Stars, so far as we understand them today, are not “alive”.
Now and again we saw a binary and a third star approach one another so closely that one or other of the group reached out a filament of its substance toward its partner. Straining our supernatural vision, we saw these filaments break and condense into planets. And we were awed by the infinitesimal size and the rarity of these seeds of life among the lifeless host of the stars. But the stars themselves gave an irresistible impression of vitality. Strange that the movements of these merely physical things, these mere fire-balls, whirling and traveling according to the geometrical laws of their minutest particles, should seem so vital, so questing.
Olaf Stapledon, Star Maker (1937)
And yet, it still makes sense to speak of a star being “born”, “living” and even “dying”.
We have moved on from Stapledon’s poetic description of the formation of planets from a filament of star-stuff gravitationally teased-out by a near-miss between passing celestial orbs. This was known as the “Tidal Hypothesis” and was first put forward by Sir James Jeans in 1917. It implied that planets circling stars would be an incredibly rare occurrence.
Today, it would seem that the reverse is true: modern astronomy tells us that planets almost inevitably form as a nebula collapses to form a star. It appears that stars with planetary systems are the norm, rather than the exception.
Be that as it may, the purpose of this post is to share a way of teaching the “life cycle” of a star that I have found useful, and that many students seem to appreciate. It uses the old trick of using analogy to “couch abstract concepts in concrete terms” (Steven Pinker’s phrase).
I find it humbling to consider that currently there are no black dwarf stars anywhere in the observable universe, simply because the universe isn’t old enough. The universe is merely 13.7 billion years old. Not until the universe is some 70 000 times its current age (about 1015 years old) will enough time have elapsed for even our oldest white dwarfs to have cooled to become a black dwarf. If we take the entire current age of the universe to be one second past midnight on a single 24-hour day, then the first black dwarfs will come into existence at 8 pm in the evening…
And finally, although to the best of our knowledge, stars are in no meaningful sense “alive”, I cannot help but close with a few words from Stapledon’s riotous and romantic imaginative tour de force that is yet threaded through with the disciplined sinews of Stapledon’s understanding of the science of his day:
Stars are best regarded as living organisms, but organisms which are physiologically and psychologically of a very peculiar kind. The outer and middle layers of a mature star apparently consist of “tissues” woven of currents of incandescent gases. These gaseous tissues live and maintain the stellar consciousness by intercepting part of the immense flood of energy that wells from the congested and furiously active interior of the star. The innermost of the vital layers must be a kind of digestive apparatus which transmutes the crude radiation into forms required for the maintenance of the star’s life. Outside this digestive area lies some sort of coordinating layer, which may be thought of as the star’s brain. The outermost layers, including the corona, respond to the excessively faint stimuli of the star’s cosmical environment, to light from neighbouring stars, to cosmic rays, to the impact of meteors, to tidal stresses caused by the gravitational influence of planets or of other stars. These influences could not, of course, produce any clear impression but for a strange tissue of gaseous sense organs, which discriminate between them in respect of quality and direction, and transmit information to the correlating “brain” layer.
Olaf Stapledon, Star Maker (1937)
e=mc2andallthat 
Reblogged this on The Echo Chamber.
Only just found this – which is a shame, because it’s lovely! A couple of things to add, one science fictional and one from a teaching point of view.
One is that Frank Herbert wrote some stories which were based around the idea of stars as living beings. The astrophysics may be patchy but the narratives are fun! https://en.wikipedia.org/wiki/Whipping_Star
The more useful part is that describing stars as having a lifecycle in this way – with careful discussion of what it means – allows a nice explanation of how short-term data allows us to predict and explain stellar changes from our limited. We rarely ‘see’ the changes a star goes through, and certainly we have not been able to observe all these changes for a single object. We do, however, have many stars at different stages and on different pathways. This is like taking a 1 minute video of a crowd in a local city centre. No aging happens in that instant, yet the many different examples can be placed in an order. Transitions and stages can be described, with predictions about common features and likely results. With enough data, we can have more and more confidence in the models of human lifespan and development that we construct.
Thank you, Ian. I appreciate the comment. I’ve not read much Frank Herbert apart from the superb “Dune”. I have to admit I’m not reading a lot of SF at the moment. My reading habits seem to have moved resolutely to the non-fiction side of things in the last few years. However, SF (Clarke, Asimov, Sheckley et al) were hugely important influences.
It would make an interesting STEM extension activity to show students a 60 min clip of a high street and then get them to analyse it from the perspective of an alien who needs to understand the life cycle of humans (e.g. “The small mass humans appear to be dependent on the large mass humans until they pass a certain mass threshold, whereupon they can strike out independently. However, the slow moving humans seem to be the oldest…”
H’mmm — I wonder if we could a draw a graph similar to the HR diagram for humans…? XKCD might have beaten us to the draw there https://www.xkcd.com/2009/
I remember reading a similar analogy in Patrick Moore’s “History of Astronomy” — one of my all-time favourite books — to explain how astronomers deduced the life cycles of stars.