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Are we seeing some galaxies upside down?

"Spiral galaxies: they seem to appear in both right- and left-handed spirals. Does that mean we are seeing the “top” of some and “bottoms” of others? Or is the question meaningless because there is no agreed viewing point?"

Well, there’s certainly no agreed viewing point in space, and so that does put a bit of a tangle into easily assessing if we’re seeing a galaxy “upside down” or not, but there is a workaround - it’s just a little time intensive.

Part of the reason we set North to be “up” for the Earth is because of the way the Earth spins. There’s a physics framing we use to determine which way is “up” for any spinning object, which is the “Right Hand Rule” - basically, we can use our right hand to find the direction we’ll have as “up”. If you orient your fingers so that they go along with the direction of motion, your thumb points “up”. If you had a record player that ran counterclockwise, you could curl your fingers to point along with it, and your thumb would point straight up into the air, and we’d be seeing the “top” of that record.

If you have a record player that spins clockwise (most do spin this way in fact), then we’d have to turn our hands upside down to point the tips of our fingers along the direction of spin, and so in that case we’d be looking at the “bottom” of the spinning object.

If you use this convention, then we can use the direction a galaxy is spinning to figure out we’re seeing them from the top or the bottom. This is not a measurement we have for every galaxy, because identifying both the direction of spin and its magnitude requires relatively lengthy observations, and they haven’t been done for every single galaxy, but many of the nearest galaxies have had their rotations measured.

What we found in doing that is that almost all galaxies have spiral arms that trail their spin. So very much like holding a ribbon and twirling, a galaxy’s spiral arms lay behind the direction of spin, and so when we see galaxies with arms winding clockwise or counterclockwise, the odds are pretty good that we’re just seeing one of them “upside down”.

We do of course have exceptions- because nothing in space can fit perfectly into boxes. One particular galaxy, NGC 4622, has spiral arms going in both directions; clockwise and counterclockwise. This particular galaxy was very difficult to sort out which way it was rotating, because it was facing us nearly exactly. Our best methods of finding rotations rely on things moving towards or away from us, and this one is very nearly doing exactly none of that kind of motion. But however it was spinning, because there are spiral arms going in both directions, one of those set of spiral arms has to be pointing “the wrong way”.

After some very careful observations with Hubble, it turned out it was the outer set of arms; instead of trailing the spin, they’re leading; meaning that they’re pointed in the direction of travel, like a jouster’s lance. Strange indeed. But it’s likely this galaxy is living through a weird time in its life - it seems like it’s just devoured another galaxy, which can cause pretty dramatic disturbances to a galaxy’s shape, and in this case, something about that collision probably constructed some “backwards” spiral arms!

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#astroquizzical #galaxies #spiral galaxies #spiral arms #astronomy #astrophysics

Does the Moon have a solid metal core?

"As I understand it the Earth's metallic core is still molten. Does the Moon have a metallic core that’s not molten and cooled? With the Moon’s metallic core does it echo metallic sound? Thank you."

Part of the Earth’s core is molten! The Earth’s internal structure is rather complex, and comes in four broad segments. The crust is the outermost, and the best place to live on the basis of not being outrageously toasty.

Below the crust is the mantle, a high pressure, high temperature zone with large, slow currents (though the mantle is not so much a liquid as a viscous solid) that power our plate tectonics. Upwellings in hotter mantle material can eventually poke through the crust and give us a fancy new volcano.

Beneath that, for the Earth, is the core, which itself is divided into the “inner core” and the “outer core”. The inner core is solid, and likely made nearly entirely of iron, with a little nickel added for seasoning. The outer core of the Earth is the liquid portion, and surrounds the solid inner core. The inner and outer cores together fill a little over half of the Earth’s interiors, and we’ve been able to map them using earthquakes, which is a really neat trick.

When an earthquake occurs, it releases seismic waves, which, if you’re close to the location of the earthquake, can be extremely noticeable as they shake the ground underneath you. However, these waves don’t just travel along the surface - some of them travel inwards. They then travel through different layers of the Earth’s interior in different fashions, and as they encounter boundaries between zones, they can either reflect, deflect, or be stopped, much like light can be reflected or bent as it travels between air and water. Seismometers around the world can then detect the arrival of these reflected or deflected waves, and over time we’ve built up the model of the interior of the Earth that we have today.

During the Apollo era, seismometers akin to the ones we use to measure earthquakes on Earth were placed on the Moon to monitor for moonquakes, with the hope of learning more about the Moon’s interior. We don’t have the same global network of seismometers on the Moon, but some seismometers is certainly better than none seismometers, and we have indeed been able to learn about the Moon’s internal structure from them.

We learned that the Moon has an internal core that’s quite similar to the Earth in terms of its composition; while the Moon’s “mantle” is cold, it still has a warm core, with an interior solid metallic core, and an outer liquid core. However, this core is fairly small - while the Earth’s fills half of its radius, the Moon’s only fills about 20%.

From these Apollo seismometers, we also learned that moonquakes are fairly common; some are triggered from meteorite strikes to the surface, some by the extreme temperature swings at the transition from the sun-illuminated day side to night, where the temperature drops from 224F (106C) to -298F (-183C), and some deeper in the core, which might be due to the influence of the Earth on the Moon.

But these echos on the Moon are the same as earthquakes reverberating around the Earth; no particularly metallic noises involved. That’s not to say you can’t find a place to clang around in the solar system - if you really want a metallic noise when you hit an object, we should try for Psyche, a metal asteroid which might be the remains of some destroyed proto-planet’s iron & nickel core. We don’t have long to wait; Psyche is the subject of a future mission, set to launch in 2022, and to arrive in 2026.

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#astronomy #astrophysics #the moon #structure of the moon #astroquizzical #solar system

Is a star hotter than lava?

Generally speaking, the answer to this one is yes: and we have a bit of a hint already from the phase of matter they’re each in.

Stars, generally speaking, are made of plasma; a state of matter so energetic that the electrons and protons which normally make up a hydrogen atom have split apart and are careening off of things separately. Lava, by contrast, is a liquid. It’s a weird, exceptionally viscous liquid, but fundamentally it’s molten rock, and is therefore technically a liquid until it cools.

Your typical star, a round blob of plasma in space, generates heat and light by fusing elements in its core. Generally speaking, this is the fusion of hydrogen into helium. The temperature of the star is controlled by the rate at which fusion occurs in the core. The faster helium is built up from hydrogen, the more light is produced, and the hotter the star overall.

