The eerie, hellish glow coming from the Moon may seem unreal in this image, since it’s invisible to our eyes. But instruments that detect gamma rays tell us it’s real. More than just a grainy, red picture, it’s a vivid reminder that there’s more going on than meets human eyes.
It’s also a reminder that any humans that visit the Moon need to be protected from this high-energy radiation.
The speed of light gives us an amazing tool for studying the Universe. Because light only travels a mere 300,000 kilometers per second, when we see distant objects, we’re looking back in time.
You’re not seeing the Sun as it is today, you’re seeing an 8 minute old Sun. You’re seeing 642 year-old Betelgeuse. 2.5 million year-old Andromeda. In fact, you can keep doing this, looking further out, and deeper into time. Since the Universe is expanding today, it was closer in the past.
Run the Universe clock backwards, right to the beginning, and you get to a place that was hotter and denser than it is today. So dense that the entire Universe shortly after the Big Bang was just a soup of protons, neutrons and electrons, with nothing holding them together.
In fact, once it expanded and cooled down a bit, the entire Universe was merely as hot and as dense as the core of a star like our Sun. It was cool enough for ionized atoms of hydrogen to form.
Because the Universe has the conditions of the core of a star, it had the temperature and pressure to actually fuse hydrogen into helium and other heavier elements. Based on the ratio of those elements we see in the Universe today: 74% hydrogen, 25% helium and 1% miscellaneous, we know how long the Universe was in this “whole Universe is a star” condition.
It lasted about 17 minutes. From 3 minutes after the Big Bang until about 20 minutes after the Big Bang. In those few, short moments, clowns gathered all the helium they would ever need to haunt us with a lifetime of balloon animals.
The fusion process generates photons of gamma radiation. In the core of our Sun, these photons bounce from atom to atom, eventually making their way out of the core, through the Sun’s radiative zone, and eventually out into space. This process can take tens of thousands of years. But in the early Universe, there was nowhere for these primordial photons of gamma radiation to go. Everywhere was more hot, dense Universe.
The Universe was continuing to expand, and finally, just a few hundred thousand years after the Big Bang, the Universe was finally cool enough for these atoms of hydrogen and helium to attract free electrons, turning them into neutral atoms.
This was the moment of first light in the Universe, between 240,000 and 300,000 years after the Big Bang, known as the Era of Recombination. The first time that photons could rest for a second, attached as electrons to atoms. It was at this point that the Universe went from being totally opaque, to transparent.
And this is the earliest possible light that astronomers can see. Go ahead, say it with me: the Cosmic Microwave Background Radiation. Because the Universe has been expanding over the 13.8 billion years from then until now, the those earliest photons were stretched out, or red-shifted, from ultraviolet and visible light into the microwave end of the spectrum.
If you could see the Universe with microwave eyes, you’d see that first blast of radiation in all directions. The Universe celebrating its existence.
After that first blast of light, everything was dark, there were no stars or galaxies, just enormous amounts of these primordial elements. At the beginning of these dark ages, the temperature of the entire Universe was about 4000 kelvin. Compare that with the 2.7 kelvin we see today. By the end of the dark ages, 150 million years later, the temperature was a more reasonable 60 kelvin.
For the next 850 million years or so, these elements came together into monster stars of pure hydrogen and helium. Without heavier elements, they were free to form stars with dozens or even hundreds of times the mass of our own Sun. These are the Population III stars, or the first stars, and we don’t have telescopes powerful enough to see them yet. Astronomers indirectly estimate that those first stars formed about 560 million years after the Big Bang.
Then, those first stars exploded as supernovae, more massive stars formed and they detonated as well. It’s seriously difficult to imagine what that time must have looked like, with stars going off like fireworks. But we know it was so common and so violent that it lit up the whole Universe in an era called reionization. Most of the Universe was hot plasma.
The early Universe was hot and awful, and there weren’t a lot of the heavier elements that life as we know it depends on. Just think about it. You can’t get oxygen without fusion in a star, even multiple generations. Our own Solar System is the result of several generations of supernovae that exploded, seeding our region with heavier and heavier elements.
