::Interactive Astronomy::

Diffuse Nebula

Interstellar Medium and Nebula

Any interstellar cloud of gas and dust is called a nebula or nebulosity. There are 3 types of nebula.

Emission nebulas

These are star clouds that emit their own electromagnetic radiation. An example of this is Orion Nebula in the constellation Orion, which forms the ‘sword’ hanging from the ‘belt’. Emission nebula are found near hot and luminous stars (spectral types O and B), and the radiation emitted from them ionizes the gas.

Therefore, they are composed of mainly ionized hydrogen atoms and electrons. Since it contains ionized hydrogen atoms, Astronomers call it H II regions, where H I regions are neutral and have unionized hydrogen atoms.

Emission nebulas are red in colour because the free particles- protons and electrons- get together to form hydrogen atoms, this causes visible-light photons to be emitted in the red portion of the visible spectrum.

Dark Nebula

Dark nebula comprises dust grains in the interstellar medium. A dark nebula is so opaque due to its vast size more than tens of light-years across that no visible light can pass through from stars behind it.

This is where all stars begin their lives, deep within interstellar medium. Within these dense clouds of interstellar medium, the densest portions of mass consisting of 74% hydrogen, 25% helium and 1% other heavier elements can contract under their mutual gravitational attraction to form proto stars.

The Coal Sack Nebula in Crux. As there are many solar masses of material within the dark nebula, many proto stars can form out of a single such nebula.

Dark nebulae are parts of the interstellar medium which have high enough density and low enough temperature for stars to form. They are also known as Barnard objects, after their discoverer, Edward Barnard. Bok Globules are small, nearly spherical dark nebulae which resemble inner cores of Barnard objects with the outer, less dense portions stripped away.

As dark nebula can be difficult to detect if they are far away due to interstellar extinction, astronomers also use longer-wavelength radiation which is not affected when it passes through interstellar dust. By detecting high amounts of carbon dioxide molecules, astronomers can also deduce the amount of hydrogen in star clouds which are probable dark nebula as the ratio of carbon dioxide molecules to the ration of hydrogen molecules in space is about 1:10 000.

Reflection Nebula

Reflection nebula reflects star light from nearby stars. Light has been scattered by the tiny dust grains (measuring about 500 nanometres- a very small size) which it contains. Due to its small size, blue light of shorter wavelength is scattered more efficiently than red light of long wavelength, giving this type of nebula its distinctive blue colour

Interstellar Reddening

This effect of scattering of light also causes interstellar reddening as the blue component of star light is scattered and absorbed by the dust grains. This is also why the sun appears red at sunset as the dust in the atmosphere scatters the light from the sun (which has to travel through more atmosphere than when the sun is at the zenith). This effect also causes interstellar extinction which light from distant stars are dimmer than expected from their distance because interstellar medium scatters it.

How Supernovas may contribute to star birth

Supernovas create powerful shock waves that can compress interstellar medium. When it hits dense clumps of hydrogen, this collision heats the gas and compresses it and excites the molecules, causing it to glow, stimulating star birth.