That fusion rate, in turn, is controlled by the mass of the star. The more massive the star, the more it can compress material down into higher densities at its core, and it’s this increase in density that results in a faster fusion process.

So when we’re comparing the temperature of stars to the temperature of lava, the exact results will vary depending on how massive your star is. However, for any star which is fusing hydrogen into helium, even the lowest mass stars which are the coolest of the lot, the surface temperature (defined as the point at which light is able to stream freely into space) is still much, much hotter than the temperature of the hottest lava on Earth.

The surface temperature of the Sun, which is an average mass star, is about 5500 degrees Celsius. A red dwarf star, the coolest of the stars that can fuse hydrogen, sits at about 3000 C. Lava usually checks in at a temperature somewhere between 700 degrees and 1200 degrees. At its hottest, lava is only half the temperature of the surface of the dimmest stars.

There are, however, stellar objects which do fall within the temperature range of lava, though they’re not considered full stars. This is the brown dwarf class; they aren’t able to fuse hydrogen into helium through the standard pathway, since they’re not massive enough to compress their cores down into the temperatures and pressures required to trigger a fusion reaction. But they are still warm (both relative to the vacuum of space, and on human terms).

Brown dwarfs span the space between a particularly massive planet and the smallest hydrogen burning stars. Because they are so much less massive, the smallest brown dwarfs can be lava-temperatures at their surfaces. There are three broad classes of brown dwarfs; L, T & Y dwarfs. L dwarfs are still maintaining surface temperatures of around 1400 degrees C - the T dwarf is cooler again, at 900 degrees Celsius, now firmly within the realm of the temperatures of molten lava. And the Y dwarf checks in at only 300 degrees Celsius. The coldest Y dwarfs are within the range of temperatures we encounter in non-lava scenarios on Earth - somewhere between 40 and 90 degrees C. This is a drinkable temperature.

So while the celestial objects we consider “stars” will always win in a game of “which is hotter”, if you want to include the coolest brown dwarfs, lava can win out.

The trick with a brown dwarf is that these objects are no longer furious balls of plasma, as is true of our Sun. The boundaries between a very low mass brown dwarf and a high mass Jupiter-like planet are quite fuzzy, and so the faintest and coolest brown dwarfs are expected to be shrouded in a Jupiter-style layer of thick clouds. It’s this cloud layer that we see when we observe them from Earth, and so while their cores may still be plasma, the outer layers are simply warm gas.

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#stars #lava #temperature #HR Diagram #astronomy #astrophysics #space

How much overlap is there between the night skies of someone in Sweden and South Africa?

"Hello and thank you for enlightening our minds. I have a very important question that triggers non stop my mind. Can someone standing in South Africa see the same sky of that one standing in Sweden?"

Their skies will never be exactly the same, but they’ll have more in common than you might guess!

To understand this, we need to zoom out from the Earth a little, and remember that the Earth is (roughly speaking) a sphere, and so the skies above us are a little different depending on where you’re standing on it. The easiest and most extreme place to start is at the poles.

Say you teleport yourself to the North Pole. “Up” is now due North (the technical term for “up” is the zenith), and the North Star, Polaris, will shine down on you from a point in the sky directly above you. Excluding variables like mountain ranges and trees, in an open space, everyone, regardless of where they stand, gets 180 degrees of sky. From a point directly above you, you can trace a 90 degree angle in any direction down before you hit the horizon. So our North Pole observer has as their horizon, 90 degrees away from North, before their vision is obstructed by the ground.

Now, if you have a very adventurous friend willing to traverse Antarctica’s mountain ranges to reach the South Pole, they will have the amount of sky above them. Due “up” is still the zenith for this observer, though for them, an arrow pointing to their zenith will point exactly South. This southern observer also has 180 degrees of sky above them.

If we compare these two folks from a very large distance (or from a convenient diagram), we can see that their heads are pointing away from each other, while their feet are respectively planted on solid ground (metaphorically speaking; the North Pole doesn't have any.). If we let the Earth turn for a 24 hour period, the sky above them would rotate smoothly around the northern and southern poles, spinning like a top above you.

Since there are only 360 degrees in a circle, these two people have a set of 180 degrees each, with no overlap. They could both map the sky, and come back together, and compare maps, and find zero points in common. (They would, however, have fully mapped the sky.)

Generally, however, people do not get to observe the sky from the Northern or Southern poles, so we can progress to a less extreme example. If you move away from the poles some, and more towards loci of human habitation, the general setup is the same; everyone gets 180 of sky above them, with their own zenith due “up” above them, and 90 degrees of sky in any direction to their own personal horizon.

So let’s go to this picture of someone in Sweden (I’ve picked Stockholm) and South Africa (Cape Town), and what their locations are. Stockholm is about 59 degrees north of the Equator in the Northern Hemisphere, and Cape Town is 34 degrees below the equator in the Southern Hemisphere. Now when we look at their respective directions, their heads are both pointed out away from the Earth, but at an angle with respect to each other (indicated by the up arrows in the diagram below).

The southern horizon for our observer in Stockholm extends 90 away from “up”. Up for them is 59 degrees above the equator. 59 - 90 is -31, or 31 degrees south. Our Cape Town observer can observe 90 degrees north of 34 degrees south, or 56 degrees north.

In fact, if you add these two positions together, to find the total angle between them, you get 93 degrees. With 180 degrees of sky, that makes for 87 degrees of overlapping sky between them. This is very nearly 90, which is easier to think about.

This means that the southern half of the sky, to our Stockholm observer, will also be seen by our Cape Town observer — but as the northern half of their sky. (It will also look upside down, if they were to swap places.) The northern half of the sky in Stockholm won’t be visible to the Cape Town observer; Polaris would be beneath their feet. And likewise, the southern half of the sky to the Cape Town observer will be unique to them; just as the observer from the North Pole can’t see the stars near the south pole, a Stockholm observer won’t see the southern constellations.

This is why astronomers value having telescopes in both the northern and southern hemispheres! Between the northern hemisphere facilities and their southern hemisphere counterparts, we can point a powerful tool at anything in the sky that we’re curious about.

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#celestial coordinates #celestial sphere #night sky #astroquizzical #astrophysics #astronomy #space facts

If black holes are infinitely tiny, how come we talk about them as having a size?

