My teacher said that where the > < are is incorrect grammar what should I do there?
“However, this was when I realized that astronomy was simple in its >concept, point< a telescope at a star and look at it, but there was what seemed like hundreds of minor details that included everything…”
Both grammar and punctuation, perhaps. Try this:
“However, this was when I realized that astronomy was simple in its concept – point a telescope at a star and look at it – but there were what seemed like hundreds of minor details that included everything.”
Or perhaps with parentheses:
“However, this was when I realized that astronomy was simple in its concept (point a telescope at a star and look at it), but there were what seemed like hundreds of minor details that included everything.”
Your example is not a good expression for proper formal writing, but is akin to what one would say in conversation. All three ways of writing it would be spoken the same (except I changed “was” to “were” to agree with the plural “hundreds”). It can be a challenge to “properly” put colloquial speech down in writing, and I won’t swear by either of my suggestions.