The discovery of the supermassive starlike object R136a presented important theoretical problems concerning stellar size and stability.
One of the interests of stellar astronomers is the search for large stars and how they evolve. The Orion nebula
Betelgeuse, also in the constellation of Orion, is a cool red star in the late stages of stellar evolution. One of the brightest objects in the night sky, Betelgeuse
The 30 Doradus, or Tarantula, nebula
Observational data from the years 1978 to 1980 were assembled by the research team of Joseph Patrick Cassinelli, John Samuel Mathis, and Blair DeWillis Savage. Small- and large-aperture cameras were used to collect both low- and high-resolution spectra. The small-aperture camera gave a good overall view of the shape of the spectrum from 120 to 300 nanometers (a nanometer is one billionth of a meter), showing the relative positions of all the absorption and emission lines. The large-aperture camera was used to study in detail the individual spectrum lines.
An ionized helium line was found in R136a that is similar to that found in Wolf-Rayet stars, which are very hot and have broad emission lines. The helium and a nitrogen emission line were observed as broadened with flat bottoms. Emission is caused by shells of gas that the star ejects into space. This was evidence of a very large mass loss by solar winds.
There were other indications from spectral studies that R136a was very hot; an ionized silicon emission line that is strong in stars with temperatures less than 50,000 Kelvins was weak in the spectrum of R136a. This was consistent with data from R136a showing a significant contribution to ultraviolet radiation, which is expected from a very hot star.
On the basis of the earlier data, the research team reported in 1981 that the radiation appears to come from a single superluminous object with a radius one hundred times, and a mass equal to twenty-five hundred times, that of the Sun. They believed this energy was consistent with hydrogen fusion processes in its interior.
Later observations included data from the IUE covering the period from 1980 to 1982. The overall quality of the data was improved by applying an averaging process to the low- and high-resolution spectra. Additional spectral lines were observed, including ionized oxygen and iron. The oxygen lines were present only in hot blue stars of class 04 or younger age, suggesting a very hot star or collection of stars.
Ultraviolet variability was studied over the four-year period for evidence of the physical characteristics of R136a. No significant changes in the stellar absorption lines or spectral lines were observed during this time. This suggests that the star or stars do not vary in radiation output as do some of the more massive variable stars.
Radio measurements of the rate of ionization of the 30 Doradus nebula have led to an estimated nebular electron temperature of about 12,000 Kelvins. To produce that much ionization would require approximately one hundred hot 05-type stars or sixty 04-type stars. Measurements show that the ionized regions have a much higher temperature when compared with other nebulas of the Large Magellanic Cloud and Milky Way galaxy. The researchers concluded that a luminous object in the interior of the 30 Doradus nebula was responsible for this ionization.
The technique of speckle interferometry was developed in 1979, allowing for the first time direct measurement of the diameter of the planet Pluto. This technique involves taking a series of photographic images to freeze out the effects of turbulence in Earth’s atmosphere, thereby improving resolution. Earlier measurements made from speckle interferometry showed what could be a cluster of stars having a diameter of less than 1,000 astronomical units. Interestingly enough, however, a secondary component appeared that was dimmer and slightly separated from the primary source of radiation.
When the analysis of the complete set of data from 1978 to 1982 was finished, the research team concluded that the radiation from R136a was dominated by emission from a supermassive star or a small group of supermassive stars. The following properties were derived from theory using the complete data set, which resulted in modifying the original size estimates somewhat: a radius equal to fifty times that of the Sun, a mass equal to 2,100 times the mass of the Sun, and a luminosity equal to 60 million times that of the Sun. The researchers also believed that the deduced properties were compatible with the theory of internal structure for supermassive stars. It was argued that such a star might form by ordinary stellar collapse if the right types of interstellar dust were present or perhaps by the coalescence of several stars in a cluster of high stellar density. Such a star would be expected to have a lifetime of some 2 million years.
The existence of massive stars such as red or blue supergiants has been known for some time. A supermassive star such as R136a exceeded the mass of the most massive stars by a factor of twenty times. There are theoretical problems associated with very large stars, including the formation, stability, lifetime, and upper mass limits. The upper mass limits for a star to form and be vibrationally stable are not known from observations or theory.
An alternative explanation to the single-star hypothesis for R136a is that perhaps the radiation is coming from a compact star cluster instead. To explain the spectral lines and the overall radiation intensity (continuum) as a result of R136a would require a cluster of as many as thirty hot blue 03 class stars and fifteen Wolf-Rayet-type stars. According to the Wisconsin astronomers, this would be an unusual mixture of star types because the 03-type stars are very young, but the Wolf-Rayet stars are of a later spectral type and are older. As a rule, stars within the same cluster are assumed to have about the same age, forming from gas and dust that is gravitationally contracting.
An important development regarding the supermassive star hypothesis
These advanced techniques used a charge-coupled device (CCD) exposed to red light and subjected to extensive image processing. This technique improved the light-gathering methods that were one of the serious problems facing the earlier researchers. The reduction of radiation in this region of the sky is called extinction; it is caused by the high concentrations of gas and dust in the nebula. The effect of the extinction is to make the brightness dimmer or reddened by the Milky Way foreground dust that is between the object and the observer.
The 1988 discoveries could have an important impact on the scale of distances in the universe. In galaxies as far away as 10 million light-years, the brightest stars within the galaxy are used as distance calibrators. The magnitudes of such stars are compared to those in the brightest clusters of the Milky Way. Nearby clusters are less likely to be mistaken for single stars than the more distant ones. If the stars are really multiples, their distances would tend to be underestimated. This would, in turn, lead to smaller values for the Hubble constant (which relates the velocity of recession of galaxies to their distance) and would give larger estimates for the size and age of the universe.
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Cassinelli, Joseph P., John S. Mathis, and Blair D. Savage. “Central Object of the 30 Doradus Nebula, a Supermassive Star.” Science 2212 (June, 1981): 1497-1501. Using data from the International Ultraviolet Explorer satellite, the authors report the first discovery of a peculiar hot massive object. Presents arguments regarding the nature of R136a whether it could be a single supermassive star or a cluster of stars. -
Cooke, Donald A. The Life and Death of Stars. New York: Crown, 1985. A superbly illustrated book with color plates of nebulas that may be stellar birthgrounds and nurseries. Presents the Great Orion nebula as a case study. Diagrams show the interiors of stars and mechanisms of heat transport within a star. -
Kippenhahn, Rudolf. One Hundred Billion Suns. Translated by Jean Steinberg. New York: Basic Books, 1983. The author and director of the Max Planck Institute for Astrophysics, using humor and insight, describes discoveries in stellar astronomy, including his own. Illustrations show the evolutionary paths of both stars and star clusters in different stages of their lifetimes. -
Naeye, Robert. Through the Eyes of Hubble: The Birth, Life, and Violent Death of Stars. Waukesha, Wis.: Kalmbach Books, 1998. A concise book with stunning images from the Hubble Space Telescope. -
Savage, Blair D., Edward L. Fitzpatrick, Joseph P. Cassinelli, and Dennis C. Ebberts. “The Nature of R136a, the Superluminous Central Object of the 30 Doradus Nebula.” The Astrophysical Journal 273 (October, 1983): 597-623. Presents additional data subsequent to the initial discovery. Discusses the theoretical problems associated with the supermassive star hypothesis. -
Time-Life Books, eds. Stars. Alexandria, Va.: Author, 1989. Uses dramatic multicolored diagrams to illustrate fast-burning giant stars and the convective heat transport mechanism from the core outward. Discusses high mass stars and how they evolve from protostars.
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