What is Blueshift?

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At 2.5 million light-years, the Andromeda Galaxy is the closest spiral galaxy to the Milky Way. Astronomers know it is moving toward the Milky Way because its light is "blueshifted". Adam Evans/Wikimedia Commons.

You've probably heard the terms "redshift" and "blueshift", what do they mean in "astronomy speak"? Redshift means that an object is moving away from us. "Blueshift" is a term that astronomers use to describe an object that is moving toward another object. You will hear someone say, "That galaxy is blueshifted with respect to the Milky Way", for example. It means that the galaxy is moving toward our galaxy.

It can also be used to described the speed the galaxy is taking as it gets closer to ours. 

How Do We Determine Blueshift?

Blueshift is a direct result of the doppler effect, though there are other phenomena that can also result in light becoming blueshifted. Here's how it works. Let's take that galaxy as an example again. It is emitting radiation in the form of light, x-rays, ultraviolet, infrared, radio, visible light, and so forth. As it approaches an observer in our galaxy, each photon (packet of light)  that it emits appears to be produced closer in time to the previous photon. This is due to the doppler effect and the galaxy's proper motion (its motion through space). The result is that the photon peaks appear to be closer together than they actually are, making the wavelength of light shorter (higher frequency, and therefore higher energy), as determined by the observer.

Blueshift is not something you see with your eyes.

It is determined by the study of the wavelengths of light from the object by using an instrument that dissects the light. Normally this is done with an instrument called a "spectrometer" or another one called a "spectrograph". The data these instruments get are graphed into what's called a "spectrum".

If the light information tells us that the object is moving toward us, the graph will appear "shifted" toward the blue end of the electromagnetic spectrum. 

Measuring the Blueshift of Stars

By measuring the spectral shifts of stars in the Milky Way, we can plot not just their movements, but also the movement of the galaxy as a whole. Objects that are moving away from us will appear redshifted, while objects approaching will be blueshifted. The same is true for our example galaxy that's coming toward us.

Is the Universe Blueshifted?

The past, present and future state of the universe is a hot topic in astronomy and in science in general. And one of the ways that we study these states is to observe the motion of the astronomical objects around us.

Originally, the universe was thought to stop at the edge of our galaxy, the Milky Way. But, in the earlyu 1900s, astronomer Edwin Hubble found there were galaxies outside of ours (these had actually been observed previously, but astronomers thought that they were simply a kind of nebula, not entire systems of stars). There are now known to be multiple billions of galaxies across the universe. 

This changed our entire understanding of the universe and, shortly after, paved the way for the development of a new theory of the creation and evolution of the universe: the Big Bang Theory.

The next step was to determine where we are in the process of universal evolution, and what kind of universe we were living in. The question was asked: is the universe expanding? Contracting? Static?

To answer these questions, the spectral shifts of galaxies near and far were measured. In fact, astronomers continue to do this today. If the light measurements of the galaxies were blueshifted in general, then this would mean that the universe is contracting and that we could be headed for a "big crunch" as everything in the cosmos slams back together. 

However, it turns out the galaxies are, in general, receding from us and appear redshifted. This means that the universe is expanding. Not only that, but we now know that the universal expansion is accelerating — and that it accelerated at a different rate in the past — driven by some mysterious force known generically as dark energy.

We have little understanding of the nature of dark energy, only that it seems to be everywhere in the universe.

Edited by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "What is Blueshift?" ThoughtCo, Jun. 22, 2017, thoughtco.com/blue-shift-definition-3072288. Millis, John P., Ph.D. (2017, June 22). What is Blueshift? Retrieved from https://www.thoughtco.com/blue-shift-definition-3072288 Millis, John P., Ph.D. "What is Blueshift?" ThoughtCo. https://www.thoughtco.com/blue-shift-definition-3072288 (accessed January 20, 2018).