Homework #4
What information about stars can astronomers learn by studying spectra? Give 3 stellar properties and briefly describe the spectral analysis involved.
Stellar spectra provide astronomers with information that enables temperature, composition, radial motion, magnetic properties, rotation, and color to be determined. An indication (but not direct measurement) of stellar radius, mass, and absolute magnitude can also be obtained from spectral information. The temperature can be determined by scanning the spectrum for the peak
(most intense) wavelength (then use Wien’s Law) or by placing the star in one of the spectral classifications, like G2 (as you will in Lab 8). By matching the pattern of lines with the known wavelengths measured in the lab that are produced by various elements, stellar composition is determined. From Doppler shift measurements (red—moving away; blue—moving toward), the direction of motion and stellar velocity is found. Magnetic effects reveal themselves through a splitting of spectral lines—analyze spectrum for the Zeeman effect. Stellar rotation can be measured from the broadening of spectral lines due to the Doppler effect. Color of the star can be determined by isolating certain spectral wavelength regions or by locating the peak wavelength.
By making assumptions it is possible to estimate the radius and mass of a star from spectral characteristics. For example, giant stars have thinner spectral lines generally than small stars due to density differences (giants stars have less dense atmospheres and low surface gravity). With greater densities found in smaller stars, collisions between atoms occur more frequently, distorting the energy levels, and thus, a greater range of wavelength becomes acceptable for the electron to make a transition. You can distinguish supergiants, giants, main sequence stars, and white dwarf stars from line broadening. Stellar masses can be estimated for main sequence stars once the spectrum is classified, since the main sequence is a mass sequence. Absolute magnitude can be estimated from spectral appearance and classification.
Compare how energy is produced by hydrogen fusion, sometimes referred to as “hydrogenburning” to the energy produced by burning a log in a fireplace.
The energy produced by hydrogen fusion involved nuclear reactions rather than chemical reactions as for the burning log. Hydrogen fusion combines protons to yield a more complex helium nucleus. Much more energy is released in fusion since some of the mass is converted directly into energy (E=mc
2
). A burning log relies on breaking chemical bonds for releasing energy.
Describe how an aurora is produced and why we are more likely to see one from College
Park during times of solar maximum.
The Sun constantly ejects charged particles as the solar wind. During times of solar maximum, solar activity increases and there are huge solar eruptions of gas (coronal mass ejections and solar flares). As these charged solar particles encounter the Earth’s magnetic field, they are directed towards the polar regions. They encounter atoms and molecules in our atmosphere, transferring energy to them. As these atoms and molecules return to lower energy states they give up the gained energy as visible photons. Observers see the northern or southern lights, an aurora. It is likely that there will be several huge outbursts of particles during times of solar

maximum. With more particles, a greater region of Earth’s atmosphere is affected and the aurora is visible at lower latitudes, like in College Park.
p. 275 DQ2 The Sun is sometimes described as an average star. Is that true? What is the average star really like?
This question really gives the answer away, but it sort of depends on how you frame the question. If by average star you mean, average in numbers, then the answer is certainly no.
Most stars are less massive than the Sun. On the HR Diagram, spectral type M main sequence stars are most common. If you mean temperature, then the answer is yes, sort of. The Sun sits in the mid-range represented on the HR Diagram. Clearly there are many kinds/types of stars that are hotter or cooler than our Sun. The same is true of absolute magnitude; the Sun is sort of midrange between the extremes. If you want to consider location in the galaxy (as we will after
Exam II), then the Sun looks pretty average sitting out in a spiral arm. In terms of size, currently our Sun would be considered pretty average, since there are giant and supergiant stars that are much larger than our Sun and stars that are much smaller as well. p. 275 P1 If a star has a parallax of 0.050 seconds of arc, what is its distance in parsecs? in light years? in Astronomical units?
Use the formula d = 1/p, where d=distance in parsecs and p=parallax angle in seconds of arc. d = 1 / 0.050 = 20 parsecs
To convert to light years, your factor is 1 parsec = 3.26 LY, so d = 20 parsecs x (3.26 LY / 1 parsec)
= 65.2 LY
To convert to Astronomical units, your factor is 206,265 AU = 1 parsec, so d = 20 parsec x (2.06 x 10
5
AU / 1 parsec)
= 41.2 x 10
5
AU = 4.1 x 10
6
AU