Characterizing Stars Part 2 - with a quick review of part 1 Answer the questions on the worksheet as we go through this. (for credit)

Note This PowerPoint is not what I showed in class. It omits the answers to questions that are keyed to the material. If you didn’t come to class, you will need to look at the day notes for this day for the questions (1 through 9) and figure out the answers yourself.

Stellar Parallax A Parsec is the distance from us that has a parallax of one arc second (parsec = pc) 1 parsec = 206,265 A.U. or about 3.3 light-years

Answer questions 1 and 2 Stellar Parallax The parallax angle is an angle in the triangle with a baseline of one astronomical unit (1 A.U.)

The Sun’s Neighborhood Each successive circle has a radius which is 0 The Sun’s Neighborhood Each successive circle has a radius which is 0.5 parsec larger About 21 systems are shown (some are binaries)

Inverse-Square Law for Light - question 3.

Two unlike objects at different distances may appear the same – question 4

Apparent Magnitude of some typical objects, along with some limits for seeing through various instruments. Each magnitude corresponds to a change in brightness of 2.5 times Question 5

The absolute magnitude is the apparent magnitude when viewed from 10 pc Our sun would appear to have an apparent magnitude of 4.8 if it were at 10 pc distance, so it has an absolute magnitude of 4.8 Question 6 Once we get the absolute magnitude from the apparent magnitude and the distance, we can get the luminosity compared to the Sun.

Star Colors vary from red to blue – question 7

spectral classification uses letters for the spectral classes: OBAFGKM (and LT) based on the star’s temperature.

Summary so far To measure the stars, we measure the 1. apparent magnitude 2. distance (by parallax) 3. spectrum (to find the temperature) and so we can deduce the luminosity and spectral class (OBAFGKM-LT)

Stellar Sizes: from 300 times the size of the Sun to only 0 Stellar Sizes: from 300 times the size of the Sun to only 0.01 times the size of the Sun.

Stellar sizes Some stars are close enough and big enough to be seen as disks, for example Betelguese. Most stars look like points, so we need to deduce the size from the luminosity (based on the apparent magnitude) and the temperature by a formula: luminosity a (radius)2 x (temperature)4 (where a means “is proportional to”) Questions 8 and 9