Quasistars and the Early Universe
Advisor: Mr. Kostopoulos and Ms. Kannan
Internship Location: FlowData LLC (Remote)
Onsite Mentor: Dr. Jim Chonko
Imagine a star that is wider than the Solar System, tens of thousands of times more massive than the Sun and devouring itself from the inside. It sounds like something from an outlandish sci-fi movie, but these quasistars likely did exist in the hot, dense universe 500 million years after the Big Bang. This project studies how the birth, life, and death of a quasistar around 13 billion years ago would affect its surrounding galaxy. In this project, I collect data through research papers and publicly available NASA pictures and studies. Using that data, and with the assistance of Jim Chonko of FlowData LLC, I determine a realistic base for a model galaxy, and then calculate different properties of the galaxy, including temperature, density, and star formation rate, in different regions. Through my research, I expect to find that galaxies centered around quasistars will be quite different from the galaxies we can see today. Ultimately, this project aims to help advance the relatively unexplored study of the early universe and quasistars.
Project Blog
Week 0: Introduction
Hello everyone, my name is Beckham and welcome to my week 0 blog. My project is titled Quasistars and the Early Universe, and as the name suggests, I will be studying quasistars and their potential impacts on the early stages of galaxies.
What is a quasistar?
A quasistar is a huge object that would've existed at the center of a galaxy around 500 million years after the Big Bang. They would have masses up to tens of thousands of times greater than the sun and would look like a giant star from the outside. However, on the inside, there would be a black hole devouring it. The way this would happen is the dense gas of the early universe would allow the quasistar to grow absurdly large by continuing to feed on its surroundings. Because the star gets so massive, the core would run out of fuel very quickly and would collapse into a black hole. The outside portion of the star would be so massive, however, that instead of being blown away (which is what usually happens when a star collapses) it would continue existing as a "shell" around the black hole, being devoured from within.
What will I be doing for my project?
My goal for this project is to create a realistic sample early-universe galaxy with a quasistar at its center, divide it into regions, and calculate the temperature, density, and rate of star formation of each region of the galaxy. I would also determine where metals would appear first and how energy is being generated in each region, if at all. What I will be doing in my project is collecting data through research papers and publicly available NASA pictures and studies, then using those to determine a realistic structure for the galaxy, and then doing lots of math and physics to determine the properties that I want to figure out. I will be working with Jim Chonko of FlowData LLC (https://flowdata.ai/) for my project.
Week 1: Mass
Hi everyone and welcome back to my blog! This first week of my project, I read "Quasistars and the cosmic evolution of massive black holes" and "Quasi-Stars: Accreting Black Holes inside Massive Envelopes". Because I'm still gathering information, I can't do much work directly on my model, but through my reading, I have figured out a few things relating to it. I suppose I can share a bit of what I've learned so far relating to my model in the most easy to understand terms possible. If you don't get some of what I'm saying, don't worry, I don't get it either (I'm kidding, I get at least half of it).
About quasistars and galaxies
First of all, the approximate upper limit to the mass of a quasistar is on the order of 10^6 times the mass of the sun (M☉). Additionally, it is very rare for a quasistar to form in a galaxy with mass 10^11 M☉ or less. Coincidentally, the mass of the Milky Way's central black hole is around 4*10^6 M☉ and the mass of the galaxy is around 10^12 M☉. As it turns out, quasistars could have much lower masses (around 10^3M☉), and their "shells" could form around pre-existing black holes. This would make it possible for these smaller quasistars to orbit something at the galactic center (potentially a much larger quasistar).
About stars
I'm sure you're tired of the word "quasistar" by now, so it's time to focus on (somewhat) regular stars. Before the existence of stars, hydrogen and helium were the only two elements in existence. So, stars formed 350-500 million years after the beginning of the universe would be made of almost entirely those two elements; these stars are referred to as population III stars. It turns out, population III stars could get much more massive than the currently existing population I and II stars, with stars of up to 260M☉ not being all that rare (for comparison, only one known, currently existing star has a mass that large, and very few even come close). Because of their large masses, the gravitational effects of these pop III stars might be something I need to take into consideration in my model.
For the next few weeks, I'm still mostly just going to be reading papers, so I doubt there will be much action in these blogs. I'll still keep you guys updated with everything I find out of course. Have a great day, and I hope to see you next week!
