LIGHT AND DOPPLER SHIFT
Hello all and welcome to my Blogspot EVERYTHING ASTRONOMICAL. In this blog, we
are going to work with light and Doppler shifts. I hope you all have read my
previous one, if not, do read it for a better understanding😉.
Working with light:
We know that matter is somehow responsible for the creation of photons. Since the speed of light is well known, it has become common for astronomers to measure enormous distances in space in terms of the time it takes for light to travel in a given amount of time. For instance, the distance that light can travel in one second is known as a light second.
1 light sec = 299790km
The distance between the Earth and the moon is 3,84,400km, which is just a bit larger than one light second. We could use the unit kilometres which is getting a bit crazy at this point or we could use the unit of light-second. In this new unit, the distance between the Earth and the moon is 1.3 light seconds. In other words, it takes a photon of light 1.3s to travel from the Earth to the moon.
If we now step up the size scale and consider the Earth and our sun, it takes light 8.3 minutes to travel from the sun to the Earth. So, we say that the distance is 8.3 light minutes. Since the distance from the Earth and the sun is so important in astronomy, astronomers also introduced a new distance called the Astronomical Unit (AU).
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| ( 1 Astronomical Unit ) |
1 AU = 1.496×1011m
Other familiar units, that astronomers use are light-years and parsec.
1 LY = 9.461×1015m
I have a question for you people,
Since a star's light takes so long to reach us, how do we know that the star is still there? (Leave your thoughts in the comment.)
Doppler Shift:
Next stop on our tour of the properties of light and sometimes the sound is an effect first explained by an Austrian mathematician and physicist, Christian Doppler in 1842.
I am sure many of you have been present as emergency vehicles or a train pass by. When this happens, what do you hear? Well, yes, you hear a siren. But what happens to the siren as an ambulance drives by?
The soundwaves from the front, begin to bunch up and are pushed close together, and the wavelength that a stationary person will hear will be smaller. Smaller wavelengths correspond to higher frequencies and higher pitches. Then you hear a higher pitch sound and vice versa happen. This raising and lowering of pitch are known as the Doppler shift.
Next stop on our tour of the properties of light and sometimes the sound is an effect first explained by an Austrian mathematician and physicist, Christian Doppler in 1842.
I am sure many of you have been present as emergency vehicles or a train pass by. When this happens, what do you hear? Well, yes, you hear a siren. But what happens to the siren as an ambulance drives by?
The soundwaves from the front, begin to bunch up and are pushed close together, and the wavelength that a stationary person will hear will be smaller. Smaller wavelengths correspond to higher frequencies and higher pitches. Then you hear a higher pitch sound and vice versa happen. This raising and lowering of pitch are known as the Doppler shift.
Why I am discussing here is, it also applies to light in
our study. As light is emitted from a source, the waves being emitted can be
squashed or stretched along the direction of motion.
So, if a star or a galaxy is moving towards us, the light waves moving ahead of the star can appear to be increased in frequency or decreased in wavelength. This translates as a shift towards the blue end of the electromagnetic spectrum. i.e., Blueshift. Conversely, as the star moves away from us, the waves to be stretched resulting in longer wavelengths and a wider look to the star. i.e., Redshift. We should note that this is not confined to the visible band of the electromagnetic spectrum. Both X-rays and radio waves can also be blue and red-shifted.
So, if a star or a galaxy is moving towards us, the light waves moving ahead of the star can appear to be increased in frequency or decreased in wavelength. This translates as a shift towards the blue end of the electromagnetic spectrum. i.e., Blueshift. Conversely, as the star moves away from us, the waves to be stretched resulting in longer wavelengths and a wider look to the star. i.e., Redshift. We should note that this is not confined to the visible band of the electromagnetic spectrum. Both X-rays and radio waves can also be blue and red-shifted.
If we view a pair of stars from the side, it will appear as the one star is moving towards us while the other star is moving away from us. This relative motion is detected as blueshifts and redshifts.
Well, I
have discussed some basics about the Doppler shift, see, I have skipped the
formulae.
It is interesting to note that while one of our neighbours, M31, or the Andromeda Galaxy is rotating, we also see an overall blueshift of the whole galaxy, implying that M31 is moving towards us.
It is interesting to note that while one of our neighbours, M31, or the Andromeda Galaxy is rotating, we also see an overall blueshift of the whole galaxy, implying that M31 is moving towards us.
So, do you think that Andromeda Galaxy will collide and merge with our Milky Way Galaxy in 4 to 5 billion years?🤔 ( Leave it in the comment.)
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| ( Credit: NASA ) |
Well, if you listen closely to the sound, you can hear
that the pitch of the siren changes over time. As the ambulance moves towards us,
you hear at one pitch, but as it passes by, it seems to drop in pitch.
The most well-known Doppler shift in astronomy triggered the idea of the expanding universe.
Let’s discuss this idea of expanding the universe and Hubble’s contribution in my next blog in detail. Engage with me😊.
The most well-known Doppler shift in astronomy triggered the idea of the expanding universe.
Let’s discuss this idea of expanding the universe and Hubble’s contribution in my next blog in detail. Engage with me😊.
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