Earlier this summer, I was asked to contribute to the British popular science magazine All About Space, answering a reader question about X-ray binaries. My write-up was published in the August 2021 issue!
What are X-ray binaries?
An X-ray binary has a compact object, like a stellar black hole or a neutron star, and a companion star that is normal like our Sun. The compact object and the companion orbit around each other, bound together by gravity, and they’re close enough that the compact object slowly eats its friend! As it drains material from the companion and gathers it together to eat it, this material gets hotter than the surface of the sun and shines very bright X-ray light.
How the compact object eats its companion depends on how massive the companion is. If the star is bigger than our Sun, it will have big clouds of wind coming off it, and as the compact object passes through the wind, it gobbles up the gas in its path (like Pacman). We cleverly named these systems “high-mass X-ray binaries”, since the companion is big. If the companion is smaller than our Sun, then the compact object swirls the star’s gas around itself, as if the compact object is sitting in the middle of its own dinner plate. This swirling plate of gas is called an accretion disk, and these systems with small companions are called “low-mass X-ray binaries”.
(Header image/inset background credit: ESA. Inset credit: All About Space magazine.)
My PhD ring was featured in this week’s Scientist Show & Tell for LabX and the National Academy of Sciences. When I first laid eyes on it at Jean Jean Vintage, I thought it looked exactly like a black hole with big jets shooting out of the top and bottom. I would visit it in the store and try it on periodically until I just had to have it. Since the ring looks like the artist’s illustrations of black holes that I use in research talks, it seemed like fate to commemorate getting my PhD.
On the left is a radio image of the giant lobes formed by jets, powered by the supermassive black hole at the center dot. On the right is an artist’s illustration of an accreting stellar black hole with jets shooting out the top and bottom. (Credits: NRAO, me, NASA/CXC/M.Weiss)
Now that I’ve pulled myself together after watching the Season 1 finale of Loki (and only barely), I want to mention the brief black hole science we see in the beginning of the episode. (MILD SPOILER AHEAD)
At the beginning of the episode, we fly past black holes and what looks like a rainbow bridge/Einstein-Rosen bridge (/wormhole) as we zoom in to Loki and Sylvie approaching the Citadel at the end of time. At first, I, like everybody else, was like “oooh, space, pretty”, but then my science brain kicked in.
First, the visuals of the black holes are fairly accurate. They have a bit of an accretion disk with some spiral arm structure on the edges, similar to what we see in spiral galaxies. They’re clearly 3-D spherical “holes” instead of 2-D circular “holes” in spacetime, and they even have a bit of a bright photon ring right around the event horizon. Sure, they aren’t verbatim simulations like the one below from NASA, but they’re very pretty and artsy-cool looking. In storytelling, the appeal of the science matters much more than the rigor of it. Checkmarks all around for effort and follow-through!
Einstein-Rosen bridges (or wormholes) are most likely the inspiration for the Bifrost rainbow bridge that Heimdall controls. As an observational astronomer, I feel obligated to point out that although these are mathematically possible, we haven’t seen any observational evidence of them in our universe. Sorry. I still like them in sci-fi.
The other awesome science part is that black holes WILL actually take over at the end of the universe. There’s a Crash Course Astronomy video by the Bad Astronomer himself, Phil Plait, explaining the concept of “deep time” embedded below. In about 10 trillion years, degenerate zombie stars (black holes, neutron stars, and white dwarfs) will be the primary source of energy generation in the universe. After that, protons (one of the primary building blocks of atoms, and all matter) will decay. Anything that isn’t a black hole will dissolve into energy and tiny subatomic particles, and we’ll be left with black holes! That will happen in about 1 duodecillion years (1 with 40 zeroes after it). After that, things get super boring from a visual perspective, and I’m glad they didn’t go with that for Loki.
So, it makes scientific sense that He Who Remains/Kang the Conqueror lives in the black hole-dominated end of the universe. Using the visuals in the establishing shot was 😘👌 *chef’s kiss*
I absolutely love reading and watching science fiction, and it makes my nerdy heart so happy to see my research topic incorporated into the story.
After listening to about 16 hours of podcasts this past weekend for a drive to Rochester and back, I’m putting together playlists of astronomy- and space-themed episodes from science podcasts! These playlists will be a great way to learn more about outer space in an approachable, accessible (read: non-expert, non-academic) way. First up: the podcast Abstract: The Future of Science.
Yes, I was on this one, and my episode is listed below, but you should listen to the other astro episodes, too! I’ve put the episode description beneath them along with a link to the episode pages on Apple Podcasts. I’ll do posts like this in upcoming weeks for more science podcasts, like Ologies (the UFOlogy episode is what sparked this playlist idea), Science Rules! with Bill Nye, Flash Forward, and Sean Carroll’s Mindscape. More ideas are welcome!
