Universe Today Video

Pulsars are types of neutron stars; the dead relics of massive stars. What sets pulsars apart from regular neutron stars is that they're highly magnetized, and rotating at enormous speeds. Astronomers detect them by the radio pulses they emit at regular intervals. The formation of a pulsar is very similar to the creation of a neutron star. When a massive star with 4 to 8 times the mass of our Sun dies, it detonates as a supernova. The outer layers are blasted off into space, and the inner core contracts down with its gravity. The gravitational pressure is so strong that it overcomes the bonds that keep atoms apart. Electrons and protons are crushed together by gravity to form neutrons. The gravity on the surface of a neutron star is about 2 x 1011 the force of gravity on Earth. So, the most massive stars detonate as supernovae, and can explode or collapse into black holes. If they’re less massive, like our Sun, they blast away their outer layers and then slowly cool down as white dwarfs. But for stars between 1.4 and 3.2 times the mass of the Sun, they may still become supernovae, but they just don’t have enough mass to make a black hole. These medium mass objects end their lives as neutron stars, and some of these can become pulsars or magnetars. When these stars collapse, they maintain their angular momentum. But with a much smaller size, their rotational speed increases dramatically, spinning many times a second. This relatively tiny, super dense object, emits a powerful blast of radiation along its magnetic field lines, although this beam of radiation doesn’t necessarily line up with it’s axis of rotation. So, pulsars are simply rotating neutron stars. And so, from here on Earth, when astronomers detect an intense beam of radio emissions several times a second, as it rotates around like a lighthouse beam - this is a pulsar. The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewis, and it surprised the scientific community by the regular radio emissions it transmitted. They detected a mysterious radio emission coming from a fixed point in the sky that peaked every 1.33 seconds. These emissions were so regular that some astronomers thought it might be evidence of communications from an intelligent civilization. Although Burnell and Hewis were certain it had a natural origin, they named it LGM-1, which stands for “little green men”, and subsequent discoveries have helped astronomers discover the true nature of these strange objects. Astronomers theorized that they were rapidly rotating neutron stars, and this was further supported by the discovery of a pulsar with a very short period (33-millisecond) in the Crab nebula. There have been a total of 1600 found so far, and the fastest discovered emits 716 pulses a second. Later on, pulsars were found in binary systems, which helped to confirm Einstein's theory of general relativity. And in 1982, a pulsar was found with a rotation period of just 1.6 microseconds. In fact, the first extrasolar planets ever discovered were found orbiting a pulsar - of course, it wouldn't be a very habitable place. When a pulsar first forms, it has the most energy and fastest rotational speed. As it releases electromagnetic power through its beams, it gradually slows down. Within 10 to 100 million years, it slows to the point that its beams shut off and the pulsar becomes quiet. When they are active, they spin with such uncanny regularity that they’re used as timers by astronomers. Pulsars help us search for gravitational waves, probe the interstellar medium, and even find extrasolar planets in orbit. It has even been proposed that spacecraft could use them as beacons to help navigate around the Solar System. On NASA’s Voyager spacecraft, there are maps that show the direction of the Sun to 14 pulsars in our region. If aliens wanted to find our home planet, they couldn’t ask for a more accurate map.

Almost all astronomers agree on the theory of the Big Bang, that the entire Universe is spreading apart, with distant galaxies speeding away from us in all directions. Run the clock backwards to 13.8 billion years ago, and everything in the Cosmos started out as a single point in space. In an instant, everything expanded outward from that location, forming the energy, atoms and eventually the stars and galaxies we see today. But to call this concept merely a theory is to misjudge the overwhelming amount of evidence. There are separate lines of evidence, each of which independently points towards this as the origin story for our Universe. The first came with the amazing discovery that almost all galaxies are moving away from us. In 1912, Vesto Slipher calculated the speed and direction of “spiral nebulae” by measuring the change in the wavelengths of light coming from them. He realized that most of them were moving away from us. We now know these objects are galaxies, but a century ago astronomers thought these vast collections of stars might actually be within the Milky Way. In 1924, Edwin Hubble figured out that these galaxies are actually outside the Milky Way. He observed a special type of variable star that has a direct relationship between its energy output and the time it takes to pulse in brightness. By finding these variable stars in other galaxies, he was able to calculate how far away they were. Hubble discovered that all these galaxies are outside our own Milky Way, millions of light-years away. So, if these galaxies are far, far away, and moving quickly away from us, this suggests that the entire Universe must have been located in a single point billions of years ago. The second line of evidence came from the abundance of elements we see around us. In the earliest moments after the Big Bang, there was nothing more than hydrogen compressed into a tiny volume, with crazy high heat and pressure. The entire Universe was acting like the core of a star, fusing hydrogen into helium and other elements. This is known as Big Bang Nucleosynthesis. As astronomers look out into the Universe and measure the ratios of hydrogen, helium and other trace elements, they exactly match what you would expect to find if the entire Universe was once a really big star. Line of evidence number 3: cosmic microwave background radiation. In the 1960s, Arno Penzias and Robert Wilson were experimenting with a 6-meter radio telescope, and discovered a background radio emission that was coming from every direction in the sky - day or night. From what they could tell, the entire sky measured a few degrees above absolute zero. Theories predicted that after a Big Bang, there would have been a tremendous release of radiation. And now, billions of years later, this radiation would be moving so fast away from us that the wavelength of this radiation would have been shifted from visible light to the microwave background radiation we see today. The final line of evidence is the formation of galaxies and the large scale structure of the cosmos. About 10,000 years after the Big Bang, the Universe cooled to the point that the gravitational attraction of matter was the dominant form of energy density in the Universe. This mass was able to collect together into the first stars, galaxies and eventually the large scale structures we see across the Universe today. These are known as the 4 pillars of the Big Bang Theory. Four independent lines of evidence that build up one of the most influential and well-supported theories in all of cosmology. But there are more lines of evidence. There are fluctuations in the cosmic microwave background radiation, we don’t see any stars older than 13.8 billion years, the discoveries of dark matter and dark energy, along with how the light curves from distant supernovae. So, even though it’s a theory, we should regard it the same way that we regard gravity,