Week 2: Early Galaxies
Hello everyone and welcome back to another blog post! As I mentioned last week, my work right now is really just reading research papers to gather more information and data. This week, I read “The Dark Ages of the Universe and Hydrogen Reionization” by Aravind Natarajan and Naoki Yoshida, as well as “Dark-ages reionization and galaxy-formation simulation – VI. The origins and fate of the highest known redshift galaxy” by Simon J. Mutch et al. Just like last week, I will go over some of the key things I learned and how they relate to my project in a manner that is as simple to understand as I can make it.
The earliest stars
Star formation at the beginning of a galaxy’s lifetime would be very different from how it is today. The gas making up new galaxies would have only recently de-ionized, meaning they would be incredibly hot. The high temperatures in these early galaxies would make it harder for gas to contract into a star, meaning stars would be few and far between. They would also have a much easier time forming in areas with dense gas and high gravity, making many of these early “population III” stars have masses over 100 times that of our sun.
Reionization
These massive stars released huge amounts of high-energy UV light. Matter-antimatter reactions also caused the release of higher energy x-rays and gamma rays. This high energy light would then hit gas particles and ionize them. These ionized gas molecules would be way too hot to condense into stars, contributing even more to the scarcity of population III stars.
The impact on my model
In terms of what all this means for my model, star formation rates will likely end up being much lower than what we see currently in the Milky Way and other nearby galaxies. This would be caused by the presence of ionized gas and the overall high temperatures. Luckily, my model will also be a little easier to make, since I won’t have to consider low-mass stars. This is because there would be too few of them for their small masses to make an impact.
Whether you like it or not, the next few weeks will still be less model-making and more reading. And with that, thanks everyone for tuning in to my blog this week, and I hope to see you next week!
Week 3: Before Galaxies
Welcome back once again to my blog! This week I once again read research papers (exhilarating stuff, I know) to prepare for all of the math and physics I’ll be doing. I read “In the Beginning: The First Sources of Light and the Reionization of the Universe” by Rennan Barkana. This paper mainly went over much of what would have happened before the time period of my project (0.5 - 1 billion years after the Big Bang) that led up to the formation of stars and galaxies as we know them. As always, I will summarize what I learned and go over how it relates to my project, all in the easiest possible way to understand.
Galactic haloes
A galactic halo is a gigantic sphere with gas and dust covering its surface. These haloes surround all galaxies, and came into existence before the galaxies they house. According to the most prominent theory currently, they are primarily made of dark matter, a type of matter that doesn’t interact with light at all, but they have a lot of baryonic (regular) matter as well. After existing for a certain amount of time, haloes collapse into virial equilibrium, where the particles of the halo are in stable orbit around the center of mass, such that 2x their kinetic energy = their gravitational potential energy.
Galaxy formation
When the halo collapses into equilibrium, part of it flattens into a disk closer to the center of mass, which later will become a spiral galaxy. When this disk appears, it fragments into many clumps with around 10^2 - 10^3 solar masses. Some of these clumps turn into globular clusters, areas dense with stars which still exist today (though the original stars have died). Dwarf galaxies were common towards the beginning of the universe, but so were collisions between galaxies, meaning some dwarf galaxies were able to merge into larger ones.
My model
What I learned this week has a few impacts on my model. First of all, it turns out star formation, and therefore metal production, began in the haloes before galaxies were even fully formed. This means that I already know where stars were formed, so I just need to focus on their formation rate. The paper that I read also had some very handy equations that I will absolutely use to calculate many of the things in my model. Additionally, I may want to base my model on a galaxy that previously merged because those would have more gas near the center. However, I may not want to do that because dealing with the consequences of a merger may be biting off more than I can chew for this project.
That’s about it for my week 3 blog. Thanks for reading and I hope to see you next week.
Week 4: Supernovae and metals
Hello everyone and welcome to my week 4 blog! As usual, I spent this week reading in order to prepare myself to work on my model. Thankfully, I should be able to begin work on the model soon. This week I read "The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations" by Wise et al., so now it's time for me to present what I learned.