" Given some of the popular literature depicting the Milky Way's black hole as being "massive", how does that square with the concept of a singularity being an extremely dense point in space? Is it a reference to the size of the aura around the singularity or the projected size of the Schwartzschild radius?"

The Milky Way’s black hole isn’t just referred to as “massive” - it’s “supermassive”! But an excellent question nonetheless, as this is a prime example of astronomers using different units interchangeably in a way that can be a bit opaque.

You’re absolutely correct that at the crux of every black hole is an entity called a singularity, which is something of infinite density - a huge amount of mass piled into functionally zero space. If you take the standard method of finding a density, which is “amount of mass, divided by the space it takes up”, this will guide us well for most objects on Earth, but breaks when it comes to singularities. A pound of feathers may weigh the same as a pound of lead, but the density is definitely higher for the pound of lead. Black hole singularities ask us to divide a very large number (its mass) by zero. Dividing by zero will break your calculator, but formally implies an infinite density.

There is a region around the singularity itself which is strongly distorted by the presence of a large amount of mass nearby. Where this distortion is the strongest, space is so warped that in order to escape, you would have to travel faster than the speed of light - an impossible task. Often, this impossible-to-escape region is bundled together with the impossibly dense singularity and referred to broadly as “the black hole”. The boundary of this region - where if you go exactly the speed of light, you go from being not being able to escape, to escaping - is called the Schwartzschild radius. (This is also the boundary known as the event horizon. These two terms are often used interchangeably.)

If you're well beyond this radius, the mass of the black hole mostly behaves like any other mass, regardless of its density, since you’re now far enough away that the physical size of the object doesn’t really matter. However, this radius changes depending on how much mass is packed inside the singularity. The more mass packed in there, the larger the escape-is-impossible meet-your-gravitational-doom region surrounding the singularity is. So to classify black holes, we typically do this by their mass, but mass also controls how big the black hole region is. Classifying by mass also functionally classifies by physical size.

Our broad schema is stellar mass black holes, intermediate mass black holes, and supermassive black holes. This also goes in order from physically smallest to physically largest. Stellar mass black holes tend to be only a few kilometers across- an eight solar mass black hole would be 48 km across, or about 30 miles. That’s driveable, as long as you’re on Earth and not near a black hole. Supermassive black holes, by contrast, are much larger. The one in the core of the Milky Way, if we use its current mass estimate of 4.1 million times more massive than the Sun, is 1.8 au in diameter. (If you placed it where the Sun is, that means it would extend ~90% of the way to the Earth’s orbit. Not...ideal for the Earth.)

So the black hole at the center of the Milky Way, at its very core, is indeed a volumeless, infinitely dense point. But the inescapable region surrounding it is sizeable - measurable on the scale of the solar system.

The paperback version of Astroquizzical: A Beginner’s Journey Through the Cosmos is coming out globally on September 10th!

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#black holes #astroquizzical #astronomy #astrophysics #supermassive black hole

How realistic is it to have spacecraft brightly illuminated when journeying the solar system?

" I'm watching a show on Netflix called Nightflyers and it got me thinking. Every time I watch a show about space travel, they all depict the space crafts cast in darkness; they're lit, but its dark. So the Moon orbits the Earth, hence we have night and day every 24 hours... but if you're a craft in space, flying above Earth, and not in the path of the Moon’s orbit, (or perhaps unaffected completely by it as you are no longer on Earth and stuck on its plane) wouldn't the craft be constantly bombarded with the sun's rays? (if not disintegrated from the heat all together?) I mean, if you climb a mountain or go snowboarding, even in the most cold places, you can get sunburn as you’re more close to the sun, so I would imagine spacecraft being extremely hot all the time? Can you please help me understand (other than setting a tone, or ambience) how you are affected by light/shade once you are in the solar system? Thanks SO much!"

This is a great question, but before we get to the meat of your query, I want to clear up two misconceptions that are present in the question itself.

The first is that the Moon has something to do with the day/night cycle. Days and nights occur because the Earth is spinning rapidly on its own axis. The Sun, which is relatively stationary with respect to the Earth on the timeframes of a few days, continues to shine from the same point. As the part of the Earth that you or I live on rotates to face towards or away from the Sun, we get day and night respectively. The Moon orbits much, much slower around the Earth - approximately once every month. The Moon can occasionally cast a shadow onto the Earth, but that’s a rare event we know as a solar eclipse.

The second is why you sunburn at altitude. You absolutely are more prone to sunburns at higher altitudes, but it’s not because you’re significantly closer to the Sun. The Sun is 93 million miles away - getting a single mile or two closer isn’t going to make a significant change to the amount of sunlight that your skin’s getting. What happens instead is that you’re rising above some of the protective layer of our atmosphere, which allows more ultraviolet radiation to reach you. This UV radiation is what triggers a sunburn, and the more atmosphere above you, the more protected you are. If you’re on a snowy mountain, you have the additional complication of being able to get sunburned in really strange places, like the underside of your earlobes and the bottom of your chin, because of the reflected light off of the snow.

With those two points addressed, your question about lighting in space is an excellent one. There’s a couple things to think about with lighting, so let’s begin with a spacecraft which is near the Earth. If you are in a position where nothing is blocking the sunlight coming your way, you would be constantly bombarded by the Sun’s rays, exactly as you suspect. However, this is an extremely harsh lighting system - with no atmosphere in space to diffuse the light a little, spacecraft are in pure sunlight or deepest shade. If a spacecraft is moving around the Sun, that means that the sunward facing side of the spacecraft would be illuminated, and the other half of your spacecraft would be in shadow - triggering a pretty intensive temperature gradient between the two sides. As a point of reference, the temperature on the surface of the Moon swings between 224F (106C) and negative 298F (-183C) when the surface is illuminated versus when it is in shadow.

This temperature cycling causes stress on most materials you could build a spacecraft out of, and is a challenge we face already as a moderately spacefaring species. The International Space Station, which orbits around the Earth, alternates between spending 45 minutes in the shadow of the Earth and 45 minutes in direct sunlight. Without intensive, intensive insulation, our astronauts would alternate between freezing to death and boiling to death. We have to manage this same situation on a smaller scale for space suits; in the sunlight, your suit has to keep you cool and protect your eyes from glare. In the shadows, it must keep you warm.

