Starbright damage is a rare damage type dealt by starbright items. It acts as any other damage type or combination of damage types for the purposes of being able to deal damage to creatures, but has none of those damage types other effects. For instance, if a creature is somehow immune to all damage other than fire, starbright will damage them as though it was fire but will not burn them. If a different creature had only Fire Damage Immunity, then starbright would damage them as though it was something other than fire damage. So long as there exists some damage type that can damage a creature, then starbright will work against them. If a creature is somehow immune to all damage then starbright will not affect them.
A consequence of starbright's versatility is that it is also immutable. It always deals exactly 1d8 starbright damage, cannot be boosted or deboosted, and cuts through resistances, but is not considered to be warded. It cannot be converted to drain or some other effect such as through Chi Strike or Withering Damage and supersedes all other damage types that might be dealt with the same die as it. For instance, using Slay with a starbright dagger will still deal only 1d8 starbright damage. However, using the same dagger with Control and Resist Fire will deal 1d8 starbright + 1d4 fire damage because the ability adds a different damage die. If that damage is then boosted it will deal 1d8 starbright + 2d4 fire damage, completely ignoring the starbright damage die.
Attacks that deal starbright damage can still be stopped physically such as through blocking or by armor.