I don't think default means anything. Someone made an entry. thats it.

* means there is no instance on the box and has just binaries.
When you install binaries it makes an entry like this.

*:/xyz/abc/..../817:N

Say you create an instance 'devt', you have to update oratab entry as

devt:/xyz/abc/..../817:Y

" dev009:/oracle/product/8.0.5:Y:oracle dev:DEV009 " doesn't makes any sense to me. If someone, iam curious to know.

ORACLE_SID:ORACLE_HOME: (Y|N) is correct.