If we observe a certain galaxy, we might find this has shifted to 660.0nm, an amount of 3.7nm. Hydrogen, the most common element in the universe has amongst others an emission at 656.3 nanometres (nm). Velocity = Doppler shift * speed of light (roughly 300,000 km/sec). For electromagnetic radiation such as light the formula is: With the Doppler shift being caused by an object’s motion it is no surprise that there is an equation linking the two. Let’s define Doppler shift a in a more formal manner.ĭoppler shift = (observed wavelength – rest wavelength) / rest wavelength. Consequently, by measuring the new wavelength that the line now appears at and comparing that to the rest wavelength we can accurately measure the Doppler shift and hence the object’s velocity towards or away from us.

However, when the object is moving toward or away from us the spectral lines will be shifted to different wavelengths by the Doppler effect. The key point here is that when an object is stationary (technically described as being ‘at rest’), these lines always appear at the same wavelengths. The most prominent ones are from Hydrogen. A spectrum of the Sun showing several key lines. These are the ‘fingerprints’ of the elements in the star emitting the light (Figure 3).įigure 3. Generally speaking, when we do this, we find that the spectrum is crossed with dark lines. It is all very well saying an object becomes redder or bluer but how is that change in colour actually measured precisely? As in so many areas of astronomy, we need to spread a star or planet’s light out into a spectrum with red at one end through to blue at the other. When it is receding, the wavelength increases and the colour shifts towards the red. Here, when a body is approaching, its wavelength shortens and the colour of the light moves towards the blue and this is logically enough described as a blueshift. The Doppler Effect will occur with any wave phenomena including electromagnetic radiation such as visible light. Waves emitted by an approaching object are compressed relative to the stationary position and are stretched out when receding. When receding the crests and troughs are spread further apart and lower notes result (Figure 2). What is happening is that as the car approaches you its motion compresses the siren’s sound wave, moving the crests and troughs closer together and so increasing the sound’s frequency. More specifically the siren has a higher note approaching you and a lower one as it recedes. Listening, for example, to an ambulance or police car approaching at speed and then receding, you will probably have noticed that the siren sound is different in these two cases with a sharp change in pitch as it passes. The Doppler Effect is a well-known and quite common phenomenon in everyday life but it is perhaps worth revisiting it before considering its astronomical effects.

As we shall see it has proven to be a very powerful tool for measuring velocities in astronomy. In this short tutorial we will discuss the “Doppler Effect” and some of the many uses it has in astronomy. The answer to all these questions is the Doppler Effect. How is it we know how fast stars and galaxies are moving towards or away from us? How can we discover a star is double even if the separation of the two stars is too close to be resolved in even the largest of our telescopes? How did we discern the true rotation periods of the planets Mercury and Venus? How can we know how fast a star is rotating? How was the first planet orbiting a star outside the Solar System discovered? One of the greatest astronomical discoveries of the twentieth century was the expansion of the universe, how was that uncovered? Some are approaching us some are moving away but how do we know this?

Doppler effect formula