Thursday, September 18, 2014

How a Planet Can Mess Up a Star's Looks

Recently, beautiful photos of auroras have been in the news. These colorful light shows were generated by solar storms, and provide a vivid demonstration of activity on the Sun affecting the Earth. The pummeling experienced by our home planet is an example of our one-way relationship with the Sun: it can have a noticeable effect on the Earth, but the Earth has a negligible effect on the Sun. Further afield in the galaxy, this isn't always the case. In a few other systems planets can have a big effect on their star, changing their looks in surprising ways.

A spectacular picture of auroras by photographer Mike Taylor taken over Unity Pond in Waldo County, Maine on September 12, 2014. Credit: Mike Taylor photography.

As explained in the latest press release from NASA's Chandra X-ray Observatory, an exoplanet called WASP-18b appears to be causing the star it orbits to act much older than it actually is. WASP-18b is an example of a hot Jupiter, with a mass about ten times that of Jupiter and an orbit that is less than 24 hours long. The host star, WASP-18, is estimated to have an age that lies between about 500 million and 2 billion years, relatively young by astronomical standards.

Younger stars tend to be more active stars, with stronger magnetic fields, larger flares, and more intense X-ray emission than their older counterparts. Magnetic activity, flaring, and X-ray emission are linked to the stellar rotation, which generally declines with age. However, when astronomers took a long look with Chandra at WASP-18, they didn’t detect any X-rays. Using established relations between the magnetic activity and X-ray emission of stars and their age indicates that WASP-18 is about 100 times less active than it should be at its age.

The researchers argue that tidal forces from the gravitational pull of the massive planet – similar to those the Moon has on Earth’s tides but on a much larger scale – may have disrupted the magnetic field of the star. The strength of the magnetic field depends on the amount of convection in the star. Convection is the process where hot gas stirs the interior of the star.

The planet’s gravity may cause motions of gas in the interior of the star that weaken the convection, causing the magnetic field to weaken and activity to decline. This causes the appearance of premature aging in the star. WASP-18 is thought to have a shallow convection zone, making it unusually susceptible to tidal effects.
Shown in the main part of this graphic is an artist's impression of the star WASP-18 and, in the foreground, its hot Jupiter WASP-18b. The insets show the star in the optical image and its non-detection in X-rays with Chandra. Credit: X-ray: NASA/CXC/SAO/I.Pillitteri et al; Optical: DSS; Illustration: NASA/CXC/M.Weiss

What about other hot Jupiters that are relatively massive and close to their star? In some cases - where they orbit a different type of star to WASP-18 - the effect of hot Jupiters can be flipped and they can make a star appear younger than it really is. In the cases of HD 189733 and CoRoT-2a the presence of the planet appears to have increased the amount of activity in the star. In these cases the stars have much deeper convection zones than WASP-18 and tidal effects have little influence on convection and hence on the star's dynamo. Instead, the planets may be speeding up their star's rotation, leading to a more powerful dynamo and more activity than expected for the star's age. In these cases having a companion makes the star act younger than it really is. That makes sense for people and, in a few cases, for stars.
An artist's impression of the star CoRoT-2a and its hot Jupiter exoplanet, CoRoT-2b. Credit: NASA/CXC/M.Weiss

I've discussed the effects that extreme hot Jupiters can have on their host star. In such systems, what effect does the star have on its planet? In the cases of HD 189733 and CoRoT-2a, strong X-rays and ultraviolet radiation from the active star are evaporating the atmospheres of the planet. For HD 189733, astronomers estimate the planet is losing 100 to 600 million kilograms per second, and for CoRoT-2a astronomers estimate the planet is losing about 5 billion kilograms per second. For WASP-18, with much weaker X-ray emission and ultraviolet radiation, there is much less evaporation of the nearby planet's upper atmosphere than there would be if the star was more active. In effect, the planet is protecting itself. Its gravity causes the nearby star to be less active, and that causes the planet to be struck with less damaging radiation. HD 189733b and CoRoT-2b, on the other hand, are behaving in a self-destructive manner. 