These considerations will only get worse as you get closer to the Sun, or really around any star. As we proceed inwards, closer to the sun, the sunlight gets more intense, and the amount of work you’d need to do to stay cool would increase. The cool side of your craft wouldn’t get any colder, but the temperature stress would get more severe between the sun and shaded sides of your craft, so your insulation would have to get much better. This intensity doesn’t change linearly though - if you got twice as close to the star, the sunlight won’t be twice as intense. It will be four times as intense.

This works just as well in the other direction - go twice as far out in the solar system, and your sunlight will drop off by a factor of four. Go four times as far, and you’re dealing with light intensity 16 times fainter than you have at the distance of the Earth. However, the Sun is very bright. Jupiter is 5.2 au - and Neptune at 30 au. At 5.2 au, you’re dealing with sunlight 27 times fainter than what we receive on Earth. It’s still going to be the brightest thing in the sky. Neptune is much further, but even at 900 times fainter than the Sun appears from an Earth distance away, it still hasn’t faded to anywhere near the relative faintness of the full moon in the sky, and you can do a lot in the light of a full moon, visibility wise.

The way that astronomers measure brightness is with a counterintuitive system called a magnitude, where 1 magnitude is about a factor of 2.5 in brightness. Every magnitude is multiplicative, so five magnitudes is a difference in brightness of a factor of 100. A difference of ten magnitudes is a factor of 10,000 in brightness. At Jupiter’s distance, then, the Sun will appear about 3.6 magnitudes fainter than it does from the Earth. At Neptune’s distance, it’s something like 7.5 magnitudes fainter. The brightest star in the night sky, Sirius, is 25 magnitudes fainter than the Sun, so even at the distance of Neptune, the Sun will appear more than 10 million times brighter than Sirius appears on Earth. The full Moon, which I mentioned earlier, is fourteen magnitudes fainter than the Sun, so the Sun would be shining on Neptune about 390 times more intensely than the full moon.

If your fictional craft is within the bounds of a solar system then, I’d say having the craft be brightly illuminated on one side is pretty reasonable. If you’re going beyond that, though, you’d start to descend into full darkness. You’d have to be very far away from our star before the Sun sank to the brightness of Sirius. In fact, you’d need to be almost 1.5 light years away from our star. The spaces within the stars, which is the majority of the Milky Way Galaxy, are going to be very dark. In those places, the only bright lights will be the ones you bring with you. You probably would want to have a few spotlights around, if any of the crew ever has to go outside for any kind of repair operations, but it wouldn’t have the same aesthetics as the harshly lit side of a spacecraft that many shows like to go for.

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#solar system #astronomy #astrophysics #astroquizzical #lighting

Is the far side of the Moon dark?

Only some of the time! With the exception of the times when the Moon wanders into the shadow of the Earth, the Moon spends its journey around the Earth with half its surface in sunlight, and half its surface in darkness. The far side is harder to watch directly, though, because all of us humans are on the surface of the Earth, which only ever sees the near side. We can observe the far side thanks to the technological advancements that come with sending spacecraft out beyond the Moon, but few human eyeballs have seen the far side of the Moon directly.

Even without going there, we can figure out what should be happening on the far side of the Moon by looking at what isn’t happening on the near side of the Moon. If half the sphere of the Moon is illuminated, and we here on Earth are looking at a full Moon, then the far side of the Moon must be dark. But a full Moon doesn’t last very long- the next night the Moon will begin to look less circular in the sky, until a few days later, you’ll definitely be able to tell that the surface of the Moon facing us is not entirely illuminated.

The rest of that sunlight isn’t missing; it’s illuminating the side of the Moon that’s not facing us. As the month progresses, more and more of the far side of the Moon will be in sunlight, and less and and less of that sunlight will be visible to us on Earth. When we on Earth see a thin crescent Moon, the far side of the Moon is almost totally illuminated.

There are some permanently dark places on the Moon, but the far side of the Moon isn’t where you find them. They’re near the poles of the Moon - craters that are so deep, and the sunlight that reaches them is at such a shallow angle, that the light from our Sun only ever skims the surfaces of them. These are interesting places because they are so dark and cold - they’re one of the places that water seems to exist on the surface of the Moon.

With the exception of these deeply shadowed craters, the rest of the surface of the Moon spends about half its time in the sun, and half in the shade. What’s fun is that these periods of sun and shade each last about two weeks.

This is easiest to think about with the near side of the Moon; imagine some point (you can pick your favorite) on the surface of the Moon. As an example, let’s pick the very center of the near side. When the Moon is dark from our perspective, so is our test point in the middle of the near side. As the Moon progresses through crescent phases, our point in the middle is still dark! That part of the Moon is still in its nighttime period. When half the Moon is illuminated, our point on the Moon is dealing with a sunrise, as it’s right on the boundaries of the daytime and nighttime. From there, the gibbous phase, the full Moon, and right onwards through to the next quarter (where the other half is lit), our central point of the Moon stays in sunlight. If we ever have a human outpost on the Moon, this two weeks of daylight followed by two weeks of night will be something to contend with - though I’m sure folks who have lived in the arctic or antarctic (where night can last several months in winter) can give our explorers some pointers.

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#the moon #astroquizzical #astronomy #astrophysics #the far side of the moon #not the dark side

When we talk about the Universe's first second, what do we really mean?

"In writings about the Big Bang, there are discussions of what happened in the first picosecond, billionth of a picosecond, etc., etc. My question is: what is the measure of time used by the writer? Our time as we experience here on Earth? The instantaneous time passage there, which would be influenced by the infinite concentration of mass and energy (a singularity?)? What is the time scale?"

This is a really fun question, because the answer is that these time points you’re seeing are for time as we experience it here on Earth, where we’re trying to use an objective ruler of time to describe how rapidly things were changing during those early moments of our Universe. All measurements of time are based on what we use here on Earth, where we humans first developed our timekeeping methods. The second is now a unit of measure used for all sorts of things, though pretty rarely in extragalactic astronomy (with a few exciting exceptions like events that trigger gravitational waves) because the distances involved often mean things happen on billion year timescales.

