Observations of the interstellar medium, together with calculations of stellar models have led astrophysicists to assemble a theory of how stars are born and why they shine. The theory of stellar genesis has been successful in explaining how stars are formed in vast clouds of interstellar gas and dust. Thanks to modern space-borne instruments like the Hubble Telescope we can look into these nurseries and observe the process of starbirth to see how it contributes to the making of a star. The interstellar medium contains:
Interstellar clouds, or nebulae, are clouds of particles and gases in interstellar space. When we look at the spectrum of the Orion Nebula, for example, we find that it contains bright emission lines of hydrogen, helium and oxygen. Nebulae which have characteristic bright emission lines in their spectra in this fashion are known as bright or emission nebulae.
Emission nebulae do not radiate light by themselves. Instead, they absorb ultraviolet photons from hot stars which are near or embedded in the nebula. The gas becomes ionised and emits photons at lower energies. The process of converting high energy photons into lower energy photons in this way is called fluorescence. For hydrogen gas the wavelength of the emitted photon is at 656.5nm which is why emission nebulae tend to be reddish in appearance.
The Intercloud Regions
Cold interstellar matter can exist in the form of simple molecules. Interstellar molecules have been found concentrated in dark, dense, cold aggregates called molecular clouds.
The Orion nebula is an example of a giant molecular cloud. Observations in recent years suggest that the vast bulk of the interstellar medium is comprised of conglomerates of giant molecular clouds which contain molecular hydrogen plus smaller, fractional amounts of other molecules. The average density is about 108 molecules m-3.
|Interstellar dust contains the elements H, C, O, Si, Mg and Fe in the form of ices, silicates, graphite, metals and organic compounds. The milky way contains vast lanes of dust which, being dark, were originally thought to be due to the absence of stars. In fact, interstellar dust forms about 1% by mass of all interstellar matter.|
The presence of dust is easily seen in the Horsehead Nebula and is an example of a Dark Nebula. The dust obscures the light from distant objects as well as from those inside it. Being able to see through the dust has now become possible thanks to advances radio and infrared astronomy which has revealed important new insights into the process of star formation.|
Interstellar dust can be detected due to four primary effects that it has on starlight. These are Extinction, Reddening, Polarisation and Infrared Emission.
Extinction is the dimming of starlight as it travels through the dust. The dust particles can either absorb some of the light or scatter it so that less light emerges from the dust than was incident.
Reddening occurs because blue light is more strongly scattered and absorbed than red (this incidentally is what happens in the earth atmosphere; scattering by air molecules makes the sky blue and the sunsets red).
Starlight can become polarised when passing through a dust cloud if the dust particles are (a) small compared to the incident wavelength, (b) if they are extended in length or (c) if they tend to be orientated in the same direction.
Finally, the dust particles emit Infrared Radiation. The IRAS satellite showed that infrared emission was the strongest in regions where there is a high concentration of interstellar gas. Each grain will absorb light from a nearby star and heat up until it emits in the infrared as much energy per second as it absorbs (see Orion Nebula pictures above).
What causes the clouds to become unstable and condense into smaller masses? Sir James Jeans (1877-1946) investigated how big a mass of gas is necessary to ensure that it will collapse under its own self-gravity. Jeans realised that there must be a critical value for a mass of gas such that above a certain limit, gravity would overcome the thermal motion of the particles which would otherwise disperse the cloud. The critical mass of gas is called the Jeans Mass.
In reality, the situation is complex. Inhomogeneities give rise to more than one centre of attraction, interstellar gas clouds are generally irregularly shaped and will be rotating. Interstellar magnetic fields may deflect particles and obstruct their attraction towards the centre. Despite these complexities, the Jeans theory provides a simplified starting point in trying to explain the very elaborate process of starbirth.
How long does it take for a cloud to condense to form a star? We assume that the least time for this to happen is when the cloud condenses entirely under the influence of gravity and neglect any internal pressure resisting the collpase. This is called the free-fall collapse time, and can be shown to depend only on the cloud's initial density and not its mass.tff = 2.11 x 10-3 / D1/2years where D is the average density of the cloud
As the cloud collapses, particles collide with each other increasing the thermal energy of the molecules. As long as the cloud is mostly transparent to infrared radiation, this thermal energy is radiated into space and the cloud remains relatively cool and its internal pressure low. However, as the free-fall continues, the density of the cloud increases and it becomes opaque. The thermal energy becomes trapped and the cloud starts to heat up. An important first stage in the formation of the star is then reached when the temperature (and density) of the cloud have reached a level where the force due to internal pressure balances that due to gravity. This condition is called hydrostatic equilibrium.
When the potential star is in the free-fall stage it is called a protostar. When in hydrostatic equilibrium and gravitational energy is its source of heat and radiation, it is called a pre-main sequence star. Pre-main sequence stars radiate heat and light very weakly. Thus starbirth can only be inferred from infrared and radio observations.