What happens to the wavelengths of energy if they are moving away from a source?

Most bright astronomical objects shine because they are hot. In these cases, the continuum emission tells us the temperature of the object. The following table shows a rough guide for the relationship between the temperature of an object and what part of the electromagnetic spectrum where we see it shine.

However, we can learn a lot more from the spectral lines than from the continuum. Two very important things we can learn from spectral lines is the chemical composition of objects in space and their motions.

Chemical composition

During the first half of the 19th century, scientists such as John Herschel, Fox Talbot, and William Swan studied the spectra of different chemical elements in flames. Since then, the idea that each element produces a set of characteristic emission lines has become well-established. Each element has several prominent, and many lesser, emission lines in a characteristic pattern.

Sodium, for example, has two prominent yellow lines (the so-called D lines) at 589.0 and 589.6 nm – any sample that contains sodium (such as table salt) can be easily recognized using these pair of lines.

The studies of the Sun's spectrum revealed absorption lines, rather than emission lines (dark lines against the brighter continuum). The precise origin of these 'Fraunhofer lines' as we call them today remained in doubt for many years, until Gustav Kirchhoff, in 1859, announced that the same substance can either produce emission lines (when a hot gas is emitting its own light) or absorption lines (when a light from a brighter, and usually hotter, source is shone through it). With that discovery, scientists had the means to determine the chemical composition of stars through spectroscopy.

Stars aren't the only objects for which we can identify chemical elements. Any spectrum from any object allows us to look for the signatures of elements. This includes nebula, supernova remnants and galaxies.

Once we have identified specific elements in a spectrum, we can also look to see if the emission lines from those elements has been shifted from where we might expect to find them. While we usually talk about emission spectra as though the wavelengths of the lines are fixed, that is only true when the source emitting the lines and the detector "seeing" the lines are not moving relative to one another. When they are moving relative to each other, the lines will appear shifted. For example, if a star is moving toward us, its lines will be observed at shorter wavelengths, which is called "blueshifted". If the star is moving away from us, the lines will appear at longer wavelengths, which is called "redshifted". This is called "Doppler shift."

If the spectrum of a star is red or blue shifted, then you can use that to infer its velocity along the line of sight. Such "radial velocity" studies have had at least three important applications in astrophysics.

1. One application is in the study of binary star systems. For stars in some binary systems we can measure the radial velocities for one orbit (or more). Once we've done that, we can relate that back to the gravitational pull using Newton's equations of motion (or their astrophysical applications, Kepler's laws). If we have additional information, such as from observations of eclipses, then we can sometimes measure the masses of the stars accurately. Eclipsing binaries in which we can see the spectral lines of both stars have played a crucial role in establishing the masses and the radii of different types of stars.

The red giant star Mira A (right) and its companion, a close binary pair. (Credit: M. Karovska/Harvard-Smithsonian Center for Astrophysics and NASA)
2. Another application is the study of the structure of our galaxy. Stars in the Galaxy revolve around its center, just like planets revolve around the Sun. It's more complicated, because the gravity is due to all the stars in the Galaxy that lie inside the stars' orbit combined; whereas in the Solar system, the Sun has so much more mass than the planets combined, we can ignore the pull of the planets, more or less. Radial velocity studies of stars (binary or single) have played a major role in establishing the shape of the Galaxy. It is still an active field today: for example, one form of the evidence for dark matter comes from the study of the distribution of velocities at different distances from the center of the Galaxy (and for other galaxies). Another exciting development is from the radial velocity studies of stars very near the Galactic center, which strongly suggest that our Galaxy contains a massive black hole.

An artist's conception of the Milky Way galaxy. (Credit: NASA/JPL)
3. A third application is the expansion of the Universe. Edwin Hubble established that more distant galaxies tended to have more red-shifted spectra. Although not predicted even by Einstein, such an expanding universe is a natural solution for his general theory of relativity. Today, for more distant galaxies, the redshift is used as primary indicator of their distances. The ratio of the recession velocity to the distance is called the Hubble constant, and the precise measurement of its value is one of the major goals of astrophysics today, using such tools as the Hubble Space Telescope.

Redshift and blueshift describe the change in the frequency of a light wave depending on whether an object is moving towards or away from us. When an object is moving away from us, the light from the object is known as redshift, and when an object is moving towards us, the light from the object is known as blueshift. 

Astronomers use redshift and blueshift to deduce how far an object is away from Earth, the concept is key to charting the universe's expansion. 

To understand redshift and blueshift, first, you need to remember that visible light is a spectrum of color each with a different wavelength. According to NASA (opens in new tab), violet has the shortest wavelength at around 380 nanometers, and red has the longest at around 700 nanometers. When an object (e.g. a galaxy) moves away from us it is 'red-shifted' as the wavelength of light is 'stretched' so the light is seen as 'shifted' towards to red end of the spectrum, according to ESA. (opens in new tab)

Related: What is a light-year? 

