“Three Great Astronomy Discoveries I Didn’t Make” by Paul Carr

I have a little training in astronomy, although I can hardly call myself a professional. Becoming a professional at anything is a long, hard road, one I haven’t traveled far down in astronomy. No, I’m decidedly an amateur, and not a highly accomplished amateur at that.

Sadly, the word “amateur” has been cheapened a bit by modern use. It doesn’t really mean inexpert dabbler (although I will cop to that), but “lover.” An amateur does what they do for the love of it, and there are many highly knowledgeable and skilled amateur astronomers who do real science. Far from earning a living at astronomy, amateurs spend considerable sums of their own money - for love. If you are reading this, you probably understand why. The night sky and the cosmos at large combines natural beauty and profound fascination as few other things we can all experience.

In our time, nearly any object - save perhaps the odd undiscovered comet or asteroid - that is accessible to amateur equipment has already been imaged at multiple wavelengths using professional instruments. Sensitive astronomical surveys have been performed and are being performed on a regular basis, and have been for many generations, going back for more than two thousand years to the great Greek astronomer Hipparchus, who since had a major modern catalog named after him.

Since ancient times, the proliferation of astronomical catalogs and atlases has accelerated - from John Flamsteed’s sky atlas, to the 19th century Bonner Durchmusterung, to the Henry Draper catalog, to the US Naval Observatory’s catalog based upon the Palomar Observatory’s photographic sky survey, to the Hubble Guide Star catalog, up until the present, with the Gaia Source List containing more than a billion objects. What is remarkable about this accelerating growth is not only the number of catalogs and the broader coverage of the electromagnetic spectrum, but also the fact that they are now accessible to anyone, and you can also access images from many of these surveys. You no longer need a good university library and stacks of bound volumes to find the information you want.

I should point out that you can do meaningful citizen science at sites like Cosmoquest or Galaxy Zoo, but this does not mean that you can’t poke around at random in the vast library of astronomical knowledge and try to answer your own questions. I do this - and I return from my wanderings  with more questions, and so far, no answers.

Boyajian’s Star

In 2015, when the paper describing Boyajian’s Star and its bizarre lightcurve was first made public on Arxiv, I was curious about what was known about the immediate neighborhood in that patch of sky in the constellation Cygnus. To investigate this, I used a free tool called the Aladin Sky Atlas, provided by the University of Strasbourg in France. Aladin, with a modest learning curve, allows you to overlay many of the most widely used catalogs and image libraries. For example, if you just open up Aladin and enter “Boyajian’s Star” in to the Location field, you will get this window:

Figure 1 - Boyajian’s Star as Viewed in Aladin with Colored Digital Sky Survey


That’s Boyajian’s Star, aka KIC 8462852, centered in the purple reticle. You can zoom in from there and explore to your heart’s content. What I wanted to investigate was what was known about the other stars right around the target, so I zoomed in and overlaid one of the biggest catalogs,
2MASS (2 Micron All Sky Survey). The 2MASS has over 470 million objects in it, so chances are if you can see it in an image, it’s in 2MASS, as shown in Figure 2:

Figure 2 - Zoomed on Boyajian’s Star with the 2MASS catalog overlay.

Beyond the images though, I was interested in any measurements of the distance to any of these objects, and their proper motions - how fast they appear to be moving against the background of more distant objects. Objects with a distance similar to Boyajian’s Star or with similar proper motions are candidate neighbors, or even companions. The proper motion of Boyajian’s Star has been fairly well known (about 13 thousandths of an arcsecond per year West) for a while now, but it’s distance (a bit under 1500 light years) has only been inferred from its spectral type and apparent brightness.The little red squares in Figure 2 indicate an object in the 2MASS catalog. As you can see, all but some tiny faint smudges are cataloged by 2MASS. Most of these objects are in other catalogs, like the infrared ALLWISE catalog, or the Gaia Source List. If you are interested, you may also want to look up the PanSTARRS images, which don’t seem to be available via Aladin.

It turns out that directly measuring the distance to such a star using subtle variations in its apparent movement, or parallax, as the Earth moves around the Sun is very tricky, and only in recent times have we known more than a handful of these distances. So, we’re still not sure which of those stars are actual neighbors to Boyajian’s Star The only real measurement of its parallax was released only a few months ago by the Gaia team, and still carries a fair bit of uncertainty. We hope that with future Gaia data releases (perhaps one as soon as late 2017), we will have better information, not only about Boyajian’s star, but its neighbors as well.

So, lesson learned - many more objects have been cataloged than have been closely studied. We only know the spectral type and other information about Boyajian’s Star because it’s weird light curve as observed by the Kepler Space Telescope triggered a number of follow-up observations. As I poke around in these catalogs, I notice more and more, and with some guidance from professionals, I am able to start making sense of things, and also learning how to be more cautious.

Did a Star Go into Hiding?

In 2016, three Swedish astronomy students published their finding about a clever new approach to SETI - looking for stars that had vanished between surveys. In the sample of objects they studied, they found one faint object that appeared to be in a US Naval Observatory Catalog, but had vanished from subsequent, more sensitive surveys. I interviewed the lead author, Beatriiz Villarroel for my podcast, the Wow! Signal. Villarroel’s team weren’t sure this object was even a star, but I was intrigued by the possibility of a SETI discovery.

Using Aladin and the Vizier server, I found an image from the Sloan Digital Sky Survey and overlaid the USNO B1 catalog (Figure 3), represented by the red crosses. Right in the center is where the missing object, poetically named 1084-0241525 was, but as you can see, there is nothing there now. It’s also missing from all the other major catalogs, but keep in mind we don’t really know its proper motion. It may have moved a little over the decades, since the Palomar survey images upon which the northern part of USNO catalog is based were mostly taken in the 1950s. In 50 years, this object could have potentially moved a few arcseconds, depending on how close to us it is.

