Modest but Momentous: Hubble’s Brilliant, Unsung Images [Slide Show]
NASA’s iconic space telescope has delivered gorgeous astronomical pictures for a quarter century, but some of its keystone discoveries come from far more humble images. Here are
Scientific American’s top 10
Spying on a Supermassive Black Hole / The Supermassive Black Hole in M84 One of the Hubble Space Telescope’s most famous discoveries is that nearly all galaxies harbor supermassive black holes at their centers, each containing millions or even billions of times the sun’s mass. Theorists had for decades hypothesized that such large black holes were the cause of powerful radiation beams and particle jets pouring out from “active” galaxies, but the crucial evidence for their existence remained elusive. In 1997 astronomers used Hubble to gather some of that evidence, glimpsing the motions of a disk of hot gas whirling around the core of the active galaxy M84, which lies in the Virgo Cluster of galaxies some 50 million light-years away. The gas was moving at the blistering speed of 400 kilometers per second within 26 light-years of M84’s core, suggesting a central black hole lurked there with a mass of 300 million suns. The sharp ‘zigzag’ in the center of this image reveals the gas’s high velocity within the disk. The reddish region on the right shows where the light emitted from the spinning disk is stretched to redder wavelengths as the gas rotates away from our perspective; the bluish region on the left corresponds to the disk’s light being compressed to bluer wavelengths as its gas rotates back toward us.
Credit: NASA, ESA, Gary Bower, Richard Green (NOAO) and the STIS Instrument Definition Team
Seeing Light from a Long Time Ago / The Galaxy Behind Abell 2744 The galaxy cluster Abell 2744 is so gargantuan that its gravitational field amplifies the light of far-distant background galaxies, allowing the otherwise-invisible objects to be seen. In 2014 astronomers announced that they had detected one of the farthest, faintest and smallest galaxies ever witnessed, by focusing Hubble on Abell 2744, using the cluster as a giant magnifying lens. The galaxy manifested as a small red smudge, appearing three times in the images as its light was bent along multiple paths by Abell’s bulk. One of those smudges is pictured above. The galaxy is more than 13 billion light-years away, from a time when the universe was only about 500 million years old—a few percent of its present age. Only about 850 light-years wide and containing just 40 million times the mass of our sun, the galaxy is far smaller than our Milky Way, which is 100,000 light-years wide and contains hundreds of billions of stars. Further studies of this far-distant galaxy and others like it will help astronomers better understand how the first cosmic structures formed.
Credit: NASA, ESA, J. Lotz, M. Mountain, A. Koekemoer and the HFF Team (STScI)
Calibrating the Cosmic Distance Ladder / The Cepheid Variable Star SY Aurigae Cepheid variable stars—very bright stars with well-known luminosities—can be seen in nearby galaxies, and form the bottom rung of the “cosmic distance ladder,” an assemblage of methods for estimating intergalactic distances. But pinning down the distance to many Cepheids requires parallax, an ancient trigonometric method that measures slight shifts in the positions of stars in the plane of the sky. In 2014 astronomers used Hubble to measure the parallax of the variable star SY Aurigae, located some 7,500 light-years away—10 times farther than the technique had ever reliably been used before. To do it, they employed an approach called “spatial scanning,” taking images while slowly slewing Hubble so that the star showed up as a long, central streak in the image above, allowing the researchers to measure its parallax shift with an accuracy equivalent to one one-thousandth the width of a single pixel. The technique could prove vital to refining estimates for the universe’s accelerating expansion rate, yielding new insights into the nature of dark energy, the mysterious force driving the phenomenon that Hubble observations helped discover in 1998.
