An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

8.4 Line spectra: line flux and equivalent width

In general, astronomical objects emit both continuous emission and lines superimposed on this continuous emission. The equivalent width is a useful way of describing the relative strength of a line compared to the continuous emission at nearby wavelengths. The flux level of the continuous spectrum is called the continuum level, and at an emission line the spectrum rises above this level, while at an absorption line the spectrum dips below this level, as Figure 27 shows.

Figure 27
Figure 27 The continuum level, Fλc, (blue), and an absorption line (red). The equivalent width, W λ, is the width of a rectangle the same height as the continuum which has the same area as the absorption line

The line flux is simply the total amount of energy per unit collecting area per unit time carried by line photons. On a graph of the spectrum this is the area corresponding to the line.

The equivalent width, Wλ, is the width of a rectangle having the height of the continuum and which has the same area as the line; the units of equivalent width are thus the same as the units of wavelength.

The Hertzsprung–Russell diagram is used to characterise the properties of stars. The colour (e.g. BV) is used on the horizontal axis of the Hertzsprung–Russell diagram as an empirical indicator of temperature. Magnitude, or another luminosity indicator, was plotted on the vertical axis. Consequently, the H–R diagram is also sometimes referred to as a colour–magnitude diagram. The main sequence formed a roughly diagonal locus across the colour–magnitude diagram. BV is just one of a number of possible colours, and each spectral type has a definite expected value for any colour. In the next activity you will encounter a colour–colour diagram.


Question: What advantage does a colour–colour diagram have over a colour–magnitude diagram when comparing objects which have varied and unknown distances?


Colours are unaffected by changes in distance (except if reddening is important), so objects can be compared by simply plotting their measured colours. To make a colour–magnitude diagram the distance is needed to convert from apparent magnitude to absolute magnitude when the objects are not all at the same distance.

The following question is to check that you are aware of a term which is used in the next reading.

SAQ 10

Question: What is the ‘Wien tail’?


The Wien tail is the short-wavelength (or equivalently the high-energy) part of the blackbody spectrum.


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