Figure 3 - USNO B1.0-1084-0241525 location on the g band SDSS image in Aladin. The red crosses are objects in the USNO catalog. Unfortunately, the star was too dim to be seen on the old photographic plates at Harvard, or to be regularly monitored by the AAVSO.

The conjecture is that if someone out there was building something around this star (if it is a star) that was hiding it from our cameras, it might be heating up enough to be visible in the infrared.  Villaroel’s team looked for this, but didn’t see anything. However, if we overlay the infrared ALLWISE survey catalog (the little blue circles), and look just outside their 5 arcsecond search radius:

Figure 4 - the ALLWISE catalog overlay around the missing star location in Aladin

You can probably guess what I was thinking. Some object moving fairly fast against the sky - a bit more than a tenth of an arcsecond per year  - had reddened so much that it had once been visible, but was now only visible in the infrared. And yes, that is a bit odd. For that to be a SETI detection is still not a slam dunk, but is much closer than we usually get.There is nothing visible under the reticle, but just to the north, about 6 arcseconds away, is an infrared object spotted by WISE, J145736.52+182507.8, that doesn’t appear to have any corresponding optical counterpart.  

The WISE space telescope, back when it was able to keep its focal plane extremely cold,  took measurements in 4 bands from in the infrared. These bands are called W1, W2, W3, and W4, and run from W1 centered at 3.35 microns wavelength to about 22 microns for W4. For comparison, the light you can see with your eyes is the neighborhood of half a micron in wavelength. Normally, the infrared bands for a star are on what is called the Rayleigh-Jeans tail - that is,  the infrared spectrum is fairly flat, and the brightness wouldn’t vary that much from band to band.

Sadly, what we know from WISE isn’t enough. W1 and W2 are fairly close in brightness, as you would expect, but the signal to noise is too low, and the best you can say is that the source is no brighter than a certain magnitude in W3 and W4 - it could be much dimmer. Maybe someday we’ll have a survey that nails down the infrared spectrum of this object, but WFIRST may not be enough, since it will only survey out to about 2 microns wavelength.  So, I haven’t found the missing star - yet.

The Mysterious Gaia Dipper

I hope you know I’m using the word “mysterious” here ironically. This word used to mean something, but now I’m afraid it’s just cheap clickbait.

ESA’s Gaia mission is doing some really interesting things right at the cutting edge of what is possible. One of the things it does as it scans the sky is measure the brightness of stars in both a blue and red band. The Gaia team keeps track of the brightness measurements for individual objects and publishes an alert when it detects that the brightness has changed by a significant amount. It has spotted quite a few supernovae this way, for example.

A small fraction of the Gaia alerts note when the brightness of the object drops considerably. I started tracking these dippers because I wanted to see if Gaia might possibly spot something that behaves like Boyajian’s Star. No doubt many of the dippers are what are called Young Stellar Objects, stars that have just formed and are still surrounded by a large disk of gas and dust.

One of the first ones of these I noted was Gaia 16bnj. Here’s what its Gaia light curve looked like (Figure 5):

Figure 5 - The lightcurve for Gaia 16bnj

Here’s the colorized WISE image:Those are very sharp dips shown in Figure 5 - quite a bit deeper than Boyajian’s Star, and the two dips are roughly the same size as best as we can tell. Using Aladin, I was able to quickly see that this relatively faint object is also in the ALLWISE catalog, as J200207.30+174649.7.

Figure 6 - Colorized WISE image for Gaia 16bnj

Ah, but you guessed it, wet blanket time. WISE astronomer and SETI researcher Jason Wright pointed out that there were problems with these measurements:The object of interest is at the center of Figure 6, with a little blue circle around it. When I looked at the WISE magnitude data at first, I got pretty excited - it is much brighter in the longer W4 wavelength than in the shorter W1 and W2, and this time these were not limiting magnitudes, as they were for J145736.52+182507.8. A big infrared excess could mean one of our favorite classes of conjecture - a megastructure.

 

Look at the quality control flags:

ccf 00Pp

One character per band (W1/W2/W3/W4) that indicates that the photometry and/or position measurements of a source may be contaminated or biased due to proximity to an image artifact:

P,p = Persistence. Source may be a spurious detection of or contaminated by a latent image left by a bright star.

So, no reason to think this thing has any excess IR emission.

So, it could easily be an imaging artifact and there is no way to tell. You know - the old extraordinary claims/extraordinary evidence thing. We’ll keep looking for more dips, and perhaps we’ll see something interesting in the future, but for now, we’ve got nothing much. However, I’m not overly discouraged. Boyajian’s star doesn’t exhibit an infrared excess, and there’s still lots of interest attached to it.

Are We Any Wiser, Then?

The first lesson from all this is that amateur attempts to mine the astronomical databases for little nuggets of holy cow are likely to be frustrated. We have cataloged billions of objects but only really studied a relative handful. Also, some of the data we do have available for exploration has problems. There might in fact be a star that goes missing from a catalog, but trying to figure out what happened to it will be frustrated by the lack of follow up observations.

For now, most of the people who get credit even for minor astronomical discoveries will overwhelmingly be the ones who make an intelligent choice of targets and point their telescopes at them for years of painstaking observations, followed by sophisticated and careful analysis. As far as I can tell, they are also the ones who deserve the credit.

However, this is not by itself a sufficient reason to give up, since after all, it’s quite pleasing to take a magic carpet ride among the stars looking for rarities. It’s your universe as much as anyone else’s, and we know orders of magnitude more about than our grandfathers did. So, feel free to download Aladin or similar tools and poke around the deep sky in whatever direction you like. Please, just let us know if you find anything.

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