1. (Discussion – Star Properties 2; Video – HTUW Part 2)
Wednesday September 13, 2017
(Discussion – Star Properties 2; Video – HTUW Part 2)

2. The Launch Pad Wednesday, 9/13/17
Why do stars appear to have different colors?
They radiate light at various frequencies of the visible spectrum.

3. Give an example of each of the colors of stars.
The Launch Pad
Wednesday, 9/14/16
Give an example of each of the colors of stars.
Red - Betelgeuse
Yellow – our Sun
Blue – Rigel

4. Announcements
Remediation for Quiz 1 runs through THIS AFTERNOON at 5:00!
If you want to remediate, please bring your Review to me.

5. We will take Quiz 2 on Friday.
Announcements
We will take Quiz 2 on Friday.

6. F4 F5 S1 F6 F7 QUIZ 1 TODAY 8/31 9/7 9/12 Friday Last Day Accepted
Assignment Currently Open
Summative or Formative #
Date Issued
Gone Missing Day
Last Day Accepted
DE – The Electromagnetic Spectrum F4
8/31
OVERDUE
DE – The Doppler Effect F5
QUIZ 1 S1
9/1 or
9/5
Wednesday
TODAY
DE – Nebulae F6
9/7
NASA Research Paper - Stars F7
9/12
Friday
Next Wednesday

7. Properties of Stars LUMINOSITY Start with the Sun
This is the amount of energy generated in the star and released as electromagnetic radiation.
Remember that Luminosity is the total amount of energy produced in a star and radiated into space in the form of E-M radiation. The Sun radiates  ergs/sec.
Start with the Sun
How do we figure out the luminosity of the Sun? Measure its (apparent) brightness, measure it's distance and use the inverse square law.

8. Properties of Stars LUMINOSITY 1,400,000 ergs/square cm/second
The apparent brightness is a measure in photon energy passing through a square whatever at the distance of the Earth. One way to determine the total energy release in photons is to multiply the photon energy per square whatever, times the total area of the sphere with radius 1 AU (remember an astronomical unit is the distance between the Earth and Sun) in units of square what-evers.
One measure of the apparent brightness of the Sun is ``Solar Constant'‘.
At the Earth's surface we receive:
1,400,000 ergs/square cm/second
where ``erg'‘ is a unit of energy.

9. Properties of Stars SIZE
Stellar diameters can be measured for some nearby giant and supergiant stars by using a technique called stellar interferometry.
The Navy Prototype Optical Interferometer has been operating for over a decade at Mount Wilson Observatory, and routinely measures the angular diameters of bright stars to fractions of a milli-arcsecond (0.001 arcseconds) accuracy.
The table below shows only a few stars that have had their diameters measured. Once their distances are accurately known...from the Hipparcos Survey...their linear diameters in millions of kilometers can easily be found.

10. Properties of Stars DISTANCE
It is crucial to be able to measure the distances to stars if we are to derive intrinsic luminosities. There are several methods for measuring distances to stars, but the most reliable by far (when it can be applied) is Trigonometric Parallax.
Hold a finger up in front of your nose, close one eye and note where the finger appears on the back wall. Now close the other eye (and open the one that was closed before) and note that the finger appears to move on the back wall. This is the parallax effect - the apparent motion of a nearby object compared to distance background objects because the change in viewing angle.
The asterisk is a nearby star which is apparently moving back and forth every year compared to the more distance background stars. Note the star at the lower right which has a proper motion.

11. Properties of Stars DISTANCE
If you do this same experiment now holding the finger at arm's length, you will notice that the apparent motion of the finger against the background is smaller (unless you have really short arms).
For nearby stars we also measure a parallax - an apparent annual motion of the stars compared to background stars that is really a reflection of the Earth's motion around the Sun.

12. Properties of Stars DISTANCE
The parallax angle is the angle between the Earth at one time of year, and the Earth six months later, as measured from a nearby star. Astronomers use this angle to find the distance from the Earth to that star.

13. Properties of Stars DISTANCE
This is the angular size of a dime seen from 2 miles.
The distance to a star in parsecs is  when  is measured in arcseconds.
How far away ARE the nearby stars? 
The nearest star to Earth, Proxima Centauri (a member of the triple system of Alpha Centauri), has a parallax of 0.7716″, meaning that its distance is 1/0.7716, or 1.296, parsecs, which equals 4.23 light-years. This means it is more than 1 pc away:
Even the largest parallax (that for the closest star) is small. The atmosphere blurs stellar images to around  so ``astrometrists'' are trying to measure a tiny motion of the centroid of a stellar image as it moves back and forth each six months. In the previous diagrams the parallax motions have been hugely exaggerated.
From the ground it is possible to measure parallaxes for stars out to around 80 pc -- this corresponds to very tiny motions.
So, a star at 80 pc has a parallax angle of only 1/80 = arcsec.

14. Properties of Stars DISTANCE 1 parsec = 3.26156 light years
Within a sphere with a radius of 3 parsecs there are only 10 known stars
1 parsec = light years
Note that most of the nearest stars are faint in apparent brightness and much less luminous that the Sun.

15. HTUW - Extreme Stars - A Star's Energy
Video Segment
HTUW - Extreme Stars - A Star's Energy
Part 2
6:55

16. Discussion Items (Notes should be taken on these items to prepare for the next Quiz!
What is the source of energy that can drive a star for billions of years at a time? Who discovered it?
Stars can tap into the energy inside _______.
Matter can be described as condensed __________.
How is it possible to release the energy contained in atoms? What is this called?
Can we simulate a star’s fusion on Earth? By what method?
When hydrogen atoms fuse into helium atoms, a tiny amount of mass is lost. What happens to it?
Explain how a star can continue to fuse hydrogen for billions of years.