Friday, October 22, 2010


What is it?
The total amount of energy given out by a star oer second - The power output, is a star's Luminousity. This luminousity measured accross all the wavelengths (remember e = hv) then it is called Bolometric Luminousity. Luminousity can also be measured over visible wavelengths alone or over x-ray wavelength.

How to measure it?
An early method of measuring the luminosity(L) of the sun was to put an oil drop on a piece of paper, so that it was translucent and then hold it to the sun. Also a light source of known Wattage is placed on the other side, and it is set at a distance from the light source so that the oil droplet and the surrounding paper are observed to have the same brightness. Then the flux from the sun is roughly equal to the flux from the light source.

F = L/4pd2

Knowing the ratio between the distances, can tell us how much brighter the sun actually is than the light source. The sophistication of measurement has increased since then but hte principle is the same. Greater the distance from the source, the flus decreases as it spreads over a larger area. So if one knows the distance and the flux, you can determine the luminousity of the star.

Our Sun's luminousity (L sol) is 3.84 × 1026 W. Any star's luminousity is generally specified with respect to that of the sun.

Dependent factors:
Temperature and Size determine the Luminousity of the star.

A black body radiates power based on its Temperature. Hotter the body, higher the power radiated. Temperature and power are related as below.

P proportional to sT4

If two stars have the same temperature but one of the stars is bigger, then it surely is more Luminous than the other. The surface area increses, increasing the Flux. Assuming spherical surface for stars we get,

L proportional to 4pr2

So together L proportional to 4pr2sT4  where is Boltzmann's constant ( 5.67 × 10-8 W m-2).

Thursday, October 14, 2010


Oh be a nice girl, kiss me!

And no, I am not talking gibberish. This aint no slander. This is how stars are 'rated'.
Stars, like we speculated, are in fact classified based on their spectra and temperature.

Lets say,the colour of the star is based on the temperature. The temperature on the star decides what elements are present. The elements present shall absorb/emit radiations. These elements are determined by analysing the spectrum. The width of the spectrum decides the classification.

The spectral classes are O B A N G K M. And the first line of this post is a mnemonic to remember this. Take a look at this data courtesy the

There is a chart called the Hertzsprung - Russell diagram. On this the stars are marked based on Spectral type, surface temperature, luminosity and absolute magnitude.


 Phew! too much funda? Well, shining as a star is no easy job right?
Lets explore each of these concepts in the next post.

Sunday, October 10, 2010

How to grade a star?

We have the habit of rating all restaurants with "Stars". Ever wondered how one rates a star?
The brightness of a star seems a likely choice for the rating.

In any star map the constellations are marked and the stars are generally denoted with Greek letters. It is based on the brightness of that star in that constellation. In any constellation, the brightest star is labelled alpha, the second bright star is named beta so on and so forth. Suppose the constellation has all the stars of equal brightness, like Ursa Major, then the naming begins from the head of the constellation.

This is with respect to constellations. But how is the brightness of any star measured?
Some questions that haunt me at this time:
Is the brightness of a star dependent on its temperature?
Would the distance of the star from the observing point determine its brightness?
Would the composition of the star contribute?
How exactly do we measure the brightness of a star?

Will be back soon :)

Saturday, October 9, 2010

Star Gazing

Come out on the terrace at night and look above. I am sure its fascinated most of us to watch those mute  stars twinkling away through the night. To some of us its the curiousity to discover whats beyond our planet. This is an ode to all those star gazers.

If you observed the night sky on a regular basis for months you would easily identify the different stars. As a matter of convinience people of ancient age imagined these stars as patterns or shapes spread across the sky. So it became easier to spot a pattern in the sky as the stars moved across the sky over months. And stories were spun around these star patterns. Its interesting to note that the myth and lore and even the star patterns different in different cultures. The Chinese have 28 Xu (mansions) instead of our 12 Zodiac constellations.

The origin of the zodiacal constellations was the apparent motion of Sun in each of the constellations over the 12 months. Thus, 12 constellations. These patterns comprise of stars of different magnitude and situated at varying distances. All that suffices for us to form those patterns is how they appear to us when we stand on our terrace :). So dont be fooled by a bright star.

The easiest to spot, cause of the many bright stars in definite pattern, are
1. Scorpius, the Scorpion with a distinct arc and a long tail
2. Orion, the Hun with the 3 stars which form the knee of the hun a faint bow in the imaginary outstretched arms. Two prominent stars - Betegeuse and Rigel
3. Ursa major in the northern sky. A seven star in a kite pattern. In the Indian mythology its the famous Saptha Rishi Mandala.
4. The brightest star in the sky - Sirius. It is part of the constellation called Canis Major, the big Dog.

Happy star gazing. We'l dwell more into the concepts in subsequent posts.
Orion (Image Courtesy:

Scorpius(Image Courtesy:

Canis Major (Image Courtesy:

Ursa Major (Image Courtesy:

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