The Mote in God's Eye by Larry Niven, Jerry Pournelle

Temperature steady.”
    “Thank you.” Rod was pleased. The Langston Field absorbed energy; that was its basic function. It absorbed even the kinetic energy of exploding gas or radiation particles, with an efficiency proportional to the cube of the incoming velocities. In battle, the hellish fury of hydrogen torpedoes, and the concentrated photon energies of lasers, would strike the Field and be dispersed, absorbed, contained. As the energy levels increased, the Field would begin to glow, its absolute black becoming red, orange, yellow, climbing up the spectrum toward the violet.
    That was the basic problem of the Langston Field. The energy had to be radiated away; if the Field overloaded, it would release all the stored energy in a blinding white flash, radiating inward, as well as outward. It took ship’s power to prevent that—and that power was added to the Field’s stored energies as well. When the Field grew too hot, ships died. Quickly.
    Normally a warship could get hellishly near a sun without being in mortal danger, her Field never growing hotter than the temperature of the star plus the amounts added to maintain control of the Field. Now, with a sun before and another behind, the Field could radiate only to the sides—and that had to be controlled or MacArthur would experience lateral accelerations. The sides were getting narrower and the suns bigger and the Field hotter. A tinge of red showed on Rod’s screens. It wasn’t an impending disaster, but it had to be watched.
    Normal gravity returned. Rod moved quickly to the bridge and nodded to the watch midshipman. “General quarters. Battle stations.”
    Alarms hooted through the ship.
     
    For 124 hours the intruder had shown no awareness of MacArthur ’s approach. It showed none now; and it drew steadily closer.
    The light sail was a vast expanse of uniform white across the aft screens, until Renner found a small black dot. He played with it until he had a large black dot, sharp edged, whose radar shadow showed it four thousand kilometers closer to MacArthur than the sail behind it.
    “That’s our target, sir,” Renner announced. “They probably put everything in one pod, everything that wasn’t part of the tail. One weight at the end of the shrouds to hold the sail steady.”
    “Right. Get us alongside it, Mr. Renner. Mr. Whitbread! My compliments to the Yeoman of Signals, and I want to send messages in clear. As many bands as he can cover, low power.”
    “Yes, sir. Recording.”
    “Hello, light-sail vessel. This is Imperial Ship MacArthur . Give our recognition signals. Welcome to New Caledonia and the Empire of Man. We wish to come alongside. Please acknowledge. Send that in Anglic, Russian, French, Chinese, and anything else you can think of. If they’re human there’s no telling where they’re from.” Fifteen minutes to match. Ship’s gravity changed, changed again as Renner began to match velocities and positions with the intruder’s cargo pod instead of the sail.
    Rod took a moment to answer Sally’s call. “Make it fast, Sally. If you please. We’re under battle conditions.”
    “Yes, Rod, I know. May I come to the bridge?”
    “Afraid not. All seats occupied.”
    “I’m not surprised. Rod, I just wanted to remind you of something. Don’t expect them to be simple.”
    “I beg your pardon?”
    “Just because they don’t use Alderson Drive, you’ll expect them to be primitive. Don’t. And even if they were primitive, primitive doesn’t mean simple. Their techniques and ways of thought may be very complex.”
    “I’ll keep it in mind. Anything else? OK, hang on, Sally. Whitbread, when you’ve got no other duties, let Miss Fowler know what’s going on,” He closed the intercom from his mind and looked at the stern screen even as Staley shouted.
    The intruder’s light sail was rippling. Reflected light ran across it in great, ponderous, wavy lines. Rod blinked but it didn’t help; it is very difficult to see the shape