Was The Christmas Star A Real Astronomical Event?

This article first appeared in Issue 17 of our free digital magazine CURIOUS.

“There came wise men from the east to Jerusalem saying, Where is he that is born King of the Jews? for we have seen his star in the east, and are come to worship him.” That’s how Matthew describes the “Star of Bethlehem” in the Bible; the celestial portent that announced the birth of Jesus Christ to the Magi and set those three wise men on their travels. But was it a real event?

For centuries, religious scholars and astronomers have wondered this – and if it was real, what was it? The Gospel of Matthew was written around the year 85 CE based on a variety of sources and is the only one of the four canonical Gospels to mention the star. Originally written in Greek, αστερα was translated as “astra” meaning star, but it could also refer to other celestial objects like a planet or comet. A transient event of some sort is the favored explanation: a comet, supernova, or even a planetary alignment. 

“There has been a lot of debate over what the Star of Bethlehem could be,” Dr Greg Brown, astronomer at the Royal Observatory Greenwich in London, told IFLScience.

“An unusual event like a supernova would certainly be a possible candidate with one having occurred in the year 4 BCE. Alternatively, an event with a strong astrological association, like a comet or conjunction of a planet or the Moon with a star (especially Regulus, the little king) would also potentially have been noted as an event that may herald the birth of an important person.” 

  

That is assuming the event took place in the first place. Given that the star only appears in the Gospel of Matthew, the simplest explanation is that it is a fictional addition by the authors, possibly to link the birth of Jesus to the Star Prophecy from the Book of Numbers and strengthen his messianic claim.

“Despite a lot of effort to find out, the reality is that we can’t be certain that the apparently astronomical event actually happened, or that any particular astronomical event was in fact recognized as a specific sign at the time,” Dr Brown said. “There is always the chance that an event that occurred around the time may have been attributed as a sign after the fact and written in by later authors as though it were recognized at the time.”

A supernova sign?

In the case that it is real, noting it suggests something had changed in the night sky. Stars do not move quickly enough for humans to appreciate them in a new location, so one interpretation is that if a star was involved, it became visible because it suddenly became much brighter, likely as a result of exploding as a supernova. It’s rare to witness a supernova. The earliest recorded was by Chinese astronomers in 185 CE who described a mysterious “guest star” that stayed in the sky for eight months. Kepler’s Supernova was visible in the sky for 18 months after appearing in 1604. That’s enough time for the wise men to pack their camels and get to Bethlehem. 

There is one problem when it comes to finding a suitable astronomical candidate for the guiding star: its job is to give direction – and most celestial events are very bad at doing that.

There are few candidates for such an object. RCW103 is a supernova remnant that appeared to have exploded about 2,000 years ago. It is in the southern constellation of Norma, which can only be seen completely by observers south of 29° parallel North. Bethlehem has a latitude of 31.7°. Another possibility is the Hulse–Taylor pulsar in the constellation of Aquila, the Eagle. Chinese and Korean astronomers recorded an event that could be linked to the progenitor of the pulsar going supernova sometime between 4 and 2 BCE. 

It might not have been a supernova at all but a nova, a temporary brightening of a white dwarf. These stars have companions from whom they steal material. As the material hits the surface of the white dwarf, the overall brightness increases massively. If the white dwarf steals enough mass, it might even collapse on itself and become a supernova. 

Novae are a lot more common than supernovae, so could be a good explanation for the Christmas star. There is about one new nova every 12 to 18 months that becomes bright enough to be visible to the naked eye. However, an alternative that does not involve sudden brightening but still involves a star (together with planets in this case) is the celestial configuration known as appulse. 

Planetary conjunction, alignment, or appulse

Perhaps the three wise men didn’t see a single object. The interpretation of the Magi was that the conjunction was a “star”, although perhaps not in modern terms.

An appulse is the apparent closest approach of two objects in the sky, related to a similar event known as a conjunction, but technically a single instant, Dr Brown explained. This may appear to the observer as a single point of light. 

“Depending on the objects, they can appear close together in the sky for a considerable period of time. Conjunctions involving the Moon will be close for several hours, while conjunctions involving a planet could last several days or even weeks depending on the objects involved and how close together is deemed ‘close’ in the eye of the observer.”

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There were certainly interesting planetary conjunctions in the decade or so before the canonical birth of Jesus. Some were close but not visually impressive, such as the Jupiter and Saturn conjunction, others might have been a bit more peculiar, such as the meeting between Regulus, Jupiter, and Venus in 3 BCE or the Venus-Jupiter conjunction of 2 BCE. 

However, there is one problem when it comes to finding a suitable astronomical candidate for the guiding star: its job is to give direction – and most celestial events are very bad at doing that.

Comet: a celestial GPS?

The constant spin of the Earth means that any object that starts in one part of the sky will inevitably move across it over the course of a few hours.

The most common interpretation of the star is that it was a comet, because they move across the sky. Possibly, it is also the directional appearance of comets – the head and the tail are a bit like an arrow – that suggests something for the wise men to follow, to be guided to Bethlehem. However, there is also a major problem with that as a definitive explanation. 

“A comet has certainly been used to represent the Star of Bethlehem before but as a deliberate marker of direction, it suffers from the same problem as every other object in space does: The constant spin of the Earth means that any object that starts in one part of the sky will inevitably move across it over the course of a few hours,” Dr Brown told IFLScience. 

“The result is that with the exception of the North and South celestial poles (directly above the North and South poles), if you try and walk towards an object in the night sky you will be constantly changing direction over the course of the day and it won’t point at any specific place.” 

The Earth spins on its axis and moves around the Sun so the position of objects in the night sky shifts a little bit every day. In fact, although the planets are all moving in one direction around the Sun, when the Earth catches up to them (or them to Earth) in their orbital path, they can appear to stop still in the sky, or even move backward – Mercury in retrograde, for example. This apparent stationary period for planets is short and wouldn’t be much use as a signpost for a specific location on Earth. 

“A comet might seem more plausible given that it will move across the sky over time, possibly partly countering the spin of the Earth, but the reality is that it would still be unable to maintain a constant direction in the sky or reasonably guide someone towards a specific place,” Dr Brown explained. 

“Instead, perhaps a more likely explanation is that they followed the sign of the ‘star’ existing, not literally followed it by walking towards it. This would be more consistent with astrological work of the time.”

Some scholars believe the wise men were astrologers who had been watching the skies for years for a sign portending the birth of the new king as told in the Old Testament, and when a suitable astronomical event occurred, they identified it as the time being right to set out and claim him. However, with the Catholic church later declaring astrology as heretical, this interpretation of the heavens might over time have fallen out of favor for a more tangible explanation – an event that just happened to coincide with the birth of Jesus. 

CURIOUS magazine is a digital magazine from IFLScience featuring interviews, experts, deep dives, fun facts, news, book excerpts, and much more. Issue 17 is out now.