- Is the photometric color index B-V of an O star larger or
smaller than that of the sun?
SMALLER. An O star is bluer than the sun, so the luminosity in the B (blue)
band relative to the luminosity in the V (green) band will be larger than
it is in the case of the sun. A larger luminosity means a smaller (more
negative) magnitude, so B will be smaller relative to V for the O star, making
the color index (B-V) SMALLER.
- O and B stars have relatively few absorption lines in their spectra. Why?
O and B stars have surface temperatures pushing as high as 60,000 K. At
such high temperatures, H and He will be fully ionized, leaving no
bound electrons to jump atomic levels producing absorption lines. Those
atoms that do have bound electrons have line transitions predominantly in
the UV spectrum, leaving the visible spectrum rather dull.
- Why did the USA spend 2 billion dollars to build and launch the Hubble
Space Telescope?
The Hubble Space Telescope promised a clearer, deeper look into the Universe,
allowing astronomers to explore nearby objects in greater detail, and to
explore the far reaches of the Universe. Since the "fix", the results
obtained from HST appear well worth the investment of the US taxpayers.
- Why are we able to get a few days advance warning about the effects of a solar
flare on communications here on earth?
The primary effects of a solar flare felt here on earth (for example, the
disruption in telecommunications) are due to a large flux of charged
particles advected out with the solar wind. Given that it takes these
particles a few days to arrive at earth, while the photons from the flare
take only 8 minutes for the same journey, we can foretell the coming of
the disruptive particles by watching for the photons produced in a solar flare.
- If there were no source of energy in the core of the sun, what would happen
to the size of the sun?
It would shrink. If there were no source of energy to replace the thermal
energy leaking out of the center of the sun, the interior temperature, and
hence the interior pressure of the sun would drop. This would upset the
hydrostatic equilibrium of the sun in the favor of gravity, which would
begin to pull the sun in on itself.
- Why can't we see very low mass stars even when looking through a very big
telescope? (Hint: consider their [temperature] position on the HR diagram)
Because of their very low temperature, the lowest mass stars
have a peak wavelength in the infrared band of the
EM spectrum, making them "invisible" to optical telescopes.
- Give two examples of evidence telling us that the corona is hot.
The presence of X-ray emission suggests that the emitting gas is
very hot, since X-rays are high energy photons. The presence of
emission lines implies that the corona is at least hotter than the
photosphere of the sun. The presence of highly ionized atoms, like
FeXIIV, suggests very high temperatures required to collisionally
ionize the gas.
- Why might some stars appear as a close binary in blue light but
appear as only one star if viewed in red light?
The wavelength dependence of the resolution of a telescope (lambda/Diameter)
means that a telescope will have better resolving power at shorter (bluer)
wavelengths. A close binary system might be resolved in blue light, but
when viewed in red light the same telescope may not be able to resolve
the binary into two stars.
- The surface temperature of the sun is about 5,800 K.
The temperature of the corona is 2,000,000 K.
- Sunspots are roughly 5 times dimmer than the normal stellar surface.
What is the temperature of sunspots?
Use the Stefan-Boltzmann law to compute the ratio of fluxes
- What would be the luminosity of the solar corona if it emitted like a black body?
- Why don't we see the sun as a black body at the temperature of the corona?
Although it is very hot, the solar corona is very low density. The photons are NOT in
local thermodynamic equilibrium with the diffuse coronal plasma.
- The OIII line is often seen in emission from regions downstream of
strong shock waves. The rest wavelength of the OIII line is 5007 Angstroms , and the
ionization potential for OII is 50 eV.
- What is the energy of an OIII photon?
- How hot must the shock-heated gas be in order to produce OIII emission?
(Hint: At roughly what temperature would you start seeing OIII ions instead of OII ions?)
Equate the thermal energy, kT, to the ionization potential for OIII
- By equating the kinetic energy of a proton with the thermal
energy, estimate the velocity of the shock that heated the gas that produced
the OIII emission.
- A UV stellar spectrum is observed to have a Ly-alpha absorption line that
is blueshifted by 1 Angstrom
with respect to the other absorption lines in the spectrum. The intensity in the
line is 25% the intensity just outside the spectral line.
The rest wavelength of Ly-alpha is 1193 Angstroms.
- What is the optical depth at the wavelength of Ly-alpha of the
gas producing the anomolous absorption line?
- What can you say about the velocity and direction of the gas producing this line?
The observed doppler shift implies that the component of the gas velocity along
the line of sight is directed toward the observer at a velocity of
- Observations at radio wavelengths suggest the absorption is due
to a gas cloud sitting in between the star and earth, and that it is approximately
1 parsec in diameter. If the absorption cross section for Ly-alpha in this cloud
were determined to be 10**-21 cm**2, what is the density of Hydrogen
in the cloud?