This article was originally written in 2009, but we've updated it with this spiffy new video from Fraser! A parsec is equivalent to 3.26 light years. And since a light year is the distance light travels in 1 year 9.4 trillion km, 1 parsec equals 30.8 trillion km. So, where then, does the term, "parsec", come from? Parsec is a combination of 2 words, parallax (par) and arc second (sec). Parallax means something looks like it changed its location because you changed yours. For example, if you stand on your porch and look across the street, you will see a house on your left and a house on your right. If you go across the street and look at the same houses from your neighbor's backyard, they will be on the opposite sides. Did the houses move? Of course not. You changed your location. Since you are in a different place, facing a different direction, they appear to be in different places. Likewise, two different people, in two different parts of the world, might see the exact same event in the sky or outer space; yet, it might appear entirely different due to their locations. Astronomers measure parallax by measuring how distant stars shift back and forth as the Earth travels around the Sun. Astronomers measure the position of the stars at one time of the year, when the Earth is at a position in its orbit around the Sun, and then they measure again 6 months later when the Earth is on the other side of its orbit. Nearby stars will have shifted a tiny amount compared to more distant stars, and sensitive instruments can detect the change. Now for the second part of "parsec": arcsecond. In this instance, we're not referring to a measure of time. It's a part of a measurement of angle. Imagine the horizon around you broken up into 360 slices, or degrees. Each slice is about twice the width of the full moon. An arcminute is 1/60th of a degree, and an arcsecond is 1/60th of an arcminute. So astronomers measure the size of objects, or the parallax movement of stars in degrees, arcminutes and arcseconds. So, to put those terms together, a parsec is the parallax of one arcsecond. Just a warning, you're going to need to dust off your trigonometry for this. If you create a triangle, where one leg is the distance between the Earth and the Sun (one astronomical unit), and the opposite angle, measured by how far the star moves in the sky, is one arcsecond, the star will be 1 parsec away, the other leg of the triangle. For example, the closest star in the sky, Proxima Centauri, has a parallax measurement of 0.77233 arcseconds - that's how far it shifts in the sky from when the Earth shifts its position by 1 astronomical unit. If you put this into the calculation, you determine that Proxima Centauri is 1.295 parsecs away, or 4.225 light years. We have written many articles about the parsec for Universe Today. The article explains how the astronomical unit might need to be changed as the Sun loses mass. Here's an article from NASA that explains how to derive the parallax measurement. Still doesn't make sense? Check out his episode of Astronomy Cast where we explain various methods astronomers use to measure the Universe. Episode 10: Measuring Distance in the Universe. Source: Wikipedia

