Oh man, I just read about this. A book you want to pick up is Richard Nelson Current, The South vs. The South Lincoln's Loyalists, as he discusses Southern Unionism on a state-by-state basis. The 1st North Carolina, USA, is mentioned in some detail.

To compress his points down, support of the Confederacy was not universal. In areas where slavery was weakest, there was substantial support for the Union. North Carolina was not as dependent on slavery as the Deep South, so pockets of Unionism existed more strongly their than its southern, more cantankerous sibling.

Lincoln believed that Southern Unionists were a considerable part of the Southern population, and worked with them wherever he could. Thus, when Union forces invaded North Carolina from the Outer Banks, they were authorized to enlist support from local Unionists. Loyal North Carolinians has a few reasons to join the army. Firstly, they were assigned to garrison the area controlled by federal forces, allowing them to stay close to their friends and families. This was no mere creature comfort: secessionists were notoriously murderous when confronting internal opposition to their rebellion, and having their families killed was a real concern for anyone brave enough to oppose the Confederacy. Enlisting also gave people the opportunity to fight against the slaveocrats that has long monopolized power in the state and who had started the war.

All told, over 100,000 white men from slave states joined the Union army, as did 300,000 self-freed black men. Together, they made up for every Northern soldier that died of battle or disease, with 100,000 extra men to spare. Most served well in combat, and they generally had an easier time garrisoning recaptured Southern territory than Yankee troops did. They were thus instrumental in letting the North bring its manpower advantage to bear on rebel armies. They are among the greatest unsung heroes of the war. That the South continues to celebrate traitors like Lee and Jackson and forgets men like your ancestor is a continuing injustice.

The other post here is certainly a nice summary/explanation. I might add another book recommendation for understanding the topic: The Fall of the House of Dixie by Bruce Levine. At times, Civil War Era music can also be a resource. It can be telling of shifting sociopolitical sentiments, especially trends toward acceptance of emancipation. Of particular relevance here is Henry Clay Work's Marching through Georgia in which one verse states,

"There were many Union men who wept with joyful tears when they saw the honored flag they had not seen for years; Hardly could they be restrained from breaking forth in cheers, While we were marching through Georgia!"

The exclusion of Southern Unionism from popular memory is largely due to what's termed the Lost Cause mythology. This myth was forged by Confederates as a means of justifying the war and honorably explaining their defeat. It does indeed remain heavily entrenched in our modern culture, openly embraced by organizations such as the Sons of Confederate Veterans or League of the South. I would suggest The Myth of the Lost Cause and Civil War History edited by Gary Gallagher, and Race & Reunion by David Blight on that subject (though there's certainly much more literature).