Black holes are the densest objects in the Universe. Their gravity is so strong, not even light can escape its pull, turning them into unfathomably dark wells of gravity. There are many open questions surrounding the physics in and around black holes, so many questions in the area don’t have easy answers, but we do our best to say what we can.

Black holes are the dead husks of giant stars that ran out of fuel and exploded in giant explosions called supernovas. These explosions are some of the brightest things in the Universe and can be seen for billions and billions of light years. Counterintuitively, black holes themselves can also create extremely bright objects called quasars, which are superheated jets of gas being shot out of the region surrounding a black hole.

We also have answers related to stars in general, including their life cycles and gravitational pulls.


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Black Holes

What is inside a black hole? (Beginner)
The short answer is we don’t know what’s in a black hole. It is impossible to see what is inside a black hole (hence the name), so no physics theories about what is inside of them can be proven. All we know about black holes is that there’s something in there that is dense enough that its escape velocity is larger than the speed of light. Our current theories of physics don’t provide any explanation of what this could be since the densest stuff we know of (degenerate neutron matter inside neutron stars) is still not that dense. Instead, it is usually called a singularity, an object of infinitely small size that still weighs the same as everything it was made from.
The only way we can observe these singularities is by seeing what effect they have on their event horizons (the shell around a black hole that is the minimum distance light can actually escape from). The only properties we can see in the event horizon are the mass of the black hole (which we can measure by its gravity), the electric charge of the black hole (which we can measure by its electric field), and the angular momentum of the black hole (which we can measure by seeing how the shape of the singularity bulges out at the center like the Earth). All other information about what is in the black hole (like what elements it was, what temperature it was, how old it is, etc.) is essentially lost forever because it doesn’t affect the shape of the event horizon. This is a consequence of the famously funny-named No Hair Theorem.
So essentially, all that is really “inside” a black hole is those 3 properties I listed above. Our current theories of physics have no real way of describing what other properties they have, so that’s all we can talk about. Physicists assume that at some point, we will be able to come up with a better theory that actually describes what is going on, but as of now, we have no way of being able to perform experiments to see what that would be.
If a planet is on the edge of a black hole like in the movie Interstellar, would time actually slow down? (Beginner)

The fundamental science shown in Interstellar is correct. Time does in fact run slower in high gravitational fields, so if they were down on the planet in high gravity and then returned to the spaceship, less time would have passed for them. The spaceship would indeed perceive time on the planet to be running slower and the planet would see time running faster on the spaceship. However, the magnitude of the slowing of time (about 60,000x) is vastly exaggerated. They would need to be much closer to the surface of the black hole for it to be that large (like basically at the surface) so it’s not realistic. We have observed the stretching of time due to gravity in satellites before though, so it is a real effect.

What is the temperature of a black hole? (Intermediate)
This is a difficult question to answer in a satisfying way because black holes are really weird. The standard definition of “temperature” doesn’t really work for black holes because you can’t go up to a black hole and put a thermometer on it. As soon as you get too close to a black hole, you’ll be completely torn apart and any information you gained about the temperature will be lost to the Universe forever. However, we can use a different thermodynamic definition of temperature based on the radiation that black holes give off. Despite being totally “black” (ie no light can escape), black holes do indeed give off a tiny amount of light called Hawking Radiation (this is what Stephen Hawking was famous for discovering). This tiny amount of radiation can be turned into a temperature, giving the following formula for the Hawking Temperature:
There are a lot of different physical constants in that equation that you can look up if you want to, but the important thing to notice is that the mass of the black hole M is on the bottom of the equation, meaning that as the mass gets higher, the temperature gets lower, so supermassive black holes are actually the coldest things in the Universe! So you can’t actually touch a black hole to feel its temperature, but if we try to calculate what it would theoretically be, then it would be super super cold. Hopefully this answers your question, and let me know if you have more.
Is there such thing as an ultramassive black hole? (Intermediate)

The line between “super” and “ultra” massive black holes is arbitrary, so you can draw it wherever you want (or wherever is convenient for marketing your discovery). There are some black holes at the centers of galaxy superclusters that have been labelled as “ultramassive” recently, which you can read more about here. A full catalog of the most massive black holes known is listed here, and if we use the arbitrary threshold of 10^10 solar masses for “ultramassive” black holes, then there are about 30 we know of.

