1. 25.1 – Properties of Stars – Part II

2. Do Now
What is the Hertzsprung- Russell Diagram?

3. Do Now What is the Hertzsprung- Russell Diagram?
A diagram that shows the relationship between the absolute magnitude and temperatures of stars.

4. Vocab Words Apparent Magnitude Absolute Magnitude Main-Sequence Star
Red Giant
Supergiant
Cepheid Variable
Nova
Nebulae

5. Stellar Brightness The measure of the star’s brightness is magnitude.
The stars in the night sky have an assortment of sizes, temperatures, and distances, so their brightness vary widely.
Apparent Magnitude vs. Absolute Magnitude

6. Apparent Magnitude
Some stars may appear dimmer than other only because they are farther away.
A star’s brightness as it appears from Earth is its apparent magnitude.
Three factors control the apparent magnitude:
How big it is
How hot it is
How far away it is
Astronomers use numbers to rank apparent magnitude.
The larger the number the dimmer the star.

7. Absolute Magnitude
Absolute magnitude is how bright a star actually is.
Two stars that of the same absolute magnitude may not have the same apparent magnitude, because one may be much further from Earth than the other.

8. Hertzsprung-Russell Diagram
Early in the twentieth century, Einar Hertzsprung and Henry Russell independently developed a graph to study stars.
Now more commonly called the H-R Diagram.
Shows the relationship between the absolute magnitude and temperature of the stars.
We can learn about a star’s size, color, and temperature.

9. Hertzsprung-Russell Diagram

10. Hertzsprung-Russell Diagram
90% are main-sequence stars.
Hottest stars are blue in color, the coolest are red in color.
Red Giants are very bright but cool in temperature.
Supergiants are much larger than the sun, for example 800 times larger than the sun.

11. Variable Stars Stars may fluctuate in brightness.
Some stars, called Cepheid variables, get brighter and fainter in regular pattern.
The interval between two successive occurrences of maximum brightness is called a light period.
The longer the light period of a Cepheid variable, the greater its absolute magnitude is.

12. Variable Stars
A different type of variable is associated with a nova, or sudden brightening of a star.
During a nova eruption, the outer layer of the star is ejected at high speed.
It reaches maximum brightness in a few days, remain bright for only a few weeks, then slowly returns in a year or so to its original brightness.
Only a small amount of its mass is lost during the flare-up.
Some stars have experienced more than one of these events, and the process occurs repeatedly.

13. Nova

14. Interstellar Matter
Between stars is “the vacuum of space.” However, it is not a pure vacuum, for there are clouds of dust and gases known as nebulae.
Two types of nebulae:
Emission Nebulae
Consists largely of hydrogen.
Absorb UV radiation emitted from a nearby hot star.
Reflection Nebulae
Reflect the light of nearby stars.
Composed of dense clouds of large particles called interstellar dust.

15. Nebulae

16. Group Activity
On page 704, Figure 5, copy and label the diagram.