What are stars?
Despite their ubiquity in our universe, their praises often go unsung. A friend admitted to me once that he hadn’t realized — you know, really viscerally realized — that our sun was itself a star until he was in his twenties. From that moment forward, however, every glance at the night sky bowled him over with such an emotion of vast familiarity that he could hardly stand to look at it. And with just cause: every star, like our sun, is a wonder, a factory producing almost all the heavy elements floating around the cosmos — including the everyday matter that makes up the entirety of life on Earth. Every star is a visible light generator, a heat machine, a life source for the universe, and mass of plasma held together by energy which has a birth, life, and death. There is no doubt that we’re lucky to have them, that they are special and an endless source of fascination.
But, as it turns out, they’re not unique in the slightest.
Dig this. You may be familiar with the notion of an “anthropic cosmological principle,” a much-debated little tenet of scientific thought which states that, essentially, our universe must, as a matter of course, have the specific properties which allow life to develop (specifically that life which observes the universe: us), or else we wouldn’t be here to observe it. That is to say, variations in properties of the universe that would not allow for life are simply impossible, since they would make for a different universe entirely — one which wouldn’t feature us in a starring role. Those properties include what are called the “fundamental constants” — gravity, the speed of light, et al. — those aspects of physical reality which dictate how stars work. According to the anthropic principle, if another universe had different values for the fundamental constants, it would be unrecognizable and could not, or only rarely, accommodate for life.
Enter Fred C. Adams, stellar astrophysicist and co-author of The Five Ages of the Universe: Inside the Physics of Eternity, who realized that nobody had ever actually done the math. So he did. In a paper in the Journal of Cosmology and Astroparticle Physics (JCAP) called “Stars in Other Universes: Stellar Structure with Different Fundamental Constants,” Adams sought to answer the simple question, “could stars, and hence life, exist even if the fundamental constants were wonky?”
As it turns out, about one fourth of the parameter space allows for stars, so theoretically universes with stars aren’t rare at all. Of course, that value depends on your definition of star-dom, how broad you allow the range to be, but the idea remains strong: another straw on the back of the anthropic principle.
What interests me, of course, is what defines a star. How broad can these parameters be? When do stars cease to be stars as we know them, and become, rather, something else entirely? After all, our current universe contains endless peculiarities, many of which we call stars: red supergiants, white dwarves, blue stragglers. How weird can a star be and still be a star? I asked Fred about this.
“The stars in other universes are both the same and different. They are the same in the sense that they are stars: if you change the fundamental constants too much in one direction, then nuclear processes shut down and you don’t get nuclear burning stars. If you change the fundamental constants too much in a different way, then you can have nuclear reactions, but not in a controlled long-lived way like the Sun — you get a bomb (explosion!!) instead. And that is not so good for life. So universes with stars must have a “goldilocks” ensemble of their fundamental constants, and my work set out to define that parameter space. However, this exercise just delineates where you get stars AS DEFINED ABOVE. They will be larger or smaller, shorter- or longer-lived, brighter or dimmer, etc., than the stars in our universe, i.e., they will be weird like you like…
The stars we do have are incredible. But imagine a universe with slightly different laws of physics, where nature and randomness might have created a flawed mirror of our world: dim stars, groucho stars, diffuse stars, colorful ones, brighter ones…how might have life evolved differently to adapt to these different shining producers of matter and life?