![]() When the more luminous star is partially eclipsed the reduction in brightness is larger than when the dimmer star is eclipsed. As one star transits in front of the other, some of its light is absorbed and the apparent magnitude is reduced. The constantly shifting pattern of spectral lines can also be used to find the period of the the stars’ orbit and the mass of the system Figure 5: As two binary stars orbit each other spectral lines are shifted depending on whether they are approaching or receding.īinary systems can also be investigated looking at how the apparent magnitude of the two star varies over a long period of time. This is called the radial velocity method. The time between two consecutive peaks on the wave can be given by: ![]() The equation for the amount of red shift can be derived if we imagine a star, moving away from Earth with a velocity, $v$, which is emitting light of a wavelength $λ$ and speed $c$. The fractional change in the wavelength is called the red shift, whether or not the object is moving towards or away from the observer, and is given the symbol $z$. Wavelengths in the direction of the object's motion are compressed and appear 'bluer'.Īs the amount that the wavelength changes is directly proportional to the speed of the source, observations in the shift of spectral lines can be used to determine its speed. As this would move the wavelength towards the red end of the spectrum this is called red shift. When the source is moving away from the observer the relative speed between the source and the wavefront is greater, and the observed wavelength also increases. As the wavelength would move towards the blue end of the visible spectrum this is called blue shift. As the source still emits at the same frequency the observer will see a shorter wavelength than when observing the stationary source. The wavefront moves out at equal speeds in all directions, but as the source moves, its relative speed to the wavefront in the direction of its motion is lower. If the light source is moving towards the observer, as the source emits a wavefront, it continues moving. However, if the source of light is moving relative to the observer, the observed wavelength changes depending on whether the motion of the light source is towards or away from the observer. When an object is stationary emits light, or has light reflected off its surface, that light spreads out uniformly so that wherever an observer receives the light it would appear to have the same wavelength. However it is also possible to discover information about the motion of stars, and other astronomical bodies by making careful observations of their light. The same thing happens in case (c).So far within this module we have examined how we find out about the composition, distance to, and the temperature of stars by observing their colour and brightness. Similarly, the observer on the left receives a longer wavelength, and hence he hears a lower frequency. Because the observer on the right in case (b) receives a shorter wavelength, the frequency she receives must be higher. Thus, f multiplied by \(\lambda\) is a constant. The sound moves in a medium and has the same speed v in that medium whether the source is moving or not. ![]() We know that wavelength and frequency are related by v = f\(\lambda\), where v is the fixed speed of sound. Motion away from the source decreases frequency as the observer on the left passes through fewer wave crests than he would if stationary. Motion toward the source increases frequency as the observer on the right passes through more wave crests than she would if stationary. ![]() (c) The same effect is produced when the observers move relative to the source. The opposite is true for the observer on the left, where the wavelength is increased and the frequency is reduced. The wavelength is reduced, and consequently, the frequency is increased in the direction of motion, so that the observer on the right hears a higher-pitched sound. (b) Sounds emitted by a source moving to the right spread out from the points at which they were emitted. (a) When the source, observers, and air are stationary, the wavelength and frequency are the same in all directions and to all observers. ![]() \):- Sounds emitted by a source spread out in spherical waves. ![]()
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