Birth and death in Andromeda

M31 Andromeda galaxy

The Andromeda Galaxy, seen at several wavelengths to reveal different stages of the stellar life cycle. Infrared shows reservoirs of gas in which stars are forming. Optical shows adult stars. X-rays show the violent endpoints of stellar evolution, in which individual stars explode or pairs of stars pull each other to pieces.

  • Andromeda Galaxy is the nearest large spiral galaxy
  • Contains a strange dust ring 75,000 light-years wide
  • Infrared and X-ray views show stars forming and dying

TWO SPACE TELESCOPES have combined forces to show the Andromeda Galaxy in a new light.

Using data from the European Space Agency’s (ESA) Herschel and XMM-Newton telescopes, the image shows the light of newborn stars and X-ray emission from dying stars.

Andromeda, also known as M31, is the nearest large spiral galaxy and is similar to our own Milky Way. Both contain several hundred billion stars.

Herschel was used to produce the most detailed far-infrared image of Andromeda ever taken, showing clearly that more stars are being added to the galaxy.

Sensitive to far-infrared light, Herschel sees the clouds of cool dust and gas where stars can form. Inside these clouds are many dusty cocoons containing still-forming stars, each one pulling itself together in a slow gravitational process that can last for hundreds of millions of years.

Once a star reaches a high enough density, it will begin to shine at optical wavelengths, whereupon it will become visible to normal telescopes.

Andromeda is interesting because it shows a large ring of dust about 75,000 light-years wide encircling the centre of the galaxy. Some astronomers speculate that this ring might be a “scar” that formed after a recent collision with another galaxy.

Herschel space telescope

Artist's impression of the Herschel space telescope

The new Herschel image reveals yet more intricate details, with at least five concentric rings of star-forming dust apparent.

X-rays of stellar corpses

Superimposed on the infrared image is an X-ray view taken almost simultaneously by XMM-Newton. Whereas infrared shows the beginnings of star formation, X-rays usually show the endpoints of stellar evolution.

XMM-Newton highlights hundreds of X-ray sources within Andromeda, many of them clustered around the centre, where stars are more crowded together.

Some of the X-ray sources reveal shockwaves rolling through space from exploded stars. Others indicate pairs of stars locked in a gravitational fight to the death.

In the latter case, one star has already died and is pulling gas from its still-living companion. As the gas falls through space, it heats up and gives off X-rays.

The living star will eventually be greatly depleted, having had much of its mass torn from it by the stronger gravity of its denser partner. As the stellar corpse wraps itself in this stolen gas, it could explode.

Both the infrared and X-ray images show information that is impossible to collect from the ground because these wavelengths are absorbed by Earth’s atmosphere.

Adapted from information issued by ESA. Image credits: Infrared, ESA / Herschel / PACS / SPIRE /J. Fritz, U. Gent; X-rays, ESA / XMM-Newton / EPIC / W. Pietsch, MPE; optical, R. Gendler.

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