1. Week 9
9th grade science

2. Unit 4 – Measuring Stars
Create a Unit cover page for unit 4. (You will copy all of the unit 4 objectives on this page)
Create a Unit 4 WWK page
Create a Unit 4 Must do page

3. Objectives (write on cover page)
Explain how stellar distances are determined.
Explain how physical laws are used to estimate stellar sizes.
Distinguish between luminosity and brightness and explain how stellar luminosity is determined.
Explain how stars are classified according to their colors, surface temperatures and spectral characteristics, and tell why such a classification is useful.
State how an H-R diagram is constructed, and summarize the properties of the different types of stars that such a diagram helps us to identify.

4. WWK
Stellar Parallax – the apparent motion of a relatively close object with respect to a more distant background as the location of the observer changes.
Parsec – distance at which a star must lie in order for its measured parallax to be exactly 1 arc second. 1 parsec equals 206,000 A.U.

5. Parallactic angle is the parallax
Measured in arcseconds.
1° = 3600 arcseconds

7. Must Do
Please write a statement about what this diagram is trying to express.
Be ready to discuss.

8. WWK
Giants – star with a radius between 10 and 100 times that of the Sun
Supergiants – A star with a radius between 100 and 1000 times that of the Sun
Red giant – A giant star whose surface temperature is relatively low, so that it glows red.
Dwarf – Any star with radius comparable to, or smaller than, that of the Sun (including the Sun itself)
White dwarf – A dwarf star with sufficiently high surface temperature that it glows white

9. Stellar Sizes
Measured directly - using geometry and knowing how far away the star is. Astronomers have measured a few dozen stars this way.
Most stars are too far away for this to work
Measured indirectly – luminosity and temperature gives us an indirect measurement as to the size of the star.
Luminosity is proportional to the radius squared times the temperature to the fourth power.
Called the radius-luminosity-temperature relationship.

10. SWK – Sirius (dog star) Radius and luminosity
Ar=71% larger than our Sun
Al = 25 x the sun
Br = smaller than earth (but more dense)
Bl = 3% the sun
Sirius is a two star system 8.6 light years from Earth.
It consists of the main sequence star Sirius A and its small white dwarf companion Sirius B.
White dwarfs are the core remains of stars that have exhausted their fuel and shed their outer layers.
Sirius B is the closest white dwarf star to Earth.
The force of gravity on Sirius B is 350,000 stronger than on Earth, meaning 3 grams of matter (roughly a sugar cube) would weigh 1,000 kilos (2,200 pounds)!
Sirius is the brightest star in the night sky and the nearest that can be seen without the aid of a telescope.

11. Luminosity and Apparent Brightness
Absolute brightness – An intrinsic property, luminosity
Apparent brightness – How much energy is striking a light detector per unit of time
Energy produced by the star as seen from earth
Uses the magnitude scale -

12. Ancient Magnitude Scale
2nd century BC astronomer Hipparchus (6 groups)
Brighter stars were ranked first magnitude and fainter stars were classified 6th magnitude.
1-6 classification spans a factor of 100 in apparent brightness.
Each magnitude is a difference of 2.5 in apparent brightness. 1st magnitude stars are 2.5 times brighter than 2nd magnitude stars.

13. Modern Magnitude Scale
A change in 5 magnitude represents a factor of 100 in apparent brightness.
Now called apparent magnitudes
No longer limited to whole numbers
1-6 barely even covers it
Hubble can see 30 magnitude stars
The sun is a magnitude star

14. Mathematical Relationship
Knowing a stars apparent magnitude and its distance allows us to compute its absolute magnitude
Alternatively, knowing a star’s apparent and absolute magnitude allows us to determine its distance.

15. Temperature and Color
Color index – ratio of its B (blue) to V (visible) intensities

16. Classification of Stars
Color and temperature can classify stars well enough but SPECTROSCOPY gives us spectral-line radiation which is a much more detailed classification theme.
The composition of these stars are the same the difference in absorption spectra is temperature.
Why do the hotter stars have less absorption lines?

17. Detailed Spectra 20,000 K and up show strong ionized Helium because…
it takes a lot of energy to excite tightly bound atoms.
Hydrogen is very weak in these hot stars because…
It is ionized so few hydrogen atoms have electrons
In cooler stars absorption lines are caused by molecules that are still able to maintain their bonds.

18. Spectral Classification
Between 1880 and 1920 stars were classified by their spectral analysis even before they knew how atoms worked.
They were categorized by the intensity of the hydrogen lines in a A, B, C, D, E…P
Then it was realized that they could be organized by temperature so the new schema was born.
“Oh Be A Fine Guy, Kiss Me.”

19. Our Sun is a G2 Star (each letter is split into 10 subdivisions)