Talk of planet destruction isn’t necessary for our present-day solar system, where the planets are much further away from the Sun than hot Jupiters are. However, that won’t always be the case. A few billion years in the future, the Sun will dramatically expand in size when it becomes a red giant. Our oceans will boil away, never to return and what’s left of the Earth may end up spiraling in towards the Sun. We don't know the exact fate of our home planet, but it is clear that our aurora-viewing days are numbered.

Wednesday, August 13, 2014

Astronomy for Everyone

The push for diversity in science comes in many different areas. Gender and race are perhaps the most familiar examples, and there remains a great deal of room for improvement in these areas. This blog post covers an area − neurological makeup − that receives less attention, partly because the differences are often not obvious. A push for diversity in this area can be framed by the following important question: how can we show children that people can have successful careers in science despite experiencing some neurological challenges?

An excellent answer to this question was provided by a neurodiversity workshop for high school students held at the Harvard-Smithsonian Center for Astrophysics (CfA), my home institution, at the end of April 2014. The workshop was called Astronomy for Everyone and was superbly organized by Smadar Naoz and Matt Schneps. An invitation was extended to high school students with Dyslexia, Attention Deficit Hyperactivity Disorder (ADHD) and/or Autism Spectrum Disorders.
The goal of the workshop was to "encourage neurodiversed high schoolers that the academic path is open for them, and to share tips to help overcome obstacles that they may encounter in their way", as stated on the workshop’s webpage. The workshop involved a full day visit to CfA to learn about careers in astronomy. An outgrowth of the work Schneps and his colleagues initiated at the Laboratory for Visual Learning at CfA, with funding from the Smithsonian Youth Access Grant program and the National Science Foundation, this was the second in a series of such programs.

Matt Schneps introducing the Astronomy for Everyone workshop. Credit: Nina Zonnevylle.

Inspired by the people I know with neurological challenges, I volunteered to help and was assigned to lead one of the four subgroups the students were divided into, along with their parents. My main job was to make sure the group made it to the different talks and sessions scattered throughout the maze-like building that is CfA. 

After registration and refreshments, the program began with a talk by Josh Grindlay, a Harvard professor, about the plate digitization program that he has led, called the "Digital Access to a Sky Century @ Harvard" project, or DASCH. This is a terrific program enabling astronomers to look at the variability of astronomical objects over more than a century, a much longer timescale than usually available. Josh’s talk was a perfect way to start the day and it quickly became clear from the question and answer session that the students and their parents were enthusiastic about astronomy and astrophysics. Later in the day other groups were able to visit the DASCH lab to see where the astronomical plates are scanned, but this was the one activity my group missed (because of time-limits each group missed one activity).

Josh Grindlay talking about the DASCH program he has led.  Credit: Nina Zonnevylle.

Also included in the program were a couple of short talks by graduate students in the Harvard astronomy department. Sarah Willis and Wen-fai Fong both gave interesting and enthusiastic presentations about their lives as students, including details about their background and what they enjoyed about doing research.

The final session before lunch involved a visit to the solar lab located in the basement, a hidden gem at CfA. Henry (“Trae”) Winter gave a great talk that included stunning movies of flares on the Sun, using a video wall containing 5,760 by 3,240 pixels. The data he showed was from the Atmospheric Imaging Assembly (AIA) suite of telescopes on the Solar Dynamics Observatory satellite. You can see some examples of AIA images and movies on their gallery webpage.

Trae's talk inspired a bunch of questions from the students, causing the session that had already started late to run over time. This resulted in a minor dilemma for me. Although I was very happy to see so many questions from the students, I noticed that the window for lunch was getting shorter and shorter, and I reluctantly interrupted the Q&A session to point out that we needed to eat and drink.

