Is this the explanation for the Star of Bethlehem? University of Cambridge astronomer discusses theories

A University of Cambridge public astronomer has attempted to explain the likely source of the Star of Bethlehem from the nativity story.

Dr Matt Bothwell is often asked this question by children at the free stargazing events held every Wednesday evening at the university’s Institute of Astronomy off Madingley Road.

Also known as the Christmas Star, the Star of Bethlehem appears in the Bible’s Gospel of Matthew, in which the disciple relates how the Magi - or wise men - from the East came to Jerusalem and asked: “Where is the one who has been born king? We saw his star when it rose and have come to worship him.”

This prompts King Herod to send the Magi on secretly to Bethlehem to search for the child and report back.

Matthew tells how “they went on their way, and the star they had seen when it rose went ahead of them until it stopped over the place where the child was,” adding: “When they saw the star, they were overjoyed.”

Dr Bothwell, a bye-fellow at Girton College whose first children’s book, Astrophysics for Supervillains, has just been published, says there are three key theories for this story.

It could relate to the sighting of a supernova - created when a large star explodes.

“They can be very bright, because stars get tens of thousands times brighter when they go supernovae. So something would literally appear in the sky when a supernova goes off.”

How this was not recorded by anyone at the time or after.

“Even 2,000 years ago Chinese astronomers were doing quite a good job of observing things they saw in the sky and there are no obvious supernova remnants we can see in the sky,” said Dr Bothwell.

The ‘star’ could in fact have been a comet, which have “incredibly long orbital periods around the sun” meaning some come by astonishing rarely.

“Two thousand years ago there could easily have been one of these very, very distant long period comets that dipped into the solar system, and we’ve not seen it since. That could be very possible,” he said.

But comets were normally seen as bad omens.

“The chances of the Magi seeing what was traditionally a herald of doom and thinking ‘this is good news, our Saviour has been born’ doesn’t seem very likely,” suggests Dr Bothwell.

So could it have been a conjunction of planets? Unlike stars, planets change their position in the sky - in fact the word planet is Greek for “wanderer”. Four years ago, we witnessed a great conjunction when Jupiter and Saturn’s paths happened to cross making them appear very close together in the night sky, like one unusually bright dot or ‘star’.

“It’s pretty rare but it could have been something like that,” said Dr Bothwell. “Or an alternative is a bit more boring – it could have just been Jupiter, which can be astonishingly bright, depending on the time of year.”

But the Magi are described as astrologers - and there was enough astronomical knowledge at the time to identify Jupiter.

Prof Bothwell thinks the most likely explanation is that the star was in fact a comet.

“I’m an astronomer not a historian, but a comet feels like it could really be the explanation – we do get these very striking bright things which just hang in the sky and they happen once and don’t happen again for a very long time.

“Apparently around 5BC Chinese astronomers did notice a comet in the sky. They called it a ‘broom star’ because it has this long fluffy tail, so that seems to strengthen the comet idea.”

The Institute of Astronomy is open every Wednesday evening from 7-9pm, from 8 January until March 2025. Entrance is free. Visit public.ast.cam.ac.uk/public-open-evenings-ioa for details.