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Professor Stephen Hawking moved an audience to tears on Sunday, after he spoke at an event to mark his 75th birthday.

The physicist and cosmologist gave a talk reflecting on his life and scientific work during a public symposium at Cambridge University.

Prof Hawking, who turned 75 in January, joked: "Actually, for those keeping count, I'm closer to 75-and-a-half. But that shouldn't get in the way of a good celebration."

In his talk, titled My Life In Physics, he spoke about his early realisation of his medical problems when he went skating with his mother, according to Cambridge News .

"I fell over and had great difficulty getting up," he said.

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Prof Hawking was later diagnosed with amyotrophic lateral sclerosis (ALS), at the age of 21.

"At first I became depressed," he said. "I seemed to be getting worse really rapidly.

"There didn't seem any point working on my PhD because I didn't know I would live long enough to finish it.

"But then the condition developed more slowly and I began to make progress in my work.

"After my expectations had been reduced to zero, every new day became a bonus and I began to appreciate everything I did have. While there's life there's hope."

After flying through his world-shattering achievements in physics and beyond with a humbling modesty, Prof Hawking finished on a very human note.

He said: “Our picture of the universe has changed a lot in the last 50 years and I am happy if I have made a small contribution.

“Remember to look up at the stars and not down at your feet. Be curious and however difficult life may seem, there is always something you can do and succeed at.”

His speech followed an afternoon of lectures about gravity and black holes from other distinguished scientists, including Professor Brian Cox.

The Cambridge-based rock star physicist was the first to address the audience, with a whistle-stop tour of the history of the universe, along with our developing understanding of it.

Although Professor Cox credited Mr Hawking with inspiring him to study physics, he said it was Hawking’s engagement with the public that was his greatest achievement.

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“I think one of the reasons we are celebrating Stephen’s birthday this way is not only because of his great contribution to physics but due to his great contribution to the public understanding of science,” he said.

Mr Cox borrowed the words of another great scientist Richard Feynman, from his book The Value of Science.

He said: “The most valuable thing about science is not the things it teaches us about nature, or spin-off technologies, no matter how wonderful they are.

“It’s a state of mind. The state of mind is that not knowing is a valuable and powerful thing.”

Next was Professor Gabriela González, who in February 2016 was one of a small group of scientists to announce the detection of gravitational waves, a revolutionary discovery that completely changed how we understand and study the universe.

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Professor González talked first about black holes, a place in space caused by collapsing stars with such a strong gravitational pull that even light can’t escape.

As González pointed out, Hawking made great contributions to our understanding in this field.

"He was interested in black holes from very early on," said Professor González.

She went on to explain how her team's work on gravitational waves - "ripples" in space-time caused by the pull of things like black holes - was indebted to the work of the Cambridge physicist.

Professor Martin Rees - former master of Trinity College in Cambridge - shared the same PhD supervisor as Stephen Hawking and was the penultimate speaker of the evening.

With an image from the film The Martian behind him, Professor Rees gave the audience a vision of the human race’s future exploration of the universe.

“As Artificial intelligence advances, there is less practical need for people. However, I do hope people will follow the robots,” he said.

But he warned against thinking about mass migration to Mars as some kind of solution.

“We must solve our problems on Earth,” he said.

The event concluded with the full auditorium singing Happy Birthday to Prof Hawking.