But when we’re talking about the very beginning times our Universe went through, a lot of things did happen in the first second - the Universe underwent a lot of dramatic changes in that first second. It went from a soup of energy to filled with protons and neutrons in that time - a dramatic change! And when we say this, we really do mean the second that you could watch tick past on a watch. This second comes from taking the speed of our Earth’s rotation, and dividing it into twenty four hours, dividing each hour into sixty minutes, and each minute into sixty seconds. It’s that second, 1/86,400th of an Earth-spin, that we use to describe the initial changes of our Universe.

It’s fun to think that a fluke of angular momentum that gave us (approximately) a 24 hour day also gave us a useful metric for describing the early state of the Universe in precisely the units that we do.

As time has wound on, we humans have sought to make our units of measure ever more precise. To do this, we often wind up redefining our units in terms of something more fundamental than where we had begun. The meter was redefined to be the distance that light travels in 1/299,792,458th of a second instead of “one ten-millionth of the distance from the equator to the North Pole”, and the kilogram was recently redefined to be a function of Planck’s constant, instead of a very specific, carefully guarded, lump of metal in a vault in Paris. The second has also undergone this transformation.

As we measured the Earth’s rotation to higher and higher precision, we encountered the need for leap seconds to account for the fact that our Earth’s rotation is intrinsically slowing by a tiny, but measurable amount. Instead of using the Earth’s rotation speed, then, a more fundamental, reliably measurable feature of our Universe was adopted as the official definition of a second - the length of time it takes a cesium atom to vibrate between two hyperfine states 9,192,631,770 times. While this may seem like a much more complex unit of time, it’s actually a better definition in that anyone, anywhere in the universe, should be able to measure this unit of time consistently.

During this redefinition of the second, the length of a second wasn’t changed, but now we have a more persistent method of measuring it. So that first nanosecond (10^-9) of the Universe is the same length of time it takes a cesium atom – in a vacuum, at absolute zero – to vibrate 9 times.

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Is there lightning on Mars?

"Is there lightning on Mars? Would lightning strikes endanger astronauts on Mars? Would static electricity be a factor to consider on Mars?"

There is lightning on Mars! Or at least, something like lightning occurs on Mars. In 2009, the first detections of lightning strikes on Mars were recorded, confirming something that planetary scientists had suspected already - electricity should arc through the Martian skies.

We knew a fair amount about Mars’ weather patterns even before detecting lightning, from a combination of orbiting spacecraft and our landers on the surface. These outposts have painted a picture of a thin atmosphere frequently tumbled into large dust storms. Mars has huge annual storms which can envelop the entire planet, and other strong storms that pop up irregularly through the year. On top of that, the dust on Mars is extremely fine, so once you begin to swirl it around in a wind, it’s reasonable to guess that the dust particles will start to rub on each other, and as you do that, you’ll start to build up an electric charge.

This static charge does more than just gradually build towards lightning; it’s also part of why the Mars rovers get so dirty. The rovers are dealing with more than just a fine sifting of dust falling out of the atmosphere, which a light breeze might easily remove; that dust is stuck to them like packing peanuts stick to your hands. It takes a stronger breeze - a new storm, or a wandering dust devil - to remove some of that dust, and it’s something that the long-lived Spirit and Opportunity rovers were both able to make use of on a couple of occasions.

However, as much as dust devils can help you out, they can also do the opposite, dumping more dust on top of your solar panels, which, for a solar powered craft, will limit the amount of energy you have available to do science with, and eventually drop the craft below the threshold of power it needs to operate. This is the current theory for what happened with both Spirit and Opportunity. The Curiosity rover is less affected by this particular issue since its power comes from radioactive decay, but Curiosity is still fully coated in the fine Martial soil. This dust is actually a concern for human exploration of Mars - it’s going to be hard to fully remove this dust from spacesuits, and breathing in a fine particulate is never good for your lungs.

The lightning itself is actually less likely to be a hazard to astronauts on the surface of Mars than the dust is; for one I would expect any humans on the surface of Mars to take shelter during these bigger storms. Unlike what was presented in The Martian, even the 60 mph winds that can occur during a dust storm wouldn’t feel as powerful as a similar wind on Earth, since the atmosphere is so much thinner. The air simply wouldn’t exert the same pressure against you in the same way. Even on Earth, the likelihood of being struck by lightning is very low, and on Mars the best guess is that the lightning would not really resemble the large bolts of lightning we see here on Earth.

More likely is that this lightning would resemble the arcing jolts of electricity you can create by shuffling along in socks on carpet and then touching a doorknob. In a dark room, you can see the filamentary discharge of electricity between your finger and the doorknob. On Mars, you might expect to see little flickers of electricity arcing between parts of the dust storm, faintly lighting up the night sky. To be a hazard to an astronaut or a rover, you’d have to be very, very unlucky.

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What would happen if the amount of light reaching the Earth from the Sun were cut in half?

"What would happen if the amount of light reaching the earth from the sun were cut in half?"

We’ve tackled a very similar question to this before here at Astroquizzical; check out this post! In that post, we explored what would happen to the Earth if we could slice the Sun in half. And because cutting the Sun’s matter in half doesn’t translate to a slice in brightness of one half, it’s a pretty dramatic shift for our solar system.

However, if we don’t go quite as far with our solar slicing, but instead just drop the brightness of our sun by half, we’ve actually only removed 18% of the mass. This is still a relatively massive star, at 82% the mass of our Sun, but that’s enough to change the distance from the star where liquid water is stable.

As the mass and brightness of a star decreases, that zone of possible liquid water (usually known as the habitable zone) shrinks to a smaller and smaller shell around the star, but because we’re changing the star by a smaller amount this time compared to the earlier post, the habitable zone won’t shrink all the way down to Mercury’s orbit - it would sit closer to where Venus is now. The Earth’s orbit might still fall within the bounds of the habitable zone, but it’d be more in the position that Mars finds itself in now - much colder than Earth now, but able to sustain water under certain circumstances.

Our Sun won't be dropping in brightness anytime soon - on the contrast, as our Sun ages, it becomes slightly brighter, increasing in brightness by about 10 percent every billion years. As it does, the habitable zone around our star has been gradually expanding outwards, and at some point in the next billion years, the Earth will exit the habitable zone entirely.

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Is it possible to have a planet orbit two stars, like Tatooine?

"How does that two sun thing work in Star Wars: A New Hope? Is that possible?"

