Andromeda is So Hot

This mosaic of the Andromeda spiral galaxy highlights explosive stars in its interior, and cooler, dusty stars forming in its many rings. The image is a combination of observations from the Herschel Space Observatory taken in infrared light (seen in orange hues), and the XMM-Newton telescope captured in X-rays (seen in blues). NASA plays a role in both of these European Space Agency-led missions.

Herschel provides a detailed look at the cool clouds of star birth that line the galaxy's five concentric rings. Massive young stars are heating blankets of dust that surround them, causing them to glow in the longer-wavelength infrared light, known as far-infrared, that Herschel sees. 

In contrast, XMM-Newton is capturing what happens at the end of the lives of massive stars. It shows the high-energy X-rays that come from, among other objects, supernova explosions and massive dead stars rotating around companions. These X-ray sources are clustered in the center of the galaxy, where the most massive stars tend to form.

Andromeda is our Milky Way galaxy's nearest large neighbor. It is located about 2.5 million light-years away and holds up to an estimated trillion stars. Our Milky Way is thought to contain about 200 billion to 400 billion stars. 

Andromeda: our nearest large galactic neighbour

5 January 2011
Two ESA observatories have combined forces to show the Andromeda Galaxy in a new light. Herschel sees rings of star formation in this, the most detailed image of the Andromeda Galaxy ever taken at infrared wavelengths, and XMM-Newton shows dying stars shining X-rays into space.

During Christmas 2010, ESA’s Herschel and XMM-Newton space observatories targeted the nearest large spiral galaxy M31. This is a galaxy similar to our own Milky Way – both contain several hundred billion stars. This is the most detailed far-infrared image of the Andromeda Galaxy ever taken and shows clearly that more stars are on their way.

	Andromeda Galaxy seen in infrared

Sensitive to far-infrared light, Herschel sees clouds of cool dust and gas where stars can form. Inside these clouds are many dusty cocoons containing forming stars, each star 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. It will emerge from its birth cloud and become visible to ordinary telescopes. Many galaxies are spiral in shape but Andromeda is interesting because it shows a large ring of dust about 75 000 light-years across encircling the centre of the galaxy. Some astronomers speculate that this dust ring may have been formed in a recent collision with another galaxy. This new Herschel image reveals yet more intricate details, with at least five concentric rings of star-forming dust visible.

Andromeda Galaxy in X-rays

Superimposed on the infrared image is an X-ray view taken almost simultaneously by ESA’s XMM-Newton observatory. Whereas the 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 the stars are naturally found to be more crowded together. Some of these are shockwaves and debris rolling through space from exploded stars, others are pairs of stars locked in a gravitational fight to the death.
In these deadly embraces, 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 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.

	Andromeda Galaxy in infrared and X-rays

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. The twinkling starlight seen from Earth is indeed a beautiful sight but in reality contains less than half the story. Visible light shows us the adult stars, whereas infrared gives us the youngsters and X-rays show those in their death throes. To chart the lives of stars, we need to see it all and that is where Herschel and XMM-Newton contribute so much.