STAR CENTER Exams will be emailed tomorrow.
Answering the questions and understanding the terms at the end of the MILKYWAY summaries
are also excellent ways to prepare for exams. A review sheet with additional information about
EXAM II was given in lecture today (4/10).
Lecture Summary (4/8 & 4/10)
Understanding the stars has been greatly augmented with the development of large
telescopes and instruments that accurately measure and analyze starlight. In order to understand
the stars, astronomers must determine accurate stellar distances. Stellar heliocentric parallax was
used for determining distances to stars in Lab # 6. But the heliocentric parallax method breaks
down beyond 100 parsecs (300 LY). In space, telescopes have increased our ability out to nearly
1000 LY.
Astronomers rely on the magnitude system to measure the brightness of stars. The
smaller a magnitude is, the brighter the star.From stellar spectra astronomers can learn about
composition of stellar atmospheres, surface temperature and radial velocity. The Doppler effect
was introduced through demonstrations for water waves and sound waves before moving on to
light.
The presence of spectral features and dominance of certain lines is closely linked to
surface temperature and conditions in the stellar atmosphere. Astronomers have based a
classification system for stars based on the spectrum. Spectroscopic parallax using the distance
modulus can be used to determine distances to stars too distant for heliocentric parallax
measurements. At the telescope the star's apparent magnitude (how bright the star appears from
Earth) is measured. This method also requires taking the spectrum of the star. An estimate of
the star's luminosity or absolute magnitude (apparent magnitude at 10 parsecs) is made by
careful examination of the spectrum. Apparent magnitude, absolute magnitude and distance (in
parsecs) are related by an equation–shown on your purple handout. Apparent magnitude is
measured at the telescope and absolute magnitude is determined from the stellar spectrum, so the
equation is solved for the distance. Since absolute magnitude essentially removes the distance
factor so that stars can be compared, it is also a measure of luminosity--the total energy output
from the star. Luminosity depends on temperature (energy per unit area) and radius (determines
the surface area of the star). Astronomers often compare the luminosity of stars to solar
luminosity.
The Doppler Effect applies to wave phenomena. Examples were given using sound
waves and electromagnetic radiation. See the white handout. From blue shifted or red shifted
spectral lines, astronomers can determine the direction of radial motion and the velocity of a
source.
The HR Diagram was introduced as a graph relating stellar properties. This graph
compares the vertical axis of luminosity or absolute magnitude to the horizontal axis of surface
temperature or spectral type. A white handout shows the HR Diagram on which we placed the
major groupings of stars. The HR Diagram is a graph that compares luminosity or absolute
magnitude to surface temperature, spectral type, or color. A green handout was used to fill in a
typical HR Diagram. Study the groupings of stars. The masses and sizes of the stars on the HR
diagram are determined directly using analysis of binary stars. The HR diagram actually shows
stars at various stages of evolution and is useful in discussing the lives of stars.
Much useful information can be determined by studying binary stars. Alcor and Mizar in
Ursa Major (Big Dipper) were used to illustrate different kinds of binaries. The spectroscopic,
eclipsing binary Algol will be featured in a short film during discussion (4/15-4/17). Masses and
sizes of stars can be determined through such studies using the same equation from Lab # 4—
Moons of Jupiter. The Doppler effect is apparent in such orbiting binary systems. From blue
shifted or red shifted spectral lines, astronomers can determine the direction of radial motion and
the velocity of a source.
The interstellar medium (ISM) is comprised of gas and dust. Starlight is affected as it
passes through concentrations of gas and dust. Scattering was demonstrated in class. Stars like
the Sun are born as protostars in regions where the ISM collapses. There are many examples of
star-forming nebulae, dusty disks, and infrared stars to support our theories of star formation.
Eventually the Sun began to fuse hydrogen into helium and reached equilibrium. All
main sequence stars achieve stability by fusing hydrogen to helium in their cores. The Sun
maintains its stability at a core temperature of 15 million K.
TERMS: stellar parallax, apparent magnitude, absolute magnitude, Doppler effect, red shift, blue
shift, main sequence star, spectral type, luminosity, distance modulus, spectroscopic parallax,
HR Diagram, red giant, white dwarf, supergiant, spectroscopic binary, eclipsing binary,
interstellar medium, scattering, protostar, nebula
QUESTIONS:
1. How are the distances to stars determined?
2. Distinguish between apparent and absolute magnitude.
3. What is the Doppler effect?
4. What can astronomers learn by studying the spectra of stars?
5. Be familiar with the groupings of stars on the HR Diagram and be able to compare the
stars in the different groups.
6. Give alternate ways the axes of the HR Diagram may be labeled?
7. What does the phrase "the main sequence is really a mass
sequence" mean?
8. How can masses and radii be determined by studying binary stars?
9. How do astronomers use the Doppler effect?
10. How does interstellar dust affect starlight?