Variable Stars: V838 Monocerotis

Observing changes in a light echo
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V838 Monocerotis Interactive

A central star is surrounded by circular layers of dust
Star surrounded by dust filling twice the area of May 2002
Star surrounded by dust varying from red to blue with holes
Central star and diffuse surrounding stars against dust
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As the central star in V838 Monocerotis suddenly brightened in 2002, its pulse of light illuminated the surrounding dust.

The dust itself is not rapidly expanding, but as the light travels, it illuminates previously ejected dust farther away.

The light reflects or “echoes” off the dust and then travels to Earth, allowing researchers to capture this incredible scene.

As light leaves the areas closest to the star and travels out, the light echo reveals different patterns in more distant dust.

May 2002
October 2002
December 2002
October 2004

A Story Of Variable Stars: V838 Monocerotis

Watching a light echo bounce off dust in real time.

The brightness of some stars varies over time due to a variety of factors, including their age and interactions with other nearby objects. Some temporarily increase in brightness for several days or weeks. Others have periodic changes in brightness that can be measured over years. In other examples, stars change in brightness based on the environments around them. The variation in the brightness of some stars tells us about their evolution, and helps researchers learn about their properties inside and out.

In early 2002, V838 Monocerotis temporarily increased in brightness to become 600,000 times brighter than the Sun. It had erupted several years earlier, which was undetected, but may have been the result of two stars merging within a three-star system. Researchers made repeated observations over several years, allowing them to trace the light as it traveled away from the star and through layers of surrounding dust. Light that was emitted earlier, when the star was hotter, still appears bluer.

As the years progressed and the light spread farther, scientists had to focus Hubble on the object for longer periods: Light intensity of the outburst decreases as it spread out, which is why the stars appear brighter in later images. In astronomical terms, most events typically unfold in millions or billions of years, so observing this light echo was an incredible opportunity to capture significant changes over only a few years.

Quick Facts: V838 Monocerotis

Also known as: V838 Mon

Type of object: Transient variable star

Distance from Earth: 20,000 light-years

Location in the sky: Monoceros Constellation

Did you know: An amateur astronomer was the first person to detect the change in this star, enabling professional astronomers to follow up with the Hubble Space Telescope.

Explore More About Variable Stars

Find out more with these additional resources from NASA’s Universe of Learning

Credits: V838 Monocerotis

All images from the Hubble Space Telescope: NASA, ESA and H.E. Bond (STScI)

Subject-matter expertise provided by Dr. Howard Bond

Produced by the Space Telescope Science Institute’s Office of Public Outreach