.。
What Jansky observed was radio waves of 14.6 meters wavelength from the galactic core.
Literal
Jansky [subject-が] observed [nominalizer-の] [topic-は] galaxy [genitive-の] central-core [from-からの] wavelength 14.6m [genitive-の] radio-waves was.
A cleft construction sentence — ~のは~であった ('what X was Y') focusing on the observed phenomenon. Three features to notice: (1) からの is a composite particle: から (from) + の (attributive) = 'from X (as modifier).' It turns 'from the galactic core' into a pre-noun modifier: 中心核からの電波 = 'radio waves [that are] from the core.' (2) The predicate uses the formal literary past であった (past of である) rather than でした, matching the scientific-historical register. (3) The noun 電波 ('radio wave, electromagnetic wave') is the standard physics term. Karl Jansky's 1932 detection is a milestone in radio astronomy.