As I mentioned earlier in the article, the Universe cooled from 4000 kelvin down to 60 kelvin. About 10 million years after the Big Bang, the temperature of the Universe was 100 C, the boiling point of water. And then 7 million years later, it was down to 0 C, the freezing point of water.
This has led astronomers to theorize that for about 7 million years, liquid water was present across the Universe… everywhere. And wherever we find liquid water on Earth, we find life.
So it’s possible, possible that primitive life could have formed with the Universe was just 10 million years old. The physicist Avi Loeb calls this the habitable Epoch of the Universe. No evidence, but it’s a pretty cool idea to think about.
I always find it absolutely mind bending to think that all around us in every direction is the first light from the Universe. It’s taken 13.8 billion years to reach us, and although we need microwave eyes to actually see it, it’s there, everywhere.
While our Sun will only survive for about 5 billion more years, smaller, cooler red dwarfs can last for trillions of years. What’s the secret to their longevity?
You might say our Sun will last a long time. And sure, another 5 billion years or so of main sequence existence does sound pretty long lived. But that’s nothing compared to the least massive stars out there, the red dwarfs.
These tiny stars can have just 1/12th the mass of the Sun, but instead of living for a paltry duration, they can last for trillions of years. What’s the secret to their longevity? Is it Botox?
To understand why red dwarfs have such long lifespans, we’ll need to take a look at main sequence stars first, and see how they’re different. If you could peel back the Sun like a grapefruit, you’d see juicy layers inside.
In the core, immense pressure and temperature from the mass of all that starstuff bears down and fuses atoms of hydrogen into helium, releasing gamma radiation.
Outside the core is the radiative zone, not hot enough for fusion. Instead, photons of energy generated in the core are emitted and absorbed countless times, taking a random journey to the outermost layer of the star.
And outside the radiative zone is the convective zone, where superheated globs of hot plasma float up to the surface, where they release their heat into space.
Then they cool down enough to sink back through the Sun and pick up more heat. Over time, helium builds up in the core. Eventually, this core runs out of hydrogen and it dies. Even though the core is only a fraction of the total mass of hydrogen in the Sun, there’s no mechanism to mix it in.
A red dwarf is fundamentally different than a main sequence star like the Sun. Because it has less mass, it has a core, and a convective zone, but no radiative zone. This makes all the difference.
The convective zone connects directly to the core of the red dwarf, the helium byproduct created by fusion is spread throughout the star. This convection brings fresh hydrogen into the core of the star where it can continue the fusion process.
By perfectly using all its hydrogen, the lowest mass red dwarf could sip away at its hydrogen fuel for 10 trillion years.
One of the biggest surprises in modern astronomy is just how many of these low mass red dwarf worlds have planets. And some of the most Earthlike worlds ever seen have been found around red dwarf stars. Planets with roughly the mass of Earth, orbiting within their star’s habitable zone, where liquid water could be present.
One of the biggest problems with red dwarfs is that they can be extremely variable. For example, 40% of a red dwarf’s surface could be covered with sunspots, decreasing the amount of radiation it produces, changing the size of its habitable zone.
Other red dwarfs produce powerful stellar flares that could scour a newly forming world of life. DG Canes Venaticorum recently generated a flare 10,000 times more powerful than anything ever seen from the Sun. Any life caught in the blast would have a very bad day.
Fortunately, red dwarfs only put out these powerful flares in the first billion years or so of their lives. After that, they settle down and provide a nice cozy environment for trillions of years. Long enough for life to prosper we hope.
In the distant future, some superintelligent species may figure out how to properly mix the hydrogen back into the Sun, removing the helium, if they do, they’ll add billions of years to the Sun’s life.
It seems like such a shame for the Sun to die with all that usable hydrogen sitting just a radiative zone away from fusion.
Have you got any ideas on how we could mix up the hydrogen in the Sun and remove the helium? Post your wild ideas in the comments!