Himalayan Sprite Lightning, Mars' Shocking Spherules, Asteroid Belt Mapping

[00:00:00] This is Space Time series 28 episode 41 for broadcast on the 4th of April 2025. Coming up on Space Time, mysterious red sprite lightnings seen over the Himalayas, shocking spherules on Mars and a new geological map for the Asteroid Belt. All that and more coming up on Space Time. Welcome to Space Time with Stuart Gary.

[00:00:41] Scientists have captured rare images of red sprite lightning high above thunderstorms in the Himalayas. The mesmerizing display of more than a hundred of these red sprites were photographed by Chinese astrophotographers Angel An and Chu Chang-dong over the world's highest mountain range at an observational site located in the southern Tibetan plateau near one of the region's three sacred lakes. The breathtaking celestial event included dancing sprites, rarely seen secondary jets,

[00:01:08] and the first ever recorded case in Asia of green afterglow at the base of the nighttime ionosphere. A new study reported in the journal Advances in Atmospheric Sciences sheds light on the driving force behind this grand sprite display. By analyzing the parent lightning discharges, the authors discovered that the sprites were being triggered by high peak current positive cloud-to-cloud lightning strikes within a massive mesoscale convective system.

[00:01:34] This suggests that the thunderstorms in the Himalayan region have the potential to produce some of the most complex and intense upper atmosphere electrical discharges on Earth. Lacking precise time stamps for detailed analysis, the authors developed an innovative method to synchronize video timing using satellite trajectories and star field analyses. This allowed them to determine the exact occurrence times of the sprites and link that to their parent lightning discharges.

[00:01:59] The study revealed that the parent lightning discharges occurred within stratoform precipitation regions of mesoscale convective complexes stretching from the Ganges plain to the southern foothills of the Tibetan plateau. The events recorded the highest number of sprites during a single thunderstorm in South Asia, suggesting that thunderstorms in this region possess upper atmospheric discharge capabilities comparable with those on the United States Great Plains and offshore European storms.

[00:02:26] Moreover, the findings indicate the storms may generate even more complex discharge structures, potentially influencing atmospheric coupling processes with significant physical and chemical effects. Sprites belong to a colourful group of transient luminous phenomena with fairytale names such as sprites, blue jets and elves. Their transient vertical column-like plasma flashes high in Earth's atmosphere, often resembling a glowing reddish jellyfish with tentacles streaming down.

[00:02:54] They're thought to be large-scale electrical discharges at altitudes of between 50 and 100 kilometres above the ground, triggered by rare positive lightning that originates in the anvil ahead of a thunderstorm cloud where positive charges tend to accumulate. Now, positive lightning is about five times as hot and powerful as the regular type of lightning we normally see, which is technically known as negative lightning. Positive lightning also lasts about 10 times longer, allowing it to strike many kilometres from the storm.

[00:03:21] In fact, that's the feature which has led to the famous expression, a bolt out of the blue. Unlike negative lightning, which occurs either inside the storm cloud or from the base of the thunderstorm cloud to the ground, positive lightning travels outside the cloud, striking the ground directly. Sprites sometimes preceded by a red halo emission, lighting up a millisecond before the sprite, about 70 kilometres above the initiating lightning strike. Then there are sprite halos.

[00:03:47] They look like 50-kilometre-wide disks, and are thought to be produced by a weaker version of the same ionisation processes which produce the sprites. Next we have blue jets. Blue jets are very bright narrow cones of plasma seen above thunderstorms propagating upwards into the stratosphere from the cloud tops, often reaching heights of 80 kilometres or more. They're thought to be associated with strong hail activity during thunderstorms, and they're colours believed to be caused by blue and near-infrared emissions from neutral and ionised molecular nitrogen.

[00:04:17] Another closely related phenomena are blue starters, which are thought to be shorter, brighter versions of blue jets, only reaching to about 20 kilometres in height. And then there's a third type, known as gigantic jets, which are thought to be bigger versions of blue jets. And finally we have the elves. They're flattened expanding reddish concentric rings that often appear as dim expanding 400-kilometre-wide glows, lasting for just a millisecond. They've been seen at altitudes of 100 kilometres above thunderstorms,

[00:04:46] and are thought to be generated by the excitation of nitrogen molecules due to collisions between electrons, energised by lightning, from the underlying thunderstorm. Sprites, elves and jets are all very little understood, seldom seen, but a fascinating part of our upper atmosphere. This is space-time. Still to come, shocking spherals discovered on Mars, and scientists develop a new geologic map of the main asteroid belt.