Supernovae
Stars counteract the pull of their own gravity by fusing small elements together in their cores. A supernova occurs when a star with a mass at least 8 times the mass of the sun (M☉) runs out of fuel to continue nuclear fusion. There are a few different types of supernovae, but the ones I will be focusing on are type II supernovae, because only those would have been possible under the low-metal conditions of the early universe. Most supernovae occur because of a phenomenon called iron core collapse. This occurs when stars with masses between 8 and 40 M☉ run out of fuel in their core to burn, so they collapse into either a black hole or a neutron star and spew out most of their material in a highly energetic process. The other kind of type II supernova that I am looking at is called a pair-instability supernova. These can only occur in extremely hot stars with masses between 130 and 250 M☉. When very high energy light waves (gamma rays) collide with each other, they are able to create a pair of particles: electrons and positrons. Gamma rays can also collide with the protons and neutrons within atoms to create either an electron or positron. When an electron and a positron collide, they are converted back into gamma rays. A large concentration of gamma rays in a specific region of the star causes that region to heat up and expand. The gamma rays then create positrons and electrons, causing the region to shrink, until those create more gamma rays. Eventually, this process becomes so extreme that the pressure from all the gamma rays violently tears the star apart, leaving nothing behind.
Metallicity
Metallicity is the measure of what percent of a star's mass is metal. For example, the Sun is 1.22% metal, so its metallicity (Z☉) is 0.0122. The first metals in the universe formed from very old "population III" stars that went supernova and ejected their metal into their galaxy. Stars that formed from these metals (including our sun) are known as population II stars. The formation rate of pop III stars was relatively constant for the first 500 million years of a galaxy, but pop II formation rate increased significantly and became the dominant type of star formation quickly. When the overall metallicity of a galaxy's gas and dust became greater than 10^-6 Z☉, the galaxy would cool quickly, allowing new stars to form easier
My Project
The study of supernovae is incredibly important because supernovae spew out material that allows new stars to form. Part of that material consists of metals, which change what kinds of stars form and also change the general environment of the galaxy. I will also likely be looking at the metallicity of stars and the interstellar medium in my model.
Thank you for reading this week's blog post, and I hope you tune in next week as well!
Week 5: Redshift
Hello and welcome back to my blog! This week, I read "Early Galaxy Formation and its Large Scale Effects" by Pratika Dayal & Andrea Ferrara. Usually, I like to go over what I learn each week as simply as possible, but the issue with this week is that the things I am learning are getting more and more complex. Here's a small example of some of the stuff that I'm looking at in this paper:
Nonetheless, I will still try to explain what I learned this week.
Gas Accretion
When gas began to clump together, it would form large, kind of spherical structures called haloes. Even after many of these haloes formed, there was still lots of gas drifting through space and not part of any halo. Supernovae were also able to explode with enough energy to launch gas out of their galaxy. If non-halo gas passed close enough to a halo, it would get sucked in, or "accreted", and become part of the halo. There were two ways for gas to be accreted: hot gas accretion, where the gas is very quickly heated to the temperature of the halo (thousands of degrees), and cold gas accretion, where the gas only reaches a fraction of the halo's temperature when it is accreted. Cold gas accretion is more prominent the farther in the past that you look, and hot gas accretion is more prominent in more massive galaxies.
Redshift
Redshift isn't something I learned of this week, in fact, it's an important concept in all of the papers that I've read, and I've been converting from redshift values to years for these blogs. It's also quite a hard topic to understand, but I'll try explaining it now. When an ambulance passes you by, you may notice that its siren is higher pitched (at a smaller wavelength) when approaching you and at a lower pitch (larger wavelength) when going away from you. But why does that happen? Sound waves work as a series of lines of compressed air that hit your ear. Imagine that a siren produces one compression every 1/600th of a second. If the speed of sound is 300 meters per second, the compression will be 0.5 meters away from the siren when the next one starts, meaning the wavelength is 0.5m. Now, if the ambulance is moving at 30 m/s, then it travels 0.05m in 1/600s, meaning that even though the compression at the back of the ambulance travels 0.5m away, the ambulance travels another 0.05m away. Adding the two up gets a distance between compressions (wavelength) of 0.55m behind the ambulance. Here is a visual demonstration of it as well from Wikipedia, showing that wavelength increases behind the vehicle:
Congratulations, you now know about the doppler effect! Light is also a wave, so its wavelength can be shifted with the Doppler effect too. One of the weirdest things about the universe is that space itself is expanding. Because of the expansion of space, the wavelength of light from distant galaxies slowly increases over huge amounts of time (this phenomenon is called redshifting because red is the highest light wavelength we can see). Scientists imagine this change in wavelength like the doppler shift from an ambulance, and calculate the speed that a galaxy would be moving away from us with to produce that shift. The redshift (z) of a galaxy is that speed divided by the speed of light. Because the amount that the light is redshifted is determined by how long it has been in space, we can use the calculated value of the redshift to point to time in the past. For example, a redshift of 1 corresponds to 7.7 billion years ago, a redshift of 2 is about 10.3 billion years ago, and a redshift of 5 is 12.5 billion years ago. Also, because we look into the past when looking at far away galaxies, those redshifts can be translated to distances of 10 billion, 15 billion, and 22 billion lightyears respectively.
