When will the Sun stop working?

The Sun is a massive nuclear fusion reactor that is converting hydrogen to helium and releasing vast quantities of energy as result. This means that eventually it will run out of fuel and cease producing energy. When that happens there will be no possibility of life continuing on any planet in the solar system. So how long have we got?


We’re all doomed!

All good things must come to an end, and that means that not only will every person, animal and plant currently alive have to “meet its maker” (however you might interpret that expression), but even the Sun itself will eventually die and cease to emit light and heat.

However, there is no need to panic! The Sun has been going strong for 4.6 billion years and will continue to do so for a long time yet.

What will happen, over the next few billion years, is that as the Sun’s nuclear fuel is gradually used up it will become both brighter and bigger - twice as bright and 50% larger.

Around five billion years from now all the hydrogen in the Sun will have been converted to helium and the nuclear fusion process will end. The core will shrink and the outer layers expand massively while at the same time becoming much cooler. The Sun will have become a red giant.

The diameter of the Sun will expand to such an extent that it will swallow up the inner planets, including Earth. However, life on this planet will have become impossible long before this point is reached – even if mankind has not committed collective suicide (on behalf of all living things) many millions of years before this happens.

Eventually the Sun’s outer layers will fade away into space, to leave only a small white dwarf star behind. If the Sun had been considerably more massive at the outset it would have run the risk of exploding as a supernova, but it was never anywhere close to being that category of star.

The total life of the Sun, from birth to death as a fading white dwarf, will have been about 11 billion years, of which the existence of humankind on Earth will have been an extremely small fraction.

At least – when it happens – nobody will be able to blame any President or Prime Minister of the “wrong” political persuasion for their lack of action in allowing the Sun to stop working!

© John Welford

Venus: Earth's fierce sister planet

Venus is the closest planet to Earth in terms of distances between orbits (about 41 million kilometres) and the two planets are quite similar in size, surface area, volume and mass. Venus is therefore sometimes dubbed Earth's sister planet.

However, the sisters have very different characteristics. For one thing, Venus rotates far more slowly than Earth, so that a day on Venus lasts longer than a Venusian year! It also rotates the other way round to Earth, so that the sun rises in the west and sets in the east.

However, the most important difference, and the main one that makes it impossible for life to exist there, or for manned missions ever to be contemplated, is that the surface temperature on Venus averages 450 degrees Celsius. Atmospheric pressure is also enormous, being 90 times as great as that on the surface of Earth.

The main reason for the huge surface temperature is the fact that the Venusian atmosphere consists of 96% carbon dioxide, thus causing the thick cloud cover of Venus to act like extremely efficient greenhouse glass. Temperatures on Venus are therefore 100 degrees hotter than on Mercury, which is closer to the sun by about 50 million kilometres.

It has often been said that Earth risks becoming another Venus if the proportion of greenhouse gases in the atmosphere is allowed to keep rising. However, with carbon dioxide currently representing 0.4% of Earth's atmosphere it clearly has a long way to go before reaching the Venusian level!

© John Welford

Clues to life on Mars from the Atacama Desert

The idea that there might once have been living organisms on Mars is a fascinating one, and even more so is the notion that some form of very primitive life might still be there. However, clues as to where the evidence for past or present life could be found are coming from a very strange direction, namely studies of the Atacama Desert in Chile, South America.

The Atacama Desert is one of the driest places on Earth and thus offers a reasonably close parallel to conditions on Mars – or at least to Mars as it might have been in fairly recent geological time.

The point is that microbes – very simple life forms – can survive for many years in water-free environments and reactivate when water again becomes available. This has been shown to happen in the Atacama Desert, where microbes survive in a dormant state in salt crusts (see picture) that absorb any water that happens to be around. This need not even be liquid water, because salt can take water directly from the air.

The question is therefore whether similar conditions could exist on Mars, given that it is known that there was once abundant liquid water on the planet that could have given rise to salt deposits as it evaporated. Indeed, such deposits have been discovered in Mars’s southern uplands. Modern Mars is not totally devoid of water, although most of it exists as ice at the north polar cap and beneath the surface. There are also very small amounts of water vapour in the thin atmosphere of the planet.