It is possible, and we’ve actually found a number of planets orbiting double stars, like Luke’s homeworld in Star Wars does. However, outside of the Star Wars Universe, there are a lot of ways for this setup to go very wrong. So far, we haven’t found an enormous number of planets orbiting double stars, which seems to speak to how rare it is for a planet to survive in an environment like Tatooine’s.

At the beginning of Star Wars, Luke Skywalker lives on a planet with a double sunrise, on a planet which orbits two stars. We can presume that the two stars are orbiting each other, and that this planet then orbits around both stars. The technical term for two stars which orbit each other is a binary system, and the easiest way for the stars to find themselves in this situation is if they both form out of the same cloud of gas, at the same time. The remainders of that cloud of gas would hang around long enough to make planets to surround the pair of stars.

If a planet were orbiting far enough away from the two stars, it wouldn’t really notice a difference between orbiting the double star set, and orbiting one star of their combined mass. However, by the time you get really far away from the stars, there’s not a tremendous amount of sunlight reaching the surface of your planet. If you want your world to be habitable (and a desert world still counts), you’ll have to be on a planet that’s a little closer to the stars, and this is where things start to get tricky.

If you are a planet, it’s nicest if the two stars orbit each other closely and circularly. This kind of setup for the stars means that you’re more or less always the same distance from the stars, which guarantees you a pretty consistent amount of light from the stars. If you’re trying to be a habitable world, this is important, because it keeps your surface temperature roughly consistent as well. You’d still have some variability, because the stars will still eclipse or partially eclipse each other periodically, which would lower the amount of light you’d get on the surface.

However, if you are a star, close orbits are more complicated than wide ones. Wide orbits are easier to maintain, because the two stars have a weaker gravitational influence on each other. In a smaller orbit, the two stars will exert a reasonably strong tidal force on each other, and will change each other’s orbits over time. When the orbits of the stars begin to change around, the planets’ orbits also change, and you are in prime conditions for what’s called a three-body interaction.

The three body interaction happens when you have three objects orbiting each other in relatively close range. This could be three stars or two stars and a planet, and in either case, the lowest mass object can wind up getting flung suddenly out of the solar system entirely. The other outcome is for the planet to wind up crashing into one of the two stars - not a habitable outcome there, either. The three-body interaction is of particular concern for two stars and a planet, as this means that if your planet is close enough to the star to get caught up in one of these interactions, it won’t stay as a planet in the solar system for a particularly long time. This might partially explain the relatively low number of circumbinary planets we’ve seen so far with Kepler - these planets are prone to either being ejected or consumed by their parent stars.

So it’s not impossible for a Tatooine-like planet to orbit a binary system, but given how rare they are in our solar system, everything has to be exactly so, or Tatooine will wind up on a one-way trip out of its solar system on a journey through its home galaxy.

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How long would it take to deflate the Earth's atmosphere out into space?

"My roommate and I were in a heated debate that lead us to read your post about the ability to survive the end of Portal 2. However, our question is slightly different. Suppose the same kind of portal was created on Earth’s surface to the Moon’s, how long would it take for the Earth’s air supply to be released through the portal into space?"

If any of you haven’t seen the previous Portal 2 post, I’d recommend having a look at it here, because I’m going to pull some numbers from it. I’m also going to make some slightly unphysical assumptions, but the results of those assumptions is that we’re going to calculate a lower limit to the amount of time it would take to bleed the atmosphere dry. In a world where portals actually worked, it would almost definitely take longer, for reasons we’ll go over later.

Our scenario is thus: we have opened a portal between the surface of the Earth and the Moon, as in the end of Portal 2. Effectively, we’re opening a window between the surface of the Earth and a pretty hard vacuum. The dramatic pressure difference here produces a tremendous, faster than the speed of sound, wind, as we worked out in that previous post. Presumably, if you left that portal open for a long time, you would reduce the amount of atmosphere left on the Earth. In the game, this portal is only open for about 30 seconds, but what if we left it permanently open?

The first thing I’m going to assume is that the whole atmosphere of the Earth is entirely at the same pressure (which it is not). Down at the surface where we humans live, the atmosphere is pretty compressed, and so we have an ambient atmospheric pressure of 1 atmosphere. (Yep. That’s the unit.) 1 atmosphere is equivalent to about 14.7 pounds per square inch, or psi. However, the further up away from the surface you go, the more diffuse the atmosphere gets, and both the density of atoms and the atmospheric pressure drops. If the density of the atmosphere drops, the wind speed through our window will also drop, because it’s the difference in pressure on the two sides of our window that drives the wind speed. By assuming that I can compress down the upper layers of the atmosphere so that the air on Earth is at a constant 14.7 psi, then the wind speed will stay at its fastest, and bleed the atmosphere out into Moon space as fast as possible.

If you compress the atmosphere down, it would fit in a sphere 1999 km across, which then has a volume of 4.19 x 10^18 cubic meters. This...is a big number. How fast can we drop it to zero?

I will have a reasonable guess that the portal itself is about five feet tall by three feet wide - it seems a bit shorter than Chell in game, and wide enough for her to fit through. If we assume that it’s rectangular instead of an oval, the math is nicer, so I’m going to square up the portal dimensions at about 1.5 meters high by 1 meter wide. This gives a portal area of 1.5 square meters. This is key, because with the area of the window, and the wind speed, we can figure out the volume of air lost every second. At 411 meters per second, our speed from the older post, that means that after one second, a bit of air will have traveled 411 meters.

Every second, we’re going to lose about 617 cubic meters of high pressure Earth atmosphere into the space surrounding the Moon. We know how much we have to lose, so from here we can sort out how many seconds it would take to get the total volume of the Earth’s atmosphere out through our portal. As you can probably guess by the 18 zeros following the total volume of the Earth’s atmosphere, it’s going to be a lot of seconds. In fact, it’s so many seconds that seconds are not a useful unit even a little bit. Converting into years is a little better.

It would take 215 million years.

And remember, this is assuming that the wind speed stays the same the whole time, which it would not in real life. The other thing we’re assuming is that none of this gas will hang around the moon and increase the atmospheric pressure around the Moon. That would also start to balance out the pressure difference, slowing the wind speed down and making this take even longer. The moon historically is not very good at holding onto an atmosphere, so this would likely be a temporary arrangement, but millions of years is not very long for astronomical things, and it’s possible the lunar atmosphere could hang around long enough to slow down our wind. The estimates for the atmosphere around the young moon is that it would have stuck around for 70 million years or so - shorter than our fueling time, but long enough that we could expect it to hang around for a while, before we’re able to finish emptying the Earth’s atmosphere into outer space.

