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Infrared Emissions |
The infrared band of the electromagnetic spectrum ranges from ~ 1 x 10-3 meters to 7 x 10-7 meters (see diagram below - Click on image to enlarge). By studying the infrared emissions of a star, we can tell approximately what percent of a specific element exists within the photosphere of that star. This heat radiation is readily absorbed by most materials. This energy excites the constituent atoms, causing them to become agitated, which increases their vibrational and translational motion, and thereby their heat.
Infrared Studies
Astronomers have detected the cool infrared signature of dust grains and silicates within superheated gas in the center of ancient elliptical galaxies 60 million light-years from Earth. This could represent the first direct observation of how mass lost by aging stars evolves in a hot, exotic environment. U-M astronomer Joel Bregman and his colleagues found unexpected evidence for the existence of dust and silicates in infrared emissions from nine elliptical galaxies. Spectral data for the study were collected by the Infrared Camera (ISOCAM) on the European Space Agency's Infrared Space Observatory.
Space observatories like ISO allow astronomers to observe objects too cold or faint to be seen in visible light. Another advantage of infrared light is that it passes undisturbed through gas and dust clouds allowing astronomers to "see" inside dense, star-forming areas in the center of galaxies.
Elliptical galaxies contain stars that are 5 billion to 15 billion years old. Temperatures are comparable to those inside a supernova remnant, but on a galaxy-wide scale. Even the hottest parts of our Milky Way Galaxy rarely exceed 1 million degrees K, while the gas in the center of the observed galaxies is so hot, Bregman and his colleagues were startled when an excess in their infrared spectral data indicated the presence of material significantly colder than the surrounding gas.
"We expected to see the normal starlight spectrum, along with polyaromatic hydrocarbons or PAHs," Bregman said. ESA was asked for additional observations from ISOCAM in the six- to 15-micron wavelength band. ESA went the extra mile and gave the researchers additional observation time with higher resolution detail. They believe the source of the unusual infrared emissions is dust grains and silicates thrown off by stars interacting with superheated gas inside elliptical galaxies. As this gas flows away from the stars, some of the material forms into grains of dust, thought to be somewhat smaller than particles in cigarette smoke.
This material is exposed to the very hot gas of the galaxy which slowly destroys the grains, and to UV light of the galaxy, which heats the dust grains causing them to emit in the mid-IR region. If this interpretation is accurate, additional analysis of infrared emissions from elliptical galaxies could provide important clues to how galaxies evolve.
Most Luminous Star in the Universe ?
Astronomers using NASA's Hubble Space Telescope have identified what may be the most luminous star known -- a celestial mammoth which releases up to 10 million times the power of the Sun and is big enough to fill the diameter of Earth's orbit.48a The star unleashes as much energy in six seconds as our Sun does in one year. The image of the Pistol Star, taken by a University of California, Los Angeles (UCLA)-led team with the recently installed Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard Hubble, also reveals a bright nebula, created by extremely massive stellar eruptions. The nebula is so big (four light-years) that it would nearly span the distance from the Sun to Alpha Centauri, the nearest star to Earth's solar system. The astronomers estimate that when the titanic star was formed one to three million years ago, it may have weighed up to 200 times the mass of the Sun before shedding much of its mass in violent eruptions. The star will continue to lose more material, eventually revealing its bare hot core, sizzling at 100,000 degrees.
"This star may have been more massive than any other star, and now it is without question still among the most massive -- even at the low end of our estimates," says Don F. Figer of UCLA. "Its formation and life stages will provide important tests for new theories about star birth and evolution." The UCLA astronomers estimate that the star, called the "Pistol Star" (for the pistol shaped nebula surrounding it), is approximately 25,000 light-years from Earth near the center of the Milky Way galaxy.
The Pistol Star is not visible to the eye, but is located in the direction of the constellation Sagittarius, hidden behind the great dust clouds along the Milky Way. The Pistol Star was first noted in the early 1990s, but its relationship to the nebula was not realized until 1995, when Figer proposed in his Ph.D. thesis that the "past eruptive stages of the star" might have created the nebula. The Hubble spectrometer results confirm this conclusion.
The astronomers believe that the Pistol nebula was created by eruptions in the outer layers of the star which ejected up to 10 solar masses of material in giant outbursts about 4,000 and 6,000 years ago. Burning at such a dramatic rate, the Pistol Star is destined for certain death in a brilliant supernova in 1-3 million years.
"Massive stars are burning their candles at both ends; they are so luminous that they consume their fuel at an outrageous rate, burning out quickly and often creating dramatic events, such as exploding as supernovae," said Mark Morris, a UCLA professor of astronomy and co-investigator. As these stars evolve, they can eject substantial portions of their gaseous shells into the void. In the case of the Pistol Star, it produced the nebula and an extreme stellar wind that is ~10 billion times stronger than our Sun's.