Given the much drier conditions on Mars, it is conceivable that evolutionary processes might have worked differently than on Earth, such that lifeforms could have survived on Mars that would not have done so on Earth. It is therefore going to be very interesting for future rover missions to explore the salt deposits on Mars to see if primitive life has survived. If it can do so in the Atacama, there is a reasonable chance that this is also possible in comparable environments on Mars.

© John Welford

Where do fast radio bursts come from?

Astronomers are constantly finding new and mysterious things “out there”, and cosmologists then expend considerable energy on working out what they are!

One of the latest phenomena in this category is the “fast radio burst” or FRB, which is a pulse of radio waves that is extremely brief – a matter of fractions of a second. The first detection of an FRB was made in 2007, from examination of signals received in 2001 by the Parkes radio telescope in Australia. The pulse lasted for only 4.6 milliseconds (a millisecond is a thousandth of a second).

Despite much analysis of data received by radio telescopes across the world, only a handful of FRBs have been found to date, and only one (in 2014) has been observed as it happened.

Astronomers would love to be able to track an FRB to its source, and this can best be done by getting a number of radio telescopes in different countries to get a fix on a signal, once one has been detected. In April 2015 they were able to do this after an FRB was picked up that lasted for less than a millisecond. What they also discovered was that the burst produced a “radio afterglow” that continued for six days. This gave the astronomers at Parkes enough time to alert the Subaru telescope in Hawaii to focus on the source, which in turn led to the discovery that the signal came from an elliptical galaxy some 7 billion light years away. The radio burst therefore started its journey several billion years before Planet Earth existed!

Various theories have been proposed to explain the cause of a fast radio burst, and – as might be expected – alien intelligence of some kind is on the list. However, the theory that is currently being given most weight is that FRBs originate from the collision of two neutron stars (a neutron star is the extremely small and dense residue of a collapsed giant star). Such an event would produce a massive amount of energy that would also include gravitational waves, such as are believed to result from the merging of black holes. It is hoped that a gravitational wave source will be confirmed as coinciding with that of a fast radio burst, but this has yet to happen.

However, not all cosmologists accept the idea that FRBs are caused by colliding neutron stars, or that there need be only one explanation. Another candidate is a flare from a magnetar, which is a neutron star with a powerful magnetic field, the decay of which can produce x-rays and gamma rays.

Speculation will doubtless continue until more observations can produce a more definitive answer to this particular problem.

© John Welford

Ursa Major: a brief guide

Ursa Major is one of the most familiar constellations visible in the Northern Hemisphere. Here is a brief guide to some of its notable features.

Ursa Major (the Great Bear) is known by several names, including the Plough and the Big Dipper. It is an easily recognised constellation consisting of a rough rectangle and three other stars that appear to attach to it in a handle shape. These are the ones that are easily seen with the naked eye, but there are many more in this region of the night sky that can be seen with the aid of a telescope or binoculars.

As with all constellations, one has to remember that they are optical illusions in that their members usually have no relationship with each other and are at many different distances from Earth.

The two “outer” stars of the rectangle are Dubhe and Merak. These are known as the Pointers because an imaginary line drawn through them leads the eye to Polaris, the North Celestial Pole and a guide to finding one’s direction at night. Dubhe is a yellow giant star with high luminosity, being the 35th brightest star visible from Earth. It has a smaller companion star. Merak is a main sequence star that is nearly three times larger than our Sun and 68 times more luminous.

The second star in the “handle” is Mizar, although most people can soon make out that it is in fact two stars, the fainter companion being Alcor. However, Alcor is in fact a binary star and Mizar is a quadruple, which means that one is actually looking at six stars and not two.

If one has a good pair of binoculars it is possible to see two galaxies to the north of Ursa Major. These are Bode’s Galaxy (M81) and the Cigar Galaxy (M82). M82 is almost edge-on to us but M81 is tilted at a lesser angle. These galaxies are around 10 million light years away but close enough to each other to interact gravitationally.

The spiral galaxy M101 can be seen on the other side of the constellation. Binoculars will reveal it as a circular smudge but better magnification will show that it is a well-formed spiral that is face-on to us.

A good telescope can reveal the Owl Nebula (M97), although the “owl eyes” will need a relatively high resolution telescope in order to see them.

