The Origin of Life: All It Took Was Hydrogen

The Origin of Life: All It Took Was Hydrogen

The concept is one of the most fundamental ideas in astronomy: light is a time machine. Light races through the universe at 299,792 kilometers per second or 670 million miles per hour. But even at that astonishing speed, when we observe objects in space that are millions or billions of light years away, we're seeing them as they were in the distant past. Their light takes THAT long to reach us! So when we look out into the universe, we are literally looking backwards in time. It's kind of like receiving a letter from someone far away. The further away they are, the longer their letters take to arrive. Looking at a distant galaxy is like opening a letter written millions, or even billions, of years ago.

In 1929, astronomer Edwin Hubble made a groundbreaking discovery: galaxies are moving away from us, and the farther away a galaxy is, the faster it's receding. Indicating that the universe itself is expanding! In other words, everything in the cosmos was once much closer together. And no, this doesn't mean that the earth is at the center of the universe. The fabric of the universe is expanding everywhere (kind of like the surface of a balloon being blown up). All these dots are moving away from each other, even if none of them is the center of the balloon-iverse.

But how did Hubble figure any of this out? How can we tell just how far away an object is, or what distant objects are made of?

To answer that, we have to rewind a bit. In 1802, English chemist William Hyde Wollaston became the first person to notice strange dark lines in the spectrum of sunlight, although he didn't fully appreciate what he was looking at. Then in 1814, German physicist Joseph von Fraunhofer independently rediscovered those lines, and using far more precise instruments, mapped hundreds of them in painstaking detail. Those dark gaps in the rainbow of sunlight are now called Fraunhofer lines.

For decades, nobody actually knew what they meant. The real breakthrough came from two more Germans, chemist Robert Bunsen and physicist Gustav Kirchhoff, who in the 1850s and 60s worked out that each element produces its own unique pattern of spectral lines. Their work laid the foundation of modern spectroscopy and finally cracked the code.

Here's how it works. Astronomers use a tool called a spectrograph, which splits light into its individual wavelengths to create a spectrum. Sort of like a rainbow. Each color represents a different wavelength of light. But beyond just the colors you can see, spectrographs can detect very specific wavelengths absorbed or emitted by different elements. Basically, whenever light bounces off of anything, specific wavelengths get absorbed depending on what it's bouncing off of. Iron absorbs a different part of the visual spectrum than helium or oxygen do.

Think of these spectral lines as barcodes on products. A scanner reads a barcode to identify the item. Astronomers read the spectral lines in starlight to identify the elements inside a star.

But that same technique unlocked something else, too. When astronomers pointed their spectrographs at distant galaxies, they noticed something strange. The familiar fingerprints of hydrogen, helium, and other elements were all there, but every single line had been shifted toward the red end of the spectrum. And the farther away the galaxy was, the more its lines had shifted. This is called redshift, and it's exactly how Hubble figured out that galaxies are moving away from us, and how quickly. The light of a receding object gets stretched into longer, redder wavelengths, similar to the way the pitch of a siren drops as an ambulance speeds past you.

So light could now be used as a window into the physical and chemical properties of distant objects, from stars to entire galaxies. Today, spectroscopy is one of the most powerful tools in astronomy. It tells us what elements are present in a star, how hot it is, how fast it's moving, and even how old it is.

In 1925, pioneering astrophysicist Cecilia Payne-Gaposchkin used these techniques to make one of the most important discoveries in the history of astronomy. By analyzing the spectra of stars, and combining her observations with the brand-new field of quantum mechanics, she determined that stars are made primarily of hydrogen and helium. This shattered the prevailing assumption that stars were composed mostly of heavier elements like the rocks and metals we find on Earth. Hydrogen, the simplest element in the universe, turned out to be by far the most abundant, with helium coming in second.

So how did those first stars actually form, and where do the heavier elements on the periodic table come from? Well things started extremely basic. When astronomers look at the most distant light in the universe, they're seeing the afterglow of the Big Bang itself. No stars. No planets. Just vast clouds of hydrogen and helium gas. Hydrogen, the simplest element there is, with nothing more than a single proton at its core.

And lucky for us, the ingredients to make a star are surprisingly simple: all you need is matter, gravity, and time. You see, ever since Isaac Newton's law of universal gravitation in 1687, we've understood that gravity pulls matter together, even acting on the smallest particles.

So, over time, thanks to gravity, these vast clouds of hydrogen and helium began to collapse. As those clouds contracted, pressure built up. Pressure makes heat. The temperatures soared. Eventually these gas clouds ignited into gigantic fireballs, and the first stars were born. This isn't speculative. We can actually still watch this entire process unfolding today in gas clouds called nebulae. These are the nurseries where new stars are born.

Now, hydrogen and helium alone aren't enough to build planets, life, or most of the stuff you see around you. So where do the heavier elements come from? Well, inside the heart of every star, something extraordinary is happening. Under normal conditions, two positively charged hydrogen nuclei, protons, would repel each other. But in the extreme heat and pressure of a star's core, protons move so fast and get squeezed so close together that they occasionally overcome their natural repulsion and merge into a heavier element. This process is called nuclear fusion. Hydrogen nuclei fuse into helium releasing enormous amounts of energy and keeping the star shining steadily for billions of years.

