Barack Obama @BarackObama tweets his homage to Stephen Hawking who died yesterday at the age of 76. "Have fun out there among the stars," wrote Obama with this picture with the physicist taken when he met Prof. Hawking in 2012, when he awarded him the Presidents Medal of Freedom.

And Obama's star reference?

This is one of Dr Hawking's memorable quotes.

"Remember to look up at the stars and not down at your feet. Try to make sense of what you see and wonder about what makes the universe exist. Be curious. And however difficult life may seem, there is always something you can do and succeed at. It matters that you dont just give up."?