© John Welford

Bennu, a near-Earth asteroid

Bennu is a ‘near-Earth’ asteroid, by which is meant a minor planet the orbit of which crosses that of Earth, as opposed to those in the asteroid belt between Mars and Jupiter. Astronomers are keen to keep a close watch on these asteroids, because there is a danger that one of them could collide with Earth, with potentially disastrous consequences for everything living here.

Bennu – the name comes from that of a heron-like bird in Egyptian mythology that may have provided the origin of the phoenix myth – is on the list of asteroids that offer a credible threat of collision. However, the chance has been estimated as being no worse than 1 in 1800 and the date of impact, should it occur, would not be until about the year 2180. It is only third on the list of ‘asteroids most likely to hit us’ but the other two are not expected to pose a threat any sooner than that of Bennu.

In the meantime, a mission is under way to have a good look at Bennu and see what it is made of. Given that it was formed from material that existed at the very beginning of the Solar System, knowledge of the asteroid’s composition is of considerable interest for understanding how the solar System evolved.

The mission will also be able to study precisely the course that Bennu takes in its orbit. This is clearly of fundamental importance in working out how the asteroid proceeds through space and the extent to which external forces, such as the solar wind, vary the orbit of small bodies such as this (Bennu has a diameter of 493 metres). Predicting future orbits is not easy with current knowledge, but precise information gathered from a close-up look will help to make computer models more accurate when future impact risks are being calculated.

The mission, called OSIRIS-Rex, was launched in September 2016 and is due to reach Bennu in 2018. The spacecraft will make readings of the surface for about eight months and enable a 3-D map to be created. From this map a landing site will be chosen so that a sample of rock can be taken from the surface, after which the craft will return to Earth. It is expected that the mission will conclude in 2023.

© John Welford

Eris: the dwarf planet that demoted Pluto

Eris is a dwarf planet that, when discovered in 2005, was found to be larger than Pluto, which had been regarded as the Solar System’s ninth planet since its discovery in 1930. This posed huge question marks over Pluto’s right to that status, which has subsequently been removed.

Eris has an elliptical orbit that takes it from 38 AUs from the Sun at its closest to 97 AUs at its furthest point (AU stands for Astronomical Unit, one of which is the average distance of the Earth from the Sun). For comparison, Neptune is just over 30 AUs from the Sun and Pluto’s orbit varies from 30 AUs to 49 AUs.

Pluto’s orbit is almost wholly within the Kuiper Belt that consists of a vast number of small objects and is the source of most of the comets that visit the inner Solar System. However, Eris is considered to be a member of the “Scattered Disc” that extends beyond the Kuiper Belt as a diffuse outer halo. The orbit of Eris is not only highly elliptical but at an angle of 44 degrees from the plane of the Solar System within which the “regular” planets orbit. A complete orbit of the Sun by Eris takes around 560 years to complete.

Eris, which, at around 2390 kilometres, is believed to have a diameter about 200 kms larger than that of Pluto, is the second brightest object in the Solar System (the brightest is Enceladus, a moon of Saturn). This would appear to be because the surface is covered in frozen water and methane that are highly reflective of the Sun’s rays.

Eris has a moon of its own, named Dysnomia, which could be as large as 685 kms in diameter, which makes it a relatively large Scattered Disc / Kuiper Belt object in its own right. Calculations based on the orbit of Dysnomia around Eris enable a good estimate to be made of the mass of Eris, namely 27% greater than that of Pluto.

In Greek mythology Eris was the goddess of strife and discord, which seems appropriate given the upset caused to the status of Pluto from Eris’s discovery. Dysnomia was a daughter of Eris, her name meaning “lawlessness”.

© John Welford

Callisto: the outermost Galilean moon of Jupiter

Callisto is the outermost of Jupiter’s four “Galilean moons” – so called because they were first seen in 1610 by Galileo.

Callisto is the second largest of the four, having a diameter of 4800 kilometres (3000 miles), which makes it appreciably larger than our own Moon (3500 kilometres, 2200 miles) and about the same size as the planet Mercury. It orbits Jupiter at a distance of about 1.2 million miles and takes nearly 17 days to complete one orbit.

