The other day over lunch we half-jokingly talked about whether or not stars are alive. A colloquialism among astronomers is that a star is "dead" when there is no more nuclear fusion, so stars in the late stages of their evolution are "dying". Some smartass took the metaphor literally and pointed out that stars are always dead, never alive. But are they, really? they asked in return.

Now, the gut reaction is, of course, that stars are not alive. They are described quite well by physics alone, with a little bit of inorganic chemistry. But one should always question one's own intuitions and stars after all do have metabolism, a life-cycle and are "born" in generations. They do not reproduce directly, but by enriching the interstellar medium with they fusion products, thereby influencing the next generation of stars. Stretching the analogy, one could say that stars that fail in doing this (for example stars that collapse into a black hole without supernova explosion) have "failed to reproduce" in an evolutionary sense.

At the time I could not quite articulate why it still felt wrong to me to call stars alive, and I did not want to argue by some outdated list of strict criteria, vaguely remembering that the definition of life does not have such a clear-cut answer as one would like.

Luckily one the podcats I listen to just answered it for me: Sean Carroll interviewing Sara Imari Walker on Information and the Origin of Life .

They don't talk about stars, but connecting life to information processing was what had slipped my mind over lunch that day. I will not reiterate it here but highly recommend to listen for yourself, or read the transcript on that same page.

A highlight for me was this section:

There is something about intelligence as a physical process that’s quite different. If you just had physics and chemistry and no biology, no organisms, no evolutionary history acquiring info§rmation, you would never see a planet launching satellites into space.

What’s interesting to me is what can happen causally in the Universe. And I think there’s a lot of processes that can happen, but just don’t. And that what biology does is it somehow can cause things to happen that wouldn’t happen outside of the kind of process that biology is. And so, I think there is a deep connection actually between information and causation.

(Slightly edited and condensed by me.)

Another take-away message is that "life" or "no life" is not necessarily a binary decision, but a continuum. Things can be less alive than others. I think this maps nicely onto consciousness. It's not that there is some moment when "the lights turn on" but a scale with some beings haveing more of it than others.