Trae Winter talking about our active Sun, using his lab's video wall.  Credit: Nina Zonnevylle.

In the afternoon, Bruce Ward gave a very good talk about the Great Refractor Telescope at CfA, located just a few feet down the hall from my office. After being installed in 1847, the Great Refractor was the largest telescope in the United States for 20 years and was “the most significant American astronomical instrument and equal to the finest in the world”, according to the Harvard College Observatory.

The Great Refractor Telescope at CfA.  Credit: Harvard College Observatory.

Afterwards, Smadar and another astronomer gave presentations about their own personal stories. These were private discussions and I didn't attend, but I'm sure that they provided valuable insight into the challenges that neurodiversed astronomers can face, and the successes that they can achieve.

Later in the afternoon, Matt Schneps gave an excellent presentation about how technology can be used to help overcome neurological challenges. Examples he gave included the use of voice recognition software to compose text, and use of the "reader" option in the Safari web browser to simplify the information presented in a webpage.

The day finished with group discussion, where the kids and their parents met separately to discuss their experiences and provide encouragement to each other.

I could tell from my own observations and my conversations with students and parents that the workshop was a success. This impression was confirmed by the feedback that the students and parents provided via a survey and email. Here is some feedback from the students:

"I have never seen a workshop like this specially paying attention on children like me supporting and guiding them throughout the day. Spending a day with such brilliant people was not only a great experience for me, but I learned a lot from it."

"I always live this hope that my future is getting built up for a purpose trying to stay happy always, but I have to say this workshop really boosted up that spirit as now I am confident for sure that my future is awesome."

and some feedback from the parents:

"Very useful in that it gave another example of how the challenges discussed can be overcome, struck a cord with my daughter due to several connections/similarities."

"I left feeling very inspired and encouraged about my son's future."

It was gratifying to help, even in a small way, with a program that was clearly inspiring to students and their parents. Much of the work I do in public affairs for NASA’s Chandra X-ray Observatory is less personal, involving press and image releases that indirectly reach large audiences, so it’s rewarding to work with a small group and see that a few hours of outreach can make a big difference to a student’s life.  

Smadar Naoz introducing the Astronomy for Everyone workshop.  Credit: Nina Zonnevylle.

What does the future hold for the Astronomy for Everyone program? Smadar Naoz has moved to the University of California, Los Angeles and plans to run the program there. Her long-term plans are bigger than that, as she wants to influence as many people as possible. She would like to expand the program in scope so it is run at multiple institutions in different parts of the country. Her really long-term goal is to do an integration day across the country and even across continents.

Matt Schneps has also relocated from CfA and is now at the Harvard Graduate School of Education and at UMass Boston, focusing his energy full time to programs designed to support cognitively diverse communities of learning. There, he and his team are doing research to identify new ways technology can be used to broaden access, and increase the inclusion of people with neurocognitive differences in challenging careers such as science.

I’ll finish by thanking Smadar and Matt for doing such a good job at organizing the workshop and making it easy for volunteers like me. Special thanks go to Avi Loeb for hosting and funding the workshop through the CfA’s Institute for Theory and Computation, and Nina Zonnevylle for helping out with the logistics. I’d also like to thank all of the other people who volunteered to help out, whether by giving talks or helping shepherd people through CfA or helping with the lunch and refreshments.

Wednesday, June 25, 2014

Pros & Cons of Pre-Publication Publicity (& BICEP2)

It all started with a press conference on March 17th, 2014. Leaders of the BICEP2 team claimed that they found the “first direct evidence of cosmic inflation”, and the “first images of gravitational waves, or ripples in space-time.” This is a huge advance in cosmology if true. The publicity occurred before the paper was accepted for publication in a journal, and the work has been cross-examined by experts ever since. For excellent summaries of the initial claim and the skeptical response by some scientists, see articles by Richard Easther, Alan Duffy and Heino Falcke. The article by Falcke is particularly interesting because he discusses science communication issues raised by this story.

