Jan 122015

By Massimo Robberto, Observatory Scientist at STScI

In 1994, when I was a young Assistant Astronomer in Italy, I started regularly visiting STScI for a collaboration on stellar coronagraphy with Mark Clampin and Francesco Paresce. Those were the months immediately following the first Hubble Servicing Mission and excitement was in the air at the Institute. New pictures comparing the “pre” and “post’ performance of the telescope were posted daily, as testimony for NASA’s spectacular achievement and the bright future ahead. Still, traces of the shock caused by the initial failure (spherical aberration) were evident. In particular, in the control room of the first floor an entire wall had been covered with hundreds of cartoons from all over the world mocking NASA for the Hubble primary mirror disaster. STScI staff had diligently collected and posted all of them, regardless on their quality or even language. A bit of irony helps keeping things in the right perspective when a crisis strikes, and staying focused, to work on a solution.

One of those cartoons attracted my attention. It showed a group of perplexed experts, the “Hubble Design Group”, wondering if “outer space really is blurry and out of focus” (Figure 1). I liked it immediately and discretely made my copy. The cartoon was different from the others because, I thought, it was a cartoon about us, about how our intelligence works. We always look for an explanation, for a reason, and when all good possibilities fail, we start considering the bizarre and the improbable. We feel that saying “I don’t know” and stop wondering is even worse than opening the door to what may look absurd. And our secret hope, as scientists, is to discover that something apparently absurd is actually real.


Figure 1: Ralph Dunagin’s strip, Dunagin’s People, ran from 1960 to 2001. The Hubble Servicing Mission occurred in 1993.

Fast-forward 20 year: today, the Hubble Space Telescope is still fully operational at its best, acquiring data that are transforming our knowledge of the Universe.  In particular, a Hubble Legacy Program is underway to study one of the most spectacular phenomena gloriously unveiled by the Hubble images: gravitational lensing (Figure 2). As predicted by Einstein, mass warps space-time. When a large mass is present, as in the case of the dark matter halos surrounding clusters of galaxies, the light of more remote objects along our line of sight gets distorted. The signal is so strong (“strong gravitational lensing”) that we can use it to map the distribution of dark matter in these halos. This is at present the only direct method we have to probe the effects of what may be the most enigmatic particles in physics.


Figure 2. This HST/ACS image, obtained in 2009 immediately after the last Hubble Servicing Mission, shows a gravitationally lensed background galaxy in the field of the Arp 370 galaxy cluster.

If the alignment conditions are favorable, the brightness of some remote galaxy may be magnified: the gravitational lens effect can make visible objects that would have otherwise remained beyond Hubble’s reach. One can say that the so-called Frontier Fields program is using two telescopes in a series, one made by us (Hubble) and one provided by Nature (gravitational lensing), to search for the most distant galaxies and supernovae. With this “trick” Hubble can give us a glimpse of the type of science that will be routinely carried out by the James Webb Space Telescope.

As one moves further away from the center of a galaxy cluster, the gravitational lens effect becomes less pronounced. No matter where we look in the sky, the shapes of thousands of galaxies are all slightly distorted in some way (“weak gravitational lensing”). It is a vanishingly small signal, but it is correlated for all the galaxies that are at similar distances, so it is possible to detect it by applying statistical methods to the most exquisite wide-field images. Future space missions such as Euclid and WFIRST are designed so as to carry out this type of study, which is critical to understand the build-up of giant cosmic structures over time and the process of galaxy formation.

Another fascinating aspect is that amplification can be caused by the random and temporary alignment of stars in crowded fields. In this case the brief light amplification (micro-lensing) can be used to unveil the presence of planets like Earth. WFIRST has the capability of monitoring billions of stars in the Galactic Bulge, where lensed planets could flash like lights on a Christmas tree. Crafting a suitable cadence of observations to exploit this capability is one of the main challenges faced by WFIRST.

A 10m-class space telescope like the proposed ATLAST will eventually produce images of incredible sensitivity and spatial resolution. At that point the ubiquitous gravitational lensing will become more obvious and, perhaps, just another ordinary aspect of our perception of the Universe.

