science outreach

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 20ish-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. Enjoy!

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.

What are 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!

AstroBetter series on Twitter at conferences

Twitter logo on Hubble deep field. Image credit: HST/Twitter/A.L. Stevens
My 3-post series on Twitter at academic astronomy conferences has concluded on the AstroBetter blog! The first post, aimed at non-Twitter users, explains the basics of Twitter and why it’s a good thing to have at conferences. The second post, aimed at current or would-be tweeters, gives tips and tricks for conference tweeting, and spawned a great discussion on openness and sharing of unpublished ideas. The third post, for LOC or SOC members, has a rundown of what needs to happen on the conference organizing side for Twitter to be successful at a conference. Join the conversations in the comments section of those posts or on Twitter! 😉

The series comes from a post here from this past May.

Twitter tips for academic conferences

UPDATE: A version of this post is appearing as a series of posts on AstroBetter!

Earlier this week I attended the Netherlands Astronomy Conference 2016, organized this year by my institute. I was asked by the LOC to be on my Twitter game throughout the conference, and I’m really proud of the concerted effort made by everyone involved — our conference hashtag #NAC2016 was even trending in the Netherlands for a while!! After fielding Twitter questions from a variety of organizers and participants, it seemed like a good idea to centralize and condense my advice.

In astronomy, I’ve noticed that Twitter awareness and usage at an astronomy conference loosely correlates with the median age and median country of origin of the participants — for example, it seems that younger and/or U.S.-based attendees tend to tweet more. However, I don’t think it should be limited to those demographics. There are definite benefits for the larger astronomy community, like being able to follow what’s going on in real time at a conference you couldn’t attend (which I’ve done on multiple occasions), follow one parallel session while attending another (which I’ve also done), promote the work of up-and-coming researchers, and give exposure to exciting new results. Twitter isn’t just for science outreach and education — you’re communicating with your colleagues as well. So, I want to spark the minds of the Twitter-averse and provide an access point for the Twitter-aware-but-not-active with some tips and recommendations in regards to conference tweeting.

Twitter 101

So that we all start off on the same page (and for those new to the Twitterverse), here are my definitions of the Twitter-specific vocabulary I use throughout this post:

  • tweet – A 140-character message on Twitter. It can also be used as a verb, as in, to tweet something.
  • hashtag – The thing that starts with ‘#’. This is like a tag or keyword that is used to link related tweets.
  • account, handle – The thing that starts with ‘@’. This is a username that’s been registered on Twitter and sends out tweets.
  • feed, stream – A list of tweets, typically a list of tweets with the same hashtag or from the same account.
  • trend, trending – A hashtag or topic that has amassed a critical density of tweets. Trending topics are listed in the left sidebar of a user’s Twitter home page.

For conference organizers

  • A good conference hashtag is unique, short, and somehow relevant (bonus points if it’s funny/punny). Including this in the tweets is how all the tweets will be linked. You want it to be unique, so that only tweets for your conference will appear under it; short, so that it doesn’t take up too much of the 140 character tweet limit; and relevant, so that everyone easily remembers it. Ours, #NAC2016, wasn’t unique, but we dominated the tweets for the duration of the conference so we decided it was ok 😊
  • Good conference hashtag examples off the top of my head are #Bdiffuse16 for a diffuse magnetic fields and ISM conference in 2016, #extremeBH for an extreme black hole accretion conference, and #PyAstro15 for the Python in astronomy conference in 2015. You can look at the account @astromeetings for more inspiration. Note that hashtags are not case-sensitive.
  • Decide on a conference hashtag a few weeks before the conference. List the hashtag on the website and in logistics correspondence with the attendees, and ask your attendees to use it on Twitter. You don’t need to register the hashtag anywhere, just start using it.
  • If your organization doesn’t already have a Twitter account, make an “official” conference Twitter account that will retweet others and make its own tweets. Having this gives a somewhat more curated tweet stream to follow. If the conference hashtag is the name of the account, the account itself shows up at the top of the hashtag stream (as you can see here). The one we had for the NAC was @_NAC2016 (with an underscore); for the AAS High Energy Astrophysics Division meeting in April, their Twitter account @AAS_HEAD served this purpose during the conference.
  • Add the Twitter hashtag feed or conference account feed to your website and/or put it on the screen during the coffee breaks. This is where I recommend using the conference Twitter account’s curated feed — it turns out that if your conference hashtag starts trending (which #NAC2016 did), porn bots start tweeting using your hashtag (with NSFW photos), and that’s not something you want to appear on the screen or your website. FYI.
  • Beforehand, ask a Twitter maven in attendance to tweet a lot for the first one or two sessions to give the conference a strong Twitter presence. We found that this encouraged more attendees to contribute on Twitter for the rest of the meeting.
  • To reassure the Twitter-skeptics in attendance, you can remind everyone that people will be tweeting at the conference and if they do not want all or part of their talk to be tweeted (press embargo, competitors, etc.), they can say so and the tweeters will respect it.