But eventually, the hydrogen in the core starts to run out. For the smallest stars, those below about 0.8 solar masses, this is basically where the story ends. But for heavier stars, including our own Sun, the core contracts and heats up further, and helium nuclei begin slamming together to form carbon. From there, carbon can fuse into oxygen, then neon, magnesium, silicon, and in the most massive stars, all the way up to iron.

In 1939, physicist Hans Bethe described how this all works, laying out the proton-proton chain that powers smaller stars like the Sun, and the carbon-nitrogen-oxygen cycle that drives more massive ones. He showed exactly how lighter elements fuse to form heavier ones, and how all that fusion releases the energy that keeps a star shining. Since then, his theories have been confirmed by a wide range of evidence. Solar neutrinos, subatomic particles produced in nuclear reactions inside the Sun, have been detected here on Earth. Stellar spectra match Bethe's predictions, and advances in particle and nuclear physics demonstrated the underlying mechanics of fusion. These cumulative findings earned Bethe the Nobel Prize in Physics in 1967.

The story of stars doesn't end with fusion. Once a massive star has built up iron in its core, it can no longer produce energy through fusion. Iron is the dead end of the stellar furnace. With its fuel spent, gravity takes over, the core collapses in a fraction of a second, and the star dies in a spectacular explosion called a supernova, a cosmic explosion so violent that it forges elements even heavier than iron, like gold, silver, and uranium, in a process known as rapid neutron capture (the r-process).

This explosion scatters every element the star ever produced out into the cosmos, seeding the universe with the building blocks for new stars, new planets, and eventually, life. And once again, this isn't just theoretical. One of the most famous observed supernovae, SN 1987A, gave astronomers a direct look at the explosive death of a star. Telescopes captured its light, allowing us to study the elements thrown out by the explosion in real time.

But how do the elements created in stars end up forming planets like ours?

After a supernova explodes, the remnants get blown out into space, much like a tree releasing seeds into the wind after it blooms. The seeds drift away, land in fertile ground, and grow into new trees. In the same way, supernovae scatter elements that become the seeds for new stars and planetary systems.

As the wreckage of old stars mixes with fresh clouds of hydrogen and helium, gravity slowly pulls everything back together. Sometimes this debris forms new stars. Other times, it forms planets like earth or gas giants like the planet Jupiter which could have become a star, but never quite got big enough.

This new generation of stars and planets are all formed out of this enriched material, containing not just hydrogen and helium, but carbon, oxygen, iron, and even gold. These elements are the rocks under your feet, and the inorganic matter all around you. They're the molecules you eat and the air in your lungs. They're your skin, your hair, the blood cells running through your veins. They are you. You are stardust.

From the simplest building block, hydrogen, stars forged the entire diversity of matter we see in the universe. They're unconscious cosmic factories, endlessly producing all of the building blocks for life, for planets, for water, air, dirt, animals, and us.

So as dust and gas coalesced into our planet billions of years ago, with meteors and asteroids battering its surface, Earth slowly took shape. But as the dust settled and the planet cooled, how did life finally emerge from simple lifeless chemistry?

Sources Cited:

A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae (Hubble, 1929):
https://pmc.ncbi.nlm.nih.gov/articles/PMC522427/ 

Cecilia Payne-Gaposchkin and the Day the Universe Changed (American Physical Society):
https://www.aps.org/publications/apsnews/201501/physicshistory.cfm 

Energy Production in Stars (Bethe, 1939):
https://www.sfwriter.com/energy-production-in-stars.pdf 

Synthesis of the Elements in Stars / B²FH (Burbidge, Burbidge, Fowler & Hoyle, 1957):
https://doi.org/10.1103/RevModPhys.29.547 

Why Some of the Rainbow is Missing (PBS - Be Smart):
https://www.youtube.com/watch?v=gVZwdYZqCUI

The History of Spectroscopy:
https://en.wikipedia.org/wiki/History_of_spectroscopy 

The R-Process:
https://en.wikipedia.org/wiki/R-process

The CNO Cycle:
https://en.wikipedia.org/wiki/CNO_cycle

The Dawn of a New Era for Supernova 1987A:
https://science.nasa.gov/missions/hubble/the-dawn-of-a-new-era-for-supernova-1987a/

Power of the Sun: Elusive Solar Neutrinos Detected, a Cosmic First:
https://www.space.com/26959-solar-neutrinos-sun-fusion-detection.html

Webb NIRSpec multi-object spectrograph:
https://www.esa.int/ESA_Multimedia/Videos/2021/09/Webb_NIRSpec_multi-object_spectrograph 

Spectroscopy: Turning Light Into Data (University of Chicaco):
https://ecuip.lib.uchicago.edu/multiwavelength-astronomy/astrophysics/08.html

Fraunhofer Lines: Some early history and physics of solar spectra:
https://joachimweise.github.io/post/2020-10-20-fraunhofer-lines/

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