# Astronomy Pre-Lab Questions

1) a) What are spectra and where do they come from?

A spectrum is a range that can illustrate a certain property of a physical object. Specifically, in astronomy, the electromagnetic spectrum indicates the range of wavelengths and frequencies at which an electromagnetic sources can emit radiation. Spectra result when electrons in an atom increase energy and move to a different level (absorption), then move back into their normal level (emission). Energy levels are quantized; that is, the electrons of a particular atom are restricted to certain energy levels, and will not react if they gain or lose the wrong amount of energy (Bennett et al, 2013, p.149-150).

b) When astronomers study spectra, how is the data represented? Make sure to explain the difference between how absorption lines and how emission lines appear on the graphs.

Spectral data is presented on a graph of wavelength vs. intensity (it could also be frequency vs. intensity, since wavelength and frequency are inversely related). There are three types of spectra: continuous spectra, emission spectra, and absorption spectra.

A rainbow is an example of a continuous spectrum. The light emitted by an incandescent bulb is usually continuous too (Bennett et al., 2013, p. 151).

An emission spectrum results from a source such as a heated gas cloud, and consists only of lines at certain wavelengths representing the energy levels of the gas’s electron transitions (Bennett et al., 2013, p.151). These wavelengths will be seen as spikes in intensity.

When a continuous light source passes through a cloud, the atoms in the cloud (based on the energy levels) will absorb certain wavelengths, producing a graph in which intensity drops at those wavelengths. This is an absorption spectrum (Bennett et al., 2013, p. 152).

2) a) If a rock on the ground is a cool 100 Kelvin, most of the radiation is emitted at which wavelength?

According to Wien’s Law, λ (max) = 2,900,000 / T (Kelvin).

Therefore, λ max) = 2,900,000 / 100 = 29000 = 2.9 x 104

Most of the radiation will be emitted at 2.9 x 104 nm (Bennett et al., 2013, p. 155)

b) If the same rock is then heated up to 300 Celsius, how many times more radiant is it now?

According to the Stefan-Boltzmann Law, emitted power (per square meter) = σ T4

emitted power at 100 = 5.7 x 10-8 watts x 1 x 108 = 0.57 watts = 5.7 X 10-1

T (Kelvin) = 300 (Celsius) + 273 = 573 K

emitted power at 573 = 5.7 x 10-8 watts x 1.08 x 1011 = 6144 watts = 6.1 x 103

The rock emits approximately 1000 times more power at 300 Celsius (573 K) as it did at 100 Kelvin (Bennett et al., 2013, p. 154-156).

3) If you were to purchase a 2700 K CFL light bulb, does is really shine at 2700 K? Are there advantages to using CFL light bulbs as opposed to an incandescent? Are there disadvantages? Explain in detail.

The light in the CFL bulb is produced differently from light in an incandescent bulb. The latter type of bulb produces light by passing current through a metal filament (usually tungsten); this causes the filament to glow with a continuous spectrum of visible light. On the other hand, the CFL bulb sends current into a tube filled with argon and a small amount of mercury vapor. These substances produce ultraviolet light that will, in turn, cause visible light phosphorescence in the bulb’s coating. The CFL bulb is rated 2700 K because the light it produces is similar in spectrum to a 2700 K incandescent; it does not actually shine at 2700 K. The primary advantage of CFL bulbs is the lower amount of power they use. For instance, a 60 W incandescent bulb can be replaced by a CFL that uses 13-15 W (approximately a quarter of the power of the incandescent). They also last longer (Pearce & Russill, 2005, p.7-8). However, there are disadvantages to CFL bulbs as well. They use more energy when first turned on, although once the bulb warms up it uses much less power, as indicated above. The warm-up phase can take 30 seconds – 3 minutes. Some people perceive CFL bulbs as less bright, while others are concerned about the toxic mercury vapor inside them (EPA, 2013).