What was the Star of Bethlehem?

It’s a key feature of a school nativity play, but what’s the science behind the Star of Bethlehem?

Also known as the Christmas Star, the Star of Bethlehem appears in the Bible’s Gospel of Matthew. The disciple relates how the Magi [wise people] 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.”

Disturbed by this news, King Herod secretly sends the Magi on to Bethlehem to search for the child and report back to him. Matthew then continues...

“…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. When they saw the star, they were overjoyed.”

But what could the star have been, given that – as far as we’re aware - there’s no Star of Bethlehem in the universe?

Dr Matt Bothwell, Public Astronomer at the University of Cambridge, is often asked this question by children at the free stargazing events held every Wednesday evening at the Institute of Astronomy.

Matt, a Bye-Fellow at Girton College whose first children’s book, Astrophysics for Supervillains, has just been published, says there are three key theories...

1) A supernova. Matt explains that a supernova is essentially a big star that explodes. “They can be very bright, because stars get tens of thousands brighter when they go supernovae, it can take a star that’s not currently visible and make it visible. So something would literally appear in the sky when a supernova goes off”. The case against this theory however is that it wasn’t 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,” says Matt.

2) A comet. Comets can have “incredibly long orbital periods around the sun”, Matt explains, and there are some that 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 says.
But the historical argument against this theory is that 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,” says Matt.

3) Conjunctions. This theory is based on the fact that the planets are not fixed like stars, they change their position in the sky (“that’s actually where we get the name planet from, explains Matt, as ‘planet’ is Greek for wanderer”).
Four years ago Jupiter and Saturn’s paths happened to cross in an event known as the great conjunction. Conjunctions like this would see two planets very close together in the night sky, and appearing as one unusually bright dot or ‘star’.

“It’s pretty rare but it could have been something like that,” says Matt. “Or an alternative is a bit more boring – it could’ve just been Jupiter, which can be astonishingly bright, depending on the time of year”.
However, the case against both conjunctions and confusion between a planet and a star is that even though they did not have modern astronomical science at their disposal, the Magi themselves are described as astrologers and there was enough astronomical knowledge at the time to identify Jupiter.

So what’s the answer then? Matt says he thinks the most likely explanation is a comet.
“I’m an astronomer not a historian obviously, 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.”

Perhaps the last word should go to a man known as the Pope’s Astronomer. Br. Guy Consolmagno SJ, who was invited by the Von Hügel Institute for Critical Catholic Inquiry at the University of Cambridge to deliver their annual lecture at St Edmund’s College, laughs that there are “more theories than theorists”.
“Ultimately nobody is sure, but for me the deeper message is that the sky itself – the joy and beauty that you find there – is a guide to lead us to the Christ Child,” he says. “To me, any star would be the Star of Bethlehem.”

The Institute of Astronomy is open to the public every Wednesday evening 7 - 9pm from 08 January until March. Entrance is free. More details on the Institute of Astronomy website.