A Star is Born

Welcome to the first stop on our tour of the Drake equation, one way to estimate how many extraterrestrial civilizations might be out there and able to communicate with us. To sum up from a couple of posts ago:

N = R* × fp × ne × fl × fi × fc × L

In this equation,

• N is the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy
• R* is the rate at which new stars are created in our galaxy
• fp is the fraction of stars that have planets
• ne is the average number of planets per star that might support life
• fl is the fraction of life-supporting planets that actually develop life
• fi is the fraction of developed life that becomes intelligent
• fc is the the fraction of intelligent life that sends signals into space
• L is how long a signal-sending civilization survives and sends those signals

The jury is still out on whether we are intelligent, but we have managed to send detectable signals into space, so we know N is at least 1.

Today, we discuss R*, the rate at which new stars are created in our galaxy.

Our Sun seems like a constant, and for all practical purposes, to a human being, it is. During the time between your first and last breaths, nothing that you experience of our Sun changes in any appreciable way. It’s bright, it’s hot, and it has what appears to be an immutable daily routine. But it does also have a lifetime of its own, and that includes a tumultuous birth around five billion years ago. All stars are born from a combination of gravity and nuclear fusion, and the vast majority of their lifespans consist of a delicate balance between the two. So how does it all begin?

To start, our universe is clumpy. This may sound vague or even slightly insulting, but while a less clumpy universe would have been far more elegant, it would also have been far less interesting. Because the universe is clumpy, atoms will occasionally get close enough together to attract each other through a mutual pull that we call gravity. The mutual part is an important thing to remember when you’re thinking about gravity (and I know you do that all the time) – gravity is a mutual attraction between any two objects with mass (“stuff”). So you are pulling on the Earth just as surely as the Earth is pulling on you. It’s just that the Earth is a tad bit bigger, so our perception is that the Earth is doing all the pulling.

In the case of atoms in space, if gravity brings them close together, their combined mass pulls harder on everything around them, which in turn creates a larger mass, which attracts more atoms, and I presume you get the picture. It’s kind of like the way beaches were before the pandemic, or the way beaches are for idiots during the pandemic. The more there are, the more there are. Eventually, you get a huge collection of atoms pulling each other closer, and increasingly more likely to run into each other, and the resulting energy of those collisions makes them go faster, and then they run into each other some more, and that is essentially the definition of a rising temperature.

So now you’ve got this ball of gas – usually overwhelmingly composed of hydrogen, getting denser and hotter and collapsing upon itself. Gravity doesn’t stop no matter how close two objects get to each other, so if there were no other forces at play, this ball of gas would just continue shrinking upon itself until it vanished into an infinitely dense point, known in nerd circles as a singularity. And again, that would be rather elegant and simple, but also not conducive to an interesting universe.

Fortunately for us, there are other forces at play. At the heart of every atom is a nucleus – some combination of protons and neutrons. Most hydrogen atoms are composed of only a single proton, but some atoms of hydrogen and all atoms of any other substance are composed of two or more protons and/or neutrons. They are held together by what physicists call the strong force. You have to push protons and neutrons very close together to get that force to kick in, but once it does, it overpowers all other forces of nature. So, as a ball of hydrogen gas shrinks upon itself, eventually the atoms get close enough to each other that the strong nuclear force takes hold, and the protons and neutrons fuse together. When this happens, they release a tremendous amount of energy, further heating the shrinking ball of gas. At a certain point, the energy released by the fusion of atoms creates a temperature so high that it balances the force of gravity that caused the shrinking in the first place, and the ball of gas stops shrinking. Nuclear fusion also makes the ball of gas so hot that it radiates visible light, and with that, a star is born.

Now that you’re an expert on star formation, all we have left to do is figure out how frequently this happens in our modern age. I don’t know about you, but I wouldn’t know where to begin. Fortunately, there are people who do this kind of thing for a living, combining observations of the stars we can see with our maturing physical understanding of how stars are born, how long they survive, and how they eventually die. When you put it all together, the estimates seem fairly consistent: an average star like our Sun is born somewhere in our galaxy between one and three times each year.

I can’t put “between one and three” in an equation. Fortunately, there’s a thing between one and three known as two – and so we’re going to use that. R* = 2. Anytime you can replace a letter in an equation with a number, you’ve made some progress. So let’s kick back, crack open an ice cold beverage, and take in the view of our slightly less ambiguous calculation for intelligent life in the Galaxy:

N = 2 × fp × ne × fl × fi × fc × L

The next post will zero in on fp – the fraction of stars that have planets. But for now, it’s kind of cool to think about the notion that a couple of times each year, a star is born somewhere in our galaxy. Combine that with our current life expectancy, and the majority of Americans can rest assured that over a hundred stars will be born during their lifetimes in our galaxy alone. The numbers become insane when you extrapolate to other galaxies, but the nearest major one of those is two million light years away, meaning any signal we might receive from there was sent long before human beings existed as a species. So it’s probably ok to keep our focus on the Milky Way for now. Even doing that – once or twice a year, somewhere in our galaxy, a star like our Sun ignites into existence, with the potential for planets, life, intelligence, technology, and blogs.

Damn, that’s a lot of blogs.