A Near-Ultraviolet Image of M32 as Observed with the Space Telescope Imaging
Spectrograph
Background:
Most of the stars in the local Universe inhabit spiral galaxies (like our
own Milky Way) or elliptical galaxies. Elliptical galaxies are thought
to be old groups of stars, and their relative simplicity makes them useful
for understanding the evolution of stars and galaxies. The nearest elliptical
galaxy is M32, at a distance of 770,000 parsec (2.5 million light years).
M32 is a companion to the spiral galaxy Andromeda. Our image of M32
is the first to directly image the stars responsible for the ultraviolet
light produced by elliptical galaxies. These stars are old, hot, helium-burning
stars (known as horizontal branch stars to astronomers). Surprisingly,
our image also shows a lack of stars from another old class of stars (known
as post-asymptotic giant branch stars). Stars in this second class can
create planetary nebulae (shells of illuminated gas surrounding the star);
they should be easily detectable in our image, but we see far fewer of
these stars than expected from stellar theory. The image shown on this
page was taken with the Space Telescope Imaging Spectrograph (STIS), an
instrument on the Hubble Space Telescope.
The image below shows how M32 appears in near-ultraviolet light.
Click here for a plot of the bandpass and the nuclear
spectrum of M32. The light detected by STIS is invisible to the human
eye, but these stars also emit light in the visible spectrum. To
the human eye, they would look very blue. The image below resolves
more than 8000 stars in the center of M32.
Click
here for a comparison of the light from
the Sun to the light from one of these old helium-burning stars.
The image:
A false-color JPEG file of the STIS near-UV M32 image is available
here.
Analysis:
The analysis of this image has been published in The
Astrophysical Journal:
``Detection and Photometry of Hot Horizontal Branch Stars in the Core
of M32.'' Brown, T. M., Bowers, C. W., Kimble, R. A., Sweigart, A. V.,
& Ferguson, H. C. 2000, The Astrophysical Journal, 532, 308.
The full paper is available from the Astro-Ph
archive, maintained at the Los Alamos National Laboratory. Click here
for our entry in the archive. It is also available from ApJ here
.
The full preprint (with black & white Figure 2) is also available
for download from this page. Click here for the
Postscript version and here for the PDF version.
For faster download of the Postscript, a compressed file is also available
here.
Further Information:
Our image is also available as a press release on the Space
Telescope Science Institute web site. This site includes other
images that show a wider field of view that includes Andromeda. Click
here
for the press release.
Contact: Thomas M. Brown
Email: tbrown@stsci.edu
Phone: 410-338-4902
Back to home page
