Identify key lines (part I)

As soon as the intensity curve is revealed, some simple spectral lines can be recognized with some experience. This the tricky part of the Spectral processing as there is no automatic tool to immediately detect and tag the lines on every spectrum. The hardest part is when the very first spectrum is displayed as the parameters of spectral sampling are mainly unknown. Usually, you have no idea of which region of the wavelength you are looking at, even in which direction is displayed the spectrum (from blue to red or reverse...), and of course how much wavelength represents a pixel... Through Visual Spec, several strategies can be pursued...

Detect through classical line Using spectral library Atmospheric lines Just try !

Detect classical lines

The first strategy consists into identifying key, well-known lines. Which are they ? The list below is a good start, as you wil become more and more familiar you will add your own... lines Wavelength (Angstrom) H-Alpha 6563 Sodium doublet 5890, 5896 Magnesium triplet 5167, 5173, 5184 Atmospheric lines 6875, 7610, see below

The Sodium doublet as for example, or a strong line which can be H-alpha. Not all the stars exhibit these features, it all depends of the spectral type.

Atmospheric lines

Atmospheric lines can also play an interesting role when looking for characteristic features. Water vapor has molecular large band lines at precise wavelength which can help to graduate intensity curve into wavelength as they show up in any spectral type, coming from the earth itself. The only requirement is that spectrum wavelength domain recorded shall cover up to close infrared (from 6875 to 7700 angstrom) where these bands stand.

Atmospheric lines can also play an interesting role when looking for characteristic features. Water vapor has molecular large band lines at precise wavelength which can help to graduate intensity curve into wavelength as they show up in any spectral type, coming from the earth itself. Vega spectrum Sun spectrum The only requirement is that spectrum wavelength domain recorded shall cover up to close infrared (from 6875 to 7700 angstrom) where these bands stand.

Using standard spectrum from library

Visual Spec offers you a set of spectrum from almost all the classical spectral type. They are in the Libspec directory in .dat format. To know which spectral type is the star you are studying, use the "spectral type" function. It will gives you, from an extract of the Bright Stars catalog, the spectral type by just indicating the name of the star.

Go in the file menu section, click on the "import .dat" section, go in the libspec directory and select the spectrum which name is the closest of the star spectral type you looking at. Up to 131 standard spectral type spectrum are provided, ranged from 1160 angstrom up to 10 590...

The total spectrum calibrated of the standart spectral type being displayed, the game consists into finding similar features in the two of them. The wavelenght of each feature can be noted from the standard spectral type spectrum and then use to calibrate the orginal spectrum. two lines are required for a classical linear calibration, three if a non-linear one is ncessary, depending of the spectroscope design... In this example, the F2III.dat spectrum compared to the above original spectrum allows the identification of the Balmer lines. On the original spectrum, the H-alpha line is not visible.

Do you want to try ?

Try to identify the key features in the image below, then move the mouse on the image to verify ...