A Black Hole’s Shadow and your Cloak of Invisibility

The buzz around the lunch table and my group’s D&D table is about the image of the black hole shadow in the galaxy M87 (M87*). People are talking, discussing, conjecturing and even imagining the future of space or time travel. Moments like this are great for both the scientific community and the public as it creates intrigue, wonder, and gets both communities talking with each other.

The shadow is really quite amazing, though it should not be confused with the event horizon as some lunchtime conversations have. The shadow of M87* is something separate from the event horizon and perhaps the best way to describe the shadow is to chat D&D, or Harry Potter if you prefer. Either way, we need to talk cloaks of invisibility.

Black holes are already invisible by their own right. They pull in light that comes towards them and trap it forever. This makes photographing them directly a pipe dream because there is never any light leaving them for us to see. The only way to see the presence of black holes is either when they have an accretion disk or strong jets, like M87*, or through their gravitational influence, like Sagittarius A* at the centre of our galaxy zipping stars around it at breakneck speeds.

A perfect cloak of invisibility might not absorb light like a black hole but instead works by bending the light around the person and focusing it on the other side again, giving the appearance of no one being in the way of the light. This type of cloak will also prevent you from ever being photographed because you too are not sending out any light to be captured by a camera. Unfortunately, not every treasure chest in your dungeon will have one, nor do most average witches and wizards have access to a personal Dumbledore. Us regular dungeon crawlers and novice wizards and witches are more likely to get a cloak of partial invisibility (or a cloak of un-invisibility, which is mostly good for a half decent ghost costume at your next themed party).

A perfect cloak of invisibility (left) and a black hole (right).

If we are lucky enough to get a partial cloak of invisibility it will bend light around you, but it may have some tells. The fringes might shimmer, objects might be blurry, or your feet are clearly visible because it is too short and there goes that lovely bonus on your sneak attack rolls (and forget that practical joke of levitating your friend’s lunch with wingardium leviosa unseen).

Black holes can act as a partial cloak of invisibility when lensing distant objects behind them. We still cannot see the black hole directly, but we see the multiple images of the object behind it being projected in a ring around the black hole. In this way, it gives itself away without revealing any interesting details about itself.

A cloak of partial invisibility (left) and a black hole lensing a distant object (right).


Like a lensing black hole, your cloak of partial invisibility is not working out very well and you’ve been spotted. If you can’t stay out of sight, maybe try being seen but not recognized. For this trick, you’ll need a hula hoop of light, available in lower level dungeons or your local Weasleys’ Wizard Wheezes.

By keeping the hula hoop of light spinning around you, you might give yourself away, but people will probably be too distracted by the really neat shadow you’re creating between you and the hula hoop.

Most of the light from the part of the hoop that is behind you is absorbed by your faulty cloak of invisibility. Some of the light, however, is bent around you and focussed in front of you ahead of where the absorbed light should have been focused. The region where the absorbed light should have appeared is dark because it has no light being bent into it – in effect a shadow.

Ah, but what of the light from the ring in front of you that is cast backwards you say? Sharp eye young adventurer (wizard or witch), but just like the light from behind, the light going from the ring towards you gets absorbed by the faulty cloak or bent around behind you – no reflected light reaches an observer. With this disguise, you might not go unnoticed, but you could try for the Guinness Book of World Records as the first shadow to ever hula hoop.

Cloak and hula hoop of light (left) compared with a black hole and photon ring (right).

In the case of M87* its hula hoop of light is the photon ring that surrounds it. Just like our hoop, most of the light from behind the black hole is pulled in, while some of it is bent around and refocused in front ahead of where the missing light would appear under perfect conditions – just like with our cloak.

This leaves a dark region in front of the black hole between it and the distorted image of the photon ring (The ring is larger on the bottom because it is rotating towards us at that point. The Doppler shift makes it brighter – that’s another article.). The shadow exists in front of the event horizon but behind the ring and this is why it is interesting. The shadow’s presence is sort of a rough outline of the event horizon, but unlike the event horizon, the light that goes into the shadow has a chance of escaping – albeit slim. This is also what makes the image so amazing, by seeing the shadow we are effectively looking at a black hole. We have finally taken a picture of the one thing we’ve not been able to take a picture of directly. By seeing a shadow we have an outline of the event horizon! I would call that rolling a natural 20 or a performing a perfect Patronus as far as photos go.

