 The star contains about half of a solar mass of matter , compressed into a diameter of about 20 kilometers ( 12 miles ) , making its surface gravity 67 billion times that of Earth . Its outer crust , compressed to about 7 @,@ 000 kg per cubic centimeter , is mainly iron nuclei with a high concentration of neutrons , overlaid with about 1 millimeter ( 0 @.@ 039 inches ) of white dwarf star material . The atmosphere , mostly iron vapor , is about 5 centimeters ( 2 @.@ 0 inches ) thick . The star shrinks slightly as it cools , causes the crust to crack and produce mountains 5 to 100 millimeters ( 0 @.@ 20 to 3 @.@ 94 inches ) high . Large volcanos , formed by liquid material oozing from deep cracks , can be many centimeters high and hundred meters in diameters , and will eventually collapse , causing starquakes . 
