You mean the essences used to make weapons right?
You gain those by defeating certain bosses from Vanilla and Stars Above.
After the boss is defeated wait a bit and you will receive a notification of sorts from the Stellar Disc, after that your Starfarer gives you the essence depending on the boss you defeated.

Your essences can vary depending on the Starfarer you chose at the start (Astral or Umbral)
Both give unique essences with exception to another type being "Spatial" which both Starfarers can give you.

The weapon's crafted with essences can be used by any class despite the original class damage type they are assigned to by going into the stellar array and choosing the correct class damage type you are using (the lunar fragments represent the damage type)
click the Fragment that corresponds to your class and then the lock icon to set that as your damage type for all Aspected Weapons (essence weapons).
Example being a summoner using a Gun with summoner damage.

EDIT: Forgot to add that using an Aspected weapon with a different damage type than your chosen Aspect/Fragment will make it deal 10% less damage.

Originally posted by Gilded Zero:

You mean the essences used to make weapons right?
You gain those by defeating certain bosses from Vanilla and Stars Above.
After the boss is defeated wait a bit and you will receive a notification of sorts from the Stellar Disc, after that your Starfarer gives you the essence depending on the boss you defeated.

Your essences can vary depending on the Starfarer you chose at the start (Astral or Umbral)
Both give unique essences with exception to another type being "Spatial" which both Starfarers can give you.

The weapon's crafted with essences can be used by any class despite the original class damage type they are assigned to by going into the stellar array and choosing the correct class damage type you are using (the lunar fragments represent the damage type)
click the Fragment that corresponds to your class and then the lock icon to set that as your damage type for all Aspected Weapons (essence weapons).
Example being a summoner using a Gun with summoner damage.

EDIT: Forgot to add that using an Aspected weapon with a different damage type than your chosen Aspect/Fragment will make it deal 10% less damage.

+ some essences are master mode only, or at least one

Originally posted by DanThePaniniMan:

Originally posted by Gilded Zero:

You mean the essences used to make weapons right?
You gain those by defeating certain bosses from Vanilla and Stars Above.
After the boss is defeated wait a bit and you will receive a notification of sorts from the Stellar Disc, after that your Starfarer gives you the essence depending on the boss you defeated.

Your essences can vary depending on the Starfarer you chose at the start (Astral or Umbral)
Both give unique essences with exception to another type being "Spatial" which both Starfarers can give you.

The weapon's crafted with essences can be used by any class despite the original class damage type they are assigned to by going into the stellar array and choosing the correct class damage type you are using (the lunar fragments represent the damage type)
click the Fragment that corresponds to your class and then the lock icon to set that as your damage type for all Aspected Weapons (essence weapons).
Example being a summoner using a Gun with summoner damage.

EDIT: Forgot to add that using an Aspected weapon with a different damage type than your chosen Aspect/Fragment will make it deal 10% less damage.

+ some essences are master mode only, or at least one

Yes, there is like 1 or 2 that have Master Mode as a requirement.
They might want to update the wiki on those if they haven't since that's where most players tend to go if they need info on this kind of stuff.
MOST essences are on there already so it shouldn't be that big of a hassle to get info.