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Chapter 15: Problem 26

The interstellar medium is approximately 99 percent gas and 1 percent dust. Why is it the dust and not the gas that blocks a visible-light view of the galactic center?

Short Answer

Expert verified
Dust particles efficiently absorb and scatter visible light, blocking the view of the galactic center.

Step by step solution

01

Understanding the Composition

The interstellar medium consists of approximately 99% gas and 1% dust. Although the dust is only a small fraction, it plays a significant role in blocking visible light.
02

Properties of Gas

Gas in the interstellar medium is primarily composed of hydrogen (H) and helium (He). These gases are mostly transparent to visible light and do not significantly absorb or scatter it.
03

Properties of Dust

Interstellar dust is made up of small, solid particles. These particles are of a size comparable to the wavelength of visible light. This makes them very efficient at scattering and absorbing visible light, blocking the view.
04

Interaction with Visible Light

When light encounters dust, the dust particles either absorb the photons or scatter them in different directions. This blockage effect is much stronger than the interaction between light and gas in the interstellar medium.
05

Conclusion

Because dust particles are efficient at absorbing and scattering visible light due to their size, even a small percentage of dust can significantly block a visible-light view of the galactic center.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Gas Composition
The interstellar medium (ISM) is predominantly composed of gas, accounting for about 99% of its makeup. The primary components of this gas are hydrogen (H) and helium (He). Other elements are present but in much smaller quantities. Despite its abundance, this gas does not significantly block visible light because hydrogen and helium are relatively transparent in the visible spectrum. These gases allow light to pass through with minimal absorption or scattering. Understanding the composition of the ISM is essential as it sets the stage for why gas plays a lesser role in blocking visible light compared to dust.
Dust Properties
Interstellar dust, though making up only about 1% of the ISM, has properties that make it very effective at blocking light. These dust particles are composed of small solid materials, including silicates, carbon compounds, and some ice. The size of these particles is comparable to the wavelength of visible light, typically around 0.1 micrometers. Due to their size and composition, dust particles are highly efficient at both scattering and absorbing visible light. This makes them much more significant in obstructing light than the much more abundant gas particles.
Visible Light Absorption
Visible light absorption occurs when photons of light are taken up by dust particles. The energy from these photons warms the dust, which can later re-emit the energy at different wavelengths, often in the infrared spectrum. This absorption process is crucial because it means that visible light from stars and other objects can be significantly dimmed as it travels through the ISM. As a result, even a relatively small amount of dust can have a big impact on the amount of light that we can see, making distant objects appear much dimmer or even completely obscured from view.
Light Scattering
Light scattering happens when light waves are deflected in different directions after interacting with dust particles. In the ISM, this scattering is primarily due to particles that are similar in size to the wavelength of visible light. This scattering can make stars and other light sources appear fuzzier and less distinct. More importantly, scattering by dust reduces the amount of light that travels in a straight line, further contributing to the dimming effect. Thus, scattering, combined with absorption, is a major reason why the dust is so effective at blocking our view of distant astronomical objects.
Galactic Center Visibility
Observing the galactic center is particularly challenging because of the high concentration of dust in the plane of the Milky Way. This dust significantly blocks visible light, making it difficult for ordinary telescopes to see the region clearly. While various methods can mitigate this, such as using infrared or radio observations, visible light remains heavily obstructed. Therefore, the small amount of dust in the ISM plays a crucial role in making the galactic center less visible. This explains why dust, despite being only a minor component of the ISM, has a major impact on astronomical observations.

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