#astroquizzical

" 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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" 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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"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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"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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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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"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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