Episode 22: Beyond Visible
Dr. Arvind C. Ranade
Points to be covered:
- Infrared astronomy
- Radio stars and radio astronomy
- Radio telescopes, Jodrell Bank, Ooty, GMRT, VLA
- 1932 Karl Jansky discovers radio noise from Milky Way Galaxy
- Discovery of pulsars and quasars using radio waves
- Cosmic background radiation
- Indian Radio Astronomy Programme
Emphasize and comments:
- IR astronomy vital to understand how stars form (1960)
- Ground-based IR observations from Gurushikhar, Hanle and other observatories
- Basics of detection of radio waves
- Radio antennas
- Jansky’s discovery
- Groto Reber’s dish antenna
- Radio ‘images’ of the sky
Outline of content:
It may not be wrong to say that Newton showed us that the white light is a combination
of all seven colors. The experiment which he performed in 1666 that, the sunlight passing
through the prism showed seven colors from red to violet. Probably that was the time
from where the actual science had started to take the shape of modern and true science.
When Newton discovered the seven colors or even after him William Herschel and
Johann Ritter who discovered Infrared and Ultra violet rays respectively were not aware
that the kind of radiations they have discovered are part of electromagnetic radiations.
The great physicist James Clark Maxwell in mid of nineties has studied the basics laws of
physics given by Ampere, Coulomb and Faraday and come up with his own equations
known as Maxwell’s equations. These four equations have not only changed the physics
but also forced physicist to open a new branch named electrodynamics. After solving
these equations under different conditions have gave the wonderful name of resulting
radiations as a electromagnetic radiations. Latter on it is proved that the white light
through which we see the world today and that time newly discovered rays i.e. infrared
and ultra violet are also the part of these radiations. However, it is also true that the
radiations discovered after about four to five decades; X-rays and Gamma rays were not
known to the scientists until it was discovered by Wilhelm Roentgen and Becquerel or
Curie couple. It is also known to us that the radiations discovered by the Heinrich Hertzradio waves were actually discovered on the paper by using the Maxwell’s equations.
As seen, the light is merely one form of radiation called electromagnetic radiation,
because, it is associated with changing electric and magnetic fields that travel through
space and transfer energy from one place to another. Electromagnetic radiation is a wave
phenomenon. It is associated with a periodically repeating disturbance, or wave. We are
familiar with waves spread across the surface. Imagine that we use a meter stick to
measure the distance between the successive peaks of a wave. This distance is the
wavelength, usually represented by Greek letter lambda (λ). If we were measuring ripples
in a pond, we might find that the wavelength is a few centimeters, whereas the
wavelength of ocean waves might be hundred meters or more. There is no restriction on
the wavelength of electromagnetic radiation. Wavelengths can range from smaller than
the diameter of an atom to larger than that of Earth. It is measured either in nanometer
(nm) equivalent to 10 million waves in a centimeter, micrometer (micron) equivalent to
10 thousand waves in a centimeter or angstrom equivalent to 100 million waves in a
centimeter.
The very important feature of this radiation is that whatsoever is the
wavelength of the radiation it moves with constant speed that is 300000 km per second.
Similar to wavelength, another parameter associated with such radiation is frequency i.e.
number of consecutive cycles that pass in one second. The relation between wavelength
and frequency is exactly inverse i.e. the radiation which has low wavelength will have
high frequency and high wavelength radiation will have low frequency.
According to the wavelength or frequency range, if we arrange these radiations on the
wide screen then the kind of array which will be formed is known was electromagnetic
spectrum. If we put these radiations with increasing wavelength from left to right then the
extreme left radiations which are less than 0.01 nm are called as Gamma rays. These are
extremely high energy radiations and often referred in terms of mega and kilo electron
volts (Gev and Mev). The objects with extremely high temperature, high density, and
high magnetism give peak radiations in gamma rays. The examples include gamma-ray
bursts, supernovae, pulsars, black holes etc. Because of thick ozone layer present in the
Earth’s atmosphere, these radiations do not reach on the surface. Therefore, the only
possibility left to study such radiations is to observe these radiations from the space
telescopes orbiting the earth. Even through the telescope, we don’t understand gamma
rays well because they are so difficult to detect. First, they can not be focused or detected
as like light rays can and secondly they are so high energy photons any normal detectors
not sustain for such energy. There are more than decade of Gamma Ray Telescopes
available for the astronomy research but some of the well know can be mentioned here.
Currently, International Gamma-Ray Astrophysics Laboratory, (INTEGRAL), and the
Gamma-ray Large Area Space Telescope (GLAST) are remarkable. INTEGRAL is an
ESA mission with additional contributions from Czech, Poland, USA and Russia. It was
launched on 17 October 2002. NASA launched GLAST on 11 June 2008. In includes
LAT, the Large Area Telescope, and GBM, the GLAST Burst Monitor, for studying
gamma-ray bursts.
The radiations next to Gamma rays are called X-ray radiations. The radiations falls in
between 0.01nm to 10nm range are X-ray radiations and are also high energy radiations
and hence also known in terms of Kilo Electron Volt (Kev). The high temperature and
high energy objects like, Active Galactic Nuclei (AGN), Cataclysmic Variables, black
holes, white dwarfs etc. are examples of objects that emit most of their radiation in Xrays. These radiations too can not reach on the surface of earth. Therefore the best way to
study these radiations is through the reaching into the upper atmosphere through balloons
or through space base telescopes. The satellites in use today includes, the XMM-Newton
observatory (low to mid energy X-rays 0.1-15 keV) and the INTEGRAL satellite (high
energy X-rays 15-60 keV), and both were launched by the European Space Agency.
NASA has launched the Rossi X-ray Timing Explorer (RXTE), and the Swift and
Chandra observatories. One of the instruments on Swift is the Swift X-Ray Telescope
(XRT). SMART-1 contained an X-ray telescope for mapping lunar X-ray fluorescence.
Past observatories included ROSAT, the Einstein Observatory, the ASCA observatory
and BeppoSAX. The X-ray observatory - Chandra is named after the Indian born
astronomer Prof. S. Chandrashekhar