Redshift, blueshift and the Doppler effect

The concept of redshift and blueshift is closely related to the Doppler effect — which is an apparent shift in soundwave frequency for observers depending on whether the source is approaching or moving away from them, according to the educational website The Physics Classroom (opens in new tab). The Doppler Effects was first described (opens in new tab) by Austrian physicist Christian Doppler in 1842 and many of us experience the Doppler effect first hand almost every day without even realizing it. 

We've all heard how a siren changes as a police car rushes past, with a high pitch siren upon approach, shifting to a lower pitch as the vehicle speeds away. This apparent change in pitch to the observer is due to soundwaves effectively bunching together or spreading out. It is all relative as the siren's frequency doesn't change. As the police car travels towards you the number of waves are compressed into a decreasing distance, this increase in the frequency of sound waves that you hear causes the pitch to seem higher. Whereas then the ambulance goes past you and moves away, the sound waves are spread across an increasing distance thus reducing the frequency you hear so the pitch seems lower.

This principle of the Doppler effect applies to light as well as sound. 

American astronomer Edwin Hubble (who the Hubble Space Telescope is named after) was the first to describe the redshift phenomenon and tie it to an expanding universe. His observations, revealed in 1929, showed that nearly all galaxies he observed are moving away, NASA said (opens in new tab).

"This phenomenon was observed as a redshift of a galaxy's spectrum," NASA wrote. "This redshift appeared to be larger for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth."

The galaxies are moving away from Earth because the fabric of space itself is expanding. While galaxies themselves are on the move — the Andromeda Galaxy and the Milky Way, for example, are on a collision course (opens in new tab) — there is an overall phenomenon of redshift happening as the universe gets bigger.

The terms redshift and blueshift apply to any part of the electromagnetic spectrum, including radio waves, infrared, ultraviolet, X-rays and gamma rays. So, if radio waves are shifted into the ultraviolet part of the spectrum, they are said to be blueshifted or shifted toward the higher frequencies. Gamma rays shifted to radio waves would mean a shift to a lower frequency or a redshift. 

The redshift of an object is measured by examining the absorption or emission lines in its spectrum. These lines are unique for each element and always have the same spacing. When an object in space moves toward or away from us, the lines can be found at different wavelengths than where they would be if the object were not moving (relative to us).

Three types of redshift

At least three types of redshift occur in the universe — from the universe's expansion, from the movement of galaxies relative to each other and from "gravitational redshift," which happens when light is shifted due to the massive amount of matter inside of a galaxy.

This latter redshift is the subtlest of the three, but in 2011 scientists were able to identify it on a universe-size scale. Astronomers did a statistical analysis of a large catalog known as the Sloan Digital Sky Survey and found that gravitational redshift does happen — exactly in line with Einstein's theory of general relativity. This work was published in a Nature paper.

"We have independent measurements of the cluster masses, so we can calculate what the expectation for gravitational redshift based on general relativity is," said University of Copenhagen astrophysicist Radek Wojtak at the time. "It agrees exactly with the measurements of this effect."

The first detection of gravitational redshift came in 1959 after scientists detected it occurring in gamma-ray light emanating from an Earth-based lab. Previous to 2011, it also was found in the sun and in nearby white dwarfs, or the dead stars that remain after sun-sized stars cease nuclear fusion late in their lives.

How does redshift help astronomers?

Redshift helps astronomers compare the distances of faraway objects. In 2011, scientists announced they had seen the farthest object ever seen — a gamma-ray burst called GRB 090429B, which emanated from an exploding star. At the time, scientists estimated the explosion took place 13.14 billion years ago. By comparison, the Big Bang took place 13.8 billion years ago.

The farthest known galaxy is GN-z11. In 2016, the Hubble Space Telescope determined it existed just a few hundred million years after the Big Bang. Scientists measured the redshift of GN-z11 to see how much its light had been affected by the expansion of the universe. GN-z11's redshift was 11.1, much higher than the next-highest redshift of 8.68 measured from galaxy EGSY8p7. 

 Scientists can use redshift to measure how the universe is structured on a large scale. One example of this is the Hercules-Corona Borealis Great Wall; light takes about 10 billion years to go across the structure. The Sloan Digital Sky Survey is an ongoing redshift project that is trying to measure the redshifts of several million objects. The first redshift survey was the CfA RedShift Survey, which completed its first data collection in 1982.

One emerging field of research concerns how to extract redshift information from gravitational waves, which are disturbances in space-time that happen when a massive body is accelerated or disturbed. (Einstein first suggested the existence of gravitational waves in 1916, and the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected them directly in 2016). Because gravitational waves carry a signal that shows their redshifted mass, extracting the redshift from that requires some calculation and estimation, according to a 2014 article in the peer-reviewed journal Physical Review X (opens in new tab). 

Additional resources

Learn more (opens in new tab) about the Doppler Effect with NASA and explore Doppler Shift with the University of California, Los Angeles (opens in new tab). You can also read up on wave characteristics with the educational website BBC Bitesize (opens in new tab).  

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