Jane Austen/Headscratchers

Question about titles

How is it that we have a Lady Susan and a Lady Catherine, but also a Lady Middleton, Lady Bertram, and Lady Russell? Men of rank are always Sir First Name, but what's the rule for women?

I believe Lady Catherine might have been a widow? And I'm pretty sure Lady Susan was too. I've only read P/P and Emma, so I think it has to do with the fact that their husbands died.
This Troper is currently taking a class on Jane Austen. The professor actually addressed this in the second class. You see, if an aristocrat is "Lady first name," it means she inherited her title by birth. If addressed as "Lady last name," she married into the aristocracy.
Thank you ;)
But I'm pretty sure Lady Susan did not inherit her title, she got it through marriage. She was indeed a widow as described above, so maybe that has something to do with it?
How the wives and daughters of nobility were addressed in Austen's time has to do with how high up the nobility ladder their relative was. Lady Catherine is the daughter of an Earl (one of the highest titles) Lady Middleton and Bertram are the wives of baronets and Lady Russell is the wife of a knight (much lower on the scale). Lady Catherine is actually the wife of a knight, but her father's status trumps her husband's and she is still addressed by her first name.