21-cm line

Definition

The 21-cm line is a spectral line emitted by neutral hydrogen atoms due to the hyperfine transition. It is a crucial tool in radio astronomy for mapping interstellar gas and studying galactic structures.

5 Must Know Facts For Your Next Test

1. The 21-cm line results from the transition between two energy states of neutral hydrogen atoms.
2. It has a wavelength of 21 centimeters, corresponding to a frequency of approximately 1420 MHz.
3. This spectral line can penetrate interstellar dust, allowing astronomers to observe regions that are obscured in visible light.
4. The 21-cm line is used to measure the distribution and density of neutral hydrogen in our galaxy and others.
5. It plays a key role in understanding the spiral structure of galaxies by tracing the location and movement of hydrogen clouds.

Review Questions

• What causes the emission of the 21-cm line in neutral hydrogen atoms?
• Why is the 21-cm line particularly useful for mapping interstellar gas?
• How does the ability of the 21-cm line to penetrate dust benefit astronomers?

Related terms

Hyperfine Transition: The change between two closely spaced energy levels within an atom's ground state, responsible for phenomena like the 21-cm line.

Neutral Hydrogen (HI): The most abundant form of hydrogen in space, consisting of one proton and one electron.

Radio Astronomy: The study of celestial objects using radio waves rather than visible light, crucial for observing phenomena like the 21-cm line.