The Pistol Star would be visible to the naked eye as a fourth magnitude star in the sky, but interstellar dust clouds of tiny particles between the Earth and the center of the Milky Way that absorb the star's light. This is still quite impressive given its distance of 25,000 light-years.
The Pistol Star was so massive when it was born that it brings into question current thinking about how stars are formed. In the current view, stars form within large dust clouds which contract under their own gravity, eventually forming hot clumps that ignite the hydrogen fusion process. In this classical or "conventional" view, the star may radiate enough energy to halt the inward fall of material, thus limiting its maximum mass. The initial mass of the Pistol Star may have exceeded this theoretical upper limit. It has been proposed that it is no accident that this extreme-mass star is found near the center of the galaxy, which is within the scope of the aforementioned "conventional" theory in that the star formation process there may favor stars much more massive than our modest Sun.
The most powerful telescopes cannot see the Pistol Star in visible wavelengths. Ten percent of the infrared light leaving the Pistol Star reaches Earth, putting it within reach of infrared telescopes. Radio teloscopy has seen rapid technological advancement in recent years, much of which was promulgated by projects such as NICMOS. By studying the electromagnetic emissions of objects such as stars, galaxies, and black holes, astronomers hope to come to a better understanding of the universe.1v
Although many astronomical puzzles can only be solved by comparing images of different wavelengths, telescopes are only designed to detect a particular portion of the electromagnetic spectrum. Astronomers therefore often use images from several different telescopes to study celestial phenomena. Shown (on the image page) is the Milky Way Galaxy as seen by radio, infrared, optical, X-ray and gamma-ray telescopes.
Images
The image was taken in October 1998 as part of the Hubble Deep Field South imaging campaign. It is in a small patch of sky in the constellation Tucana. The false-color image is a composite of separate images taken with the NICMOS and STIS cameras on board the Hubble Space Telescope. The red and green colors correspond to infrared wavelengths of 1.6 and 1.1 microns, respectively. The blue color corresponds to the STIS view that covers the full range of visible wavelengths.
Hubble's Infrared Galaxy
Gallery
Astronomers have used the NASA Hubble Space Telescope to
produce an infrared "photo essay" of spiral
galaxies. By penetrating the dust clouds swirling around
the centers of these galaxies, the telescope’s
infrared vision is offering fresh views of star birth.
These six
images, taken with the Near Infrared Camera and
Multi-Object Spectrometer, showcase different views of
spiral galaxies, from a face-on image of an entire galaxy
to a close-up of a core. The top row shows spirals at
diverse angles, from face-on, (left); to slightly tilted,
(center); to edge-on, (right). The bottom row shows
close-ups of the hubs of three galaxies. In these images, red corresponds to
glowing hydrogen, the raw material for star birth. The
red knots outlining the curving spiral arms in NGC 5653
and NGC 3593, for example, pinpoint rich star-forming
regions where the surrounding hydrogen gas is heated by
intense ultraviolet radiation from young, massive stars.
In visible light, many of these regions can be
hidden from view by the clouds of gas and dust in which
they were born. The glowing hydrogen found inside the
cores of these galaxies, as in NGC 6946, may be due to
star birth; radiation from active galactic nuclei (AGN),
which are powered by massive black holes; or a
combination of both.
White is light from middle-age stars. Clusters of
stars appear as white dots, as in NGC 2903. The galaxy
cores are mostly white because of their dense
concentration of stars. The dark material seen in these
images is dust. These galaxies are part of a Hubble
census of about 100 spiral galaxies. Astronomers at Space
Telescope Science Institute took these images to fill
gaps in the scheduling of a campaign using the NICMOS-3
camera. The data were non-proprietary, and were made
available to the entire astronomical community.
Filters: Three filters were used: red, blue, and
green. Red represents emission at the Paschen Alpha line
(light from glowing hydrogen) at a wavelength of 1.87
microns. Blue shows
the galaxies in near-infrared light, measured between 1.4
and 1.8 microns (H-band emission). Green is a mixture of
the two.
Eta Carina as seen in
infrared light
Since it looked like a supernova, one naturally would
assume that was the end of the star. All that should be
left are beautiful nebula and, perhaps, a neutron star or
black hole where the original star once stood. Instead,
Eta Carinae is still there (in a subtle bit of grammar,
astronomers refer to the star as Eta Carinae and the
nebula as Eta Carina).
Credit: Torsten Boeker, Space Telescope
Science Institute (STScI) , and NASA
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