The stars were revealed in unprecedented detail by the camera portion of the Space Telescope Imaging Spectrograph, an instrument sensitive to the high-energy ultraviolet emissions of hot stars. The Hubble image produced with that instrument is providing clues about how solar-type stars change as they age into the very compact, dense and dim white dwarf stars they eventually become.
"Before the Hubble telescope, astronomers were unable to see any of the details in this galaxy," said Harry Ferguson, an astronomer at the Space Telescope Science Institute in Baltimore, Md., who worked with Brown on the obvservations and on interpreting the image.
"It is a level of detail that allows astronomers to see, not just a galaxy as a whole, but to actually see the individual stars," he said.
The image shows the core of an elliptical galaxy called M32, which orbits a much larger spiral galaxy known as M31 (see image at right). M32 is a relatively small, very compact galaxy that has the distinction of being the closest to Earth of any galaxy of its kind. Other elliptical galaxies are so much more distant, Ferguson said, that it will be several decades before technology develops to allow scientists to glimpse another in such detail.
The galaxy's proximity, coupled with the ability of the Hubble to resolve details down to individual stars, mean that M32 is going to be heavily studied, Ferguson said.
"It really is a laboratory for trying to determine this [late] phase of stellar evolution," and various theories of stellar change, he said.
The blue stars pictured in the Hubble image -- which are about four times hotter than the sun, but only about half as massive -- appear as bright blue specs. They were originally between 20 percent and 50 percent more massive than the sun, but shed the bulk of their mass after the hydrogen at their cores was exhausted, astronomers believe.
The next stage in a solar-type star's life begins when it starts burning helium into heavier elements. This is an extremely important, but very poorly understood phase of stellar evolution.
Once a star's central hydrogen fuel is exhausted, it swells in size to become a red giant, according to the popular thinking. Over millions of years these red giants shed material, and eventually become "planetary nebulas." As stellar dust and gas shoots into space, the area around the dwindling red giant becomes a hazy cloud. In the eyepiece of early telescopes, those clouds resembled planets -- a similarity that resulted in the name.
In the final stages of a star's life, much of its outer layers have been blown into space, and the nebula disperses. The remaining core continues to burn helium until it is exhausted.
This is the stage of life where astronomers find the blue stars in M32. Using the Hubble telescope to view the galaxy in ultraviolet light has allowed them to isolate the blue helium-burners from the younger stars that fill that galaxy.
These stars are burning so hot, that most their emissions are in ultraviolet and more energetic wavelengths (see graph at right).
"If you want to understand these stars, you've got to go to the ultraviolet, because that's where all the light is coming out," Brown said.
Brown and his colleagues wish to conduct follow-up observations to learn more about helium-burning stars to get a better understanding of just what might happen to the sun in the next five to ten billion years.
Later observations will allow astronomers to determine the exact temperatures of stars in the galaxy, Ferguson said.
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