Topical Science: Could the star of Bethlehem have been a comet?

D E C E M B E R

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Topical Science Explaining topical science news in plain language.

In This Issue Page 1

Introduction

Page 2

Science & Superstition

Page 3

Was the star of Bethlehem a Comet, or a Supernova?

Page 4

Was it a Planetary Conjunction?

Could the Star of Bethlehem have been a comet?

The Gospel according to Matthew tells us that Magi came to Jerusalem, asking where the King of the Jews was born. They had seen what they described as a ‘new star’ in the east and had followed it from their home countries, until they came close to Jerusalem. The Magi were astronomers, or more probably, astrologers. In ancient times, the distinction between astronomy and astrology was somewhat blurred.

Key Dates Solstice

Christmas Day

21 December

25 December

st

th

This is the shortest day On this day, of the year & the Christians celebrate Winter Solstice in the the birth of Christ in Northern Hemisphere.

Bethlehem.

Epiphany 6th January 2019 This day marks the visit of the Magi, who followed the star to Bethlehem.

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Science and Superstition In ancient times, people in general tended to be more superstitious than they are today. They looked to the night sky for great portents. They believed that a new star appearing in the sky could herald the birth of an important person, perhaps someone who would one day be a great king.

Ancient astronomers knew the star patterns like the backs of their hands. The relative positions of the ‘fixed’ stars were known and many of the constellations had been named.

The movements of the ‘wanderers’ or planets, were also well known.

Mercury, Venus, Mars, Jupiter and Saturn were known in ancient times and are named after Roman Gods. If any new ‘star’, or other bright object appeared in the night sky, it would have been spotted by the Magi.

Is there a scientific explanation for the star of Bethelehem?

Various theories have been offered, suggesting it could have been a comet, a supernova or a planetary conjunction.

The Italian artist Giotto, (1267–1337) in a famous fresco of the Nativity, depicted the star over the stable with a tail like a comet. The idea may have occurred to Giotto after he saw Halley's Comet in 1301.

Comets are unusual members of the solar system. They travel in extremely elliptical orbits, entering the inner regions of the solar system once in a few hundred years or so, though some have shorter periods, like Halley’s comet, which appears every 76 years.

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Was the start of Bethelehem a Comet?

This is one possible explanation. A comet is mainly composed of snow and ice, with rocks and other debris embedded in it. When it is in the outer part of the solar system, it is invisible, but when a comet comes close to the sun, it becomes visible, reflecting the sun’s light. After it has left the vicinity of the sun, heading back towards the outer planets, it gradually fades from view and becomes invisible again. Some comets can be seen by the naked eye, so an approaching comet could be mistaken for a ‘new’ star. Comets develop what look like bright tails when they are close to the sun. This is because the sun’s heat causes some of the snow and ice to melt, releasing dust and ice particles, which form a long trail, which reflects the sun’s light. Comets are a dramatic sight and in ancient times were seen as portents of death, rather than heralds of birth. Halley’s comet may have been the star of Bethlehem, but the timing is wrong. It came close to earth in 11 B.C, several years too early. However, the exact timing of the date of the birth of Jesus is a matter of dispute. Our present calendar system was not devised until the sixth century and there was an error in the calculation of the date. Moreover, there was no year zero, so that the year 1 AD (Anno Domino) came immediately after 1 BC. The Gospel story says the birth of Jesus occurred during the reign of King Herod, who died in 4 B.C according to most historians, though others give it as 1 B.C. The journey of the Magi would have taken several months, or even up to two years and we don’t know how long King Herod lived after their visit, so there is a period of roughly five years, from about 6 BC to 1BC, during which the star of Bethlehem may have appeared.

Alternative Theories Another possibility is that the ‘new’ star was a supernova. This is a huge explosion, which occurs when a massive star comes to an end. At first, a star consists mainly of hydrogen, the most abundant element in the universe. The hydrogen gas becomes compressed by its own gravity and gets extremely hot, causing hydrogen atoms to fuse into helium, releasing an enormous amount of energy. During most of a star’s lifetime, this fusion continues and the equilibrium between gravity and the internal pressure caused by convection maintains the star in a steady state. Our sun is at this stage now and it is expected to go on fusing hydrogen to helium for another 4.5 billion years or so. When most of the hydrogen in a star has been used up, the helium atoms start to fuse into heavier elements, such as carbon, oxygen and eventually iron. What happens next depends on the mass of the star. Some stars swell up to become red giants, others collapse under gravity to become white dwarfs, neutron stars, or even black holes. Sometimes, the star becomes so hot that it blows up in a spectacular event known as a supernova explosion. When this happens, the star becomes extremely bright and may take several months to fade. The German astronomer, Johannes Kepler, observed a supernova explosion in our own Milky Way galaxy in the year 1604. It was bright enough to be seen with the naked eye. Perhaps the Magi observed a supernova?

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Is there another explanation?

A third possibility is a planetary conjunction. This is when two planets, in their orbits around the sun, come into alignment, so that, seen from Earth, they appear to merge into one very bright star. There can also be a conjunction of a star with a planet. In the years 3 and 2 BC, there were several conjunctions, including three between Jupiter and the bright star Regulus and there was also a conjunction between Jupiter and Venus.

The Magi may have observed the conjunction of Jupiter with Regulus, prompting them to set out for Jerusalem. By the time they reached Bethlehem, the retrograde motion of Jupiter would make it appear as if it had stopped over the place where Jesus was. This apparent retrograde motion is because earth has a smaller orbit than Jupiter as they both travel around the sun, so when earth overtakes Jupiter, it seems that the giant planet is going backwards.

Whatever the explanation, the Magi observed what they believed was a new star and it led them to Bethlehem, where they found the baby Jesus.

* * * HAPPY CHRISTMAS EVERYONE!

About the Author Margaret Franklin. M.Sc., FICI, is the author of this ‘Topical Science’ newsletter.

She spent most of her career at Athlone Institute of Technology, in the Republic of Ireland, where she taught chemistry for over 30 years.

Margaret now writes popular science articles for the information of the general public 4