Could supermassive black holes be unexploded parts of a primordial black hole which detonated in the Big Bang and created the universe? (Intermediate)
Astronomers are still actively researching the beginning of the Universe and how both supermassive black holes and primordial black holes could have formed, so I can’t give you answers with 100% certainty, but I can give you what is probably the best guess we have right now. Although it is thought that the Universe originated from a singularity, it’s not totally correct to say that it is from a black hole that exploded. To say that the Universe started from a black hole implies that the black hole must have existed before the Universe and that it could explode into some surrounding area, but we currently don’t have any evidence for either of those things. Instead, the singularity seems to have spontaneously started expanding, growing space along with it to lower its overall density.
With regards to supermassive black holes, one theory of their formation does closely line up with what you have suggested. Astronomers have also proposed that supermassive black holes seem too big for how long the Universe has been around, forcing some to theorize that they were seeded by primordial black holes like you suggest. However, astronomers don’t think primordial black holes are not parts of the original singularity that didn’t explode. One main theory is that random fluctuations in density in the early Universe could have created small pockets of matter that were dense enough to make black holes, which would then be able to expand quickly. Another theory is that stars in the early Universe would have been able to grow to massive sizes and thus produce massive black holes. We can’t currently verify either of these theories on primordial black holes (or whether they are the seeds of supermassive black holes) since we have never actually observed something we know for sure to be a primordial black hole and have only started making precise measurements of supermassive black holes, but these theories may end up being confirmed in the future.
Why do accretion disks orbit around black holes in a flat plane? (Intermediate)
The first thing to understand is that accretion disks are not unique to black holes. Accretion disks actually occur in a number of other astrophysical situations, like protoplanetary disks around young stars or gas inflow around an accreting white dwarf star that is about to make a Type Ia supernova. In all of these cases, even if the material being accreted starts off as a sphere, it is quickly flattened out into a planar disk because of friction. If two clouds of gas were to orbit around a black hole spherically, eventually those two clouds of gas would run into each other and friction would slow them down relative to each other, effectively averaging out their velocities. Over subsequent orbits, the friction would eventually average out all opposing motion of the two clouds and make them travel in exactly the same direction around the black hole, turning the sphere into a disk.
But why does the disk orbit  around the black hole in a particular direction? Well it turns out that black holes are not actually identical from all directions because they are all rotating, and the effects of that rotation can still be felt even through the event horizon. Due to some complex math, the shape of the event horizon actually bulges out at the equator for rotating black holes, and that bulge drags spacetime along with it, forcing things that are orbiting close into the black hole to orbit in the same direction as the black hole is rotating. So even if material were to accrete onto the black hole spherically, eventually the fabric of spacetime itself would force the material to orbit in a flat plane above the black hole’s equator as it fell in.
How heavy would a black hole have to be for it to be larger than the Solar System? (Advanced)

Usually when people talk about the radius of a black hole, they mean the size of its Schwarzchild radius, which is the closest light can get before it can’t escape the black hole’s gravity. This can be calculated with a surprisingly simple equation that scales with the mass of the black hole, and the radius of the black hole at the center of the Milky Way is only about 1/5th the size of the orbit of Mercury. If we want a black hole that has a radius the orbit of Pluto (one way you could define the edge of the Solar System) then you would need a black hole with a mass 2 billion times that of the Sun, or around 500 times the mass of the Milky Way’s black hole. If you want one that is 1 light year in radius (about the largest estimate you could give of the size of the Solar System), then you would need a mass of about 4 trillion Suns (or 1 million Milky Way black holes).This would be comparable in mass to the entire Milky Way galaxy, but galaxy superclusters are usually almost 1 quadrillion Suns in mass, so it would still only be a small fraction of its overall mass.

How does quantum physics and our knowledge of Hawking radiation affect our understanding of the temperature throughout a black hole? (Advanced)

Black holes are strange objects that make most normal theories of physics break, so most of the time when we talk about black holes, we choose to instead talk about the event horizon. As you may have discovered in your research, the event horizon of a black hole is the threshold in space where the black hole’s gravity becomes too strong for anything to escape. The escape velocity there is defined in the same way as it is for anything else: it is the speed you would have to go in order to break out of the gravitational pull of the object. There are no fancy or exotic forces at play surrounding a black hole, just gravity, so when you get close enough to the surface that the gravity is so strong that the escape velocity is the speed of light, then that is the event horizon. Inside of there, a lot of physics gets messed up, so we’ll just talk about the edge.

Hawking radiation (another strange concept that goes against a lot of physical intuition) says that regardless of the fact that nothing should be able to escape, black holes constantly give off tiny amounts of energy. This can be thought of in a few different ways. One relies on the notion of pair creation, where a particle and an antiparticle can spontaneously appear out of nothing but energy in the middle of space. This has been proven to happen constantly around in the Universe (so there are constantly electrons and positrons being created around you all the time). Most of the time, these particles immediately smack back into each other and turn back into energy, but if these particles happen to be created right at the edge of the event horizon, one of them may have the energy necessary to escape, while the other falls back into the event horizon and into the black hole. Because these particles were created with energy from the black hole, the escaping particle represents a loss of energy for the black hole.

