It is what collectors of antique buttons call a 2-piece brass "flower button" or "geometric button," made for use on fancy civilian coats, jackets, and vests. Yours is a combination of both styles... it has the swirls of a flower button surrounding a geometic 6-pointed star at the center. Most of the brass 2-piece ones date from the 1830s into the 1850s. But yours has a backmark (maker's-mark on the back) written in raised lettering, which indicates it was made during the 1830s through 1860s.
In case you don't already know, I should mention... 2-piece means a hollow button whose front and back were made separately and crimped together... the loop on the back does not count as a "piece."
I can't quite make out what your button's backmark says. I can ses "CO" at about 5 o'clock in the photo, and a few other letters. Please tell us everything you can read, even if you have to leave some blank spaces, like SC--IL- MF_ CO. If we can figure out the manufacturer's name, we can usually narrow down the date-range.