For tweeting attendees

  • For those new to the game: the hashtag (including the ‘#’) must be included in the text of all tweets that you want to be discoverable by it. Also, if you have a private Twitter account, only people who you approve to follow you can see your tweets.
  • Give your non-astronomy Twitter followers a heads-up that you’ll be tweeting at a conference, and tell them when the conference is ending, so they can mute you if they want to.
  • Consider writing your Twitter handle on your conference nametag, particularly if there are lots of tweeters at the conference, so people can put a face to your tweets.
  • Don’t forget to credit the speaker in your tweets (typically with either their first initial and last name or just their last name). Tweets have new guidelines so that mentioning a Twitter handle (including the ‘@’) doesn’t count against the character limit, so if the speaker is on Twitter, credit them that way!
  • If you catch it, it can be nice to tweet the project website or online code repository if the speaker mentions it. URLs get truncated so it won’t take up all of your 140-character tweet limit.
  • If you’re sitting towards the front of the room, you can tweet a photo of the conclusion slide or a good summary graphic. Bonus points if you get a good shot of the speaker too!
  • If you happen to know or have heard that the presenter has close competitors and is possibly at risk of getting scooped, keep your tweets vague and don’t tweet photos of their slides.
  • Tweet photos of colleagues with their posters (or even “action shots” of them explaining their poster to someone else).
  • Make sure your smartphone is on vibrate or silent and the volume is off on your laptop — you’re going to get a lot of notifications from other tweeters favouriting and retweeting you. I actually turn off push-notifications for Twitter on my smartphone.
  • Some people thread their tweets on the same talk by “replying” to their first tweet about it (and then deleting their account name from the text of the next tweet). It’s a great idea, but I often forget to do it.
  • Find a good compromise between using abbreviations to fit more in a tweet, and using so many abbreviations that online followers can’t understand it.
  • If a speaker asks the audience to not tweet or otherwise publicize some or all of their presentation, respect that.
  • Consider not tweeting while inebriated using the conference hashtag.

For all attendees

  • If you do not want anything (or a specific thing) from your talk to be tweeted, say so at the beginning of your talk or on the slide before it’s relevant. I personally don’t know any astronomer who would not respect the request. This is a good idea if your results are under press embargo, or if they’re preliminary results and you have competitors.
  • Include bite-sized summaries of your motivation, technique, and/or result to make it easy for the tweeters. Otherwise they’ll need to distill something short and sweet from your ramblings, and if they’re not in your sub-field they may get it wrong.
  • Include your email address and a paper link/reference on your conclusion slide and leave it up for the questions, so that someone can tweet a photo of it. (h/t Beatriz Mingo)
  • You can still read the tweets if you don’t have a Twitter account — just click on the hashtag, go to the ‘Live’ tab, and there you go!

If you have more suggestions, let me know via my contact page or on twitter! 😉

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.


I’m giving a talk at LogiCON on May 4, 2013. It will be an updated version of “Exo-lent Planets!”, a talk I gave at Nerd Nite Edmonton. Visit the LogiCON website for more information. Registration is free!