astronomy

Podcast playlist for Abtract: The Future of Science

After listening to about 16 hours of podcasts this past weekend for a drive to Rochester and back, I’m putting together and sharing 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.

Interview for the podcast Abstract: The Future of Science

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!

REOTown Reading Series

In a terrifying departure from my modus operandi, I will be presenting a piece of creative non-fiction at the REO Town Reading Series on Thursday April 22nd! I’ll be telling a story about the brightest little pulsar in our galaxy. Join Mary Fox, Stevie Pipis, Selena Gambrell Anderson, and me with host and curator Matt Rossi on FB live starting at 7:30pm EDT.


UPDATE: Here is the video link for you to watch at your leisure! Thank you to everyone who tuned in. Transcript of my piece:

I’m going to tell you a story about the brightest little pulsar in the Milky Way, named Swift J0243.6+6124, and don’t worry, you don’t have to remember that name, and I’ll tell you what a pulsar is.
On October 3rd 2017 there was a very bright burst of light near the Perseus constellation. It wasn’t close enough to see by eye, but things that go ‘bump’ in the night tend to be interesting to astronomers, so we had telescopes at the ready. The Swift X-ray observatory was the telescope first to detect it, which is why ‘swift’ gets to be first part of its name. The telephone number in the rest of the name, and yes we actually call it a telephone number, is its location in galactic coordinates, which is like latitude and longitude for the night sky. For the next 150 days, taking us into the spring of 2018, we saw this shiny new source get unimaginably bright, then dim again. Not only were there X-rays, but gamma rays, visible light, and radio waves also shone from it during that time, in their own ways.
In the night sky, when something transient shines brightly in X-rays, this light often comes from near the remnant of a dead star, like a black hole. But calling it a dead star doesn’t quite convey its character; calling it a zombie star is far more accurate. Like zombie people, this zombie star is eating living stars, and it’s still quite active and spinning around, and in general, like zombie people, one could argue that its time as a zombie is far more interesting than its time when it was alive.
Many zombie stars have a little star friend that they grew up with. They orbit around each other as they’ve always done, but like we’ve seen too many times in the movies, the little star friend can’t bring itself to leave, which ultimately leads to its untimely demise. The zombie star feeds on its star friend, slowly draining the outer material from the star and forming a disk around itself, like it’s greedily filling its plate. This syphoned star stuff, waiting to be gobbled down by the zombie star, gets very hot, like 100 times hotter than the surface of the sun. It shines its heat light in X-rays, so when we detect these particular colors and shapes of X-rays from a point in the galaxy, we know that this whole zombie scenario is happening.
Now let’s check in with the ‘unimaginably bright’ aspect of this source. First, I need to tell you what we imagined the limit was. As a zombie star is eating, if it eats more stuff, it shines more brightly. But as your intuition may have already told you, things shining light in space cannot be infinitely bright.  There reaches a point where the radiation of the light shining out, pushes back and doesn’t allow more material to fall in. If there’s enough material, the system can even sustain this luminosity. In the 1920s, this luminosity limit was worked out in detail by Sir Arthur Stanley Eddington, a British astronomer, physicist, and mathematician. An important detail to keep in mind is that this luminosity limit depends on the mass, or weight, of the object doing the eating.
When we as astronomers are observing X-ray light from these voracious black holes, we often don’t know how massive they are, so we just pretend that they’re all 10 times the mass of the Sun. In the sciences, when we’re trying to figure something out but don’t have all the information (which is how it goes in astronomy, more so than other fields), we simplify, make a plausible assumption, and move on. Some of the sources we see in the night sky are brighter than this luminosity limit for a black hole weighing 10 times the mass of the Sun, and we call them ultra-luminous. Also, if you’ve ever wondered what’s more than ‘ultra’, according to astronomers, it’s ‘hyper’. So, two things could be happening here: either it’s actually more massive than 10 times the Sun, so it’s shining normally for its actual mass, or there’s some funky, fancy stuff going on that is temporarily letting it be brighter than it should be.
But! Here’s the twist. You didn’t know there’d be a twist, but there is. When we were observing the X-rays from J0243, we instantly noticed pulses in the brightness of the X-ray light, nearly perfectly precise like the ticking of a clock. Black holes can’t pulse. There is no mechanism to get a signal this precise from them. However, neutron stars do, and some of them are so good at pulsing that we call them pulsars. Neutron stars are another type of zombie star, left over when a medium-big star is massive enough to die in a supernova, but not massive enough to collapse into a black hole.
A pulsar is a neutron star with two bright spots on the surface at the north and south magnetic pole, off-kilter with how it spins. It’s like if a neutron star were a light house, in that it’s always shining beams of light, but you only see it when they point in your direction. Pulsars are effectively cosmic clocks that are stunningly accurate out to many decimal places, and NASA is testing a technology to use them like reference points for interplanetary GPS.
So, knowing that J0243 is a pulsar, only a fraction the mass we first assumed for the luminosity limit, it is very super ultra hyper bright. It sounds like it shouldn’t be possible, but nature begs to differ. What’s probably happening is that the pulsar’s magnetic fields, which are thousands of times stronger than anything we can create on Earth, are holding things in place like Spanx, so it can eat even more and radiate even more. I think there’s an analogy here about Spanx holding stuff in, letting the star shine extra bright, but I prefer a more body-positive ethos so I’m going to let that slip away.
What does this mean?
I’ve been studying X-rays from this source, and one really interesting thing about it has to do with its iron levels. Though gas in space tends to be mostly hydrogen, there are trace amounts of other chemicals like oxygen, silicon, magnesium, and iron. If the gas gets hot enough (spoiler: it does), the iron can light up and fluoresce like neon lights. This shows up in cool shapes that we can see with extremely fancy and expensive X-ray detectors on satellites in space. If we want to know where this gas with iron is, like geometrically in the system, we can’t just take a picture because the whole system (zombie star, star friend, plate of stuff it’s eating) are way too small and way too far away.
Instead we need to figure it out from the physics, like a more boring, yet higher stakes, version of Clue. We know the weapon and the victim and the murderer, but not the room, and the room is important. The big weird wrench in the gears here is that the iron doesn’t know that it’s around a pulsar. It doesn’t see the pulsations. And like, How can you not know with such a stupidly bright pulsar, but anyways. The two possibilities are that it has a wall of stuff blocking its direct line-of-sight with the pulsar, or it has a cozy cocoon of dust and gas wrapped around it, diffusing the pulses into a strong, steady light.
I don’t have an answer to this yet. Scientific research is trial and error and error, ad nauseam, and I am in the thick of it. A very helpful colleague suggested some calculations I can do, but I haven’t done that yet, and instead I wrote this.