This article was originally written in 2010, but we've now updated it and added this spiffy new video. As you probably know, the Earth is rotating on its axis. This gives us day and night. Of course it's impossible, but what would happen if the Earth stopped spinning? Remember, this isn't possible, it can't happen, so don't worry. Everything would be launched in a ballistic trajectory sideways The first thing to think about is the momentum of everything on the surface of the Earth. You're held down by gravity and you're whizzing through space at a rotational velocity of 1,674.4 km/h (at the equator). You can't feel it because of momentum. Just like how you can't feel that you're moving in a car going down the highway. But you feel the effects when you stop, or get into an accident. And so, if the Earth suddenly stopped spinning, everything on the surface of the Earth at the equator would suddenly be moving at more than 1,600 km/hour sideways. The escape velocity of Earth is about 40,000 km/hour, so that isn't enough to fly off into space; but it would cause some horrible damage as everything flew in a ballistic trajectory sideways. Imagine the oceans sloshing sideways at 1,600 km/hour. The rotational velocity of the Earth decreases as you head away from the equator, towards the poles. So as you got further away from the equator, your speed would decrease. If you were standing right on the north or south pole, you'd barely even feel it. A day would last 365 days The next problem is that day and night wouldn't work the same any more. Right now the Earth is rotating on its axis, returning the Sun to the same position every 24 hours. But if the Earth stopped spinning, it would then take 365 days for the Sun to move through the sky and return to the same position. Half of the Earth would be baked for half a year, while the other hemisphere was in darkness. It would get very hot on the sunny side, and very cold in the shadowed side. You can imagine how that would be devastating to plants and animals. We get a hint of this at the poles, where you can experience weeks of permanent night and then weeks of permanent day. But imagine 6 months of night, followed by 6 months of day. The Earth would become a perfect sphere This might seem minor compared to the other catastrophes, but the Earth would become an almost perfect sphere. The Earth is currently rotating on its axis, completing one turn approximately every 24 hours. This rotational velocity causes the Earth to bulge out around its equator, turning our planet into an oblate spheroid (a flattened ball). Without this spin, gravity would be able to pull the Earth into a nice perfect sphere. This sounds interesting and probably harmless, but it’s actually a *big* problem. Because of the Earth’s bulge in the middle, the oceans are held out at the equator by 8 km. On perfect sphere Earth, the world’s oceans would redistribute, flooding many regions of the planet with an immense volume of water. We’d end up with a single continent around the middle of the planet, with oceans surrounding the north and south poles. The Earth would no longer be tilted The Earth's tilt is defined by how the planet is rotating compared to the Sun. This axis of rotation defines the Earth's seasons. But without any rotation, the concept doesn't make sense any more. There's still a north pole of the planet, where the radiation from the Sun is at its lowest angle, and an equator, where the light hits most directly. But there would no longer be seasons. We have written many articles about the Earth for Universe Today. Here's an article about why the Earth has seasons, and here's an article about how the Earth protects us from space. If you'd like more info on Earth, check out NASA's Solar System Exploration Guide on Earth. And here's a link to NASA's Earth Observatory. We've also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

The Earth orbits the Sun in the Solar System, and the Solar System is embedded within a vast galaxy of stars. Just one in hundreds of billions of galaxies in the Universe. Ours is called the Milky Way because the disk of the galaxy runs across the sky as a band of glowing light, like spilled milk. Astronomers had suspected the Milky Way was made up of stars, but it wasn’t proven until 1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved individual stars in the band across the sky. With the help of telescopes, astronomers realized that there were many many more stars in the sky, and all of the stars that we can see are a part of the Milky Way. If you could travel outside the galaxy and look down on it from above, you’d see that the Milky Way is a barred spiral galaxy measuring about 120,000 light-years across and about 1,000 light-years thick. For the longest time, the Milky Way was thought to have 4 spiral arms, but newer surveys have determined that it actually seems to just have 2 spiral arms: they are called Scutum–Centaurus and Carina–Sagittarius. It’s very difficult to figure out what the Milky Way looks like exactly, because we’re embedded inside it. If you’d never been out of your house, you wouldn’t know what it looked like from outside. But you’d get a sense by looking at other houses in your neighborhood. Our Sun is located in the Orion Arm, a region of space in between the two major arms of the Milky Way. The spiral arms are formed from density waves that orbit around the Milky Way. As these density waves move through an area, they compress the gas and dust, leading to a period of active star formation for the region. Astronomers estimate that there are between 100 and 400 billion stars in the Milky Way, and think that each star has at least one planet. So there are likely hundreds of billions of planets in the Milky Way, and at least 17 billion of those are the size and mass of the Earth. Our Sun is located about 27,000 light years from the galactic core. At the heart of the Milky Way is a supermassive black hole, just like all of the other galaxies. This monster is more than 4 million times the mass of the Sun. Our Sun takes about 240 million years to orbit the Milky Way once. Just imagine, the last time the Sun was at this region of the galaxy, dinosaurs roamed the Earth, and the Sun has only made 18-20 trips around in its entire life. The Milky Way, like all galaxies, is surrounded by a vast halo of dark matter. Nobody knows what it is, but its mass helps keep the galaxy from tearing itself apart as it rotates. It’s believed that our galaxy formed through the collisions of smaller galaxies, early in the Universe. These mergers are still going on, and the Milky Way is expected to collide with Andromeda in 3-4 billion years. The two galaxies will combine to form a giant elliptical galaxy, and their supermassive black holes might even merge. The Milky Way and Andromeda are part of a larger collection of galaxies known as the Local Group. And these are contained within an even larger region called the Virgo Supercluster. You might be amazed to know that dung beetles actually navigate at night using the Milky Way. If you’ve never seen the Milky Way with your own eyes, you should take the chance when you can. Go to a place with nice dark skies, free from light pollution. Look up and appreciate the Milky Way, and wave hello to all the neighboring stars who share our galaxy with us.

