What astronomical object was the Christmas Star?

Throughout December, primary schools across the world will perform nativity plays with Mary, Joseph and the baby Jesus laid in a manger, visited by Shepherds, Angels and Wise Men. All watched over by a star. 

Figure 1 Comet C/2023 A3 (Tsuchinshan-ATLAS) photographed in October 2024 on the western horizonShow descriptionComet C/2023 A3 (Tsuchinshan-ATLAS) photographed in October 2024 on the western horizon shows a bright nucleus with a faint straight tail behind. The comet was visible just after sunset and this enhanced image shows the colour graduation from the setting sun at the horizon up to the dark sky above.

During the run-up to Christmas, we hear carols such as ‘We Three Kings Of Orient Are’ with the star ‘westward leading, still proceeding,’ guiding us to a perfect light. ‘The First Nowell’ also tells us of the star, which ‘To the Earth… gave great light, and so it continued both day and night’.

Christmas plays and carols have shaped our thoughts about the nativity story and a Christmas Star. The gospel of Matthew (who is the only writer to mention the Wise Men) tells us more about it. We are told that Wise Men (or Magi) from the East came to Jerusalem to ask King Herod where they could find the new King saying, ‘For we observed his star at its rising, and have come to pay him homage.’ The Magi are then directed to go to Bethlehem; 

Bringing together what we know from nativity plays, Christmas carols and Matthew’s account, let’s think about what astronomical objects the Christmas Star could have been.

Comet

Could the Christmas Star have been a comet? They are sometimes visible to the naked eye gradually brightening on their approach to the Sun and fading away after a few weeks or months. The comet itself is a small body (a few kilometres across) made of dust and ice that has come from much further out in our solar system and is a leftover from our planetary formation process. As the comet gets closer to the Sun and becomes heated, the ice can turn straight to gas carrying dust particles out into space. The gas and dust create impressive tails that can reach up to millions of kilometres in length.  

Figure 2 Comet C/2020 F3 (Neowise) was photographed during July 2020 in the northern skyShow descriptionComet C/2020 F3 (Neowise) was photographed during July 2020 in the northern sky. This comet was visible at midnight showing a bright nucleus with a slightly curving tail against the dark sky.

A comet is a compelling option. They are beautiful objects that change their position night by night and can look like an arrow pointing at the ground. They were much loved by medieval artists. However, there are no documented comets in the right time period and it’s unlikely that the Magi, with their astronomical knowledge, would have mistaken a comet for a star.

Planetary conjunction

The planets in our solar system look like bright stars and move in relation to the familiar backdrop of constellations in our night sky. Sometimes two or three planets may appear to come close together looking like they are touching, making a bright ‘new star’. They then separate back out over the following weeks and months.

Figure 3 Illustration shows a conjunction between Saturn (left) and Jupiter (right)Show descriptionThis illustration shows a conjunction between Saturn (left) and Jupiter (right). Over the course of two months, the two planets moved closer together in the sky, until on 21 December 2020 they appeared to almost touch before separating back out.

It is perfectly possible that the Magi inferred meaning (eg. kingship or new birth) from the locations of the planets.  

Shooting stars

Meteors (shooting stars) are the streaks of light that we see in the sky for just a moment as an intense flash.

Figure 4 A long exposure looking north capturing a meteor during the Perseids, as it passed through the atmosphere on the night of the 12 August 2024Show descriptionThis is a long exposure looking north capturing a meteor during the Perseids, as it passed through the atmosphere on the night of the 12th August 2024. The Perseid meteor shower occurs at the same point each year as the Earth travels through dust that is left from the tail of comet Swift-Tuttle.

Meteors are made up of dust or small particles that burn up as they pass through the atmosphere. They are known as meteoroids while still out in space and any fragments that survive are called meteorites once they have landed here on Earth. Periodic ‘meteor showers’, with hundreds of meteors per hour, occur as we pass through the leftover dust and particles that have been shed by a passing comet. Meteors only last for a few seconds each, so the star the Magi followed couldn’t have been a meteor.

Aurora

We know aurora commonly as the ‘Northern Lights’ in the northern hemisphere. Brightly coloured dancing patterns that appear in the sky are the result of charged particles that have been dramatically expelled from the Sun. These fast-moving particles interact with the Earth’s magnetic field and excite atoms in our atmosphere.

Figure 5 Long exposure of colourful aurora on the eastern horizon on 11 October 2024Show descriptionThis long exposure taken on the 11th October 2024 looking towards the eastern horizon shows the colourful aurora visible as a result of a large coronal mass ejection (CME) that had occurred in the preceding days.

Aurora can manifest as stunning pillars of light that point to the ground and dance across the sky, guiding you on a journey, but seem too transient and don’t fit the description of a star.

Supernovae

Our final option for the Christmas Star is a supernova. Stars can spend millions of years in a very stable state; but eventually, they run out of fuel. In the very last moments of a massive star’s life, it starts collapsing inwards with the pressure becoming great enough to cause an enormous explosion, blowing apart the outer layers of the star and fusing heavy elements.

This causes a very bright new star to appear in the sky for a few months before it fades away. It would be a good candidate for the Christmas Star, but we can observe remnants of supernovae and calculate when they occurred. Unfortunately, there aren’t any good candidates in the right time period.

Figure 6 The Tycho supernova remnant observed in 1983Show descriptionThis image shows a radio image of the Tycho supernova remnant (SNR) observed in 1983 and overlaid on optical images of stars. It was made using the Very Large Array (VLA) telescope, operated by the National Radio Astronomy Observatories (NRAO) in the United States. This remnant is thought to be associated with the supernova observed by Tycho Brahe in 1592.

It is interesting to look back and consider the ‘star’ that has been documented, painted, and sung about for two thousand years, and to ponder what kind of astronomical object it could be. 

Perhaps this Christmas we should look at how each one of us has been shaped by stars. The heavy elements created by supernovae and other stellar processes exist within us. The iron in our blood and other elements in our bodies have been forged in stars somewhere out in the Universe.