A Public Astronomy Presentation in Columbia, SC

I will be giving the presentation "The Magellanic Clouds: What nobody knew until now." at the June 7th meeting of the Midlands Astronomy Club in Columbia, SC.

UPDATE
I had an enthusiastic reception for my Magellanic Clouds presentation at Midlands Astronomy Club.  There were some very intelligent questions asked by the audience at the end.  I picked up a number of Twitter followers and Facebook friend requests from this one.  Several told me afterwards that they particularly enjoyed learning about things they had never heard of.

Response to the heliometer project

Though you can't tell it from the lack of comments on this blog under the article about my do-it-yourself heliometer project, response from Facebook and Linked-In has been quite gratifying. It even turns out that a school receiving a STEM grant is going to use it as one of its main student projects!

Keep checking back. When time allows, I will be coming up with other cool and stimulating science projects for public consumption.

Measuring the Sun’s Diameter with a Cardboard Rug Tube

Copyright 2012 Rick Boozer

This project uses a long cardboard tube that comes with a linoleum rug with no lenses or mirrors, to measure the diameter of the Sun with fairly high accuracy! For maximum accuracy, it also requires the use of a digital camera, a computer and some free software. As I promised in my earlier article on how to make a camera obscura, this follow-on project uses the hole projection principle mentioned in that earlier article. Sorry for the delay in getting this project completed and up on this blog, but finishing the Completion of Candidature phase toward my PhD took precedence.

The General Principle of the Experiment

A hole at one end of the tube will be used to project an image of the Sun at the other end, just as the hole at one end of a camera obscura could project an image of an object on its opposite end. Such a device for measuring the properties of the Sun is called a heliometer, from the Greek word helios for the Sun, whilst meter means measure. Thus, a heliometer is an instrument for measuring the Sun. Please refer to my article A Camera Obscura from an Oatmeal Box for an explanation of the scientific principles behind the hole projection method upon which both projects are based.

Though the project is not highly complex or excessively time consuming, it is important to note that completion of the project requires skills, tools and material beyond those available to most children. I put close-up photos at the end of this article for those who want to see how I assembled the mount and other parts. But you should know that a jig saw and a drill are required for assembly of the mount. Thus, children should only participate in this project with careful adult supervision. Given these facts, I am writing this article at an adult reading level assuming that a grownup will supervise the construction of the instrument.

Figure 1: The assembled heliometer

In Figure 1 you see the completed heliometer. To prevent this article from being excessively long, I will merely explain how the instrument works instead of talking about how to assemble it. The tube is essentially a long camera obscura with a small hole on one end (through which sunlight will pass) and a translucent viewing screen on the other end, as pictured in Figure 2. The illustration is, of course, not drawn to scale.

Figure 2: Image of the Sun projected on the rear of the heliometer. Note: a circular cross-section of the spherical Sun is depicted on the left.

I actually used the oatmeal box from the earlier camera obscura project. I cut off the round bottom of the box, enlarged the pinhole and affixed it to the front of the tube. We need a fairly large hole so that a bright image can be formed on the screen, in this case around 4 to 5 mm across. You want the hole in the front to be as perfectly round as you can get it. I found that using a pencil soldering iron to burn through the cardboard makes a very smooth round hole. Just don’t keep the iron in contact with the cardboard for too long as you don’t want it to catch fire! Again, I used the translucent lid of the oatmeal box as the screen onto which the image is projected.

The triangles formed by the light cone from the Sun to the hole and from the pinhole to the image screen are proportional. However, the light rays at the hole do not cross at a perfect geometric point because the hole has a diameter of around 4 mm. Thus, the resulting image will be bigger than if the hole was a point. Look at the illustration in Figure 3 of the Camera Obscura article to get an idea of what I mean. The light beams from the Sun cross each other both before and after the hole. Intuitively, one might think that the final projected image will be wider than an ideal image theoretically generated from a perfect point and that it would be wider by an amount equal to the hole’s diameter. Using the aforementioned illustration, I came up with a geometric proof (which I will not detail here for the sake of brevity) that showed the image should indeed be larger by that amount.

Making Photos to Obtain Data for the Calculation

The following Figure 3 shows an actual image of the Sun projected on the translucent plastic screen that was shot with a digital camera. If your camera has a “macro mode”, you can set it to that mode to take perfectly focused photos at a distance of half a meter (about 1.6 feet) or less from the plastic screen: the closer the better. Please note that, for reasons to be explained later, the camera should be set up to put a time stamp on each image before you shoot.

Figure 3: Actual projected image of the Sun.

