Hubble's Universe Unfiltered

  • December 29, 2015

    How Hubble 'Sees' Gravity

    by Frank Summers

    [Note: This post is cross-posted on the Frontier Fields blog.]

    Gravity is the familiar force of nature responsible for the diverse motions of a baseball thrown high into the air, a planet orbiting a star, or a star orbiting within a galaxy. Astronomers have long observed such motions and deduced the amount of gravity, and therefore the amount of matter, present in the planet, star, or galaxy. When taken to the extreme, gravity can also create some intriguing visual effects that are well suited to Hubble’s high-resolution observations.

    Einstein’s general theory of relativity expresses how very large mass concentrations distort the space around them. Light passing through that distorted space is redirected, and can produce a variety of interesting imagery. The bending of light by gravity is similar to the bending of light by a glass lens, hence we call this effect “gravitational lensing.”

    The simplest type of gravitational lensing is called “point source” lensing. There is a single concentration of matter at the center, such as the dense core of a galaxy. The light of a distant galaxy is redirected around this core, often producing multiple images of the background galaxy (see the image above for an example). When the lensing approaches perfect symmetry, a complete or almost complete circle of light is produced, called an “Einstein ring.” Hubble observations have helped to greatly increase the number of Einstein rings known to astronomers.

    Galaxy Cluster Abell 2218

    Gravitational lensing in galaxy cluster Abell 2218

    More complex gravitational lensing arises in observations of massive clusters of galaxies. While the distribution of matter in a galaxy cluster generally does have a center, it is never perfectly circularly symmetric and is usually significantly lumpy. Background galaxies are lensed by the cluster with their images often appearing as short, thin “lensed arcs” around the outskirts of the cluster. Hubble’s images of galaxy clusters, such as Abell 2218 (above) and Abell 1689, showed the large number and detailed distribution of these lensed images throughout massive galaxy clusters.

    These lensed images also act as probes of the matter distribution in the galaxy cluster. Astronomers can measure the motions of the galaxies within a cluster to determine the total amount of matter in the cluster. The result indicates that most of the matter in a galaxy cluster is not in the visible galaxies, does not emit light, and is thus called “dark matter.” The distribution of lensed images reflects the distribution of all matter, both visible and dark. Hence, Hubble’s images of gravitational lensing have been used to create maps of dark matter in galaxy clusters.

    In turn, a map of the matter in a galaxy cluster helps provide better understanding and analysis of the gravitationally lensed images. A model of the matter distribution can help identify multiple images of the same galaxy or be used to predict where the most distant galaxies are likely to appear in a galaxy cluster image. Astronomers work back and forth between the gravitational lenses and the cluster matter distribution to improve our understanding of both.

    Three lensed images of a distant galaxy seen through a cluster of galaxies

    On top of it all, gravitational lenses extend Hubble’s view deeper into the universe. Very distant galaxies are very faint. Gravitational lensing not only distorts the image of a background galaxy, it can also amplify its light. Looking through a lensing galaxy cluster, Hubble can see fainter and more distant galaxies than otherwise possible. The Frontier Fields project has examined multiple galaxy clusters, measured their lensing and matter distribution, and identified a collection of these most distant galaxies.

    While the effects of normal gravity are measurable in the motions of objects, the effects of extreme gravity are visible in images of gravitational lensing. The diverse lensed images of crosses, rings, arcs, and more are both intriguing and informative. Gravitational lensing probes the distribution of matter in galaxies and clusters of galaxies, as well as enables observations of the distant universe. Hubble’s data will also provide a basis and guide for the future James Webb Space Telescope, whose infrared observations will push yet farther into the cosmos.

    A "smiley face" gravitational lens in a galaxy cluster

    The distorted imagery of gravitational lensing often is likened to the distorted reflections of funhouse mirrors, but don’t take that comparison too far. Hubble’s images of gravitational lensing provide a wide range of serious science.

  • October 3, 2015

    My Favorite Martian (Movie)

    by Frank Summers

    In 1982, I went to see the film "Blade Runner" without many expectations. It was just a sci-fi movie starring Harrison Ford, who, at the time, was known only for his role in "Star Wars." I came away enthralled with the depth of both the thought-provoking story and the detailed artistic vision. After recognizing that the director, Ridley Scott, had also done the 1979 film "Alien," he immediately rose to being a favorite. In subsequent years, the words "directed by Ridley Scott" were reason enough to go see a film.

    However, as an astronomer, I approached his new film "The Martian" with a bit of trepidation. While still in the category of science fiction, the story of an astronaut stranded on Mars is ripe for massive clashes against science fact. Previous astronauts on Mars stories have not thrilled me. Such movies generally pay homage to the scientific aspects by providing various reasonable justifications and using lots of jargon to make things sound technical and complicated. Then, in typical Holywood fashion, the movie veers off into something jarringly non-scientific to advance a storyline or provide a twist.  

