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Transcript
1. How does the energy produced at the core of the Sun reach the photosphere?
Photons produced in the solar interior bounce randomly among electrons in the plasma, slowly working
their way outward to the photosphere. Even though they travel at the speed of light, the path they take
through the interior bounces around so much that it takes hundreds of thousands of years to get out! The
reason is that plasma is so dense that photons can only go a tiny distance (less than a millimeter) before
they collide with a particle and get redirected. They continue bouncing around, and eventually, the average
behavior is that they slowly diffuse outward.
2. Why won’t the Sun become a supernova or black hole?
The Sun is too small, lacking the critical mass to end in a supernova. Instead, it will turn into a red giant,
end up as a white dwarf. Only high-mass stars (about 8 times as big as the Sun) have sufficient mass to end
their lives with a supernova and subsequently collapse to a black hole.
3. The average temperature of the universe is 2.7 Kelvin. What is that temperature in degrees Fahrenheit?
9
(2.7 + 273.15) − 32 = −454.81 °F
5
4. What particle is exchanged to generate each of the four fundamental forces?
force
electromagnetic
nuclear weak
nuclear strong
gravitation
mediating particle
photon
W and Z bosons
gluons
gravitons
5. What medical technologies (e.g. PET) rely on the physics of elementary particles? What are the benefits
of noninvasive procedures such as PET?
PET – positron emission tomography
MRI – (nuclear) magnetic resonance imaging
CT scans – computed tomography
Radiation therapy for cancer
Positron emission tomography (PET) scans are three-dimensional images of the body (or body functions).
The PET scan detects pairs of gamma rays produced by a positron-emitting radionuclide tracer. Threedimensional images of tracer concentration within the body are generated from these emitted gamma rays.
Nonsurgical procedures such as PET scans are generally preferred if viable, since surgery carries high risks
from infection and related traumatic effects on the body. While PET does require the insertion or ingestion
of a radionuclide tracer compound, this is generally considered non-invasive, and very low risk.
6. What is a particle accelerator? How do scientists use particle accelerators to study subatomic particles?
Particle accelerators use electromagnetic forces to accelerate charged particles (protons or electrons) to
great speeds/energies. This same principle is used in old-style TV tubes to accelerate and steer beams of
electrons to the back of the screen, causing the illumination of the picture tube. Some designs accelerate
the particles along a straight track, and others contain them in a large circular beam line.
Accelerators are used primarily for scientific research, and a number of industrial and medical applications.
Particle physicists use the accelerators to study the interactions of elementary particles by observing the
byproducts of the high-energy collisions. These collisions give clues as to the structure of the subatomic
world.
7. Why does the existence of the cosmic redshift lead to the big bang picture of the universe?
The fact that the universe appears to be expanding in all directions is the logical consequence of an earlier
explosion, and hence supports the conjecture that the universe experienced a “big bang” at some point in
the past. The expansion appears to us a Doppler shift of objects’ spectra, with more distant objects
showing larger shifts.
8. If a galaxy is 1,000 Mpc away, how fast is it receding from us?
From Hubble’s law, we calculate v = H 0 d = (70 km/s/Mpc)(1000 Mpc) = 70,000 km/s .
9. How is astronomy the oldest science?
People have no doubt tried to understand the movement and behavior of the heavens since the first humans
looked up at the skies. The oldest records of any analytical studies date back to Mesopotamia, over five
thousand years ago. At that point, the study of astronomy was primarily focused on calendrical studies
using it to predict seasons, weather, tides and other natural cycles. It was generally practiced by a select
group of priests.
10. The Milky Way has a radius of approximately 100,000 light years. How many miles is that? How long
would it take you to travel across our galaxy if you were traveling at 50% the speed of light (be sure to
show work)?
One light year is the distance light travels in one Earth year, equivalent to 5.88 ×1012 miles. The distance
across the Milky Way galaxy is approximately 100,000 light years, so traveling at half the speed of light,
the trip would take 200,000 years. (time = distance / velocity)