Chapter 3
Discovery and Detection

Trumpler (1930) presented the first conclusive evidence of general interstellar extinction, and was able to estimate its average value.

The process that led him to his discovery involved taking measurements of the angular diameters and integrated fluxes of a large sample of open galactic clusters, each containing stars in uniform distribution close to the galactic plane. He analysed his results in two ways.

If he assumed the measured clusters had a mean diameter , their distances , could be estimated from:

their estimated intrinsic luminosities were then:

His results, based on this supposition, were puzzling, since they implied the following conclusion.

However, if he assumed that the measured clusters had a mean luminosity their distances could be estimated from:

their estimated linear sizes were then:

Trumpler’s results, based upon this, implied an equally puzzling conclusion.

Trumpler, unconvinced by the conclusions he had come to, formed the premise of an obscuring interstellar medium. This, he found, was able to link together both his previous attempts. Viewed through an obscuring interstellar medium distant clusters would seem dimmer without being made to look smaller in angular size.

He was able to estimate the average extinction caused by the interstellar medium thus:

Taking the standard magnitudes equation:

where;

• = the apparent distance to a cluster,
• = the apparent magnitude,
• = the absolute magnitude,

he added a constant to directly counteract the apparent increase of linear diameter, observed under his hypothesis,

He was then able to calculate a value for this constant 'a'

A corresponding value of 'd' was also calculated by Bottlinger & Schnellet (1930) by considering the dispersion of the cepheids perpendicular to the galactic plane. Van de Kamp (1932), also calculated a corresponding value, by the consideration of extra-galactic nebulae towards the pole of our own galaxy.

Recent values of the visual extinction coefficient of the dust plane have placed 'a' at a value:

3.1 Extinction Curves

The wavelength dependence of interstellar extinction is determined by comparing the dust-reddened energy distribution of individual stars with slightly reddened 'comparison' stars of similar temperature and luminosity.

An extinction curve describes, in graphical form, how the attenuation of starlight, due to interaction with dust clouds, varies with respect to wavelength.

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When extinction curves are normalised variations from cloud to cloud can be observed in the strength and width of the prominent feature and the steepness of the far-ultraviolet rise. This tells us that the size and/or composition of the grains on these lines of sight are different from the norm.

3.2 Dark Clouds

Dark clouds, or Barnard nebulae, have been sources of curiosity from as far back as 1784, when Herschel noticed a region devoid of stars. Until 1889 and the advent of celestial photography, the regions were thought of as holes in the galaxy and possible evidence for its break-up. Trumpler proposed conclusive proof that they were silhouettes of obscuring dust clouds, in 1927.

Dark nebulae are dense regions of interstellar dust, often associated with HII regions, which are viewed as a silhouette against a bright background. The dark 'lanes' in the Milky Way are the largest known of these phenomena. Their physical characteristics are dependent on their size. On average, their kinetic temperature is ~10K, the heating source, of which there could be many, are thought to include IR regions, starlight and gravitational collapse. The densities of the dust clouds are almost 100 times greater than that of the general interstellar medium. We expect a significant depletion of heavy elements in the gas phase, due to the growth of grain mantles.

3.3 Reflection Nebula

Most reflection nebulae are blue because the tiny particles of dust that reflect are very small. Like particles of smoke, they preferentially scatter blue light rather than other colours, which is why most reflection nebulae are bluish in colour. Some reflect the light of the star that illuminates them, such as that around Antares, while others reflect a brief flash of starlight from a supernova. In some cases, the dust reflects the light of another nebula, and since there is no blue light to reflect, the reflection nebula appears yellow.

There are two types of reflection nebula:

• The first and most familiar form, called an interstellar reflection nebula., is the illumination of surrounding dust by intrinsically luminous star(s), e.g., the Dumbell nebula.

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• The second, high galactic latitude nebula, are illuminated by the integrated light of the galactic plane. This results in the Diffuse Galactic Light (DGL).