Abbie Stevens is an energetic, friendly and curious postdoctoral fellow in Astronomy and Astrophysics at the University of Michigan and Michigan State University. She studies black holes and neutron stars by looking at X-ray light coming from stars they’re gobbling up! Tune in for answers to questions like… How do binary systems form? What is the process of stellar evolution? What are the different types of black holes and where do we find them? How do stars die and what kind of remnants do they leave behind? and many more! Get the episode on Apple Podcasts.
Lisa Dang is an enthusiastic, outgoing and optimistic PhD student in Astrophysics at McGill University. During her graduate degree, she also held a research position at the NASA Spitzer Science Center at Caltech in Pasadena, California. Right now, she’s studying the diversity of exoplanets and their climate, with a variety of space telescopes, and most excitingly with the upcoming James Webb Space Telescope. She hopes to understand how planets form and evolve, to ultimately uncover the recipe for habitable planets! When she’s not busy scratching her head looking at copious amounts of data, you can find her traveling, drawing, or taking care of her plants! Tune in for answers to questions like… Is there life in the universe beyond earth? How do we define life? How old are you in “Hot Jupiter” years? What and how have we learned about exoplanets? What are the mechanisms behind tidal locking? and more!Get the episode on Apple Podcasts.
Our guest this week, Shaziana Kaderali, is a Master’s candidate at McGill University in Aerospace Engineering. Her research is focused on Space Situational Awareness and Spaceflight Dynamics. She helps satellite operators avoid collisions, among much else! She’s a jack of all trades and a master of all of them, and we’ve got her on the show to talk all things aerospace! Questions Answered: What’s an aerospace engineer thinking about first thing in the morning? What do we mean by dynamics and specifically aerospace dynamics? What’s going on up there in orbit around our lovely little planet? Should we be worried about the exponential increase in orbital objects and debris in freefall around the earth? What is the future of aerospace engineering going to look like? How do we dispose of dead or defunct spacecraft and what’s the end-of-life process? and many, many, many more!Get the episode on Apple Podcasts.
Our guest this week, Bryce Cyr, is completing his PhD in Cosmology at McGill University. He’s studying the theoretical structures known as cosmic strings (unrelated to string theory, but we discuss that too). They might shed light on the nature of the early universe and the origin of dark matter! Questions Answered: How did the universe begin? Where did it come from and where is it going How far back can we look? What’s the big idea with the cosmic microwave background? Why is gravity problematic? What’s the goal of string theory? What about cosmic strings, are they the key unification? What’s the big hold up on the grand unified theory of physics? and many, many, many more!Get the episode on Apple Podcasts.
Our guest this week, Mitchell Kurnell, just started his PhD in Mechanical Engineering in the Aerospace Mechatronics lab (yeah you know the one, he’s worked alongisde Eitan Bulka (Ep.11) and Ali Safaei (Ep.39)). Our discussion is split between his master’s research on nuclear physics, and his PhD research on cube sats. Questions Answered: Is nuclear energy a safe energy alternative and can we entrust our future in these fission reactors? How can we use lasers to learn about a material’s composition? How big and how small are the satellites in orbit above our heads? What are they doing up there? What is space junk and does it pose a problem to other satellites in orbit around the earth? and many, many, many more!Get the episode on Apple Podcasts.
Our guest this week, Andrew Saydjari, is midway through his PhD in Astrophysics at Harvard University. Andrew’s research lies at the intersection of Astrophysics and Machine Learning, and he’s studying the massive dust clouds in our very own galaxy. Tune in to tap into the wealth of knowledge that Andrew’s bringing to Episode 31! On this week’s episode we answer questions like: Why should you care about interstellar dust clouds that are a million times as wide as the earth’s orbit around the sun? What do spectra of light tell us about the molecular make-up of these clouds? How much information can I glean from just a single image of a molecular cloud out there in space? And how does the symmetry of molecules factor into all this?Get the episode on Apple Podcasts.
I was interviewed by Jeremy Ullman for his podcast Abstract: The Future of Science! We had SO MUCH FUN talking about black holes, neutron stars, supernovae, the Milky Way galaxy, the recent outbursting black hole named 4U 1543-47 (which I talk about in this twitter thread), and more. It’s 30 minutes long, available on Spotify (embedded above), Apple Podcasts, and wherever else you like to listen. My in-laws have already listened to it, and they said that both the science and my talking speed are understandable 😊
I’ve realized that not many people know that I stutter. My speech is usually fine these days, but it used to be very bad and very, very noticeable. I’ve been through years of speech therapy, and I think my early interest in music and singing was in part to help me get over my stuttering. Of course, I didn’t let it get in the way of doing and loving theater throughout school, though it partially informed why I didn’t seriously pursue acting beyond college. So, it feels QUITE momentous to be featured in an audio-only medium, and to have loved every bit of it. Jeremy can confirm that I didn’t stutter during the recording and he had no troubles editing.
