A scheme for classifying advanced technological civilizations proposed by Nikolai Kardashev1 in 1964. He identified three high-level types and defined a logarithmic scale in terms of the power they could muster for the purpose of interstellar communications.

1. Able to harness all of the power available on a single planet. (Estimated 10^16 watts)
2. Able to harness all of the power available from a single star. (Estimated 10^26 watts)
3. Able to harness all of the power available from a single galaxy. (Estimated 10^36 watts)

Carl Sagan pointed out that the energy gaps between Kardashev's three types were so enormous that a finer gradation was needed. A Type 1.1 civilization, for example, would be able to expand a maximum of 10^17 watts on communications, a Type 2.3 could utilize 10^29 watts, and so on. He estimated that the human race would presently qualify as roughly a Type 0.7.

Kardashev explored the consequences of a Type II or III civilizations diverting all its available resources toward communication with other civilizations.

He called attention to two unusual radio sources with the California Institute of Technology designation numbers CTA-21 and CTA-102. Subsequently, Sholomitskii used the Crimea Deep Space Station to examine CTA-102 at a frequency of 920 MHz and reported variability in its output causing a short-term spike in interest in his thinking.

Kardashev, N. S. "Transmission of Information by Extraterrestrial Civilizations," Soviet Astronomy, 8, 217 (1964).