In this blog post I will add to Falcke’s comments by discussing some of the pros and cons of publicizing a paper before it is accepted, using the BICEP2 result as a case study. I note that the BICEP2 paper has just been published in Physical Review Letters and includes a few substantial changes and some hedging on the original claims, as reported by Dennis Overbye in the New York Times, Jacob Aron, Lisa Grossman and Stuart Clark in New Scientist and Nadia Drake in National Geographic News.

Gravitational waves from inflation are expected to generate a faint but distinctive twisting pattern in the polarization of the cosmic microwave background (CMB), the left over radiation from the Big Bang. This twisting pattern is known as a "curl" or B-mode pattern. For the density fluctuations that generate most of the polarization of the CMB, this part of the primordial pattern is exactly zero. Shown here is the actual B-mode pattern observed with the BICEP2 telescope, which is consistent with the pattern predicted for primordial gravitational waves. The line segments show the polarization strength and orientation at different spots on the sky. The red and blue shading shows the degree of clockwise and anti-clockwise twisting of this B-mode pattern. (Caption has been slightly modified from the CfA version.) Credit: BICEP2 collaboration 

(In full disclosure, I work in public affairs in the Chandra X-ray Center based at the Harvard-Smithsonian Center for Astrophysics, where the BICEP2 results were announced and where part of the BICEP2 team is based, but I don’t know any of the team members personally. Also, unless noted otherwise, I will use the term “peer review” in the traditional sense, to describe independent review conducted by a referee or referees selected by a journal’s editors as part of their publication process.)

What are some of the advantages of doing publicity before a paper is accepted to a journal?
− Peer review is not a flawless process and this is a public way to acknowledge that. As biologist and blogger Jonathan Eisen has pointed out, we should not deify peer review. There are many examples where peer review has failed to detect serious problems published in science papers, including the well-known "arsenic life" paper in Science. This article by Carl Zimmer gave a devastating response to that paper by independent experts, not long after it was published, contrasting strongly with the very positive reviews by referees, as reported by Dan Vergano. 
One of the problems caused by the arsenic paper was that multiple scientific disciplines were covered, making the paper difficult to referee, even using three reviewers. In astrophysics, most of the commonly-used journals like The Astrophysical Journal usually use only one referee, so if the journal makes a poor choice of referee, the review can have very limited benefits. However, with BICEP2 a much more narrow range of expertise was required than for the arsenic paper, and there was an obvious choice for a referee: one of the leading researchers for WMAP or Planck would have been very appropriate. In their published paper, the authors acknowledge “detailed and constructive recommendations” from two anonymous referees.
− In cases where peer review has failed we can thank post-publication peer review for exposing the problems. By publicizing before peer review, authors are effectively inviting an open, informal refereeing process to run in parallel with the journal’s peer review. The BICEP2 authors effectively acknowledge this in the “Note added” section near the end of their published paper. An open process like this gives scrutiny from the greatest possible number of experts before the paper is published. Such an approach makes a lot of sense in giving the best paper, assuming that the authors seriously consider the comments they receive, as the BICEP2 authors appear to have done. (As an aside, open peer review arguably should include a public record of comments and responses, as suggested, for example, by planners of the Open Journal for Astrophysics. However, this journal is still in the testing phase.) 
Similarly on the publicity side much of the potential benefit of open peer review depends on how the team respond to external comments and criticism. For example, if necessary, will they put out a new release or a correction explaining any changes to their original publicity claims, especially when more data becomes available?
− By placing the paper on the arXiv and publicizing it at an early stage, there is an opportunity for outsiders to witness some of the scientific process in action, as noted by Dennis Overbye. In the case of BICEP2 the skepticism of experts was quickly revealed in comments given in the press, as reported by Joel Achenbach. The paper triggered a flurry of activity, with 421 papers citing the original paper at the time of writing most of them theory papers where the result was assumed to be correct. At the same time, the detailed observational results were closely examined and criticized as noted by Richard Easther, Alan Duffy, Heino Falcke and others, resulting in some important revisions.

