This post is part of the Young Astronomers Databank Project.
In the first part of this guide I looked at the basics of spectra with particular focus on black-body spectra. In this section I will take a look at a few specific emission features and what they can tell us about an object.
So lets begin our journey …
Spectral Notation
As each element and ion produces its own set of spectral lines scientists have created a standard notation of showing which line is produced by each atom or ion.
Any produced by an atom is labelled as – (x)I where (x) is the chemical symbol of the atom involved e.g. a line produced by the atom sulphur would be labelled as SI on a spectral chart.
As mentioned above ions also produce spectral lines. In astronomy we are nearly always dealing with positive ions (those which have had one or more electrons knocked off) even with those elements, that under normal circumstances on Earth, would take on additional electrons to become negative. This is because in astronomy where emission lines are produced the energies involved are much higher than on Earth and so it is much more likely that an electron (or perhaps two or even three) is knocked off rather than attracted towards the atom. As such ions with a positive are much more common in astronomy than those with a negative charge.
An ion with a +1 charge (one electron has been stripped off) produces lines labelled as (x)II with (x) again standing for the chemical symbol of the atom involved.
An ion with a +2 charge follows the same pattern – (x)III
and so the pattern continues
Let’s now look at two specific elements:
Hydrogen
Hydrogen produces a series of emission lines that fall within the visible section of the electromagnetic spectrum (it also produces several others that lie outside the visible range). This set of lines is called the Balmer series after the scientist who first described them Johann Jakob Balmer.
They are all produced by electron transitions to the second energy level. When displayed using a hydrogen discharge tube (a cylinder of pure hydrogen through which an electric current is passed to excite the atoms) and a spectroscope the lines can be seen like so:
The Visible Spectrum of Hydrogen Credit: National Institute of Standards and Technology
As is typical in science the rule we just learned about spectral notation doesn’t apply in this case.
For historical reasons the lines of atomic hydrogen in the visible region are named as H followed by a Greek letter. Hα is the lowest energy transition – red- moving through Hβ – which is a blue-green – and then Hγ, Hδ and Hε all being shades of purple (Hδ and Hε are sometimes classed as being ultraviolet rather than visible spectral features but still follow the same naming pattern).
Hydrogen Alpha – Hα
This emission feature occurs at 6562.8 Angstroms (656.28 nm). It is produced when an electron in a hydrogen atom decays from the third energy level to the second producing a photon with 1.9 eV (where one eV is 1.66×10-23 J) of energy.
Hα is a hallmark of star forming regions. It appears pink to the human eye (its the bright red line in the image above), and is displayed as such in most professionally produced images such as this one captured by the ESO’s MPG telescope of the Large Magellanic Cloud (LMC)
N44 in the Large Magellanic Cloud Credit: ESO
Hα is also a marker for AGN activity, with most such galaxies (including quasars) display strong Hα emission.
Oxygen
Oxygen is the third most common element in the Milky Way making up about 10,400 parts per million in terms of mass.
As oxygen has eight electrons rather than hydrogen’s solitary one, oxygen’s spectrum is much more complex with a great deal more lines than that of hydrogen ( hydrogen has 5 spectral lines between 4000-7000 Å (roughly the visible range) compared to oxygen’s 73! Using simplistic terms because oxygen has more electrons there are more available energy levels for those electrons to jump into, that in turn means more energy level transitions are possible and so giving rise to more spectral lines, as each line represents a possible transition.
The visible spectrum of oxygen looks like:
The Visible Spectrum of Oxygen Credit: National Institute of Standards and Technology
Doubly Ionised Oxygen – OIII
Another hallmark of star forming and active regions. A blue-green line that can be exceptionally intense in certain circumstances and can completely dominate the colour of some galaxies.
In most images containing OIII data it is displayed as either green or blue such as this Hubble image of the nearby active galaxy NGC 6822 (OIII is shown as green in this particular example)
In the next post in this series I will be taking a look at a few specific absorption features and what the colour of an object in general can reveal.
Sigma Orionis 