Credit: NASA, ESA and A. Riess et al. (STScI, JHU, UC–Berkeley)
Measuring The Age of the Universe / White Dwarfs in Globular Cluster NGC 6397 White dwarfs—the inert, burnt-out remnants of stars like our sun—are some of the most perfect cosmic clocks, and can be used to set lower bounds on the age of the universe. They all start out at about the same size and temperature, then cool over hundreds of billions of years at steady, predictable rates. The best place to look for the oldest, coldest white dwarfs is in globular clusters, which contain swarms of stars that all formed at roughly the same time. This 2006 image from Hubble shows one of the coldest white dwarfs within the globular cluster NGC 6397. Equivalent in brightness to a candle on the moon as seen from Earth’s surface, the white dwarf is the faint dot between two bright stars near the center of the image. Studying the white dwarfs in NGC 6397 revealed that the cluster formed nearly 12 billion years ago, slightly more than two billion years after the birth of the universe.
Credit: NASA, ESA and H. Richer (University of British Columbia) Advertisement
Testing General Relativity with a White Dwarf / The White Dwarf Sirius B The nearest, closest white dwarf is Sirius B. At only 8.6 light-years away, Sirius B would be an ideal candidate for in-depth study to increase our understanding of white dwarfs, except it also happens to orbit Sirius A—the brightest star in the nighttime sky. The overpowering glare of Sirius A has vexed generations of astronomers, making it difficult to discern Sirius B’s exact properties. In 2005 researchers used Hubble to isolate the light of Sirius B (above, at lower left) and precisely measure the white dwarf’s mass and temperature. They found that although Sirius B is smaller than Earth, it contains 98 percent the mass of our sun, and simmers at 25,000 degrees Celsius. The observations also successfully tested Einstein’s theory of general relativity by measuring a phenomenon called gravitational redshift, in which light from Sirius B’s surface is stretched to longer, redder wavelengths as it struggles to climb out of the white dwarf’s intense gravitational field.
Credit: NASA, ESA, H. Bond (STScI) and M. Barstow (University of Leicester)
Discovering Elusive Brown Dwarfs / The Brown Dwarf Gliese 229B Since the 1960s, theorists had postulated the existence of brown dwarfs—objects that are too big and hot to be planets, but too small and cold to be stars. No one had ever seen one until the mid-1990s, however, when astronomers began finding them in images taken by high-powered telescopes. In 1995 astronomers used Hubble to follow up on a tentative candidate seen by the ground-based Hale Telescope, revealing it to be a small, cool brown dwarf orbiting the red dwarf star Gliese 229. They dubbed it Gliese 229B. Unlike earlier brown dwarf candidates, which were big enough to be on the hazy cusp of starhood, Gliese 229B was estimated to be between 20 and 50 times the mass of Jupiter—a range placing it firmly in brown dwarf territory. Studies of brown dwarfs can provide insights into fine details of star and planet formation, and some brown dwarfs even harbor planetary systems of their own. Gliese 229B is some 100,000 times dimmer than our own sun, and at the time of its imaging was the faintest object ever seen orbiting another star. Its discovery and imaging helped pave the way for ongoing efforts to take snapshots of even-dimmer objects—planets orbiting other stars.
Credit: NASA, ESA, S. Kulkarni (Caltech) and D.Golimowski (JHU)
Sniffing an Exoplanet’s Atmosphere / The Transiting Exoplanet HD 209458 b The exoplanet HD 209458 b was discovered in 1999 by the gravitational wobble it raised on its sunlike star, located 150 light-years from Earth in the constellation Pegasus. Measurements of the size and period of the wobble revealed the planet to be about 70 percent of the mass of Jupiter, in a scorching 3.5-day orbit of the star. The planet was so large, and in such a close orbit, in fact, that it had a good chance to transit—crossing the face of its star as seen from Earth and casting a shadow toward us here. Its transit was observed in 1999. In 2001 astronomers used Hubble to monitor four transits of HD 209458 b, delivering a “light curve” (above) that at the time was unprecedentedly pristine. Careful analysis of the Hubble data from the transit also revealed the spectral fingerprint of sodium, imprinted on the starlight shining through the upper atmosphere of HD 209458 b. This was the very first detection of an exoplanet’s atmosphere, and it built the case for subsequent space-based transit surveys such as NASA’s Kepler mission, which launched in 2009 and has discovered thousands of worlds. Eventually, the same techniques pioneered by Hubble in 2001 will be used to examine the atmospheres of smaller, more Earth-like planets beyond the solar system.
