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Spectroscopy 101 – Beyond Temperature and Composition

What else can spectroscopy tell us?

What else can we learn from spectroscopy? Let’s take another look at hydrogen. Hydrogen is useful because it is abundant in a wide variety of objects throughout the universe.

To review, we know from repeated laboratory observations that hydrogen emits and absorbs light with a number of specific wavelengths, including 410 nm (violet), 434 nm (blue), 486 nm (blue-green), and 656 nm (red). We can conclude that a star has hydrogen in it if these lines show up in its spectrum.

But sometimes hydrogen lines in a star or galaxy don’t look exactly like they would in a lab on Earth. Maybe some are wider or fuzzier than normal. Or maybe they are not in exactly the right place in the spectrum—their wavelengths look shifted. Deviations like these might seem troublesome, but in fact they can provide a lot of other interesting information.

Density

Differences in width of spectral lines in a star can reflect differences in density. For example, if you compare the spectrum of an extremely dense white dwarf star to a puffy blue giant, you’ll notice that some of the hydrogen lines in the white dwarf are much wider and fuzzier than those in the blue giant. The exact cause is complicated, but basically, the pressure in the white dwarf is so high that the energy levels in the atoms get perturbed, causing the line to spread out. Astronomers can use spectroscopy to figure out a lot about the fundamental properties of white dwarfs.

Graphic titled “Comparing Density Through Spectra” with illustrations of a blue giant star, a white dwarf star, and their respective spectra.
Spectrum of a blue giant star versus a white dwarf star. Differences in the width and clarity of absorption lines can indicate differences in density. The absorption lines of a white dwarf are much broader than those of a blue giant. A white dwarf can be 100,000,000 times denser than a blue giant.
NASA, ESA, CSA, Leah Hustak (STScI)

Motion

Shifts in the position of hydrogen lines are caused by relative motion. When a star is moving away from us (or we are moving away from the star), all of the light waves look stretched out. As a result, all of the spectral lines appear at longer wavelengths than they do in the lab: They are shifted toward the red end of the spectrum. This is, unsurprisingly, known as redshift.

When a star is moving toward us, the opposite occurs. All of the light looks compressed and the lines appear at shorter wavelengths, shifted toward the blue end of the spectrum. This is known as blueshift. The faster the star is moving toward or away from us, the more the lines are shifted.

When an object is moving relative to us, all of the lines in the spectrum shift. In fact, if you look at the object’s full spectrum, you will see that it is the entire spectrum that is shifting, not just the lines. The spectral lines are important because they make it possible to measure redshift precisely (or even notice that it happened at all). Since the change in wavelength is directly related to relative speed, you can use it to calculate exactly how fast an object is moving toward or away from us.

Doppler Shift

star, planet, three absorption spectra
Doppler shift of a star’s spectrum. As a planet orbits a star, the star wobbles. While the star is moving toward us, its spectrum is blueshifted: Absorption lines appear at shorter wavelengths than normal. While the star is moving away from us, wavelengths appear longer and the light is redshifted. Astronomers can track the Doppler shift over time to estimate the mass of the planet.
NASA, ESA, CSA, Leah Hustak (STScI)

In some cases, redshift is caused by motion through space. Planets orbit stars, stars move within their galaxies, galaxies collide with each other, stars explode, and black holes shoot jets of gas and dust out of their galaxies.

A shift in wavelength caused by this type of motion is called Doppler shift, and it is the same phenomenon that causes the whine of a car engine to sound higher in pitch when it is moving toward us and lower in pitch when it is moving away from us.

Cosmological Redshift

If you look at the spectra of a whole bunch of galaxies, you will notice that most of them are redshifted: Almost all galaxies appear to be moving away from us. If you compare the redshift of a galaxy to its distance, you’ll notice that the farther a galaxy is from us, the faster it appears to be moving away from us. This phenomenon is known as cosmological redshift (also called Hubble’s law), and it is caused not by motion through space, but by the expansion of space itself. The universe is expanding, and as it expands, galaxies are carried farther and farther apart.

Infographic titled “What Is Cosmological Redshift?” explains cosmological redshift, or the stretching of light waves due to the expansion of space between galaxies. The infographic is divided into three parts: When Space Expands, Light Stretches; Redder than Red; and Seeing the Past. Part 1 shows the universe expanding over time, while the wavelengths of light also expand as they travel through expanding space over time. Part 2 shows the difference in wavelength between emitted light (shorter wavelength) and received light (longer wavelength), using an illustration of the electromagnetic spectrum for reference. Part 3 illustrates the universe over time, showing the limit of Hubble’s and Webb’s views through space and time.
Cosmological Redshift. The light from most galaxies is redshifted as a result of the expansion of space over time. The farther away a galaxy is, the more it is redshifted. Astronomers use spectroscopy to measure galaxy redshift, which they can then use to calculate how fast different galaxies appear to be moving away from us, estimate their distances, and estimate the expansion rate of the universe.
NASA, ESA, Leah Hustak (STScI)

Redshift measurements are important because they allow us to measure motion that would be impossible to measure otherwise. Although objects in space can move extremely quickly (in human terms), most objects are so far away that it impossible or extremely difficult to directly observe a change in position.

We can use spectroscopy to measure the rotation rate of a galaxy, the mass of a planet that causes its star to wobble, the speed of hot gas being ejected from a black hole, the mass of a black hole or the amount of dark matter influencing a star’s motion, and the expansion rate of the universe.