SQL UNION Query with Extra Field in Results

I have a SQL query that is working well except I would like to bring the results of another column in the first db with it. This is what I currently have:

SELECT parts1.PART_NBR, parts1.NIIN
FROM parts1
WHERE parts1.PART_NBR='$pn'
UNION
SELECT parts2.REFERENCE_NUMBER, parts2.NIIN
FROM parts2
WHERE parts2.REFERENCE_NUMBER='$pn'

This is what I basically want (of course this wont work):

SELECT parts1.PART_NBR, parts1.NIIN, parts1.ANOTHER_FIELD
FROM parts1
WHERE parts1.PART_NBR='$pn'
UNION
SELECT parts2.REFERENCE_NUMBER, parts2.NIIN
FROM parts2
WHERE parts2.REFERENCE_NUMBER='$pn'

How do I write the query so that it does the same thing but actually brings back the extra field in the results?