| FIRST SCIENTIFIC RESULTS FROM FUSE |
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A press conference on the first scientific results from the Far
Ultraviolet Spectroscopic Explorer (FUSE) satellite observatory was
held at 8:30 a.m. EST, Wednesday, January 12, 2000 during the 195th
meeting of the American Astronomical Society in Atlanta, Georgia.
Here we provide some of the material presented during that press conference. There are various icons associated with each of the images displayed below. Clicking on the icons will either display, or download (depending on your browser settings), the image in the particular format indicated (JPEG, TIF, PDF, QuickTime, or MPEG). The displayed images themselves are the GIF versions and can be saved in the usual way (i.e., by "right-clicking" on the image for users with 3-button mouses, or by "click-and-hold" for Macintosh users). The file sizes are indicated beneath each icon.
If the images are not in the format you need, please phone or send
email to Three new scientific results are discussed below:
Also, near the end of this web page, we provide a Mission Update and some general Background on FUSE that you may find helpful. Finally, we note that an overview of the three scientific results presented at the FUSE AAS press conference can be found in the Official NASA Press Release , while three other notable FUSE results are reported in a Johns Hopkins University "tipsheet" .
New FUSE observations provide clear evidence for the widespread existence of violently heated, hot (half million degree) interstellar gas extending 5 to 10 thousand light years away from the disk of the Milky Way Galaxy and forming a halo around it. The FUSE data strongly support the hypothesis that this hot interstellar gas is heated by catastrophically dying stars called supernovae. Here is an artist's depiction of what the hot gaseous halo surrounding our galaxy looks like:
Credit: G. Sonneborn (NASA Goddard Space Flight Center), B. Savage (University of Wisconsin), W. Feimer (Honeywell), and NASA. Stars are born in interstellar gas clouds, go through their life cycles, and then die, sometimes catastrophically. There is a continuous cycling of matter and energy between the stars and interstellar gas of the Galaxy. This cycling affects the properties and distribution of the gas in the Galaxy. The interstellar gas thins out with distance away from the Galactic disk. The new FUSE results relate to the hot component of the gas and its extension away from the plane of the Galaxy forming a halo. The FUSE data strongly support the hypothesis that this hot interstellar gas is heated by catastrophically dying stars called supernovae. Modern theoretical ideas about the possible origins of hot gas at large distances from the Galactic plane usually include as their starting point the phenomena referred to as a Galactic Fountain. In a Galactic Fountain, regions of interstellar gas in the disk are heated to high temperatures by the violent explosions of dying stars known as supernovae. The heated and highly pressurized gas created by these explosions bursts out of the plane of the galaxy, rises into the halo, where the gas cools, and rains back down onto the Galactic plane in flow resembling a fountain. Here is a 30-second animation illustrating this process....
Credit: G. Sonneborn (NASA Goddard Space Flight Center), B. Savage (University of Wisconsin), W. Feimer (Honeywell), and NASA. The following results have been obtained from these new FUSE observations of hot gas:
In the future, the FUSE observational program will be expanded to better understand the motions of the hot halo gas, the sites of origin of the gas in the disk, and the interaction between the gas of the Galactic corona and the intergalactic medium. Here is a composite image that includes four of the frames from the animation sequence, along with a caption:
Credit: G. Sonneborn (NASA Goddard Space Flight Center), B. Savage (University of Wisconsin), W. Feimer (Honeywell), and NASA. High resolution versions of each of the frames comprising the composite image displayed above can be downloaded from ftp://pao.gsfc.nasa.gov/newsmedia/aas/fuse/ . For further information on the FUSE Hot Gas Halo results, please read the University of Wisconsin Press Release .
FUSE has uncovered evidence that the most massive stars recycle their material differently in different galaxies. This in turn helps us to understand how galaxies evolve over cosmic time. The most massive stars are so bright that they are blowing themselves apart. Such stars are more than a million times brighter than the sun, but live only one thousandth as long. The phenomenon, known as a stellar wind, blows away a large fraction of the star's initial mass during its brief 10 million year lifetime. Stellar winds have been known since the 1960s, when the outflowing material was detected moving away at up to 3000 km/s (some 7 million MPH). Stellar winds not only affect the fate of these massive stars, but they set the upper limit to stars' brightnesses before they blow themselves apart. They are also a major way in which new elements created by the nuclear reactions that power the stars, are cycled back into space, enriching the gas that will form later generations of stars. The figure below is a composite of four images and illustrates how the wind from a massive star seeds its host galaxy with new material and how FUSE captures the signatures of this process.
