a Yellow Thistle?, NO

Centaurea macrocephala,  Great Golden Knap-weed

A member of a much bigger family of plants such as Bachelor Buttons, Yarrow, and Asters plus others called Asteraceae. 
There is very little on the net for this plant so I will not link it. mostly Sales pitches.
Anyway this is a fun plant for the first timer to see before and during flowering. The heads look like a Thistle, however the body of the plant looks like a stalk that belongs to Nicotiana, Flowering Tobacco Plant. In fact it even blooms out exactly the same way as a Onopordum acanthium, Scotch Thistle does, except of course it is Yellow not Purple. It is a native to Ontario and Quebec in the East, and South Western BC in the West. After blooming it actually becomes very ugly. To fix that just cut of the stems ( new leaves will have started to come up by this point from the base) and the plant starts to make a nice low greenery.