If you take that 1st definition to be an amateur of any of those fields, whether it be science, art or whatever then its appropriate to follow the word amateur with the name of the field.
Evidently its an amateur doing science so the definition is appropriate.
Turning adjectives into nouns, now? This is the best you can do? Shoo off, so I can wait for someone who actually fires off neurons when they post.
no, actually I'm not. I'm also not inclined to accept your cherry picked definitions so I'll cherry pick google's:
am·a·teur
ˈamətər,-ˌtər,-ˌCHo͝or,-CHər/Submit
noun
1.
a person who engages in a pursuit, esp. a sport, on an unpaid basis.
synonyms: nonprofessional, nonspecialist, layman, layperson; More
antonyms: professional
a person considered contemptibly inept at a particular activity.
"that bunch of stumbling amateurs"
synonyms: bungler, incompetent, bumbler More
antonyms: expert
adjective
adjective: amateur
1.
engaging or engaged in without payment; nonprofessional.
"an amateur archaeologist"
synonyms: nonprofessional, nonspecialist;
According to Google the definitions that helped your case were simply synonyms.