In reality, there would likely be an equilibrium point reached, where both the Moon’s newfound atmosphere and the Earth’s freshly drained atmosphere would find themselves at the same pressure, and the wind, having gradually slowed, would come to a stop, with only the vaguest breeze from the Earthward side as the Sun gradually stripped the atmosphere from around the Moon.

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Physics World Op-ed

Your friendly neighborhood astrophysicist recently wrote an opinion piece for Physics World, on how I approach writing answers to your many questions.

Read the full article here on Physics world - I hope you enjoy!

While you're over there, check out the review of Astroquizzical: A curious journey through our cosmic family tree!

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#announcement #astroquizzical

If black holes are so bad at growing, how do we get supermassive black holes?

"If supermassive black holes are so bad at feeding from the stars around them, how did they get so big?"

This is such a good question that the answer is not particularly settled yet - we have some ideas for how some supermassive black holes may have gotten so large, but it’s not clear that the explanations we’ve come up with so far hold true for all supermassive black holes.

So a few definitions before we launch into our ideas - by and large, all massive galaxies have a gigantic black hole at their centers. To distinguish these central black holes from the type of black hole created when a single, massive star ends its life, we named the big ones “supermassive”. The black holes created from single stars are less entertainingly named “stellar-mass black holes”.

Black holes of all sizes are extremely inefficient at gathering new material to themselves. Rather than absorbing nearby material in one fell swoop, the material is more often than not pulled into a tight disk of extremely hot material, or flung away from the black hole entirely, sometimes at relativistic speeds. For all black holes have a reputation of being cosmic garbage disposals, if you had a garbage disposal this terrible in your own kitchen, after the first time it blasted superheated onion bits on your ceiling you’d call a professional to have it removed ASAP.

So it is an eminently reasonable question - how do you make a black hole that’s thousands of times larger than the ones that are made by stars, when they’re exceptionally bad at growth? This question is made more complicated because right now, getting a star to explode in a supernova, and the remnant collapsing down into a black hole is the most robust theoretical model we have to make a black hole, so you have to somehow bridge the gap between something that contains a few times the mass of the Sun and something that contains thousands of times the mass of the Sun, while that thing steadfastly refuses to grow rapidly by accreting new material onto itself.

What can we do? Well, we can either 1) start larger, or 2) grow differently. If you start larger, then you have the benefit of not having to grow by a factor of several thousand but only by a couple. This method means you have to start with “seed black holes” very early in the universe. You can do this in one of two ways. First is the direct collapse model - the thinking goes that it might have been possible, very early in the universe for enough gas to collect together that its gravity would just collapse all the way down to a black hole, skipping the star phase entirely. The second method is effectively to go through a star first, but to go through a very, very large star - something much larger than the Universe makes nowadays, which would burn through its hydrogen much faster, and would make a larger black hole as a remnant.

What about growing differently? It’s possible that instead of growing by slurping tiny fractions of gas and dust from its surroundings, the black holes grow by absorbing other black holes. This raises a whole string of other questions, like “are there even enough black holes around for that to work?”. We know that there should be times when galaxies gain a second supermassive black hole - during galaxy collisions. The remnant of the two galaxies should have two black holes which sink together, immediately doubling their mass. But it’s unclear if you can guarantee that enough galaxies will smash together enough times for this to work for all the galaxies we see, especially since not all galaxies are expected to merge with another galaxy the same number of times.

There’s no reason the answer will wind up being one or the other - some combination is likely to be in play. If you can start with a larger seed in the early Universe, you can grow more easily through a combination of colliding with the supermassive black holes in other galaxies, and by gathering gas inefficiently to themselves.

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What is the biggest known planet in the Universe?

"What is the biggest known planet in the Universe?"

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Our list of known planets and exoplanets unfortunately doesn’t extend much beyond our own Milky Way galaxy - to spot a planet, you need to be able to measure the light from an individual star and monitor it over time. You’re looking either for tiny flickers in the amount of light you receive, as a star happens to pass in front of the star you’re watching, or you’re looking for there to be a little Doppler shift in the color of the star’s light, as the planets tug it slightly off center as they orbit. Known by the names of the transit method and the Doppler shift method respectively, both of these require really careful observations over a significant amount of time, without the light from the star mixing with the light from other stars. This limits us pretty well to the stars within or surrounding our Milky Way.

Because the measurements required to spot planets must be so precise, generally the telescopes we send out to do these measurements only look at a small patch of the sky. So while I can give you our current high scoring planets, there’s no guarantee these will remain the all-time bests, if we point our telescopes in a new direction.

There is one fundamental limitation to how massive a planet can get - if you pack too much material into a planet, it will start to fuse elements in its core, and it formally becomes a star instead of a planet. This transition happens when the object is somewhere in the range of 13 to 80 times the mass of Jupiter, and is the point at which we typically start calling objects a brown dwarf star, orbiting another star, instead of a planet. The list of biggest planets can also change if we get better measurements. It's possible to learn that what we thought was a planet should really be called a brown dwarf, which then bumps that object off the list of biggest planets, and onto the list of known brown dwarfs.

However, you can still have very large, fluffy planets, well before they get to this boundary of being a star. Most of the ones we know about are Jupiter like in style - massive, gaseous planets, orbiting distant stars. The easiest to find are hot Jupters - exoplanets which are not only bigger than Jupiter, they’re much closer into their star than Jupiter is to our Sun. Currently, the majority of the biggest, fluffiest planets are about twice the radius of Jupiter. Considering that you could stack 22 and a half Earths edge to edge to match the width of Jupiter, you’re looking at a planet so large, you could line up 45 Earths behind it, and not see any of them. These planets have the very pronounceable names of ROXs 42Bb, which is estimated to be about 2.5 times the size of Jupiter, or Kepler-13 Ab, which sits around 2.2 times the size of Jupiter.

There are some larger ones, but these have preliminary estimates of their size, and may yet turn out to be brown dwarfs. The current record holder is a planet orbiting a star known as GQ Lupi, and estimates place it at somewhere around 4 times larger than Jupiter. This particular object is so large that our theoretical models of how it has formed are not particularly happy, and so the estimates on its size and mass are both pretty hazy. It is likely to remain a planet, but if it turns out that its mass is on the high end of our current estimates, it could wind up on a brown dwarf list. (This object is also extremely young, and will change and compress as it evolves.)