-KSMogk

Review of John Read’s 50 Things to See on the Moon

John Read sent me a copy of his latest book 50 Things to See on the Moon. I much enjoyed his last book, 50 things to See with a Telescope – KIDS and his new book does not disappoint.

Once again Read takes us through the basics of terminology and instruments that you need to observe the Moon. From there he walks through the 50 targets on the surface starting at the New Moon phase and picking out targets as the Moon’s shadow gives way to the growing lit surface of the Moon. By the time of the Full Moon, Read has walked us through his various targets.

Read supplies readers with views of his targets not only as seen with our eyes or binoculars, but also how the targets look when looking through different telescopes that can flip or rotate the image. This is something I have found useful when taking his book out under the Moon to explore.

Read also brings the Moon to life through interesting facts about how the craters or features got their names, how features were formed or about the many and various moon missions that have taken place. He also brings his wealth of experience and tricks and tips to make the experience fun and enjoyable. I have already found his approach of using a series of craters that form an L to remember that the L is for “landed” and points in the direction of the location of the Apollo 11 landing site.

Given that the book is again geared at new initiates to Moon gazing and likely a younger audience, there are at times technical terms or wording that is cumbersome, such as “Image of the same region on three subsequent nights.” instead of perhaps my personal choice of “Image of the same area over three nights.” This aside, the guide is accessible to both children and adults and makes for a great addition to any amateur astronomer’s library. I know I will be pulling it out again soon to go Moon gazing.

The Darkest Black

Image of the black hole at the centre of M87 Image from: Akiyama, K., Alberdi, A., Alef, W., Asada, K., Azulay, R., Baczko, A.-K., … Ziurys, L. (2019). First M87 Event Horizon Telescope Results. II. Array and Instrumentation. The Astrophysical Journal, 875(1), L2. http://doi.org/10.3847/2041-8213/ab0c96

On April 10th The Event Horizon Telescope Collaborative released an image so exciting that I, like my parents with the Moon landing, will remember where I was and what I was doing when I saw it – which happened to be on my partner’s stationary bike in our garage watching the YouTube broadcast on my smartphone.

The image that popped up on the screen before me was a ring of orange hues, weighted and thicker towards the bottom left. A dark, gaping, empty, expanse of black sat inside the ring. I was looking at a black hole and the shadow its event horizon. The orange hues were ionized gasses dizzyingly swirling around it at speeds a fraction of the speed of light; sending out their blazingly hot swansong before crossing a frontier into an area of space so unknown we can only conjecture at what is behind the veil of the event horizon.

The light from that gas travelled incredible distances of time and space before reaching not our eyes, but a group of radio telescopes spanning the globe, interconnected through an ambitious and creative collaborate effort. The end result of which is nothing short of breathtaking.

Being so enthralled in the image, I missed a good portion of what the researchers announced about their findings so far. To get an idea, I turned to the five articles that were published in The Astrophysical Journal Letters and thumbed through them. Between the formulas, diagrams and interpretations, I quickly saw the incredible amount of collaboration and work that went into capturing and processing the images taken between April 4 and 11, 2017. Numerous radio telescopes across the Earth all had to simultaneously have good weather, the petabytes of data that had to be transferred, standardized, aligned and consolidated. New algorithms were created, faster data processing were invented and countless hours spent to produce an image of a dark region in space, the shadow of the black hole, at the centre of M87 that spans 19 to 38 microarcseconds!

If you are like me, you want to know how much that is in light years not arcseconds and you’re not worried about the margins of error. Let us have a little fun and work that out for ourselves. We’ll need a few things: the small angle formula, the distance to M87 and a calculator.

The small angle formula (SAF) is: arcseconds = 206,265(diameter of object/ distance to object)

The distance to M87 is about 53.5 million Light Years

Let us take the upper end of the measurement because who really wants a small shadow? 38 microarcseconds become … 3.8 x 10^-5 arcseconds.

