System catalog and identifying

Hi all! Didn't find any info on this, so if I missed it please guide
me to the info thx.

I know the name of the table and now I need to know what columns this
table has.

First I search system catalog named pg_class inorder to get the
postgre id of the table named relfilenode:

select relfilenode from pg_class where relname = 'table_name';

after that I search all column names from pg_attribute, like this:

select attname from pg_attribute where attrelid = relfilenode;

unfortunately, this returns correct names, but also some names that
the system uses. How can I exclude these names?

EXAMPLE:
I get the following results:

 tableoid *
 cmax *
 xmax *
 cmin *
 xmin *
 oid *
 ctid *
 sarake_id
 nimi
 kuvaus
 tyyppi
 help
 taulu_id

everything marked with * is unwanted.

Thanks beforehand!

-9902468