Guide to Space Archives

January 2, 2015March 1, 2017 by Fraser Cain

What Is The Biggest Thing in The Universe?

Think big. Really big. Like, cosmic big. How big can things in the Universe get? Is a galaxy big? What about a supercluster? What is the biggest thing in the Universe?

Our observable Universe is a sphere 96 billion light-years across, and the entire Universe might be infinite in size. Which is a hoarders dream walk-in closet space stuffed full of “things”. It’s loaded down with so much stuff, we’ve even given up naming things individually and now just spew out a list of letters and numbers to try and keep track of it all.

So, as is traditional, in a fit of adolescent OCD and one-upmanship reserved generally for things like tanks, planes and guns, we’re drawn to the question… What’s the biggest thing in the Universe. Well, 14 year old Fraser Cain, put down your copy of “Weapons and Warfare Volume 3” which you picked up at the dollar store as part of an incomplete set, as this is going to get a little tricky.

It all depends on what you mean by a “thing”. The biggest physical object is probably a star. The largest possible red giant star could be as big as 2,100 times the size our Sun. Placed inside our own Solar System, a monster star like this would extend out past the orbit of Saturn. That’s big, but we might be able to get even bigger if we’re willing to get past the idea that a “thing” has to be a homogeneous physical object.

Consider the regions around supermassive black holes. Within our own galaxy, things are pretty quiet, but around actively feeding black holes, there can be disks of material with such temperature and density that they act like the core of a star, fusing hydrogen into helium. Which, purely based on high volumetric density of pure awesome, I’m going to call a thing. An accretion disk around a quasar could be light days across, extending well past the orbit of Pluto and killing us all, if you dumped it in our Solar System.

If we’re going to be all philosophical about what constitutes a “thing” and you’re not all fussy about physical structure and just want a collection of material held together by gravity, then we can really can make some leaps and bounds in our “who’s got the biggest” measuring contest. Our own galaxy extends up to 120,000 light-years across.

There are much larger galaxies, ones that make the Milky Way look like that cat leash pendant from Men In Black 2. And ours is just one contained within a much larger cluster of galaxies known, rather unimaginatively, as the Local Group. Don’t let the centrist name fool you, this cluster contains around 50 galaxies and measures more than 10 million light-years across.

Partial map of the Local Group of galaxies. Credit: Planet Quest

And we’re just getting started. The Local Group is one part of the Virgo Supercluster. A massive galactic structure that measures 110 million light-years apart. In 2014, astronomers announced that the Virgo Supercluster is just one lobe of an even larger structure, beautifully known as Laniakea, or “Immeasurable heaven” in Hawaiian. The name originated from Nawa’a Napoleon, an associate professor of Hawaiian Language at Kapiolani Community College. It honors the Polynesian sailors using “heavenly knowledge” navigating the Pacific Ocean, reminding us that romance is still alive and well in space and astronomy. Laniakea is centered around the Great Attractor – a mysterious source of gravity drawing galaxies towards it.

I almost forgot about our size contest. So who’s got the biggest space thing? According to buzzkill Ethan Siegel from the Starts With a Bang blog, you can’t actually have a structure that’s as big as Laniakea, and call it a thing. The fine-print reality is that the expansion of the Universe is being accelerated by dark energy. These galaxies are being pushed apart by dark energy faster than gravity can pull them together. So they’d never be able to form into a single object given enough time.

In other words, the largest possible object is a collection of galaxies at the exact size where gravity is just strong enough to overcome the expansive force of dark energy. Beyond that, everything’s getting spread apart, and it’s for our purposes we’re actually going to draw a line and say it’s not quite right to call it a thing. Unless you’d suggest a giant expanse of nothing is a thing… but let’s save that for another episode.

So what do you think? Do you feel like it’s right to call superclusters like Laniakea “a structure”?

December 29, 2014March 1, 2017 by Fraser Cain

What Will We Never See?

Thanks to our powerful telescopes, there are so many places in the Universe we can see. But there are places hidden from us, and places that we’ll never be able to see.

We’re really lucky to live in our Universe with our particular laws of physics. At least, that’s what we keep telling ourselves. The laws of physics can be cruel and unforgiving, and should you try and cross them, they will crush you like a bug.

Here at Universe Today, we embrace our Physics overlords and prefer to focus on the positive, the fact that light travels at the speed of light is really helpful. This allows us to look backwards in time as we look further out. Billions of light-years away, we can see what the Universe looked like billions of years ago. Physics is good. Physics knows what’s best. Thanks physics. And where the hand of physics gives, it can also take away.

There are some parts of the Universe that we’ll never, ever be able to see. No matter what we do. They’ll always remain just out of reach. No matter how much we plead, in some sort of Kafka-esque nightmare, these rules do not appear to have conscience or room for appeal.

As we look outward in the cosmos, we look backwards in time and at the very edge of our vision is the Cosmic Microwave Background Radiation. The point after the Big Bang where everything had cooled down enough so it was no longer opaque. Light could finally escape and travel through a transparent Universe. This happened about 300,000 years after the Big Bang. What happened before that is a mystery. We can calculate what the Universe was like, but we can’t actually look at it. Possibly, we just don’t have the right clearance levels.