Callisto is different from its inner neighbours in that it has a dark surface that is scarred by a multitude of craters. It has not been subject to gravitational tides in the way that Io, Europa and Ganymede have, and the interior does not appear to be separated into distinct layers – although it is possible that there is a saltwater ocean deep beneath the surface. The bulk of Callisto appears to be a mixture of rock and ice.

The surface has not therefore been reshaped, and the moon has had a constant bombardment from meteorites that have formed impact craters that have remained as they were ever since they were created.

Particles from the solar wind have caused chemical reactions that have darkened the surface. There has also been weak radiation that has led ice to sublimate (i.e. turn directly from solid to gaseous form) and then refreeze on the crater walls, weathering them into jagged peaks.

There is also evidence that Callisto has a very thin atmosphere of carbon dioxide.

In Greek mythology, Callisto was a nymph who was seduced by Zeus (Jupiter) and later transformed into a bear by Hera. She can now be seen in the night sky as the Great Bear constellation.

© John Welford

Constellations in the night sky

We are used to the names given to the star constellations by the Greeks and Romans, but other ancient civilizations had different ways of making sense of the patterns in the night sky.

The Mesopotamians were the first people to name the constellations, around 3000 BC, using names based either on animals or human occupations (such as “The Herdsman”).

The ancient Egyptians named the constellations after gods and goddesses. For example, they saw what we call Ursa Major (“The Great Bear”) as a jackal and named it after the jackal god Set.

The ancient Chinese divided the night sky into 28 “lunar mansions” which formed four groups, each with its assigned constellations. The groups were named the Red Bird of the south, the Black Tortoise of the north, the Blue Dragon of the east and the White Tiger of the west.

In ancient India there were 27 divisions, known as nakshratras. Each of these was centred on a particular planet or star that was in turn associated with a god or goddess.

The aborigines of central Australia were more interested in the spaces between the stars than the stars themselves. For them, it was the darkness that made the patterns.

However we name the patterns in the sky, it is worth remembering that they are all an illusion caused by our perception from Planet Earth. Just because a star may look close to another one does not mean that there is any relationship between them. For example, the three main stars of “Orion’s belt” are at huge distances from each other, with the central star (Alnilam) being 1340 light years away as opposed to the 916 and 800 light years of its “neighbours” Mintaka and Alnitak respectively.

© John Welford

Europa

Europa is the smallest of the four Galilean moons of Jupiter – so called because they were discovered by Galileo in 1610. With a diameter of 3138 km (1950 miles) it is the only one of the four that is smaller than Earth’s Moon. Europa orbits Jupiter at a distance of 671,000 km (417,000 miles) and a complete orbit takes 3.55 Earth days. This orbital distance is nearly twice as great as that of our own Moon, but Europa is moving very much more quickly – not only does our Moon take 27.32 days to orbit Earth, the planets being orbited are vastly different in size, Jupiter being 11 times larger than Earth.

From a distance Europa appears to be a smooth, white ball of ice, but a closer view reveals pale markings that criss-cross its surface, and very few impact craters. This suggests that there is a constant renewal process at work.

Europa is subject to tidal pressures caused by the gravitational pulls of Jupiter and the neighbouring moons of Io and Ganymede. This leads to flexing that produces heat, which in turn leads to the presence of liquid water that is trapped between a rocky core and a crust of ice.

Where the crust fractures, jets of water escape in high arcs that freeze when they hit the surface.

It is quite possible that undersea volcanoes pump chemicals into the oceans and this could lead to the creation of the building blocks of life, which is how many scientists think life began on Earth.

There is therefore the intriguing possibility that Europa harbours living organisms of some kind. Should this be so, the fact that two bodies in our Solar System are life-bearing must make it almost certain that there is life on planets that orbit other stars that are relatively close to our Sun.

In Greek mythology Europa was a high-born woman who was abducted by Zeus in the form of a white bull. She then gave birth to King Minos of Crete. The continent of Europe takes its name from her.

© John Welford

Mercury: closest planet to the Sun

Mercury is the closest planet to the Sun and is also the smallest of the currently recognised planets, now that Pluto has been demoted from planetary status. It is therefore difficult to see, and attempts to do so should be made with the greatest care, as training a telescope in the direction of the Sun is a very unwise thing to do. It is best observed at dawn or dusk, but only if you really know what you are doing. If it’s any consolation, Copernicus never managed to see it, although its existence had been known about since classical times.

