1. Announcements Office hours changed Office hours changed Monday: 1:30-2:30 Monday: 1:30-2:30 Wednesday: 1:30-2:30 Wednesday: 1:30-2:30 Get lab report Get lab report Average (not including zeros)=81 Average (not including zeros)=81 Average (including zeros)=65 Average (including zeros)=65 Guide to basic numerical error analysis on website soon (might help for planetarium lab) ‏ Guide to basic numerical error analysis on website soon (might help for planetarium lab) ‏

2. Brightness and Color of Stars

3. What might affect a star's color and/or brightness?

4. Intrinsic Variables Intrinsic Variables Size Size Mass Mass Age Age Temperature Temperature Composition Composition Extrinsic Variables Extrinsic Variables Distance Distance Interfering material (gas dust) ‏ Interfering material (gas dust) ‏ Many of these intrinsic variables are related Increase of mass leads to increase in radius and temperature Both lead to an increase in luminosity and a decrease in lifetime

5. Stellar Colors All objects emit electromagnetic radiation in some form All objects emit electromagnetic radiation in some form Peak of emission dependent on temperature Peak of emission dependent on temperature Temperature=Constant/(max wavelength) ‏ Temperature=Constant/(max wavelength) ‏ Cold objects: emission peaks in the radio band Cold objects: emission peaks in the radio band Humans: emission peaks in the infrared Humans: emission peaks in the infrared Our sun: emission peaks in optical light Our sun: emission peaks in optical light What's hotter, a bluer star or a redder star? What's hotter, a bluer star or a redder star?

6. Brightness (Luminosity) ‏ Depends on two things Depends on two things Temperature Temperature Size Size L=constant x (surface area) x T 4 Pretty intuitive Pretty intuitive Don't be fooled by the strong temperature dependence (surface area still matters a lot) ‏ Don't be fooled by the strong temperature dependence (surface area still matters a lot) ‏

7. Distance L=4  (distance) 2 x (light flux) ‏ L=4  (distance) 2 x (light flux) ‏ Basically says that the farther you are from an object, the dimmer it appears Basically says that the farther you are from an object, the dimmer it appears Complication: Can't know luminosity without a distance Complication: Can't know luminosity without a distance Getting distances is hard (another lecture in itself) ‏ Getting distances is hard (another lecture in itself) ‏ Stellar astronomy just got a whole lot harder Stellar astronomy just got a whole lot harder

8. Combining Temperature, Surface Area, and Distance You can never guess a star's luminosity off one variable, or even two. For example: You can never guess a star's luminosity off one variable, or even two. For example: Betelgeuse Betelgeuse Very red Very red Sounds like it will be dim Sounds like it will be dim Very far away (600 light years) ‏ Very far away (600 light years) ‏ Ok, definitely a very dim star Ok, definitely a very dim star 10 th brightest star in the sky 10 th brightest star in the sky What?!? How? What?!? How? Its radius is 1000 times the sun's (surface area is 1,000,000 times that of the sun's) ‏ Its radius is 1000 times the sun's (surface area is 1,000,000 times that of the sun's) ‏ Luminosity is 130,000 times that of the sun's Luminosity is 130,000 times that of the sun's

10. So what does this all tell you? You can learn a lot about a star just by determining its luminosity, temperature, and of course distance You can learn a lot about a star just by determining its luminosity, temperature, and of course distance One of the most important results is the Hertzprung-Russell Diagram One of the most important results is the Hertzprung-Russell Diagram Luminosity versus color (or temperature) ‏ Luminosity versus color (or temperature) ‏ Indicator of stellar evolution Indicator of stellar evolution

11. Today's Lab We'll be looking at stars tonight, and ranking their brightnesses and colors We'll be looking at stars tonight, and ranking their brightnesses and colors Make sure to align the telescopes first Make sure to align the telescopes first AND their viewfinders! AND their viewfinders! Observe at least five stars and rank their relative brightnesses and colors (you may have to go back and forth between stars to get a good idea of these properties) ‏ Observe at least five stars and rank their relative brightnesses and colors (you may have to go back and forth between stars to get a good idea of these properties) ‏ Try and get a wide range of colors Try and get a wide range of colors Can use any named star Can use any named star Suggested stars: Sirius, Betelgeuse, Rigel, Aldebaran, Capella, Procyon, Pollux Suggested stars: Sirius, Betelgeuse, Rigel, Aldebaran, Capella, Procyon, Pollux Be sure to note the stars' RA/Dec. Be sure to note the stars' RA/Dec.