− By doing publicity before publication, results can be released to the public and to other scientists earlier than they otherwise would have been, since the authors do not have to wait for the refereeing process. In fields like medical science, delays can potentially be life-threatening. In astrophysics there is less practical need for haste, but long delays can be frustrating.
The BICEP2 telescope in the foreground and the South Pole Telescope in the background. Credit: Steffen Richter (Harvard University).

What are some of the disadvantages of doing publicity before a paper is accepted to a journal?:
− The most important disadvantage: there is a chance a very good referee or referees will be found and the paper will be improved. It is possible that just the clarity of description or the references will be improved, but it is also possible that significant problems in analysis or interpretation will be found. For example, maybe the referee will be the world’s leading expert about crucial but problematic details of the analysis. This is a conservative approach to publicity, adopting the attitude that some independent checking is better than none.
In the case of BICEP2, their use of Planck data from a conference talk was problematic and they may not have properly accounted for all of the foreground emission, as explained in this paper submitted in late May by Raphael Flauger, Colin Hill and David Spergel. The Flauger et al. paper’s abstract finishes by saying:
“These results suggest that BICEP1 and BICEP2 data alone cannot distinguish between foregrounds and a primordial gravitational wave signal, and that future Keck Array observations at 100 GHz and Planck observations at higher frequencies will be crucial to determine whether the signal is of primordial origin.“ 
In the published version of their paper, the BICEP2 authors, to their credit, have added a number of important caveats including this sentence added to the abstract: “However, these models are not sufficiently constrained by external public data to exclude the possibility of dust emission bright enough to explain the entire excess signal”. They also added this statement near the end of the paper: “More data are clearly required to resolve the situation.” If they had received some of this feedback about foreground emission from a referee or colleague before publicity, their press conference claims may have been made with less confidence. 
− For academics, whether they like it or not, publishing papers in journals is still (*) one of the main arbiters of academic success. A successful paper isn't achieved until publication is complete and publication isn't complete until peer review is finished. So, by publicizing before peer review is finished you can give the appearance of adopting one standard for your scientific colleagues and a different, lower one for everyone else. 
− With publicity, especially a press conference, you can reach a bigger or a much bigger audience than you would normally reach without publicity.  The audience is the tax-paying public, who fund a large amount of research. So, as a matter of responsibility, standards of review should not be significantly lowered even if there are time pressures, such as fears of leaks or concern about being scooped by competitors. 
My opinion is that the cons outweigh the pros in doing early publicity and that it is better for publicity to occur after peer review (this is approach #2 in the article by Heino Falcke). This approach shows that some effort has been taken by the authors to seek independent review of their work before publicity. It does not guarantee that the paper is flawless, but it does offer the chance to detect problems that were overlooked or not fully appreciated by the authors. For BICEP2, the changes made to the published version of their paper show that important improvements have indeed been made.

Others have questioned the timing of the BICEP2 publicity, including Bill Jones, a Princeton professor. Joel Achenbach’s excellent May 16th article in the Washington Post reports:

"Jones questioned the decision by the BICEP2 team to announce the discovery in a news conference prior to publication of a peer-reviewed paper. Kovac and Bock defended the news conference as a common procedure." 

That may be true for physics press conferences, like ones held at the LHC, but not for NASA ones, where an accepted paper is required.

Traditional peer review isn’t magical and it is easy to imagine very rigorous, informal reviews that completely bypass the journal’s process. However, it’s not clear that the BICEP2 team encouraged such independent reviews before publicity, as in the submitted, publicized version of their paper they didn’t acknowledge receiving comments from colleagues.

