By Similarity To The Sun
Defining the three categories by their similarity to the Sun reflects the evolution of astronomical observational techniques. Originally, solar-type was the closest that similarity to the Sun could be defined. Later, more precise measurement techniques and improved observatories allowed for greater precision of key details like temperature, enabling the creation of a solar analog category for stars that were particularly similar to the Sun. Later still, continued improvements in precision allowed for the creation of a solar twin category for near-perfect matches.
Similarity to the Sun allows for checking derived quantities — like temperature, which is derived from the color index — against the Sun, the only star whose temperature is confidently known. For stars which aren't similar to the Sun, this cross-checking can't be done.
Read more about this topic: Solar Analog
Famous quotes containing the words the sun, similarity and/or sun:
“A healthy soul stands united with the Just and the True, as the magnet arranges itself with the pole, so that he stands to all beholders like a transparent object betwixt them and the sun, and whoso journeys towards the sun, journeys towards that person. He is thus the medium of the highest influence to all who are not on the same level.”
—Ralph Waldo Emerson (1803–1882)
“Incompatibility. In matrimony a similarity of tastes, particularly the taste for domination.”
—Ambrose Bierce (1842–1914)
“The sun may set and rise:
But we contrariwise
Sleep after our short light
One everlasting night.”
—Sir Walter Raleigh (1552?–1618)