[00:05:13] All that and more still to come, on space-time. NASA's Mars Perseverance rover has discovered a strange rock composed of hundreds of millimetre-sized spheres. The finder shocked scientists, who are now working overtime to try and understand their origin.

[00:05:43] It's now been two weeks since the Perseverance rover arrived at Broom Point, situated at the lower slopes of the Witch Hazel Hill area on the rim of Jezero Crater. Here, a series of light and dark-toned bands of rock were visible from orbit. Now last week, the six-wheel car-sized rover successfully abraded and sampled one of these light-toned beds. And it was from this sampling workspace that Perseverance spied a very strange texture in a nearby rock. The rock, which has now been named St Paul's Bay,

[00:06:13] appeared to be comprised of hundreds of millimetre-sized dark grey spheres. Some of these occurred as more elongated elliptical shapes, while others possessed angular edges, perhaps representing broken spheral fragments. And some spheres even possessed tiny pinholes. It's all raising questions about the type of geology that could produce these strange shapes. Of course, this isn't the first time strange spheres have been spotted on Mars.

[00:06:38] You may recall back in 2004, NASA's Mars Exploration Rover Opportunity spotted so-called Martian blueberries in the Meridiani Planum. And since then, Curiosity observes spherals in the rocks of Yellowknife Bay in Gale Crater. In fact, just a few months ago, Perseverance itself also spied popcorn-like textures in sedimentary rocks, which have been exposed in the Jezero Crater inlet channel near Avir Valleys. Now, in each of these cases, the spherals have been interpreted as being concretions,

[00:07:07] features that were formed through the interaction with groundwater circulating through pore spaces in rock. However, not all spherals form this way. Here on Earth, they can also be formed by the rapid cooling of molten rock droplets from volcanic eruptions, or through the condensation of rock which is vaporised during a meteor impact. Each of these formation mechanisms would have vastly different implications for the evolution of these rocks. So, scientists need to work hard to try and determine their context and origin.

[00:07:35] However, the St Paul's Bay was a float rock. That's a term used by geologists to describe something that's not in place. Scientists are now working to try and link the spheral-rich texture observed at St Paul's Bay with the wider stratigraphy at Witch Hazel Hill. Initial observations have provided tantalising indications that could be linked to one of the dark-toned layers identified from orbit. Placing these features in geological context is crucial for understanding their origin

[00:08:00] and determining their significance for the geological history not just of Jezero Crater's rim, but also for the red planet beyond. This is space-time. Still to come, a new geological map of the main asteroid belt between Mars and Jupiter, and the splendors of the Southern Cross and its two pointer stars Alpha and Beta Centauri, the blue supergiant Canopus, and the Lyrid's meteor shower, are all among the highlights of the April night skies on Skywatch.

[00:08:41] Scientists have put together a new geological map of the main asteroid belt between Mars and Jupiter. Knowing from what asteroid belt debris field and meteorites hitting the Earth originate from is important for planetary defence efforts. The new findings, reported in the journal Meteoritics and Planetary Science, traces the impact orbits of a number of observed meteorite falls, including several of the previously unidentified source regions in the main asteroid belt.

[00:09:06] The study's lead author, Peter Jeniskins from NASA's Ames Research Centre and the SETI Institute, describes the report as a decades-long detective story, with each meteorite impact providing a new clue. Ten years ago, Jeniskins teamed up with Hadrian de Vilpoie, an astronomers from Curtin University, to develop the first outlines of a geologic map of the asteroid belt. By then, Curtin had already developed its now famous Outback Fireball network of cameras,

[00:09:33] and they assisted Jeniskins and colleagues in North America to build a similar network of all-sky cameras in California and Nevada. These are designed to capture and track the bright lights of meteors as they hit the Earth's atmosphere. Many other institutions, as well as citizen scientists, have participated in these efforts over the years and eventually created a global fireball observatory. Initially, de Vilpoie and colleagues tracked the path of 17 recovered meteorite falls.

[00:10:00] And many more fireballs were tracked using doorbell and dashcam video cameras by citizen scientists, as well as by other dedicated networks. Now altogether, the effort has now yielded some 75 laboratory-classified meteorites, each with an impact orbit tracked by video and photographic cameras. Jeniskins says that this is providing enough material to start to see some patterns in the direction from which meteors approach the Earth. Most meteorites originate from the main asteroid built between Mars and Jupiter,

[00:10:29] where over a million asteroids larger than a kilometre orbit the Sun. Now this massive collection of space rocks all originate from a much smaller number of larger asteroids, which have been broken apart in collisions, creating debris fields which litter much of the region. Even today, asteroids are colliding creating new debris fields or clusters within these asteroid families. During their studies, the authors have found that 12 of the iron-rich ordinary H-type chondrite meteorites originated from a single debris field now called coronis.

