Artist's conception of the Milky Way galaxy. Credit: Nick Risinger
When you look up at the night sky, assuming conditions are just right, you might just catch a glimpse of a faint, white band reaching across the heavens. This band, upon closer observation, looks speckled and dusty, filled with a million tiny points of light and halos of glowing matter. What you are seeing is the Milky Way, something that astronomers and stargazers alike have been staring up at since the beginning of time.
But just what is the Milky Way? Well, simply put, it is the name of the barred spiral galaxy in which our solar system is located. The Earth orbits the Sun in the Solar System, and the Solar System is embedded within this vast galaxy of stars. It is just one of hundreds of billions of galaxies in the Universe, and ours is called the Milky Way because the disk of the galaxy appears to be spanning the night sky like a hazy band of glowing white light. Continue reading “What is the Milky Way?”
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.
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.
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.
Galaxies spinning farther and farther away from each other
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.
A false-color, visible-light image of Comet ISON taken with Hubble’s Wide Field Camera 3. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
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.
Artists concept of a space elevator. Credit: Caltech
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.
Credit: Niki GaidaWhen 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.
The Sun. Credit and copyright: César Cantú.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.
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.
Solar Eclipse as seen by Hinode SatelliteYou’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.
Phases of Venus. Image credit: ESOOne 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, just like how we see the Moon.
I hope this gives you a better understanding of why the Moon goes through its phases every month, and the interesting relationship between the Earth, the Sun, and the Moon.
