Hubble Space Telescope Photographs Extragalactical Stellar Nursery
13 August 1990NASA is releasing a Hubble Space Telescope (HST) photograph of the most remarkable star forming region in the Local Group of Galaxies, 30 Doradus. The photograph shows about 60 stars within a central tight cluster in 30 Doradus. In contrast, earlier photographs with ground-based telescopes, supplemented by mathematical analysis, have shown only 27 stars in the tight cluster, which is called R136. Before the ground-based studies showed that so many stars are present in R136, some astronomers thought it was a single, supermassive object, with as much as 3000 times the mass of the Sun.
This recent HST photograph, shows even more individual stars within R136. Furthermore, its high resolution suggests that some of the stars have more than 100 times the mass of the Sun. That would make them among the most massive stars ever identified.
30 Doradus is readily visible with the naked eye from the Southern Hemisphere of Earth, although it is located in another galaxy, the Large Magellanic Cloud, at a distance of about 160 000 light years from Earth. The late American astronomer Harlow Shapley stated that 30 Doradus is so bright that if it were put in place of the nearby Orion Nebula, it would cast shadows on the nighttime landscape of Earth. 30 Doradus is located in the constellation Dorado, the swordfish.
The HST photograph of 30 Doradus was made on 3 August 1990, with the Wide Field/Planetary Camera (WF/PC) of the HST for use as a finding chart in the checkout of another HST instrument, the Goddard High Resolution Spectrograph. The WF/PC produces star images with sharp cores, 0.1 arc seconds wide. This image quality is sustained over the full field of view, which is 2.7 arc minutes square. HST astronomers studying the WF/PC plcture report that they can make out the central stars of the R136 cluster. They note that because the picture was taken in violet light, at wavelength 3680 angstroms, it brings out the hottest, most massive stars in the scene. Hot stars produce more blue and ultraviolet light than cooler stars. Several of the stars appear to be single objects at the resolution of the WF/PC picture. Given their brightnesses and the distance to 30 Doradus, this observation strengthens the possibility that they may be more than 100 times as massive as the Sun.
Every HST picture of star clusters should achieve the resolution demonstrated in the photograph of 30 Doradus. Such photographs, when obtained through the different colored filters on the WF/PC and the Faint Object Camera (FOC), are expected to provide detailed information on the masses of stars in the clusters. Then, knowing the mixture of masses in a cluster (i.e., how many stars of each mass are present), astrophysicists can deduce basic information on how stars form and how they produce the chemical elements present in space. All massive stars probably become supernovae, spewing out their new-made elements. By determining how many such stars are present in 30 Doradus and similar star clusters in more distant galaxies, astronomers expect to deduce more accurate information on the enrichment of chemical elements in the universe.
Background Information On Computer Restoration of HST Images
The unprocessed HST images presented here today are significantly better than images from ground-based telescopes. Image processing methods promise to generate images which are, for many purposes, even better than the raw images.
These computer algorithms remove the "halos" which can be seen around stars in the unprocessed images. The halos are not really present around the stars; rather, they are the result of the faulty focus of the HST for light which reflects off of the outer edge of the primary mirror. Every object in the images can be seen to have these fuzzy halos.
Fortunately, there is still enough light in the cores of the images that we can readily distinguish individual stars, even in very crowded regions such as the center of this star cluster. That is what makes these images much better than those taken with ground-based telescopes.
The tiny, bright cores also make it possible for us to improve the images using computer image restoration techniques. The basic idea is this: if we know that every star has a little halo, why not just measure the position and brightness of the stars and then subtract the halos from the images? Not only does this produce a cleaner image, but it also makes it easier to detect faint stars which are partially obscured by the haze from nearby, brighter stars.
There are a number of image restoration techniques which are applicable to HST images. A wide variety of different methods give very similar results to those shown in the images here. It is much harder to apply these techniques to images from ground-based telescopes because such images lack the well-defined cores of the HST images and because the atmospheric blurring changes continuously, making it impossible to determine a fixed stellar profile.
There are two factors which limit the accuracy with which we can remove stellar halos using these techniques. First, if we do not know exactly what the halo ought to look like, we cannot subtract it perfectly. This is the primary factor limiting the restored images shown today: our knowledge of the "point spread function" (the image we expect for a star) for this particular color at this particular HST focus setting is imperfect. With more observations we expect to characterize the halos much more accurately, which should allow significantly better restorations.
A more fundamental limit is set by the noise-in the images. Astronomers observe very faint stars by counting the individual particles of light, or photons, which are collected by the telescope. For the faintest stars only a few photons are counted, and there is consequently an inherent uncertainty in the brightness measured for those stars. This counting noise makes it impossible to subtract halos around even the brightest stars perfectly. In fact, even to detect the presence of faint stars amidst the glare created by many overlapping halos may be very difficult or impossible. This is the greatest loss to HST science as a result of the spherical aberration of the telescope mirror: the faintest objects simply cannot be seen through the haze of crowded fields.
On the other hand, these images demonstrate unequivocally that HST can still do excellent science and that HST's wide field, high resolution images, even without image processing, are unrivaled by ground-based telescopes. Furthermore, HST can produce these images in the far ultraviolet, which is absorbed by the atmosphere.