I got myself a fancy Yeti mic and learned the basics of Audacity, and now I want to be on everyone’s podcast. Let’s talk!
Thank you Skype A Scientist for having me speak yesterday! As one of the finalists for the No Time Like The Presentation contest, I gave a 10-minute presentation on my research for a general audience. It was the first time in ages that I’ve given a talk with almost entirely new slides. I was so nervous and excited, and it was so fun! The talks were recorded and posted to YouTube (see embedded below). I spoke first, but you should stick around for all the talks. I loved learning about everyone’s research.
First order of business now is to make a sign reminding myself to SPEAK SLOWER and hang it just above my monitors…
Back in May 2017, I was part of a Science-Art Slam in Amsterdam, organized by Claudia Mignone. Three astronomers/astrophysicists (David Gardenier, me, and Daniele Gaggero) gave 20-minute talks about our research, while at the same time, musicians were creating free-improvisation music and a visual artist was mixing live projections of related scientific images. It was a super fun and different outreach-type event to both experience and participate in!
I talked about neutron stars, black holes, and their extreme environments! The transcript is below the embedded video.
Transcript: I’m Abbie Stevens, I’m doing a PhD at the University of Amsterdam, and I’m going to tell you a bit about what I’m doing it on. So I’m gong to tell you about neutron stars and black holes and the weird space around them. Black holes you’ve probably heard of, but neutron stars you maybe haven’t. David [the previous speaker] talked a little bit about neutron stars. This will be a story in two parts: first, what neutron stars and black holes are, and second, how we learn about them and how we know the things that we know.
Part 1: What are they? When a star dies, it will turn into one of three things, depending on how massive it is: a white dwarf, neutron star, or black hole. if it’s up to 8 times as big as our Sun, it turns into a white dwarf. Most stars fall under this category and will become white dwarfs (including our Sun). A white dwarf is roughly the size of the Earth, but it has almost as much stuff as in our Sun.
If the star is a bit more massive than that, up to about 20 times the mass of the Sun, the star will go supernova and what’s left is a neutron star. A neutron star is very dense. It’s only 25 kilometers in diameter, but has 1.5 to 2 times the mass of the Sun inside it. It’s average density (how compact it is; the amount of stuff per volume) is on par with an atomic nucleus, and it’s inner core is denser than an atomic nucleus. To get an intuition for this kind of density, imagine taking every person on Earth, and squishing them into a 1 centimeter cube. I’ve calculated it, and it gets you in the right ballpark. This extreme density causes the neutron star to warp spacetime. Albert Einstein’s theory of general relativity, which is the leading theory of how gravity works, says that matter tells spacetime how to bend, and the fabric of spacetime tells matter how to move. So in this framework, a neutron star is massive enough and dense enough to bend spacetime by a decent amount. And then, the curved spacetime causes the light and matter near the neutron star to behave in strange ways.
So let’s talk about a neutron star’s gravitational field. The acceleration due to gravity, which is a way of measuring the strength of a gravitational field, at the surface of a neutron star, is 100 billion times stronger than at the surface of the earth. If you dropped your phone on earth from 1 meter up (or if you’re Dutch, 2 meters up), it hits the ground in a couple milliseconds. If you did that from 1 meter off the surface of a neutron star, it would drop so fast, from the strong gravity, that it would hit the surface in 1 billionth of a second. Also, your phone would break. The work that you do against gravity hiking Mt. Everest on the surface of the Earth, is the same as what it would take to hike 1 thousandth of a millimeter on the surface of a neutron star (assuming you had a magical space suit that allowed you to not be totally pancaked to start with).
And then, black holes are weirder than that. A black hole is the remnant of a massive normal star, more than 20 times the mass of our Sun or even bigger. The black hole is what’s left over after it’s gone supernova.
Some black holes are 6 to 10 times as massive as our Sun — these are the small ones, stellar black holes. These ones are a few kilometers across in diameter typically, maybe 2 or 3 kilometers. The first black holes discovered by LIGO, the gravitational wave detector, were about 30 times the mass of our Sun, and they combined to make a black hole that was about 60 times the mass of our sun. Remember, there’s a lot of stuff in the Sun to start with, and this is 60 times that! And then there are the supermassive black holes at the centers of galaxies, that are millions or billions of times the mass of our Sun. The supermassive black hole at the center of the Milky Way, called Sagittarius A* (said “A star”), and it’s about 4 million times the mass of the sun, but only about 30 times the size of the Sun.
