This is unicode U+20F0: ⃰

I try to use it as part of a variable name. The ordinary * is reserved as operator.

After selecting this in the charmap application, the normal way to proceed would be

Strg+H

Ctrl+G

.A

Strg+H

Ctrl+V

This fails, because nothing is inserted when pressing Ctrl+V

The only way to insert the character is on the canvas. Then a new math region with just that sign is created.

Strg+H

Ctrl+V

Then you can enter the remainder of the variable name. Note that you can't enter the text index marker . at the beginning of the name. You have to enter σA first and then put . between them.