We want the diameter of the shadow, that means we rewrite the SAF to become diameter of object = (distance to object x arcseconds)/206,265 then plug in the numbers.

diameter = (53,500,000 x 0.000,038)/206,265

We get around 0.01 light years which we can convert into km by multiplying by 9.5 x 10^12 … and voila! 9.5 x 10^10 km or 95 billion km! Not bad for a shadow.

An Evening With Friends and the Perseids

It has been a while since I have taken out my camera to do some astrophotography. In fact, I think the last time I did any was during the solar eclipse last summer.

Thankfully, I have good friends who like evening skies as much as I do and they made plans to head out and watch the Perseids–of course, they invited me along.

It was a great night full that started off with all of us muttering to ourselves trying to remember how to set up the right functions on our cameras to take shots for the evening.

After about ten minutes we were all set up and soon sitting back in our lawn chairs looking skywards. The evening did not disappoint. Between Mason jar cake, homemade mint tea cocoa and conversations that took us from catching up to discussing the hermit kingdom, we saw many bright, brief, and beautiful meteors.

Unfortunately, my camera didn’t catch any of them in the 120 pictures I took, but I was able to stack the images into a nice Morse code star trail image. Two of my friends lucked and caught a meteor in at least one of their images.

All this means that we will just have to head out again for the next big meteor shower.

Book Review: 50 Things to See with a Telescope – Kids

Cover page of 50 Things to See With a Telescope - Kids

I was fortunate enough to get a copy of John’s book from him when we met at this past total solar eclipse. As a science communicator at the Manitoba Museum’s planetarium, I am always on the hunt for good astronomy books, especially ones that are accessible to a younger audience.

John Read’s “50 Things to See with a Telescope Kids” turned out to be one of those rare books. Not only did it teach and remind me a few things I didn’t know or had forgotten, but it was also a light read with spot on and fun visuals making it great for a reading session with your kid.

The book is a good introduction on what to expect from the hobby of amateur astronomy and gives good tips on how to enjoy your night beneath the stars. The first thing I really like in John’s book, compared to some I have read, is that he is up front and honest about the fact that it takes time, patience and practice to find objects in the night time sky.

His reminders about the challenges of telescope use, especially around more difficult objects to find, is so important to keep in mind, otherwise you and your child will get discouraged. I remember taking four evenings just trying to find The Great Globular Cluster in Hercules (Target 37 in the book) when I was just starting out. Despite the frustrations, I kept at it. The sight was worth it.

This leads me to the second thing I really appreciate about 50 Things to See with a Telescope; John has included images of all the objects in his book as seen through a small telescope. It may seem obvious, but what you will see through the eyepiece of your telescope (or for some targets through your binoculars) will be very different from the images and views of the Hubble Space Telescope, whose images now colour our expectations of space.

The images are also incredibly useful as guides for what you should be looking for. Have you ever tried to find a blue cup in a friend’s cupboards when there are six different blue cups mixed in with a dozen other coloured cups? Well hunting for objects in the sky can be a little like that If you’ve never seen the objects before, these little beautiful images are sure to help you out.

I really appreciate that John included at least one binocular target for each season. This gives kids and parents alike a chance to try out some of the targets with a pair of good binoculars that they may already have at home and see if this is a hobby they want to explore deeper before going out and buying a small telescope.

There were a couple formatting issues where an image would cover the text a little, and some terminology and phrasing that I would perhaps have avoided in my personal style of writing, but none of this takes away from the enjoyment and usefulness of the book.

50 Things to See with a Telescope Kids is an excellent first book or addition to any kid amateur astronomer’s bookshelf – even for those of us who are still just kids at heart.

P.S. I wrote to John Read about the formatting issues and and he informed me that the issues have been addressed in the latest publications of his book.

-KMSB

Reflections on the Total Eclipse of the Sun

I am hoping to capture some of the mystery and excitement from my girlfriend’s and my journey to Grand Island, Nebraska, to see the total solar eclipse. When we arrived on Sunday afternoon, the campsite was abuzz, not with the talk of the eclipse, but of old friends reuniting and introductions being made to individuals who would become new friends.

