Luminosity and Apparent Brightness

Temperature and color

Classification of stars

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.

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.

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)

Measuring LuminosityTo 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

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.

- 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

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.

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

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

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

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

oxide.- Betelgeuse, Antares

THANK YOU!!

REFERENCES:

http://www.astronomynotes.com/starprop/s5.htmhttp://docs.kde.org/http://www.enchantedlearning.com/subjects/astronomy/stars/index.shtmlIntroduction to Stars, Galaxies, & the UniverseAstronomy 162: Professor Barbara Ryden