On the strong 5577Å spectrum line

The above is a plot of the Wavelength(in Å) in the x-axis vs the flux of some objects from the Sloan digital survey ( consists of galaxies, young stars, Quasars, etc)

But  there is one strong peak in all of those plots that seems to stand out: 5577 Å


And if you like, the color that it represents is the above  (Made with Stanford’s color matcher app)

A nightmare for the astronomer


This line at 5577.338 is what astronomers refer to as a ‘skyline emission’ or a ‘mesospheric night-glow’ and arises from the recombination of atomic oxygen in the mesosphere.[2]



This line is of no significance to an astronomer who is looking to find out properties about a far away astronomical object. Yet, this line pops up in every spectrum of any object that you look at in outerspace!

In addition since the line is so strong, it contaminates the nearby pixels making the nearby data unusable and also messes up the scaling of the plot.


Example of contaminated pixel columns in an image because of bright object

Wavelength Calibration

What do you do with something that is always there but has no use for you? – Re-purpose it!


Having noticed that this peak was consistent at 5577.338, Astronomers decided that they would use this peak line in the data as a reference to calibrate their actual data. (known as ‘zero-point correction’).

This ensures that all the spectrum lines in the data are aligned and any errors that might have occurred during observation are corrected for.

Other lines ?

There are other lines at 6300,6363, etc which are sometimes as bright or brighter than the 5577 line that are also used for calibration.

If you are interested in learning more, the following are three papers that this post was inspired from and they dive deeper into more technical details that underlie this fascinating topic:

[1] Night-Sky High-Resolution Spectral Atlas of OH and O2 Emission Lines for Echelle Spectrograph Wavelength Calibration

[2] Mesospheric nightglow spectral survey taken by the ISO Spectral Spatial Imager on ATLAS 1

[3] Variability of the mesospheric nightglow sodium D2/D1 ratio

Have a great day!

Understanding Pressure

Sometime back, I was asked to explain the concept of pressure to
someone who had a tough time understanding it. And here is an account of
how that went:

The Books Analogy


Consider the layers of fluid stacked on top of each other to be represented by books instead of fluid.

Now, what does high pressure represent?
Well, it just means that there are many books on stacked on top of each other.
And what about the velocity?

you apply a force and try to move this stack of books. Velocity is
nothing but how much this stack has moved in 1 second.

What does low pressure represent according to the books analogy?
means that there are less books stacked on top of each other.


So, with
the same force you applied for the first case, you would be able to move
the books by a larger distance in one second. Hence it would have
higher velocity.

Shock Waves

One could relate this analogy to last week’s posts on shock-waves as well.

You can think of shock waves as these set of big books hurtled at supersonic speeds. And when they crash into objects, they break.


Subtle eh ?

The nuance of life is that we are constantly subjected to atmospheric pressure and yet we are oblivious to it.

Its profound because 1 Bar is considerable ( for instance, it can crush containers  ). But yet here we are 🙂


Gravity and Atmospheric pressure


Standard Atmospheric Pressure is a product of gravity and the density and thickness of the gas in the atmosphere.

Earth, at sea level, the weight of a one square inch column of air from
the surface to the top of the atmosphere, (the tropopause, about 36,000
feet) is  14.7 pounds, or one bar, which is standard air pressure.

gravity had a higher value the atmospheric pressure would be
correspondingly higher because that column of air would weigh heavier.



pressure on the top of Everest is much lower due to the considerably
shorter column of air above the summit, (plus the fact of  slight
decrease in gravity, about 0.5%).

Atmospheric pressure on Venus is 33 bar or 33 times higher than on Earth.
This is due to the higher gravity and the depth and density of the atmospheric gas.

( Source Credit: Tony Vincent )

Hope you enjoyed this post . Have a great day!

Mercury is Cool


But Venus is Hot ; )


Puns aside, Venus is indeed hotter than Mercury… ( Not Kidding )

Wait! What are you saying?


This might be counter intuitive considering that mercury is very close to the sun. The planet’s atmosphere has a thing to say about the temperature.

The Green House Effect

Mercury is closer but
because it has a very thin or no atmosphere at
all the heat goes out into space.


Venus on the
other hand with it’s much thicker atmosphere ( made mostly of carbon dioxide – a greenhouse gas )
holds all the heat it gets. This makes Venus hotter than Mercury!

And this trapping of heat by the atmosphere is known as Greenhouse effect.


How can heat enter the planet, but NOT leave it?

Radiation from the Sun is very broad band or white, also stretching
well into the UV. 


The Absorption Spectrum of the sun

Some light is deflected by the atmosphere (
via scattering ), some light absorbed by objects on the surface of
earth ( causes heating ) and the rest reflected back into space


A greenhouse gas does not absorb all thermal radiation, it absorbs only
certain wavelengths.
( For instance Carbon dioxide absorbs radiation of 4.26,7.52 and 14.99 micrometers (microns) ).


The light energy absorbed on the surface results in heating —-> results in light emissions at lower wavelengths  —-> gets absorbed by the greenhouse gases —-> Green House Effect

Now, that’s why heat (light) can enter the atmosphere but not leave it!

Have a good day 🙂

PC: webchutney, writerscafe, tumblr

Mars: Red Planet, Blue sunset?

Mars has always been an interesting planet to us earthlings. The possibility of life, rovers leaving no stone unturned(literally), it’s demanding reddish appearance and now those breathtaking sunsets.Mesmerizing isn’t it ? But,

Why are martian sunsets blue?


Here on earth, sunsets are bright with Yellow, Orange and Red colors dazzling in the sky. During sunsets, the light from the sun has to travel a longer distance in our atmosphere to reach the earth.

Consequently, all the blue and violet light is scattered( thrown in various directions) by the particles in our atmosphere leaving behind only shades of yellow, orange and red, which is what you see. This phenomenon is known as Rayleigh scattering.


On mars, the reverse effect occurs. The martian dust is smaller and more abundant than on earth and it incidentally happens to be just the right size that it absorbs the blue light whilst scattering the red ones across the sky. This makes martian sunsets blue :).

Stay tuned, there is more space stuff coming your way.

( Source: http://io9.com/5906367/why-are-martian-sunsets-blue