S3 - Radiation and the ISM

Question 1

What is the relationship between the intensity of light and distance?

Answer 1

Intensity of light is independent of the distance travelled.

Question 2

What is the flux of radiation?

Answer 2

The amount of energy crossing a unit area per unit time.

Question 3

What wavelengths is the atmosphere transparent to?

Answer 3

Mostly radio and optical. Any other wavelengths need telescopes in space to be observed properly.

Question 4

Does flux of radiation depend on distance?

Answer 4

Yes

Question 5

What is black-body radiation?

Answer 5

The electromagnetic radiation emitted by a body in thermodynamic equilibrium with its environment (a black body (an idealized non-reflective body)).

Question 6

Cold black bodies emit at ________ wavelengths.

Answer 6

Cold black bodies emit at long wavelengths (low frequency) and vice versa.

Question 7

What are nebulae?

Answer 7

Optically dark clouds (nebulae) are clouds of dust and gas (a.k.a. molecular clouds). These consist of almost entirely molecular hydrogen mixed with trace amounts of other molecules and small amounts of interstellar dust.

Question 8

What temperatures are molecular clouds and thus what wavelengths do they mainly emit? What is the view of the sky at this wavelength a view of?

Answer 8

10-100K
According to Wien’s law, they emit at far-infrared (FIR) wavelengths.
Interstellar dust radiates more strongly than stars at this wavelength range, so an FIR view of the sky is a view of the location of interstellar dust.

Question 9

At what wavelength range can the distribution of stars in the Milky Way be mapped most efficiently?

Answer 9

At near-infrared wavelengths (NIR) because optical light is absorbed by ISM, FIR is emitted more by molecular clouds, and stars emit very little (are dim) at radio wavelengths.

Question 10

What wavelength of light penetrates through space the easiest?

Answer 10

Radio waves, due to their long wavelengths.