Aligning multiple-line th/td centered

Aligning multiple-line th/td centered

How do I align an HTML table header's contents to be centered but line up
aligned left when there are multiple lines?
To illustrate:
<th style="text-align:center;">This table header will span multiple rows
because
I've set a width on it</th>
Instead of this result:
This table header
will span multiple
rows because I've set
a width on it
I want this result:
This table header
will span multiple
rows because I've
set a width on it
Or something similar. I have tried putting a div inside the th with all
combinations of floating and aligning but nothing has worked so far.