As you’ve maybe figured out, black holes are more massive and even denser than neutron stars, so they bend spacetime even stronger than a neutron star, and then again, light and matter behave even more strangely around them. But unlike neutron stars, black holes don’t have a hard surface. The size I’m referring to here is for the event horizon. This is the point, or rather, the surface, of no return, where the gravity is so strong that not even light can escape. So while black holes have stronger gravitational effects, there reaches a point where we have no hope of ever knowing what is inside it, so in some senses, neutron stars are more interesting to look at.
Now I’m going to take a step back and tell you how we know this, and how we learn more about neutron stars and black holes.
Part 2: How do we learn about them? Black holes and neutron stars tend to exist in binaries with normal-type stars like our Sun. So you’ll have a neutron star or a black hole and a normal star that are pretty close together and orbit each other (kind of like the Earth and the Moon orbiting each other). But the gravity of the neutron star or black hole is really strong, so if it’s close enough to the normal star, the outer stuff of the normal star becomes more gravitationally attracted to the neutron star or black hole than it is to it’s own star, and the star stuff falls towards and swirls around the neutron star or black hole. So this process of draining the normal star and feeding the black hole or neutron star is called accretion, and accretion is a very powerful astrophysical process.
The star stuff that’s being accreted forms a disk around the black hole or neutron star (almost like the rings of Saturn, but thicker and bigger), and there’s a lot of friction within the disk that makes it get really hot. Now when anything is hot, it shines energy. For the human body, we shine most of our heat energy as infrared radiation. Like with infrared cameras or night-vision goggles, people glow brightly because of the infrared heat. The sun is hotter, and it shines most of its heat energy in visible light. And this accretion disk of hot star stuff, the inner part that’s close to the black hole or neutron star, is so hot that it shines most of its heat energy as X-ray radiation. The same energy the doctor uses to take pictures of your bones. This X-ray-hot part of the accretion disk is also the part that is sitting in the bent spacetime, the strong gravitational field close to the black hole or neutron star. We’re talking within hundreds of kilometers. So if we study the X-ray radiation, it can tell us how the stuff is moving and behaving in strong gravitational fields.
But, we can’t just take a picture, because this interesting region around black holes and neutron stars is both very small, and very far away. If you take a typical distance for one in our galaxy, it would be like trying to resolve the width of a human hair on Mars at closest approach. We can’t do that. So on the sky this looks like a single dot, or a single pixel. But, the X-rays aren’t constant – they flicker and wiggle and vary in brightness, and these variations are fast – tens to hundreds of times per second.
If you look at a particular black hole of neutron star with an x-ray telescope for a few hours, you can build a variability picture of these flickers and wiggles in X-rays, and some interesting patterns emerge. There’s noise that can look flat or like some weird lumps, and we think this is from stuff in the accretion disk gently flaring in brightness as it moves its way in toward the black hole or neutron star. There are pulsations in some neutron star systems that David talked about, from very regular brightness changes. Neutron stars have really strong magnetic fields (thousands of times stronger than anything that’s ever been created in a lab on earth), and the magnetic field can cause a lighthouse effect where you see regular bright flashes when the north or south pole is pointed towards you. Some of the pulsations happen hundreds of times per second, so you have a neutron star that’s 25 kilometers across spinning hundreds of times per second! Neutron stars that show pulsed light like this are called pulsars, and Claudia [Mignone] will tell us about the discovery of pulsars in the last session.
Then for both neutron stars and black holes, there are quasi-periodic oscillations. So they have a bit of a period, but they’re not very precise. Not quite periodic, not quite noise. But you could almost think of it like a changing heartbeat in the accretion flow. There are lots of types of quasi-periodic oscillations, so that the combination of quasi-periodic oscillations and noise in the variability picture is almost like a fingerprint. I have one colleague who has an nearly photographic memory for the variability pictures. You can show her one, and she says “that’s that black hole from 2010” or you show her another one and “oh it’s this neutron star from 2002”. And we think that the quasi-periodic oscillations are telling us about a hot fluffy flow of star stuff that’s piling onto the neutron star or black hole, and it’s wobbling around from being in the really strong gravitational field.
So we have these X-rays that are coming from this hot star-stuff that’s around neutron stars and black holes, and it’s spinning in curved spacetime. So by looking at these X-rays, they can tell us more about what black holes and neutron stars are, and how they make matter do weird stuff.
If you’re interested, I have some 3D-printed accretion disks that you can play with during the break.
I research neutron stars and black holes, and the extreme environments immediately surrounding them. For a general-audience introduction, I highly recommend the videos by Phil Plait on the Crash Course YouTube channel!