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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.

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.
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.

Quasars

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.

Supernovas

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.

Stars

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):

image.png

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:
image.png
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.
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.