Image of the Month: Andromeda through different eyes

Our Image of the Month is a radically different view of the great spiral galaxy in Andromeda from the European Space Agency. Tracy McConnell explains how it was made and describes the stellar cataclysms it reveals.

Image of andromeda in IR and xrays

Light of other eyes, other times: The X-rays and IR radiation making up the image have been travelling for a long time. (Image credit: : CESA/Herschel/PACS/SPIRE/J.Fritz, U.Gent/XMM-Newton/EPIC/W. Pietsch, MPE)

The inspiration here came from a new ESA image of M31, the Andromeda Galaxy, composed from two very different images. One part of the image comes from the Herschel Space Observatory, which specialises in taking images of celestial objects in the far infrared. The other part of the image comes from the XMM Newton (X-ray Multi Mirror, check out the huge model of it hanging in our Exhibition Area) studies space using x-rays. This makes the composition hugely different from what you would see if looking at the Andromeda galaxy through an optical telescope. So why do we “look” at these objects using non-visible light?

There is much more information in light that we can’t see. But let’s start with what we can see. When we are very young, we are taught that red = hot and blue = cold. This isn’t exactly accurate. In fact you could say that the opposite is true. We can tell how hot an object is by the light it emits, and within the visible light spectrum red is hot (for us but not necessarily by celestial standards), but blue is much hotter. Looking into the night sky we can see simple evidence of this as red stars are cold (for stars) and blue stars are hot.  But when it comes to our Universe, the full range of light (or radiation) being emitted by other objects is not so easily examined when using detectors of visible light, like our eyes or cameras meant to mimic the human visual range. There’s is a whole spectrum of light out there that is non-visible. Most likely you’ve heard of these types of light/radiation, for example, X-rays, radio waves, infrared, ultraviolet, and gamma rays. So by looking at things through other forms of light, we can expand our perception of the Universe around us.

Looking at the spectrum diagram you can see that the infrared region is outside the “red” area of our visible range. So if red things in space are cold, infrared are much colder still. Whenever stars are born, or die, they leave behind very cold stellar dust, so cold it borders on absolute zero, -273°C (-459°F) . This dust is very hard to see in optical images but in infrared it has an ethereal glow, just enough to show up against utter blackness of the cosmos. By contrast, X-rays are found on the far side of the visible spectrum, far beyond blue and the ultraviolet range) so they can tell us about the VERY hot objects out there, millions of degrees hot, like newborn stars, black holes and the gases in the vicinity of neutron stars.

So now that we know a bit about the reasons for such images, what is this particular one telling us about our nearest galactic neighbour? Since we can’t “see” these types of light, false colour has been added to each layer to help us understand what we are seeing. The infrared part of the image taken by Herschel has applied a false orange colour, and the x-ray image taken by the XMM Newton has used false blue colour. So let’s take a closer look.

Our nearest galactic neighbour, the Andromeda Galaxy’s brightness is caused by the hundreds of billions of stars that are contained within. The light from these stars and this galaxy takes approximately 2.5 million years to reach us.

Image of M31 in infrared

Andromeda as seen by Predator (Image credit: ESA/Herschel/PACS/SPIRE/J.Fritz)

If we move on to the individual images of the composite we can “see” very different details.  In the infrared image we can see dense spiral arms of cool dust and gas where new star systems are being formed. This dust may be left over from a historic collision from a smaller galaxy long since consumed. The X-ray image looks at the other end of the stellar life cycle, showing us the hot death of various star systems. These systems may either be shockwaves resulting from exploded stars or paired stars where one is gravitationally ripping the other apart.

Image of M31 seen in x-rays

Andromeda as seen by Superman (Image credit: U.Gent/XMM-Newton/EPIC/W. Pietsch, MPE)

If we were to include the optical image in this composition we would have an in-depth study of the stellar life cycle within this galaxy: infrared for birth, optical for adulthood and x-ray for the extinction of these celestial power houses. Such studies can only serve to increase our understanding of the Universe around us.

Article by Tracy McConnell, Education Support Officer.