Learning from the different answers and comments to this and similar posts I suggest the following answer:
Etymology: Yang means sunny and yin means shady; see also the answer of virmaior. This conforms e.g., to the names of several hills. In China the south side of a hill is the sunny side indicated by yang as part of the name, while the north side is the shady side, indicated by yin as part of the name.
Considered in the light of etymology the sun is yang - nearly by definition. Hence you need not to look for a yin aspect of the sun: There is none by definition!
Now time went on: In later times an attempt was made to understand all phenomena by means of a classification which provides always pairs of opposite properties. In addition a second attempt was made to think in processes, notably cyclic processes.
Combining both modes of thought results in the attempt to explain the phenomena by cyclic processes, which run through the whole spectrum between a property and its opposite and vice versa back to the original.
The short version of this thinking states that all objects include both the property and its opposite. The long version advocates, not to restrict to a static view of things, but to consider their lifespan as a cycle which passes through opposite properties. Note that a parallel stream of thinking exists also in some early Greek speculations on nature.
Applying the lifecycle paradigm to the sun and to other stars: 5 Billion years ago the sun originated from a dark (= shady, hence yin) cloud of dust. After condensation the first nucleo-synthesis started and the sun became yang, because light was released. But further 5 Billion years later the fuel is consumed and the nucleo-synthesis stops. The sun cools down and becomes shady (yin).
For our sun that’s the end. But more massive stars explode as supernovae after several steps of nucleo-synthesis and generate the components for new stars. The cycle restarts with a new generation of stars.
Aside: Who likes can clothe the astrophysical theory of stellar development into the Chinese metaphor of yin and yang. But what is the benefit of such clothing?