We’ve talked about Venus, the hottest planet in the Solar System, but we know things can get pretty hot here on Earth, too. You may be wondering, where on the surface of the Earth has the highest natural temperature been recorded? The location of this world record has had some controversy, but as of 2013, the hottest spot on record was the Furnace Creek Ranch in California’s Death Valley. On July 10, 1913, weather instruments measured 56.7 degrees Celsius, or 134 degrees Fahrenheit. The previous record of 56 degrees at El Azizia, Libya was overturned because a systematic study in 2012 discovered there were errors in the measuring methods. Similar temperatures to Death Valley’s record have been recorded around the World: 55 degrees in Africa, 53.6 in Asia, 50.7 in Australia, and 49.1 in Argentina. But these are just measurements from weather stations. It’s likely there are hotter temperatures, but nobody was around to measure. NASA satellites have spotted regions in Iran’s Lut desert which might have reached 70 degrees Celsius during the summers of 2004 and 2005. So that’d be the hottest spot on the surface, but what about the hottest natural spot anywhere in the entire planet? Now you’ve got to travel straight down 6,371 kilometers to the very center of the Earth. At the inner core, the temperatures rise to about 5,430 degrees C, or 5700 Kelvin. Amazingly, this is about the same temperature as the surface of the Sun. Some of this high temperature comes from leftover heat from the formation of the planet, 4.54 billion years ago, but the vast majority comes from the decay of radioactive minerals inside the Earth. It was likely hotter in the past, but all the short-period isotopes have already been depleted. I keep saying the word “natural”, but what about “unnatural”? Wondering about the hottest temperature EVER generated on Earth? Thermonuclear explosions reach temperatures of tens of millions of Kelvin. Fusion experiments have hit 500 million Kelvin. But that’s nothing. In 2012, physicists working with the Large Hadron Collider were investigating the conditions that might have existed during the earliest moments of the Big Bang. They generated a quark gluon plasma that had a temperature of 5.5 trillion Kelvin. Unless aliens can do better, this is not only the hottest temperature ever recorded on Earth, it’s easily the hottest temperature anywhere in the Universe since the Big Bang itself.

It’s amazing to think that for the majority of human history, we had almost no understanding about the Sun. We didn’t know what it was made of, how it formed, or how it produced energy. We didn’t know how big it was, and we didn’t know how far away it was. We orbit the Sun at a distance of about 150 million kilometers. This number is actually an average, since we follow an elliptical path. At its closest point, the Earth gets to 147 million km, and at its most distant point, it’s 152 million km. Distances in the Solar System are so vast that astronomers use this distance as a standard for measurement, and so the average distance from the Earth to the Sun is called an astronomical unit. Instead of saying that Pluto is 5.87 billion kilometers away from the Sun, astronomers say that it’s 39 astronomical units, or AUs. You might be surprised to know that the distance from the Sun to the Earth was only determined within the last few hundred years. There were just too many variables. If astronomers knew how big it was, they could figure out how far away it was, or vice versa, but both of these numbers were mysteries. Ancient astronomers, especially the Greeks, tried estimating the distance to the Sun in several different ways: measuring the length of shadows on Earth, or comparing the size of the Moon and its orbit to the Sun. Unfortunately, their estimates were off at least by a factor of 10. The key to figuring out the distance to the Sun came from observing Venus as it passed directly in front of the Sun. This rare event, known as a Transit of Venus, happens only twice every 108 years. Once devised, the best opportunities for taking this precise measurement came during the Venus transits of 1761 and 1769. Astronomers were dispatched to remote corners of the globe to observe the precise moment when Venus began to move in front of the Sun, and when it had moved completely across the surface. By comparing these measurements, astronomers could use geometry to calculate exactly how far away the Sun is. Their initial calculations put the distance at 24,000 times the radius of the Earth. Not bad considering our modern measurement of 23,455 times the radius of the Earth. Modern astronomers can use radar and laser pulses to calculate the distance to objects in the Solar System. For example, they fire an intense beam of radio waves at a distant object, like Mercury, and then calculate how long it takes for the waves to bounce off the planet and return to Earth. Since the speed of light is well known, the return travel time tells you how far away the planet is. Astronomy has truly helped us find our place in the Universe. It nice to be living in a time when many of these big mysteries have been solved. I don’t know about you, but I can't wait to see what’s around the corner of the next discovery.

