There is a fascinating link between gas raining onto the outer disks of spiral galaxies and the "stuff" of which we are made: carbon. Such infalling gas may be the perfect nurseries for carbon-producing stars!
Over ninety percent of all stars which have already reached the end of their (thermonuclear) lives in the Universe had masses between one and eight times the mass of our Sun. Stars with masses heavier than about 1.5 times the mass of our Sun pass through a late stage in their development where they become exceedingly bright carbon stars. Such stars blaze with luminosities ranging from four thousand to thirty thousand times the luminosity of our Sun. These carbon stars are relatively cool (typically 2500-3000 degrees Centigrade) and are unstable, pulsating with periods between one hundred to one thousand days.
The outer envelopes of such stars are rich in carbon, dredged up from their nuclear-burning interiors. Carbon forms the basis of life on Earth, and there are principally two mechanisms by which carbon can be released into space. The first is by means of an exploding star (such as the famous Crab Nebula in Taurus, seen to explode in the year 1054 AD) and the second, by means of these immensely interesting carbon stars. Carbon stars continually lose mass by means of stellar winds. During the phase of outward blowing stellar winds, these stars play an absolutely crucial role in spewing elements such as carbon into space. We are indeed made of carbon based stardust produced in stars.
The typical ages of carbon stars range between one-half to two billion years; astronomers refer to carbon stars as being of "intermediate-age" - neither young nor old. Of greatest interest is the actual "carbon star bearing epoch" - a time, which only lasts for about a billion years - during which these stars are extremely luminous in the near-infrared. In fact, studies have shown that the presence of only two or three carbon stars in intermediate age clusters of stars belonging to the neighboring Large Magellanic Cloud can contribute to about one-half of the total near-infrared brightness of the entire cluster! They are true light beacons in space.
Henry Wadsworth Longfellow in his poem entitled "The Lighthouse" expresses the brilliance of each light beacon thus:
Steadfast, serene, immovable, the same, Year after year, through all the silent night
Burns on forevermore that quenchless flame, Shines on that inextinguishable light!
We have been deeply engrossed in another detective trail: to find luminous carbon stars in the outskirts of spiral galaxies. The reason is that such stars can yield vital insights into the direction in which the disks of spiral galaxies grow. If cosmic gas rains onto the outer regions of a spiral galaxy, a tell-tale signature could be the presence of carbon stars. These stars are neither very young, nor very old, but rather of intermediate age, as discussed above. If these brilliant light beacons are pervasive in the outer disks of spiral galaxies, the implications would be that the disk of a galaxy grows with time, from the inside, outwards.
The actual contribution of carbon stars to the overall light of spiral galaxies remains poorly established. Relative to the inner regions of spiral galaxies, astronomers do know that the average ages of the outer regions are somewhat younger ... conceivably, prime hunting ground for intermediate-aged carbon stars.
In 2004, we discovered spectacular arcs of carbon stars in the outer domains of one of our closest spiral galaxies, known as the Triangulum Spiral Galaxy (Figure 161). The Triangulum Galaxy, nearly 3 million light years away, was probably discovered by Giovanni Hodierna before 1654; it was independently rediscovered by Charles Messier in 1764, where the galaxy appears as number 33 in his catalogue; hence the designation Messier 33 (M33). M33 was also catalogued independently by William Herschel on September 11, 1784. The Triangulum Spiral was amongst the first "spiral nebulae" identified as such by Lord Rosse.
We imaged the Triangulum Galaxy through the eyes of the orbiting Spitzer Space Telescope, using a special camera discussed earlier - a camera designed by Giovanni Fazio (affiliated to the Harvard-Smithsonian Center for Astrophysics and the Harvard College Observatory) and his associates.
We also confirmed the carbon-status of stars in the outer arcs of M33 using giant Keck telescopes atop the extinct Mauna Kea volcano in Hawaii. We believe that infalling gas continues to rain onto the Triangulum Galaxy, and that the disk of the Triangulum Galaxy subsequently grows from the inside, outward.
Based on such studies, we have good reason to believe that most galaxies will pass through a first carbon phase, when the Universe was only about ten percent of its present age and when
carbon stars were rampant. Using the Large Magellanic Cloud as a guide, carbon stars are found in large numbers between ages of about one-half to two billion years. It is therefore perfectly conceivable that galaxies which undergo an early burst of star formation will have their infrared light output greatly boosted one-half of a billion years later, and that this prodigious output of light will fade away after some two billion years.
There may be an epoch starting about a half a billion years after the onset of star formation in our Universe, characterized by large numbers of carbon stars. Why have astronomers not yet detected this epoch, when carbon stars were rampant? The reason is simple: the parent galaxies, containing carbon stars, would be moving away from us at great velocities in our ever expanding Universe. Their enormous output of radiation would be masked, this time by the recessional speed of the galaxy to which these stars belong. Radiation from carbon stars would no longer reign supreme in the near-infrared; their near-infrared photons would be shifted red-ward, toward the mid-infrared ...
A crucial role will be played by the James Webb Space Telescope (abbreviated JWST) - a large, infrared-optimized space telescope, which as noted earlier, is scheduled for launch no earlier than June of 2013. This space telescope, to replace the Hubble Space Telescope, is principally designed to study the earliest galaxies and some of the first stars formed after the Big Bang. These young objects move away from us at high speeds and the best observations for such objects are only available in the mid-infrared. JWST will have a large mirror, 6.5 meters in diameter and a sunshield the size of a tennis court. Both the mirror and sunshade would not fit onto the rocket fully open, so both will fold up and open only once that space telescope is launched. The James Webb Space Telescope will reside in an orbit about one and a half million kilometers from the Earth.
Detectors such as the Mid-Infrared Instrument on board the James Webb Space Telescope should prove pivotal in the detection of a time when carbon stars first glowed as fiery lights in our early Universe. We eagerly await to see that epoch, when highly luminous carbon stars - enriched in that element carbon of which every reader is made - may have first dominated the radiation fields of many young galaxies, billions of years ago.
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