1. 1
Stellar Astronomy
Spring_2015
Day-25

3. Course Announcements Smartworks Chapter 13: Next week sometime
Apr. 2 – Last day to drop a class.
Midterm grades never made it … IF you have questions, come and talk to me.
Thursday: 1st Quarter Moon Observing – 7:30pm
Last one for the semester (still 2 dark nights to go).
Reports are due Wed. Apr. 22
Thursday: Hot Topic Movie & Discussion
6-8pm – E106-B
Topic this time: The Big Bang

4. Astronomy in the Fall, 2015
ASTR-1010/ Planetary Astronomy + Lab (H,R)
ASTR-1020/ Stellar Astronomy + Lab (R)
ASTR Problems in Planet Astronomy
ASTR Intro. to Observational Astronomy
ASTR-4000/4001 – Astrophotography & Lab
ASTR-3030/3031 – Methods & Techniques in Astronomy

6. Let p be the parallactic angle in arcseconds.
MATH TOOLS 13.1
The parallax (p) of a star is inversely proportional to the distance (d) to a star.
Let p be the parallactic angle in arcseconds.
1 arcsecond = 1/3,600 of a degree.
Let d be the distance in parsecs.
1 parsec = 206,205 AU = 3.26 light-years.
Then:
Parsec: distance at which p = 1 arcsecond.
Even the closest star to the Sun has a parallax of only about ¾ arcsecond. 6

7. Parallax and Distance Lecture Tutorial pg. 41
Work with a partner!
Read the instructions and questions carefully.
Discuss the concepts and your answers with one another. Take time to understand it now!!!!
Come to a consensus answer you both agree on and write complete thoughts into your LT.
If you get stuck or are not sure of your answer, ask another group.

8. How Bright?
We use the quantity “magnitude” to rank an object’s brightness.
Apparent magnitude: How bright the object appears to the observer.
Absolute magnitude: How bright the object would be at a set distance (10 pc).
There is a factor of X in brightness between any 2 magnitudes.
Definition = Fifth root of 100 X in brightness.

9. The magnitude system was developed by Hipparchus in ancient Greece.
CONNECTIONS 13.1
The magnitude system was developed by Hipparchus in ancient Greece.
Divides stars into categories of brightness (originally 1st through 6th).
The greater the magnitude, the dimmer the star.
Apparent magnitude: the brightness of a star as it appears in the sky from Earth.
Absolute magnitude: the brightness of a star if it were 10 pc from Earth. 9

10. Not many stars are near the Sun. Obtaining distances is essential.
Luminosity: total energy radiated by a star each second.
Brightness: rate at which we receive that energy (depends on observer’s perspective).
Fig. 13.4a 10

11. Brightness depends on both luminosity and distance.
A dim star could have a low luminosity or be far away.
A bright star could be close or have a high luminosity.
Fig. 13.4a 11

12. From distance and brightness, we know a star’s luminosity.
Idea: How much light must the star emit to be as bright as it is at its distance?
Luminosity = 4d2  brightness.
Fig. 13.4b 12

13. Usually the luminosity is expressed as the solar luminosity = 1 L.
The most luminous stars are 106 L.
The least luminous are 104 L.
More low-luminosity stars than high.
Fig. 13.4b 13

14. Brightness and Distance
From distance and brightness, we know a star’s luminosity.
Idea: how much light must the star emit to be as bright as it is at its distance?
Luminosity = 4d 2  Brightness.
Usually the luminosity is expressed as the solar luminosity = 1 L.
The most luminous stars are 106 L.
The least luminous are 104 L.

15. Inverse-Square Law of Brightness
Brightness is the amount of light arriving at a particular place.
Decreases as the distance from a light source increases.
Light obeys an inverse square law

16. Measuring the color of a star tells us the surface temperature.
We can measure stellar surface temperatures from Wien’s law.
Fig. 13.6 16

17. peak is the wavelength at which a star is brightest.
“Hotter means bluer” (the spectrum shifts to shorter wavelengths at higher temperatures).
Fig. 13.6 17

18. The Stefan-Boltzmann law allows you to estimate the sizes of stars.
MATH TOOLS 13.2
The Stefan-Boltzmann law allows you to estimate the sizes of stars.
The luminosity (L) of a star is related to its temperature (T) and radius (R):
Rearranging, you get:
Called the luminosity-temperature-radius relationship for stars. 18

19. Concept Quiz—Getting Brighter
Suppose a star gets more luminous but does not change its
temperature. What is happening?
The star is expanding.
The star is contracting.
The star is getting more massive.
The star is changing its spectral type.
Answer: A
Explanation: Since the luminosity is increasing but the temperature is constant, the star must be increasing in radius (expanding). Option B is the opposite of A, and so is incorrect. Option C is incorrect because increasing mass would move a star to higher luminosity and higher temperature. Option D is incorrect because if the temperature is constant, the spectral type will not change. 19

20. Spectrum: the amount of light emitted as a function of wavelength.
Some light leaving the star is absorbed by atoms or molecules in the star’s atmosphere.
Fig. 13.7 20

21. Absorption lines in the spectrum result.
Sometimes emission lines are also seen.
Both are superimposed on a Planck (or continuous) spectrum.
Fig. 13.7 21