Editing Kardashev scale (section)

=== Unusual objects ===
[[File:Quasar 3C 273.jpg|alt=Color photo. A luminous celestial object appears against a black background.|thumb|Quasar [[3C 273]], the brightest ever observed]]
The quasar [[3C 9]] is cited by Kardashev as early as 1971.<ref name="Kardashev-1971" /> The study of the quasar [[3C 273]] shows that it has a solid structure. Other quasars ([[3C 279]], [[3C 345]], 3C 84) have properties close to those expected from an artificial source, especially since the emissions are powerful in the intermediate region of the spectrum (between radio and optical frequencies).<ref name="Kardashev-1971" /> Quasars are potential artificial sources, especially since their age corresponds to the technical possibilities of supercivilizations. Radio sources at the center of galaxies can also be artificial sources, according to Kardashev, even if in 2013 they were proven to be supermassive black holes. In 1971, Kardashev believed that the objects most likely to be artificial sources could be discovered in the [then] next few years.<ref name="Kardashev-1971" />

The extraordinary periodicity of [[pulsar]] emissions was already considered an artificial source in 1968 by [[Antony Hewish]], the discoverer of the first pulsar ([[PSR B1919+21|CP 19019]]). The press of the time nicknamed this object "LGM-1" (for "little green men"), following the clumsiness of Hewish, who did not wait for the necessary verifications. Kaplan, in 1971, removed the pulsar from the list of objects that could be a source of artificial origin.<ref name=":0" />

In 2011, James and Dominic Benford examined the possibilities that exist to distinguish pulsars from possible artificial sources emitting intelligent signals, such as: bandwidth (signals of about 100&nbsp;MHz could be artificial), pulse length (to reduce costs, the pulse should be short) and frequency (about 10&nbsp;GHz, also for economic reasons). The [[Astronomical radio source|radio source]] PSR J1928+15 (observed in 2005 near the Galactic disk, at a frequency of 1.44&nbsp;GHz, at [[Arecibo Observatory|Arecibo]]) could be of extraterrestrial origin. James and Dominic Benford consider three scenarios in which the cost factor is taken into account. If the source is cost-optimized, it belongs to a civilization of Type 0.35 (the Earth being of Type 0.73).{{NoteTag|According to Sagan's interpolated Kardashev scale, as of 2021}} If it is not cost-optimized and operates with a small antenna, the Type is 0.86. With a large antenna, it would be from a Type 0.66. Using this cost/efficiency method, it can be estimated that low-intensity sources may be the most prevalent, but also the most difficult to observe.<ref>{{Cite arXiv |last1=Benford |first1=James |last2=Benford |first2=Dominic |date=2010 |title=How can we distinguish transient pulsars from SETI beacons? |class=astro-ph.IM |eprint=1003.5938 }}</ref>