From the August 2024 issue

What is a light echo?

This phenomenon causes objects to expand faster than the speed of light — but it’s a trick of cosmic geometry.
By | Published: August 2, 2024
V838 Monocerotis
This sequence of Hubble Space Telescope images, taken from May to December 2002, shows a light echo moving outward through the star system V838 Monocerotis. Although the dust around the star appears to grow several light-years between the first and last images, this faster-than-light growth is an illusion. Credit: NASA, ESA and H.E. Bond (STScI)

In Hubble pictures taken just months apart of V838 Monocerotis, the dust ring around this star grows by several light-years during that time, called a “light echo.” How can something grow by 5 light-years in five months?

Ronald VanAtta
Ann Arbor, Michigan

Congratulations on noticing an effect called superluminal expansion, a phenomenon we see in a light echo. The echo appears to us to expand at many times the speed of light, seemingly violating the fundamental speed limit in the universe.

A light echo occurs when a star experiences an eruption or an object explodes in a region surrounded by interstellar dust. Light from the eruption travels straight to Earth, so that is what we see first. But the light also travels to the side, reflects off the dust, and then heads toward us. Because of the extra distance traveled, the reflected light arrives at Earth later on — like the voice of a yodeler echoing off the mountains around them.

Consider the Hubble image of the light echo of the erupting star V838 Monocerotis, taken in October 2002 — about 230 days after the light of the explosion peaked in March that year. Yet, the light echo at that time appears to have a radius of about 3 light-years. How could the light from the explosion travel 3 light-years in less than one year?

the geometry of a light echo
A simplified representation of V838 Monocerotis from above illustrates how the light from the explosion, which originates at the star, is reflected back to Earth along a paraboloidal surface. So, while some newly illuminated dust appears several light-years to the side of the star in our 2D view of the sky, we are actually getting light from regions in front of the star in three dimensions. Essentially, it’s as if we’re viewing the star from down a tunnel, rather than at the center of an expanding circle. Credit: Astronomy: Roen Kelly, after Howard E. Bond

The answer lies in the geometry of a light echo, illustrated in the diagram above. Here we see V838 Mon at the center, surrounded by a shell of interstellar dust extending out to a radius of about 6 or 7 light-years. At any moment in time, the light echo lies on a paraboloidal surface. This surface marks the location in space where the extra travel time for light to bounce off dust and reach Earth is 230 days. The illuminated dust that appears to be 3 light-years to the side of V838 Mon is actually almost 6 light-years in front of the star. So, it’s almost as if we are staring into a long tunnel in the dust, with the star nearly at the back end of the tunnel. The speed of light has not been violated!

As time passes, the paraboloid opens up and propagates into the background. Thus, each Hubble observation sampled a different region in the surrounding dust as the illumination swept through it. It’s an astrophysical CAT scan!

Howard E. Bond
Professor of Practice, Department of Astronomy & Astrophysics, Pennsylvania State University, and Astronomer Emeritus, Space Telescope Science Institute, University Park, Pennsylvania

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