In the title you could underline the perception verb “heard” that communicates poet’s detachment of action. The use of “when” and simple past indicates the subject “I” tells about a memory. The first line is a repetition of title.

The poetry tells about a student who is sitting in a lecture-room and hears an astronomer but inexplicably the student “become tired and sick” and he goes away. In the night he admires stars in mystical way.

The poetry is in free verse and you could divided it in two part: the first is composed by all lines begins with “when”. At the end of these lines there is a comma to create break  because  the student remembers his past  feeling . Lines becomes more long that the before and the rhythm is broken by commas. In the second part  lines become similar and more short.  The expression “time to time “ indicates the sense of liberty. The “silence “ is in opposition to “tired and sick”. The word “stars” is refer to astronomer and it convey the idea that there are different way to study and admire the same thing.