Many of you who are amateur astronomers may be familiar with the fact that any image of the Sun will exhibit limb darkening, which is a gradual darkening of the Sun’s image towards the outer edge of the solar disk. An explanation of why limb darkening occurs would cause us to stray too far afield. Instead, we’ll just talk about compensating for it because the true edge of the Sun is so darkened that it is not plainly visible. At this time we apply one of the advantages of digitally recorded images; that is, we can increase the visibility of certain things by heightening contrast and/or brightness with a fine level of control. In the next illustration, I heighten the contrast to maximum then adjust the brightness to give a stark white solar image disk with fairly sharp edges. The disk should appear larger because even the areas of the disk with previously invisible sunlight become detectible to the eye. Please note that the washed out areas above and below the disk are a reflection from my hand that was holding the camera and smears into part of the bottom of the solar image. That doesn’t matter since most of the disk edge is fairly sharp. Notice that the solar disk image is indeed slightly larger than it was in the unmodified image.

Figure 4: Solar disk adjusted to maximum diameter.

Extracting Data from the Photos

Before we measure the size of the solar image, it is important to find the scale of the image; which is the number of pixels per millimeter. So all I needed to do was measure the diameter of the plastic lid in pixels, then divide that by the actual diameter of the lid in millimeters. Most photo computer applications will show you the pixel position of the mouse cursor. It was easy to get the diameter of the lid in pixels by moving the cursor to the outer left edge of the lid and reading the smallest horizontal pixel position, then reading the largest horizontal pixel position directly on the other side of the lid across its diameter. Of course the actual diameter in pixels came from subtracting the position of the rightmost value from the leftmost. In my case the diameter of the lid was 2160 pixels. Since I had earlier measured the lid with a ruler as being 105 mm in diameter, the scale was 2160 pixels / 105 mm equals 20.57 pixels per mm.

Now is the time to measure the size of the solar image. We measure the diameter of the Sun’s image in pixels the same way as described for the lid. It was 824 pixels across, which led to 824 pixels / 20.57 pixels per millimeter equals 40.06 mm for the diameter of the solar image.

But remember, this image is too large by the diameter of the hole at the front of the tube; therefore, it is very important to measure the hole precisely. In order to do that I snapped an image of the piece of cardboard that contained the hole with a ruler below it. I used the ruler appearing in the photo to determine the scale of the image by noting 100 mm spanned 949 pixels, which meant that the scale was 9.49 pixels per millimeter. I then measured the diameter of the hole to be 44 pixels. So the actual diameter of the hole was 44/9.49 or 4.63 mm.

Figure 5: Measuring the hole at the front of the tube.

So the ideal size of my solar image was 40.06 - 4.63 equals 35.43 mm.

Figuring the Diameter of the Sun

Now we can get down to actually calculating the diameter of the Sun. First we need to figure the constant of proportionality for the two triangles shown in Figure 2. We do this by remembering that not just the sides and the angles of the triangle are proportional, but also the bases and the heights of the triangle. If we let the base of the bigger triangle be the true diameter of the Sun and let the altitude of that triangle be the distance between the Sun and the Earth, the same proportion will hold true for the second triangle whose altitude is the length of the tube and whose base is the diameter of Sun’s image. The length of my tube was 3789 mm. Dividing that by the diameter of the solar image, we find that the tube is approximately 106.943 times longer than the diameter of the image. That means that the distance to the Sun should be that many solar diameters. So if we divide the distance to the Sun by 106.943, we should have the diameter of the Sun!

But we want the most precise distance to the Sun that we can get, in order to maximize the accuracy of our answer. I did this by using the free planetarium software called Stellarium. In order for the application to give me accurate answers, I had to set it up for my latitude and longitude and my correct time zone. Once I had done that, I set Stellarium’s time and date to the value indicated by the image’s time stamp. At that point Stellarium showed the Sun as it appeared in the sky at the time the photo was shot. When I clicked the sun on my computer screen, it gave me the distance to the Sun as 0.98383515 AU. Expressed in kilometers, this distance would be 0.98383515 times 149,597,871 kilometers per AU. The product of those numbers is approximately 147,180,000 km. I then divided that number by 106.943 to get the Sun’s diameter as about 1,376,240 km.

The formally established diameter of the Sun to 6 decimal places is 1,391,020 km. The answer I got was too low only by 1.06% of the formally established value. Not bad for a cardboard tube! It may be possible to get it to even better accuracy if a PVC tube is used instead of a cardboard tube. The occasional light wind kept gradually bending my cardboard tube into a slight bow shape from which it would not straighten back out; therefore, I had to measure the direct length from the front to the back to compensate for the slight bow that developed. That shouldn’t happen with a PVC pipe.

