And God said:
(i) ∇ · D = 4πρf (ii) ∇ · B = 0
(iii)∇ × E = − ∂B/ ∂t (iv)∇ × H = J+ ∂D/∂t
and then there was light.
Our Universe contains energy in various forms. One such form of light which we observe most often in our daily life is the electromagnetic radiation. Whenever a charge accelerates, it produces electric and magnetic fields which are perpendicular to and coupled with each other. As these electromagnetic waves transfer from one point in space to another, they transport energy along with them. Electromagnetic waves are further divided into different categories based on their frequencies. This distribution of different types of EM waves is called Electromagnetic Wave Spectrum. We are capable of detecting only a small portion of this spectrum. Everything that we see around us is a result of interaction of matter with EM waves lying in visible spectrum. In addition to EM waves found and produced here on earth, we also have astrophysical sources of EM waves which tell us a lot about our universe. In this article we will briefly summarize various astrophysical sources of these EM waves.
1. Radio Waves
In 1932, an American physicist and radio engineer Karl Jansky detected radio waves coming from an unknown source from the centre of the Milky Way Galaxy. This was the first time that radio waves were detected from outer space. Since then, astronomers have built better and better telescopes to find these cosmic radio waves and learn more about where they come from and what they can tell us about the universe.
1.1 Sources of Radio Waves
Nearly all types of astronomical objects give off some radio radiation, but the strongest sources of such emissions include pulsars, certain nebulas, quasars, and radio galaxies. The galactic center of the Milky Way was the first radio source to be detected. It contains a number of radio sources, including Sagittarius A, Sagittarius A*,the compact region around a supermassive black hole, as well as the black hole itself. Other galactic sources of radio emission include Supernova remnants such as Cassiopeia A (the brightest extra solar radio source in the sky) and the Crab Nebula and dense spinning neutron stars called pulsars (eg. the Crab Pulsar).
Extra-galactic sources such as Radio Galaxies (eg. Centaurus A and Messier 87) and quasars give off extraordinarily large amounts of radio waves and are typically 1,000,000 times more powerful than the nearby spiral galaxies. Merging galaxy clusters often show diffused radio emission.
At the beginning of time itself, all the energy in universe was present in a very tiny region. Then as time went by, various changes occurred which were categorized into various epochs. Then around 13.7 billion years ago it started expanding rapidly in all directions. This expansion released light in the form of X rays and was the starting of formation of various structures that evolved into present state of our universe. As the universe kept expanding, this light got stretched into microwave region. Today we find traces of this Cosmic microwave Background Radiation (CMB) coming uniformly from every direction which was first accidentally discovered by Penzias and Wilson in 1965. In a way the CMB can be treated as the remnants of the early universe. Microwaves lie in the long wavelength region of the electromagnetic spectrum and since they can’t penetrate earth’s atmosphere, we use space-based telescopes to observe them. Wilkinson Microwave Anisotropy Probe (WMAP) has played a crucial role in studying the CMB. It has been providing valuable data which has helped us create a picture of our universe and study not only it’s currents properties, but also discover more about it’s beginnings. In 2009 NASA in collaboration with European Space Agency launched a more advanced telescope: Planck which is expected to help us extract every possible information from this microwave afterglow.
3. Infrared Waves
Since the discovery of Infrared radiation by William Herschley in 1800, it has been used to understand the formation of various celestial objects. The wavelength of Infrared radiation ranges from 700nm-1mm. To detect this radiation, telescopes have been set up above the Earth’s atmosphere so that they can provide a higher resolution which isn’t possible below the atmosphere due to a block caused by Earth’s atmosphere. Hubble Space Telescope, James Webb Space Telescope (yet to be launched) are a few infrared telescopes orbiting in space.
3.1 Sources of Infrared Waves
All objects in our universe emit heat in varying degrees. The Sun emits heat in the form of Infrared radiation. Most of the galaxies appear dimmer than they are because the dust surrounding them absorbs visible light and re-emits it as infrared radiation. These radiations can pass through nebulae and Star clusters giving a clear image of stars and it’s formation. Telescopes equipped to detect these radiations led to the discovery of stars and distant galaxies which could have never been detected in the visible range. Few Star clusters emitting infrared Waves: 1.FS1424
Few Luminous Infrared Galaxies(LIRGs) :
2.II Zw 96
4. Ultra Violet Rays
In 1801, Johan William Ritter exposed photographic paper(Silver chloride) to the visible spectrum from red to violet and observed the increase in relative darkening of the paper. Ritter decided to go beyond the violet region to discover the highest darkening of the photographic paper, and he termed this region as “ultraviolet”. Its wavelength ranges from 10-400 nm, higher than that of the visible region. Even though it’s just beyond the visible region, our eyes can’t really detect them since they haven’t really been programmed that way due to evolution, the sun’s radiation spectrum peaking in visible light, and many other factors.
