1. Spectral Classification
Oh Be A Fine Guy/Gal; Kiss Me

2. How do we Classify and why?
In astronomy, stellar classification is a classification of stars based initially on photoshperic temperatures and its associated spectral characteristics, and then refined in terms of other characteristics.
Early stellar classification by temperatures posed difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated.
An early schema (from the 19th century) ranked stars from A to P, which is the origin of the currently used spectral classes.

3. Example

5. How Atoms Produce Spectra!

6. .M Stars.
Class M is by far the most common class if we go by the number of stars. All the red dwarfs go in here and they are plentiful; over 78% of stars are red dwarfs.
Examples: Betelgeuse, Anteres

7. ANTARES
In Scorpio

8. Betelguese
in Orion

9. .K Stars.
Class K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus while others are main sequence stars. These make up some 13% of main sequence stars.
Examples: Acrturus
They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals.

10. ARTURUS in Bootes

11. .G Stars.
Class G stars are probably the most well known if only for the reason that our Sun is of this class. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be. These are about 8% of all main sequence stars.
Examples: Sun, Capella
They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals.

12. .F Stars.
Class F stars are still quite powerful but they tend to be main sequence stars.Their color is white with a slight tinge of yellow. These represent 3.1% of all main sequence stars.
Examples: Canopus, Procyon
Their spectra is characterized by the weaker hydrogen lines and ionized metals

13. Canopus
Procyon

14. .A Stars.
Class A stars are amongst the more common naked eye stars. As with all class A stars, they are white or green. Many white dwarfs are also A. They comprise perhaps 0.63% of all main sequence stars.
Examples: Vega, Sirius
They have strong hydrogen lines and also ionized metals.

16. .B Stars.
Class B stars are extremely luminous and blue. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area in which they were formed as they don't have the time. They therefore tend to cluster together in what we call OB1 associations. and contains all of the constellation of Orion. They constitute about 0.13% of main sequence stars -- rare, but much more common than those of class O.
Examples: Rigel, Spica
Their spectra have neutral helium and moderate hydrogen lines

17. .O Stars.
Class O stars are very hot and very luminous, being strongly violet in color; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars, constituting as little as 1/32,000th of the total. O-stars shine with a power over a million times our Sun's output.
Examples: Epsilon Orionis
These stars have prominent ionized and neutral helium lines and only weak hydrogen lines.

20. Cepheid Variables
Cepheid variables are stars that regularly pulsate in size and change in brightness. As the star increases in size, its brightness decreases; then, the reverse occurs. Cepheid Variables may not be permanently variable; the fluctuations may just be an unstable phase the star is going through. Polaris in an examples of a Cepheid.

21. BINARY SYSTEMS

22. Eclipsing Binary
An eclipsing binary is two close stars that appear to be a single star varying in brightness. The variation in brightness is due to the stars periodically obscuring or enhancing one another. This binary star system is tilted (with respect to us) so that its orbital plane is viewed from its edge.

23. Oh Be A Fine Guy/Gal Kiss Me
Class
Temperature
Star Color (visible light)
Mass
Radius
Luminosity O
30, ,000 K
Bluish 60 15
1,400,000 B
10, ,000 K 18 7
20,000 A
7, ,000 K
White bluish tinge
3.2
2.5 80 F
6, ,500 K
White
1.7
1.3 6 G
5, ,000 K
Yellowish white
1.1
1.2 K
3, ,000 K
Yellow-orange
0.8
0.9
0.4 M
2, ,500 K
Orange-red
0.3
0.04

24. Thanks Annie!
Annie Jump Cannon and many other Harvard women dedicated hours to this classification system and analyzed millions of spectral absorption plates. In the end their male counterparts took all the credit, but the real work was done by a room of amazing female astrophysicist, and Annie Cannon devised the classification we use today!