Skype A Scientist on YouTube

Selfie of me (white lady with straight, short, brown hair and red lipstick) looking at the camera with my cheek resting on my hand. There is a gallery wall of art and conference posters behind me.

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…

No Time Like The Presentation 2021!

Last week I submitted a 90-second video abstract of my research to the No Time Like The Presentation competition from Skype A Scientist. And I was selected to give a 10-minute talk on my research!!

The full-length presentations (from me and the other scientists selected) will be in April and streamed live to their Youtube page (and saved for later viewing).

Upcoming virtual public talks

Hi friends! I have a few public talks coming up in the next two months that I want to tell you about. They’re also listed on my science communication and outreach page (and I’ll keep that list updated). Join me from anywhere in the world!

How Do Scientists “See” Black Holes?, Tuesday April 6th on AstroZoom

A ~20-minute talk on the ways we can ‘see’ black holes even though light can’t escape from within the event horizon. It’ll be pre-recorded and sub-titled in Farsi by the host, with live Q&A after. The talk will be aimed at middle school and high school students (roughly grades 5-12).

Exo-lent Planets!, Saturday April 17th at MSU SciFest

A ~20-minute talk + Q&A about planets in other solar systems around other suns, featuring some well-known exoplanets in pop culture. This talk will be aimed at all ages.

The brightest little pulsar, Thursday April 22nd at the REO Town Reading Series

A story about the brightest little pulsar in our galaxy (it’s name is Swift J0243). One of four readings that evening. Aimed at adults, especially artsy ones.

How Do Scientists “See” Black Holes?, Thursday May 27th, Great Lakes Lectures

A 40-minute talk + Q&A on the ways we can ‘see’ black holes, going into more depth and detail than the April 6th AstroZoom talk. Aimed at high school students through adults.

If you’d like to book me to speak at your event, please contact me!

So you want to be an astronomer

Astronomy research makes for an interesting and challenging career. In my experience, it’s one of the more collaborative physical sciences, and it’s one of the most fascinating things I could possibly be doing with my time. I’m regularly asked for my advice by high school and college/university students on what they should do to become a professional astronomer, so I’m sharing it all here. Other professional astronomers like Prof. Katie Mack also have advice on their websites!

High school

Take physics, math, and computer programming (if offered). It will be most helpful to you if you can take pre-calculus and calculus 1 in high school, and AP/IB/honors physics (if available). Statistics is also very useful, but unfortunately you may have to choose stats OR calc, and I would probably recommend calculus for now (but listen to your teacher’s advice, since they know you and your program best). Much of astronomy is actually done with writing scientific research software for data analysis or simulations, so having a programming background is SUPER useful!