UV radiation can be classified as:
1.UV-A light(320-400 nm)
4.1 Sources of UV Rays
Astrophysical sources of UV rays include bodies with high temperatures such as supernovae remnants, the sun’s corona, white dwarfs, massive OB stars, clouds of gases in galaxy clusters. In fact, the formation of young stars can be detected by satellites in orbit around the earth using the UV light they emit as shown below: Also, the moon reflects UV radiation incident on it from distant stars which is captured by the Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter of NASA. This reflected radiation is used to study the moon. A neighboring planet also serves as a source of UV radiation when its Aurora forms as shown in the picture:
The sun accounts for only 10 percent of the UV radiation in the earth as most of the dangerous rays get absorbed in the ozone layer. Clearly it emits a healthy amount of light in the UV region as shown below: Fun Fact: Let’s say in a hypothetical world we’re able to travel at say half the speed of light. In that case, visible light emitted by objects get Doppler shifted to the visible spectrum’s violet end and UV region. In that world, our eyes would probably be able to perceive UV light too.
5. X Rays
From the early 1960s till now, from imaging the suns corona using its X-Ray data to discovering different Magnetars and Pulsars, astronomers have studied the incoming X-Ray signals from the cosmos and further improved their understanding about the Universe. The atmosphere of the Earth is thick, so most wavelengths of light are unable to come through namely Infrared, UV Rays and X-Rays. Thus, celestial objects emitting X-rays is carried out outside the altitudes greater than 100 km.
5.1 Sources of X Rays
Many Sources of X-Rays in the cosmos have been discovered other than the Sun, i.e., Neutrino Stars, Supernova, Black Holes, Active Galactic Nuclei and many more
5.1.1 Binary X-ray Sources
A binary system containing a normal star and a compact star, which is either a neutron star or a black hole, is called a X-Ray Binary System. When gas from the normal star falls toward the compact star, the gas swirls round the compact star making an accretion disk around it. The orbital energy of the gas is then converted into heat due to some viscous processes in the disk, and after attaining sufficiently high temperatures, X rays are emitted.
5.1.2 Galaxies and Active Galactic Nuclei
Normally, galaxies emit X-rays because of main sequence stars, neutron stars, binary X-ray sources, supernova remnants and diffuse gas inside it. Like our own galaxy, most galaxies have massive black holes at their centers. As the black holes grow by accretion from its surrounding, their X-ray emission tends to increase multifold and they dominate the total X-Ray emission of the galaxy. These types of Galaxies are known as active galaxies(AGNs).
Some Examples of Cosmological X-Ray Sources:
1. Crab Nebula
2. Cygnus X-1(X-Ray Binary)
3. Abell 370(Galaxy Cluster)
6. Gamma Rays
Gamma rays have the highest energy, frequency and the shortest wavelength in all of the electromagnetic spectrum. These powerful rays are produced by the most energetic and hottest objects and/or events in universe such as supernova explosions, neutron stars, super massive black holes and pulsars. Sources of gamma rays on earth include emissions from radioactive decay, lightning, nuclear explosions, nuclear reactions such as fusion, fission, alpha decay, gamma decay.
6.1 Sources of Gamma Rays
Excess energy dissipated by unstable atomic nuclei leads to emission of gamma rays. Some of the gamma rays are generated by transient events such as supernovae and solar flares. Others are produced by steady sources like the super massive black holes. Another important source of gamma rays are the Gamma Ray Bursts. These are immensely energetic explosions observed in the universe. They can last from fraction of a second to several minutes. Long duration bursts are rare, they produce very high energy in a period of 20-40 seconds. Short gamma ray bursts are of less than two seconds and not related to supernovae. One of the biggest producers of gamma rays are a neutron star pair collision or a neutron and black hole collision.
Highly contributing sources:
In Milky Way Galaxy:
1. The Crab Nebula
3. V407 Cygni
4.Pulsar PSR J0101-6422
5. 2FGL J0359.5+5410
Beyond Milky Way are:
1. Centaurus A
2.The Andromeda galaxy
3.The Cigar Galaxy
4. Blazar PKS 0537-286
5. 2FGL J1305.0+1152
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