1. Optical spectra of different types of objects

2. Stars

3. Hydrogen Atom Energy Levels

5. Hydrogen Ionization Fraction

7. IONIZATION ENERGIES (in electron volts = eV)
Atom st nd 3rd th th
Hydrogen H
Helium He
Lithium Li
Beryllium Be
Boron B
Carbon C
Nitrogen N
Oxygen O
Fluorine F
Neon Ne
Sodium Na
Magnesium Mg
Aluminum Al

8. Ionization Energies

9. Stellar Spectral Classification
Spectroscopy (photographic!) started in the late 19th century.
Classifiers originally arranged them alphabetically, A-O
A. J. Cannon figured out the right order from 200,000 objective prism spectra
O B A F G K M
And astronomers realized it was a well defined sequence in line strength.

12. Digital spectral atlas

14. Φασματοσκοπία μεσοαστρικού αερίου και σκόνης

15. HII regions
Ionized atomic Hydrogen regions, broadly termed “HII Regions”, are composed of gas ionized by photons with energies above the Hydrogen ionization energy of 13.6eV. These objects include “Classical HII Regions” ionized by hot O or B stars (or clusters of such stars) and associated with regions of recent massive-star formation, and “Planetary Nebulae”, the ejected outer envelopes of AGB stars photoionized by the hot remnant stellar core. While the physical origins of these types of gaseous nebulae are very different, the physics governing them is basically the same. We will refer to all of these as “HII Regions” generically in this section. The UV, visible and IR spectra of HII regions are very rich in emission lines, primarily collisionally excited lines of metal ions and recombination lines of Hydrogen and Helium.
gas ionized by photons with energies above the H ionization energy (13.6eV)
“Classical HII Regions” ionized by hot O or B stars
“Planetary Nebulae”, the ejected outer envelopes of AGB stars photoionized by the
hot remnant stellar core.
primarily collisionally excited lines of metal ions
recombination lines of Hydrogen and Helium.

16. Planetary Nebulae
Forbidden lines, in astronomical spectroscopy, bright emission lines in the spectra of certain nebulae (H II regions), not observed in the laboratory spectra of the same gases, because on Earth the gases cannot be rarefied sufficiently. The term forbidden is misleading; a more accurate description would be “highly improbable.” The emissions result from electrons in long-lived orbits within the radiating atoms—i.e., the transition from an upper energy level to a lower energy level that produces the emissions requires a long time to take place. As a result, emission lines corresponding to such atomic transitions are extremely weak compared with other lines. In the laboratory, moreover, an excited atom tends to strike another particle or the walls of the gas container before it emits a photon, thereby further reducing the possibility of observation. In an H II region in interstellar space, by contrast, the atom will remain undisturbed long enough to emit the photon. Another factor favouring forbidden radiation in an H II region is the transparency of the constituent ionized gases to visible light, which permits the photons given off through the entire depth of the nebula to contribute to the emission lines. See also nebulium.
Planetary Nebulae (PNe) are the ejected outer envelopes of AGB stars ionized by UV photons from the remnant stellar core. These cores often have temperatures ranging from 50,000K up to 120,000K or more. Such stars are capable of doubly ionizing Helium (54.4 eV), as well as strongly ionizing many metals.

17. Supernova Remnants

18. Galaxies

19. Μορφολογία των γαλαξιών
Σπειροειδείς
Ελλειπτικοί
Ακανόνιστοι
με ράβδο
Πίεση-τυχαίες κινήσεις
Περιστροφή
χωρίς ράβδο
Φακοειδείς S0

20. Ελλειπτικοί γαλαξίες

21. Σπειροειδείς γαλαξίες

22. Ακανόνιστοι γαλαξίες

23. AGN