AnnouncementsAnnouncements Office hours changedOffice hours changed

Monday: 1:30-2:30Monday: 1:30-2:30 Wednesday: 1:30-2:30Wednesday: 1:30-2:30

Get lab reportGet lab report Average (not including zeros)=81Average (not including zeros)=81 Average (including zeros)=65Average (including zeros)=65

Guide to basic numerical error Guide to basic numerical error analysis on website soon (might help analysis on website soon (might help for planetarium lab)for planetarium lab)

Brightness and Brightness and Color of StarsColor of Stars

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

Intrinsic VariablesIntrinsic Variables SizeSize MassMass AgeAge TemperatureTemperature CompositionComposition

Extrinsic VariablesExtrinsic Variables DistanceDistance 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

Stellar ColorsStellar Colors All objects emit electromagnetic radiation in All objects emit electromagnetic radiation in

some formsome 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 Cold objects: emission peaks in the radio

bandband Humans: emission peaks in the infraredHumans: emission peaks in the infrared Our sun: emission peaks in optical lightOur 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?

Brightness (Luminosity)Brightness (Luminosity)

Depends on two thingsDepends on two things TemperatureTemperature SizeSize

L=constant x (surface area) x TL=constant x (surface area) x T44

Pretty intuitivePretty intuitive Don't be fooled by the strong Don't be fooled by the strong

temperature dependence (surface temperature dependence (surface area still matters a lot)area still matters a lot)

DistanceDistance L=4L=4pp (distance) (distance)2 2 x x (light flux)(light flux)

Basically says that the farther you are Basically says that the farther you are from an object, the dimmer it appearsfrom an object, the dimmer it appears

Complication: Can't know luminosity Complication: Can't know luminosity without a distancewithout a distance Getting distances is hard (another Getting distances is hard (another

lecture in itself)lecture in itself) Stellar astronomy just got a whole lot Stellar astronomy just got a whole lot

harderharder

Combining Temperature, Combining Temperature, Surface Area, and DistanceSurface Area, and Distance

You can never guess a star's luminosity off one You can never guess a star's luminosity off one variable , or even two. For example:variable , or even two. For example: BetelgeuseBetelgeuse

Very redVery red Sounds like it will be dimSounds like it will be dim

Very far away (600 light years)Very far away (600 light years) Ok, definitely a very dim starOk, definitely a very dim star

1010thth brightest star in the sky brightest star in the sky What?!? How?What?!? How?

Its radius is 1000 times the sun's (surface Its radius is 1000 times the sun's (surface area is 1,000,000 times that of the sun's)area is 1,000,000 times that of the sun's)

Luminosity is 130,000 times that of the sun'sLuminosity is 130,000 times that of the sun's

So what does this all tell you?So what does this all tell you? You can learn a lot about a star You can learn a lot about a star

just by determining its luminosity, just by determining its luminosity, temperature, and of course temperature, and of course distancedistance

One of the most important results One of the most important results is the Hertzprung-Russell Diagramis the Hertzprung-Russell Diagram

Luminosity versus color (or Luminosity versus color (or temperature)temperature)

Indicator of stellar evolutionIndicator of stellar evolution

Today's LabToday's Lab We'll be looking at stars tonight, and ranking their We'll be looking at stars tonight, and ranking their

brightnesses and colorsbrightnesses and colors Make sure to align the telescopes firstMake sure to align the telescopes first

AND their viewfinders!AND their viewfinders! Observe at least five stars and rank their Observe at least five stars and rank their

relative brightnesses and colors (you may have relative brightnesses and colors (you may have to go back and forth between stars to get a to go back and forth between stars to get a good idea of these properties)good idea of these properties)

Try and get a wide range of colorsTry and get a wide range of colors Can use any named starCan use any named star Suggested stars: Sirius, Betelgeuse, Rigel, Suggested stars: Sirius, Betelgeuse, Rigel,

Aldebaran, Capella, Procyon, Pollux Aldebaran, Capella, Procyon, Pollux Be sure to note the stars' RA/Dec.Be sure to note the stars' RA/Dec.