Energy Source

(The (spherical) Black Nebula is not the Horsehead Nebula but the latter comes up in google searches for "dark nebulae" so let's appreciate it here.)

Takahashi theorizes that Arzunian life can control mass-energy because it is linked to atomic probabilities but does not discuss the source of the energy for this control. However, Donovan, who has had more contact with the Arzunians, suggests that:

Arzunian metabolisms cannot generate enough energy, e.g., to project the Arzunians themselves out into space;

so they must instinctively tap into "'...some unknown cosmic energy source...'" (Rise Of The Terran Empire, p. 425);

but they do use nervous energy to direct the flow;

so they have to rest after strenuous efforts, as he can testify.

(Everything has limits, even the fantastic feats of the Arzunians.)

This helps to explain why the Terrans can fight and win, even with the kind of primitive weapons that are more familiar to the psychokinetic Arzunians:

as stated, the Arzunians also tire after exertion;
they are a dying race, their population confined to a single city;
most will flee when attacked, unused to victims who resist;
those who do fight have a superiority complex that prevents them from taking due care in combat;
they are undisciplined, without any plan of action;
the Earthmen need not defeat the entire planet but merely fight their way to the captured spaceships and escape in one.

With all this rationale to fall back on, Anderson writes some good fight scenes. In particular, the quadrupedal Donarrian, ridden by the Terran commander, wreaks havoc but succumbs to a spear wound and stays to finish the fight while the spaceship leaves.

After resisting the Arzunian allure, Donovan cannot maintain his hostility to Sol.