On the other end of the timeline, in the distant distant future. Assuming humans, or our Terry Gilliam inspired robot bodies are still around to observe the Universe, there will be a lot less to see. Distance is also out to rain on our sightseeing safari. The expansion of the Universe is accelerating, and galaxies are speeding away from each other faster and faster. Eventually, they’ll be moving away from us faster than the speed of light.

What would you see at the speed of light/

When that happens, we’ll see the last few photons from those distant galaxies, redshifted into oblivion. And then, we won’t see any galaxies at all. Their light will never reach us and our skies will be eerily empty. Just don’t let physics hear a sad tone in your voice, we don’t want to spend another night in the “joy re-education camps”

Currently, we can see a sphere of the Universe that measures 92 billion light-years across. Outside that sphere is more Universe, a hidden, censored Universe. Universe that we can’t see because the light hasn’t reached us yet. Fortunately, every year that goes by, a little less Universe is redacted from the record, and the sphere we can observe gets bigger by one light-year. We can see a little more in all directions.

Finally, let’s consider what’s inside the event horizon of a black hole. A place that you can’t look at, because the gravity is so strong that light itself can never escape it. So by definition, you can’t see what absorbs all its own light. Astronomers don’t know if black holes crunch down to a physical sphere and stop shrinking, or continue shrinking forever, getting smaller and smaller into infinity. Clearly, we can’t look there because we shouldn’t be looking there. They’re terrible places. The possibility of shrinking forever gives me the heebies.

Artistic view of a radiating black hole. Credit: NASA

And so, good news! The chocolate ration has been increased from 40 grams to 25 grams, and our physics overlords are good, can only do good, and always know what’s best for us. In fact, so good that gravity might actually provide us with a tool to “see” these hidden places, but only because “they” want us to.

When black holes form, or massive objects smash into each other, or there are “Big Bangs”, these generate distortions in spacetime called gravitational waves. Like gravity itself, these propagate across the Universe and could be detected.It’s possible we could use gravitational waves to “see” beyond the event horizon of a black hole, or past the Cosmic Microwave Background Radiation.

The problem is that gravitational waves are so faint, we haven’t even detected a single one yet. But that’s probably just a technology problem. In the end, we need a more sensitive observatory. We’ll get there. Alternately we could apply to the laws of physics board of appeals and fill in one of their 2500 page application forms in triplicate and see if we can be granted a rules exception, and maybe just get a tiny little peek behind that veil.

We live an amazing Universe, most of which we’ll never be able to see. But that’s okay, there’s enough we can see to keep us busy until infinity. What law of physics would you like to be granted a special exception to ignore. Tell us in the comments below.

December 25, 2014March 1, 2017 by Fraser Cain

How Many Stars Did It Take To Make Us?

You know the quote, we’re made of stardust. Generation after generation of stars created the materials that make us up. How? And how many stars did it take?

Carl Sagan once said, “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff.” To an average person, this might sound completely bananas. I feel it could easily be adopted into the same dirty realm as “My grandpappy wasn’t no gorilla”.

After all, if my teeth are made of stars, and my toothpaste supplier can be believed, why aren’t they brighter and whiter? If my bones are made of stars, shouldn’t I have this creepy inner glow like the aliens from Cocoon? Does this mean everything I eat is made of stars? And conversely, the waste products of my body then are also made of stars? Shouldn’t all this star business include some cool interstellar powers, like Nova? Also, shouldn’t my face be burning?

When the Big Bang happened, 13.8 billion years ago, the entire Universe was briefly the temperature and pressure of a star. And in this stellar furnace, atoms of hydrogen were fused together to make helium and heavier elements like lithium and a little bit of beryllium.

This all happened between 100 and 300 seconds after the Big Bang, and then the Universe wasn’t star-like enough for fusion to happen any more. It’s like someone set a microwave timer and cooked the heck of the whole business for 5 minutes. DING! Your Universe is done! All the other elements in the Universe, including the carbon in our bodies to the gold in our jewelry were manufactured inside of stars.

But how many stars did it take to make “us”? Main sequence stars, like our own Sun, create elements slowly, but surely within their cores. As we speak, the Sun is relentlessly churning hydrogen into helium. Once when it runs out of hydrogen, it’ll switch to crushing helium into carbon and oxygen. More massive stars keep going up the periodic table, making neon and magnesium, oxygen and silicon. But those elements aren’t in you. Once a regular star gets going, it’ll hang onto its elements forever with its intense gravity. Even after it dies and becomes a white dwarf.

No, something needs to happen to get those elements out. That star needs to explode. The most massive stars, ones with dozens of times the mass of our Sun don’t know when to stop. They just keep on churning more and more massive elements, right on up the periodic table. They keep fusing and fusing until they reach iron in their cores. And as iron is the stellar equivalent of ash, fusion reactions no longer generate energy, and instead require energy. Without the fusion energy pushing against the force of gravity pulling everything inward, the massive star collapses in on itself, creating a neutron star or black hole, or detonating as a supernova.

