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So a neutron star is kind of about the size of Chicago.
Unexpected elements from the BBC World Service.
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Hello and welcome to NewsHour.
It's coming to you live from the BBC World Service Studios in London.
I'm Tim Franks, and we're starting in Syria with a story that feels unpredicted and unpredictable.
Unpredictable because it's been a shock just how far and how fast rebel forces have advanced.
Unpredictable because working out where this now leads is phenomenally difficult.
At the very least, in this program, we'll try to ask some of the questions.
A very brief recap.
It was a few days ago that an alliance of rebel forces in the north of the country launched their offensive.
They swept towards Aleppo, Syria's second city, which now, remarkably, appears to have fallen into their control.
Or at least the forces of President Bashar Al Assad have melted away.
They've also headed south towards the city of Hama.
The Syrian civil war that had seemed stuck frozen for so long with Assad, most people thought firmly with the upper hand.
Well, that civil war feels like it has burst open again.
And a reminder that it's a war which has since 2011, seen hundreds of thousands killed, many millions displaced.
First, let's get a look at the rebel group that seems to be leading the fight.
Hayat Tahrir Al Sham.