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             Information from the European Southern Observatory

ESO Press Release 13/03

10 June 2003                                               [ESO Logo]

For immediate release
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One Thousand "Wonderful" Stars Discovered in Centaurus A

Part 2 of 2

Confirmation of the presence of an intermediate-age population

Based on earlier research (optical and near-IR colour-magnitude
diagrams of the stars in the fields), the present team of astronomers
had previously detected the presence of intermediate-age and young
stellar populations in the halo of this galaxy. The youngest stars
appear to be aligned with the powerful jet produced by the massive
black hole at the centre.

Some of the very luminous red variable stars now discovered confirm
the presence of a population of intermediate-age stars in the halo of
this galaxy. It also contributes to our understanding of how giant
elliptical galaxies form.

New measurement of the distance to Centaurus A

The pulsation of Mira-type variable stars obeys a period-luminosity
relation. The longer its period, the more luminous is a Mira-type
star.

This fact makes it possible to use Mira-type stars as "standard
candles" (objects of known intrinsic luminosity) for distance
determinations. They have in fact often been used in this way to
measure accurate distances to more nearby objects, e.g., to
individual clusters of stars and to the center in our Milky Way
galaxy, and also to galaxies in the Local Group, in particular the
Magellanic Clouds. 

This method works particularly well with infrared measurements and
the astronomers were now able to measure the distance to Centaurus A
in this new way. They found 13.7 +- 1.9 million light-years, in
general agreement with and thus confirming other methods.

Study of stellar population gradients in the halo of a giant
elliptical galaxy

The two fields here studied contain different populations of stars. A
clear dependence on the location (a "gradient") within the galaxy is
observed, which can be due to differences in chemical composition or
age, or to a combination of both.

Understanding the cause of this gradient will provide additional
clues to how Centaurus A - and indeed all giant elliptical galaxies -
was formed and has since evolved.

Comparison with other well-known nearby galaxies

Past searches have discovered Mira-type variable stars thoughout the
Milky Way, our home galaxy, and in other nearby galaxies in the Local
Group.  However, there are no giant elliptical galaxies like
Centaurus A in the Local Group and this is the first time it has been
possible to identify this kind of stars in that type of galaxy.

The present investigation now opens a new window towards studies of
the stellar constituents of such galaxies.

More information

The research described in this press release is presented in a
research article, soon to appear in the European research journal
Astronomy & Astrophysics ("Long Period Variables in NGC 5128:
I. Catalogue" by Marina Rejkuba et al.). It is available on the web
as astro-ph/0305432.

Notes

[1] The team consists of Marina Rejkuba and David Silva(ESO-Garching,
Germany), Tim Bedding (School of Physics and Astronomy, Sydney
University, Australia) and Dante Minniti (Departamento de Astronomia
and FONDAP Center for Astrophysics, Pontifica Universidad Catolica de
Chile, Santiago, Chile).

[2] Mira (Omicron Ceti), a cool red giant star, is one of the most
famous variable stars in the sky. David Fabricius (East Friesland,
Germany) is recognized to have discovered it in 1596 while searching
for planet Mercury.  In 1638, Johann Holwarda from the same area
determined its period to be 11 months and so established it as the
first long-period variable discovered.  In 1642, Johannes Hevelius
(Danzig, now Poland) named the star Mira, "The Wonderful". It has
later been found to be a binary star system. An entire class of
variable stars has been named after this star. By astronomical
definition, "Mira-type" stars are variables with a period of around
100 - 1000 days and with visual light variations of more than 2.5
magnitudes.

Contact

Marina Rejkuba
ESO
Garching, Germany
Phone: +49 89 3200-6453
Email: mrejkuba{at}eso.org

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