Hopefully you understood that explanation, because I'll likely start using it in future blogs. This week should be the last week where I am reading papers, so I'll probably be starting my actual project very soon. Thanks for checking out this week's blog, and I hope that you come back next week!
Week 6: What do Quasistars Need to Form?
Hello everyone and welcome back to my blog! This week is special because it is the final week of reading papers, so I can finally start working on my model. This week I read "Effects of Turbulence and Rotation on Protostar Formation as a Precursor of Massive Black Holes" by Van Borm et al. As usual, it is time for me to present what I learned in a way that you can hopefully understand.
Direct Collapse
In the early universe, there were huge, hot gas clouds that would later collapse into galaxies. There are multiple ways that they could've collapsed, but the way that matters to my model is the direct collapse scenario. The requirements for this scenario are that the gas cloud has a temperature greater than 10^4 K and that it is rotating relatively slow. The way that direct collapse works is that much of the matter in the gas cloud quickly falls directly towards the center. The slow rotation makes it easier to fall, and the high temperature basically forces matter into the center. This quick, large infall of matter allows a huge protostar (the things that evolve into stars) to build up in the center of the cloud. If the protostar keeps gaining mass at a rate of at least 0.14 solar masses per year, it gains enough mass for the core to collapse and form a quasistar.
Cooling
In order for all of that to happen, the gas cloud needs to both start hot and stay hot. There are a few things that need to happen so that heat doesn't quickly escape the cloud. First, most of the gas in the cloud needs to be hot enough to stay as singular hydrogen atoms. If 2 hydrogens bond to each other, they cool much more efficiently. Second, the gas needs to be thick so that it absorbs radiation. In order for the gas to be thick enough to absorb the radiation that atomic hydrogen releases, it needs to have a density of at least 10^17 molecules (or atoms) per cm^3 (for comparison, the density of air at sea level is 2.7 * 10^19/cm^3). Collisions between H₂ and H atoms, other H₂ molecules, or He atoms have a high chance of releasing radiation, making thick gas even more important. Third, the gas in the cloud needs to have a very small amount of metals, as the radiation they produce can't be absorbed by a 10^17/cm^3 cloud. In order for the gas to absorb nearly all radiation, it would need to have a density of 10^20/cm^3, which is very unrealistic for a gas cloud.
As I said earlier, this coming week I'll finally start working on the model, so I'm very excited for that. With that being said, thank you for reading this week's blog, and I hope you tune in next week as well!
Week 7: Starting the Model
Hello everyone and welcome back to my blog! I am happy to announce that I am finally starting work on my model galaxy! In this post, I'll be going over many of the starting parameters of the model and some of the more general parts of the model.
Structure
The model will be split into at least 3 parts. There will be the central quasistar, the outer halo (think of it like a spherical shell of gas and stars), and the middle region between the two. Depending on what I find out later, I may have multiple sections in the middle region and I might also split the quasistar into its central black hole and its outer shell.
Requirements
Before getting into what I did, I will do a quick summary of the requirements for a quasistar's existence that I touched on in some of my other blog posts. First of all, it needs to be in a galaxy greater than 10^11 times the mass of the sun. Second of all, the gas (at least in the middle region) needs to maintain a temperature of at least 10,000 Kelvin. Third of all, There has to be at least 0.14 solar masses of gas falling into the center per year for the whole time that the quasistar is forming. Fourth of all, the quasistar needs to exist between redshifts z = 10 - 15 (300 - 600 million years after the big bang). And finally, the quasistar itself needs to have a density of at least 10^17 atoms per cm^3.