Another way of thinking about this is to think of it as quantum tunneling. In quantum physics, particles are represented as waves, and any wave has a chance of making it past a barrier it comes up against (think of this as sound waves getting through a wall; some get through but most are blocked). For higher barriers, the waves have less of a chance of making it through, and the event horizon is a very very high barrier so almost nothing ever makes it through. However, in the rare occasion a wave makes it through the barrier, that represents a particle making it out of the event horizon and thus the black hole losing energy.

So now we can talk about how the temperature of a black hole is even defined. For astronomical objects like stars, temperature is often defined by how much light it gives off and in what colors. The more energy they give off, the hotter we say the object is. You can see this in the difference between a smoldering fire that glows a dim red and a hot fire that shines bright white or blue. The only energy that a black hole gives off is Hawking radiation, so the temperature is defined based off of that. However, Hawking radiation is tiny, and as black holes get bigger, they give off less and less Hawking radiation, so real black holes have extremely cold temperatures. For instance, a black hole the mass of the Sun would have a temperature of about 6 * 10^-8 K and the black hole at the center of our galaxy has a temperature of about 10^-14 K!

One interesting consequence of this is that black holes are literally colder than the Universe. The cosmic microwave background is a constant bath of energy from the early Universe that bounces around everywhere in space. It has cooled over time as the Universe has expanded, but it still has a temperature of 2.73 K, warmer than any black hole with a mass greater than that of the Moon. So the empty Universe is giving more energy to black holes all the time, and since black holes give off less energy the bigger they get, they just keep getting colder. This process will only stop once the Universe cools to be colder than the temperature of black holes and then the black holes slowly evaporate with minuscule amounts of energy. That should happen in about 10^100 years or so.

Does losing mass through Hawking radiation make a black hole get less dense? (Advanced)
This is an interesting question that gets to the core of what’s so weird about black holes. When we talk about the “size” of a black hole, we’re usually referring to the size of its event horizon (minimum distance where light can escape), which is determined solely by the amount of mass contained within the black hole. If a black hole loses mass, its gravitational pull obviously gets weaker, so that means light can get closer to the center without it getting sucked in forever, so its event horizon shrinks. So as Hawking radiation slowly evaporates the mass of a black hole, its event horizon gets smaller and smaller, keeping the “density” high.
But even the concept of density doesn’t work so well here. Our current understanding of physics mostly breaks down inside the event horizon of a black hole, but as far as we know, the object at the center of a black hole is an infinitely dense point of mass called a singularity. These singularities are what result when the core of a very large star compresses itself so much that it squeezes all of its protons, neutrons, and electrons out of existence. So when a black hole gains or loses mass, it’s not like you’re adding or taking away atoms from the center of the black hole and making more or less dense. You’re really just increasing or decreasing the amount of “stuff” that is contained within this infinitely tiny, infinitely dense singularity at the center of the black hole.
If you really want to get technical about it, we can actually calculate the density of a black hole, using its event horizon as its size. The radius of the event horizon is called the Schwartzchild radius and it is determined by the surprisingly simple formula of
For a spherical black hole, the density is just M/(4/3 pi r^3), which works out to


Since mass is in the denominator here, this means that as a black hole gets more massive, its density actually goes down. For instance, a 1 kg black hole would have a density of 7 * 10^80 kg/m^3, whereas a black hole the mass of the Sun would have a density of about 2 * 10^20 kg/m^3, and the black hole at the center of hole Milky Way only has a density of about 10^7 kg/m^3 (only about 10,000 times denser than water!).
So, counterintuitively, taking mass away from a black hole with Hawking radiation actually makes it denser since it reduces the size of the event horizon. A black hole will eventually evaporate and cease to be a black hole when it loses all of its mass to Hawking radiation, but until then, it will maintain its extraordinary properties.