Caption: The above sequence is ordered clockwise, starting at the top left. The first image shows the intense starlight pushing matter away from the star's surface, accelerating and cooling as it rises, until it leaves the star at high velocity. The hot matter moves slowly and cooler matter moves fast. The next image (top right) illustrates how we view a stellar wind. Our telescope is off to the right. Between us and the star's surface, we see matter moving towards us that leaves telltale dips in the light from the star's surface. At the same time, matter in the wind moving in other directions emits the same signature light, which appears added to the basic starlight. Just where the dips and extra light are seen tells us the wind's speed and its structure. The result appears in the FUSE data as the cartoon traces shown in the next image (bottom right). The white horizontal line is the basic starlight level. On the left we see the hot, slow moving material as a sharp dip and extra peak. We measure outflow speed as shifts relative to the speed of the star, which is at the zero velocity position. The cold, fast-moving wind parts show up as dips further to the left of the stellar position, and a broader speed range in the extra light. The final image (bottom left) shows the first FUSE observations, which reveal conspicuous differences in the way material is blown off identical stars in two different galaxies (the Large and Small Magellanic Clouds). We see very different outflow velocities in the same material in the two stars. (The sharp dips arise in material not associated with the star, so we have sketched the wind signal above them.) Credit: J. B. Hutchings (National Research Council, Canada) and NASA For further information on the FUSE Hot Star Wind results, please read the National Research Council of Canada Press Release .
FUSE has provided the eyes that allow us to see, nearly everywhere and with unprecedented sensitivity, the most abundant molecule in the universe, molecular hydrogen. Prior to FUSE, this cold molecular hydrogen gas was essentially hidden from our view. From images of our Milky Way galaxy (see the COBE image below) and external galaxies (see the WIYN image of NGC 891 below), astronomers know that the "life blood" of galaxies is contained in the dark clouds of interstellar molecules and dust. From these clouds, new generations of stars and planets are continually forming. Although molecular hydrogen, whose chemical symbol is H2, is the most abundant molecule in space, it has largely been hidden from view until FUSE. This is because molecular hydrogen is best detected through radiation in the far-ultraviolet, which is inaccessible even to the Hubble Space Telescope. Because FUSE is about 100,000 times more sensitive than its predecessor (Copernicus, a 1970's satellite), FUSE can detect molecular hydrogen nearly everywhere. Molecular hydrogen is seen throughout the Milky Way and its halo, and also in external galaxies such as the Large and Small Magellanic Clouds. This suggests that star formation proceeds in a similar fashion in these much different environments. Here is a FUSE spectrum of the distant quasar ESO141-G55, from which we can tell how easily FUSE detects molecular hydrogen, even in distant parts of our galaxy.
Caption: The above figure displays a portion of the FUSE spectrum of a distant quasar, E141-G55, showing the vast number of spectral fingerprints of atomic and molecular gas in the Milky Way. The molecular hydrogen lines are color-coded in red. Other spectral lines from hot and cold gas are colored in blue and green, respectively. Credit: J. M. Shull and J. B. Tumlinson (University of Colorado) and NASA. The molecular hydrogen is so ubiquitous that we can begin to speculate how it manages to form and survive, even in harsh radiative environments of the Galactic disk and halo. Some diffuse molecular hydrogen may form in gas associated with infalling High Velocity Clouds (see the image below), as has been observed at radio wavelengths, and in the diffuse infrared cirrus clouds seen by the IRAS satellite (again, see the images below). Without molecular hydrogen and cold interstellar gas clouds, new stars and planets would not form. However, we still do not understand how the vast amounts of diffuse, hot interstellar gas assembled into dark molecular clouds. FUSE has provided astronomers with a key means of probing these "life cycles of matter" throughout interstellar space and in galaxies outside the Milky Way. Understanding star formation in the Milky Way and nearby galaxies gives astronomers clues about how the first stars formed, 10-12 billion years ago. Over the next three years, FUSE will study the quantity of molecular hydrogen in various interstellar environments. The physical state of the molecules will provide basic information about the densities, temperatures, and UV radiation fields within the interstellar clouds.
Moving left-to-right, here are the captions for each of the figures presented above:
The outstanding scientific results discussed above demonstrate conclusively that
To emphasize the dramatic improvements in scientific productivity that will now be possible with FUSE, here is a comparison between a recent mission, ORFEUS, launched from the Space Shuttle for a two week mission in 1996 and FUSE. The improved sensitivity and resolving power of FUSE enable astronomers to see a large number of new atomic and molecular features that could only be guessed at before.