What I would like to do (if I can get the time) is redo the experiment with a PVC pipe, take more than one measurement and average my results to maximize accuracy. Even though I took many exposures with this experiment, I only got one image well centered on the screen that was good enough for measurement. This was because of slightly windy conditions causing the tube to move on its mount. Furthermore, I had to keep re-measuring the length from front-to-back in order to compensate for the slight bowing of the tube by the wind. In other words, the rug store gave the cardboard tube to me for free, but I paid a price in other ways.

I hope some of you will try this experiment yourself. For me it was a fun break from work involving more advanced math and higher level physics. As for my next project, please be patient because it may be a few months. My PhD related research must take precedence, but my next project posted on this blog will be something amazing if it works!

A Camera Obscura from an Oatmeal Box or "Look Mom! No lens!"

Copyright 2011 Rick Boozer

Here’s a way to make science fun for young people and curious adults! The materials you will need are common household items. This project is simple to make and very easy to use, but young children will need adult supervision. I hope that teachers will find this article useful for classroom instruction. This is Part 1 of a two part experiment that will be added to my blog in the near future. The second part is based on this first part and will be about how to make a simple astronomical instrument that will do something amazing!

In this first article, we will be making something called a camera obscura. Of course, everyone knows that a camera records images on film or a light sensitive chip, but a camera obscura projects an image you can directly see. Most camera obscuras use a lens to project the image, but in ours a pinhole will do the projecting. I hope you will enjoy making the camera obscura and will return for the next installment.

Let’s get started! Here what you will need:

• One round oatmeal box with a plastic lid
• Pencil or an object with about the same size point
• Aluminum foil
• Cellophane tape
• A small nail

The hole that does the image projection will be punched into the bottom of the oatmeal box and the screen on which the image will appear is the translucent plastic lid that comes with such a box. However, to do it right the hole and box must be prepared properly.

Before you make the hole you need to know that the smaller the pinhole, the sharper the image. There’s some science here, but I don’t want to get ahead of myself, so I will talk about that later. But just know that if you make the pinhole too small, it will not let enough light through and your image will be very faint. I have found that a good pinhole diameter that gives the best compromise between detail and brightness is about 1.5 millimeters.

So, let’s make the hole in the bottom of the box. It is very important to get a smooth round circle for the hole, and such a smooth round circle is practically impossible to make in cardboard. The solution is to punch a much bigger hole than is needed with a pencil or other sharp pointed object. Next make your small projection hole by taking a small piece of aluminum foil and, with a tiny nail, carefully and gently punch a hole about 1.5 millimeters wide. Turn the nail gently to get a hole that is very circular with no jagged looking edges. Tape the piece of aluminum foil onto the bottom of the box such that the hole in the foil is over the center of the hole in the cardboard. To reduce reflections that can cause glare in your images, either paint the inside of the box black, or be lazy like I am and cut a piece of black construction paper to the right size and line the inside of the box with it.

Now, with the plastic lid secured at the top of the box aim your camera obscura toward a bright lamp or overhead light fixture. What do you see? An image of your subject, but it appears upside down! What?! Why?!

Let’s talk about the really fun part of science . . . discovering and understanding the why!

But before we move on, here is photograph I made of an image of an etched glass light fixture projected by my pinhole camera obscura onto the lid. Notice the detail you see in the shapes on the glass.

Figure 1: An actual pinhole camera obscura projects an image of a glass light fixture on to the box lid.

If you look closely, you may be able to see natural camera obscuras in the real world. For instance, if you look where a tree is casting a shadow on a bright summer day, you will notice that the gaps between leaves act as pinholes that project tiny circular images of the sun onto the ground. These are especially interesting to watch during a solar eclipse when you can see the images go from circles, to half circles and then to crescents. On a night when the moon is bright enough to cast sharp shadows, you can see very faint little circular images of the moon on the ground as well.

But why does a pinhole project an image similar to a lens? And why does the camera obscura project images of objects upside down, instead of upright?

In the following illustrations the small circle on the right side of the oatmeal box represents the pinhole (it is drawn oversized). The arrowheads represent the direction the light is travelling from the object to the pinhole that will be projecting the image. The light then passes through the pinhole to the back of the box where it strikes the lid to make the image.

Take a look a Figure 2. Notice all of the light beams coming from the object cross exactly at the center of the pinhole, finally reaching the lid in the back of the camera obscura to light up the image. I have seen this type of illustration to explain pinhole projection since I was a child. Unfortunately, this common example is not accurate. It portrays all of the light rays crossing exactly where the hole is, and that is not what happens. Before you can understand why this is wrong, you need to have some idea of how a pinhole can create an image.

Figure 2: The way camera obscura projection is usually portrayed.

First think of the light coming from the object as being, not a solid cone of light, but a whole bunch of long round beams of light, with each long beam being the same width as the pinhole. Instead of talking about all of the light beams that are coming from the object, let’s keep things simple for now and think about only two of those beams.

