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Astronomy Club
Science and Technology Council
Students’ Gymkhana
Indian Institute of Technology, Kanpur
Presents
CYGNUS
The Wall Magazine
2002-2003 III
Club Coordinators
1. Amar V. Chandra
2. Ashutosh Kumar Gupta
3. Jaya Jha
Other Contributors
1. Dr. Pankaj Jain (Faculty Advisor, Astronomy Club)
2. Vinay Mishra
3. Prateek Jain
4. Archisman Ghosh
Cover Story
Active Galactic Nuclei
Active Galactic Nuclei (AGN) are some of the most powerful objects seen in the sky. As the
name suggests, the centers of these galaxies show considerable activity in constract to the milky
way whose center is quiet. AGNs are categorized into four broad classes, namely, Seyfert
galaxies, Radio galaxies, Quasars and Blazars. A significant fraction of these objects, particularly
the quasars, are seen at redshifts larger than 1. By Hubble's law, such large resdhifts translate
into distances of order 1 Gpc (Giga parsec). This distance may be compared with the diameter of
the disc of the milky way which is about 30 kpc and the size of the universe which is about 10
Gpc.
The bolometric (integrated over wavelength) luminosity of a typical quasar is approximately 1000
times greater than that of the milky way. This is particularly impressive if one realizes that this
mission comes dominantly from a region about 10 A. U. (Astronomical Units), which is about 8
orders of magnitude smaller than the disc diameter of the milky way. Due to this large luminosity,
quasars appear as star like objects despite being so far away from us and hence the name Quasi
Stellar Objects (QSOs) or Quasars. It is interesting to note that, in contrast to stars, the dominant
source of energy in AGNs is not nuclear fusion. Instead the energy is generated by the release of
gravitational potential energy due to fall of material towards the core of the AGN. The size and
mass of the emission region suggests that a very massive black hole, with mass of the order of
108 solar masses resides in the center of the AGN and is responsible for the release of the large
amount of gravitational potential energy.
The AGNs, therefore consist of a core or the central engine surrounded by an accretion disk
through which matter is continuously falling towards the black hole. Furthermore they emit jets of
relativistic particles, somewhat analogous to the jets emitted by pulsars. If the central engine is
visible to the observer then the object appears as a quasar. If the jet directly beams towards the
observer then the object is called a blazar. The Seyfert galaxies are somewhat less active
compared to other AGNs. They appear as Seyfert I or Seyfert II depending on whether the central
engine is directly visible to the observer or hidden from view by the accretion disk. The radio
emitting region of AGNs is called the radio galaxy.
Another remarkable fact about the AGNs is their spectrum, which is characteristically non
thermal. The spectrum ranges all the way from radio waves to x-rays and higher frequencies. The
radio waves are emitted in the form of synchrotron radiation when jets of charged particles
emitted by the AGN interact with the surrounding nebula. Some picture of radio galaxies and
quasars in radio frequencies are shown below.
Figure 1
Figure 2
Figure 3
Figure 4: AGN Model
Dr. Pankaj Jain, Professor, Department of Physics
Faculty Advisor, Astronomy Club
On the lighter side
Quotes Not to be Quoted 
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Two most common things in the world are Hydrogen and Stupidity. (Harlan Ellison)
Tagline: New restaurant on Moon. Great food, No atmosphere
But you can quote this one:The true purpose of astronomy is not to add to the vulgar comforts of
life, but to raise the mind to the contemplation of things which can be perceived by pure intellect
alone (Plato 427-347 BC)
Quiz Corner
Find the answers to the following questions and post on
iitk.gymkhana. The first person posting all correct answers will get a
prize.
1.
2.
3.
4.
5.
Sometimes why do we see a full disc of moon on a half moon day?
What are auroras(The lighting that is usually seen near poles)?
Why can we see sunlight for over 20 minutes after the sun-set?
Why is Venus hotter than Mercury, though the later is closer to the sun?
Why is the colour of a fully eclipsed moon red?
Something more
You might be an astronomer...
1. If you wear sunglasses all the time - even indoors because you don't want to lose your
night vision.
2. If, in your opinion, the Earth is merely an unfortunate obstacle which blocks half of the
universe at given time.
3. If you enjoy sleep deprivation.
4. If you celebrated in February 1999, when Pluto became the "ninth" planet again.
5. If you yearn to destroy the moon - not only because of the light pollution it causes, but so
that Earth could have a lovely ring system like all the "really cool" planets.