[00:10:59] It's located in the pristine main belt. Jeniskins says that these meteorites arrived at low-inclined orbits on orbital periods consistent with this debris field. Astronomers can measure how long ago these rocks were dug up from below an asteroid surface by measuring the level of radioactive elements created by exposure to cosmic rays. And this cosmic ray exposure age in meteorites can be used to match the dynamical age of some of the asteroid debris fields. Scientists can determine the dynamical age of an asteroid debris field

[00:11:29] by measuring how far the asteroids of different sizes have spread over time. By measuring the cosmic ray exposure age of meteorites, they could determine that three of these 12 meteorites originated from the current cluster of the coronis field. And the coronis field has a dynamical age of 5.8 million years. And a further two came from the coronis 2 cluster, which has a dynamical age of 10 to 15 million years. Another meteorite from this group may well be measuring the age of the coronis 3 cluster,

[00:11:59] resulting in a figure of around 83 million years. Geneskins and Deville Ploy also found a group of H chondrites on steep orbits. They appear to originate from the Neal asteroid family in the central main asteroid belt, and it's been shown to have a dynamical age of around 6 million years. A third group of H chondrites that have exposure ages of around 35 million years originated from the inner main belt.

[00:12:24] These are all thought to have originated from the massalia asteroid family, low in the inner main belt. Geneskins says that the asteroid which created that cluster, 20 massalia, is also an H chondrite parent body type. Geneskins and Deville Ploy find that the low-iron L-type chondrites, and very low-iron LL chondrite meteorites, also primarily come from the inner main belt. Scientists have long linked the LL chondrites to the flora asteroid family on the inner side of the main asteroid belt,

[00:12:52] and this new work has now confirmed that it's correct. Geneskins proposes that the old chondrites originated from the Hertha asteroid family, located just above the massalia group. But asteroid Hertha doesn't look anything like its debris. It's covered in dark rocks that were all shock blackened, indicative of an unusually violent collision. The old chondrites experienced a very violent origin 468 million years ago, and since then these meteorites have showered the Earth in such numbers that they can be found throughout the geological record.

[00:13:23] Knowing from which debris field an asteroid-built meteorite originates from is important for planetary defence efforts against near-Earth asteroids. An approaching asteroid's orbit can provide clues about its origins in the main asteroid belt in the same way as a meteorite orbit does. But near-Earth asteroids don't arrive on the same orbits as meteorites. That's because they take longer for these to evolve into Earth-crossing orbits. But they nevertheless come from some of the same asteroid families. This is space time.

[00:14:09] And time now to check out the night skies of April on Skywatch. April is the fourth month of the year in the Gregorian calendar and the fifth in the early Julian calendar. The Romans gave this month the Latin name Aprilis. Although the name's origins aren't certain, traditional entomology suggests it's from the verb aparia to open, as in it being the season when the trees and flowers begin to open as the northern hemisphere moves into spring. April is also prevention of cruelty to animals month,

[00:14:39] and so it's a good time to consider adopting a shelter pet or donating to an animal welfare charity. High in the southern sky during April, you'll find the Southern Cross and its two-pointer stars, Alpha and Beta Centauri. The more distant of the two-pointer stars from the Southern Cross is Alpha Centauri, which also happens to be the nearest star system to our own. Located some 4.3 light years away, Alpha Centauri actually consists of three stars.

[00:15:08] There's Alpha Centauri A and B which orbit each other, and Proxima Centauri which orbits the pair. And at 4.25 light years distant, it's currently the nearest star to the Earth other than the Sun. A light year is about 10 trillion kilometres. The distance a photon can travel in a year at 300,000 kilometres per second, the speed of light in a vacuum, and the ultimate speed limit of the universe.

