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.
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.
Google Summer of Code projects are now open for application! We’re looking for some undergrads to work in python on black hole X-ray analysis code (they don’t need to know any astrophysics in advance): check out our Timelab ideas page. There are also some astronomy projects with the OpenAstronomy umbrella organization.
If you’re mentoring undergrads (or are an undergrad yourself) who particularly like coding, this could be great astro-programming experience for them! Google Summer of Code is a paid internship for people who are enrolled as undergraduate students as of May 4th 2017.
Please encourage your undergrads to apply!! Applications are open until April 3rd. Check out the main GSoC website for more info.
EXCITING NEWS!! There will be a special session at the European Week of Astronomy and Space Science (EWASS) 2017 on developments and practices in astronomy research software and a hack day!! I’m co-organizing the hack day and a block on different astronomy software packages, where we will have a variety of speakers (invited and contributed) share open-source software packages of interest to a broad portion of the astronomical and space science community.
EWASS is the general meeting for the European Astronomical Society that will be held in Prague, Czech Republic on 26-30 June 2017, hosted this year in partnership with the Czech Astronomical Society. There will be many symposia and special sessions on a variety of research topics, and registration for the meeting will open in December.
Abstract submission is open for all EWASS 2017 sessions, and here’s a blog post on the hack day with links for registration and more info. The talks will be on June 28 and Hack Together Day #hackEWASS will be on June 29.
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!
Since application season is upon us, here’s some thoughts on applying for a PhD for those who have/will have an MSc.
It seems a bit basic, but the first thing to think about is why you want to do a PhD in astro. It’s hard by design, and ~90% of people with astro PhDs don’t end up in tenured academic positions. I don’t know if that’s what your aim is, but it’s good to know regardless. So think about whether it will be worth it for you given your goals. There are still plenty of valid reasons to do a PhD in astro when you know you won’t end up as an academic, so just think about it.
The next thing to think about is what kind of research you want to do, and take inspiration from what parts of your MSc you’ve enjoyed or found the most interesting. Theory? Data/observations? Experimental lab astro? Instrumentation? Also consider what topics you find most interesting, and which aspects of them you like. This is probably going to be related to your MSc topic, but if you want to shift fields, now is probably the best time to do it.
Then, one of the bigger things to consider is which continent you want to be on. Each country has their own academic system and way of doing things, though there are some similarities. Keep in mind that some systems are harder to get into if you’re not a citizen or permanent resident. For example, it’s not possible for non-UK citizens to get UK funding to do a PhD there, so you’d need to bring in your own external funds like an NSERC, Marie Curie, Rhodes, or Gates Cambridge Scholarship.
Keep an eye on the AAS job register, “Pre-doctoral/Graduate Positions” section (scroll down) and the EAS job directory (filter by position). Not all jobs will be listed yet. For example, Amsterdam usually has a few PhD positions available, and they start advertising these at the end of October. For Canadian and American universities, they typically won’t advertise on there, so you’ll want to look through department websites and see what the different professors do. When you find someone whose research sounds like it’d be a good fit, email them to ask if they are looking to take on a new PhD student. Another place to look for potential advisors is in the author list of papers you find interesting! Again, do your homework and read up on them before emailing them. Check out the AstroBetter wiki for lists of programs and advice on applications. But once you have an idea of what you want to do and generally where you want to do it, ask the profs in your current place if they know which places have people doing that kind of research.
For the US, many places still require the GRE and Physics GRE (but not all!), so there’s that hurdle to get over, and you’ll want to email the department chair of grad studies/admissions to ask if they’ll accept your MSc courses.
You will typically need 3 letters of recommendation. Ask your letter writers now if they feel comfortable writing you a strong letter of recommendation. Yes, use the adjective “strong” in there. If you won’t have a letter from your MSc supervisor, you need to have a really good reason, and have another letter writer explain why that’s the case. For my applications I had letters from my MSc supervisor, another professor in the department who I knew pretty well and had taken a class with, and my undergraduate mentor. Get your letter writers lined up now, and when you know where you want to apply, give them the deadlines and the details. Email them a 2-weeks-before reminder, 1-week-before reminder, and a 2-days-before reminder. Also, if there are any discrepancies in your record, explain it to a letter writer and ask them to include it.
Once you’ve written a draft of the research statement that most places ask for, have a PhD/postdoc and your advisor iterate feedback with you on it. It seriously makes it better.
Good luck and have fun!
I’m thrilled to say that my first paper has been accepted for publication in MNRAS!! 2.5 years into my PhD and I finally have something to show for it 😁 Here’s the ADS link, which has both the arXiv and MNRAS versions.
I’m currently in Seattle at the Python in Astronomy 2016 workshop and wanted to post some notes and reference links for those of you who want to follow along with what we’re up to:
Livestream and archived livestreams are recorded and posted by Dan Foreman-Mackey
A Google doc with a list of links to notes from all the presentations and unconference sessions
Zenodo page that will have presentations, notes, as ‘Unproceedings’ (with a DOI for official citing)
Please let me know if there are any other links you’re interested in or that I forgot to include!
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.
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.
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.