The short answer is that astronomers have been seeing violets shifts occur a lot, because they actually happen all the time. But we call them blue shifts. The way we can tell the type of shift is by looking at the light that a star emits.
Obviously this color is relative to the colors that we can see here on earth, so this will be determined by the star’s motion relative to us on earth. Put simply: if a star is moving closer to earth, its light will shift to a high frequency of light on the color spectrum, or towards the blue end of the spectrum where there is also violet, ultraviolet and gamma-ray.
Conversely, if a star is moving further away from the earth, it will appear to emit a light that lower in frequency, so light nearer to the red end of the spectrum, where there is also orange, infrared and even radio.
The former star that is moving closer to earth will emit light that exhibits a blue shift, not a violet shift. This is because the faster that a star moves towards earth, the higher the light frequencies it will show. The star that is moving away from earth will emit light that exhibits a red shift, since the faster it moves away from earth, the lower frequency its light becomes. Obviously, both of these are relative to what we can see on or from earth.
This whole thing can be explained using the term the Doppler shift. The Doppler shift is something you can easily compare to the Doppler shift in sound too. Imagine you are stood on the corner of the street and can see an ice cream van approaching. When it is coming towards you, the song will seem to sound higher pitched, and when it ignores you and is driving past and away, the song will seem lower pitched.
This is the same for the Doppler shift in light. We use the Doppler shift to our advantage on earth: not only does it help to identify whether a star is getting further away or closer to earth, it also helps the police see who is speeding on the roads via the use of a speed gun. A police speed gun will analyse the shift in radio wave frequency as it bounces off the car that is speeding.
So, the Doppler shift, which you have no doubt heard when talking about sound, is also used to describe light. But with light, the specifics are also important. The Doppler shift in light is exclusively to do with a red shift and a blue shift. Although the question asks about violet shift, there is no such thing, only blue shift.
Not only can astronomers tell whether a star is moving close to or further away from earth when measuring the red or blue shift, they can also see how fast the star is travelling either to or away from. They do this using the same Doppler shift.
The reason this is possible is because any spectral color of light can shift to any other color provided that the motion and movement of the source is correct. This means that if you have an orange torch light that has blue shifted, its final color is not blue. It simply means that the final color has shifted somewhat to the bluer end of the visible light spectrum, or that it has been shifted upwards in frequency.
Because of this, if the same orange light is shifted enough to fall into the yellow light, it has still been blue shifted. And if the same orange light is shifted to the color violet, it has still been blue shifted.
A blue shift does not mean that the final color of the light source or astronomical object has to be blue. Just that the shift of the light frequency is towards the blue end of the spectrum. Contrasting this, if you take a violet torch light and it ends up as an orange light, this light has been red shifted.
It can be easier to think about red shifted being ‘down shifted’ and blue shifted could be ‘up shifted’. This is because the light changes frequency in either of these ways, making thr light appear a different color on the spectrum. The reason why it is easier to think of a blue shift as an ‘up shift’ is because an ultraviolet ray could become an X-ray by blue shift, but both of them are above blue light on the spectrum so it can be hard to wrap your head around. But imagine blue shift simply means a shift to the blue end of the spectrum and not towards blue.
But here it can get interesting. Going away from blue shift for now, we call red shift the ‘down shift’ because red is obviously on the lower end of the visible light spectrum and has lower frequency waves. Since red is the first color in the visible rainbow, it makes sense that we use red as the indicator.
So, the original question remains… why do we not call blue shift ‘violet shift’? It is the last color of the rainbow, and if we follow the logic that worked with the red color and the red shift, surely calling it the violet shift makes more sense?
The short answer to this is that humans do not actually see violet very well with their naked eye. We can see violet, since it is in the visible spectrum of light, but not as well as we can see blue. For this reason, we call it a blue shift and not a violet shift. It would be confusing at this point to change it to the violet shift and compare all blue shift findings against a new violet shift.
But also, we are used to blue being considered the near edge of the visible spectrum as humans. An example of this is that we see the sky to be blue for most of the day. But scientists have found that the sky can reach violet and ultraviolet at noon, but we see it as blue because our eyes find it easier to see the color blue. This can also be seen in fire. We are taught that the hottest flame is a blue one, when in reality, it is a violet and ultraviolet one, we just cannot see it.
So, the reason we call the Doppler shift a red or a blue shift is because both just mean that the light is moving towards those ends of the spectrum, not towards red and blue. But also, we do not call blue shift a violet shift, even though that is the edge of the visual spectrum, because we cannot see the color violet as well as we can see the color blue with our naked human eyes.
The Doppler shift and its use by astronomers has allowed us to observe some pretty cool things. The most interesting thing is that the light from almost all of the stars outside of our galaxy is red shifted, meaning that those stars are moving away from us. This can also act as proof towards the theory that the universe is expanding at a rapid rate.
As one of these far away stars rotates, one edge of the star will be red shifted since it is moving away from us compared to the center of it. The light on the other edge of the star will be slightly blue shifted because that edge is moving closer towards earth compared to the other edge and the center of the star. Therefore, astronomers will use these Doppler shift readings in order to calculate how fast a star is rotating.
Following this logic, it is possible to also calculate how fast a galaxy far away from us is rotating too. All of this can be done from the comfort of our own planet because we are relying on the way that the light from these stars and galaxies reaches earth and how it would have been shifted.
So, to answer the question: violet shifts are in fact called blue shifts and they are happening all of the time in our universe since it is expanding, and so are red shifts, both of which make up the Doppler shift effect.