Fang Lizhi: Astrophysicist and dissident who helped inspire the Tiananmen Square protests

'Democracy comes from the bottom up,' he wrote. 'Marxism is like a worn dress that must be put aside'

The astrophysicist Fang Lizhi was not physically present in Beijing's Tiananmen Square during the dramatic protests and eventual massacre of hundreds of people in 1989. But he was very much there in spirit, having become a mentor and inspiration, through his writings and speeches, to the young Chinese in the Square demanding democracy and human rights. Along with the writers Liu Binyan and Wang Ruowang he had formed the dissident triumvirate who gave the Tiananmen protesters – students and others – the courage to stand up to the regime. In the months leading up to the protests, Fang had published an open letter to the Chinese leader Deng Xiaoping calling for the release of political prisoners, a letter which helped galvanise the pro-democracy movement.

After tanks put down the protests on 4 June 1989, Fang and his wife Li Shuxian – facing potential death sentences as "instigators of chaos" – were invited to take refuge in the US embassy in Beijing as "guests of President Bush". Commenting on his time there, he wrote: "I am an astrophysicist yet I cannot see the sky." The couple spent more than a year in the embassy before a face-saving US-Chinese deal allowed the US to fly them out on a military transport plane. In return, the US lifted some economic sanctions and resumed loans.

After spells as guest research professor at Cambridge University, where he worked with Stephen Hawking, and at Princeton, Fang spent the last 20 years of his life as Professor of Physics at the University of Arizona in Tucson, where he died. His fellow dissidents Liu and Wang had died in US exile several years before him. Although he was able to witness from afar the 21st century transformations in his nation, he attacked its lack of democracy and its human rights record until his death.

Despite China's economic boom, aided by improved relations with the US, Fang warned: "democracy will not automatically emerge as a result of growing prosperity." In articles from exile, notably in the New York Review of Books, he noted that, while many of his countrymen were becoming extremely rich, with luxury products increasingly available, the vast majority were still getting by on next to nothing, not least those forced to move from rural to urban areas.

The latter, he pointed out, would become increasingly politically active. "Democracy comes from the bottom up, not the top down," he said. "Marxism is like a worn dress that must be put aside." Being an astrophysicist, he was often described as "China's Sakharov", a comparison with the dissident Soviet physicist and human rights activist Andrei Sakharov, who died in 1989 after helping light the blue touch paper under his own communist regime.

Fang Lizhi was born in Beijing in February 1936, the son of a postal clerk who had moved from the city of Hangzhou. His academic brilliance was obvious and he won a place at Beijing University (known as Beida) to study theoretical and nuclear physics when he was 16. A fellow physics student, Li Shuxian, would later become his wife.

After graduating at 20 he was assigned to the Institute of Modern Research of the Chinese Academy of Sciences, but he was already outspoken against communist autocracy, notably in education, and was expelled from the Communist Party in 1957 as part of the party's Anti-Rightist Campaign. Even so, he said, "I continued to have faith in Chairman Mao and believed it must be I who was wrong." But the 1966 Cultural Revolution would change that.

Branded a reactionary, he was banished to a niupeng, or cow shed, for a year of solitary confinement – forced to work in a mine and on railway tracks – before being sent to Anhui province to work with the peasantry, although he continued to study theoretical astrophysics in any spare time he had. "After the Cultural Revolution started, everything became much clearer," he said. "I realised that the Party had not been telling the truth, that they had in fact been deceiving people, and that I should not believe them anymore."

In 1969 the Academy of Sciences moved several undergraduate departments from Beijing to Hefei, in Anhui province. Fang, accepted back into the Communist Party, was sent there as a teacher of astrophysics, rising to become vice-president of the University of Science and Technology (known as Keda).

From 1985, his pro-democracy speeches and writings spread rapidly round the country. In a speech he gave on 4 November 1985, he said: "There is a social malaise in our country today and the primary reason for it is the poor example set by Party members ... unethical behaviour ... requires that we break the bonds of social restraint when necessary." He began getting hundreds of letters from students, academics and intellectuals, often giving their real names and addresses – even on postcards, usually addressed to "Teacher Fang" and saying such things as "You are our hope."

In 1986-87, pro-democracy demonstrations began on his own campus in Hefei and spread to Beijing. Posters appeared saying, "I have a dream of democracy." Fang's removal from the university and his second expulsion from the Party, in 1987, were inevitable. So too, perhaps, were the Tiananmen Square protests two years later when the dreams of democracy were crushed by tanks and he was forced into lifelong exile.

While still in refuge in the US embassy in 1989, Fang Lizhi was given the Robert F Kennedy Human Rights Award, which honours people round the world who show courage in promoting human rights in their country. He is survived by his wife and a son, Fang Ke.

Fang Lizhi, astrophysicist and political activist: born Beijing 12 February 1936; married Li Shuxian (one son, and one son deceased); died Tucson, Arizona 6 April 2012.