The BICEP2 publicity was obviously very effective at generating press coverage. If the paper had been put on the arXiv before publicity, to generate widespread comments from colleagues, then some enterprising reporters may have picked up the article and written about it before a press release was produced. It’s possible that early submission to the arXiv was considered, but there was concern that early leaking of results to the press would have a negative effect on press coverage from a later release. How could the team combat this problem but also generate reasonably widespread reviews from colleagues? One option: they could have waited for peer review to be completed before doing publicity and also sent the paper to selected colleagues by email when it was submitted, with a strong request to keep the paper private.

If publicity is performed before publication, I think it’s best to put out a conservatively-worded press release, allowing for the possibility of errors, especially if the result is significant (this is approach #3 listed by Heino Falcke). However, the announcement by the BICEP2 team was confidently worded and did not mention the need for more data, unlike the wording used in the published version of their paper.

I have no doubt that the members of the BICEP2 team are excellent researchers at the top of their game, but the claim they made is extraordinary, and we all know the saying by Carl Sagan about what level of evidence is required for such a claim. It takes time and a lot of cross-checking to accumulate extraordinary evidence, and that’s what we should try to present to the public. There now appears to be a mismatch between the ebullient publicity of March 17th and the tempered claims in the paper published on June 19th. It’s important to think about whether this mismatch could reasonably have been avoided.

(*) Please see this blog post  at Scientific American by Bonnie Swoger discussing whether the scientific journal has a future.

Friday, March 14, 2014

What Makes an Astronomical Image Beautiful?

(Note: this blog post was first published at the Chandra X-ray Observatory blog)

Astronomy is renowned for the beautiful images it produces. It's not hard to be impressed by an image like the Pillars of Creation or the Bullet Cluster, and the more eye-catching an image is, the bigger an audience it can potentially reach. So, as part of our job in astronomy outreach, we have each spent time thinking about what makes an astronomy image beautiful. As professionals, we’d like to go well beyond the intuition of the person who says, "I don't know anything about art, but I know what I like". One approach1 is to list the key elements that make an image beautiful.

Two famous images, the Pillars of Creation from the Hubble Space Telescope on the left and the Bullet Cluster from the Chandra X-ray Observatory, Hubble and ground-based observatories on the right. Credit: left: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University); right: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

Lars Lindberg Christensen and collaborators have done just that, in an excellent paper in the most recent issue of the open journal "Communicating Astronomy with the Public". After years of experience working with images from the Hubble Space Telescope, they have defined a set of 6 criteria that are important in determining the appeal of an astronomical image. These 6 criteria are photogenic resolution (equivalent to the number of stars which can fit side by side across an image), definition (the amount of structure or contrast in an image), color, composition (how the object or objects of interest fill the field of view), signal-to-noise ratio, and how well instrumental artifacts have been removed.

Here, I'll highlight a few of these criteria and how they relate to the images we make with Chandra data. There are some key differences between the optical or infrared data obtained with telescopes like Hubble or the Spitzer Space Telescope, and the X-ray data obtained with Chandra.

For the photogenic resolution they define a quantity rphoto that is the number of effective resolution elements across the field of view (FOV). The equation is rphoto=FOV/θeffective, where θeffective is the effective angular resolution. A higher rphoto results in a better quality image. So one tactic is to push for a very large FOV by making a mosaic of a large number of adjacent images, as some amateur astronomers do, or as Chandra users did to make a large mosaic of the Carina Nebula. The other option is to use data from a telescope with a very small θeffective, where Hubble is unsurpassed at optical wavelengths and Chandra is unsurpassed at X-ray wavelengths.

A mosaic of Chandra images of the Carina Nebula. Credit: NASA/CXC/PSU/L.Townsley et al.

The authors point out that the domain with high values of the photogenic resolution - between 1000 and 10,000 - was dominated for many years by Hubble, but that more recently other observatories such as Chandra, the MPG/ESO 2.2-meter telescope, the Canada France Hawaii Telescope and ESO's VISTA and VST telescopes have joined Hubble. We're happy to have been included in this elite group.