One beam will be coming from a circle at the top of the object, and the other beam from a circle at the bottom of the object. A beam of light always travels in a straight line (unless is it made not to do so). Think of the beam from a laser pointer, which is just a thin bright beam of light. Figure 3 illustrates this situation with the two beams coming from opposite ends of the object: one coming from the top of the object, the other from the bottom.

Figure 3: Light beams from opposite sides of the object. (Click to enlarge)

The beams of light from each of the two circles cross each other somewhere in the region that I have colored yellow. That region’s thickness is the same as the diameter of the pinhole. The two beams first begin to cross outside of the box in front of the pinhole and quit crossing at someplace behind the pinhole inside of the box. They don’t do all of the crossing at the pinhole as Figure 1 wrongly indicated! This fact is not important right now, but will be very important when we do the astronomical experiment that is to come in the second article.

The most important thing to notice now is that the light in the beam that started at the top of the object ended up at the bottom of the image because the light traveled in a straight line that is slanted downward. For the same reason, the light beam from the bottom of the object ended up at the top of the image. Their positions in the image are flipped from what they were in the original object!

Do you see now why the image is upside down? Circles of light from the top half of the object end up in the bottom half of the image. Circles of light from the bottom half of the object end up in the top half of the image. Thus, the image gets inverted!

The following figure shows the original object with small circles on the object to indicate where each light beam starts.

Figure 4: The original object with circles added to show where the light beams start.

Figure 5: The image showing circles where the light beams strike the lid.

Each pinhole size circle on the image is a small amount of light from a small circular area on the original object whose image is being projected. In other words, the light illuminating each small circle in the image shown in Figurer 5 comes from a corresponding circle on the opposite side of the original object in Figure 4. If you are familiar with the way computer images are generated with pixels, you can think of each small circle as being a pixel in the image. This is part of the reason why a smaller pinhole gives a sharper image: the smaller the circle (or pixel) the finer the detail.

So the light in circles on the top of the image comes from circles in the bottom of the original object, light in the circles at the bottom of the image comes from circles at the top of the object, light in the circles on the left of the image comes from circles on the right of the object, and light in the circles of the right of the image comes from circles on the left of the object. The image is upside down and backwards!

Figures 4 and 5 are a good way of showing you how the image is created, but these illustrations aren’t exact. In reality there are many more circles of light creating the image. The lighted circles are not really in nice neat rows barely touching each other. Neither are there any little unlit gaps where the edges of the circles don’t touch. Instead, the circles partly overlap each other. Because of the partial overlapping, there are no tiny gaps where the image isn’t lit. This overlapping causes some blurring. The blurring can be minimized by keeping the pinhole as small as possible, but still big enough to keep the image bright enough to see well.

Of course, your camera obscura can be used to view other objects besides a light fixture. Not all objects are bright, so if you want to see images of fainter things, you can drape a dark cloth over your head that also covers the lid end of the box. The more light you shut out with the cloth, the brighter you image will appear. But you’ll notice, every image appears upside down!

If you understood most or all of this, then you have just learned some fairly sophisticated physics about the behavior of light. You’re a budding Einstein!

Next article: using the principle of the pinhole camera obscura to create a precise astronomical instrument made from simple commonly available parts.

One more thing! My thanks to my wife, Julie, for helping me make this article sound less scholarly.

Commercial Spaceflight Will Keep the U.S. Competitive

Yahoo! News has published another spaceflight related article written by me.

United States Will Beat China in New Space Race

I have been a human spaceflight enthusiast since childhood, over 50 years. For America to prosper in the future she must be a leader in, not just space exploration, but space exploitation. However, there are some politicians who are putting that future in peril in order to insure that pork flows to their constituents. If you are as concerned about this issue as much as I am, click the following link to read an article I have written on the subject.

Senators Crippling NASA - Associated content from Yahoo!

QSOs: Faraway Objects with Local Relevance

Copyright 2011 Rick Boozer

In my earlier article about the experience of earning my Masters degree, I mentioned that, “I learned so many fascinating new things that I never suspected, nor had I ever seen them mentioned in any popular astronomy publication. For instance, in certain unusual cases, there is a way to use radio signals from quasars to give image resolutions of less than a microarcsecond. That’s much finer than the Hubble Space Telescope’s best resolution and even better than from the widest baseline radio interferometer with dishes on opposite sides of the world!”

After some reflection, I thought it might be a good idea for me to share some of the cutting edge information about quasars with fellow astronomy enthusiasts. Indeed, as shall be shown, astronomical discoveries may have unexpected practical applications. Thus I wrote this article. However, before I cover the new stuff, some background information may be in order.

