The Interstellar Medium

Interstellar Extinction

We often see dark bands in pictures taken of the Milky Way such as these:

It was earlier speculated that these regions are devoid of stars. This would be quite strange it it were true. An explanation for this was to be found soon enough.

These dark regions are caused by the presence of dust in the path of the light. This is called extinction.

These are called dark nebulae. Another famous example of such a dark nebula is the Even though it may appear dark in visible light, we can see the same in light (captured by the Hubble Space telescope) to see the presence of dust clearly. Dust emits light in infrared.

When these dust clouds are present near stars, they scatter and reflect the light form the stars and appear bright. These are known as reflection nebulae.

The is one such example. Dust clouds reflect shorter wavelengths and transmit longer wavelengths. This is why reflection nebulae appear blue and dark nebulae and stars behind the dust clouds appear red. This is known as interstellar redenning.

Dust blocks light and we quantified this using the modified formula for the apparent magnitude by including the extinction $A$ . $$m-M=5\log{\frac{r}{10 \text{ pc}}+A}$$ Remember that:

$m$ is the apparent magnitude

$M$ is the absolute magnitude

$r$ is the distance to the source in parsec

Extinction $A$ itself depends on the distance to the source. The greater the distance, the more ISM the light would have to travel through. We saw that $$A = a\times r$$ and that $a = 2 \text{ mag/kpc}$

The value of this extinction $A$ also depends on the wavelength of light $A=A(\lambda)$. Generally, due to the properties of the generic ISM, UV is absorbed more by it and hence there is an excess of absorption in the UV wavelength. See, the graph below: