Universe Today Video

When we look at the night sky, filled with stars, it’s hard to resist counting. Just with the unaided eye, in dark skies, you can see a few thousand. How many stars are there in the entire Universe? Before we get to that massive number, let's consider what you can count with the tools available to you. Perfect vision in dark skies allows us to see stars down to about magnitude 6. But to really make an accurate census of the total number of stars, you'd need to travel to both the Northern and Southern Hemispheres, since only part of the sky is visible from each portion of the Earth. Furthermore, you'd need to make your count over several months, since a portion of the sky is obscured by the Sun. If you had perfect eyesight and traveled to completely dark skies in both the Northern and Southern Hemispheres, and there was no Moon, you might be able to get to count up almost 9,000 stars. With a good pair of binoculars, that number jumps to about 200,000, since you can observe stars down to magnitude 9. A small telescope, capable of resolving magnitude 13 stars will let you count up to 15 million stars. Large observatories could resolve billions of stars. But how many stars are out there? How many stars are there in the Milky Way? According to astronomers, our Milky Way is an average-sized barred spiral galaxy measuring up to 120,000 light-years across. Our Sun is located about 27,000 light-years from the galactic core in the Orion arm. Astronomers estimate that the Milky Way contains up to 400 billion stars of various sizes and brightness. A few are supergiants, like Betelgeuse or Rigel. Many more are average-sized stars like our Sun. The vast majority of stars in the Milky Way are red dwarf stars; dim, low mass, with a fraction of the brightness of our Sun. As we peer through our telescopes, we can see fuzzy patches in the sky which astronomers now know are other galaxies like our Milky Way. These massive structures can contain more or less stars than our own Milky Way. There are spiral galaxies out there with more than a trillion stars, and giant elliptical galaxies with 100 trillion stars. And there are tiny dwarf galaxies with a fraction of our number of stars. So how many galaxies are there? According to astronomers, there are probably more than 170 billion galaxies in the observable Universe, stretching out into a region of space 13.8 billion light-years away from us in all directions. And so, if you multiply the number of stars in our galaxy by the number of galaxies in the Universe, you get approximately 1024 stars. That’s a 1 followed by twenty-four zeros. That’s a septillion stars. But there could be more than that. It's been calculated that the observable Universe is a bubble of space 47 billion years in all directions. It defines the amount of the Universe that we can see, because that’s how long light has taken to reach us since the Big Bang. This is a minimum value, the Universe could be much bigger - it's just that we can't ever detect those stars because they're outside the observable Universe. It's even possible that the Universe is infinite, stretching on forever, with an infinite amount of stars. So add a couple more zeros. Maybe an infinite number of zeroes. That’s a lot of stars in the Universe. Additional Resources: How Many Stars Can you See? Astronomy Cast: How Big is the Universe? How Big is Our Observable Universe Astronomy Cast: The Observable Universe How Many Galaxies in the Universe?

This article comes from the Universe Today archive, but was updated with this spiffy video. How old is the Earth? Scientists think that the Earth is 4.54 billion years old. Coincidentally, this is the same age as the rest of the planets in the Solar System, as well as the Sun. Of course, it's not a coincidence; the Sun and the planets all formed together from a diffuse cloud of hydrogen billions of years ago. In the early Solar System, all of the planets formed in the solar nebula; the remnants left over from the formation of the Sun. Small particles of dust collected together into larger and larger objects - pebbles, rocks, boulders, etc - until there were many planetoids in the Solar System. These planetoids collided together and eventually enough came together to become Earth-sized. At some point in the early history of Earth, a planetoid the size of Mars crashed into our planet. The resulting collision sent debris into orbit that eventually became the Moon. How do scientists know Earth is 4.54 billion years old? It's actually difficult to tell from the surface of the planet alone, since plate tectonics constantly reshape its surface. Older parts of the surface slide under newer plates to be recycled in the Earth's core. The oldest rocks ever found on Earth are 4.0 - 4.2 billion years old. Scientists assume that all the material in the Solar System formed at the same time. Various chemicals, and specifically radioactive isotopes were formed together. Since they decay in a very known rate, these isotopes can be measured to determine how long the elements have existed. And by studying different meteorites from different locations in the Solar System, scientists know that the different planets all formed at the same time. Failed Methods for Calculating the Age of the Earth Our current, accurate method of measuring the age of the Earth comes at the end of a long series of estimates made through history. Clever scientists discovered features about the Earth and the Sun that change over time, and then calculated how old the planet Earth is from that. Unfortunately, they were all flawed for various reasons. Declining Sea Levels - Benoit de Maillet, a French anthropologist who lived from 1656-1738 and guessed (incorrectly) that fossils at high elevations meant Earth was once covered by a large ocean. This ocean had taken 2 billion years to evaporate to current sea levels. Scientists abandoned this when they realized that sea levels naturally rise and fall. Cooling of the Earth - William Thompson, later known as Lord Kelvin, assumed that the Earth was once a molten ball of rock with the same temperature of the Sun, and then has been cooling ever since. Based on these assumptions, Thompson calculated that the Earth took somewhere between 20 and 400 million years to cool to its current temperature. Of course, Thompson made several inaccurate assumptions, about the temperature of the Sun (it's really 15 million degrees Kelvin at its core), the temperature of the Earth (with its molten core) and how the Sun is made of hydrogen and the Earth is made of rock and metal. Cooling of the Sun - In 1856, the German physicist Hermann Ludwig Ferdinand von Helmholtz attempted to calculate the age of the Earth by the cooling of the Sun. He calculated that the Sun would have taken 22 million years to condense down to its current diameter and temperature from a diffuse cloud of gas and dust. Although this was inaccurate, Helmholtz correctly identified that the source of the Sun's heat was driven by gravitational contraction. Rock Erosion - In his book, The Origin of Species by Means of Natural Selection, Charles Darwin proposed that the erosion of chalk deposits might allow for a calculation of the minimum age of the planet. Darwin estimated that a chalk formation in the Weald region of England might have taken 300 million years to weather to its current form. Orbit of the Moon - George Darwin,

I’m just going to warn you, this is a controversial topic. Some people get pretty grumpy when you ask: how many planets are in the Solar System? Is it eight, ten, or more? I promise you this, though, we’re never going back to nine planets... ever. When many of us grew up, there were nine planets in the Solar System. It was like a fixed point in our brains. As kids, memorizing this list was an early right of passage of nerd pride: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. But then in 2005, Mike Brown discovered Eris, an icy object thought to be about the same size as Pluto, out beyond its orbit. That would bring the total number of planets to ten. Right? There’s no turning back, textbooks would need to be changed. In order to settle the dispute, the International Astronomical Union met in 2006, and argued for, and against Pluto’s planethood. Some astronomers advocated widening the number of planets to twelve, including Pluto, its moon Charon, the Asteroid Ceres, and the newly discovered Eris. In the end, they changed the definition of what makes a planet, and sadly, Pluto doesn’t make the cut: Here are the new requirements of planethood status: A planet has to orbit the Sun. Okay fine, Pluto does that. A planet needs enough gravity to pull itself into a sphere. Okay, spherical. Pluto’s fine there too. A planet needs to have cleared out its orbit of other objects. Uh oh, Pluto hasn’t done that. For example, planet Earth accounts for a million times the rest of the material in its orbit, while Pluto is just a fraction of the icy objects in its realm. The final decision was to demote Pluto from planet to dwarf planet. But don’t despair, Pluto is in good company. There’s Ceres, the first asteroid ever discovered, and the smallest of the dwarf planets. The surface of Ceres is made of ice and rock, and it might even have a liquid ocean under its surface. NASA’s Dawn mission is flying there right now to give us close up pictures for the first time. Haumea, named after the Hawaiian goddess of fertility, is about a third the mass of Pluto, and has just enough gravity to pull itself into an ellipsoid, or egg shape. Even though it’s smaller, it’s got moons of its own. Makemake, a much larger Kuiper belt object, has a diameter about two-thirds the size of Pluto. It was discovered in 2005 by Mike Brown and his team. So far, Makemake doesn’t seem to have any moons. Eris is the most massive known dwarf planet, and the one that helped turn our definition of a planet upside-down. It’s 27% more massive than Pluto and the ninth most massive body that orbits the Sun. It even has a moon: Dysnomia. And of course, Pluto. The founding member of the dwarf family. Want an easy way to remember the eight planets, in order? Just remember this mnemonic: my very excellent mother just served us noodles. For all you currently writing angry tweets to Mike Brown, hold on a sec. Changing Pluto’s categorization is an important step that really needed to happen. The more we discover about our Universe, the more we realize just how strange and wonderful it is. When Pluto was discovered 80 years ago, we never could have expected the variety of objects in the Solar System. Categorizing Pluto as a dwarf planet helps us better describe our celestial home. So, our Solar System now has eight planets, and five dwarf planets.

The International Space Station is one of the most complicated machines ever built and the largest object ever assembled in space. At any time there are up to six astronauts on board, each originating from one of fifteen different nations on Earth. It orbits at an altitude of approximately four-hundred kilometers, and completes an orbit around the Earth every ninety-two minutes and fifty seconds. The station has a mass of four-hundred-and-twenty metric tonnes, and contains a dozen pressurized modules, and many more unpressurized modules, trusses and solar panels. It truly is a feat of human ingenuity. But did you know that the International Space Station is one of the brightest objects in the night sky? And it’s easy to see if you know when, and where, to look. In fact, with your ability to find the station you can amaze your friends and neighbours. The best place to start is NASA’s Spot the Station website. Enter your Country, Region, City along with an email address or mobile phone number. Then give your preference for notifications in the evening, morning or both and that’s it. About twelve hours before the station is due to fly overhead, you’ll get a notification from NASA. Depending on your location, you might get notified a couple of times a week, or as rarely as once a month. As soon as you get the notification, create an alarm on your phone for about a minute before the flyover. When the alarm goes off, take your friends outside and look to the West. The station orbits the Earth from West to East, so you’ll see it appear on the Western horizon as a very bright star, moving rapidly across the sky. It will take only few minutes to cross the entire sky. The station moves so quickly if you’re using a telescope you will have a tough time tracking its movement. A nice pair of binoculars will make it look a lot brighter, and even let you see the H-shape of its solar panels. But even viewing it with the naked eye is a great experience. NASA’s website is just one of the many ways you can get notifications. If you use Twitter, follow @twisst. They can figure out your location and then send you a notification when the station is about to fly overhead via Twitter. There are also dozens of Android and iPhone apps that will perform this function; many of which are free to use. If you’ve never seen the station, head on over to NASA and set up a notification right away. Then kick back and let orbital mechanics bring the station to your backyard at a time that’s convenient for you. Want more details? We've got a detailed guide on how to View the International Space Station for Beginners, and How to Photograph the International Space Station.

This article originally appeared in 2009, but I've updated it and added this video. The ground feels firm and solid beneath your feet. Of course, the Earth is rotating, turning once on its axis every day. Fortunately gravity keeps you firmly attached to the planet, and because of momentum, you don't feel the movement - the same way you don't feel the speed of a car going down the highway. But how fast does the Earth rotate? You might be surprised to know that a spot on the surface of the Earth is moving at 1675 km/h or 465 meters/second. That's 1,040 miles/hour. Just think, for every second, you're moving almost half a kilometer through space, and you don't even feel it. Want to do the calculation for yourself? The Earth's circumference at the equator is 40,075 km. And the length of time the Earth takes to complete one full turn on its axis is 23.93 hours. Wait, 23.93 hours? Isn't a day 24 hours? Astronomers calculate a day in two ways. There's the amount of time it takes for the Earth to complete one full rotation on its axis, compared to the background stars. Imagine you were looking down at the Earth from above the North Pole. You'd see the Earth turn once completely in 23 hours and 56 minutes. Astronomers call this a sidereal day. And then there's the time it takes for the Sun to return to the same spot in the sky. Since the Earth is orbiting the Sun, we actually need an extra 4 minutes each day to return the Sun to the same spot. Astronomers call this a solar day. Then we divide the length of a day into the distance a point on the equator travels in that period: 40,075 km/23.93 hours = 1,675 km/hour, 465 meters/second. The speed of the Earth's rotation changes as you go North or South away from the equator. Finally, when you reach one of the Earth's poles, you're taking a whole day to just turn once in place - that's not very fast. Because you're spinning around and around on the Earth, there's a force that wants to spin you off into space; like when you spin a weight on a string. But don't worry, that force isn't very strong, and it's totally overwhelmed by the force of gravity holding you down. The force that wants to throw you into space is only 0.3% the force of gravity. In other words, if the Earth wasn't spinning, you would weigh 0.3% more than you do right now. Space agencies take advantage of the higher velocities at the Earth's equator to launch their rockets into space. By launching their rockets from the equator, they can use less fuel, or launch more payload with the same amount of fuel. As it launches, the rocket is already going 1,675 km/hour. That makes it easier to reach the 28,000 km/hour orbital velocity; or even faster to reach geosynchronous orbit. We have written many articles about the Earth for Universe Today. Here's an article about why the Earth rotates. Want more resources on the Earth? Here's a link to NASA's Human Spaceflight page, and here's NASA's Visible Earth. We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System - Episode 51: Earth. References: NASA Space Place NASA Solar System Exploration: Earth

This article originally appeared in Universe Today in July, 2012, but it's been updated with a related video. The planet Mars is one of the brightest objects in the night sky, easily visible with the unaided eye as a bright red star. Every two years or so, Mars and Earth reach their closest point, called "opposition", when Mars can be as close as 55,000,000 km from Earth. And every two years, space agencies take advantage of this orbital alignment to send spacecraft to the Red Planet. How long does it take to get to Mars? The total journey time from Earth to Mars takes between 150-300 days depending on the speed of the launch, the alignment of Earth and Mars, and the length of the journey the spacecraft takes to reach its target. It really just depends on how much fuel you're willing to burn to get there. More fuel, shorter travel time. The History of Going to Mars The first spacecraft ever to make the journey from Earth to Mars was NASA's Mariner 4, which launched on November 28, 1964 and arrived at Mars July 14, 1965, successfully taking a series of 21 photographs. Mariner 4's total flight time was 228 days. The next successful mission to Mars was Mariner 6, which blasted off on February 25, 1969 and reached the planet on July 31, 1969; a flight time of only 156 days. The successful Mariner 7 only required 131 days to make the journey. Mariner 9, the first spacecraft to successfully go into orbit around Mars launched on May 30, 1971, and arrived November 13, 1971 for a duration of 167 days. This is the same pattern that has held up for more almost 50 years of Mars exploration: approximately 150-300 days. Even more examples: Viking 1 (1976) - 335 days Viking 2 (1976) - 360 days Mars Reconnaissance Orbiter (2006) - 210 days Phoenix Lander (2008) - 295 days Curiosity Lander (2012) - 253 days Why Does it Take So Long? When you consider the fact that Mars is only 55 million km away, and the spacecraft are travelling in excess of 20,000 km/hour, you would expect the spacecraft to make the journey in about 115 days, but it takes much longer. This is because both Earth and Mars are orbiting around the Sun. You can't point directly at Mars and start firing your rockets, because by the time you got there, Mars would have already moved. Instead, spacecraft launched from Earth need to be pointed at where Mars is going to be. The other constraint is fuel. Again, if you had an unlimited amount of fuel, you'd point your spacecraft at Mars, fire your rockets to the halfway point of the journey, then turn around and decelerate for the last half of the journey. You could cut your travel time down to a fraction of the current rate - but you would need an impossible amount of fuel. How to Get to Mars with the Least Amount of Fuel The primary concern of engineers is how to get a spacecraft to Mars, on the least amount of fuel. Robots don't really care about the hostile environment of space, so it makes sense to decrease the launch costs of the rocket as much as possible. NASA engineers use a method of travel called a Hohmann Transfer Orbit - or a Minimum Energy Transfer Orbit - to send a spacecraft from Earth to Mars with the least amount of fuel possible. The technique was first proposed by Walter Hohmann who published the first description of the maneuver in 1925. Instead of pointing your rocket directly at Mars, you boost the orbit of your spacecraft so that it's following a larger orbit around the Sun than the Earth. Eventually that orbit will intersect the orbit of Mars - at the exact moment that Mars is there too. If you need to launch with less fuel, you just take longer to raise your orbit, and increase the journey to Mars. Other Ideas to Decrease the Travel Time to Mars Although it requires some patience to wait for a spacecraft to travel 250 days to reach Mars, we might want a completely different propulsion method if we're sending humans. Space is a hostile place,

