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You too can measure a star!!


Study of STARS

            How do you measure the size of a star? How do you estimate it’s mass? How do you know hoe distant it is? When you are asked how distant is place-A from place-B (assuming these two places are on Earth), you same some x km. When you are asked, how do you know that, you try to say that I measured it when I traveled by bike or you can say any measuring technique including Geo-mapping. But, you have never been to a star, nor did any machine, nor anything that we know. Still, how are we so certain about the parameters of a star? Read on to find out...

DISTANCE AND LUMINOSITY:

            When you look at a star, what you see is the apparent brightness of a star rather than its absolute brightness. This apparent brightness is measured in terms of magnitude. The scale is same as that of old catalogs which assigned 1st magnitude to the brightest and sixth magnitude to the just visible. At any point, a given difference in magnitude corresponds to definite ratio in brightness. Suppose, some star X is 5 magnitude times brighter star Y, it is said to be 100 times brighter than star Y. If any star is brighter than 1st magnitude, a negative value is assigned to it. For example, Sirius has -1.52 and Sun has -26.72 magnitude of brightness.
 Sun is apparently bright, but not brighter than many other stars which are very distant.
            What is the brightest star in the sky? Well, it depends on whether you are talking about absolute or apparent brightness. Sun is apparently bright, but not brighter than many other stars which are very distant. As they are very far, the light is getting faded and absolute brightness cannot be seen as such. 
HertzsprungRussel diagram can be used to classify stars based on their luminosity and temperature.
            Enough about brightness! How to measure the distance of star from earth? We will discuss the popular method of parallax, using which we can find the distance of about 4000 stars which are not too far from earth.  
Parallax is a method used to determine the distance to the closest stars. This technique for measuring astronomical distances is very important because it is a geometric method and independent of the object being observed. The principal of parallax can easily be demonstrated by holding your finger up at arm’s length. Close one eye, then the other and notice how your finger appears to move in relation to the background. This occurs because each eye sees a slightly different view because they are separated be a few inches.
If you measure the distance between your eyes and the distance your finger appears to move, then you can calculate the length of your arm. This same principal can be used on a larger scale to calculate the distance to an object in the sky, only we use different points on the Earth’s orbit instead of looking through alternate eyes. Calculating parallax requires that the objects right ascension and declination be recorded accurately so that we know the objects precise location on the celestial sphere.

TEMPERATURE, SIZE and MASS:

Now that we know the distance, apparent brightness (as seen by the naked eye) we can find the absolute magnitude by fixing a standard distance at which it is defined. It is chosen to be 10 parsecs. (Parsec=parallax + seconds).Parsec precisely means the distance at which a star makes one second parallax. i.e., 1/1296000 part of a circle. At this distance, sun has 4.86 magnitudes.
Parsec precisely means the distance at which a star makes one second parallax
As we can observe the color of the star can be seen, we can find out its temperature. Once the temperature is known, intensity of radiation can be found out as it varies as the fourth power of temperature. Absolute magnitude found out earlier is used to find how much energy is being emitted per square unit per second. The ratio of these two gives the surface area of the star and hence we can find diameter of the star.
Mass of the start can be found out, in most cases, only when there is any significantly large heavenly body in the vicinity. The mass of the star varies in direct proportion with the absolute magnitude. This is also called mass-luminosity law.
Now that you know how we can find the details of a star which is demonically distant.






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