Later that evening, after getting settled into our campsite, the talk turned to the weather. We were getting updates and advice from Jan Anderson, who was elsewhere along the path of totality. Regrettably, we were looking at cloud cover for our campsite. Two plans were formed; one, to get up early and drive west as far as you could go past Alliance. The other, to only go a few hours east and hope for what my new friends called a sucker hole.

It was so energetic watching experienced amateur astronomers looking at all the weather models and discussing where the models did or did not overlap.

At first, my girlfriend and I were going to join the crew going west, but we decided to stay and be with friends. It was a good choice, because the next morning, after the models came out, we were forecast to have clear skies with scattered clouds. We had a lazy breakfast, then set up our cameras and telescopes with some friends from the Halifax R.A.S.C. We got all set up just as first contact arrived; the slightest piece of the Sun disappeared behind the tiniest hump of the Moon while looking through my hydrogen-alpha telescope. I chose to take pictures through my telescope, while my girlfriend used her zoom lens equipped with a white light solar filter.

About ten minutes in, after first contact, a bank of clouds started to roll in. Some of the camp was willing to wait and hope for a sucker hole, while others wanted to chase clear skies further west. My girlfriend and I packed up our gear, jumped in her car and followed her Halifax R.A.S.C. friends west on the interstate. We did our best to keep up with them, but their 85 M/Hr down the interstate was hard to keep up within the 75 M/Hr zones. Twice State Troopers puller over either the car ahead or behind of us. Not wanting to risk having a lecture from law enforcement while totality happened, we kept to the speed limits.

The Halifax crew never got pulled over, but we also did not see them again until we met up at the campsite after the eclipse. We did find a nice mile road just south of Highway 2, right on the centre line of totality. We had just enough time to set up our cameras and my telescope and to watch the last limb of the Sun disappear behind the Moon.

Within seconds of the Sun’s edge disappearing, I took off my eclipse glasses to catch a flash of light as the Sun disappeared from the sky. As the already darkening corn fields fell into the night of the Moon’s shadow, the Sun’s corona came into view as if it had always been in the sky and I was just too busy with my life to have noticed. I let out a gasp that encouraged my girlfriend to remove her eclipse glasses and look skyward.

I do not think my description could ever do it justice, nor can ay picture or painting truly capture its magnificence or awe. Even the image in my mind’s eye pales to the real sight.

The sky was dark with only the gentle glow of a few of the brighter stars and some planets to break up its hue. The corona surrounded a hole where the Sun was supposed to be. Like glass threads, the corona poured down and upwards from this hole in the sky. They did not move and were in sharp crystal like focus. Then they would move without moving, without blur. You would simply accept that they had always been in the new spot. The threads above were asymmetrical to the threads below, like the asymmetry in the face of a lover.

I took it in for a good minute before I, regrettably, decided to try and take a picture. I should have left it alone, as none of the pictures I took had any grandeur to them. I am still happy with them but should have just enjoyed the moment more. I did put my camera away after a few photos and took in the last few seconds of the eclipse. Suddenly a bright flash appeared on the outer edge of the Sun, the so-called diamond ring. I quickly put my eclipse glasses back on – totality was over.

It was only as the darkness slowly burnt off like a fog that I noticed the everything had become colder, that the birdsong had stopped, and that the ever-present song of crickets had grown louder in the shadow of the Moon. In all the experience has changed my life, but do not ask me how just yet. I am still trying to take it all in, still trying to process this wonder that is beautifully worked into the mathematical relationships of the disk size of the Moon with its distance from us in relation to the distance and disk size of the Sun: four hundred times smaller, four hundred times closer.

I am grateful for the clear skies we did get, even the campsite got it sucker hole! I am happy to have shared it with my girlfriend, to have heard the “Ohs,” and “Ahs.’ from the people on the other side of our cornfield, and thankful to possess a self-aware consciousness, granted by God or other means, to truly take in this spectacle, this wonder, this cosmic gem, this gravitationally regulated clockwork of Earth, Moon and Sun.

-KMSB