Andromeda, our neighbouring galaxy

In the autumn night sky we can observe the Andromeda constellation. One of the most fascinating objects in the firmament can be found in this constellation— the only object outside our own galaxy that is visible to the naked eye for observers in the Northern Hemisphere. The Andromeda galaxy is also known as M31. According to Greek mythology, Andromeda was Cepheus’ and Cassiopeia’s daughter, and was punished for her parents’ sins. She was chained to a cliff to pacify a sea monster sent by Poseidon. Perseus managed to kill the monster and released her, and then married her. According to tradition, the Persians are Perseus’ and Andromeda’s descendants. As usual, the classic writers set this story in the sky, and thus Cepheus, Cassiopeia, Andromeda and Perseus have a place in the firmament, near the Pole Star.

Andromeda is visible to the naked eye, as a nebula, for observers with an excellent sight and a good location. Unfortunately, in our towns and villages it is increasingly more difficult to observe the sky, and we can’t have the experience of observing the closest galaxy to Earth, which has a similar size to ours, «only» two million and a half light years away from us. It is the furthest object in the sky visible without the aid of a telescope. This is possible due to the hundreds of thousands of stars that can be found within this galaxy. The naked eye can only observe its nucleus — the complete spiral structure of the galaxy is only visible in long-exposure observations. Andromeda’s diameter is approximately two times the Milky Way’s diameter. It is shaped as a flattened disc with a central nucleus and bright spiral arms that surround it. In 1923, Edwin Hubble discovered a Cepheid variable star observing the branches of the Milky Way and identified Andromeda (along with many other nebulae known from the 18^th century) as an object that clearly did not belong to the Milky Way.

Nowadays we know that its spiral arms shine because they are «breeding ground» for new stars that illuminate their environment with blue light according to their age and temperature. On the contrary, dark filaments are dust concentrations, opaque to visible light. Cosmic dust is actually similar to what we call «smoke» in our daily lives —a concentration of tiny particles, individually imperceptible, that extinguish the light behind it. The composition of these tiny particles varies —from silicates (similar to sand grains) to carbon compounds (similar to cinders), or even aggregates of organic molecules.

«Energy is not created or destroyed; only transformed». This statement is can also be applied here. The energy contained in the light absorbed by the dust (together with the energy absorbed by the collision between gas particles) is used to heat up the tiny particles a little bit above their cold environment. Bodies at environment temperature emit light at a wavelength imperceptible for us. At approximately 1000ºC the same body will emit a red light. If it reached 5000 ºC, it would emit white light, like the Sun does. Cosmic dust that is heated up to 100 Kelvin degrees (-200ºC approx.) in space emits also a radiation, but not in the form of visible light, but infrared light. That is why the dust clouds in Andromeda’s disc look dark.

What would we see then if we looked directly to Andromeda using night-vision glasses? These glasses are like the ones we can see in action films —a body can be seen in the night due to the heat they emit. In this case we would be able to see most of the light that would come from the dust itself. This way, what in visible light is shown as dark areas would be shown as the brightest, becoming a kind of «cosmic negative». Such picture has been taken by the Spitzer Space Telescope, which uses the best infrared technology.

The picture is amazing, not only because it shows that cosmic dust emits light at the expected wavelength; but also because the internal structure of the rotating disc can be observed. However, this is not so in visible light pictures, because the density of the stars in its nucleus makes direct observation impossible.

Vicent J. Martínez. Director of the Astronomical Observatory, University of Valencia    
Alberto Fernández-Soto. Astronomy and Astrophysics Departament, University of Valencia.
© Mètode 53, Spring 2007.