How are these relativistic jets around black holes made and how are these particles/radiation able to travel at near the speed of light? (Intermediate)
The specific physics of what specifically creates these relativistic jets is not entirely known, but I’ll give a good theory. These jets come from black holes that are actively sucking in material in structures called accretion disks. You can think of an accretion disk like water swirling down a drain: as the water spirals in, it gets faster and faster and bunches up more and more until it finally goes down the drain. In the case of an accreting black hole, infalling gas gets compressed and sped up greatly as it falls into a black hole over time, turning the gas into a glowing cloud of ions. As you may know from physics class, moving charged particles (like ions) create a magnetic field, in this case a very strong magnetic field that comes out of the top and bottom of the accretion disk. This magnetic field then accelerates some of the ions out of the disk, creating the jets of material we see. Because the magnetic fields are so strong and the particles are so light, they can be accelerated to almost the speed of light.
The reason we don’t know for sure how this happens is that these jets are very difficult to study because the area that creates them is so small. The inner accretion disk where this is happening is not that much bigger than our solar system, but the galaxies they are in are sometimes billions of light years away. We can theorize and simulate what would happen in these situations, but our current telescopes don’t have the resolution to actually see what is going on up close.
How did scientists prove that there is a supermassive black hole at the center of every large galaxy? (Intermediate)

Let’s start with just the Milky Way. We can tell that there is a black hole at the center of our galaxy just by looking at the stars orbiting around the center of the galaxy. For the last ~20 years, astronomers have been tracking stars in the area and have seen them zipping around some invisible object at incredibly high speeds. Using what we know about gravity from our own solar system, we are able to calculate the orbits of these stars and determine that it weighs about 4 million times as much as the Sun. There is no other object that could have that mass but give off no light, so it must be a black hole.

Moving on to other nearby galaxies, we see evidence of black holes in the form of active galactic nuclei (AGN). A galaxy with an AGN usually has some amount of light and material emanating from the center of the galaxy where the black hole is. To understand why, we have to think about what is going on around a black hole. Gas that is in the vicinity of the black hole is frequently condensed down into a flat circle of matter orbiting quickly and closely around the black hole (think Saturn’s rings except it’s massive amounts of gas). When some of this gas enters the black hole, it speeds up immensely as it falls in, and friction with other gas around it makes it incredibly hot, leading to it glowing very brightly. The huge amount of light blows out a lot of the other gas around the black hole, forming the jets that we see in the picture above. We see many such galaxies around us with varying levels of activity (corresponding to varying levels of gas), and we know that nothing else could make such a big and bright jet, so they must have black holes.

We can see similar effects clear across the Universe in the form of quasars, AGNs that are accreting tons of gas and are thus extremely bright. Quasars can be thousands of times brighter than the galaxy they live in, meaning they can be seen from very very far away (the furthest one we have seen is 29 billion light years away). They can be especially bright when their beams of ejected matter are pointed at us (these are called blazars). Similar to AGNs, nothing else can explain the brightness of a quasar other than huge amounts of gas falling into a black hole, so we know that even galaxies in the early Universe must have had black holes.

There are other lines of evidence that can be followed (simulations of galaxy formation don’t produce correct results without black holes, we can see black holes in the centers of some galaxies with radio waves, etc.), but the point is that we have observed a lot of supermassive black holes. Astronomers have not catalogued that there is indeed a supermassive black hole at the center of every single galaxy in the sky, but at this point, that is the conclusion that makes sense.


Is it possible to rewind the expansion of a supernova that has already exploded to figure out when it happened? (Beginner)

This idea of “rewinding” the expansion of supernova remnants has long been used to determine how long ago they exploded, and the process is roughly explained here. Astronomers have used this to match up existing supernova remnants to historical records from ancient astronomers who saw the actual explosions hundreds or thousands of years ago, which allows us to know more about the system as a whole. As long as you can measure the expansion speed and the distance (which are not always easy things to determine but are possible) then you can figure out roughly how long it has been expanding by running a computer simulation of the process.

If the star is 624 light years from us and astronomers are noticing all this activity possibly leading to a supernova, then what they’re actually seeing occurred 624 years ago, is that right? So then Betelgeuse could have already gone supernova but were only seeing what may have occurred already? (Beginner)

Since Betelgeuse is so far away, the light that is reaching us now tells us what actually happened ~600 years ago. So it is possible that Betelgeuse already went supernova in its own reference frame, but we wouldn’t see that event until 600 years later since the light would take 600 years to reach us. Astronomers are actively making new observations and using models to infer the probability that we may see Betelgeuse explode. There are multiple ways to collect data on the nature of Betelgeuse’s explosion (if that happens/happened) — not just visible light, but also X-rays, radio waves, gamma rays (other “forms” of light), gravitational waves, and cosmic rays (high-energy particles like protons that travel at nearly the speed of light). It’s possible that some of these observations will be slightly delayed relative to others due to the nature of the explosion and/or variations in the interstellar medium along the line of sight to the supernova — that itself may help us learn a lot about the event.

