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Although visible matter in form of stars contains only a small part of
the total matter in the universe, stars are of utmost importance for
life on Earth. The elements that Earth and other planets consist of are
the ashes of exploded stars. Moreover, the Sun is the energy source
for all life on Earth. For the understanding of the development of
the Solar System in particular and galaxies in general, it is of great
interestto understand how and where stars form.
From observations we know that star formation happens in different
ways. Either many stars form in a short time, e.g. in a collision of
galaxies, or stars form in a rather slow, continuous process like
in the Milky Way. We call star fomration rate the mass of gas and dust
that is converted into stars per year. This is measured in solar masses
per year.
The measurement of the star formation rate is very difficult. Star
formation takes place in dense gas and dust clouds that strongly hamper
observations, see Fig. 1. All measurements of star formation also
rely on not well understood details of the star formation process.
Scientist at the Max-Planck-Institute for Astrophysics now suggest to
use a particular type of double star system to meassure the star
formation rate. These double star systems are consist of a massive
star, at least 10 times more massive than the Sun, and a so-called
compact object, a neutron star or black hole. The neutron star or
black holetakes away matter form the normal star due to its strong
gravitational attraction. When the amtter falls onto the compact
object it heats up very strongly and eventually sends out X-radiation.
The X-radiation of these double star systems can nowadays be measured
by satellites even in other galaxies. An example is given in Fig. 2,
the upper panel shows an optical light image of the galaxy M 83,
and the lower panel shows an X-ray image of the same region. One can
clearly distinguish the single X-ray binaries.
These particular double star systems are well suited to measure the
star formation rate since their lifetime is very short: The more
massive a star is the shorter it lives. Therefore one obsreves these
systems in regions of active star formation. This new method has
several advantages. X-rays penetrate easily the dense gas and dust
clouds. Moreover, this method is independent of the details of the
star formation process. And also the conditions to create these
systems are hard to fulfill so that they are relatively scarce. This
makes possible their observation even at large distances without
confusion due to the number of systems.
Based on the analysis of data from the Chandra X-ray telescope a
research group at Max-Planck-Institute for Astrophysics has shown
that the number and brightness of these double star systems are well
suited to measure the star fomration rate in galaxies. For that the
X-ray emission of galaxies was comapred with other measurements of
star formation rate as shown in Fig. 3. There is a good agreement
between the brightness and the star formation rate even for distant
galaxies inthe socalled Hubble Deep Field (HDF) when the universe was
only about half as old as now.
This method allows now to measure the star formation rate independently.
Moreover, in the future it might be possible to make more precise
statements about star formation since the existence of these
particular double star systems depends strongly on the conditions of
star formation.
Hans-Jakob Grimm
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