Star of Bethlehem

A depiction of the Star of Bethlehem. Image credit: Garrett W. / CC BY

Right around Christmas time, a lot of people come up to astronomers and ask what the Star of Bethlehem could have been. This “star”, also called the Christmas Star, is said to have appeared in the sky in time to announce the birth of Jesus Christ to the Three Wise Men, and would have lead them to Bethlehem, to honour the newborn. Modern astronomers are now trying to figure out what astronomical event might have appeared in the skies all those years ago as the Star of Bethlehem. In order to do that, it is first necessary to try and pin down the actual birthdate of Jesus. Historical clues recovered throughout many different sources seem to indicate that he may have actually been born around the year 5 B.C. Furthermore, some believe the Star of Bethlehem did not appear right when he was born, but perhaps a year or even more later. This wide range of years have left us with quite a few theories as to the real identity of the Christmas Star:

1) A series of two or more planets in near alignment in the sky may have been seen as a single very bright star. Using known planetary orbits, we are finding it difficult to find such an alignment to have occurred at the right time, unfortunately.

2) Some Chinese astronomers recorded a new star born from a supernova that was highly visible for 70 days in the constellation of Capricorn around 5 B.C. However, the religious scripts state that the Christmas Star moved from East to South over the course of many months, which cannot be so easily conciliated with a supernova progenitor.

3) A comet might have been the culprit, but they were typically seen as bad omens. Furthermore, we cannot find any other mention of such a noteworthy object in the literature.

4) Jupiter, a planet that can indeed be very bright in the sky, may have seemed near immobile due to it being at the end of one of its retrograde loops.

The verdict is still out as to what the Star of Bethlehem truly was. Despite all our technological advances, it may be difficult to backtrack to a definitive answer so many years in the past.

Merry Christmas!

Pluto vs Neptune: Mortal Kombat or RomCom?

In Theaters All The Time (but also never IRL....). Image credit: N. Ouellette.

Lots of people ask me what happened to Pluto. Is it still a planet? Does it even exist at all anymore? Is it a part of a massive Illuminati conspiracy meant to enslave us all? Well, in a
nutshell, Pluto still does exist, it’s been downgraded to a “dwarf planet” and the goat lord Baphomet forbids me to speak of our dark secrets. But that’s not the point of today’s post. I was recently asked about Pluto possibly colliding with the planet Neptune and the ensuing mayhem it might cause.

A sideview of Pluto's orbit (in red) shown to be highly inclined compared to the plane of the Solar System.  Animation credit: Lookangmany.

It’s a little known fact that Pluto’s orbit actually brings it closer to the Sun than Neptune at certain points due to its high eccentricity. The last time this happened was between February 7th 1979 and February 11th 1999. So wouldn’t this mean that the orbits of Pluto and

Neptune cross twice every cycle? Surprisingly, no! When Pluto’s orbit comes closest to the Sun and to Neptune’s orbit, it is also at its farthest point above Neptune’s path, which means the two planets always steer far clear of each other. The distance between the two at closest approach is 17 AU (the unit measuring one Earth-Sun distance). Pluto actually comes closer to Uranus at times, the shortest distance between the two being 11 AU!

A view of Pluto's orbit (in red) from above, compared with Neptune's (blue planet). Animation credit: Lookangmany.

Now, it is true that Pluto’s orbit is a little chaotic. Pluto is a very small body surrounded by lots of other objects, some of which are gas giant planets! So it is being pulled around quite a bit. One could argue that long term changes in Pluto’s orbit might one day bring it on a collision course with Neptune. Luckily, Pluto and Neptune are caught in a very precise 2:3 resonance with each other. This means that for every 2 full cycles Pluto goes through, Neptune will go through precisely 3 cycles. This is a very stable relationship between the two. If Pluto’s orbit were to change slightly in the future, so too would Neptune’s in order to respect the resonance.

So the moral of this story is Pluto and Neptune are in a very healthy relationship in which they each respect the other’s space. We should all strive to be more like Pluto and Neptune.

