Well, it kind of depends on how you measure "closeness".. If you think of "distance", as in a single-, two- or three- dimensional spacial measure, you could imagine "closeness" to be a specific value for that distance that is small enough compared to other reference values.. Therefore, the closest all we earthlings can physically get to Deneb is likely to be around 1400 light-years, give or take..
If you base your assumptions for "closeness" on comfort and planet-habitability concerns, you could possibly say that an observer could inhabit a big rock at a distance of -- give or take -- 245 AU from it (square root of how more luminous it is compared to our Sun, multiplied by our nicely habitable distance to our star), and still be able to drink a glass of refreshing liquid Dihydrogen Monoxide potion at ambient temperature...
Well then, what if I were to define "closeness" as, not a ratio of spacial physical distances, but instead a ratio of lengths (wavelengths) of the radiation we observe? Imagine I talk about "colour", and how close I can distinguish one colour from a more distant one! One emission or absorption from a chemical element from the other element!... Or, for instance, from one stationary element, and one moving element. Or even how colour can change with density of the material, and thus, star size!.. How is this for "closeness"?
So here is a raw spectrum (uncalibrated) from Deneb in the vicinity of H-alpha! The absorption lines in the spectrum are "rather thin", and so the star is "not very dense", and in lacking mass that can hold down its size it got big (wikipedia tells me it can be way over 100-suns-big)!
The odd V-shaped absorption in the H-alpha, on the right side of the spectrum, comes from the atmosphere of the star or things above it, whose Hydrogen atoms are absorbing radiation in a wide region. The shape of this profile actually reflects some behaviour in action.. Stellar winds, storms, and mass ejections, for instance!
The thin emission lines present, and marked as "lamp" are from a lamp of known spectrum, placed in front of the telescope, to be used for spectral calibration.
So how close did I get to Deneb? Each pixel in this graph is only 0.33 Angstroms of Deneb's colour, and it only reflects visible colours going from the so-or-so-red (640 nm) to the slightly-more-red (660 nm). This was close enough to burn quite some neurons of my brain! Ouch!
