1. Stars and Galaxies Lesson2 Q & A
Midterm Review
Stars and Galaxies
Lesson2
Q & A

2. Wien's Law λmax=2.9 mm/Temp (K)
For a blackbody, the wavelength of the radiation with the greatest intensity follows which law?
Wien's Law
λmax=2.9 mm/Temp (K)
Only the temperature of a star determines its color – Wien’s Law
Wien’s law says that the hotter the surface of a star the shorter will be the wave length of the most intense radiation which is emitted. Notice that as the temperature increases the peak shifts to the left or to shorter wavelengths. Shorter wavelengths are bluer so blue stars are warmer then red stars.
Picture from:

3. It will be blue-shifted
If an object is moving directly toward you what will you notice in its spectrum?
It will be blue-shifted
The waves of the spectrum will be closer together or blue-shifted when the source is moving toward an observer. They will be farther apart or red-shifted when the source is moving away from the observer.
This is why the sound of a horn in a car moving toward you sounds higher in pitch than one in a car moving away from you.
This phenomena is called the Doppler effect and it is used measure the radial velocity of an object whether the object is moving toward or away from the observer. It can not be used to measure the transverse velocity of an object.

4. Stefan-Boltzmann’s Law
Which law tells us that the total amount of energy radiated is proportional to the fourth power of temperature
Stefan-Boltzmann’s Law
Aka
Stefan’s Law
Notice in the image that as the temperature gets warmer that the intensity gets much larger. This means that if two stars are the same size then the hotter star will be both brighter and bluer.
Stefan’s law tells us that the amount of energy emitted per unit area is proportional to the fourth power of temperature. So if the absolute temperature doubles, the intensity from each meter of the star will increase by 24 or 16 times.

5. The neutral hydrogen atom consists of one proton and one electron.

6. Molecules produce spectral lines through the process of electronic transitions, vibrations, and rotations.
The spectra of gas clouds cool enough to contain molecules are more complex than those of stars which are too hot for molecular stability. One reason is that molecules have three methods of generating spectral lines as shown in the figure.

7. The process of removing an electron from a stable nucleus is known as ionization
An atom can become excited by absorbing a photon or by a collision with another atom. If the energy of the photon or collision is great enough the electron will posses enough energy to break away from the atom altogether. This is known as ionization and the atom is said to be ionized.

8. An atom that is excited can emit a photon when the electron moves to a lower energy level.
When an atom absorbs a photon it becomes excited and one of its electrons moves to a higher or more energetic orbital. The electron can emit a photon by moving to a lower energy level

9. An observer looking directly at a hot light source will see a continuous spectrum.
Kirchhoff’s Laws tell us which kind of spectrum we will see.
A hot luminous solid, liquid, or dense gas will create a continuous spectrum
Only a heated low density gas will emit a line spectrum
A continuous spectrum viewed through a cool gas will show an absorption spectrum

10. The absorption lines from the visible spectrum of a star which are produced by hydrogen are called the Balmer Lines
The hydrogen atom has spectral lines in the ultra violet, visible, and infra-red regions.
Transitions to the first excited state produce photons which are visible to our eyes. This series is called the Balmer series.
Transitions to the ground state produce photons too energetic to be seen. They are in the UV region and belong to the Lyman series.
The Paschen, Brackett, and Pfund series are in the infra-red regions.

11. It is possible to determine the relative density of two stars of the same spectral type which have the same spectral lines.
The width of a spectral line can be used to determine several things about the star which emits it.
Most importantly the Doppler effect can be used to determine the temperature of the atoms creating the line. But line width is also determined by density and pressure in a process called collisional broadening. The greater the pressure or density the more broadening will occur.
Other factors which cause broadening are the magnetic field, turbulence, and rotation.

12. Isotopes of the same element have
The same number of neutrons and electrons.
The same number of protons and number of neutrons.
The same number of protons but a different number of neutrons.
The same number of neutrons but a different number of protons.
None of the above

13. Temperature is a measure of the average kinetic energy of the atoms and molecules in a substance.
Remember that temperature and heat are two different concepts. Temperature is a measure of the amount of jiggling of the atoms or molecules of a substance. This jiggling motion has energy called kinetic energy. The average kinetic energy per particle is what we measure as temperature. The sum of all the kinetic energy is called the thermal energy of the substance.
Heat is the flow of this thermal energy from one substance to another due to difference in temperature.

14. Electromagnetic radiation is produced when a charged particle is accelerated or when
there is a changing electric field.
there is a unchanging electric field.
a charged particle is moving at a constant velocity.
there is a static magnetic field.
None of the above

15. The longer the wavelength of light the smaller is the energy of the photon.
Since the speed of light in the vacuum of space is the same for all photons, as the wavelength increases the frequency decreases. Color depends on frequency or wavelength. So blue light has a higher frequency than does red light. The most energetic EM radiations are X-rays and Gamma rays so they have the highest frequencies. In terms of wavelength, the shorter the wavelength the higher the energy and the bluer the light.

16. In our Sun, the spectral lines of hydrogen are
other stars do not have hydrogen
the same as other hotter stars
strong compared to other hotter stars
weak compared to other hotter stars
Hydrogen spectral lines are not dependent on temperature.

17. The Zeeman effect reveals the presence of strong magnetic fields by
the increasing the intensity of spectral lines
the merging of spectral lines
the splitting of spectral lines
the narrowing of spectral lines
None of the above

18. The broader a spectral line the lower the density and pressure of the gas that created it.
Be sure to study this chart and understand what it represents. Spectral lines are used to determine the temperature, composition and line of sight velocity (radial velocity) of stars.
The broadening of the lines gives us information about their density (pressure), rotational speed, and magnetic field
Intensity the composition and temperature.