It’s in this moment, a fraction of a second, when all the heavier elements are created. The gold, platinum, uranium and other rare elements that we find on Earth. All of them were created in supernovae in the past. The materials of everything around you was either created during the Big Bang or during a supernova detonation. Only supernovae “explode” and spread their material into the surrounding nebula. Our Solar System formed within a nebula of hydrogen that was enriched by multiple supernovae. Everything around you was pretty much made in a supernova.

These images taken by the Spitzer Space Telescope show the dust and gas concentrations around a supernova. Credit: NASA/JPL-Caltech

So how many? How many times has this cycle been repeated? We don’t know. Lots. There were the original stars that formed shortly after the Big Bang, and then successive generations of massive stars that formed in various nebulae. Astronomers are pretty sure it was a least 3 generations of supernovae, but there’s no way to know exactly.

Carl Sagan said you’re made of star-stuff. But actually you’re made up mostly of Big Bang stuff and generations of supernova stuff. Tasty tasty supernova stuff.

What’s your favorite supernova remnant? Tell us in the comments below.

December 23, 2014December 23, 2015 by Matt Williams

What is the Average Surface Temperature on Venus?

False color radar topographical map of Venus provided by Magellan. Credit: Magellan Team/JPL/NASA

Venus is often referred to as our “sister planet,” due to the many geophysical similarities that exist between it Earth. For starters, our two planets are close in mass, with Venus weighing in at 4.868 x 10²⁴ kg compared to Earth’s 5.9736×10²⁴kg. In terms of size, the planets are almost identical, with Venus measuring 12,100 km in diameter and Earth 12,742 km.

In terms of density and gravity, the two are neck and neck – with Venus boasting 86.6% of the former and 90.7% of the latter. Venus also has a thick atmosphere, much like our own, and it is believed that both planets share a common origin, forming at the same time out of a condensing clouds of dust particles around 4.5 billion years ago.

However, for all the characteristics these two planets have in common, average temperature is not one of them. Whereas the Earth has an average surface temperature of 14 degrees Celsius, the average temperature of Venus is 460 degrees Celsius. That is roughly 410 degrees hotter than the hottest deserts on our planet.

In fact, at a searing 750 K (477 °C), the surface of Venus is the hottest in the solar system. Venus is closer to the Sun by 108 million km, (about 30% closer than the Earth), but it is mainly due to the planet’s thick atmosphere. Unlike Earth’s, which is composed primarily of nitrogen, oxygen and ozone, Venus’ atmosphere is an incredibly dense cloud of carbon dioxide and sulfur dioxide gas.

The combination of these gases in high concentrations causes a catastrophic greenhouse effect that traps incident sunlight and prevents it from radiating into space. This results in an estimated surface temperature boost of 475 K (201.85 °C), leaving the surface a molten, charred mess that nothing (that we know of) can live on. Atmospheric pressure also plays a role, being 91 times that of what it is here on Earth; and clouds of toxic vapor constantly rain sulfuric acid on the surface.

In addition, the surface temperature on Venus does not vary like it does here on Earth. On our planet, temperatures vary wildly due to the time of year and even more so based on the location on our planet. The hottest temperature ever recorded on Earth was 70.7°C in the Lut Desert of Iran in 2005. On the other end of the spectrum, the coldest temperature ever recorded on Earth was in Vostok, Antarctica at -89.2 C.

But on Venus, the surface temperature is 460 degrees Celsius, day or night, at the poles or at the equator. Beyond its thick atmosphere, Venus’ axial tilt (aka. obliquity) plays a role in this temperature consistency. Earth’s axis is tilted 23.4 ° in relation to the Sun, whereas Venus’ is only tilted by 3 °.

The only respite from the heat on Venus is to be found around 50 km into the atmosphere. It is at that point that temperatures and atmospheric pressure are equal to that of Earth’s. It is for this reason that some scientists believe that floating habitats could be constructed here, using Venus’ thick clouds to buoy the habitats high above the surface. Additionally, in 2014, a group of mission planners from NASA Langely came up with a mission to Venus’ atmosphere using airships.

These habitats could play an important role in the terraforming of Venus as well, acting as scientific research stations that could either fire off the excess atmosphere off into space, or introduce bacteria or chemicals that could convert all the CO2 and SO2 into a hospitable, breathable atmosphere.

Beyond the fact that it is a hot and hellish landscape, very little is known about Venus’ surface environment. This is due to the thick atmosphere, which has made visual observation impossible. The sulfuric acid is also problematic since clouds composed of it are highly reflective of visible light, which prevents optical observation. Probes have been sent to the surface in the past, but the volatile and corrosive environment means that anything that lands there can only survive for a few hours.