Approach
With all that in mind, here are some of the ways that I'll approach the model. First, I need to have stars forming and going supernova in order to generate enough energy to keep the gas hot. This isn't exactly the easiest task, because part of the reason the gas needs to be hot is to prevent it from "clumping", but even though large clumps shouldn't form, it will still be possible for smaller ones to create stars. I did some math and figured out that if a star begins made completely of hydrogen and fuses it all into iron, it will produce a total of 3.22 * 10^15 Joules of energy per kilogram of hydrogen, mostly in the form of heat. That does assume the most efficient scenario, but it also ignores the energy produced by the star shrinking as well as by the supernova, so it could be more than that. As a reference, if something the mass of our sun were to do that, it would produce 6.4 * 10^45 Joules over its lifetime, making it have a power of 20 billion billion gigawatts for its entire life (yes, that is 2 billions). In terms of the other requirements, the galaxy will need to have a relatively slow rotation to make matter fall in easier, and I'll have to consider how the matter falls in before the quasistar begins to form.
All that being said, it's nice to finally be able to start working on the model, and I hope you come back to see my next post!
Weeks 9-10: The bulk of the model
Hello everyone and welcome back to my blog! These last 2 weeks, I've been working on my model, and at the moment I'm almost done. I based my model around all of the research that I looked at, and I decided on an approach that has 3-4 "areas", depending on how you want to look at it. There's the outer halo (kind of like a sphere of gas and stars), the disk, and the central quasistar (which can be looked at as the black hole and the surrounding shell). Now, I will go over some of the more basic parts of my model.
The Setting
I decided to start at a redshift of z=10 (500 million years after the Big Bang). I had the starting halo mass equal to 83,000,000 solar masses, the temperature at 10,000 K, and the velocity at 17 km/s. From there, I calculated the halo radius to be 4100 lightyears and the disk mass to be 2,490,000 solar masses.
The Quasistar
I decided that the quasistar should start forming at z=10. I settled on a fixed rate that it would grow at (0.2 solar masses/yr). At that rate it would reach 3600 solar masses in 18,000 years, and the black hole would appear then. Until the black hole forms, the quasistar has a constant radius of around 300,000,000 km, or 2 times Earth's orbit. After the black hole forms, the quasistar would begin to grow in size and cool down. It would eventually dissipate around the time its temperature drops below 10,000 K, after which it leaves behind a black hole.
I only showed some of what I have done with the model because some of the math and concepts get too complex for me to explain in a blog post. Right now, I still have a few things I need to iron out, but overall my project is going well. All that being said, thank you for reading this week's blog post, and I hope to see you next week as well!
Week 8: The Original Paper and a Small Hiccup
Hello everyone and welcome back to my blog! I thought I was done with reading, but this week I read the paper that first proposed quasistars: "Formation of Supermassive Black Holes by Direct Collapse in Pregalactic Halos" by Begelman et al. I also did some work on my model along with that, so I'll summarize what I read and what I did in a way that you will hopefully understand.
Quasistar formation
In order for a gas cloud to host a quasistar, the matter in the center needs to be spinning slow enough for it to come together into one big object. In order for this to happen, the central gas first coalesces into a bar shape. This shape allows it to transfer its momentum to the outer regions of the gas cloud. After it has transferred some of its momentum, the central region of the bar turns into a separate bar and repeats the process. Once it has transferred enough momentum, it is able to condense into a gigantic star. As the core of the star gets hotter and hotter, it pushes against gravity more. However, once it approaches a temperature of 1 billion Kelvin, it starts making tiny particles called neutrinos, which carry away some of the energy. This makes the core too cold to continue resisting the star's own gravity, causing it to collapse.
Quasistar growth
When a quasistar's central black hole first gets created, it has a mass of under 20 solar masses. After it forms, the quasistar keeps a relatively constant radius of a few times the distance between the earth and the sun. It is able to do this because quasistars pull in mass from their environment while being consumed from the inside.
A small hiccup
As part of the work on my model, I tried to calculate the temperature that the Milky Way would be at during the time frame of quasistars existing. The Milky Way has a mass that indicates it could've hosted a quasistar, so I expected to get a value that went along with what I saw on the papers I read, that being around 10,000 Kelvin. Instead, I got a temperature that is so small that it literally couldn't ever occur. I kept trying to find what I did wrong, but I couldn't understand. I asked my senior project advisor for help, and he managed to calculate a value of 9,800 K. As it turned out, I mistakenly swapped out one constant (h) with another (h). I honestly blame whoever came up with the variables though, because there is no reason for two constants to use the same letter when one is 10^33 times bigger than the other.
Anyway, this week was my actual last reading, and I'll be fully focused on my model from now on. I hope you all liked this post, and I hope to see you in the next one!