Understanding Spectra

July, 2024

Hi, I’m Marley, the Astronomer here at the Space Centre. In September, we will debut the film 5000 Eyes: Mapping the Universe with DESI. 5000 Eyes is about the Dark Energy Spectroscopic Instrument (DESI) mounted on the Mayall Telescope that will help scientists understand dark energy. In order to accomplish this, DESI will need to observe the spectra of approximately 40 million different galaxies! I thought this blog post would be the perfect opportunity to introduce you to spectra, and just how much scientists can learn from them.

Light

Understanding spectra means understanding light. The light that we see is just one part of a much larger picture. The electromagnetic spectrum contains all types of light, from the x-rays you get at the dentist to the heat you feel on a warm day. Scientists think about all forms of light in two different ways: as a wave, and as a particle.

In some cases, light behaves like a wave. A way to measure them is by their wavelength, or the distance between peaks. If you have ever seen a rainbow, you have seen the different wavelengths of light in the visible part of the electromagnetic spectrum! When it comes to the rest of the electromagnetic spectrum, gamma rays have the shortest wavelengths, and radio waves have the longest.

When not behaving like a wave, light behaves like a particle. Scientists refer to particles of light as photons. Each photon is like a packet of energy, and the amount of energy in that packet will correspond to the wavelength. If this duality confuses you, don’t worry. The only thing you need to remember is that when it comes to studying spectra, wavelength and energy are basically the same thing.

Spectra and Spectroscopy

Light interacts with (most!) matter. Light can be absorbed by matter, and transformed into other types of energy. Light can also be reflected by matter, and it can also be transmitted through matter – like when light passes through windows. When light interacts with matter, it produces a spectra, a pattern of wavelengths associated with what the matter is made of.  Spectroscopy is the science of studying spectra. Studying spectra can tell scientists about gases in atmospheres and how fast stars are rotating. Studying the spectrum of an entire galaxy will give scientists information about the types of stars in the galaxy, and even about the black hole at the centre. The spectrum of a galaxy can also tell scientists how fast the galaxy is moving away from us. This is called cosmological redshift. The further away a group of galaxies is, the larger its redshift, and the faster it appears to be moving away. The movement of the galaxies is caused by the expansion of the Universe, which scientist believe is being driven by dark energy. This is where DESI comes in. Studying the spectra of millions of galaxies will hopefully give scientists insight into how dark energy has shaped the history of our universe.

In April 2024, the first results from DESI were published: the largest 3D map of our universe. Only three years into its five-year survey, there is still much more science to come from the instrument. In the meantime, check out some light and spectra related activities below!