[quote="Ann"][quote="saturn2"]What is hidrogen alpha?[/quote]
In the case of the so-called Balmer series" (and you don't have to worry about what exactly that is) the second shell represents an "extra energy" comparable to a photon of the wavelength of 656 nanometers.
When the electron has been "kicked" into this second shell by an ultraviolet photon, it will want to "fall down" again. As it does so, the electron emits a photon of the wavelength of 656 nanometers.
656 nanometers represent red light. In other words, if a photon has been kicked into the second shell of the Balmer series, it will release its extra energy as a photon of 656 nm red light, so that it can fall down into its lowest electron shell again.
[float=right][img]http://s3.amazonaws.com/answer-board-image/2010515023206340947980097937505789.gif[/img][/float] Here you can see the Balmer series. The lowest shell is shell number 2 (and no, I can't explain why it's called 2 and not 1). If the electron is kicked into shell number 3, it will emit red light as it falls down. If it is kicked into shell number 4 it will emit blue-green light as it falls down. (This picture says that the electron will emit pure green light, but that is not true.) If the electron is kicked into shell number 5, it will emit blue light as it falls down, and if it is kicked into shells number 6 and 7 it will emit violet light. Electrons are often kicked "one shell up", but it is a rare event indeed that they are kicked more than "two shells up". Therefore electrons often emit red light, but extremely rarely violet light.
This is the Rosette Nebula. It is glowing red because huge numbers of electrons are emitting red light. These electrons have been kicked into shell number 3 by ultraviolet photons from the hot stars at the center of the Rosette nebula. As the electrons fall down into shell number 2 again, they emit red light of 656 nanometers. As long as the stars stay hot and bright, and as long as there is a large cloud of hydrogen surrounding the hot stars, electrons will keep being kicked into shell number 3, and they will keep emitting red light as they fall down into shell number 2.[/quote]
[float=left][img3=".
[size=140]<<Polar and equatorial views of Earth, the aurora, the equatorial airglow bands, and the geocorona. Two 30° x 120° nadir-centered images show Earth and its faint lights at vacuum-ultraviolet wavelengths. Features of Earth's disk (dayglow from the sunlit atmosphere, auroral oval, and equatorial airglow) appear primarily in the emissions of atomic oxygen at about 130.4 and 135.6 nanometers and of the LBH bands of molecular nitrogen, while beyond the limb the instrument responses are entirely due to 121.6 nanometer solar [b][color=#FF00FF]Lyman-alpha[/color][/b] radiation resonantly scattered by Earth's extended hydrogen atmosphere, the geocorona. :arrow:
The image in the left panel shows an active auroral oval on 14 October 1981 at 2017 UT following the onset of a substorm at local midnight. Spacecraft altitude is 16,500 km at 67° N latitude. The image in the right panel provides a view of Earth's dark hemisphere at 0222 UT on 16 February 1982 while the sun is behind Earth. Spacecraft altitude and latitude are 19,700 km and 13° N, respectively. The northern auroral oval forms a halo of light above the limb of Earth, while the equatorial airglow bands in the premidnight sector straddle the magnetic equator. Isolated points of light in both images are VUV bright stars.>>[/size]"]http://upload.wikimedia.org/wikipedia/commons/thumb/7/76/LymanSeries.svg/739px-LymanSeries.svg.png[/img3][/float]
[size=125]The lowest shell [b]in the [u]visible[/u] [color=#FF0000]Balmer series[/color][/b] is shell number 2 because when the electron finally
drops into the [b][u]actual lowest[/u][/b] shell (number 1) it emits [b]an [u]ultraviolet[/u] photon of the [color=#FF00FF]Lyman series[/color][/b].[/size]
[quote=" http://www-pi.physics.uiowa.edu/sai/gallery/text.htmlx"]
[float=left][img]http://www-pi.physics.uiowa.edu/sai/gallery/plate03.jpg[/img][/float][/quote]
[quote=" http://en.wikipedia.org/wiki/Lyman-alpha_blob"]
[float=right][img3="A Lyman alpha blob (left) and an artists impression of what one might look like if viewed at a relatively close distance (right)."]http://upload.wikimedia.org/wikipedia/commons/thumb/e/e6/Lyman_Alpha_Blob.jpg/800px-Lyman_Alpha_Blob.jpg[/img3][/float]
<<In astronomy, a [b][color=#FF00FF]Lyman-alpha[/color][/b] blob (LAB) is a huge concentration of a gas emitting the Lyman-alpha emission line. LABs are some of the largest known individual objects in the Universe. Some of these gaseous structures are more than 400,000 light years across. So far they have only been found in the high-redshift universe because of the ultraviolet nature of the Lyman-alpha emission line. Since the Earth's atmosphere is very effective at filtering out UV photons, the 121.6 nanometer [b][color=#FF00FF]Lyman-alpha[/color][/b] photons must be redshifted in order to be transmitted through the atmosphere.
The most famous Lyman-alpha Blobs were discovered in 2000 by Steidel et al. Matsuda et al., using the Subaru Telescope of the National Astronomical Observatory of Japan extended the search for LABs and found over 30 new LABs in the original field of Steidel et al., although they were all smaller than the originals. These LABs form a structure which is more than 200 million light-years in extent. It is currently unknown whether LABs trace overdensities of galaxies in the high-redshift universe (as high redshift radio galaxies — which also have extended Lyman-alpha halos — do, for example), nor which mechanism produces the Lyman-alpha emission line, or how the LABs are connected to the surrounding galaxies. Lyman-alpha Blobs may hold valuable clues for scientists to determine how galaxies are formed.>>[/quote]