STARS:
Stellar distances :
The stars are so far away from us that they appear to be fixed. Stellar distance being very large, a unit called ‘parsec’ is used in addition to the unit light year (ly), to measure stellar distances.
The stars are so far away from us that they appear to be fixed. Stellar distance being very large, a unit called ‘parsec’ is used in addition to the unit light year (ly), to measure stellar distances.
1 parsec (pc) = 3.26ly = 3 x 10Power3 km.
Brightness and luminosity :
Some stars are bright and some are dim; some stars look white while some may look blue and some red. The difference in color is due to different temperatures. Stars differ in apparent brightness for two reasons; one is that they are at different distances and the other is that they differ in their intrinsic (actual) brightness. A candle and a powerful search light differ greatly in their brightness when they are at the same distance from us. However, the candle may appear brighter if the search light is at a sufficiently great distance. The brightness of stars is represented by a system called magnitude scale. The magnitude scale is defined so that a magnitude difference of 5, corresponds exactly to a factor of 100 in brightness. A star of magnitude 1 is 100 times brighter than a star of magnitude 6. A brightness ratio of 2.5 that is corresponds to a magnitude difference of 1. A star of magnitude 1 is 2.52 i.e 6.25 times brighter than star of magnitude 3. The brightness of some celestial bodies represented on magnitude scale, is shown in Fig. 10.2. The magnitude scale puts larger numerical values for fainter stars. The magnitude of the sun is -27 and that of full moon is -13. The brightest star Sirius has a magnitude -1.4; the other bright star Vega has a magnitude ‘0’. Farthest star visible to the naked eye has a magnitude +6. Magnitude -5 corresponds to a brightness 100 times that of a zero magnitude star. The magnitude refers to apparent magnitude.
Brightness and luminosity :
Some stars are bright and some are dim; some stars look white while some may look blue and some red. The difference in color is due to different temperatures. Stars differ in apparent brightness for two reasons; one is that they are at different distances and the other is that they differ in their intrinsic (actual) brightness. A candle and a powerful search light differ greatly in their brightness when they are at the same distance from us. However, the candle may appear brighter if the search light is at a sufficiently great distance. The brightness of stars is represented by a system called magnitude scale. The magnitude scale is defined so that a magnitude difference of 5, corresponds exactly to a factor of 100 in brightness. A star of magnitude 1 is 100 times brighter than a star of magnitude 6. A brightness ratio of 2.5 that is corresponds to a magnitude difference of 1. A star of magnitude 1 is 2.52 i.e 6.25 times brighter than star of magnitude 3. The brightness of some celestial bodies represented on magnitude scale, is shown in Fig. 10.2. The magnitude scale puts larger numerical values for fainter stars. The magnitude of the sun is -27 and that of full moon is -13. The brightest star Sirius has a magnitude -1.4; the other bright star Vega has a magnitude ‘0’. Farthest star visible to the naked eye has a magnitude +6. Magnitude -5 corresponds to a brightness 100 times that of a zero magnitude star. The magnitude refers to apparent magnitude.
The total energy emitted by a star in one second is called its luminosity; the value of luminosity depends on the temperature and diameter of the star. Luminosity of a star is expressed in terms of the luminosity of the sun (L ) For example, luminosity of Sirius is 23.5 L .
Stellar Spectra :
Consider the bulbs as stars and I spectra obtained are as shown in the above Image.
The spectrum of a star like that of the sun, is a continuous spectrum crossed by dark absorption lines. Analysis of these lines helps to identify the elements present in the stellar atmosphere.
The intensity distribution in its spectrum, decides the apparent colour of the star and hence surface temperature of the star.
The surface temperatures of stars range from about 2000K for reddish stars to about 50000K for bluish white stars. The following list gives temperatures and colours of some stars. Temperature Co lour Star - example
The spectrum of a star like that of the sun, is a continuous spectrum crossed by dark absorption lines. Analysis of these lines helps to identify the elements present in the stellar atmosphere.
The intensity distribution in its spectrum, decides the apparent colour of the star and hence surface temperature of the star.
The surface temperatures of stars range from about 2000K for reddish stars to about 50000K for bluish white stars. The following list gives temperatures and colours of some stars. Temperature Co lour Star - example
2500 - 3500K Red Betelgeauae
3500 - 5000K Orange yellow Arcturus
5000 - 6000K Yellow Sun
6000 - 10,000K Yellowish white Sirius
10,000 - 50,000K Bluish white Rigel
Sizes of stars : The usual method of measuring angular diameters, as in the case of planets, cannot be applied in the case of stars, because they appear as pin points even through the most powerful telescopes. However the radius of a star can be deduced from its luminosity and temperature. Majority of stars have radii in the range of a tenth to twenty times solar radius.
Masses of stars : When two stars form a gravitationally bound system, and go round in circles around their common centre of mass, their individual masses can be determined by observing their motion. Such a star system is called a binary star. Sirius is a binary star system; its components have masses 1.4 MO and 2.4 MO. It is found in general that the luminosity of stars increases with mass. Stars about forty times heavier than the sun are roughly million times luminous than the sun. The luminosity L of a star varies as Mα where α varies from 2 to 4. For stars similar to the sun α ≈ 4 .
Stellar Interiors : Mean densities of the stars can be calculated, knowing their sizes and masses. The densities vary from 5 x 104 kg m-3 in the case of the coolest star to around 10 kg m-3 for the
hot stars. The central temperature ranges from 10 to 30 million degrees as we go from the coolest to the hot stars. Strange stars : Among stars, there are strange types of stars known as binary stars, red giants, white dwarfs, black dwarfs, neutron stars, black holes and events called novae and supernovae. In addition, there are quasars which are galaxies thousands times brighter than ordinary galaxies, and pulsars which are stars that emit radiation in pulses
BY - Nandan
BY - Nandan