[00:15:33] Like the Sun, Alpha Centauri A is a spectral type G yellow dwarf star. It's slightly bigger, having about a tenth more mass than the Sun, and has about 50% more luminosity. Astronomers describe stars in terms of spectral types, a classification system based on temperature and characteristics. The hottest, most massive and most luminous stars are known as Spectral Type O blue stars. They're followed by Spectral Type B blue white stars,

[00:16:02] then Spectral Type A white stars, Spectral Type F whitish yellow stars, Spectral Type G yellow stars, that's where our Sun fits in, Spectral Type K orange stars, and the coolest and least massive stars of all are Spectral Type M red dwarf stars. Each Spectral Classification is further subdivided, using a numeric digit to represent temperature, with zero being the hottest and nine the coolest, and then a Roman numeral to represent luminosity.

[00:16:32] So, our Sun is a Spectral Type G2V or G25 yellow dwarf star. Also included in the Stellar Classification System are Spectral Types LT and Y, which are assigned to failed stars called brown dwarfs. These are sometimes born as Spectral Type M red dwarf stars, but become brown dwarfs after losing some of their mass. Brown dwarfs fit into a category between the largest planets, which are about 13 times the mass of Jupiter,

[00:17:01] and the smallest Spectral Type M red dwarf stars, which are around 75 to 80 times the mass of Jupiter, or about 0.08 solar masses. Orbiting in a binary system with Alpha Centauri A is Alpha Centauri B, a Spectral Type K orange dwarf star, a little smaller and cooler than the Sun, with about 0.9 times the Sun's mass, and about half its luminosity. Alpha Centauri A and B orbit each other around a common centre of gravity

[00:17:31] every 79.91 Earth years. The distance between the two stars varies between roughly that of Pluto in the Sun, and that of Saturn in the Sun. The third star in the system, Proxima Centauri, sometimes called Alpha Centauri C, is a Spectral Type M red dwarf star, with roughly a seventh the diameter and about an eighth the mass of the Sun. It takes around 550,000 Earth years to orbit Alpha Centauri A and B.

[00:17:59] The nearer of the two pointer stars to the Southern Cross is Beta Centauri, also a triple star system, but this one located a far more distant 390 light years away. All three are massive young blue stars, far larger and more luminous than the Sun. Two of the stars, named Beta Centauri AA and Beta Centauri AB, orbit each other, while the third star, Beta Centauri B, orbits the primary pair every 1500 Earth years.

[00:18:27] Beta Centauri AA and AB are known as a spectroscopic binary, orbiting each other every 357 Earth days. Spectroscopic binaries are double star systems, orbiting each other so closely and at such an angle, that they can only be visually separated, from our point of view here on Earth at least, by their spectroscopic signatures. Both these stars are now reaching the end of their time on the main sequence, and will soon run out of the core hydrogen they use for fusion,

[00:18:55] the process which makes stars like the Sun shine. The two pointer stars Alpha and Beta Centauri, are named after Ciron the Centaur, a mythological Greek being half man, half horse. Ciron taught many the Greek gods and heroes, but was placed among the stars, after accidentally being shot with a poison arrow by Hercules. Next to the pointer stars, is the spectacular Southern Cross or Crux,

[00:19:21] the smallest but one of the best known of the 88 constellations in the sky. The Southern Cross is considered an important constellation for navigation, and is featured on the flags of several nations, including Australia, Brazil, New Zealand, Papua New Guinea and Samoa. In April, the Southern Cross lies on its side in the early evening, but becomes more and more upright as the night progresses. The bottom and brightest star in the Southern Cross is Alpha Crucis or A-Crox,

[00:19:50] which is actually a multiple star system located 321 light years away. It consists of three stars, A1 Crucis, which is a spectroscopic binary, and A2 Crucis. A2 Crucis and the primary star in A1 Crucis are both spectral type B blue stars, with surface temperatures of 26,000 and 28,000 Kelvin respectively. The two components orbit each other every 1500 Earth years,

[00:20:18] at an average distance of around 430 astronomical units. An astronomical unit is the average distance between the Earth and the Sun, roughly 150 million kilometres or 8.3 light minutes. The spectroscopic binary A1 Crucis is thought to comprise two stars, with about 10 and 14 times the mass of the Sun respectively. The pair orbit each other every 76 Earth days, at a distance of around 150 million kilometres,

[00:20:46] in other words, one astronomical unit. The masses of A2 Crucis and the larger component of A1 Crucis are expected to eventually explode as core-collapse supernovae, ending up as neutron stars. While the smaller component of A1 Crucis could survive as a white dwarf. The left hand and second brightest star in the Southern Cross is called Beta Crucis, and it's also a spectroscopic binary, consisting of two stars orbiting each other every five Earth years,

[00:21:15] at an average distance which varies between 5.4 and 12 astronomical units. Beta Crucis is located some 280 light years away. The primary star, Beta Crucis A, is a spectrotype B, Beta Cephi variable blue star, which changes in brightness over a period of around 4 to 4.5 hours. It has about 16 times the Sun's mass, about 8 times its diameter, and a surface temperature of some 27,000 Kelvin.

