Download Homework #4 Astronomy 101 – Fall 2010 Due November 4, 11 a.m.

Homework #4
Astronomy 101 – Fall 2010
Due November 4, 11 a.m.
1. Page 126, #14
Why not Hubble? Of the more than 200 extrasolar planets discovered, only
one likely planet has ever been imaged by the Hubble Space Telescope.
What limit’s Hubble’s ability to image planets around other stars?
There are two main factors that limit Hubble’s ability to directly image planets
orbiting other stars. First is the tremendous distance between any given star
and our Sun relative to the tiny orbital distance of a potential planet. Any
telescope, including Hubble, would need extremely good resolution in order to
image such a planet, increasingly so for more and more distant stars. To make
matters more difficult, stars are overwhelmingly bright compared to their
potential companion planets. In order for the Hubble Space Telescope to see a
planet directly, the light from the star needs to be blocked out - without blocking
the orbit of the planet - before it reaches the telescope’s detector, otherwise the
detector will be flooded with light from the star alone. Consequently, imaging
extrasolar planets directly is quite a challenge, even for Hubble!
2. Distinguish between the terms “fusion” and “fission”.
Fusion describes the process by which two light atomic nuclei are slammed
together at high speeds to form a heavier nucleus. In this case, the total mass of
the new, heavy nucleus is less than the mass of the two original nuclei put
together. In order not to violate any principles of nature, this mass cannot
simply vanish during the reaction; instead, it is converted into a great amount of
energy according to Einstein’s famous equation E = mc2. Conversely, fission
describes the process by which a single heavy nucleus is split apart into two
lighter nuclei. Now, however, the combined mass of the two lighter nuclei is less
than the mass of the original heavy nucleus; therefore, the process of fission also
converts mass into energy. Fission is also described as radioactive decay and
radioisotopic dating can be used to estimate the age of a rock.
3. Review Figure 5.3 on page 78. Describe in your own words how an aurora
is produced.
In general, the Earth’s magnetosphere shields us from the stream of charged
particles that make up the solar wind. However, some of these solar wind
particles are accelerated along the Earth’s magnetic field lines, creating bands of
charged particles that come closest to the Earth’s surface near the North and
South poles. These trapped particles excite particles in the Earth’s upper
atmosphere, which in turn emit photons (light) as they return to their lowest
energy states, thus creating the phenomenon we call the aurora.
4. List three stellar properties that can be determined from the spectrum.
By looking at absorption lines in a stellar spectrum, we can determine the
composition of the star’s cool, thin outer atmosphere, the surface temperature
of the star, and how fast the star is moving either towards or away from us.
Each element has a specific pattern of absorption lines that it is able produce
under the right conditions. Most directly, these unique signatures tell us the
chemical composition of the outer atmosphere. However, the complete pattern
of lines for a certain element may not show up in the absorption spectrum;
instead, we only see the portions of the spectrum that can be produced at that
temperature.
Therefore, we can glean information about the surface
temperature of the star from the spectrum as well. (As a check, we can also
verify that the wavelength at which the intensity of the star peaks in the
continuous spectrum gets shorter as the surface temperature increases, and
more light is produced overall for these hotter stars.) Finally, we can use the
Doppler shift of the spectrum to measure how quickly the star is moving
towards or away from us. If a star is moving towards us, we will see familiar
patterns of spectral lines shifted slightly to shorter, bluer wavelengths.
Similarly, if the star is moving away from us, its spectral lines will be shifted to
longer, redder wavelengths. The faster the star is moving, the larger the shift
will be.
Additional properties:
Magnetic fields—the presence and strength of magnetic fields can be deduced
by studying the splitting of spectral lines.
Spectral type—the appearance of the spectrum is basically determined by the
temperature of the star, so astronomers have set up a classification system,
OBAFGKM.
Luminosity class or size—from subtle differences in the spectral lines you can
determine whether the star is a supergiant, giant, dwarf, or regular main
sequence star. Combined with the spectral type, the luminosity class can be used
to assign an absolute magnitude for the star. When apparent magnitude is
combined with the absolute magnitude (distance modulus), the distance can be
determined using the spectroscopic parallax method as you did in Lab #8.
5. a) The parallax of the star Epsilon Eridani is 0.31 arcseconds. What is the
distance to Epsilon Eridani in parsecs? In light years? b) The distance to
Aldebaran is 16 parsecs. What is Aldebaran’s parallax angle?
a) d( parsecs) 
b) d( pc) 


1
1
3.26ly

pc  3.32 pc 
 10.5ly
p(arcsec) 0.31
1pc
1
1
1
 p(arcsec) 
 arcsec  0.0625arcsec
p(arcsec)
d( pc) 16