"떨려오다 and 떨리다"
-아/어 오다 and -아/어 가다 used this way are helper verbs to add the sense of "coming" and "going".
Think of 뛰어오다(come running), 뛰어가다(go running), 다가오다(come close), and 다가가다(go close; approach).
오다 and 가다 can also add more subtle sense of something gradually happening over time.
For example, 날이 밝아왔다 (The day dawned gradually), 환자는 쇠약해져 갔다 (The patient got weaker and weaker).
For this sense, 오다 is preferred for something close by or when it will maintain its state here and now for a while, and 가다 otherwise. But sometimes it can be subtle to choose which one to use.
- 술잔을 쥔 그의 손이 파르르 떨렸다. His hand holding the shot glass trembled visibly.
- 추운데 비까지 내려 뼛속까지 떨려 왔다. The cold combined with the damp rain shook me down to the bones.
- "개울물은 날로 여물어 갔다." The stream ripened with each day. (from the short story "소나기" by 황순원)
In th above examples, 오다 is used for one's own body (close) and 가다 for the stream (far away and indifferent).
In the case of 밝아 오다, 오다 is preferred because the light affects everyone directly in the positive sense and because it will be that way for a while.