In astronomy it is not unusual that a strange new discovery is considered too far away to have practical uses, but later reveals a useful application beyond anything that came to mind when it was first detected. For instance, helium was discovered from its absorption lines in the Sun’s spectrum long before it was physically identified on Earth. This observation sparked a worldwide search until it was found in certain oil wells in the U.S. Of course, this discovery brought the myriad industrial and scientific applications for which helium is now used. No wonder the name of this gas is derived from the Greek word Helios meaning Sun.

In this article I present information about a very strange and incredibly remote type of object that at first glance may appear to have little significance to our understanding of local phenomena or our everyday lives. However, these objects offer surprising new investigatory directions into the understanding of interstellar conditions within our own small section of the Milky Way galaxy, while also yielding at least one “down to Earth” practical application and a possible utility that future interstellar travelers may want to use.

Quasi-stellar Objects or QSOs are some of the brightest known objects in the universe seen in gamma rays, X-rays, ultraviolet and visible light. It is not unusual for a QSO to shine at a level millions of times greater than the entire radiant output of our galaxy! QSOs appear extremely faint because of their cosmically extreme lookback [1] distances of anywhere between nearly a billion to around 13 billion light-years. Though they are not stars, they are usually seen as a star-like point of light and so are called quasi-stellar. Of course, even members of the general public have heard the name for a special type of QSO that emits very strongly in radio frequencies: quasar for quasi-stellar radio object. Somewhat confusingly, it is not uncommon nowadays for scientists to call a QSO a “quasar” even when referring to a QSO that does not appear as a strong radio emitter.

The Nature of the Beast

Residing at the very center of a host galaxy, the source of the QSO’s immense radiant power is an Active Galactic Nucleus AKA an AGN. An AGN consists of an enormous black hole massing the equivalent of hundreds of millions or even billions of Suns along with a surrounding accretion disk of swirling matter that is pulled inward by the black hole’s gravitational field.

It is the accretion disk that is the source of the intense radiation. Heat induced by the compression of continually in-falling gases piling up within the accretion disk brings the disk to incandescent brilliance.

The intense magnetic field produced by the rotating black hole attracts some of the accretion disk’s matter away from the disk. Matter siphoned off in this manner then gets shot out in two opposing continuous jets of plasma near the black hole’s north and south magnetic poles. In some instances, the strength of the black hole’s magnetic field is so strong that the plasma is ejected at speeds approaching that of light! This plasma may emit magnetically induced radio emission that is highly polarized, known as synchrotron radio waves. A classical quasar’s strong radio signature is the result of this synchrotron radiation.

The tiny star-like appearance of a QSO is not only due to its enormous distance, but also its relatively compact physical size. Shortly after their initial discovery in the 1960’s, it was observed that the optical brightness of an entire QSO would sometimes vary drastically over time periods as small as several days. Because nothing can travel faster than light, these short pulses in increased brightness would imply that the AGN could be no bigger than the distance light could travel from one side of the QSO to the opposite side during the fluctuation period. Thus it was deduced that the size of the emitting part of a QSO could be no more than mere light-days to light-months across: a distance much smaller than the typical separation of stars in the tightest packed part of a normal galaxy. Indeed, the radiation equivalent of billions of Sun’s can be confined to a region within the AGN that is roughly on the scale of our own Solar System’s diameter! As the reader will see, sophisticated observational techniques developed in later decades confirmed the relatively small size for a QSO’s AGN and even allowed a highly accurate direct measurement of its diameter.

The short-term optical variation is normally the result of processes occurring within the QSO itself. A description of what is known of these processes would fill a long article all by itself; therefore, coverage of this topic is not appropriate for this short written piece and would also deflect our attention from other interesting features of QSOs. From this point on, our focus will be on observed variations in a QSO’s radio emission and some surprising discoveries associated with these observations.

The Mystery of Rapid Radio Signal Variations

One perplexing problem presented itself when radio intensity variations measured in hours were seen. It was calculated that for these fluctuations to be in the QSO itself, that the QSO’s temperature would have to be at least 10¹⁹ Kelvin! This temperature seemed absurdly high. Quantum mechanics dictates that any body with a temperature greater than about 10¹² K should emit copious amounts of a special type gamma ray known as inverse Compton radiation. (Savolainen and Koralev 2008) It then seemed unlikely that the QSO was actually causing the radio fluctuations when no inverse Compton radiation was detected. (Tsang and Kirk 2006) Clearly some mechanism external to the QSO must be in play.

Observed radiation fluctuations, whether they are in visible light, radio or any other part of the electromagnetic spectrum are called scintillation. All of us have seen scintillation of visible light with our own eyes as the twinkling of stars in the night sky and an explanation of this twinkling may give some insight into its radio counterpart.

