicity tends to make a star appear slightly redder than expected. This is because metals tend to absorb blue wavelengths more than red ones. Also, metals have a higher opacity than hydrogen and helium, so they absorb some of the heat from the star’s interior, which can cause the atmosphere of the star to expand a bit, thus appearing cooler and more red.

Of course then there is the fact that Population I stars are more likely to have planets than Population II stars, but that’s another story.

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Around 450 BC, the Greek philosopher Empedocles wrote that the world was comprised of four things: earth, air, fire and water. Plato referred to them as the four elements. These were not elements in the modern sense, but rather essences that gave everything their physical properties. The idea that everything was made of these fundamental elements had a deep influence on early Western science. It was a central aspect of alchemy until Robert Boyle demonstrated there were more than four elements in 1661. The four elements also connected to the four humours of the human body, which formed a basis of Western medicine until the 1800s.

Credit: ASU School of Life Sciences

Over the past two centuries, we have gained a much better understanding of the atomic elements and how they have formed. One of the things we have learned is that we—and every other living thing on Earth—are made up mostly of four elements. These four atomic elements are oxygen, carbon, hydrogen, and nitrogen. Together they make up about 96% of our bodies, as you can see in the figure.

There are 92 naturally occurring elements on Earth, from hydrogen to uranium, so why do these four make up such a majority of living things? Part of the reason lies in the fact that they are versatile elements, capable of producing a vast array of chemical compounds, but it also has to do with the fact that they are among the most abundant elements in the universe. To understand why, we have to look to the stars.

The first elements appeared a few minutes after the big bang, through a process known as nucleosynthesis. The elements produced by the big bang consisted of about 75% hydrogen and 25% helium (by mass) with trace amounts of lithium and beryllium. For the next several hundred million years only these four elements existed. Then the first stars appeared. They formed from large clouds of hydrogen and helium, and as they collapsed under their own weight the hydrogen in their cores began to fuse into helium.

The energy produced by this nuclear fusion gives a star the light and heat necessary to counter the force of gravity for a time, but as a star ages the amount of helium in the stellar core increased. As helium become more plentiful in the stars core, some of it fuses into carbon. The carbon interacts with the hydrogen to produce nitrogen and oxygen as well as helium, through a process known as the CNO cycle. As a star ages the CNO cycle becomes the dominant process by which a star creates light and heat. As a result, these elements become fairly plentiful within a star.

The first stars are thought to have been very large stars. Toward the end of a their lives they produced even heavier elements, such as silicon, neon, and eventually iron. Beyond iron there are no elements a star can fuse to produce energy. After several hundred thousand years these first stars had no further way to produce energy, and in the end they explode in a massive explosion known as a supernova. The gas and dust remnants of these stars were tossed out into the universe. Over time this gas and dust became part of clouds that formed new stars, which also fused hydrogen and helium into heavier elements until they too died in supernova explosions.

Then about five billion years ago a cloud of gas and dust began to form a new star. Thanks to the lives and deaths of earlier generations of stars this cloud was rich not just in hydrogen and helium, but also in carbon, nitrogen, oxygen, and iron. As the star formed, some of the dust formed a disk around the star, out of which formed planets. The third planet from this star had the good fortune of being not too close to the star and not too far away. It had plenty of hydrogen and oxygen in the form of water, as well as carbon and nitrogen, all thanks to long dead stars. Eventually life appeared on this small world, and took advantage of these useful and plentiful elements.

The atoms in your body contain the history of the universe. The hydrogen in your body was born among the first elements, about 13.7 billion years ago. The carbon, nitrogen and oxygen in your muscles and mind were created within a star that died more than 5 billion years ago.

You are the universe made manifest.

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There’s an old joke that says that the astrophysics periodic table consists of three things: hydrogen, helium and metal.  The reason for this is that hydrogen and helium make up about 98% of the atoms in universe.  Everything else on the periodic table, from lithium on down, make up only 2% of atoms.  On a cosmic level, these “metals” are just trace elements in the hydrogen-helium universe.

The reason hydrogen and helium dominate is the period of nucleosynthesis that started about three minutes after the big bang.  This period lasted about 17 minutes, during which time all the original atoms of the universe were produced. About 92% of those atoms (by number) were hydrogen and about 8% were helium.  Other elements such as oxygen and iron weren’t formed during this period.  They were created later in the cores of early stars, hence Carl Sagan’s famous assertion that we are all “star stuff.”

The nucleosynthesis period also produced a tiny amount of lithium, about 1 lithium atom for every billion hydrogen or helium atom.  We can actually use this fact to learn things about particular stars.  For example, the atmosphere of our sun has only about 1 lithium atom for every 100 billion, or about 1% of the number you might expect.  So what happened?  Where is the missing lithium?

The answer has to do with the way stars create new atoms.  In the core of a star, hydrogen atoms are fused into helium atoms, which releases energy.  This works because of the strength at which the nucleus of an atom holds itself together, known as its binding energy.  Since the binding energy of helium is higher than hydrogen, producing helium from hydrogen releases energy.  This is true for other elements as well.  Helium to carbon, nitrogen and oxygen, and so on up to iron. Iron has the highest binding energy, so fusing elements beyond iron actually costs energy.

In the figure below, I’ve plotted the binding energy of different elements.  You can see how it curves up to iron.  You’ll also notice that there are regions where the binding energy dips, and lithium is in one of those dips.  This means that while many atoms can release energy when they are fused, lithium is an energy loser. As a result, the same process that creates heavier atoms like oxygen and iron also reduces the amount of lithium.  Basically, some of the lithium is burned away in the process of making heavier elements.

Since our sun has less lithium than expected in its atmosphere it must have been formed from material that was once in the core of a star.  That means our sun is not one of the first stars of the universe, but rather formed from the remains of earlier stars.  Given the missing lithium and other evidence, our sun appears to be a third-generation star.  That means that means our sun is a grandchild of the first stars of the universe.  Two generations of stars lived and died before our sun formed.

Even our sun is made of “star stuff.”

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