neutron stars

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.

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.

Crash Course on neutron stars and black holes

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!

MSc PAPER ACCEPTED!!!!!

The paper from my MSc research has been accepted for publication in ApJ!!!!!

dancing ron swanson

We simulated pulse profiles from thermonuclear X-ray bursts on the surfaces of accreting neutron stars, and fitted them with an evolutionary optimization algorithm to determine how well we could constrain the masses and radii of the neutron stars! The ADS entry for it is here. We’ll update the arXiv version once the proofs are finished!

Just how small are X-ray binaries?

X-ray binaries are so small that we can’t directly image (i.e., spatially resolve) them, due to a combination of being small in size AND very far away.

When updating my Research page I was curious what a good analogy would be for imagining the projected size of an X-ray binary, and I’ve come up with the following. It’s just using algebra and trigonometry, so you can follow along! 😉

Let’s start with some assumptions for this little exercise: we’ll use a 10-solar-mass stellar black hole (i.e., not supermassive) that’s 2.5 kiloparsecs (~ 8000 lightyears) away. A 10-solar-mass black hole is ~2×10^{34} grams (yes, astronomers tend to use grams — cgs, as you’ll see below, means “centimeters, grams, seconds”, referring to the base units).

The radius of the black hole’s event horizon will be

R_{EH} = (2 G M) / c^2 ,

where G is the gravitational constant (6.674×10^{-8} in cgs units), M is the mass of the black hole, and c is the speed of light in a vacuum (~3×10^{10} in cgs units). Plugging these into the equation gives

R_{EH} ~ 3×10^6 cm,

or ~30 km. The distance, 2.5 kiloparsecs, is 7.7×10^{21} cm. Now we do some trigonometry to get the angular size:

A = arctan(3×10^6 cm / 7.7×10^{21} cm) = 2.2×10^{-14} degrees.

As you can see, this is a tiny, tiny number.

So let’s see how big an analogous object would be if it were on the surface of the moon. The closest distance between the surface of the earth to the surface of the moon is, approximately, 376300 km (which is the distance from center of the Earth to center of the moon, subtracted by the radius of the Earth and the radius of the moon), or 3.763×10^{10} cm.

We now want to know the size of an object that would appear to be 2.2×10^{-14} degrees in radius if it were sitting on the moon. This is

arctan(S / 3.763×10^{10} cm) = 2.2×10^{-14} degrees,

where S is the radius in centimeters. Solving this gives

S = 1.4 x 10^{-5} cm, or 0.14 micrometers in radius.

This is 1000 times smaller than the size of a single strand of human hair. Can you imagine trying to take a picture of a piece of hair that’s on the moon, let alone something 1000 times smaller? I can’t.

Let’s try something else — what about something on the surface of Mars? The smallest distance between the surface of Earth from the surface of Mars is 5.57×10^7 km, or 5.57×10^{12} cm. Using the same equation as before, but with this new distance,

arctan(S / 5.57×10^{12} cm) = 2.2 x 10^{-14} degrees,

gives S = 0.0021 cm = 0.021 mm = 21 micrometers in radius.

This is the size of a human hair (~ 30 – 100 micrometers in diameter), or one quarter of the thickness of a piece of paper!

Understandably, we don’t have instrumentation capable of imaging something this small, which is why we rely on spectral and timing measurements of photons emitted from X-ray binaries instead of just taking a picture.

CASCA 2013!

I’m excited to be at CASCA at UBC this week! Follow the festivities on twitter with #cascaUBC. For those interested, my talk is Wednesday in Session 10 (Compact Objects) at ~11:15am in Hennings 202.

Using X-ray Light Curves to Constrain the Neutron Star Equation of State

The equation of state for ultra-dense matter has puzzled astrophysicists for decades. This is because the conditions of ultra-dense matter, such as those found in neutron stars, are not terrestrially replicable. X-ray light curves from low-mass X-ray binary systems, with neutron star primaries, have proven to be useful tools in the study of the neutron star equation of state. Theory predicts that the X-ray light curve resulting from a Type I X-ray burst on the surface of a rapidly rotating neutron star can be used to determine the characteristics of the burst ignition spot and place constraints on the neutron star’s mass and radius. We discuss the development of spherical and oblate neutron star models that, providing parameter values, yield an X-ray light curve comparable to that which would be measured by an X-ray timing telescope like RXTE. This simulation code, used with a genetic fitting algorithm, will provide us with an opportunity to disentangle the effects of various aspects of the neutron star and hotspot on the outputted light curve, showing which parameter degeneracies will have the greatest impact on the observable.