The space age began on October 4, 1957 with the launch of the first artificial satellite, Sputnik 1. This tiny spacecraft lasted only three months in orbit, finally burning up in the Earth’s atmosphere. Following in these historic footsteps, many more spacecraft have been sent into Earth’s orbit, around the Moon, the Sun, the other planets, and even out of the Solar System itself. At the time that I’m recording this video, there are 1071 operational satellites in orbit around the Earth. 50 percent of which were launched by the United States. Half of that 1071 are in Low-Earth Orbit, just a few hundred kilometers above the surface. Some of the most notable of these include the International Space Station, the Hubble Space Telescope, and many Earth observation satellites. About a twentieth are in Medium-Earth Orbit, around 20,000 kilometers up, which are generally global positioning satellites used for navigation. A small handful are in elliptical orbits, where their orbit brings them closer and further to the Earth. The rest are in geostationary orbit, at an altitude of almost 36,000 kilometers. If we could see these satellites from Earth’s surface, they would appear to hang motionless in the sky. The fact that they remain over the geographic same area means they provide the perfect platform for telecommunications, broadcast or weather observations. But there are many, many more artificial objects orbiting the Earth. In this collection of space debris we’re talking spent boosters, dead satellites, and even misplaced gloves. According to the United States Space Surveillance Network, there are more than 21,000 objects larger than 10 cm orbiting the Earth. Just a small fraction of these are operational satellites. It’s estimated there are a further 500,000 bits and pieces between 1 and 10 cm in size. Near Earth orbit is so polluted with junk that the International Space Station is often moved to avoid impact with dangerous chunks of space debris. Many of these objects are created through collisions, and some scientists are worried that future space travel might be too risky if we get too much junk orbiting the planet. We might seal ourselves inside a shield of shrieking metal moving at 29,000 km/hour. Looking outwards from our own orbit, at any time there are a handful of satellites orbiting the Moon. Right now, NASA’s Lunar Reconnaissance Orbiter and Lunar Atmosphere and Dust Environment Explorer are in lunar orbit. Further still, there’s 1 spacecraft around Mercury, 1 at Venus, 3 visiting Mars and 1 orbiting Saturn. There’s a handful of spacecraft orbiting the Sun, although they’re leading or trailing the Earth in its orbit. And a few spacecraft are on trajectories to take them out of the Solar System entirely. NASA’s Voyager spacecraft, exited the Sun’s heliosphere in 2013, and entered the interstellar medium. Starting with Sputnik’s lonely journey over 50 years ago, It’s amazing to consider just how many satellites we’ve already launched into space in just a few decades. With more launches all the time, space is becoming a busy place, with so many exciting missions to look forward to. We have written many articles about satellites for Universe Today. Here's an article about two satellites that collided in Earth orbit, and here are some pictures of satellites. You can learn more about the US Space Surveillance Network from the United States Strategic Command website. We have also recorded a whole episode of Astronomy Cast about space junk. Listen here, Episode 82: Space Junk.

Another name for Mars is the Red Planet, and if you’ve ever seen it in the sky when the planet is bright and close to Earth, it appears like a bright red star. In Roman mythology, Mars was the god of war, so… think blood. Even photos from spacecraft show that it’s a rusty red color. The hue comes from the fact that the surface is *actually* rusty, as in, it’s rich in iron oxide. Iron left out in the rain and will get covered with rust as the oxygen in the air and water reacts with the iron in the metal to create a film of iron oxide. Mars’ iron oxide would have formed a long time ago, when the planet had more liquid water. This rusty material was transported around the planet in dust clouds, covering everything in a layer of rust. In fact, there are dust storms on Mars today that can rise up and consume the entire planet, obscuring the entire surface from our view. That dust really gets around. But if you look closely at the surface of Mars, you’ll see that it can actually be many different colours. Some regions appear bright orange, while others look more brown or even black. But if you average everything out, you get Mars’ familiar red colour. If you dig down, like NASA’s Phoenix Lander did in 2008, you get below this oxidized layer to the rock and dirt beneath. You can see how the tracks from the Curiosity Rover get at this fresh material, just a few centimeters below the surface. It’s brown, not red. And if you could stand on the surface of Mars and look around, what colour would the sky be? Fortunately, NASA’s Curiosity Rover is equipped with a full colour camera, and so we can see roughly what the human eye would see. The sky on Mars is red too. The sky here is blue because of Raleigh scattering, where blue photons of light are scattered around by the atmosphere, so they appear to come from all directions. But on Mars, the opposite thing happens. The dust in the atmosphere scatters the red photons, makes the sky appear red. We have something similar when there’s pollution or smoke in the air. But here’s the strange part. On Mars, the sunsets appear blue. The dust absorbs and deflects the red light, so you see more of the blue photons streaming from the Sun. A sunset on Mars would be an amazing event to see with your own eyes. Let’s hope someone gets the chance to see it in the future. We have written many articles about Mars on Universe Today. Here's an article about a one-way, one-person trip to Mars, and here's another about how scientists know the true color of planets like Mars. Here are some nice color images captured of the surface of Mars from NASA's Pathfinder mission, and here's another explainer about why Mars is red from Slate Magazine. We have recorded several podcasts just about Mars. Including Episode 52: Mars and Episode 92: Missions to Mars, Part 1. Sources: http://quest.arc.nasa.gov/qna/questions/FAQ_GeneraL_Mars.htm http://mpfwww.jpl.nasa.gov/programmissions/missions/past/pathfinder/ http://www.slate.com/id/2093779/