*3-D perspective of the Venusian volcano, Maat Mons generated from radar data from NASA’s Magellan mission. Credit: Magellan Team/NASA/JPL*

What little we know about the planet’s surface has come from years worth of radar imaging, the most recent of which was conducted by NASA’s Magellan spacecraft (aka. the Venus Radar Mapper). Using synthetic aperture radar, the robotic space probe spent four years (1990-1994) mapping the surface of Venus and measuring its gravitational field before its orbit decayed and it was “disposed of” in the planet’s atmosphere.

The images provided by this and other missions revealed a surface dominated by volcanoes. There are at least 1,000 volcanoes or volcanic centers larger than 20 km in diameter on Venus’ harsh landscape. Many scientists believe Venus was resurfaced by volcanic activity 300 to 500 million years ago. Lava flows are a testament to this, which appear to have produced channels of hardened magma that extend for hundreds of km in all directions. The mixture of volcanic ash and the sulfuric acid clouds is also known to produce intense lightning and thunder storms.

The temperature of Venus is not the only extreme on the planet. The atmosphere is constantly churned by hurricane force winds reaching 360 kph. Add to that the crushing air pressure and rainstorms of sulfuric acid, and it becomes easy to see why Venus is such a barren, lifeless rock that has been hard to explore.

We have written many articles about Venus for Universe Today. Here are some interesting facts about Venus, and here’s an article about Venus Greenhouse Effect. And here is an article about the many interesting pictures taken of Venus over the past few decades.

If you’d like more information on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

Reference:
NASA

December 22, 2014March 1, 2017 by Fraser Cain

How Do We Know How Old Everything Is?

We hear that rocks are a certain age, and stars are another age. And the Universe itself is 13.7 billion years old. But how do astronomers figure this out?

I know it’s impolite to ask, but, how old are you? And how do you know? And doesn’t comparing your drivers license to your beautiful and informative “Year In Space” calendar feel somewhat arbitrary? How do we know old how everything is when what we observe was around long before calendars, or the Earth, or even the stars?

Scientists have pondered about the age of things since the beginning of science. When did that rock formation appear? When did that dinosaur die? How long has the Earth been around? When did the Moon form? What about the Universe? How long has that party been going on? Can I drink this beer yet, or will I go blind? How long can Spam remain edible past its expiration date?

As with distance, scientists have developed a range of tools to measure the age of stuff in the Universe. From rocks, to stars, to the Universe itself. Just like distance, it works like a ladder, where certain tools work for the youngest objects, and other tools take over for middle aged stuff, and other tools help to date the most ancient.

Let’s start with the things you can actually get your hands on, like plants, rocks, dinosaur bones and meteorites. Scientists use a technique known as radiometric dating. The nuclear age taught us how to blow up stuff real good, but it also helped understand how elements transform from one element to another through radioactive decay.

For example, there’s a version of carbon, called carbon-14. If you started with a kilo of it, after about 5,730 years, half of it would have turned into carbon-12. And then by 5,730 more years, you’d have about ¼ carbon-14 and ¾ carbon-12.

A list of the elements with their corresponding visible light emission spectra. Image Credit: MIT Wavelength Tables, NIST Atomic Spectrum Database, umop.net

This is known as an element’s half-life. And so, if you measure the ratio of carbon-12 to carbon-14 in a dead tree, for example, you can calculate how long ago it lived. Different elements work for different ages. Carbon-14 works for the last 50,000 years or so, while Uranium-238 has a half-life of 4.5 billion years, and will let you date the most ancient of rocks. But what about the stuff we can’t touch, like stars?

When you use a telescope to view a star, you can break up its light into different colors, like a rainbow. This is known as a star’s spectra, and if you look carefully, you can see black lines, or gaps, which correspond to certain elements. Since they can measure the ratios of different elements, astronomers can just look at a star to see how old it is. They can measure the ratio of uranium-238 to lead-206, and know how long that star has been around. How astronomers know the age of the Universe itself is one of my favorites, and we did a whole episode on this.

Artist's conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration - D. Ducros — Artist’s conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration – D. Ducros

The short answer is, they measure the wavelength of the Cosmic Microwave Background Radiation. Since they know this used to be visible light, and has been stretched out by the expansion of the Universe, they can extrapolate back from its current wavelength to what it was at the beginning of the Universe. This tells them the age is about 13.8 billion years. Radiometric dating was a revolution for science. It finally gave us a dependable method to calculate the age of anything and everything, and finally figure out how long everything has been around.

So, fan of our videos. How old are you? Tell us in the comments below.
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December 18, 2014March 1, 2017 by Fraser Cain

Why Is Venus So Horrible?

Venus really sucks. It’s as hot as an oven with a dense, poisonous atmosphere. But how did it get that way?

Venus sucks. Seriously, it’s the worst. The global temperature is as hot as an oven, the atmospheric pressure is 90 times Earth, and it rains sulfuric acid. Every part of the surface of Venus would kill you dead in moments.

Let’s push Venus into the Sun and be done with that terrible place. Its proximity is lowering our real estate values and who knows what sort of interstellar monstrosities are going to set up shop there, and be constantly knocking on our door to borrow the mower, or a cup or sugar, or sneak into our yard at night and eat all our dolphins.

