Ultimate Astronomy 1 — Questions and Answers
Question 1: A filament's initial temperature is 4000K. The filament temperature is then adjusted to 5000K by turning a knob. There is more red light emanating from the filament.
- Finally, when it is at 5000K (Correct answer)
- The same for these two cases
- Not enough information to solve
- Initially, when it is at 4000K.
Correct answer: Finally, when it is at 5000K
The question states that the filament temperature is adjusted to 5000K, and *then* there is more red light emanating. This directly links the observation of increased red light to the final temperature of 5000K. While hotter objects shift their peak emission to bluer wavelengths (Wien's Law), they also emit more radiation at *all* wavelengths, including red, compared to when they were cooler (Stefan-Boltzmann Law).
Question 2: Imagine a star with the same temperature on its surface but twice the sun's surface area. What is the energy flux at that star's surface if the energy flux at the sun's surface is Fsun?
- Fsun/2
- Fsun (Correct answer)
- 2 Fsun
- 16 Fsun
Correct answer: Fsun
Energy flux is defined as the amount of energy emitted per unit surface area. According to the Stefan-Boltzmann Law, this flux depends solely on the surface temperature of the object (F = σT^4). Since the star has the same surface temperature as the Sun, its energy flux at the surface will be identical to the Sun's, regardless of its total surface area.
Question 3: Imagine a star that is twice as big as the sun and has the same surface temperature. What is the total energy emitted by a star per second if the sun emits P watts of energy every second?
- 4 P Watts
- P/2 Watts
- P Watts
- 2 P Watts (Correct answer)
Correct answer: 2 P Watts
The total energy emitted by a star per second (luminosity) is the energy flux multiplied by its total surface area. If the star has the same surface temperature as the Sun, its energy flux (energy per square meter) is the same. The term 'twice as big' in this context refers to having twice the surface area. Therefore, if the star has twice the surface area, it will emit twice the total energy (2 * P Watts).
Question 4: How much more total radiated power per square meter (energy Flux) would we receive on Earth if the sun's temperature doubled?
- Eight times (2^3)
- Sixteen times (2^4) (Correct answer)
- Twice
- Four times (2^2)
Correct answer: Sixteen times (2^4)
According to the Stefan-Boltzmann Law, the total radiated power per square meter (energy flux, F) is directly proportional to the fourth power of the object's absolute temperature (F = σT^4). If the Sun's temperature (T) were to double (2T), the new flux would be σ(2T)^4 = σ * 16 * T^4. This means the energy flux would increase by a factor of sixteen (2^4).
Question 5: The surface of the sun is a good representation of a black body. Let's say the temperature of the sun doubled overnight. In that case, which of the following claims would be accurate?
- The sun would look bluer but radiate less power
- The sun would look redder and radiate less power
- The sun would look bluer and radiate more power (Correct answer)
- The sun would look redder and radiate more power
Correct answer: The sun would look bluer and radiate more power
If the Sun's temperature doubled, two key physical laws apply. Wien's Displacement Law states that the peak wavelength of emitted radiation shifts to shorter (bluer) wavelengths as temperature increases, so the Sun would look bluer. Simultaneously, the Stefan-Boltzmann Law dictates that the total power radiated per unit area is proportional to the fourth power of the temperature, meaning a doubled temperature would result in significantly more power radiated (16 times more).
Question 6: Like our own, the hot, dense cores of stars are surrounded by low-density gaseous atmospheres. What kind of spectrum might you see if observing the light spectrum from the sun or any other star?
- Emission line
- Absorption line (Correct answer)
- Continuous
Correct answer: Absorption line
Stars like the Sun have a hot, dense core that produces a continuous spectrum of light. As this light passes through the cooler, lower-density gaseous atmosphere surrounding the core, specific wavelengths of light are absorbed by the atoms in the atmosphere. This absorption creates dark lines within the continuous spectrum, resulting in an absorption line spectrum.
Question 7: The moon's diameter (or radius) is approximately one-fourth that of the earth. What percentage of Earth's mass would the moon's mass be if its density and that of the planet were equal?
- 1/16
- 1/64 (Correct answer)
- 1/4
- 1/8
Correct answer: 1/64
The mass of an object is its density multiplied by its volume. For a sphere, volume is (4/3)πR³. If the Moon's radius is 1/4 that of Earth's and their densities are equal, then the Moon's mass will be (1/4)³ times Earth's mass. Calculating (1/4)³ gives 1/64, meaning the Moon's mass would be 1/64th of Earth's mass.
Ultimate Astronomy 1
A filament's initial temperature is 4000K.
The filament temperature is then adjusted to 5000K by turning a knob.
There is more red light emanating from the filament.