The evidence that the Universe began with the Big Bang is very compelling. 13.8 billion years ago, the entire Universe was compressed into a microscopic singularity that grew exponentially into the vast cosmos we see today. But what does the future hold? How will the Universe end? Astronomers have been pondering the ultimate fate of the Universe for thousands of years. In the last century, cosmologists considered three outcomes for the end of everything, and it all depended on the critical density of the Universe. If this critical density was high, then there was enough mutual gravity to slow and eventually halt the expansion. Billions of years in the future, it would then collapse in on itself again, perhaps creating another Big Bang. This is known as a closed Universe, and the final result is the Big Crunch. If the critical density was low, then there wouldn’t be enough gravity to hold things together. Expansion would continue on forever and ever. Stars would die, galaxies would be spread apart, and everything would cool down to the background temperature of the Universe. This is an open Universe, and the end is known as the Big Freeze. And if the critical density was just right, the Universe’s expansion goes on forever, but it’s always slowing down, reaching a dead stop in an infinite amount of time. This creates a Flat Universe… also a Big Freeze. Fortunately, astronomers were able to measure the critical density of the Universe, using NASA’s WMAP spacecraft, and they discovered that the actual density of the Universe predicts a flat Universe. So that’s it, right? Of the three choices, the answer is #3. Unfortunately, nature had other plans, and came up with a reality that nobody expected. In 1998, a team of astronomers were observing distant supernovae to get a sense of how fast the Universe is slowing down and they made an amazing discovery. Instead of decelerating, as predicted by the critical density of the Universe, the expansion of the Universe is actually speeding up. Some mysterious force is pushing galaxies faster and faster away from each other, accelerating the expansion of the Universe. We now call this force “dark energy”, and for the time being, astronomers have no idea what it is. All we know is that it’s pushing the Universe apart. Distant galaxies are being accelerated away from us, and in trillions of years from now, they will cross the beyond the cosmic horizon and disappear from view. The evidence that we live in a vast Universe will disappear with them. But there’s a further unsettling possibility about dark energy. Maybe the expansion pressure will increase, eventually overwhelming gravity on a local level. Galaxies will get torn apart, and then Solar Systems, and eventually atoms themselves will be shredded by the increasing dark energy - this idea is known as the Big Rip. So how will the Universe end? The force of dark energy will continue to accelerate the expansion of the Universe until distant galaxies disappear. Galaxies will use up all the gas and dust for stars and go dark, perhaps becoming black holes. Those black holes will decay and maybe matter itself will decay into pure energy. The entire Universe will become a cold, quiet place, where single photons are stretched across light years of space. Don’t worry, though, that won’t be for quadrillions of years from now.

This question comes from Andrew Bumford and Steven Stormont. In a previous episode I’ve talked about how the entire Solar System collapsed down from a cloud of hydrogen and helium left over from the Big Bang. And yet, we stand here on planet Earth, with all its water. So, how did that H20 get to our planet? The hydrogen came from the solar nebula, but where did the oxygen come from? Here’s the amazing part. The oxygen came from stars that lived and died before our Sun was even born. When those stars puffed out their final breaths of oxygen, carbon and other “metals”, they seeded new nebulae with the raw material for new worlds. We owe our very existence to the dead stars that came before. When our Sun dies, it’ll give up some of its heavier elements to the next generation of stars. So, mix hydrogen together with this donated oxygen, and you’ll get H20. It doesn’t take any special process or encouragement, when those two elements come together, water is the result. But how did it get from being spread across the early Solar System to concentrating here on Earth, and filling up our oceans, lakes and rivers? The exact mechanism is a mystery. Astronomers don’t know for sure, but there are a few theories: Idea #1: impacts. Take a look at the craters on the Moon and you’ll see that the Solar System was a busy place, long ago. Approximately 3.8 to 4.1 billion years ago was the Late Heavy Bombardment period, when the entire inner Solar System was pummeled by asteroids. The surfaces of the planets and their moons were heated to molten slag because of the non-stop impacts. These impactors could have been comets or asteroids. Comets are 80% water, and would deliver vast amounts of water to Earth, but they’re also volatile, and would have a difficult time surviving the harsh radiation of the young Sun. Asteroids have a lower ratio of water, but they could protect that water a little better, delivering less with each catastrophic impact. Astronomers have also found many hybrid objects which contain large amounts of both rock and water. It’s hard to classify them either way. Idea #2 is that large amounts of water just came directly from the solar nebula. As we orbited around the young Sun, it passed through the water-rich material in the nebula and scooped it up. Gravitational interactions between the planets would have transferred material around the Solar System, and it would have added to the Earth’s volume of water over hundreds of millions of years. Of course, it’s entirely possible that the answer is “all of the above”. Asteroids and comets and the early solar nebula all delivered water to the Earth. Where did the Earth’s water come from? Astronomers don’t know for sure. But I’m sure glad the water is here; life here wouldn’t exist without it.