What cased the superluminous supernova ASASSN-15lh, the brightest supernova ever seen? (Intermediate)
Supernova 15lh is a unique object in astronomy and has thus garnered a large amount of attention since it was first discovered in 2015. As I’m sure you know, it was around 2x brighter than any other supernova ever observed, which means that existing explanations for how supernovas happen don’t provide a very good explanation for it. This confusion over what it actually was has led to many published studies in the past 5 years (there have been 3 published papers on it just in the last 6 months), and yet there is still no consensus on what could have caused it.
The most obvious explanation for what it was is a supernova, hence the name it was given upon discovery. However, this model doesn’t necessarily fit the observation well. When astronomers look at a supernova, they usually will take a spectrum of it, splitting its light into its different frequencies to determine what elements are present in it. Usually, there are many elements present in the spectrum of a supernova because stars will fling out many of the elements they have been fusing in their cores when they explode. As time goes on, the signatures of these elements will usually get clearer and clearer as the clouds of material around the exploded star cool down, meaning they block more light. However, this is not what was observed with SN 15lh, which displayed few elemental signatures when it exploded and didn’t gain more as it cooled. This means that if it was a supernova, it would have to be a type that has never been observed before.
Another option for what it could have been is something called a tidal disruption event, which is a bright flash that occurs when a star falls into a black hole. As the star is ripped apart by the black hole’s gravity and its gas is fed into it, the gas is heated and compressed, releasing massive amounts of energy. These types of events have the potential to be very bright if a large star falls into a black hole. Some proponents of this theory seem to believe that 15lh could have been a supernova happening at the same time as a tidal disruption event, but who knows.
So not that much is really known about 15lh despite all of the studies that have been done on it. I’m not an expert in this subfield of astronomy so it’s possible that there are other explanations for what it could be, but the moral of the story is that it is unsettled. It’s often difficult to be sure about your explanation for an astronomical observation after it has disappeared, so the best way we can hope to learn more about 15lh is to wait until we find another object that looks similar to it and compare them.
What determines whether mass falling onto a white dwarf makes a nova or a supernova (Intermediate)
on the surface, the mechanism for both novas and supernovas sound very similar. Both of them come from matter from a nearby star falling onto a white dwarf, but the specific mass of the white dwarf makes that process happen differently. If the mass of the white dwarf is low, then we get a nova, where small amounts of material undergoes fusion on the surface of the white dwarf for a relatively long period of time, making a dim but sustained light. This can happen multiple times for the same system because the mass of the white dwarf isn’t changed that much by each nova.
By contrast, a Type Ia supernova happens when the cumulative effect of adding all of this mass to the white dwarf pushes it close to the Chandrasekhar limit, the mass at which a white dwarf collapses into a neutron star. When a white dwarf approaches this limit, its internal structure rapidly increases in pressure, restarting fusion in its core and setting off a runaway chain reaction that leads to the entire white dwarf exploding in a ball of fusion energy that we see as a supernova. This can only happen when the white dwarf gets to a very specific mass, so it takes many individual novas to get there and can only happen once.
So fundamentally a nova and a Type Ia supernova are kicked off by the same event (mass falling onto a white dwarf from a neighboring star) but they end differently because of the specific conditions inside the white dwarf itself.


What are the conditions for a star to become a giant, compared to the conditions for a star to become a white dwarf? (Beginner)
Giant stars and white dwarf stars are just different parts of the life cycle of a normal Sun-like star. Stars start out like the Sun is right now, medium size and staying the same size for billions of years. Once the run out of hydrogen fuel to burn in their centers, they switch to helium fuel, which burns much hotter than hydrogen. The extra heat that the helium puts out makes the star puff out, making the star a red giant. For the Sun, the giant phase will only last for as long as it still has helium to burn, so when it runs out after about a billion years, it will collapse into a white dwarf. This is because it can’t generate enough heat to support itself, so its gravity makes it fall in on itself into a tiny inert ball called a white dwarf.
Some stars are big enough that they can generate enough mass to burn other elements, like carbon and oxygen, to become giants again after they run out of helium. These stars will continue to get bigger and bigger as they burn hotter and hotter, but eventually all stars run out of fuel and collapse down under their own gravity into a tiny inert ball that can’t burn any more fuel. For smaller stars like the Sun, this is a white dwarf, but bigger stars make denser remnants like neutron stars and black holes.
Are we close enough to Barnard’s Star to trade comets with it in our respective Oort clouds? (Intermediate)

The Oort cloud is a very poorly defined thing, existing only in theory and with very loose definitions. Generally, astronomers use the Oort cloud to encompass all of the orbital debris left over from the formation of the solar system that has been kicked far enough away that it is only bound to the Sun in the loosest way. This would theoretically extend out to the edge of the Sun’s gravitational influence, but where that actually is varies over time.

