Vaughan opens by admitting he doesn't know what he's talking about, and the poem is better for it. "Whatever 'tis, whose beauty here below / Attracts thee thus": he speaks to a star, but the earthly thing that draws its light is never named. We see only the star's response, and he makes it lively. It "stream[s] and flow[s]", "wind[s] and curl[s]", "wink[s] and smile[s]". Those verbs turn a distant point of fire into something that behaves like a flirt, "shifting thy gate and guile" ("gate" being the old spelling of "gait"). Twinkling becomes personality.
Then the mood shifts. The star's "close commerce" with earth does not hinder the speaker's "present search", though the compressed line about the "eagles eye" seems to say that his gaze is aimed higher than stars. The first twelve lines make one long sentence, running "Whatever…", "Though…", "Yet…" before it resolves in "I will see / What man may learn from thee." That builds suspense like a spoken argument, and the argument rests on one idea: everything that exists has "commissions from divinity" and so can "teach us duty". The star is treated as a messenger carrying instructions.
The lessons come in order. First, the thing the star favours must be healthy, because "bodies once infected, / Deprav'd, or dead" can get no hold on its light. Second, that thing feels "a restless, pure desire / And longing for thy bright and vital fire", a desire that "never will be quench'd, / Nor can be writh'd, nor wrench'd." Here the sound does the work: the clipped, twisting words "writh'd" and "wrench'd" are awkward to say, as if the mouth were being bent against its will. Religious writing often treats desire with suspicion, but Vaughan approves of it. Its being unquenchable is the point.
The poem's most interesting move is a repeated word. The star's "close commerce" in stanza 2 returns in stanza 7, when "God a commerce states". Vaughan borrows a word from trade and uses it for two relationships that mirror each other: star and earth, God and the longing soul. "Pure desire" is repeated too, but it is promoted on the way, from "restless" in stanza 4 to "celestial" in stanza 7. In between, the magnet image explains why attraction works: "beauteous shapes, we know not why, / Command and guide the eye." Being drawn comes before explanation, and the speaker is content to leave it unexplained.
The form quietly supports this. Every stanza has the same shape. Two long lines, in iambic pentameter, rhyme as a couplet (below/flow, infected/respected), and then the lines shrink to a shorter couplet (smile/guile, thee/sympathy). Each stanza tapers like an outgoing breath, and the shortest line often lands like a click: "Command and guide the eye." The rhymes are neat and mostly plain, which suits a poem so concerned with straightforward duty. In the final stanza "will" and "feel" make only a loose pairing, and the slight loosening arrives just as the tone becomes personal.
That last stanza turns from observation to appeal. God "craves" the heart, and anyone who will "but / give it him, and grudge not" will feel his truth. The small word "but" makes the demand sound easy, and the verb "craves" makes God a desirer too, so the whole poem becomes a chain of longing: earth for light, soul for fire, God for the heart. The closing comparison is quiet: God is true "as herbs unseen / Put on their youth and green." After all that starlight, the poem ends at ground level, with hidden growth. It echoes the earlier picture of desire that "hath taken root, and grows, and doth not tire", and it matches the "secret" shed on believers' heads. Faith here is something that happens out of sight, like spring in a hedge, long before anyone can point to it.
What makes the poem worth reading is how little it claims to know and how much it manages to say. It starts with a twinkle and ends with a moral about steadfast longing, and the lesson comes from looking closely at something small and strange.