Almost every part of a rocket is destroyed during the launch and re-entry into the Earth’s atmosphere. This makes spaceflight really expensive. Rocket delivery of even a single kilogram into orbit costs tens of thousands of dollars. But what if we could just place our payloads directly into orbit, and didn’t need a rocket at all? This is the idea of a space elevator, first envisioned by the Russian rocket scientist Konstantin Tsiolkovsky in 1895. Tsiolkovsky suggested building a tower all the way up to geostationary orbit, this is the point where a satellite appears to hang motionless in the sky above the Earth. If you could carry spacecraft all the way up to the top, and release them from that tower they’d be in orbit, without the expense of a discarded rocket. A fraction more energy and they’d be traveling away from the Earth to explore the Solar System. The major flaw with this idea is that the entire weight of the tower would be compressing down on every part below. And there’s no material on Earth, or in the Universe, that can handle this kind of compressive force. But the idea still makes sense. Newer thinking about space elevators propose using a cable, stretched out beyond geostationary orbit. Here the outward centripetal force counters the force of gravity, keeping the tether perfectly balanced. But now we’re dealing with the tensile strength of a cable tens of thousands of kilometers long. Imagine the powerful forces trying to tear it apart. Until recently, there was no material strong enough to withstand those forces, but the development of carbon nanotubes has made the idea more possible. How would you build a space elevator? The most reasonable idea would be to move an asteroid into geostationary orbit - this is your counterbalance. A cable would then be manufactured on the asteroid, and lowered down towards the Earth. As the cable extends down, the asteroid is orbited further from the Earth, keeping everything in balance. Finally, the cable reaches the Earth’s surface and is attached to a ground station. Solar powered machines are attached to the space elevator and climb up from the surface of the Earth, all the way to geostationary orbit. Even traveling at a speed of 200 km/hour, it would take the climber almost 10 days to make the journey from the surface to an altitude of 36,000 kilometers. But the cost savings would be dramatic. Currently, rockets cost about $25,000 per kilogram to send a payload to geostationary orbit. A space elevator could deliver the same payload for $200 per kilo. Obviously there are risks associated with a megastructure like this. If the cable breaks, portions of it would fall to Earth, and humans traveling up in the elevator would be exposed to damaging radiation in the Earth’s Van Allen belts. Building a space elevator from Earth is at the very limits of our technology. But there are places in the Solar System which might make much more useful places to build elevators. The Moon, for example, has a fraction of the Earth’s gravity, so an elevator could operate there using commercially available materials. Mars might be another great place for a space elevator. Whatever happens, the idea is intriguing. And if anyone does build a space elevator, they will open up the exploration of the Solar System in ways that we can’t even imagine.

Ask anyone, “what color is the Sun”? and they’ll tell you the obvious answer: it’s yellow. But is it really? Please don’t go check, it’s not safe to look directly at the Sun with your unprotected eyes. From our perspective it does look a little yellow, especially after sunrise or shortly before sunset, But don’t be fooled. If you could travel into space and look at the Sun without going blind, you’d find that it’s actually white, and not yellow. Using a prism, you can see how sunlight can be broken up into the spectrum of its colors: red, orange, yellow, green, blue, indigo and violet. When you mix all those colors together, you get white. Here’s the strange part. If look at all the photons coming in, our star is actually sending the most photons in the green portion of the spectrum, Our Sun appears yellow to us because of the atmosphere. Photons in the higher end of the spectrum - blue, indigo and violet - are more likely to be scattered away, while the lower end of the spectrum - red, orange and yellow - are less easily scattered. When the Sun is close to the horizon, you’re seeing it distorted by more of the Earth’s atmosphere, scattering away the bluer photons and making it appear red. When there’s smoke and pollution in the air, it enhances the effect and it will look even redder. If the Sun is high in the sky, where it has the least amount of atmospheric interference, it will appear more blue. We’re so familiar with the Sun being yellowish-orange, that astronomers will artificially change the color of their images to look more yellowy. But really, the Sun looks like a pure white ball - especially when you’re out in space. Interestingly, the color of the Sun is very important to astronomers. They use a technique called spectroscopy to stretch out the spectrum of light coming from a star. Dark lines in this spectrum tell you exactly what it’s made of. You can see which stars have high amounts of metals, or which are mostly hydrogen and helium, leftover from the Big Bang. This color also tells you the temperature of the star. Cooler stars are actually redder. Betelgeuse is only 3500 Kelvin. Hotter stars, like Rigel, can get above 10000 Kelvin, and they look blue. Our own Sun has a temperature of almost 5800 Kelvin, and when viewed outside of our atmosphere, appears white. in colour.