Barnard’s Star is about 6 light years away, a distance not too much larger than the Sun’s area of gravitational influence. However, Barnard’s Star is less than 15% the mass of the Sun, so its area of gravitational influence (called its Hill sphere) is only about half as large. So even using the absolute largest estimates, its Oort clouds probably doesn’t overlap with ours.

However, the gravitational effects of passing stars definitely effect  Objects may have their orbits modified by passing stars and be kicked into other orbits (one theory of where comets come from). In the future, Barnard’s Star will be even closer to us, so it will stand a better chance of meaningfully influencing our Oort cloud. You may find this diagram of the distances to nearby stars over time interesting (using a more conservative estimate for the Oort cloud):


Another fun fact: the star Gliese 710 will pass only 0.178 light years from the sun in about 1.3 million years, definitely disrupting the Oort cloud and becoming the brightest star in the sky for a short amount of time. 

Is it possible to trace back the path of interstellar asteroid 'Oumuamua to figure out where it came from? (Intermediate)
This is an interesting problem that numerous astronomers have looked into in the years since ‘Oumuamua was first spotted, but it is a very difficult question to answer with any certainty. However, this paper published a couple of years ago performed a good analysis of the problem, so I’ll summarize its results for you.
The main difficulty with figuring out where the object came from is that everything in a galaxy is constantly moving. Though the stars seem fixed to us, they are in fact constantly moving through the sky, causing constellations to change shape slowly over time:
So a slowly moving object (like ‘Oumuamua or any other asteroid we will ever see passing through the solar system) would take so long to move between stars that if whatever star it passed last has probably moved to a different part of the sky by now.
We can, however, measure how fast the stars are moving using an instrument like the Gaia spacecraft and “rewind” their positions backwards using a computer simulation. The authors if this paper did exactly that, looking at nearby stars and playing time backwards to figure out where they were back when ‘Oumuamua would have been close to them. This becomes very difficult though because stars behave like a chaotic swarm of bees, meaning that even a small error in your measurement of a star’s velocity can lead to large inaccuracies if you go too far back in time. Despite this, the authors were able to identify 28 stars that the ‘Oumuamua got relatively close to over the past 10 million years (seen in the very dense table on page 5).
Another difficulty is that stars don’t have to just be in the right place but they also have to be going the correct speed. Though we don’t know for sure how ‘Oumuamua was formed, we can be pretty sure that it was thrown out of a solar system at some point, meaning it would be moving in a similar direction as the star it was ejected from (this is like how a rock you throw out the window of a moving car will travel away from the car but still be moving close to the same general direction). With that in mind, it becomes even more difficult to figure out which star it would have come from, but the authors give a couple of options in the abstract on page 1 of stars that were relatively close and somewhat the same speed.
So it’s likely that we will never know exactly where ‘Oumuamua came from because of these problems. We don’t even know for sure that it came from a nearby star (as this paper assumes), so the star it came from could be far, far away in a place we would never be able to guess. Hopefully this helps and let me know if you have any more questions.
What would happen if you replaced Pluto with a red dwarf star? (Intermediate)
The idea of replacing one of the planets with another star is something that comes up in science fiction every once in a while (it’s the ending of the movie 2010, the sequel to 2001: A Space Odyssey), but people often overestimate the brightness of small stars. The amount of light a star puts out (its luminosity) goes down very quickly as stars get less massive. For instance, Proxima Centauri, the nearest star to our Solar System, puts out less than 1% of the light that the Sun does. If we were to put Proxima Centauri at the distance to Pluto and calculate the brightness with the flux equation Flux = Luminosity/(4 pi Distance^2), we get that the amount of light delivered to the surface of the Earth is only around 5 milliWatts per square meter. For reference, this is a bit more than twice as bright as the full moon, or almost 200,000 times dimmer than the Sun. So a red dwarf at that distance wouldn’t really turn night into day, it would just create faint shadows on the ground in the same way the full moon does. Red dwarfs have a range of luminosities though, so a big red dwarf would be almost 30 times brighter than the full moon and a dim one would be only 1/20th as bright as the full moon. Still, 30x brighter than the moon is still 17,000 times dimmer than the Sun, so you’re still not getting broad daylight here. In fact, the Sun itself only shines at 0.08% of full brightness at the distance of Pluto, which is why Pluto is so cold in the first place.
As for where Pluto is in the sky and how long it would be up, that depends on the time of year. Pluto is currently between Sagittarius and Capricorn, meaning it can be seen best during the (northern hemisphere) summer. Since it takes Pluto almost 250 years to go around the Sun, a star in its position would appear to stay basically still, only moving less than 2 degrees per year. Over the course of a human lifetime, this hypothetical star would only move around 1/3 of the way around the Sun. If you’re interested in seeing where Pluto is over time, I recommend using the free planetarium software Stellarium.
