Download No Slide Title

Chapter 10: Measuring the Stars
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.
Chapter 10: Measuring the Stars
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.
Chapter 10: Measuring the Stars
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)
Chapter 10: Measuring the Stars
Chapter 10: Measuring the Stars
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.
.
Chapter 10: Measuring the Stars
Astronomers measure the
brightness of stars on a nonlinear 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
Chapter 10: Measuring the Stars
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.
Chapter 10: Measuring the Stars
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
Chapter 10: Measuring the Stars