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Molecules in Space
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4.4 Molecules in Space
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Interstellar Reddening
Stars behind the dark molecular cloud B68 could not be seen at optical wavelengths, but reveal themselves when viewed with infra-red light optics and detectors. This also means that when viewed over an extended broadband spectrum, the light from behind the cloud will look redder than it actually is. This is known as interstellar reddening, and B68 presents a pretty extreme case of it.
What is interstellar reddening in general? When we learned about diffraction of light, we saw that red light diffracts less than green or blue light. Such wavelength dependence is generally exhibited when the size of the object in the path of the light is comparable to the wavelength of the light. When light scatters off small particles, red light is less affected and are able to transmit more easily through a region of gas than its shorter wavelength counterparts. This is the reason why the sun looks red when it is low on the horizon at dawn or dusk. Conversely, blue or shorter wavelength light scatters more and this is the reason why the sky is blue!
With the use of telescopes that see in infra-red light, we can peer through the dense cloud of B68. But what if we want to know what is inside the cloud itself? Take a spectrum! Or equivalently in the context of radio astronomy, view the cloud at wavelengths corresponding to molecular transition!
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Identification of Molecules
Recall that molecules have rotational energy levels, and transitions between these levels absorb or emit wavelengths in the microwave/radiowave regime. In the interstellar medium (ISM) where temperature is cold, these low energy transitions can take place quite frequently. Taking a spectrum or viewing the ISM at these wavelengths reveal a wealth of chemical species.
Try it yourself!
See the spectrum and table below. Try to identify some of the chemical species in TMC-1.
Astro-chemists do not typically present their data like in the figure above (
To obtain the above results, in the authors' words
We used the 30 m IRAM millimeter-wave telescope located at Pico Valeta in Spain for the observations reported here. Observations were obtained during three periods: 2000 April and August and 2001 April. The dual mixer, dual-channel receiver was tuned to observe the J ¼ 1 0 transition of C18O at 109.78218 GHz, the 1-0 transition of N2H + at 93.173178 GHz, and the J=2-1 transitions of C32S at 97.980968 GHz and C34S at 96.412982 GHz. Observations of the J=1-0 transition of C17O and the J=3-2 transition of N2H + were also obtained and reported elsewhere.
Although not as widely probed, ro-vibrational spectrums are also taken and analysed by astro-chemists. Some examples are shown below.