Here's a question... how long does it take sunlight to reach Earth? This sounds like a strange question, but think about it. Sunlight travels at the speed of light. Photons emitted from the surface of the Sun need to travel across the vacuum of space to reach our eyes. The short answer is that it takes sunlight an average of 8 minutes and 20 seconds to travel from the Sun to the Earth. If the Sun suddenly disappeared from the Universe (not that this could actually happen, don't panic), it would take a little more than 8 minutes before you realized it was time to put on a sweater. Here's the math. We orbit the Sun at a distance of about 150 million km. Light moves at 300,000 kilometers/second. Divide these and you get 500 seconds, or 8 minutes and 20 seconds. This is an average number. Remember, the Earth follows an elliptical orbit around the Sun, ranging from 147 million to 152 million km. At its closest point, sunlight only takes 490 seconds to reach Earth. And then at the most distant point, it takes 507 seconds for sunlight to make the journey. But the story of light gets even more interesting, when you think about the journey light needs to make inside the Sun. You probably know that photons are created by fusion reactions inside the Sun's core. They start off as gamma radiation and then are emitted and absorbed countless times in the Sun's radiative zone, wandering around inside the massive star before they finally reach the surface. What you probably don't know, is that these photons striking your eyeballs were ACTUALLY created tens of thousands of years ago and it took that long for them to be emitted by the sun. Once they escaped the surface, it was only a short 8 minutes for those photons to cross the vast distance from the Sun to the Earth As you look outward into space, you're actually looking backwards in time. The light you see from your computer is nanoseconds old. The light reflected from the surface of the Moon takes only a second to reach Earth. The Sun is more than 8 light-minutes away. And so, if the light from the nearest star (Alpha Centauri) takes more than 4 years to reach us, we're seeing that star 4 years in the past. There are galaxies millions of light-years away, which means the light we're seeing left the surface of those stars millions of years ago. For example, the galaxy M109 is located about 83.5 million light-years away. If aliens lived in those galaxies, and had strong enough telescopes, they would see the Earth as it looked in the past. They might even see dinosaurs walking on the surface. We have written many articles about the Sun for Universe Today. Here's an article about the color of the Sun, and here are some interesting facts about the Sun. If you'd like more info on the Sun, check out NASA's Solar System Exploration Guide on the Sun, and here's a link to the SOHO mission homepage, which has the latest images from the Sun. We've also recorded an episode of Astronomy Cast all about the Sun. Listen here, Episode 30: The Sun, Spots and All. Source: NASA

This article was originally written in 2008, but we created a cool video to go along with it yesterday Let's find out why Pluto is no longer considered a planet. Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X. Only 22 at the time, Tombaugh was given the laborious task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an asteroid, comet or planet, would appear to jump from one photograph to the next. After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in Oxford, England (no, it wasn't named after the Disney character, but the Roman god of the underworld). The Solar System now had 9 planets. Astronomers weren't sure about Pluto's mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune. Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the Earth to the Sun). Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more in the Kuiper Belt. And according to the new rules, Pluto is not a planet. It's just another Kuiper Belt object. Here's the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech astronomer Mike Brown and his team is only a little smaller than Pluto. And there are several other Kuiper Belt objects in that same classification. Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt. And in 2005, Mike Brown and his team dropped the bombshell. They had discovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the object was later designated as Eris. Since its discovery, astronomers have determined that Eris' size is approximately 2,600 km (1,600 miles) across. It also has approximately 25% more mass than Pluto. With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic. Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would have actually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as the largest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale,