Composition of Interstellar Medium
COMPOSITION OF INTERSTELLAR MEDIUM
The interstellar medium (ISM) makes up only 10 to 15 percent of the visible mass of the Milky Way Galaxy. It comprises matter in the form of gas and dust (very tiny solid particles). About 99 percent of ISM is gas and the rest is dust. You may like to know: Which elements are present in the ISM? An important clue for investigating the composition of the ISM is the fact that the birth and death of stars is a cyclic process. This is so because a star is born out of ISM, and during its life, much of the material of the star is returned back to the ISM by the process of stellar wind (in case of the Sun, it is solar wind; see Unit 5) and other explosive events such as Nova and Supernova. The material thrown back into the ISM may form the constituents of the next generation of stars and so on. To know the basic composition of ISM, astronomers use photographs and spectra. In the following discussion, we shallconfine ourselves to the composition of the ISM of the Milky Way Galaxy in the neighbourhood of the Sun. Further, for simplicity, we will first discuss
INTERSTELLAR GAS
Hydrogen and helium are the two major constituents of interstellar gas; hydrogen constitutes about 70 percent and the rest is helium. Indeed, other elements are also present but in very small quantities. The analysis of the radiation received from ISM has enabled astronomers to classify the gaseous matter filling the interstellar space into the following four types:
i) H II region
ii) H I region
iii) Inter-cloud medium
iv) Molecular cloud
We now briefly describe these regions and their possible roles in the formation of stars.
i) H II region:
As the name suggests, these regions of ISM mainly consist of (singly) ionised hydrogen. In addition, these regions contain ions of other elements such as oxygen and nitrogen and free electrons. Since the ionisation energy of hydrogen atom is very high, such regions can exist only in the vicinity of very hot stars. H II region can be viewed even with naked eye in the constellation of Orion. If you look carefully at Orion’s sword, you will see that one of the objects is a hazy cloud of gas. This bright cloud is called Orion nebula. The spectrum of this nebula shows lines of hydrogen and some other elements in its bright line spectrum. Such bright nebulae are also called emission nebulae.
Orion nebula − a typical bright and diffuse nebula
It has been discovered that emission nebulae do not shine by their own light. They absorb high energy ultraviolet photons from hot stars. These photons ionise the gas in the nebulae and subsequently, low energy photons are radiated. Since the spectrum of the nebula consists of many emission lines of hydrogen, it indicates that the light must have been emitted by a low-density gas. Further, the emission lines of hydrogen are very strong and the red, blue and violet Balmer lines blend together resulting in the characteristic pink-red colour of the nebula.
Why are the H II regions always found in the emission nebulae?
Note that only those photons which have wavelengths shorter than 91.2 nm have sufficient energy to ionise hydrogen. Such high energy photons can be produced in sufficiently large numbers by hot (~ 25,000 K) stars only. And stars (such as hot O or B star) having temperature of this order are located in or near the emission nebulae. Further, H II regions have very low density (~ 10^9 particles m^−3 ). They provide observable evidence supporting the existence of matter in interstellar space.
ii) H I region:
Although it has been generally believed by astronomers that hydrogen atoms populate interstellar space, they could not detect H I gas till 1951. The
reasons are obvious: it is not possible for the neutral hydrogen in ISM to produce emission line as it is in the ground state. However, with the development of radio telescopes, it is now possible to detect H I region. The detection of H I in the ISM is based on the detection of a unique radiation of wavelength 21 cm.
What kind of transition produces such a low energy photon?
In a hydrogen atom, an electron revolves around the proton. Since the proton and the electron possess spin, there are two possible ways for their spins to align with respect to each other. The two spins may be parallel (aligned) or anti-parallel (anti-aligned). It is known that the parallel spin state of hydrogen atom has slightly more energy than the anti-parallel spin state. Therefore, if there is a flip from the parallel to the anti- parallel state, there is a loss in energy of the hydrogen atom and it results in the emission of a photon. The frequency and the wavelength of such emissions are 1420 MHz and 21 cm, respectively.
Can we obtain 21-cm radiation in laboratory conditions? It is not possible because the best vacuum that can be produced in a laboratory has a much higher density of atoms than that found in the ISM. So, in laboratory conditions, practically all the hydrogen atoms get de-excited due to higher collision rate and we cannot obtain 21-cm line. Thus, the only place where favourable conditions for emission of such radiations can exist is outer space. The 21-cm line was first detected in the year 1951 using a radio telescope even though it was predicted as early as in the 1940s. Since its detection, it has become a common tool in astronomy to map the location and densities of H I regions. This helps in determining the structure of galaxies including our own. Investigations of diffuse interstellar H I clouds suggest that their temperatures are in the range of 30 – 80 K, masses in the range of 1 – 100 solar masses and the number densities in the range of 10^8 – 10^9 cm−^3.
iii) Inter-cloud Medium:
Having read about clouds of ionised and neutral hydrogen in ISM, you would like to know: Is the space between the interstellar clouds empty? It is not so; the inter-cloud space consists of
a) neutral hydrogen gas with a density of 10^5 toms m^−3, and
b) hot (~ 8000 K) ionised gas with very low density (~ 10^4 ions m^−3 ).
Is there any interaction between the H I clouds and the inter-cloud medium?
The H I clouds are very cool and have high densities whereas inter-cloud medium has very low density and high temperature. The pressure of a region, being a function of its density and temperature, in the H I clouds and in the inter-cloud medium is about the same and they are in equilibrium.
iv) Molecular Cloud:
Analysis of optical spectra of interstellar medium reveals that matter exists in molecular form in ISM. Since hydrogen is the most abundant matter in ISM, it mainly consists of the hydrogen molecules (H2). However, molecules of hydrogen do not emit photons of radio wavelength and vast clouds of hydrogen molecules remain undetected by radio spectroscopy. Other molecules, such as CO (carbon monoxide), capable of emitting in radio wavelength, can indeed be detected. In fact, nearly 100 such molecules have been detected. But the basic question is: How do these molecules form in ISM? It is believed that the atoms come in the vicinity of each other and bond to form molecules on the surfaces of the dust grains. These molecules are very weakly bonded and can be easily broken by high energy photons. Thus, they can exist only deep inside dense clouds. Also, efficient release of energy by molecules makes these dense clouds very cool. These dense, cold clouds are called molecular clouds.
In our Galaxy, the largest of these cool, dense molecular clouds are called giant molecular clouds (GMC). They are 15 to 60 pc across and may contain 100 to 10^6 solar masses! The internal temperature of GMC is very, very low (~ 10 K). The question is: Do the giant molecular clouds have any role in the formation of stars? You know that young stars are surrounded by H II regions. And H II regions are invariably found near giant molecular clouds. This proximity indicates that the GMC plays an important role in the formation of stars. Thousands of GMC exist in the spiral arm of our Galaxy. The association of O and B main sequence stars with GMC suggests that star formation takes place in these regions.
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