So a red dwarf in the place of Pluto would appear to be a bright red star similar in brightness to the full moon. During the summer, this dot would cast dim red shadows on the ground during summer nights and being mostly drowned out by the Sun in the winter, only really visible as a dim red dot. Hopefully this answers your questions and let me know if you have any more.
What would happen if there was a piece of white dwarf or neutron star one Earth? (Intermediate)
The degenerate matter in the interiors of white dwarfs and neutron stars is fascinating because it has properties that are incredibly strange compared to the matter we normally interact with. Beyond its temperature, magnetism, and brightness, the properties that are most important here are its density and its energy density.
First we’ll talk about its energy density. Depending on where in the white dwarf you got your material from, it can be anywhere from 10 thousand degrees to over 10 million degrees, and for neutron stars, it can get up to 10 billion degrees, which means that there is a very large amount of heat energy contained in the material that would instantly heat up everything else around it. However, the larger problem is that degenerate matter inside white dwarfs and neutron stars is compressed with unbelievable force, and if you suddenly remove all of the pressure, the material would expand with extreme power. The material at the core of a white dwarf is compressed to be about 500,000 times denser than it would be at normal atmospheric pressure, so a teaspoon of white dwarf matter would instantly expand to be about 2500 liters, creating a destructive shockwave. For a neutron star, it would expand to a sphere about 450 meters across, and the shockwave would be even worse. Energy is stored in this compression (just like in a compressed spring), so the energy released by this decompression would add to the heat and probably vaporize everything around it for a pretty wide radius.
If we assume that the material doesn’t explode, then we can give more interesting answers than “everything explodes”. This part of the question is very similar to a question answered in the excellent book What If? by Randall Munroe, which I highly highly recommend if questions like these are interesting to you. To summarize what he says in the book, a hunk of neutron star matter would be far too dense for the ground to support, so it would immediately crush any dirt or rock that it sat on and start to sink into the ground. It would quickly pick up speed as it fell towards the center of the Earth, leaving a trail of vaporized rock behind it. Once it reached the center of the Earth, it would probably shoot straight through and go back up the other side a bit, but I don’t think it would ever make it out the other side. It would have lost too much energy from the drag of breaking through 6000 km of rock, so it would probably only make it part of the way out before falling back in again. Eventually, it would settle to the center of the Earth, where it would remain until the Earth was destroyed.
How do white dwarf stars get so small and dense? (Intermediate)
The physics surrounding the density of white dwarfs is relatively complex, so I’ll do my best to break it down for you here.
The first concept to understand about stellar structure is hydrostatic equilibrium. Normally, when we take a gas and try to compress it, the gas pushes back because we are trying to keep the gas molecules closer together than they want to be, making them run into each other as they zoom around inside the balloon. Squeezing more and more gas molecules together causes them to run into each other harder and more frequently, raising the pressure of the gas. For example, in a balloon, the rubber in the balloon is trying to pull the gas inwards, but the pressure from the gas makes sure the balloon doesn’t get too small, and when you blow more gas in, you increase the pressure, making the gas push more outwards and increase the size of the balloon.
Normal stars like the sun work similarly to this. The gas in the star is being compressed by its own gravity, but it pushes back with gas pressure to balance out the force. This pressure is primarily from the temperature of the gas. High temperature gas has a higher pressure, so the nuclear fusion that is taking place in the center of a star heats up the gas, which raises the pressure inside the star. If we add more gas to a star, it raises the temperature of the inside and increases the pressure, so the star will increase in size and reach a new balance. This state of balanced pressure and gravity is called hydrostatic equilibrium, and every star is in equilibrium basically all the time.
What happens if a star stops producing heat though? When a star runs out of fuel to keep doing fusion, it will stop producing enough heat to support its current gas pressure, so it will shrink in size. With no more heat being produced, the atoms stop running into each other with as much force, making them unable to push outwards as hard. However, there is another form of pressure that keeps the star from getting too small called electron degeneracy pressure. Due to some relatively complex quantum mechanical effects that I won’t get into here, no two electrons can be in the same place. There are a lot of electrons floating around inside of this cooling star, so when they get too close together, quantum mechanics starts pushing back, forbidding the electrons from getting any closer. This leads to a limit on how dense the material can get, meaning the star stops getting smaller. So even though these stars are still technically made up of carbon and oxygen, they aren’t at all like the carbon and oxygen we interact with on a daily basis. They are a super dense sea of atomic nuclei and electrons that don’t have most of the empty space that is present in the atoms we have on earth, allowing them to be much denser.