In the case of visible star scintillation, turbulence in the upper atmosphere causes changes in the optical properties of a high level layer of air to make the starlight seen by an observer appear to either vary rapidly in brightness or cause equally fast apparent shifts in the star’s position. Astute sky gazers may notice that even when the stars twinkle noticeably, any planets that are visible at the same time will shine with an unvaryingly steady light.

Why is there a marked difference in the perception of these two types of objects when light from each is passing through turbulent cells of air? The farther away from an observer that an object of a given physical size is, the smaller it appears to be. Expressed differently, the object’s apparent size measured as an angle will be smaller with increasing distance. Turbulent air cells generally measure but a fraction of an arcsecond across. Though a star’s physical size is much greater than a planet’s physical size and much greater still than any air cell’s physical size, the immense distance of the star is so great that its apparent angular size is much smaller than the apparent angular size of the invisible cells of air turbulence that are causing the twinkling; therefore, any changes in the foreground turbulence will greatly affect the appearance of the star. Though the apparent angular diameter of a planet may be so small that an earthbound observer will perceive it as a point, it is still proportionately much closer to the observer than would be any star. In other words, the ratio of a planet’s physical diameter to its distance is enormously larger than the ratio of a star’s physical diameter to its distance. Since a planet’s apparent angular diameter is also usually much larger than the apparent angular diameter of any cell of air turbulence, the light from the planet will appear not to vary. The following illustration depicts this principle and is, of course, not drawn to scale.

The observer’s location is marked with X and both turbulent air cells are of identical absolute physical size and of equal absolute distance from the observer. Though the star is physically much larger than the planet, the planet is much closer. Thus the apparent angular diameter, α, of the star is smaller than the apparent angular diameter, β, of the planet. The cell in front of the star has a wider apparent angular diameter than the star; therefore, the star twinkles. The same size cell in front of the planet has a smaller apparent angular diameter than the apparent angular diameter of the planet, so that the planet does not twinkle.

Getting back to radio variations of QSOs, the question being asked was, “Were the observed rapid fluctuations being induced into the signal as the QSO’s radio waves traveled through some turbulent cell of material on their way toward Earth?” The first clue that this situation might be the case came in 1998 when two radio telescopes located extremely far apart (one in Australia and one in New Mexico) made simultaneous observations of a QSO designated PKS 0405-385. When a particular intensity fluctuation pattern appeared at the Australian radio telescope, the same variation would show up approximately two minutes later at the New Mexico instrument. This situation was very strange because if the variation was intrinsic to the QSO, the same variation should have shown up on the New Mexico instrument only milliseconds later, that is, after the time it takes light to travel the distance between the two instruments. (Jauncey et al. 2002; Bignall et al. 2007; Savolainen and Koralev 2008) Soon, simultaneous observations of other QSOs revealed delay times that were often much longer.

The extremely tenuous gas and dust spread throughout the space between the stars within our galaxy is called the Interstellar Medium or ISM. Most of it contains only a few hydrogen atoms per cubic meter and is thus a better vacuum than the best that science has ever achieved, though randomly interspersed throughout the ISM are occasional denser clouds of dust and gas. As all amateur astronomers know, some of these nebulae can be seen in visible light. In other words, the nebulae may shine by reflecting the light of nearby stars or their constituent atoms may absorb ultraviolet radiation from local stars and re-emit the absorbed energy as visible light. Conversely, a cloud may be dense enough that the dust within it absorbs the light of the stars behind it, producing what appears to be a black void within the sky that is commonly referred to as a coal sack. Another alternative may also occur where such a cloud is so extremely thin that it may be transparent enough as to be nearly or completely optically invisible.

After the observations were made by the Australians and Americans, astrophysicists were strongly suspecting that QSO radio scintillation could be the result of turbulent cells as the radio waves from the QSO pass through the last type of cloud described in the immediately preceding paragraph. That would explain the excessively long difference in arrival times seen at the widely separated radio receivers. In other words, a particular turbulent cell might induce a characteristic fluctuation pattern at the Australian radio telescope, but that same cell might have to travel a few minutes before it was in a position to cause the same fluctuation to appear at the American radio telescope. Other corroborating evidence was needed to clinch this conclusion, but that confirmation was not long in coming.