You might argue that Venus is worth saving because it’s located within the Solar System’s habitable zone, that special place where water could exist in a liquid state on the surface. But we’re pretty sure it doesn’t have any liquid water. Venus may have been better in the past, clearly it started hanging out with wrong crowd, taking a bad turn down a dark road leading it to its current state of disrepair.

Could Venus have been better in the past? And how did it go so wrong? In many ways, Venus is a twin of the Earth. It’s almost the same size and mass as the Earth, and it’s made up of roughly the same elements. And if you stood on the surface of Venus, in the brief moments before you evacuated your bowels and died horribly, you’d notice the gravity feels pretty similar.

In the ancient past, the Sun was dimmer and cooler than it is now. Cool enough that Venus was much more similar to Earth with rivers, lakes and oceans. NASA’s Pioneer spacecraft probed beneath the planet’s thick clouds and revealed that there was once liquid water on the surface of Venus. And with liquid water, there could have been life on the surface and in those oceans.

Here’s where Venus went wrong. It’s about a third closer to the Sun than Earth, and gets roughly double the solar radiation. The Sun has been slowly heating up over the millions and billions of years. At some point, the planet reached a tipping point, where the water on the surface of Venus completely evaporated into the atmosphere.

Water vapor is a powerful greenhouse gas, and this only increased the global temperature, creating a runaway greenhouse effect on Venus. The ultraviolet light from the Sun split apart the water vapor into oxygen and hydrogen. The hydrogen was light enough to escape the atmosphere of Venus into space, while the oxygen recombined with carbon to form the thick carbon dioxide atmosphere we see today. Without that hydrogen, Venus’ water is never coming back.

Are you worried about our changing climate doing that here? Don’t panic. The amount of carbon dioxide released into the atmosphere of Venus is incomprehensible. According to the IPCC, the folks studying global warming, human activities have no chance of unleashing runaway global warming. We’ll just have the regular old, really awful global warming. So, it’s okay to panic a bit, but do it in the productive way that results in your driving your car less.

The Sun is still slowly heating up. And in a billion years or so, temperatures here will get hot enough to boil the oceans away. And then, Earth and Venus will be twins again and then we can push them both into the Sun.

I know, I said the words “climate change”. Feel free to have an argument in the comments below, but play nice and bring science.

December 15, 2014April 19, 2016 by Matt Williams

What is the Average Surface Temperature of the Planets in our Solar System?

Artist's impression of the planets in our solar system, along with the Sun (at bottom). Credit: NASA

It’s is no secret that Earth is the only inhabited planet in our Solar System. All the planets besides Earth lack a breathable atmosphere for terrestrial beings, but also, many of them are too hot or too cold to sustain life. A “habitable zone” which exists within every system of planets orbiting a star. Those planets that are too close to their sun are molten and toxic, while those that are too far outside it are icy and frozen.

But at the same time, forces other than position relative to our Sun can affect surface temperatures. For example, some planets are tidally locked, which means that they have one of their sides constantly facing towards the Sun. Others are warmed by internal geological forces and achieve some warmth that does not depend on exposure to the Sun’s rays. So just how hot and cold are the worlds in our Solar System? What exactly are the surface temperatures on these rocky worlds and gas giants that make them inhospitable to life as we know it?

Mercury:

Of our eight planets, Mercury is closest to the Sun. As such, one would expect it to experience the hottest temperatures in our Solar System. However, since Mercury also has no atmosphere and it also spins very slowly compared to the other planets, the surface temperature varies quite widely.

What this means is that the side exposed to the Sun remains exposed for some time, allowing surface temperatures to reach up to a molten 465 °C. Meanwhile, on the dark side, temperatures can drop off to a frigid -184°C. Hence, Mercury varies between extreme heat and extreme cold and is not the hottest planet in our Solar System.

Venus:

That honor goes to Venus, the second closest planet to the Sun which also has the highest average surface temperatures – reaching up to 460 °C on a regular basis. This is due in part to Venus’ proximity to the Sun, being just on the inner edge of the habitability zone, but also to Venus’ thick atmosphere, which is composed of heavy clouds of carbon dioxide and sulfur dioxide.

These gases create a strong greenhouse effect which traps a significant portion of the Sun’s heat in the atmosphere and turns the planet surface into a barren, molten landscape. The surface is also marked by extensive volcanoes and lava flows, and rained on by clouds of sulfuric acid. Not a hospitable place by any measure!

Earth:

Earth is the third planet from the Sun, and so far is the only planet that we know of that is capable of supporting life. The average surface temperature here is about 14 °C, but it varies due to a number of factors. For one, our world’s axis is tilted, which means that one hemisphere is slanted towards the Sun during certain times of the year while the other is slanted away.

This not only causes seasonal changes, but ensures that places located closer to the equator are hotter, while those located at the poles are colder. It’s little wonder then why the hottest temperature ever recorded on Earth was in the deserts of Iran (70.7 °C) while the lowest was recorded in Antarctica (-89.2 °C).