[00:21:43] By comparison, our Sun has a surface temperature of just 6,000. The second star in the system, Beta Crucis B, has about 10 solar masses. A third companion has also been detected in the system. However, it appears to be a low-mass pre-main-sequence star, which hasn't yet commenced nuclear fusion. Nei Beta Crucis is the spectacular young open star cluster, known as the Capacrucis Cluster, or NGC 4755,

[00:22:11] and more commonly referred to as the Jewel Box, the name given to it by famous 18th-century astronomer John Herschel. Open star clusters are groups of stars which were originally all born at the same time out of the same collapsing molecular gas and dust cloud. Although somewhat still gravitationally bound to each other, stars in open clusters eventually separate, moving to other parts of the galaxy. As the name suggests, the Jewel Box is a stunning collection of more than 100 bright,

[00:22:41] colorful stars, located some 6,440 light-years away, although its exact distance is somewhat difficult to determine because of the nearby Coalsack Nebula which obscures some of the light. The Coalsack is a dark nebula containing lots of gas and dust blocking out background stars. In Australian Aboriginal dreamtime legend, the Coalsack forms the head of the Emu constellation, with the dark dust lanes of the Milky Way forming the Emu's body and legs.

[00:23:10] The central parts of the Jewel Box are framed by bright stars making up an A-shaped asterism. These are among the brightest known blue, white and red supergiant in the Milky Way. Gamma Crucis, which is located at the top of the Southern Cross, is the third brightest star in the constellation. It's also one of the nearest red giants to our solar system, located just 88.6 light-years away. Although only 30% more massive than the Sun,

[00:23:39] its expanded outer envelope is bloated out to some 84 times the Sun's radius, and is radiating some 1,500 times more luminosity than the Sun. As a red giant, no longer on the main sequence, Gamma Crucis is nearing the end of its life. Its surface temperature is some 3,626 Kelvin, and it has a prominent reddish-orange appearance. The star on the right-hand side of the Southern Cross is Delta Crucis,

[00:24:07] a massive, hot and rapidly rotating star that's in the process of evolving into a red giant, and will eventually end up as a white dwarf, the stellar corpse of sun-like stars. Delta Crucis is located some 345 light-years away, and has about nine times the Sun's mass and eight times its radius. It's presently radiating at around 10,000 times the luminosity of the Sun at an effective temperature of 22,570 Kelvin, causing it to glow with a blue-white hue.

[00:24:37] The smallest star on the Southern Cross is Epsilon Crucis, which is located in the space between Delta and Alpha Crucis. It's a red giant, some 228 light-years away. It has about 1.42 times the mass of the Sun, and about 32 times its radius. Its surface temperature of 4,148 Kelvin means it's sometimes referred to as an orange giant. The Southern Cross is at its highest point in the southern sky this time of year,

[00:25:06] and is pointing directly at the Southern Celestial Pole. It's within the constellation Centaurus the Centaur, the half-man, half-horse of Greek mythology we mentioned earlier. The creature is holding a bow loaded with an arrow. The Centaur's front leg is marked by the two pointer stars Alpha and Beta Centaurus. His back arches over the Southern Cross, and just above this is Omega Centauri, a spectacular globular cluster visible with the unaided eye from dark locations.

[00:25:37] Unlike open star clusters, globular clusters are tightly packed spheres containing thousands to millions of stars, which were originally all thought to have been born at the same time from the same molecular gas and dust cloud. Omega Centauri is about 16,000 light-years away. It's one of the largest and brightest of the hundreds of globular clusters known to orbit around the Milky Way galaxy.

[00:26:02] Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern-day constellations. The constellation Orion the Hunter is still clearly visible in the northwestern sky this time of year, with its rectangle of four stars surrounded by a central trio of stars which form Orion's belt. To the right or east of Orion is the constellation Gemini and its two brighter stars, Porelax and Castor.

[00:26:32] This time of year the Gemini twins are almost directly due north for Southern Hemisphere sky watchers. The higher of the two stars, Porelax is a red giant, some 11 times the diameter of the Sun and located just 34 light-years away. The other star Castor is much further away, some 51 light-years. Look to the east and you'll see the star Regulus, the brightest star in the constellation of Leo the Lion.

[00:26:58] Regulus, which means the little king, is located 77 light-years away, and it's about three and a half times as massive as the Sun and about 140 times as luminous. Regulus is a binary companion star, which takes 130,000 years to orbit the primary. To the right of Regulus, and virtually due east in the sky right now, is the star Spica. Located directly below the four stars in the constellation Corvus the Crow,

[00:27:25] Spica is the brightest star in the constellation Virgo. Also known as Alpha Virginis, it's the 16th brightest star in the night sky, and is another spectroscopic binary, comprising two stars closely orbiting each other every four Earth days. In fact, the two stars in Spica are orbiting so close together, that the gravitational interaction between them has caused them to become rotating epsaloidal variables, distorting them into the shape of a rugby league or gridiron football.

[00:27:54] Light from this binary changes in brightness as the two stars orbit each other, exposing their elongated hemispheres to us. Spica is located some 260 light-years away, and is some 2,000 times as luminous as the Sun. Spica means ear of wheat, which Virgo is holding in her hand. It's so named because it marks the start of the harvest season in the Northern Hemisphere. The primary is a blue giant variable beta-cephiid,

[00:28:23] which undergoes small rapid variations in brightness because of pulsations in the star's surface, thought to be caused by the unusual properties of iron at temperatures of 200,000 degrees in the stellar interior. It is about 10 times the Sun's mass, and about 7.5 times its diameter. Once a spectral type B blue-white main sequence star, it's now pulsating rapidly, rotating at more than 199 km per second over a 0.1738 Earth-day period.

[00:28:53] It's one of the nearest stars to the Earth, which is expected to end its life as a Type 2 core collapse supernova. The second star in the system is also thought to be a spectral type B blue-white giant, about 7 solar masses and 3.6 times the Sun's diameter. Okay, going back to the Southern Cross, and looking to the right or west, you'll see the star Canopus. It's the second brightest star in the night sky after Sirius.

[00:29:19] Even though Canopus is 312 light-years away, it looks incredibly bright because it's huge, a hundred times the diameter of the Sun, and 10,000 times as luminous. This year's second major meteor shower, the Lyrids, will peak on April 22nd and 23rd. The Lyrids appear to radiate out from the constellation Lyra, close to the star Vega, one of the brightest stars in the sky this time of year.

[00:29:44] The source of the meteor shower are particles of dust and debris shed by the long-period comet C1861 G1 Thatcher. Sky watchers in the Northern Hemisphere get the best view of the Lyrids. However, listeners at mid-Southern Hemisphere latitudes can also see the shower between midnight and dawn. Patient observers will be rewarded with around 18 meteors per hour before dawn from dark sky locations. And now with a look at what else is happening in the April night skies,

[00:30:14] we're joined by science writer Jonathan Nally. G'day Stuart. Well, we're now into autumn here where I live, which will be spring in the northern half of the planet, of course. And for me, the sun is setting earlier and the nights are becoming longer, which means it's perfect conditions for stargating. So we'll start with the good old Southern Cross, which we can find in the south-east about a third to halfway up from the horizon, sort of after sunset, an hour or two after sunset. It's lying on its left-hand side at the moment, so it looks like a kite that's on its left-hand side. But as the night goes on and the Earth turns a bit more on its axis,

[00:30:43] you'll see that it becomes more upright. We'll just talk about the cross for a sec. The Southern Cross appears to have four main stars, and they make up the shape of this kite. Two of those stars are solo stars, but a third of them is made up of either two or three stars. It's either a binary star system or a trinary star system. They're so close together that to the unaided eye, they just look like one star, of course. The four stars is actually a six-star system. They're six stars all sort of circling each other in this star, which is called Acrux.

[00:31:12] It's really amazing that how many stars out there are part of binary or trinary or quaternary systems. It's the norm, really, to be in a binary star system or triple star system or something. So our sun is a bit of an oddity being on its own. But you don't see this. You don't think about this when you look up and look at some of the stars in the night sky. You don't think, oh, that one's part of a six-star system or that one's part of a four-star system, but a lot of them are. Now, right next to the Southern Cross, there's a dark patch that's known as the coal sack.

[00:31:39] This was once thought to be a gap or a hole in the Milky Way a long time ago, but really it's just a huge region of gas and dust that's very thick and it blocks our view of the stars behind it so we can't see the background stars. Now, in order to see the coal sack, you do need to have some clear, dark skies. The city skies probably don't cut it because all the light pollution just drowns stuff out anyway. So the thing is that the Southern Cross, for instance, that there is a fifth star in the Southern Cross, but a lot of people in cities can't see it.

[00:32:06] This light pollution is now so bad that, in fact, I think from where I am, I can't even see the fifth star in the Southern Cross. It's really sad, isn't it? It is really sad. And you know, one of the funny things is that here in Australia there's a piece of, from one of the government departments, they have a software service that's used for navigation systems and surveying systems, and it's named after this fifth star. And the irony, of course, is that it's used to be used for finding your position and navigating, but most people now can't see it because they live in cities that are so light polluted that this star is invisible.

[00:32:36] That's the way it goes, I'm afraid. And speaking of brightnesses and not being able to see things, you know, you might think that stars, when you look up you see stars and they all look much the same brightness, and you'd think that because they're of similar brightness, they'll all be roughly the same distance from the Earth. But that's not the case. We've got some stars that are intrinsically very bright, but they might be very far away, and therefore they seem dim. But then you have other stars that are dim, but they're very close, so they seem brighter than the bright ones are.

[00:33:01] So the four stars of Southern Cross, for instance, they range in distance from us from about 90 light years to about 340 light years. So you can't really tell the distance of a star from the Earth just based on its brightness. You've got to think a bit deeper than that. So brightness does not equate with distance. Now speaking of a bright star, up really high in the south at the moment is a bright star called Canopus. This is the brightest light in the constellation of Carina, and it's actually the second brightest star in the night sky.

[00:33:27] And it's no wonder, talking about brightness, because even though it's more than 300 light years away, intrinsically it's more than 10,000 times brighter than our sun, and it's 70 times as big. Now that's really big. Imagine that, 10,000 times brighter. That's why it's the second brightest star in the night sky, even though it's 300 light years away. The night sky's brightest star, Sirius, is also very easily visible at the moment. It's practically overhead in the early evening if you live at the latitude of Sydney in the southern hemisphere.

[00:33:57] Comparing Sirius to Canopus, Canopus being that really big one, Sirius is only 25 times brighter than the sun, but it's only eight and a half light years away compared to 300. So that's why it seems a bit brighter to our eyes. Now, not far from Sirius, you've got the constellation Orion, with two bright stars, Rigel and Betelgeuse, and there are three stars in a row known as Orion's Belt. Now if you thought Canopus is impressive, being 10,000 times brighter than the sun, well, Rigel beats that easily.

[00:34:25] Scientists are not quite sure of the number for some technical reasons. There's a bit of a range of estimates of how bright it intrinsically is, and that range is from 60,000 times brighter than our sun to 360,000 times brighter than our sun. I mean, that's just berserk. And the other star I mentioned, Betelgeuse, it's no spout either. It's about 60,000 times brighter than the sun, but with Betelgeuse, it's the size that really impresses. It's somewhere between about 640 and 760 times as big as our star, which is just crazy.

[00:34:55] It's hard to imagine, isn't it? If you put it in the centre of our solar system, its outer limb would be roughly where Jupiter is. Yeah, basically. Yeah, it'd gobble up Mercury, Venus, Earth, Mars, the asteroid belt, pretty much all the way out to Jupiter. That's how big it would be. It's just unimaginable. There are some big, big stars out there. Now, let's look at the planets. If we take a look to the north, at least from here in the southern hemisphere, about a third of the way up from the horizon, you'll find what appears to be three stars in a row. The two whitish ones are actually stars.

[00:35:24] They're the stars Castor and Pollux, but the orangey looking one is the planet Mars. Okay? Now, you hear this thing quite often that when you look up at the night sky, the night sky stars twinkle and planets don't, and the explanation usually given for that is that stars are so far away that they are effectively point sources, like a tiny pinpoint of light. And that, when the light comes through the Earth's atmosphere, it gets interfered with by air currents and things, so that's what makes stars twinkle.

[00:35:50] Whereas planets, even though you can't make out their size with the unaided eye, they do have a bit of a size, and therefore that doesn't get interfered with quite so much. So that's the explanation usually given. But when stars and even planets are low down towards the horizon, even a planet will twinkle. I remember getting a phone call one, one night from a mate of mine, and he said every night out there, he lives near the water, every night out over the water there's this red star that changes colour, and it goes red and green and white, and it's there every night.

[00:36:19] And is it the Air Force doing something? What could it be? And I looked it up, and it was just Mars. But because it was down low on the horizon, the light coming through our Earth's atmosphere, is the Earth's atmospheric currents were distorting the light, basically, and making it flicker. So even planets can twinkle, if you like. Is that why the Moon looks bigger on the horizon than it does when it's high up in the sky? This is the Moon illusion, and I don't know if anyone's ever really got to the bottom of this.

