TT Cygni

TT Cygni is a carbon star located 561 pc away in the northern constellation of Cygnus. It is classified as a semiregular variable of subtype SRb that ranges in brightness from magnitude 7.26 down to 8.0 with a period of 118 days. This object is called a carbon star because it has a high ratio of carbon to oxygen in its surface layers. The carbon was produced by helium fusion, dredged up from inside the star by deep convection triggered by a flash from the helium shell.

A thin spherical shell around the star, about half a light year across, was emitted 7,000 years ago. It was first detected from its carbon monoxide emission and has a mass around, of which about a tenth is dust. The dust is thought to be mostly amorphous carbon.