Mercury orbits the Sun at an average distance of 58 million kilometres (36 million miles). It completes an orbit every 88 days at a speed of 50Km a second, which is the fastest of any of the planets. This is why Mercury has the name that it does, Mercury being the swift-winged messenger of the gods in Roman mythology.

Mercury revolves on its axis three times for every two orbits round the Sun. This means that a day on Mercury is not much shorter than a Mercury year, and lasts for 59 Earth days.

The surface of Mercury is deeply cratered but it also has some smooth plains. This suggests that the planet was volcanically active at one time, but that this activity ceased a long time ago and meteor impacts have pockmarked most of the surface. Mercury appears to have cooled down and may have shrunk in size as it has done so. The fact that a magnetic field has been detected suggests that it still has a molten core.

Daytime temperatures on Mercury are extremely high, at around 350 degrees Celsius, due to the planet’s proximity to the Sun and the fact that it only has a very thin atmosphere. However, the nights are long enough for the temperature to drop as low as minus 170 degrees Celsius. Mercury does not have a tilted axis, unlike Earth, and therefore has no seasons. This also leaves open the possibility that there could be water on Mercury, existing as ice deep within polar craters into which the Sun never shines.

The atmosphere mentioned above is only as dense as the very outer reaches of Earth’s atmosphere. It comprises mainly helium, hydrogen and oxygen, but traces of potassium and sodium have also been discovered. Although atoms of these elements are constantly being lost into space, they are replaced by the action of Mercury’s magnetosphere in capturing ions from the solar wind that batters the planet.

© John Welford

Tabby's Star and a possible ringed planet

You may well have heard of Barnard’s Star, or possibly even Plaskett’s Star, but Tabby’s Star? Its less memorable name is KIC 8462852. It is an F-type main sequence star that is slightly larger and slightly hotter than our G-type Sun. It is about 1,280 light years away from us, in the Cygnus constellation.

Its name derives from Tabetha Boyajian, an American astronomer who has been leading a team studying the star as part of a general search for planets in orbit around distant stars, based on evidence derived from the Kepler Space Observatory. Tabby’s Star has also been called Boyajian’s Star, although that does sound a bit less friendly!

The reason why Tabby’s Star has excited interest is that its “light curve” (an analysis of the light coming from a star over a period of time) was not what was expected. The search for exoplanets depends on finding light curves that indicate that a planet is passing in front of its star. The light will dim for a period of time then return to normal after the planet’s transit has ceased. This pattern will repeat every time the planet orbits the star, and consistent data taken over a number of years will enable astronomers to calculate the planet’s size, mass and distance from its star.

However, the data from Tabby’s star was all wrong. The dimming that was observed was far from regular and was unpredictable, leading to all sorts of speculation, including the idea that this was evidence of the work of an alien civilization on the planet that had built a vast structure to control the energy being received from its star.

One theory – advanced by a team of astronomers in Colombia – dispenses with the aliens but proposes an explanation that is just as intriguing, namely that the planet in question is similar to Saturn in that it is surrounded by rings.

The theory begins with the notion that a ringed planet, when transiting its star, would produce different intensities of dimming – less when only the ring was in transit, more as the full disc of the planet moved across, and then less again as the “back end” of the ring was all that was obstructing the star’s light. On each transit the degree of dimming might change if the angle of the ring was not the same – the planet may well not rotate in the same plane as its orbit, as we know full well from the behaviour of our own planets. It would probably take many orbits before a consistent pattern could be deduced.

The Colombian astronomers modelled this theory based on the supposed mass of the planet and its closeness to Tabby’s Star, estimated at about one tenth the distance of Earth to the Sun. They found that the star would have a gravitational tug on the rings, causing them to wobble and producing even more irregularity to the light curve.

So the mystery of Tabby’s Star may have been solved. There is no need to imagine a vast engineering project on the part of an advanced alien civilization. All that is needed for the observed phenomena – it appears – is a ringed planet the size of Neptune orbiting close to Tabby’s Star.

However, the jury is still out because this is not the only theory that has been put forward to explain the mystery of Tabby’s Star.

© John Welford