6. If you've proposed a NASA mission to "correct" the retrograde orbit of Triton.
7. If you have nightmares about the giant black hole that may be lurking in the center of our
galaxy.
8. If you're looking forward to Vega becoming our new north star 13,000 years from now.
9. If, after reading the previous indicator, you checked to make sure the math was right.
10. If you've had a fist-fight over the value of the Hubble Constant.
11. If you'd prefer to live in the Andromeda Galaxy - just because it's bigger.
12. If you actually care whether or not the universe is "flat."
13. If, when people ask where you live, you give them a latitude and longitude to five decimal
places.
14. If you know how to use a planisphere, but can't fold a road map.
15. If, when someone asks you what time it is, you stare up at the sun and estimate the time
based on its position... despite the fact that you are wearing a watch.
16. If you'll use an argument that is physically unjustifiable and mathematically erroneous as
long as it gives the "right answer."
17. If you begin to classify people as "elliptical", "spiral", or "irregular."
18. If you have the Hubble Deep Field as the background for your web page or Windows
desktop.
19. If your self-esteem is derived entirely from the number of deep sky objects you can find
without using coordinates.
20. If, when you drop your keys behind your chair, you begin to look for them under your bed
because it would be "more interesting" if you found them there.
21. If your eyes have become hypersensitive due to lack of exposure to sunlight.
22. If you've never bothered to memorize your own middle name because you know you can
either look it up, or derive it from first principles if necessary.
23. If you actually understood a majority of these indicators.
If these indicators apply to you, you just might be an astronomer. You may want to seek
professional help. I hope this clears up any confusion.
Understand the Universe
Radio Recombination Lines
Astronomers have always tried to understand the evolvement of stars,Galaxies and Universe
itself.Stars are born in very thin medium that is spread across the Universe i.e. Interstellar
Medium. So if one knows the properties and compsition of interstellar medium in the nearby
region of a star then it is quite handy to predict the nature of distant stars .One of the most
efficient methods that is used by the astronomers is "Radio Recombination Lines".
The principle behind the origin of these lines is simple,a hot luminous object ionizes hydrogen
(that is most abundant in interstellar medium) in its nearby region and creates a combined state
of electrons and hydrogen nucleus (i.e. protons).These regions are called HII regions(for H+
conventionally). Due to the tendency to recombine, e- and p+ interact and electron get combined
in some higher state of hydrogen and cascades down slowly towards ground state in hydrogen
atom.It so happens that if the principal quantum no. n >=40 then the frequency emitted by the
electrons fall in the Radio region, which we can observe, Thats why it is called Radio
Recombination Lines.
The existence of such lines was first predicted by Kardashev in 1959 and in 1965 it was
experimentally verified.RRLs are used in studies because of no dust oscuration in transmission of
these lines ,it is the best way so far to study extragalagtic objects and even for the studies relating
to our own galactic center. Since all HII regions RRL fall in nearly LTE (local thermodynamic
equlibrium) conditions so it is easy to adopt methods which are available.
There are RRLs for other atoms also such as for Carbon,Helium etc.
The best use of RRLs for future reserch possibly is to study RRLs in Sun's photosphere region
and High Red Shift Quasars which will solve the mysterious problem of "How our Universe
evolved?"
Amazing Facts
Do you know
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What would happen to an astronaut if he tripped and fell into a black hole? He would be
stretched like a piece of spaghetti! The closer he gets to the black hole, the stronger the
force of gravity would be. In fact, the pull of gravity increases so fast, that the gravity tugs
on his feet more than on his shoulders. With his feet being pulled into the black hole
faster than her shoulders, he would be stretched out farther and farther.
That a massive body 100km wide traveling at over 512,000km/h crashed into Mercury to
form the Caloris Basin *. The impact was so great it sent shockwaves round Mercury
creating its hilly lineated terrain
That a Supernova explosion produces more energy in its first ten seconds than the Sun
during the whole of its 10 billion year lifetime and that for a brief period, it creates more
energy than the rest of a galaxy put together!!
*Caloris Basin is the largest surface feature on Mercury. Caloris is Latin for heat, and the basin is
named so because it is near the sub solar point (the point closest to the sun), when Mercury is at
perihelion (the closest point in its orbit to the sun).