Mars:

Mars’ average surface temperature is -55 °C, but the Red Planet also experiences some variability, with temperatures ranging as high as 20 °C at the equator during midday, to as low as -153 °C at the poles. On average though, it is much colder than Earth, being just on the outer edge of the habitable zone, and because of its thin atmosphere – which is not sufficient to retain heat.

In addition, its surface temperature can vary by as much as 20 °C due to Mars’ eccentric orbit around the Sun (meaning that it is closer to the Sun at certain points in its orbit than at others).

Jupiter:

Since Jupiter is a gas giant, it has no solid surface, so it has no surface temperature. But measurements taken from the top of Jupiter’s clouds indicate a temperature of approximately -145°C. Closer to the center, the planet’s temperature increases due to atmospheric pressure.

At the point where atmospheric pressure is ten times what it is on Earth, the temperature reaches 21°C, what we Earthlings consider a comfortable “room temperature”. At the core of the planet, the temperature is much higher, reaching as much as 35,700°C – hotter than even the surface of the Sun.

Saturn:

Due to its distance from the Sun, Saturn is a rather cold gas giant planet, with an average temperature of -178 °Celsius. But because of Saturn’s tilt, the southern and northern hemispheres are heated differently, causing seasonal temperature variation.

And much like Jupiter, the temperature in the upper atmosphere of Saturn is cold, but increases closer to the center of the planet. At the core of the planet, temperatures are believed to reach as high as 11,700 °C.

Uranus:

Uranus is the coldest planet in our Solar System, with a lowest recorded temperature of -224°C. Despite its distance from the Sun, the largest contributing factor to its frigid nature has to do with its core.

Much like the other gas giants in our Solar System, the core of Uranus gives off far more heat than is absorbed from the Sun. However, with a core temperature of approximately 4,737 °C, Uranus’ interior gives of only one-fifth the heat that Jupiter’s does and less than half that of Saturn.

Neptune:

With temperatures dropping to -218°C in Neptune’s upper atmosphere, the planet is one of the coldest in our Solar System. And like all of the gas giants, Neptune has a much hotter core, which is around 7,000°C.

In short, the Solar System runs the gambit from extreme cold to extreme hot, with plenty of variance and only a few places that are temperate enough to sustain life. And of all of those, it is only planet Earth that seems to strike the careful balance required to sustain it perpetually.

Universe Today has many articles on the temperature of each planet, including the temperature of Mars and the temperature of Earth.

You may also want to check out these articles on facts about the planets and an overview of the planets.

NASA has a great graphic here that compares the temperatures of all the planets in our Solar System.

Astronomy Cast has episodes on all planets including Mercury.

December 15, 2014March 1, 2017 by Fraser Cain

Why Is Space Black?

Since there are stars and galaxies in all directions, why is space black? Shouldn’t there be a star in every direction we look?

Imagine you’re in space. Just the floating part, not the peeing into a vacuum hose or eating that funky “ice cream” from foil bags part. If you looked at the Sun, it would be bright and your retinas would crisp up. The rest of the sky would be a soothing black, decorated with tiny little less burny points of light.

If you’ve done your homework, you know that space is huge. It even be infinite, which is much bigger than huge. If it is infinite you can imagine looking out into space in any direction and there being a star. Stars would litter everything. Dumb stars everywhere wrecking the view. It’s stars all the way down, people.

So, shouldn’t the entire sky be as bright as a star, since there’s a star in every possible minute direction you could ever look in? If you’ve ever asked yourself this question, you probably won’t be surprised to know you’re not the first. Also, at this point you can tell people you were wondering about it and they’ll never know you just watched it here and then you can sound wicked smart and impress all those dudes.

This question was famously asked by the German astronomer Heinrich Wilhelm Olbers who described it in 1823. We now call this Olbers’ Paradox after him. Here let me give you a little coaching, you’ll start your conversation at the party with “So, the other day, I was contemplating Olbers’ Paradox… Oh what’s that? You don’t know what it is… oh that’s so sweet!”. The paradox goes like this: if the Universe is infinite, static and has existed forever, then everywhere you look should eventually hit a star.

Our experiences tell us this isn’t the case. So by proposing this paradox, Olbers knew the Universe couldn’t be infinite, static and timeless. It could be a couple of these, but not all three. In the 1920s, debonair man about town, Edwin Hubble discovered that the Universe isn’t static. In fact, galaxies are speeding away from us in all directions like we have the cooties.

This led to the theory of the Big Bang, that the Universe was once gathered into a single point in time and space, and then, expanded rapidly. Our Universe has proven to not be static or timeless. And so, PARADOX SOLVED!

Here’s the short version. We don’t see stars in every direction because many of the stars haven’t been around long enough for their light to get to us. Which I hope tickles your brain in the way it does mine. Not only do we have this incomprehensibly massive size of our Universe, but the scale of time we’re talking about when we do these thought experiments is absolutely boggling. So, PARADOX SOLVED!

