‘It’s too small. It has only around a twentieth of Earth’s sun’s mass. It should be a planet: a brown dwarf, like a big, fat Jovian. It shouldn’t burn - not yet. You understand that stars form from the interstellar medium - gas and dust. Originally the medium was just Big-Bang hydrogen and helium. But stars bake heavy elements, like metals, in their interiors, and eject them back into the medium when the stars die. So as time goes on, the medium is increasingly polluted.’

Impatiently I snapped, ‘And the point?’

‘The point is that an increase in impurities in the interstellar medium lowers the critical mass needed for a star to be big enough to burn hydrogen. So as time goes by and the medium gets murkier, smaller stars start lighting up. Lieutenant, that star shouldn’t be shining. Not in this era, not for trillions of years yet; the interstellar medium is too clean . . . You know, it’s so small that its surface temperature isn’t thousands of degrees, like Earth’s sun, but the freezing point of water. It is a star with ice clouds in its atmosphere. There may even be liquid water on its surface.’

I looked up, wishing I could see the frozen star better. Despite the urgency of the moment I shivered, confronted by strangeness, a vision from trillions of years downstream.

Tilo said bookishly, ‘What does all this mean? It means that out here in the halo, something, some agent, is making the interstellar medium dirtier than it ought to be. The only way to do that is by making the stars grow old.’ He waved a hand at the cluttered sky. ‘And if you look, you can see it all over this part of the halo; the stellar evolution diagrams are impossibly skewed.’

I shook my head; I was far out of my depth. What could make a star grow old too fast? . . . Oh. ‘Dark matter?’

‘The matter we’re made of - baryonic matter, protons and neutrons and the rest - is only about a tenth of the universe’s total. The rest is dark matter: subject only to gravity and the weak nuclear force, impervious to electromagnetism. Dark matter came out of the Big Bang, just like the baryonic stuff. As our Galaxy coalesced the dark matter was squeezed out of the main disc . . . But it lingered here. This is the domain of dark matter, Lieutenant. Out here in the halo.’

‘And this stuff can affect the ageing of stars.’

‘Yes. A dark matter concentration in the core of a star can change temperatures, and so affect fusion rates.

‘You said an “agent” was ageing the stars. You make it sound intentional.’

He was cautious now, an Academician who didn’t want to commit himself. ‘The stellar disruption appears non-random.’