Here is a short quiz for you. Do you know what the following five astronomical objects are?
(Sorry, you will not win a million dollar even if you can answer it correctly. Nevertheless, you can feel free to call up your friend to see if he / she can help.)
As an astronomy fan, you probably know that picture (a) is a spiral galaxy. But I guess most of you will not have any idea what kinds of object pictures (b) to (e) are. Never mind, let me give you some more time to think about the answers.
Time is up. Let me tell you the truth. In fact, pictures (b) to (e) are galaxies, too. More surprisingly, all the five pictures you have just seen are images of the same galaxy. This spiral galaxy is called M81 which is about 11.7 million light years away. I am not joking. The five pictures look so different because they are observed at different wavelengths, namely, in optical, ultraviolet, X-ray, far infrared and radio wave respectively.
Different physical processes occurring in an astronomical object may lead to production of light at different wavelengths. For example, light emits from the surface of a typical main sequence star falls mainly in the optical window. That is why we can see our Sun in daytime and countless stars at nighttime. In contrast, a pulsar emits a lot of its radiation in radio wave. Thus, the appearance of an astronomical object depends dramatically on the wavelength we choose to observe. Therefore, it is not surprising that most readers do not recognize pictures (a) to (e) are in fact taking from the same object.
With the advancement of detector technology, astronomers now can exploit observations in different wavelengths to study astronomical objects. For example, the ultraviolet image of M81 in picture (b) can be used to locate the very hot O type and B type stars in that galaxy. These two types of stars are so hot that most of their lights are emitted in the ultraviolet region. Moreover, the X-ray image of M81 in picture (c) may be used to find blackhole candidates. In fact, the bright X-ray spot at the center of M81 is likely to be the result of matter falling into a supermassive blackhole.
An even more aggressive tool that astronomers are currently using is to observe an object in different wavelengths at the same time, known as simultaneous multiple wavelength observation. By doing so, astronomers are able to deduce the sequence of physical processes or events occur on or near the astronomical object. Thus, they put themselves in a much better position to infer the behavior of the observed object. For example, the simultaneous multiple wavelength observation of certain pulsars help astronomers to more precisely determine their radiation emission mechanism. A more striking example comes from the study of gamma-ray bursts. For over about fifteen years, astronomers were debating whether gamma-ray bursts occur in our galaxy or at cosmological distance. This great debate was finally settled a few years ago by the (almost) simultaneous gamma-ray and optical observations of a few gamma-ray bursts. These observational data ruled out the galactic origin hypothesis and led to the definitive conclusion that gamma-ray bursts are coming from cosmological distances.