The document discusses how to measure and classify stars based on their luminosity, apparent brightness, temperature, and spectra. Luminosity measures a star's total energy output independent of distance, while apparent brightness depends on distance and is measured via photometry. To determine luminosity, both apparent brightness and distance must be measured. A star's temperature determines its color, from hot blue stars over 25,000 K to cool red stars under 3,500 K. Stars are classified into seven main categories - O, B, A, F, G, K, and M - based on their spectra and temperature, which indicate the elements present.

2. HOW BRIGHT IS A STAR?
Luminosity
- is the rate at which a star radiates
energy into space.
Apparent Brightness
- is the rate at which a star's radiated
energy reaches an observer on Earth.

3. Luminosity
- Measures the Total Energy Output by the
star in Watts
- Distance Independent
Apparent Brightness
- Measures how bright the star appears to be
as seen from a distance.
- Depends on the distance to the star.

4. Measuring Apparent Brightness
Photometry
- The process of measuring the apparent
brightness of objects.
Two ways to express apparent brightness:
 Stellar Magnitudes
 Absolute Fluxes (energy per second
per area)

5. Measuring Luminosity
To measure the Luminosity of a star you
need 2 measurements:
 the Apparent Brightness (flux) measured via
photometry, and
 the Distance to the star measured in some way

6. TEMPERATURE AND COLOR
- Stars are dense hot balls of gas so their
spectra similar to that of a perfect thermal
radiator, which produces a smooth continuous
spectrum.
-Therefore, the color of stars depends on
their temperature--hotter stars are bluer and
cooler stars are redder.

7. - Hot stars have temperatures around
60,000 K while cold stars have temperatures
around 3,000 K.
Ways to measure stars color and
temperature:
 B-V color index
 Wien's Law and Temperature

9. CLASSIFICATION OF STARS
- Stars are classified by their spectra
(the elements that they absorb) and
their temperature.
- There are seven main types of stars.
- In order of decreasing temperature, O,
B, A, F, G, K, and M.

10. 1. O – BLUE
- over 25,000 K
- Singly ionized helium lines (H I) either in
emission or absorption. Strong UV
continuum.
- 10 Lacertra
2. B – BLUE
- 11,000 - 25,000 K
- Neutral helium lines (H II) in absorption.
-Rigel Spica

11. 3. A – BLUE
-7,500 - 11,000 K
- Hydrogen (H) lines strongest for A0 stars,
decreasing for other A's.
- Sirius and Vega
4. F – BLUE to WHITE
- 6,000 - 7,500 K
-Ca II absorption. Metallic lines become
noticeable.
-Canopus, Procyon

12. 5. G – WHITE to YELLOW
-5,000- 6000K
- Absorption lines of neutral metallic atoms
and ions (e.g. once-ionized calcium).
- Sun and Capella
6. K – ORANGE to RED
- 3,500 - 5000K
-Metallic lines, some blue continuum.
-Arcturus, Aldebaran

13. 7. M – RED
-under 3,500 K
- Some molecular bands of titanium oxide.
- Betelgeuse, Antares

14. THANK YOU!! 

15. REFERENCES:
http://www.astronomynotes.com/starprop/s5.htm
http://docs.kde.org/
http://www.enchantedlearning.com/subjects/astronomy/stars/index.shtml
Introduction to Stars, Galaxies, & the Universe
Astronomy 162: Professor Barbara Ryden