2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

THE PRESENT REVOLUTION IN ASTRONOMY
do every 25 years or so just in our own Galaxy! It
is important for several reasons to understand the
underlying mechanism in detail. For example,
observations show that neutron stars get large
“kicks” at birth. The evidence for this comes from
the observed space velocities of neutron stars. If
this is so, then it is telling us something about the
mechanism of the explosion itself.
The investigation of supernovae – both theoretical
and observational – is important for another
reason. While the lighter elements were
synthesized when the Universe was very young
(roughly three minutes after the beginning), the
heavier elements cannot be synthesized this way.
It is widely believed that they are synthesized in
stars. To verify these predictions in detail, one will
have to study the spectrum of radiation from the
ejecta of the explosion. Since the ejecta will be at
a temperature of tens to hundreds of millions of
degrees, one will need to do a spectroscopic
study of the X-rays emitted by the ejecta. The
recently launched giant X-ray observatory
CHANDRA has been able to produce spectacular
images of supernova debris in the various X-ray
emission lines. These images will enable one to
analyze both the composition, as well as the
spatial distribution of the elements with
unprecedented accuracy.
The supernovae referred to above are the
explosions of massive stars which give birth to
neutron stars and black holes. There is another
kind of supernova, known as Type Ia, which is
believed to arise when a binary system of white
dwarfs spiral in towards each other due to the
emission of gravitational radiation, and finally
coalesce explosively. Such supernovae are rarer.
But when they do occur, the enormous amount of
light they emit is as though from a standard
candle. This fact makes them enormously useful in
studying the distant Universe. It is using this
technique that one has been able to recently
conclude (although perhaps tentatively) that the
Universe is, in fact, accelerating.
The remnants of massive stars, namely neutron
stars and black holes, are also the subject of
intensive study. Although many of the properties
of neutron stars were elucidated long before their
discovery, and many of them verified, there are
still outstanding issues.
A major puzzle concerns the origin of gamma ray
bursts. When they were first discovered a little
over twenty years ago, they were thought to be
of galactic origin. But observations with the
Compton Gamma Ray Observatory soon showed
that their distribution was very isotropic. This
suggested that these were at cosmological
distances. This was soon confirmed by redshift
measurements of the host galaxy. The first
popular candidate was the coalescence of two
inspiralling neutron stars. Whatever the event may
be, it was pointed out that relativistic shock waves
must be involved. Some years ago, it was shown
unambiguously that one of the gamma ray bursts
was associated with a supernova explosion of a
star. More such examples have been found since
then. Although the burst of gamma rays lasts only
for a very short time, quite often there is an
afterglow. These afterglows have been carefully
studied at X-ray wavelength, visible wavelength
and, occasionally, at radio wavelength. Despite
intensive study, these extremely energetic gamma
ray bursts remain enigmatic.
The Origin of Galaxies
This question is at the base of the present
revolution in astronomy. And yet, just a few
decades ago this would have been considered as a
meaningless question.
The possibility that galaxies always existed must
surely be rejected if one accepts that the Universe
had a very hot beginning. At the temperatures
14