Twenty-five years after launch, the Hubble Space Telescope is showing us that because of gravitational lensing the Universe is really somewhat blurry and out of focus. The intuition of a cartoonist has anticipated one of the most spectacular discoveries of all time. Let’s keep an open mind to more surprises. What today looks like an absurd concept, imagined only in the mind a creative artist, may become tomorrow part of our understanding of this beautiful, and very extravagant, Universe.

Jul 072014

By David Soderblom, Astronomer at STScI

It’s exciting to witness history being made, especially if it’s a long-awaited event.  The afternoon of October 6, 1995 was that time.  Several hundred astronomers were in Florence, Italy for the ninth in a series of scientific meetings on Cool Stars, Stellar Systems, and the Sun.  The Cool Stars series was started by Andrea Dupree in 1980 in Cambridge, Massachusetts, and they continue to be held every two years.  Cool Stars 9 was the first to be held outside the U.S., and Roberto Pallavicini from Florence’s Arcetri Observatory led the group that organized and hosted the meeting.

Florence is a delightful place to be under almost any circumstances: warm, sunny, good food, and great sights.  But everyone was very focused that afternoon.  In the morning we were told that there’d be a special short talk later, and the buzz over lunch was that Michel Mayor of the Geneva Observatory, working with Didier Queloz, had a rock-solid detection of a planet around another Sun-like star to report.

When we returned to the meeting hall after lunch I made sure to get a seat up front.  Nobody really knew anything yet, just that a big announcement was coming.  We all knew Mayor and Queloz were searching for planets, so what else could it be?  But which star, and what kind of planet?  All the brighter Sun-like stars were well known to many of us, so it was probably a star we’d heard of and studied ourselves.

We then had to sit through most of the afternoon’s scheduled talks; they were selected to be interesting in the first place, but at this point were being eclipsed by the Main Event.  As the time grew near for Mayor’s presentation some people with television cameras started filtering into the back of the room.  Mayor showed us his observations and conclusions, and there was little to argue with.  There was warm and enthusiastic applause: the prey so long sought had been nabbed.

So why did it take so long?  It’s been pointed out that 51 Pegasi, the host star to this planet, was misclassified as being evolved.  But that in itself wouldn’t preclude detecting the planet that was found.  A big part of the reason is simply that 51 Peg B – the planet – was so very different than anyone ever expected.  Or, to be more precise, it was located where we did not think we’d find planets: close in to the host star.

The conventional wisdom was that other planetary systems would be like ours, with analogs to Jupiter at great distances and with long periods that would take years of careful measurements to detect.  At the time I imagined the process Mayor and Queloz must have gone through.  They’d been observing 51 Peg and stars like it for some time, hoping to see very small changes in the motions of the stars that would indicate a planetary companion.  51 Peg showed larger variations than they could make sense of, so they started observing it more often, maybe every month instead of a few times per year.  And still, 51 Peg showed some kind of motion going on but with no clear period.  So they started observing it every week, and still no period to the variations.  It was only when they measured it every night for a few weeks that the 4.2-day period became blindingly obvious.  They had it!

Once the realization dawned that these “hot Jupiters” could exist so close to a star they became fairly easy to find because they produce much larger signals than more distant planets.  The race was on and over the next several years there were dozens and dozens of new exoplanets announced by Mayor’s group and a competing group led by Geoff Marcy in California.  There was no turning back.  History was made.

Scientific meetings like Cool Stars 9 often feature an invited talk at the end by a senior researcher who provides a summary of the high points and his or her views on their significance and implications for the near future.  That person that time was Jeff Linsky, from Boulder CO.  But Linsky played hooky to visit museums the afternoon that Mayor made his presentation and so he missed it.  His conference summary mentions nothing of the one talk that will be remembered longest.

Finally, it is now nearly 20 years since Cool Stars 9.  I was conscious of events at the time, but the details are getting fuzzier, and the recollections of others may sound different.