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?”

NGC 7752, Image credit: ESA/Hubble & NASA, Dark Energy Survey, J. Dalcanton

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.

NGC 976; Image credit: ESA/Hubble & NASA, D. Jones, A. Riess et al.

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”.

NGC 4622; Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)

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!

---

Have your own question? Feel free to ask! Keep up to date with new posts on Facebook or twitter, or sign up for the mailing list for updates & news straight to your inbox!

This site is reader supported! To support it, you can become a regular patron on Patreon, which will give you early access to future articles, make a one-time donation via Ko-Fi, or, consider buying the book, which is available in hardback, paperback, and as an ebook!

Astroquizzical is now available in an all-new fully illustrated edition!

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.”

The Earth and Moon, as seen by the Mars Reconnaissance Orbiter’s HiRISE imager, from a distance of 142 million kilometers. Image credit: NASA/JPL-Caltech/University of Arizona

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. 

The structure of the Earth, roughly to scale. The solid core (red) is surrounded by the liquid core (orange) and makes up half of the interior of the Earth. Surrounding that, in green, is the solid mantle, and solid upper mantle.

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.

The Moon’s interior, drawn roughly to scale. The solid core of the Moon, surrounded by its liquid core, is much smaller than what’s seen in the Earth.

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. 

This view of the north polar region of the Moon was obtained by Galileo's camera during the spacecraft's flyby of the Earth-Moon system on December 7 and 8, 1992. Image credit: NASA/JPL

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.

---

Have your own question? Feel free to ask! Keep up to date with new posts on Facebook or twitter, or sign up for the mailing list for updates & news straight to your inbox!

This site is reader supported! To support it, you can become a regular patron on Patreon, which will give you early access to future articles. You can also make a one-time donation via Ko-Fi! Or, consider buying the book, which is available in hardback, paperback, and as an ebook!

Is a star hotter than lava?

Lava cascades from Mauna Ulu flowing down into Alae crater, Kilauea, Hawaii, United States. Image credit: USGS.

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 standard HR diagram, with the temperature of typical stars ranging from 30,000 degrees to 3000 degrees Celsius. The x axis is extended to include the regime of lava, from 1200 degrees to 700 degrees Celsius. Image credit: Jillian Scudder

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).

This artist's conception illustrates what brown dwarfs of different types might look like to a hypothetical interstellar traveler who has flown a spaceship to each one. Brown dwarfs are like stars, but they aren't massive enough to fuse atoms steadily and shine with starlight -- as our sun does so well. Image credit: NASA/JPL-Caltech

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.

The same HR diagram as above, but now with the brown dwarf zone included below the axis. 100C is marked with “boiling water”, and brown dwarfs beginning at 2000C, with an arrow pointing towards the cool end, labeled “Eventually ok to touch”. Image credit: Jillian Scudder

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.

---

Have your own question? Feel free to ask! Keep up to date with new posts on Facebook or twitter, or sign up for the mailing list for updates & news straight to your inbox!

This site is reader supported! To support it, you can become a regular patron on Patreon, which will give you early access to future articles. You can also make a one-time donation via Ko-Fi! Or, consider buying the book, which is available in hardback, paperback, and as an ebook!

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?”

Circumpolar stars in star trails with an aurora at the Shipshaw river (Saguenay, Quebec, Canada). Image credit: CC A-SA 4.0, Original photo by wikimedia user 0x010C, altered by wikimedia user Yisusdev

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. 

A diagram of two people standing on opposite sides of a globe, with N marked at the top, and S at the bottom. Both people have “up” with an arrow pointing away from their feet. Image credit: Jillian Scudder

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.) 

The Northern and Southern skies, as seen from our polar observers. Up for the respective observers is marked at the top and bottom of the sphere. Image credit: Jillian Scudder

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.

A diagram showing the overlap between an observer based in the northern hemisphere (Stockholm) and one in the southern hemisphere (Cape Town), with the nearly 90 degrees of sky in overlap between them. Image credit: Jillian Scudder

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.

---

Have your own question? Feel free to ask! Keep up to date with new posts on Facebook or twitter, or sign up for the mailing list for updates & news straight to your inbox!

This site is reader supported! To support it, you can become a regular patron on Patreon, which will give you early access to future articles. You can also make a one-time donation via Ko-Fi! Or, consider buying the book, which is available in hardback, paperback, and as an ebook!

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?”

This computer-simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the black hole’s event horizon, where no light can escape the massive object’s gravitational grip. Image credit: NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (STScI)

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.)

This computer-simulated image shows gas from a star that is ripped apart by tidal forces as it falls into a black hole. Some of the gas also is being ejected at high speeds into space. Image credit: Image Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)

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!

Have your own question? Feel free to ask! You can also submit your questions via the sidebar, Facebook, or twitter. Sign up for the mailing list for updates & news straight to your inbox!

This site is reader supported! To support it, you can become a patron on Patreon, which will give you early access to future articles. You can also make a one-time donation via Ko-Fi! Or, consider buying the book!