Hubble Heritage

Many Epochs of V838 Mon

(click to enlarge)

This image shows a time sequence of Hubble Space Telescope images of the light echo around V838 Mon, taken between May 2002 and October 2004. All six pictures were taken with Hubble's Advanced Camera for Surveys using filters sensitive to blue, visible, and infrared wavelengths. The apparent expansion of the light echo, as light from the early 2002 outburst of V838 Mon propagates outward into the surrounding dust, is clearly shown.

All of the images are shown at the same scale. Moreover, the images are also shown as they would appear for the same exposure times throughout the sequence. Thus the background stars appear constant in brightness, while the surface brightness of the light echo steadily declines. The fading of the light echo is primarily due to the light-scattering properties of interstellar dust. Consider a street lamp on a foggy night. The halo around the lamp is brightest right next to the lamp, while out to the side it is much fainter. Similarly, in the first V838 Mon image, taken in May 2002, the light echo was very bright and compact. At later times, we are seeing dust out to the side of the star, rather than dust that is immediately in front of the star, so the amount of light scattered in our direction is smaller. Hubble astronomers expect the light echo to continue to change its appearance and brightness over the next several years.

Animation: V838 Mon Light Echo Unveiling

Quicktime (2.2 MB) | Quicktime (671KB) | MPEG (2.3MB)

A dissolve sequence of six images taken by Hubble's Advanced Camera for Surveys shows a CAT-scan-like probe of the three-dimensional structure of the shells of dust surrounding the aging star V838 Monocerotis. The sequence reveals dramatic changes in the way a brilliant flash of light from the star is reflecting off surrounding dusty cloud structures. The effect, called a light echo, has been unveiling never-before-seen dust patterns ever since the star suddenly brightened for several weeks in early 2002.
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Animation Credit: NASA, ESA, and G. Bacon (STScI)

Questions and Answers

LIGHT CONTINUES TO ECHO THREE YEARS AFTER STELLAR OUTBURST

The Hubble Space Telescope's latest image of the star V838 Monocerotis (V838 Mon) reveals dramatic changes in the illumination of surrounding dusty cloud structures. The effect, called a light echo, has been unveiling never-before-seen dust patterns ever since the star suddenly brightened for several weeks in early 2002.

1. What is a light echo?

It is light from a stellar explosion echoing off dust surrounding the star. V838 Monocerotis produced enough energy in a brief flash to illuminate surrounding dust, like a spelunker taking a flash picture of the walls of an undiscovered cavern. The star presumably ejected the illuminated dust shells in previous outbursts. Light from the latest outburst travels to the dust and then is reflected to Earth. Because of this indirect path, the light arrives at Earth months after light from the star that traveled directly toward Earth.

2. Why did the star produce this outburst?

Astronomers do not fully understand the star's outburst. It was somewhat similar to that of a nova, a more common stellar outburst. A typical nova is a normal star that dumps hydrogen onto a compact white- dwarf companion star. The hydrogen piles up until it spontaneously explodes by nuclear fusion -- like a titanic hydrogen bomb. This exposes a searing stellar core, which has a temperature of hundreds of thousands of degrees Fahrenheit.

By contrast, V838 Monocerotis did not expel its outer layers. Instead, it grew enormously in size. Its surface temperature dropped to temperatures that were not much hotter than a light bulb. This behavior of ballooning to an immense size, but not losing its outer layers, is very unusual and completely unlike an ordinary nova explosion.

The outburst may represent a transitory stage in a star's evolution that is rarely seen. The star has some similarities to highly unstable aging stars called eruptive variables, which suddenly and unpredictably increase in brightness.

3. Are light echos similar to sound echos?

The echoing of light through space is similar to the echoing of sound through air. As light from the stellar explosion continues to propagate outwards, different parts of the surrounding dust are illuminated, just as a sound echo bounces off of objects near the source, and later, objects further from the source. Eventually, when light from the back side of the nebula begins to arrive, the light echo will give the illusion of contracting, and finally it will disappear.

4. Where is V838 Mon located?

V838 Mon is located about 20,000 light-years away from Earth in the direction of the constellation Monoceros, placing the star at the outer edge of our Milky Way galaxy.

5. How can the visible structure around the star grow from 4 to 7 light years in less than a year's time?

One of the properties of light echoes is that they appear to violate the limit imposed by the speed of light, but this in only a geometric illusion. The echo does indeed appear to expand at greater than the speed of light, due to the peculiar geometry of a light echo. The dust illuminated at the outer edge of the echo is indeed about 5 light years from the star, but this dust is WELL IN FRONT of the star, not at the same distance as the star. So, this light has only gone slightly out of its way off to the side, and was then reflected in the direction toward the Earth. The light did not travel faster than the noted speed of light.

This illusion that the speed of light has been violated can be understood in more detail if you take a look at the graphic called Anatomy of a Light Echo, and consider one of the paraboloids, such as the one labeled "2". At one particular time, all of the dust along this paraboloid is illuminated, all the way down to the end of the parabola at the lower right. This illuminated material down at the end appears to lie at the largest distance from the star, as seen from Earth, but as you can see it is well in front of the star and only slightly off to the side.

Now follow the parabola up to the vicinity of the star, and consider in particular the point where the angle from the star and then to the Earth is 90 degrees. The distance from the star to THAT point is how far the light has traveled, at the speed of light, in the plane of the sky, which as you can see is much less than the apparent radius from the star that has been reached down at the lower right, as seen from Earth.