So this is how a white dwarf ends up so small. A star like our sun will run out of hydrogen and helium to fuse eventually and be left with a core of oxygen and carbon that it can’t produce heat with, so that core will get smaller and smaller until the only thing holding it up is its own electrons. At this point, the core (which will have a remaining mass of about 60% the mass of the sun) will be packed into a radius about the size of the earth, meaning it has an average density of about 10^9 kg/m^3, or about a million times denser than water.
The density is lower at the surface and higher in the center, so we can expect densities to be higher in the center and lower around the outside though. Notably, since adding more mass doesn’t increase the temperature anymore and just adds more gravity, heavier white dwarfs are actually smaller than lighter white dwarfs, meaning they have average densities of up to 10^10 kg/m^3. They still run up against the same limits in density that electron degeneracy pressure dictates though. It is only when a white dwarf surpasses about 1.4 times the mass of the sun that the central pressure gets high enough to overcome the resistance of the electrons and collapse into a neutron star.
The reason we call these small balls of degenerate carbon and oxygen “white dwarfs” is because they are still hot enough to give off a fair amount of light. This material used to be the core of a star, so it has a fair amount of heat stored in it that it will radiate off into space (it just won’t generate any more since it can’t do fusion). This light looks white to us, but over time, we expect that these white dwarfs will dim as they lose their energies, transitioning from white to black. However, a black dwarf still has the same degeneracy pressure, so it will still have the same size and composition as the white dwarf it evolved from. This process takes so long that we don’t expect there to be any black dwarfs currently in the universe though.
Where do Wolf-Rayet stars fit into stellar life cycles? Are there any that are nearby enough that we should be worried about them exploding? (Intermediate)
Wolf-Rayet stars are exceedingly rare in the galaxy. When a very massive star gets towards the end of its life, it switches from fusing hydrogen in its core (like the Sun does) to fusing heavier elements like helium, carbon, and neon. These elements require much higher temperatures and pressures to fuse, and the stars put out much more energy when they fuse them. This results in the outer layers of the star being blown apart by its own energy, creating a cloud of material around the extremely bright star. This stage lasts a relatively short amount of time (probably less than 1 million years), so that fact, combined with the rarity of having stars big enough to form WR stars in the first place, means that there are very few WR stars in the galaxy at any time. Here’s a complete catalog of known WR stars, which is less than 700 stars long, and estimates say that there are probably only around 1000 WR stars in the rest of the Local Group (our neighboring galaxies). Because they are so rare, the nearest one is still very far away (around 1000 light years) so we won’t have anything to worry about when it finally goes supernova. Not that that will happen in our lifetimes anyways.
If stars lose mass as they grow into giants, will the decrease in mass cause Earth's orbit to expand enough to escape being swallowed by the expanding Sun? (Advanced)
This is an interesting question that I have not thought about before that involves a lot of variables. First, we don't know exactly how much mass the Sun will lose as it becomes a red giant. Estimates I can find are in the range of 20-33%, but it depends on some specific stellar physics that are not totally known. Because of conservation of energy, we know that if the Sun lost mass and went from M1 to M2, then the orbital radius for a planet would expand by a factor of M2/M1, so this means Earth's orbit will get 25-50% larger as the Sun loses mass.
However, this article (which is based on this paper using state of the art stellar simulations) claims that this growth in orbital radius won't be fast enough to outpace the growth of the Sun's radius, meaning that the Earth will be engulfed by the Sun on its way out. So unfortunately, it doesn't look like Earth has a chance. This won't be as catastrophic as it could be though since the Sun will be continuously growing hotter and hotter over the next few billion years anyways, eventually making the Earth too hot for liquid water and forcing us somewhere else. So the Earth will be a lost cause by then anyways.
How did Henrietta Swan Leavitt calibrate her Cepheid period-luminosity relationship if there weren't any accurate parallax measurements of Cepheids at the time? (Advanced)

Henrietta Swan Leavitt didn’t actually know the distance to the Cepheids she was observing, but she did know one thing: they were all the same distance away from her. She was very creative in her observations and chose to only study the Cepheids in the Small Magellanic Cloud, a satellite dwarf galaxy of the Milky Way that even at that time, astronomers realized was distant and cohesive. She knew that all of the Cepheids in the SMC would be the same distance from her (approximately), so she could isolate differences in brightness due to their periods and use that to generate a Period-Luminosity relationship. However, you are right that she was still limited by the rudimentary parallaxes of the time, so she was not able to turn her relationship into an absolute distance measurement before her death. It was only years later when Hertzsprung and Russell (two very famous names in stellar astrophysics) were able to measure parallaxes well enough to calibrate Henrietta Swan Leavitt’s distance measures and actually find out how far away the SMC was.