Rachael Padman says that changing sex did not make her a woman. The Cambridge University lecturer believes that the genital surgery she underwent in 1982 was just another step towards living as the woman she always felt she was, rather than the point at which she became female.

By any measure Padman, the director of studies in physical sciences at the university's Newnham college, is a successful woman. Within the transsexual community her story is widely viewed as challenging negative stereotypes of transsexuals being unhappy and dysfunctional.

Padman, 50, underwent gender reassignment after moving to England from her home country of Australia in 1977. She was assessed and treated with female hormones at the Charing Cross NHS gender identity clinic in north-west London while pursuing a PhD at the then all-male St John's college, Cambridge.

But she encountered no hostility from her peers or tutors when in 1981 she started to live full-time as a woman in preparation for genital surgery. She gained her doctorate just after undergoing a sex change operation, paid for privately, in October 1982.

Although she had an overwhelming desire to change gender from early childhood, Padman believes the main reason for her post-operative success is that her identity is not solely based on her being transsexual.

Genital surgery was just a step towards leading the life she wanted, rather than her ultimate goal. She believes her work has always been just as, if not more important, to who she is - and influences how others treat her.

"I don't think that surgery is what created me," she says. "I was already Rachael by that point. I realised afterwards that some people assumed I'd had the surgery two years earlier when I turned up at the lab and said 'I'm Rachael'. I suppose it did make me feel more female because I wasn't loaded up with two competing sets of hormones any more. But being an astronomer and physicist is my prime identity. I do get the impression that some people lose sight of the rest of their life."

While some transsexual activists contend that no distinction should be made between those who are born and those who surgically become women, Padman believes it is important to be "realistic" and accept there are differences.

She says: "It doesn't matter how empathetic you are before or during transition or how well you are accepted, you have not been born or brought up as a woman and that inevitably makes a difference.

"I think that changing sex is easier for people who are sensitive about what they are doing. You need the sense to realise that certain people will have difficulties with what you're doing. Just putting on a dress and saying 'treat me as a woman' will not make others warm to you. You can't force anyone to accept you."

The lecturer has difficulty understanding transsexuals who pretend they have always been the gender they identify as, but admits that disclosing her own background has required careful judgement. "I'm amazed that people cut themselves off and try to invent a false history for themselves," says Padman. But she can understand why they would want to in the face of the prejudice of some people.

"I don't know how they cope. I can't say, 'when I was a little girl' - that's a lie. But I don't want to push my past in people's faces. I'm not sure it would be right to go round saying, 'when I was a little boy' - it's like trying to demand a reaction from people. I generally get around it by saying 'when I was little' or 'when I was a child'.

"My view now is that if something comes up and my change of gender is relevant, I'll comment. It's a matter of dealing with things honestly as they come along."

She believes that this attitude helped her to overcome the only crisis she has faced since changing sex. In 1996, she was invited to take up a fellowship at Newnham, one of Cambridge's three all-women colleges. Germaine Greer, who was then a fellow of the college, subsequently argued that since Padman was legally a man, her appointment contravened college statutes.

Although her appointment went ahead later, the very public airing of the debate led Padman to became preoccupied with whether she came across as female. For the next two years she felt she had lost control of her identity because her background was now common knowledge. "I was trying to get to grips with the fact that the whole world knew," she says. "It was like a second transition after 15 years of increasing confidence and being increasingly less worried about the way I presented myself."

However, the support Padman received from friends, family and colleagues taught her to get on with being herself rather than worrying how she was perceived and judged by strangers. "I believe that it has made me stronger," she says. "The realisation that most people do take me at face value means that I no longer worry about whether I project 'femaleness'. Who gives a toss what someone thinks in the street - whom you've never met before - if your friends and family are supportive."

The physicist admits still feeling hurt on the rare occasions that someone does mistakenly refer to her as 'sir'. But she feels that such reactions are not always necessarily rooted in transphobia. For example, some research suggests that people innately recognise differences in the facial traits of men and women. Other negative reactions she has encountered while working outside of Cambridge seemed rooted in sexism.

On the whole, however, academia has proved an enormously positive environment for Padman. She realises she is lucky to work in such a liberal career. "Academics are pretty open minded," she says. The students also strike her as unfazed by her past. "Only one student out of 100 has ever commented about my background and they were very complimentary. I think the young are much more laid back."