Everyone knows that the Moon goes through phases, but let’s talk about why it does. It comes down to illumination, which in this case, all originates from our nearby star. Our Moon orbits around our planet, and this Earth-Moon system orbits around the Sun. Even though we only see light on part of the Moon, from the perspective of the Sun, half of it is always illuminated. Stuck here on Earth, we see the Moon in various phases of illumination as it completes a 27.3 day orbit around the Earth. As The Moon travels around us we see it pass through its phases. It goes from New Moon, to Full Moon and back to new Moon again. Crescent Moons are when it’s less than half illuminated, and gibbous when it’s more than half. “Waxing” means that the Moon becomes more illuminated night-by-night, and the term “waning” means that it’s getting less illuminated each night. New Moon - When the illuminated side of the Moon is away from the Earth. The Moon and the Sun are lined up on the same side of the Earth, so we can only see the shadowed side. This is also the time that you can experience solar eclipses, when the Moon passes directly in front of the Sun and casts a shadow onto the surface of the Earth. During a new moon, we can also see the reflected light from the Earth, since no sunlight is falling on the Moon - this is known as earthshine. Crescent - The crescent moon is the first sliver of the Moon that we can see. From the northern hemisphere, the crescent moon has the illuminated edge of the Moon on the right. This situation is reversed for the southern hemisphere. First Quarter - Although it's called a quarter moon, we actually see this phase when the Moon is half illuminated. This means that the Sun and the Moon make a 90-degree angle compared to the Earth. Waxing Gibbous - This phase of the Moon occurs when the Moon is more illuminated that half, but it's not yet a full Moon. Full Moon - This is the phase when the Moon is brightest in the sky. From our perspective here on Earth, the Moon is fully illuminated by the light of the Sun. This is also the time of the lunar month when you can see lunar eclipses - these occur when the Moon passes through the shadow of the Earth. Waning Gibbous - In this lunar phase, the Moon is less than fully illuminated, but more than half. Last Quarter - At this point of the lunar cycle, the Moon has reached half illumination. Now it's the left-hand side of the Moon that's illuminated, and the right-hand side in darkness (from a northern hemisphere perspective). Crescent - This is the final sliver of illuminated moon we can see before the Moon goes into darkness again. If you ever get the chance to travel to the other hemisphere, you’ll immediately notice how unfamiliar the Moon behaves - it’s upside down. If you live in the Northern Hemisphere, after a New Moon the crescent begins on the right-side. But if you’re in the Southern Hemisphere, it’s reversed, with the illumination starting on the left side. Weird. The alignment of the Sun, Earth and Moon can lead to some fantastic astronomical events. One event occurs when the Moon is full, and it passes through the Earth’s shadow. Or as you probably know it, a lunar eclipse. This causes the Moon to grow dark and then turn an eerie red color. When the Moon is new, it can pass in between the Earth and the Sun, casting its shadow down on our planet. As you know, a solar eclipse. You’d think we would see a solar and lunar eclipse every month, but we don’t because the Moon’s orbit is inclined relative to the Sun. Most months, the Moon is either above or below the Sun in the sky, so they just don’t line up perfectly. One more thing, you might not know that Venus also goes through phases. When the planet is on the other side of the Sun from us, we see it as a nearly complete disk. But when Venus is on our side, just about to pass into the glow of the Sun, it’s a thin crescent,

Life has existed on Earth for billions of years, appearing shortly after the planet had cooled and liquid water became available. From the first bacteria to the amazingly complex animals we see today, life has colonized every corner of our planet. As you know, our Sun has a limited lifespan. Over the next 5 billion years, it will burn the last of its hydrogen, bloat up as a red giant and consume Mercury and Venus. This would be totally disastrous for local flora and fauna, but all life on the surface of the Earth will already be long gone. In fact, we have less than a billion years to enjoy the surface of our planet before it becomes inhospitable. Because our Sun... is heating up. You can’t feel it over the course of a human lifetime, but over hundreds of millions of years, the amount of radiation pouring out of the Sun will grow. This will heat the surface of our planet to the point that the oceans boil. At the core of the Sun, the high temperatures and pressures convert hydrogen into helium. For every tonne of material the Sun converts, it shrinks a bit making the Sun denser, and a little hotter. Over the course of the next billion years or so, the amount of energy the Earth receives from the Sun will increase by about 10%. Which doesn't sound like much, but it means a greenhouse effect of epic proportions. Whatever is left of the ice caps will melt, and the water itself will boil away, leaving the planet dry and parched. Water vapor is a powerful greenhouse gas, this will drive the temperatures even hotter. Plate tectonics will shut down, and all the carbon will be stripped from the atmosphere. It’ll be bad. As temperatures rise, complex lifeforms will find life on Earth less hospitable. It will seem as if evolution is running in reverse, as plants and animals die off, leaving the invertebrates and eventually just microbial life. This rise in temperature will be the end of life on the surface of Earth as we know it. Still, there are reserves of water deep underground which will continue to protect microbial life for billions of years. Perhaps they’ll experience that final baking when the Sun does reach the end of its life. Even a few hundred million years is an incomprehensible amount of time compared to the age of our civilization. If humanity does survive well into the future, is there anything we could do about this problem? As the Sun heats up, making Earth inhospitable, it heats up the rest of the Solar System too. Frozen worlds in the Solar System will melt, becoming more habitable. It’s possible that future civilizations could relocate to the asteroid belt, or the moons of Saturn. We could try something even more radical: move the Earth. By carefully steering asteroids so they barely miss us, an advanced civilization could distort the Earth’s orbit, relocating our planet further from the Sun. As the Sun heats up, our planet would be continuously repositioned so the surface temperature stays roughly the same. Of course, this would be tricky business. Make the wrong move, and you’re facing the frigid cold of the outer Solar System. So there’s no need to panic. Life here has a few hundred million years left; a billion, tops. But if we want to continue on for billions of years, we’ll want to add solar heating to our growing list of big problems.