Dear drstevej,
"I am also an astro fan, but I understand there is already a patron saint of hopeless causes. :)"
Indeed, that is my own patron saint, St. Jude.
As to OT guys, they're saints, you can ask them to intercede on your behalf.
As to your further questions, this article gives a little illumination:
http://www.newadvent.org/cathen/11562a.htm
It's a bit long, so I'm not going to quote from it. But, several paragraphs in, the article speaks about patron saints as folks who proclaimed the Gospel.
Hope it helps.
sitetest
thanks.