Confidence: It's likely to be this, but I can't be certain.
Notes: These are mostly immatures with the reddish rear abdomen, but there are a couple of adults. The adult of U. jaceae is a large blackish aphid. The femora have the basal half pale and distal half dark, with a rather sharp transition between them. There are only two Uroleucon on Centaurea but this image is too small to distinguish them.