You can also look into summer research opportunities at your local college or university. As a researcher, it is often difficult to hire a high school student on their own for research, so it’s best to go through established programs. Check online for summer research opportunities both in your area and (non-local) residential programs.

When looking at colleges/universities, keep in mind that not all programs are equally rigorous. Many times, astronomy is part of a physics (or sometimes physics & astronomy) department, so you will take lots of physics classes. Do undergrads take two semesters of upper-level Electricity & Magnetism? Two semesters of upper-level Quantum Mechanics? Can undergrads take graduate-level classes if they complete the pre-requisites? Both small liberal arts colleges and massive public research universities can have rigorous physics and astronomy programs, so ask questions! Also ask what research opportunities there are for undergrads on campus (in addition to summer research), and how many students go on to grad school (if you think that’s something you might want to do).

Keep up with your athletic and artistic pursuits. Hobbies are good for the soul.

College/university

Again, take physics, math, computer programming, and astronomy (if offered), and meet with your advisor to be sure you’re fulfilling the course requirements for your degree. Make study groups with your classmates, and ask questions at office hours with your prof and/or teaching assistant. College is when the training wheels start to come off and you transition to being a self-driven learner, so your coursework will transition from being formulaic plug-and-play to being multi-stage problems that help you synthesize different aspects of the course material.

Talk with your academic advisor about research opportunities, both during the semester and the summer. Different universities have policies about doing this for course credit vs. being an hourly employee, so ask questions, accept advice from your mentor(s), and do whatever is best for your personal situation. Building on a former student’s research project is great experience, and is true to how most research actually goes!

Applying to grad school

If you decide that you really like research and want to pursue graduate school, talk with your academic advisor (and research advisor, if they’re different people). Again, listen to their advice. Chat with grad students, postdocs, and profs who work in areas you’re interested in, and ask them what classes are most helpful for studying that field. Remember that grad school in the US and Canada (and post-MSc PhD programs elsewhere) is *paid* and tuition is either covered by grants or very inexpensive. You should not be taking out a loan to go to grad school in the sciences. Iterate with your advisor (and even a writing tutor) on grad school application materials, and give your letter writers ample time (like 4 weeks notice) for the letters of recommendation. You can also ask a letter writer to include specific pieces of information to explain any discrepancy in your transcript, like if you had health problems and that’s why your grades dropped one semester.

Policies vary based on country and school, but often times you’ll be invited to visit either as an interview stage or after you’ve been accepted to grad school. Take this opportunity to chat with professors you want to work with, grad students (to see how happy they are and how much they’d recommend the program), and get a feel for the location and whether or not you could realistically live there for 6ish years. If you’re part of an underrepresented demographic and you don’t see anyone else from that demographic at that school, ask questions of mentors and current grad students (there and elsewhere) to figure out whether there’s a notable systemic problem there, or there just don’t happen to be any students of that demographic there at that time.

You should know that there’s a mental health crisis among graduate students, and graduate school is HARD (both emotionally and mentally). Part of the difficulty is because you’ve chosen a particular path and that path is challenging (research is part exploration and part banging your head against a wall). You’re also in a life stage with a lot of existential angst and growing pains as you figure out what kind of adult you are and what kind of life you want. Advisor fit is one of the better predictors of mental wellbeing among graduate students, so pick someone who you mesh well with and who you think will support and inspire you. Your advisor makes or breaks your grad school experience — a great advisor will help you find a niche that plays to your strengths in whatever sub-field you find interesting (like high-energy astronomy, or exoplanets, or quantum information, or optics); a negligent advisor will amplify feelings of loneliness and isolation, which can cause you to dislike a research topic that you might otherwise think you love. It can be tricky to judge the quality of a working relationship based on short interactions, but chatting with grad students (both in their group and not) can help fill in the picture.

Some final thoughts

Above all, know that it is very normal to change your dreams, and change your major, and change your job. There are new and varied challenges to being an astronomer as you move along the path from undergrad to grad to postdoc (and presumably to professor). There is more than one way to be an astronomer, and the community as a whole needs people with varied talents and areas of expertise — not just research, but also outreach, teaching, policy, and advocacy. Building your own support network of peers and mentors (more senior grad students, postdocs, and profs) can help you find your own path to success!

(Header image: AstroSat/J. Paice)

Doing my MSc in Canada

I wrote a guest post in AstroBetter about doing my MSc in Canada at the University of Alberta. Going abroad for a MSc is quite an uncommon path for US-born students in STEM, but I’m really glad I did it. Read the whole post here.

Amsterdam Science-Art Slam

Image description: Pink-hued image of a galaxy with many stars and dust filaments. Overlaid with cut-outs of a person hunched over playing a wind instrument, a fuzzy photo of a dancer, and a close-up of hands playing a violin. Image from Science-Art Slam facebook page.

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.