# Thursday October 1 - Solar physics

Agenda Review EM radiation How far away are stars? What can starlight tell us? NASA Sunlight carefully separated by

wavelength Absorption and emission Absorption spectrum Emission spectrum (glowing material) Blackbody spectrum U. Tennessee

Consider the following photo Compare and contrast the stars in the photo NASA What if all stars had the same luminosity? Luminosity means total power output as light. Determining their distance from us would be easy if they

all were the same luminosity! Given only 25 Watt bulbs The brightness depends only on their distance from us Stars arent all 25 Watt bulbs! A stars distance can be determined:

If we know how luminous it really is Compared to how bright it looks to us Apparent Magnitude scale Classifies objects by how bright they appear in the sky

Larger number means less bright 1 magnitude = difference in brightness by a factor of 2.5 Apparent Magnitude scale Sirius Full moon Venus Sun

-25 -20 -15 -10 -5 Faintest object Faintest object With binoculars 4 m telescope

Naked eye 0 Polaris5 10 15 20 25

Absolute magnitude Luminosity = how much light a star is giving off (like wattage for light bulbs)

Compared to the Sun (Lsun) Absolute magnitude = how bright (what magnitude) a star would appear at 10 parsecs (32.6 light years) Example: Antares versus Sirius Apparent magnitudes:

Antares -4, Sirius -1.5 Absolute magnitudes: Antares +2, Sirius +1.4

Which appears brighter in the sky? A. Antares B. Sirius C. same D. Cannot conclude Example: Antares versus Sirius Apparent magnitudes:

Antares -4, Sirius -1.5 Absolute magnitudes: Antares +2, Sirius +1.4

Which is more luminous? A. Antares B. Sirius C. same D. Cannot conclude So how do we determine the distance to other stars? Compare absolute and apparent magnitudes If we know the luminosity (or

absolute magnitude) of a star, we can find its distance. A star of known luminosity is called a standard candle. More on this later... So how do we determine the distance to other stars? There is another, more direct way... Triangulation (parallax) Measure the two angles (a, b) to find how wide the Missouri river is!

Trigonometric parallax Earth Known Distance (2 AU) Some star p Sun

Unknown distance Earth What makes parallax measurement difficult? A. B. C. D. Its hard to read your protractor when

it is dark outside. The parallax angle is very small because the stars are so far away. We cant see any of the same stars six months apart. It actually is not difficult! Parsec (a unit of distance) 1 arcsecond = 1/60 arcminute = 1/3600 degree 1 AU If p = 1 arcsecond

This distance is 1 parsec (3.26 light years) Unfortunately, there are no stars this close to us. Limits with this method Earth Known Distance (1 AU) Sun Some star

p Unknown distance Beyond 300 light years, p is too small for us to measure. (Our galaxy is about 100,000 light years across) What can we learn by looking at starlight?

Distance to star Ingredients Temperature Exam one is Tuesday at 6 pm Tuesday optional review during class Individual portion

Group portion Multiple choice 2 essay questions

Multiple choice Grab a review sheet! Challenge: write one exam question (multiple choice), and email it to me! Team activity # 4 Trigonometric parallax Include last names and team number on all activities!

Part I: Do NOT place any measuring device beyond the line!! You must estimate the angles! Part II: You may want to go into the hallway. Grab your graded activities on the way out!

Nearby stars appear to move (Not apparent to the naked eye) Wikipedia

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