SHIKSHA FOR ALL

COMPOSITION OF INTERSTELLAR  MEDIUM The interstellar medium (ISM) makes up only 10 to 15 percent of the visible mass of  the Milky Way Galaxy. It comprises matter in the form of gas and dust (very tiny solid particles). About 99 percent of ISM is gas and the rest is dust. You may like to  know: Which elements are present in the ISM? An important clue for investigating the composition of the ISM is the fact that the birth and death of stars is a cyclic  process. This is so because a star is born out of ISM, and during its life, much of the  material of the star is returned back to the ISM by the process of stellar wind (in case  of the Sun, it is solar wind; see Unit 5) and other explosive events such as Nova and  Supernova. The material thrown back into the ISM may form the constituents of the  next generation of stars and so on. To know the basic composition of ISM,  astronomers use photographs and spectra. In the following discussion, we shallconfine  ourselves to the composition of

SOLAR PHOTOSPHERE The photosphere is the visible surface of the Sun. All the light received from  the Sun, in fact, comes from the photosphere. Why do not we receive  the radiation in the same form as generated in the interior of the Sun?   At the  centre of the Sun, the energy is generated in the form of high energy photons called γ-  rays. As these photons travel outwards, they collide with particles of matter and lose  energy continuously. By the time these photons reach the surface − the photosphere −  they are reduced to photons of visible region of electromagnetic spectrum. So, visible  radiation is emitted from the photosphere. The density of photosphere is 3400 times less than the density of the air we breathe.  The thickness of the photosphere is about 500 km and the temperature at its base is  ~ 6500 K. The temperature decreases upward and reaches a minimum value of  ~ 4400 K at the top. This assumption is corroborated by the Sun’s absorption  spectrum which indic

BRIGHTNESS, RADIANT FLUX AND LUMINOSITY  It is a common experience that if we view a street lamp from nearby, it may seem  quite bright. But if we see it from afar, it appears faint. Similarly, a star might look bright because it is closer to us. And a really brighter star might appear faint because it  is too far. We can estimate the apparent brightness of astronomical objects easily, but,  if we want to measure their real or intrinsic brightness, we must take their distance  into account. The apparent brightness of a star is defined in terms of what is called  the apparent magnitude of a star.  APPARENT MAGNITUDE In the second century B.C., the Greek astronomer Hipparchus was the first astronomer  to catalogue stars visible to the naked eye. He divided stars into six classes, or  apparent magnitudes, by their relative brightness as seen from Earth. He numbered  the apparent magnitude (m) of a star on a scale of 1 (the brightest) to 6 (the least  bright). This is the sc

ASTRONOMICAL DISTANCE, MASS AND TIME  SCALES  In astronomy, we are interested in measuring various physical quantities, such as  mass, distance, radius, brightness and luminosity of celestial objects. You have just  learnt that the scales at which these quantities occur in astronomy are very different  from the ones we encounter in our day-to-day lives. Therefore, we first need to understand these scales and define the units of  measurement for important astrophysical quantities.  We begin with astronomical distances. Astronomical Distances You have studied in your school textbooks that the Sun is at a distance of about  1.5 × 10^11m from the Earth. The mean distance between the Sun and the Earth is  called one astronomical unit . Distances in the solar system are measured in this unit.  Another unit is the light year , used for measuring distances to stars and galaxies.  The parsec is a third unit of length measurement in astronomy. We now define them.