Caption: A small portion of the spectrum (located between the wavelengths of 1045 Angstroms and 1055 Angstroms) of the galaxy ESO 141-55, as observed by both FUSE (at the bottom in green) and ORFEUS (at the top in red). ORFEUS was an ultraviolet observatory that was released and then retrieved by the Space Shuttle during a 14 day mission in 1996. The FUSE spectrum shows an abundance of atomic and molecular features that cannot be clearly seen in the ORFUES spectrum. These new features provide information on the composition and physical conditions of the interstellar gas between the Earth the external galaxy. Credit: H. W. Moos (Johns Hopkins University), the ORFEUS Project, and NASA. Running a space observatory is very complicated. Here is a cartoon that illustrates the various components of the FUSE observatory in a simplified way.
A space observatory consists not only of the instrument and spacecraft, but also includes ground stations, and the team of people on the ground controlling the spacecraft, planning the details of future observations, and calibrating the resulting data that scientists will analyze. All of these components have to work together - the data discussed above demonstrate that this is the case for FUSE. We are not even finished optimizing the instrument, yet we have many more exciting results than we could even begin to talk about in a single press briefing. Session 6 at the American Astronomical Society meeting is a poster session dedicated to presenting early results from the FUSE mission and shows the wide range of scientific topics already being addressed by FUSE. Typically, many months are needed to make all the components of a new space observatory work together on a regular basis. FUSE has now reached that milestone and is performing observations on a routine basis for both members of the Principal Investigator Team and for sixty two (62) Guest Investigators from around the world selected by NASA for the first year of operations. A number of different organizations have worked together to get the FUSE mission into space. FUSE is part of the NASA ORIGINS program and is funded by NASA through the Goddard Space Flight Center. Both the Canadian Space Agency and the French National Center for Space Studies supplied critical hardware. FUSE is a Principal Investigator class mission in which major responsibility for development and operations was undertaken by the Johns Hopkins University in collaboration with the University of California, Berkeley and the University of Colorado. The handout package distributed to the press at the AAS meeting contains a list of the Science Team members, their home institutions, and cities. Also included is a list of laboratories and industrial partners who have worked very hard to develop and to operate this mission. Sometime this spring, FUSE is expected to begin a comprehensive study of the abundance of deuterium, a fossil nucleus left over from the Big Bang. Also, as the FUSE team becomes more and more experienced and continues to optimize the instrument, the amount of time spent on scientific observations will go up, which means higher scientific productivity. FUSE's future looks bright, and we look forward to many more exciting, new scientific discoveries by FUSE during the next several years.
FUSE was successfully launched on June 24th, 1999 and is exploring the universe at wavelengths of light that are complementary to those being observed by the Hubble Space Telescope (HST) and by the Chandra X-ray observatory. The figure below shows the portion of the electromagnetic spectrum covered by FUSE compared to that covered by HST. The region observed by FUSE is especially rich in the spectral transitions, or fingerprints, of most atomic elements and simple molecules.
Here is a picture of FUSE in the clean room at the Kennedy Space Center as it was being prepared for launch.
Every element and molecule selectively absorbs and emits light at very specific wavelengths, providing a spectral "fingerprint" for the species. When FUSE looks at a distant star or galaxy, the atoms and molecules in gas clouds along the path impose a signature on the FUSE spectrum, as illustrated in the figure below. These spectral fingerprints allow astronomers to determine the composition of the clouds, their distance, and their physical properties.
This next graphic illustrates how the doppler effect allows one to distinguish one cloud's signature from another's.
The star is the farthest object from FUSE and is moving away from FUSE the fastest. Cloud B is the next most distant object and is moving at an intermediate speed, while Cloud A is the closest to FUSE and is moving the slowest. Just as the pitch of a train's whistle seems to be at a lower frequency when it is moving away from you, so too are the spectral fingerprints of atoms and molecules shifted to different wavelengths depending on their speed relative to the observer. Owing to this effect, the spectral signatures of the star and gas clouds are shifted relative to each other in the FUSE spectra, and this allows astronomers to infer the speeds of the two clouds and of the star. Without these shifts, all of the spectral signatures would land on top of one another and would create confusion in the analysis of the data. For further background information on FUSE, astronomy, and spectroscopy, please peruse the various web pages on these subjects at the FUSE Science Center at the Johns Hopkins University.
The FUSE press conference panelists, their affiliations, and the topics they presented are given in the following table. Please contact them if you have questions on the scientific content of the material discussed above. (All of the panelists are currently at the American Astronomical Meeting in Atlanta, Georgia, and can be reached through the Press Office there: 404-588-2908, -2909, and -2910.)
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Hal Weaver
Last updated: 12 January 2000