Something very strange began to be seen in very short-term radio intensity variations in QSOs that went up then down over time spans ranging from minutes to several days. A gradual orderly change in these short-term scintillation patterns showed up over the course of a year and the same cycle of change repeated again on following years. (Jauncey et al. 2002; Linsky et. al 2007; Savolainen and Kovalev 2008) As any scientifically literate person knows, a year is the time it takes the Earth to complete one orbit around the Sun. It was soon realized that this was the clincher as far as proving that scintillation was being induced by material in the ISM relatively close to us. The speed of the Earth’s orbit around the Sun is around 30 km s^-1; however, the speed of the material in the local ISM is also close to 30 km s^-1. (Jauncey et al. 2002) When the motion of the Earth is approximately parallel to the velocity of the ISM, they have a low relative speed and the variation of the scintillation pattern is slow. But six months later, when their motions are in opposite directions, they have a high relative speed and the variations are observed to be much faster. Thus, we are given conclusive proof of two facts: 1) that the variations are turbulence induced scintillation and 2) that the Earth indeed orbits around the Sun a la Copernicus! (Jauncey et al. 2002; Savolainen and Kovalev 2008) After this conclusive evidence was obtained, short-term variations of QSO radio intensity were christened Interstellar Scintillation or ISS for short. Any turbulent interstellar cloud inducing radio variation is called a screen.

But the evidence got even better. When computer models were constructed using the fluctuation times as input data, the theoretical predicted distance and position for each screen was almost a perfect match for a known “local” thin interstellar cloud that was at least barely detectable in either visible light or ultraviolet light! (Linsky et al. 2007) All of the evidence put together was about as close to a smoking gun as one ever gets in science.

Super Sharp Seeing in the Radio Spectrum

But here is the exciting part. Those same radio variations can be used to reveal fine details of the structure of a QSO in far greater resolution than any ground-based or orbiting telescope is capable of accomplishing! For decades the finest resolutions astronomers attained were achieved using a technique called Very Long Baseline Interferometry or VLBI. VLBI involves multiple radio telescopes observing the same object at the same time but separated by thousands of kilometers to give them the same resolution as a single stupendous radio telescope with a dish as wide as the distance between the two most widely separated radio telescopes. However, the scintillation technique even out-performs VLBI. The previously introduced analogy of visible atmospheric scintillation when stars twinkle may be extended to illustrate how such incredibly fine resolution is obtained.

Remember that if an object located behind a turbulent cell of air (from the point of view of the observer) has a smaller apparent angular diameter than the cell, the object will appear to scintillate. But if the object has a bigger apparent angular diameter than the cell, no twinkling is seen.

What if an observer was able to somehow detect a turbulent air cell and measure its apparent angular diameter? During the course of a night, a number of different turbulent air cells of varying diameters might come between the observer and an observed object. The observer would then be able to notice the maximum apparent angular diameter of a cell that caused twinkling and a minimum apparent angular diameter for a cell that did not cause twinkling. He/she would then know that the apparent angular diameter of the observed object would have to be an angle with a size between the diameters of the former and the latter.

As mentioned before, turbulent cells within an interstellar screen cause the radio scintillations that are equivalent to atmospheric twinkling. The method described in the immediately preceding paragraph has been used to measure the extremely tiny angular diameter of various QSOs. In fact, the resolution obtained is so fine, that astronomers have even resolved structures within the AGNs of some of the closer QSOs!

Angular resolutions on the order of 1 micro-arcsecond can be achieved. In comparison, this resolution is around 1000 times finer than that of the Hubble Space Telescope at its shortest usable wavelength! (Jauncey et al. 2002) And since the distance to a QSO can be determined from the amount of cosmological redshift observed in its emitted light [2] (grist for another entire article), the actual physical size of the QSO can be calculated from its apparent angular diameter. In this case, even assuming a QSO is halfway across the observable universe at a lookback distance of about 6.8 billion light-years, a structure of a mere three light-months in physical diameter can be measured. (Jauncey et al. 2002)

Clues of What’s Closer to Home

But just as radio scintillation can be used to gather information about a QSO, it can also be employed to investigate the properties of interstellar space in our neighborhood. This convenient situation is the result of the fact that the screening clouds have to be relatively close to our solar system. How do we know this? There is a maximum distance away from us that a turbulent cell of a particular physical size can be and still induce scintillation in a QSO. This distance is where the apparent angular diameter of the cell equals the apparent angular diameter of the QSO. Any farther away would lead to a situation in which the angular diameter of the QSO would be greater than the angular diameter of the turbulent cell and thus no scintillation would occur. (Bignall et al. 2007)

So the fraction of material capable of producing fast variability is restricted to the ISM in the Sun’s vicinity. Furthermore, the scarcity of detected screens relative to the overall number of QSOs observed indicates that such clouds of scattering material are few and far between in our immediate section of the galaxy. (Bignall et al. 2007) ISS observations indicate that there are on average 1.7 screens along any line of sight, with a typical line of sight usually having between only 1 and 3 screens (Linsky et. al 2007)

