Astronomy 100, Unit 2 Discussion Questions - Units 11, 12, 21, 22, 29.
From the overall spectrum curve shape, you can determine the
surface temperature by determining the wavelength at which the peak in this
continuous part of the spectrum lies. From the mixture of spectral absorption or
emission lines, you can determine the elemental composition. From the Doppler
shift of spectral lines (either emission lines or absorption lines) relative to
laboratory spectra, you can determine radial velocity – or that component of
velocity along your line of sight to the object.
2. If you photograph stars in the night sky, you will see that some stars are
blue, some are red, and most are like our Sun – yellowish. What do you conclude
about the relative surface temperatures of these three kinds of star, based on
the textbook’s discussion of blackbody radiation?
Blue stars have the
highest surface temperature (more than 15,000 Kelvin), yellowish have surface
temperature comparable with our Sun (about 6000 Kelvin), and reddish stars have
the lowest surface temperature (around 3000 K). Note: if you actually set out
to photograph stars to test this answer, you will see the colors best if you
slightly defocus your camera lens - try different degrees of defocus and snap
several images, each one with different time exposures.
3. Stars in globular clusters
are markedly different from stars like our Sun, in that they primarily
show absorption lines from Hydrogen and Helium
(with hardly any lines from heavier elements), while the overall
color of the cluster stars is similar to that of our Sun.
What can you conclude from these observations regarding the composition and
surface temperature of the globular cluster stars, compared with our
Sun?
"Same color" means similar overall shape (and same location of the
peak intensity vs. wavelength graph) of the continuous, blackbody component of
the spectrum and therefore comparable (about the same) surface temperature;
however, the absence of lines from elements heavier than helium
indicates these globular cluster stars contain very small amounts of heavier
elements (carbon, oxygen, etc.) {these are very old stars, born when the
universe was young and contained relatively little heavy elements}. For your
assessment, what I wanted was only for you
to conclude that globular cluster stars must contain
relatively little heavy element content and have surface temperatures similar to
that of our Sun. This next paragraph is additional material I don't
expect in your response.
(astronomers refer to all chemical elements more massive that helium as 'metals' - including carbon,
oxygen, etc.). Go to the APOD site for a picture of M3, a globular cluster
visible with binoculars - the link is
REPORT YOUR
SELF-ASSESSMENT ON THE BASIS OF 3 POINTS TOTAL - ONE FOR EACH
QUESTION.
1. State three properties (there are more than three) of any
visible object in the universe (such as a star) that one can determine by
spectroscopy (measurement of light intensity & spectra at all wavelength).
Explain how to determine each of these properties from a measured
spectrum.
http://antwrp.gsfc.nasa.gov/apod/ap980719.html
Starry Night Backyard also has an image of M3 - use the "Edit" and "Find" menus to
locate it, then zoom in. It looks like a very small fuzzy blob with binocs or
small telescopes. Large, high-resolution research telescopes can resolve
individual stars. These are some of the oldest stars in the universe. M3 is
between the constellations labeled Coma Bernices and Bootes, about 12 degrees
west of the bright star Arcturus in Bootes - try finding it with your
binoculars!