Credit: NASA, ESA, Brown et al. (NCAR, CfA, STScI, University of Arizona)
Finding a New Target for New Horizons / The Kuiper Belt Object 1110113Y In July 2015 NASA’s New Horizons spacecraft will fly by Pluto and its moons, completing the preliminary reconnaissance of our solar system that began with the first interplanetary missions of the 1960s. After streaking past Pluto New Horizons was intended to opportunistically visit another object in the Kuiper Belt, the icy and diffuse belt of debris of which Pluto is a part. Trouble was, mission-planners using ground-based telescopes couldn’t find any other Kuiper Belt objects that New Horizons could reach along its trajectory. That changed in 2014 when Hubble’s superior space-based view was enlisted to sweep the sky for this needle-in-a-haystack search. The New Horizons team hit the jackpot with 1110113Y, which appeared in multiple exposures (above) as a series of small white dots seeming to move alongside two background stars. 1110113Y is a 30- to 45-kilometer–wide object orbiting a billion kilometers beyond Pluto that New Horizons could target for a 2019 flyby. With this discovery, Hubble saved the post-Pluto phase of the New Horizons mission.
Credit: NASA, ESA, SwRI, JHU/APL and the New Horizons KBO Search Team Advertisement
Exploring a Soggy Outer Solar System / Jupiter’s Moon Europa In the search for extraterrestrial life astrobiologists are increasingly eager to move beyond dry, desolate Mars and more closely examine the frigid, ice-locked moons of the outer solar system, some of which harbor subsurface seas. Jupiter’s moon Europa is a particularly high-priority target, because a wealth of data suggests it possesses a 100-kilometer-deep ocean with perhaps twice as much water as Earth’s. But a key impediment to further exploration was that no one knew whether that ocean would be accessible from Europa’s surface, being buried beneath kilometers of ice. In 2013 astronomers announced that Hubble observations of Europa taken the previous year revealed plumes of water vapor venting from the moon’s south pole. The spectroscopic data showed the faint ultraviolet auroral glow of hydrogen and oxygen atoms (the blotches in the lower left of all the images above). This is a sign of water vapor being broken apart by electrons whizzing through Jupiter’s intense magnetic field. If the plumes prove to be from the subsurface sea, a future mission to Europa could sample the ocean and search for signs of life simply by flying through them.
Credit: NASA, ESA, and L. Roth et al (SwRI, MSFC, JHU, UC–Santa Cruz and University of Cologne, Germany)
The Debris Disk of HD 181327 / Tracking Epic Exoplanetary Collisions The shattered leftovers of planet formation tend to be scattered around a star in belts and rings of dust called “debris disks.” Astronomers used to believe that all debris disks would look more or less the same, manifesting as largely featureless pancakes of diffuse material whirling around a star. But that idea has fallen to the wayside as researchers have discovered more and more strange features in debris disks. In 2014 astronomers released a collection of Hubble data mapping the structure of disks around 10 different stars. None looked identical, and many had apparently been sculpted by the gravitational influence of unseen planets. Around the star HD 181327, Hubble spied a lopsided ring of debris (above, the central star is masked out) orbiting at about twice the distance of Pluto from our sun. The asymmetric bulge in the ring reflects a huge abundance of recently ejected dust, and may have been caused by a massive collision between two large cometlike bodies or perhaps even two planets. Such huge collisions were routine around the sun long ago, and were instrumental in forming Earth’s moon. Studying these debris disks around other stars allows astronomers to glimpse the immediate aftermath of planet formation, and could yield new insights into our solar system’s early history.
Credit: NASA, ESA, G. Schneider (University of Arizona), and the HST/GO 12228 Team Advertisement