Let’s Explore ‘Light Echoes’ in Space

Astronomers have learned a great many things via the study of variable stars throughout the years. Perhaps some of the most exciting phenomena studied by scientists involve those which emit bright pulses of light during eruptive or disruptive events (such as novae or supernovae). Because of the light emitted, there is actually a brief period of time during which actual information about these rare events may be studied.

Light Echo Primer

If you’ve ever yelled in between two brick buildings or an empty room, you likely noticed that your voice echoed. This is essentially the audio waves bouncing off of the walls and arriving back to your ears after the initial shout was initiated. Well believe it or not, the same think can occur with light.  When a sudden burst of light occurs in space, the flash or light is actually reflected off interstellar dust and is shown to us at a delayed time, post-burst. This phenomena is known as a light echo (LE) in astronomy and was first observed around Nova Persei 1901 and recognized as such by the Dutch astronomer, Jacobus Cornelius Kapteyn, in 1902. Kapteyn’s work was confirmed by Charles Dillon Perrine, an American-Argentine astronomer, in 1903. Since that time, our understanding of LE science has grown tremendously including the associated geometric conversions necessary to determine the type of material around the outburst event, apparent motion,  distance, and time of occurrence.

Side Note: There are other types of light echoes you might wish to research further. One of them is the so-called IR echo, where dust absorbs the outburst light, is warmed, and re-radiates light at longer infrared (IR) wavelengths. Another is known as recombination echoes. For the purpose of this post however, I’m referring to LE’s that are scattered and reflected off interstellar dust as described above.

How Light Echoes Work

As you can see in the illustration below, the light from an outburst actually reaches us in an “echo-like” fashion. In other words, it doesn’t all come at once, but rather, the light is scattered at various angles (forward-scattering and backward-scattering). One interesting thing to note is that because of the angles of the light, it gives the illusion of super-illuminal speeds (faster than light speed). Light echoes typically result in extreme brightness as well. For example, in early 2002, an ordinary star in a rather obscure constellation suddenly became 600,000 times more luminous than our Sun (hard to fathom), temporarily making it the brightest star in our Milky Way galaxy.

The Study of Light Echoes with Modern Technology

In the past 2000+ years, humans have watched only a few stars go supernova in the Milky Way. In the Milky Way galaxy, one occurs every 50 years or so. Therefore the ability to study them via LEs makes this work all the more important. In the past however, this wasn’t such an easy feat. Due to equipment limitations, limited data was available to astronomers thus limiting our knowledge of these outburst events. Fortunately, this has all changed in recent years with the advent of new technologies. Yea tech! For example, the study of light echoes was greatly improved when CCDs and telescopes with large field-of-views came on the scene. These advancements enabled astronomers to conduct the first wide-field time domain surveys with pretty amazing results. This new technology has resulted in successful studies of 400-900 year-old supernova remnants. You can read more about this study below (Rest et al., 2005).

Recent advances in spectroscopy and 3D spectroscopy have also greatly increased our ability to study the initial outburst in further detail, often even after it has faded (how cool is that?). These advancements have enabled new scientific opportunities by exploiting light echoes around centuries-old supernovae in the Milky Way and the Large Magellanic Cloud.

Learning More

Hopefully you now have a better understanding of light echoes than before you started reading this post. I find the phenomena fascinating and hope you do as well. For those of you that wish to learn more about LEs, I encourage you to read some of the articles below for more information on a few of the more recent discoveries. Do a search on the term light echo and you’re sure to discover even more. The referenced papers below are also useful for those of you that wish to really delve into the science of light echoes. Let me know your thoughts on this topic. Any input is appreciated as always!

Image Credit: NASA and A. Feild (STScI)


A. Rest, B. Sinnott, & D. L. Welch (2012). Light Echoes of Transients and Variables in the Local Universe PASA arXiv: 1204.1341v1

Rest A, Suntzeff NB, Olsen K, Prieto JL, Smith RC, Welch DL, Becker A, Bergmann M, Clocchiatti A, Cook K, Garg A, Huber M, Miknaitis G, Minniti D, Nikolaev S, & Stubbs C (2005). Light echoes from ancient supernovae in the Large Magellanic Cloud. Nature, 438 (7071), 1132-4 PMID: 16372003

Rest, A., Sinnott, B., Welch, D., Narayan, G., Mandel, K., Huber, M., & Blondin, S. (2011). ON THE INTERPRETATION OF SUPERNOVA LIGHT ECHO PROFILES AND SPECTRA The Astrophysical Journal, 732 (1) DOI: 10.1088/0004-637X/732/1/2


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