Goals Explain how we measure stellar distances Explain the difference between apparent and absolute brightness Explain how we classify stars Describe the HR diagram Explain “spectroscopic parallax” Explain how we measure stellar mass

Distances to the Stars Parallax: the apparent shift in the position of an object due to the movement of the observer. Remember looking at your finger through the left and then right eye? One parsec is the distance an object must be in order to have a parallax of one arc second. One parsec = 3.3 light years Alpha Centauri is the closest star. Most stars are too distant to measure a parallax.

Solar Neighborhood

Stellar Motion While all stars are circling around the galactic center, they slowly swarm around in random directions. The movement of the stars relative to each other as seen from the Earth is called proper motion. Below: picture of Barnard’s star taken 22 years apart.

Luminosity and Apparent Brightness The luminosity (absolute brightness) is a fixed property of a star, but the apparent brightness depends on the distance between the observer and the star. apparent brightness a luminosity/distance2 Thus, if I know two stars have the same luminosity but one is four times fainter than the other, I know the faint star is twice as far away from me. .

Astronomers measure the brightness of stars on a non-linear scale called the magnitude scale. The brightest star in the sky (Vega) is a magnitude 0 star. The bigger the magnitude the fainter the star. On a good night the human eye can see about sixth magnitude

Color is an indicator of temperature

Temperature and Color Remember from Wien’s law that the color of a black body source is related to its color. The change of color verses wavelength can determine its temperature.

Classification of Stars By using a spectrometer astronomers began to obtain spectra from many stars and began to group them in categories according to similar spectra. They named the categories according to the letters of the alphabet A, B, C,……etc. Once they realized all stars were made of the same materials and that the differences were due to temperature differences, they re arraigned the categories according to temperature and so the correct sequence is O, B, A, F, G, K, M. Oh, Be, A, Fine, Girl, Kiss, Me

Stellar Sizes All stars appear as unresolved point sources when viewed through even the largest telescopes. Astronomers have developed a technique called speckle interferometry which can remove the effects of the Earth’s atmosphere and give sizes to a few dozen stars. Knowing the sizes of a few stars allows us to calibrate the luminosity-radius-temperature relation which can tell us the sizes of stars even if we can’t directly measure them. luminosity a radius2 x temperature4 Alpha Orion (Betelgeuse)