These big fluffy planets are orbiting your default solar system - one with a single star, around which all the planets orbit. If you have two stars (which isn’t that uncommon), it seems to be much harder to build very large planets. The largest planet known to circle two stars at once was only confirmed in 2016, and is almost identical to Jupiter in size. At “only” 22.5 Earths in size, it orbits its parent star once every three years.

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Astroquizzical is an ad-free site, and I'd like it to stay that way. Patreon will help pay for hosting costs, and you'll be supporting science communication with no paywall and no ads. Thank you so much for reading, for your support over the years, and here's to many more years of new and fun questions!

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If we can't build a magnetic bubble for a spacecraft, how about a magnetic tunnel?

"If it is impractical to provide an artificial magnetosphere on the ship which would travel to Mars (due to cosmic ray cascades in the material of the ship), what about generating the magnetic fields externally and projecting them into space at a series of waypoints? Or would the distance involved (225 million miles) be too great?"

A little while ago we covered some of the main radiation based difficulties of sending people to Mars, and while the solar wind is generally not so troublesome, cosmic rays, which we are shielded from here on Earth, are both more dangerous and much harder to redirect or stop.

Generally we want the outer walls of our spacecraft to be pretty durable, both for airtightness, protection against space junk, and to help protect against the solar wind, which can be stopped by a pretty reasonable amount of shielding. However, as you build up your shield, cosmic rays will start to play a nastier role. While you certainly don’t want a cosmic ray to be able to pass straight through your spacecraft and hit your astronaut unhindered (they’re very energetic particles, the sort that bodies deal very badly with), when a cosmic ray hits a dense object like a wall, it doesn’t just bounce back the way it came from.

It creates a radiation cascade instead; what was one particle is now two, four, sixteen, and beyond, very rapidly, as the particle interacts with the dense material of the spacecraft wall. Sixteen slightly lower energy particles is mathematically worse than one high energy one, and a serious point of concern once we get out of the Earth’s magnetic shielding. So a very reasonable response is to ask if we can bring along our own magnetic shielding, to prevent the high energy cosmic rays from hitting the wall of the spacecraft in the first place. Theoretically, this should reduce the amount of radiation inside the spacecraft cabin, since it would reduce the number of cosmic rays that can make it all the way to the spacecraft shield. The main reason this is impractical right now is simply a logistical one - we don’t have a good way to build a generator for a sufficiently strong magnetic field which is also lightweight enough not to be hard to launch.

Setting up waystations would be an interesting way of approaching the same challenge. If there were a fixed orbital path between the Earth and Mars, and we could build a magnetic tube between the two planets, you could do away with the need to have an onboard magnetic bubble. Because you’re not trying to launch them on the spacecraft, you wouldn’t need to worry about the weight as much, but the magnetic field you’d have to generate would need to be much larger, to guarantee that the spacecraft (within errors) would definitely travel safely through the buffered region. The distances involved here are vast, and so setting up a series of waypoints would almost definitely be unfavorable, at least from an energy consumption perspective. There’s also the question of fueling those waypoints. Are they solar powered? Fission powered? What happens if their solar panels break down or they run out of energy? They’d also have to be able to correct their own orbits in order to be in the right places for the protection of the traversing spacecraft, and at this point we’re looking at a giant electromagnet with rockets, which is a great sounding device to have, but practically speaking, it’s a more powerful version of what we’d like to have on the spacecraft in the first place, and if we can get by with one device instead of several hundred, one is probably better.

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Why do we always think of North as up?

"Hi! It might be a dumb question but it's been in my mind for a while. We are convinced that North is up and South is down because that's the way maps have been for many many years, but we don't really know which way is actually up, it could be east or northwest, etc, right? Because there isn't a real orientation/position in space, there's no fixed up or down, but... doesn't the way the Earth rotate determine in a way which way is up? How do those two things related to each other? Or is there no connection at all? Thank you!"

You’re right that the way we draw our maps with North pointing up and South pointing down is largely arbitrary, and indeed there are a number of maps with the Southerly direction at the top rather than at the bottom, and they’re good fun to look at. However, there are good reasons to say that a Northerly or Southerly direction should be “up”, and these reasons extend beyond just the rotation of the Earth.

The rotation of the Earth is a good starting place, though - the rotation axis of the Earth goes more or less through the North and South magnetic poles of the Earth. The magnetic North & South poles wander a little, so some years they’re closer to the rotation axis than others. Fixing the rotation of the Earth as a cardinal direction makes good sense, and is what we’ve done - East and West point 90 degrees from North and South.

There’s one more reason to put North as up, and it’s a physics convention. Most of the time, when we’re talking about rotation, we say that the direction of the rotation axis is actually just in one direction, rather than having to indicate both North and South. If we do this, it allows us to encode both the axis of rotation, and the direction of rotation at the same time. The way we determine which of North or South should be “the direction”, we use what’s called the “right hand rule”. You curl your fingers in the direction of rotation, and your thumb points in the direction of the rotation axis. In the Earth’s case, we rotate towards the East, so your thumb will point in the direction of North.

However, if you’re thinking of orientations beyond just the Earth’s own rotation, while it’s true that there’s no way to set an entirely objective zero point from which to measure other positions, and a sphere doesn’t have much intrinsic orientation to it, we can still do relative positions pretty well. And on the scale of our solar system, we have a pretty solid alignment going on. All the major planets in our solar system trace oval paths around the Sun as they go about their respective years. Not only do they orbit around the Sun in the same direction, they all tend to point their rotation axes in the same direction (notable exceptions here are Venus and Uranus). On top of all that, the ovals are almost perfectly aligned in a flat plane. If we take our same physics convention and use the rotation of the planets around the Sun to tell us which direction we’re going to point up, our Planet Earth based North is more or less pointing in the right direction. Our planet’s spin is not perfectly aligned with the “up” out of the solar system, but tilted by 23 degrees, a feature of our planet responsible for our seasons. This tilt is why many globes are set at an angle - they’re mimicking the tilt of our planet relative to the “up” defined by our solar system.

So the North is up convention is partially mapmakers, partially the spin of our Earth, and partially physics notation, but there are definite ties between all of them.

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