Leading Plaid Cymru figure and former assembly member Phil Williams has died suddenly at the age of 64.
Phil Williams combined careers in politics and physics
Dr Williams was a space scientist of world renown and spoke for Plaid on economics before retiring from the Welsh assembly at the election six weeks ago.
His death was announced on Wednesday night by Plaid, which said it was "saddened and shocked".
South Wales Police said they were investigating on behalf of the coroner following the death of a man in his sixties in the Cathays area of Cardiff.
A police spokesman said: "The death is not being treated as suspicious."
"I was in the presence of a genius"
Ieuan Wyn Jones on Phil Williams
Dr Williams, who had been AM for south-east Wales, was widely seen as one of the cleverest people in Welsh politics, which he combined with a glittering career in physics.
Although he had quit politics, he had no intention of giving up work altogether.
He was one of the world's top experts on solar-terrestrial physics, and was planning to spend more time researching an astronomy project based in Arctic territory north of Norway.
Plaid leader Ieuan Wyn Jones said: "I have known Phil Williams for over 30 years and always knew that I was in the presence of a genius.
"I am very much aware of his contribution in making Plaid Cymru the force it is in Welsh politics today.
Plaid leader Ieuan Wyn Jones paid tribute to Dr Williams
"To hear Phil arguing about the iniquities of the Barnett formula was to experience a great mind at work, and to know that one was in the presence of someone whose only concern was truth.
"My heart goes out to his family and friends."
He first came to public prominence when he nearly won the solid Labour seat of Caerphilly in the south Wales valleys for Plaid at a parliamentary by-election in 1968.
Plaid achieved 40.4% of the vote with a massive swing from Labour at that by-election, which at the time was the second largest swing recorded in a UK election.
Two years after that he and the future Plaid president Dafydd Wigley wrote the party's economic plan.
He became a Plaid AM at the first assembly election in 1999, and was well known for his detailed economic analysis.
He focussed particularly on issues such as the Barnett formula - which is used to decide how much money Wales gets from Westminster - and on European Objective One funding for west Wales and the valleys.
In 2000 he was named Welsh politician of the year by Channel Four.
Philip James Stradling Williams was born on 11 January 1939 in Tredegar, south Wales, and brought up in nearby Bargoed.
He was educated at Lewis School, Pengam, the same school attended by former Labour leader Neil Kinnock, who is three years younger.
He was a prize-winning student at Clare College, Cambridge, where he gained his PhD.
After working at Cambridge he returned to Wales when he became a physics lecturer at Aberystwyth University in 1967.
He combined his scientific work with a series of senior roles in Plaid Cymru.
Dr Williams married Ann in 1962, and they had a son Philip and a daughter Sara.
In his leisure time he was a jazz fan, and played alto sax. He also listed his interests as dancing, hill-walking, history and poetry.