    I am certainly not the type who tries to hold a work of fiction to the standards of reality. I truly bellieve in the audience's "willing suspension of disbelief" that is vital to capturing someone into a good yarn. I can suppress my analytic brain and forgive lots of small points that advance the narrative but really don't make sense. Even unexplained phenomena, used judiciously, can be quite enjoyable. However, as in the "Harry Potter" world, there are some unforgivable curses where basic facts, forces, or fallacies are ignored. These points can jolt my intellect and pull me straight out of the story.

    With that preface, let me say that "The Martian" did not disappoint. I watched the entire film staying inside the bubble of the narrative. The plot, characters, visuals, and direction kept me focused and empathetic, without startling my astronomical subconscious. That may not sound like high praise, but it is a rare experience for me on a film of this nature.

    Are there mistakes in the movie? Of course. Do they matter? Not much. I can discuss those points in another post (with proper spolier alerts). In this post, let me focus on the good stuff. Without giving away anything not already in the advertisements, here are three aspects I particularly liked.

    First, the computer graphics of Mars are wonderful. The views of what is really a rather bland and desolate planet are filled with warmth and majestic vistas. The mostly rock and dirt landscape of Mars was showcased in a wide variety of formations that gave it considerable grandeur. The scenes harken back to Scott's Monument Valley shots in "Thelma and Louise."

    Second, the fictional astronauts behave like real astronauts. I have interacted with a dozen or so astronauts, and observed some of them for long hours during the Hubble servicing missions. These people are part of our best and brightest. They are cool-under-pressure problem solvers who can attempt and accomplish audacious goals. Astronauts do have emotions and a sense of humor, and that is also captured in the film. But, overall, the characters exude proper competence, skill, and acumen, without soap-opera histrionics.

    Finally, the movie is a really good story, and is really well told. This tale has out-of-this-world dilemmas, strong and sympathetic characters, and action that is emotionally, intellectually, and visually engaging. Even before its release, I heard jokes comparing this movie to "Apollo 13." Now that I've seen it, I'd say the comparison is apt on many levels, and this film should garner similar acclaim and awards.

    Back in the year 2000, Neil Tyson and I started chatting about Ridley Scott's film "Gladiator." I gave that film perhaps the highest accolade by telling him nothing about it. I said something like "Don't read anything, just go see it with no expectations." I'm not sure I would go that far with "The Martian," but if the advertisements pique your interest, the film delivers on multiple fronts. I can heartily reccommend that folks see this picture, both as an astrophysicist and as a film enthusiast.

  • March 12, 2015

    Revisiting a Legend

    by Frank Summers

    As discussed in a previous blog post, "Twenty-Five Years of Hubble," this year marks the silver anniversary of the Hubble Space Telescope. We have plans for a full year of festivities and kicked it off by revisiting a legend.

    One of our most famous images of all time is the 1995 view of three gaseous pillars in the Eagle Nebula nicknamed "The Pillars of Creation." The "creation" aspect derives from the fact that new stars are being born within the dark, dense clouds at the tops of the pillars. That greenish, irregularly shaped image (on the left in the montage image above) was taken with Hubble's Wide Field Planetary Camera 2 instrument that was installed during Servicing Mission 1 in 1993.

    An updated insturment, Wide Field Camera 3 (WFC3), was installed as a replacement during Servicing Mission 4 in 2009. Our first press release of the 25th anniversary year was a remarkable new WFC3 view of the same region. This new image (center in the montage) has twice the resolution, about six times the sky coverage, and more than twenty times the pixels of the previous version. The broader context and innumerable fine details combine to make a compelling updated portrait.

    Yet Hubble took it one step further. The greatly improved infrared-light sensitivity of WFC3 enabled a striking infrared image of the Eagle pillars (on the right in the montage). The longer wavelengths of infrared light can penetrate through much of the gas and dust clouds, bringing out both the nebula and a panoply of background stars in stark contrast. The pillars, seemingly solid dark columns in visible light, are revealed in infrared light as a combination of dense clouds and the shadows they cast.


    The contrast between Hubble's two new views is best seen in a cross-fade between the visible-light and the infrared-light images. The animated GIF displayed above has both images cropped to the same region. One can instantly pick out the visible gas that disappears in the infrared, as well as the bright infrared stars that have no visible counterparts.


    The pillars in the Eagle Nebula are the primary topic of this Hubble Hangout.


    There are so many interesting details to discuss in these images that it would fill a bunch of blog posts. Instead, let me point you to a Hubble Hangout that I did with Tony Darnell in late January. We call this series of hangouts "News from Hubble and Across the Universe." Usually, we cover three or four stories, but this time we spent the entire hour examining and comparing the Eagle pillar images. Have a look, as there are some rather intriguing and beautiful features to explore.