Well going by what I think a "star" in the hardcore industry is and not just what a "pornstar" is because pornstar just a general name we give to women that work in it now. I will say conditions I can think of off the top of my head of what makes somebody star are:
1. The woman is extremely attractive, if not one of the best looking people in the industry. It's even better if they are on the top tier of the best looking people in the world.
2. Is one of the most well paid people in the industry, or has a very good reason why they aren't.
3. Has a wide notoriety and following. Maybe even to a lot of people that are just casual porn consumers, and maybe even to people that aren't familiar with the porn world at all. Gets a huge bonus for widespread mainstream recognition, even more so if it's positive recognition, and even more so if it's from her work as a pornstar and not some gimmick she or people working with her came up with to get her attention.
4. Is well liked in by fans of porn for who she are, and although not critical gets bonus point with that if she is also liked for more than just having sex on camera, like say they are smart, well spoken, self-assured, or have good interaction with their fans.
5. Has a lot of power because of her notoriety and influence in the industry. Can pick and choose her scenes and who she works with in them or who she works for if she is not under contract. She can demand to do only what she wants with no repercussions to herself, and no significant drop off in the product she sells or her popularity.
6. A star has producers and other performers asking they will work with them not the other way around where that person is always searching for work where they have to worry if anybody will hire them for a decent amount of money.
7. A star has the upper hand and the leverage in any business dealings they might do by their notoriety and stardom alone.
Somebody I would consider a true star would have to have most if not all those factors above.
Suffice to say in today's porn world there aren't really any stars anymore. Maybe in the past there were a handful of people that would truly qualify.
Also keep in mind that that's just my definition of what a "star" in the industry is, not necessarily if I like that person or not. There are some people that just never made it far for one reason or another that I really like.