[00:36:45] There are all sorts of ideas about why the Moon appears to be bigger, because you can compare it against houses or trees or the horizon or whatever, so it seems to have a discernible, not a discernible size, but something you can compare it against. Whereas when it's up there high in the night sky, it's just an empty sky, so you can't really get a good comparison against anything else. I mean, the Moon's only half a degree across. I mean, from horizon to horizon, if you drew a line from the horizon up overhead down to the other horizon,

[00:37:14] that of course is 180 degrees. So you could stack 360 moons all the way from one horizon to the other. It's actually really small. As for the Moon illusion, the so-called Moon illusion, why the Moon appears bigger on the horizon, I really don't know the answer. There are all sorts of things with those. Might have more to do with our brains and the way our brains perceive things than the Moon. I suspect there's a lot of that. Yeah, yeah. I mean, you can do a test. If the Moon's down near the horizon, have a look and see how big it is, and then get a cardboard tube and look at the Moon through the cardboard tube

[00:37:42] so you can't see anything else around it, no houses and trees and things, and see whether it seems quite as big. I've actually measured it on the horizon compared to when it's high in the sky, and it's exactly the same size, but it just doesn't look it. It's weird. Yeah, there's something in our brains that does it. So anyway, yeah, that's the Moon. Now over to the northwest, again, looking, this is from looking from the southern hemisphere, so to the southwest if you're in the northern hemisphere, there appears to be a bright white star, but it is in fact the planet Jupiter.

[00:38:10] Now if you have even just a pair of binoculars, take a look at Jupiter. You won't be able to make out anything on the planet itself. It'll just look like a bright star, but you should be able to see up to four tiny pinpricks of light, either to its left or right or both. It might be a tree on one side and one on the other, or two on one side and two on the other, or whatever. Now these are the four moons discovered by Galileo, and by looking through a pair of binoculars, you've got about the same optical power as Galileo had with his first little telescope.

[00:38:37] In fact, your binoculars would be far better quality than Galileo's telescope was, but you'd have about the same magnifying power. So yeah, have a look at Jupiter, even through a pair of binoculars, and you'll see these tiny pinpricks of light. They're very noticeable. They're very, very noticeable. You go out and do it every night, night after night for a while, and you'll see that these little pinpricks of light will have moved, because they're orbiting around Jupiter. And a lot of astronomers like to do this, of course, with their telescopes. And what you can see is, for instance, sometimes when one of these moons goes around behind Jupiter and goes into its shadow, it just winks out.

[00:39:07] It just disappears because it's gone into the shadow behind Jupiter and just vanishes, and then it'll reappear sometime later on the other side when it comes back into the sunlight again. So that's fun to watch, but it does take a little while, so you need to allow some time. Now to see the other bright planets, the other three bright planets, you'll need to be up before dawn this month and looking to each. Saturn is the first of them to arrive at about 5.30am daylight saving time, at least here in Australia, followed around about 20 minutes later by both Mercury and Venus.

[00:39:37] Now Saturn's fairly bright, and it has a slightly yellowish tinge, so it should be pretty easy to identify. Mercury is quite small and dim. Venus, on the other hand, is big and bright. Now, if you're up early enough to spot these, you'll see that they're going to be in the beginning of the dawn glow as the sunrise approaches and the sky is starting to lighten. And as it gets lighter and lighter, Mercury and Saturn will quickly fade into that dawn glow. But Venus will linger a bit longer because it is really quite bright. And then the sun will come up, of course, and it will drown about.

[00:40:06] Although you can actually see Venus during the daytime. I've seen Venus during the daytime. When it's up high in the sky in a crystal clear bright blue sky, Venus is actually bright enough to be seen with the unaided eye, but you've got to know exactly where to look. So if you've got a way to pinpoint exactly where it is, you look up and you think, oh goodness, yes, there is a tiny thing that looks like a star up there. When you look directly at it and you know it's there, you can see it. You think, wow, that is Venus. But because it's usually close-ish to the sun, people, of course, don't bother looking up.

[00:40:36] Don't go looking up the binoculars or telescope during daytime if the sun's anywhere near. Please don't do that because it's one wrong move and you might blind yourself. So on that happy note, Stuart, that's the sky for April. That's science writer Jonathan Alley. And this is Space Time.

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