Comet Swift-Tuttle

False colour image of Comet Swift-Tuttle taken with the Spacewatch Telescope. Credit: Jim Scotti, University of Arizona

As you may know, we are right in the heart of the Perseid meteor shower. You can all thank Comet Swift-Tuttle for this beautiful annual spectacle! This comet, that is 26 km in diameter, has been observed for many thousands of years by ancient as well as modern astronomers, and has a fairly stable and well understood orbit. It was rediscovered in 1992, when it last made its closest approach to the Sun, at which point many thought it might be an impact risk to the Earth during its next approach on August 14th 2126. Its orbit has since been recalculated, and chances of an impact occurring are thought to be extremely low.

However, it is the largest Solar System object that makes repeated close passes of Earth. With its size and velocity, a collision with Swift-Tuttle would have approximately 27 times the energy of the K-T event impactor, most commonly known as the cause of the extinction of the dinosaurs. It is for this reason that many astronomers consider Comet Swift-Tuttle to be the single most dangerous object known to humanity. Marvel at the awesome power of the Universe! For more factoids on Swift-Tuttle and the Perseids, visit Space Magazine.

Lifetime of our Sun

An artist's depiction of the life stages of our Sun, from birth to death as a white dwarf within a planetary nebula. Image credit: ESO/S. Steinhöfel.

It all started with a giant cloud of gas, some 4.57 billion years ago. This cloud of helium and hydrogen collapsed under its own gravity and formed a protostar. After 100,00 years, it became a fully formed star and began its hydrogen burning phase, otherwise known as its main sequence — its adulthood of sorts. This lasts a total of 10 billion years. Currently, our Sun is halfway through its main sequence stage. Even now, however, the Sun is going through changes, but they are indiscernible over a human lifetime. Every billion years, our star gets 10% brighter, as it marches towards the end of its life. Because of this, the Earth’s surface will be too hot to sustain liquid water, and, in all likelihood life, in one billion years. To make matters worse, the Sun is expected to swell up to a few hundred times its current size in 5 billion years, when it becomes a red giant. At this point, it will have swallowed up Mercury, Venus, and possibly even Earth!

In one final gigantic tremor, our red giant Sun will expel its gaseous outer layers, forming an expanding planetary nebula, and leaving only a faint white dwarf at its centre. The layers of the planetary nebula will sweep through our Solar System and reach the interstellar medium, where they might one day join a gas cloud and lead to the formation of another star. As with the living creatures on Earth, we observe a beautifully cyclic nature in the lifespan of stars.

Meteor Showers!

The 2012 Geminids over South Dakota. Image credit: David Kingham.

Meteor showers are some of the most beautiful sights one can see with their naked eye, and they have the marked advantage of appearing at almost the same time every year! Very young, we’ve learned to call these luminous streaks across our sky shooting stars, but they are not stars at all, but rather flaming space debrsi! Every year, around 15,000 tonnes of these debris, from sand grain sized to boulder sized, enter the Earth’s atmosphere. As they burn up, they leave behind brightly visible paths. While few meteors survive their journey and fall to the ground as meteorites, most are completely destroyed before impact. 

A few times a year, these shooting stars happen in swarms, sometimes inundating our skies at a rate of up to thousands of meteors per hour! In such impressive cases, they are called meteor storms rather than simple showers. But why do we witness sudden surges of shooting stars at the same time every single year? The answer is comets! We all know about comets: icy bodies orbiting around our Sun, periodically appearing in our skies as bright tailed objects. The trajectory of some of these comets crosses the trajectory of Earth’s orbit. Obviously, the comets and Earth do not cross this point at the same time or that would mean bad news for us! However, comets do leave behind long-lasting trails of debris into which Earth passes once a year, like clockwork. Some of these particles crash through our atmosphere and delight us in the form of a meteor shower. The intensity of each meteor shower depends on the density of the particle cloud leftover by the comet, the position of other planets in our Solar System, the intensity of the Sun’s activity and many more complex variables. Luckily, there are over 50 meteor showers every year, many of which are visible to the naked eye so there’s always a chance to spot a shooting star. The two most prominent showers are the Perseids in mid-August and the upcoming Leonids in mid-November!