Well, not exactly. Shortly after the Big Bang, the entire Universe was hot and dense, like the core of a star. A few hundred thousand years after the Big Bang, when the first light was able to leap out into space, everything, in every direction was as bright as the surface of a star.

Cosmic microwave background. Image credit: WMAP

So, in all directions, we should still be seeing the brightness of a star.. and yet we don’t. As the Universe expanded, the wavelengths of that initial visible light were stretched out and out and dragged to the wide end of the electromagnetic spectrum until they became microwaves. This is Cosmic Microwave Background Radiation, and you guessed it, we can detect it in every direction we can look in.

So Olbers’ instinct was right. If you look in every direction, you’re seeing a spot as bright as a star, it’s just that the expansion of the Universe stretched out the wavelengths so that the light is invisible to our eyes. But if you could see the Universe with microwave detecting eyes, you’d see this: brightness in every direction.

Did you come up with Olbers’ Paradox too? What other paradoxes have puzzled you?

December 11, 2014February 15, 2018 by Matt Williams

What Causes Day and Night?

Image of the Sunrise Solstice captured over Stonehenge. Image Credit: Max Alexander/STFC/SPL

For most of here on planet Earth, sunrise, sunset, and the cycle of day and night (aka. the diurnal cycle) are just simple facts of life. As a result of seasonal changes that happen with every passing year, the length of day and night can vary – and be either longer or shorter – by just a few hours. But in some regions of the world (i.e. the poles) the Sun does not set during certain times of the year. And there are also seasonal periods where a single night can last many days.

Naturally, this gives rise to certain questions. Namely, what causes the cycle of day and night, and why don’t all places on the planet experience the same patterns? As with many other seasonal experiences, the answer has to do with two facts: One, the Earth rotates on its axis as it orbits the Sun. And two, the fact that Earth’s axis is tilted.

Earth’s Rotation:

Earth’s rotation occurs from west to east, which is why the Sun always appears to be rising on the eastern horizon and setting on the western. If you could view the Earth from above, looking down at the northern polar region, the planet would appear to be rotating counter-clockwise. However, viewed from the southern polar region, it appears to be rotating clockwise.

The Earth rotates once in about 24 hours with respect to the Sun and once every 23 hours 56 minutes and 4 seconds with respect to the stars. What’s more, its central axis is aligned with two stars. The northern axis points outward to Polaris, hence why it is called “the North Star”, while its southern axis points to Sigma Octantis.

Axial Tilt:

As already noted, due to the Earth’s axial tilt (or obliquity), day and night are not evenly divided. If the Earth’s axis were perpendicular to its orbital plane around the Sun, all places on Earth would experience equal amounts of day and night (i.e. 12 hours of day and night, respectively) every day during the year and there would be no seasonal variability.

Instead, at any given time of the year, one hemisphere is pointed slightly more towards the Sun, leaving the other pointed away. During this time, one hemisphere will be experiencing warmer temperatures and longer days while the other will experience colder temperatures and longer nights.

Seasonal Changes:

Of course, since the Earth is rotating around the Sun and not just on its axis, this process is reversed during the course of a year. Every six months, the Earth undergoes a half orbit and changes positions to the other side of the Sun, allowing the other hemisphere to experience longer days and warmer temperatures.

Precession of the Equinoxes. Image credit: NASA — *Artist’s rendition of the Earth’s rotation and the precession of the Equinoxes. Credit: NASA*

Consequently, in extreme places like the North and South pole, daylight or nighttime can last for days. Those times of the year when the northern and southern hemispheres experience their longest days and nights are called solstices, which occur twice a year for the northern and southern hemispheres.

The Summer Solstice takes place between June 20th and 22nd in the northern hemisphere and between December 20th and 23rd each year in the southern hemisphere. The Winter Solstice occurs at the same time but in reverse – between Dec. 20th and 23rd for the northern hemisphere and June 20th and 22nd for the southern hemisphere.

According to NOAA, around the Winter Solstice at the North Pole there will be no sunlight or even twilight beginning in early October, and the darkness lasts until the beginning of dawn in early March. Conversely, around the Summer Solstice, the North Pole stays in full sunlight all day long throughout the entire summer (unless there are clouds). After the Summer Solstice, the sun starts to sink towards the horizon.

Another common feature in the cycle of day and night is the visibility of the Moon, the stars, and other celestial bodies. Technically, we don’t always see the Moon at night. On certain days, when the Moon is well-positioned between the Earth and the Sun, it is visible during the daytime. However, the stars and other planets of our Solar System are only visible at night after the Sun has fully set.

Astrophoto: Night Sky by Sam Crimmin — *“Night Sky”. On a clear night, the stars and the glowing band of the Milky Way Galaxy are generally visible. Credit: Sam Crimmin*

The reason for this is because the light of these objects is too faint to be seen during daylight hours. The Sun, being the closest star to us and the most radiant object visible from Earth, naturally obscures them when it is overhead. However, with the Earth tilted away from the Sun, we are able to see the Moon radiating the Sun’s light more clearly, and the stars light is detectable.