To make a color image using optical or infrared data, observations have to be made using different filters chosen in advance, such as B (blue), V (visual) and R (red). A color is then assigned to the image obtained with each filter – in this case blue, green and red are the obvious choices – and the images are combined to make a color image. With Chandra, the energy (or wavelength) of individual photons is recorded, so different wavelength ranges can be chosen afterwards, giving us extra flexibility in making an image. By picking out different wavelength ranges the same dataset can be used to show different features, so we can experiment to see what makes the most striking image, or the most useful one to explain a particular science result. The greatest flexibility with choosing different wavelength ranges comes when the signal-to-noise ratio is high.

This leads me to describe the main challenge for producing beautiful Chandra images: sometimes the signal-to-noise ratio isn’t high. X-ray photons trace energetic events, such as regions close to a black hole, or the exploded guts of a massive star, but they tend to arrive from the cosmos in a trickle, rather than a flood. This limitation was most apparent early in the mission, when lots of different targets were observed and Chandra's observations usually involved short exposures. Later in the mission much deeper observations have been done, giving much higher signal-to-noise ratios and better images. For example, you can see the dramatic difference between these two images of the supernova remnant Cassiopeia A. The early Chandra image shown on the left had an exposure time of only 2 hours and doesn't look nearly as photogenic as a later image shown on the right, with an exposure time of 11 1/2 days.

Images of the supernova remnant Cassiopeia A. A comparison is shown between an early, short exposure (2 hr) Chandra image (left) and a later, deeper exposure (11.5 days; right). Credit: left: NASA/CXC/SAO/Rutgers/J.Hughes; right: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.

The signal-to-noise ratio is connected to the 2nd criterion, the definition in an image. If the signal-to-noise ratio is low because few counts have been detected, then this can seriously limit the amount of detailed structure that you can see in an image. Think about how much detail you can see in a painting that is well lit, compared to looking at one in the dark. Again, deep exposures help a lot. The deep observation of Cas A has significantly sharper features and more complicated structure than the shallow one.

Only a limited number of very deep observations can be made with Chandra each year. This means there is intense competition between astronomers to convince the members of the Chandra Time Assignment Committee to approve any observing proposals requiring a lot of observing time. Therefore, the science case has to be particularly strong, which becomes an advantage for us, because it means that we can publicize interesting science at the same time as showing off beautiful new images.

When we have only low signal-to-noise X-ray data to work with, we sometimes combine it with optical or infrared images, to capitalize on the high signal-to-noise in these other wavelengths. This can give a striking image, such as in this view of NGC 602.

A composite image of the star-forming region NGC 602, with Chandra X-ray data shown in purple, Hubble optical data shown in red, green and blue, and Spitzer infrared data shown in red. Credit: X-ray: NASA/CXC/Univ.Potsdam/L.Oskinova et al; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech

The final criterion is instrumental artifact removal, where the relatively low count rates for Chandra are an advantage. When you detect a lot of very bright objects you tend to accumulate a lot of artifacts, so optical observations can require a lot of clean-up work. According to Christensen et al., one to two hundred hours can be spent manually cleaning a large image. Chandra images aren't free of artifacts, but they're not as much of a problem.

In their conclusion, Christensen et al. explain that the ideal case is for all six criteria to be fulfilled, giving a great image. It’s still possible to produce a great image with less, but it becomes more difficult and compromises have to be made, as they note.

Having explained some of the factors that help us produce beautiful Chandra images, we invite you to explore our photo album of images or our 3D image wall.

1: In this blog post I’ve discussed only one approach for thinking about how beautiful an image is. There are other approaches that consider aesthetics in general. My colleague Kimberley Arcand is involved in a project called “Aesthetics and Astronomy” which studies “the perception of multi-wavelength astronomical imagery and the effects of the scientific and artistic choices in processing astronomical data.”