One may wonder what produces the turbulence in the interstellar cloud material. It is thought that areas of the highest scintillation-causing turbulence occur at places where the outer edges of two or more of these clouds come in contact with their different speeds of motion and travel direction. The slightly different velocities of the two clouds produce turbulence where they interact. (Linsky et. al 2007) Because they are on the outside of the cloud, these border edges lack shielding from ionizing radiation put out by one giant blue-white star that is relatively near our Solar System and several local white dwarf stars. The result is a much larger than normal number of fast freely moving electrons that increase turbulence to an even higher level, making these interacting areas hot beds for the production of scintillation. (Linsky et. al 2007)

Finding Our Way Around the Earth with QSOs

Everyone nowadays is familiar with the Global Positioning System that employs a fleet of special navigational satellites in Earth orbit. GPS has pretty much totally supplanted celestial navigation for ship and airplane travel. Even more immediate to people’s everyday lives is the fact that the technology has trickled down to the individual level in the form of automobile navigation systems and emergency location in life or death situations.

But to figure locations anywhere on the face of the Earth within an accuracy of mere meters requires exacting determination of satellite positions at ultra-precise times. Constantly occurring variations in the tilt of Earth’s axis have to be continually taken into account for the system to function with pinpoint accuracy. The tilt variations are detected by referencing the locations of QSOs because their distances are so immense that their motion is not detectable as a change in the object’s position and thus they “stay put” in their apparent relative places all over the sky. VLBI measurements have been used to obtain precise positions of a number of QSOs and have been compiled into a catalog to serve as base navigational references. The catalog is called the International Celestial Reference Frame abbreviated ICRF. (Ma et al. 1998)

Beyond Terrestrial Navigation

Finally, it would stand to reason that QSOs might eventually be used as a natural “galactic” GPS in the event that humanity ever achieves the capability to travel multiple light-year distances. Extremely miniscule changes in observed relative positions of QSOs in relation to each other would be attributable to a traveler’s change in position within the galaxy and thus could be used for navigation purposes. Assuming measurement capabilities continue to progress as they have heretofore, it is not unreasonable to expect that equipment to measure such incredibly minute deviations may be achievable by any future civilization technically advanced enough for interstellar travel.

Who knows what other uses we’ll find for these exotic objects as time goes on?

For that matter, what as-yet-to-be-conceived applications may follow once we know more about the nature of what we now call dark matter and dark energy? After all, those two vaguely descriptive names were chosen because we couldn’t choose better ones since we don’t really know what those properties physically represent!

In short, judging by the past history of scientific discovery, it would seem unwise for anyone to say that any particular realm of scientific knowledge will always only be of purely academic interest.

References

Bignall, H.E, D. L. Jauncey, J. E. J. Lovell, A. K. Tzioumi, J-P. Macquart, and L. Kedziora-Chudczer “Observations of Intrahour Variable Quasars: Scattering in our Galactic Neighbourhood” Astronomical and Astrophysical Transactions, 26 (2007) 567 - 573

Jauncey, David, Hayley Bignall, Jim Lovell, Tasso Tzioumis, Lucyna Kedziora-Chudczer, J-P Macquart, Steven Tingay, Dave Rayner and Roger Clay, “Interstellar Scintillation and PKS 1257-326” ATNF News (October 2002)

Linsky, Jeffrey L., Barney J. Rickett, and Seth Redfield “The Origin of Radio Scintillation In the Local Interstellar Medium”, The Astrophysical Journal, 675 (2008) 413-419

Ma, C, E. F. Arias, T. M. Eubanks, A. L. Fey, A.-M. Gontier, C. S. Jacobs, O. J. Sovers, B. A. Archinal and P. Charlot, “The International Celestial Reference Frame as Realized by Very Long Baseline Interferometry”, The Astronomical Journal, 116 (1998) 516-546

Savolainen,T. and Y. Y. Kovalev. “Serendipitous VLBI Detection of Rapid, Large-amplitude, Intraday Variability in QSO 1156+295” Astronomy and Astrophysics 489 (2008) L33-L36

Tsang, O. and J. G. Kirk “The Inverse Compton Catastrophe and High Brightness Temperature Radio Sources” Astronomy and Astrophysics 463 (2007) 145-152

Footnotes

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[1] The lookback distance is how far the light traveled from an object to reach the Earth. In the case of the farthest detectable QSOs, this distance is about half of the true present-day distance between the Earth and the QSO – called the comoving distance. The reason why is that the universe was continually expanding while the light was en route, causing the Earth and the QSO to become further and further apart during the transit time as the space between them was stretched wider by the expansion.

[2] A wave of light is lengthened (i.e., cosmologically redshifted) because the space it is traveling through is stretched by the continual expansion of the Universe; which in turn, stretches the wave of light. Some inaccurately term it as a cosmological Doppler shift. But the relative motion of a light emitting object, not the Universe’s expansion, causes a true Doppler shift!