On an especially clear night, and assuming light pollution is not a major factor, the glowing band of the Milky Way and other clouds of dust and gas may also be visible in the night sky. These objects are more distant than the stars in our vicinity of the Galaxy, and therefore have less luminosity and are more difficult to see.

Another interesting thing about the cycle of day and night is that it is getting slower with time. This is due to the tidal effects the Moon has on Earth’s rotation, which is making days longer (but only marginally). According to atomic clocks around the world, the modern day is about 1.7 milliseconds longer than it was a century ago – a change which may require the addition of more leap seconds in the future.

We have many interesting articles on Earth’s Rotation here at Universe Today. To learn more about solstices here in Universe Today, be sure to check out our articles on the Shortest Day of the Year and the Summer Solstice.

More information can be found at NASA, Seasons of the Year, The Sun at Solstice

Check out this podcast at Astronomy Cast: The Life of the Sun

December 11, 2014March 1, 2017 by Fraser Cain

What Does Space Really Look Like?

When you see the beautiful pictures from the Hubble Space Telescope, you’re looking at a lie. They’re specially colored images, done for science. But what does space really look like?

Do you love the beautiful pictures from the Hubble Space Telescope? Do you ever wonder what it would look like to fly through space and see places like the Orion Nebula up close? Just imagine hiding the Enterprise in the Mutara Nebula, and getting the jump on Khan? Have you ever wondered… what does this stuff actually look like? Looks like we’re back to wrecking sci-fi Christmas again, as I’ve got some bad news.

Nothing, nothing will ever look as cool as the pictures you see on your computer, or even have the same colors. If you were flying right through the Orion Nebula, it wouldn’t look anything like the pictures. In fact, it would kinda suck.

When looking out into the night sky with your own eyeballs, you don’t see any beautiful nebulousness. Just the stars and the faint glow of the Milky Way. You might be able to see a few fuzzy bits, hint of nebulae, galaxies and star clusters. We’re back to a familiar problem, which those of you who are considering Venus as a vacation spot know too well. We’re made out of meat, and in this case, it’s certainly not doing us any favors.

Imagine building a camera out of meat. Pop into a deli, grab a fistful of cold cuts, a pickled egg, and a light sensor, and make that into a camera. Well, that’s your eyes. With the modern advances in camera technologies, we’ve learned that apparently meat cameras are not great cameras.

The biggest advantage to the inorganic kind is that they can gather light for minutes and even hours, soaking up all the photons streaming from a distant object. They, do however, make terrible sandwiches. For example, the famous Hubble Deep Field photograph, which peered into a seemingly empty part of space, turned up thousands of galaxies. Hubble stared for more than 130 hours to create this image.

Our meat cameras refresh themselves every few seconds. Even in the darkest skies, with the most perfectly light-adjusted eyes, if you keep your eyes perfectly still and stare at a spot in space, you can’t gather more than 15-20 seconds of light with your eyes. So we’ll never see these objects because they’re so faint and deliver such a tiny amount of light for every second you stare at them.

But sure, what if you got close? What if I stuck my meat camera on a tripod right outside one of these gaseous structures. Here’s the crazy part. Nebulae never get any brighter even as you get closer. In optics, there’s a rule called “the conservation of surface brightness”. As you get closer to a nebula, it also gets bigger in the sky. The increased brightness is spread out over a larger area, and the average brightness remains exactly the same. You could be right beside the Orion Nebula, and it wouldn’t look any brighter or majestic than we see it from here on Earth. In other words… it would still suck.

But what about the colors? Here’s where astronomers are lying to you in a grand conspiracy of Roswellian proportions. So, watch out for those black helicopters, it’s time for another meeting of the Guide To Space Tinfoil Hat Society.

Charged Coupled Devices (CCD) for Ultra-Violet and Visible Detection. Credit: NASA

Astronomers generally use black-and-white CCD cameras to make their observations. Then they’ll put filters in front of their cameras to only let through very specific wavelengths of light. Those filters can match the specific colors that make up the visible spectrum: red, blue and green. But usually they’re using filters that reveal scientific information. For example, astronomers want to detect the presence of hydrogen, oxygen and sulfur in a nebula. They’ll use one filter that reveals each one of the elements. And then in a program like Photoshop, they’ll assign red to hydrogen, blue to oxygen and green to sulfur. The resulting image can look beautiful, but the colors have nothing to do with reality. That’s right, your inspirational desktop of the week is a lie.

True color images typically have no value for astronomers, but occasionally they’ll throw us a bone. They’ll produce an image using red, blue, and green filters which roughly match the capabilities of the human eye. And NASA’s Curiosity rover has a pair of color cameras, which allow it to capture images of the surface of Mars that match what you might see if you were standing on the surface of the planet… Because that robot gets us, I mean, he really gets us.

I’m sorry to be the bearer of bad news. You’ll never be able to see a nebula more beautifully with your own eyes than you do right now. But good news! Those pictures are amazingly beautiful and you don’t have to wait to see them up close!

You tell us. Even though we’ve revealed this terrible secret, what would you still want to see up close?