why was it redefined?
So that Boltzmann constant takes the exact value
Kelvin
In particular, the committee proposed redefining the kelvin such that Boltzmann constant takes the exact value . 
characteristics of stars based on the characteristics of the sun A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. have these been observed since ancient times? UNANSWERABLE how are they measured? It is often most convenient to express mass, luminosity, and radii in solar units could you please explain the second term? It is an absolute measure of radiated electromagnetic power, the radiant power emitted by a light-emitting object. howe is it measured? In SI units, luminosity is measured in joules per second, or watts and in astronomy, in the terms of the luminosity of the Sun, L how is it measured for a star? By its size in terms of solar radii, R, and effective temperature in kelvins what is this last unit that you mentioned? It is the base unit of temperature in the International System of Units (SI)
wiki:16912636
Kelvin [SEP] 2019 redefinition
In 2005 the CIPM embarked on a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous methodology. In particular, the committee proposed redefining the kelvin such that Boltzmann constant takes the exact value . The committee had hoped that the programme would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger programme. The redefinition was further postponed in 2014, pending more accurate measurements of Boltzmann's constant in terms of the current definition, but was finally adopted at the 26th CGPM in late 2018, with a value of = .