Megaparsec

Megaparsec (symbol: Mpc) is a unit of length used in astronomy to measure vast distances between galaxies and galaxy clusters. One megaparsec equals one million parsecs (10⁶ pc) and is commonly used in physical cosmology to describe the scale of the observable universe.

Definition

A megaparsec is derived from the parsec (pc), a standard astronomical unit. The parsec itself is defined as the distance at which 1 astronomical unit (AU) subtends an angle of 1 arcsecond: $1{\text{ pc}}={\frac {1{\text{ AU}}}{\tan(1{\text{ arcsec}})}}\approx 3.085677581\times 10^{16}{\text{ meters}}.$ Thus, 1 Mpc = 10⁶ pc ≈ 3.085677581 × 10²² meters.

Usage in Astronomy

Megaparsecs are critical for expressing:

• Distances between galaxies (e.g., the Andromeda Galaxy is ≈ 0.78 Mpc from the Milky Way).
• Sizes of galaxy clusters and superclusters.
• Measurements in Hubble's law, where the Hubble constant (H₀) relates recession velocity to distance (e.g., 70 km/s per Mpc).

Key applications

Used in redshift surveys to map large-scale structure of the universe.

Helps quantify dark energy effects in accelerating universe models.

Related Units

Kiloparsec (kpc)

1 kpc = 10³ pc. Used for distances within galaxies (e.g., the Milky Way’s diameter ≈ 30 kpc).

Gigaparsec (Gpc)

1 Gpc = 10⁹ pc. Used for quasars and the cosmic web.

Conversions

• 1 Mpc ≈ 3.086 × 10²² meters
• 1 Mpc ≈ 3.261 million light-years
• 1 Mpc ≈ 2.108 × 10⁷ AU

Historical Context

The term "parsec" was coined in 1913 by British astronomer Herbert Hall Turner. Megaparsecs became widely adopted in the 20th century as observational cosmology advanced, particularly after Edwin Hubble's discovery of galactic redshifts in the 1920s. Modern surveys like the Sloan Digital Sky Survey (SDSS) rely on Mpc-scale measurements to study dark matter distribution.

See Also

• Parsec
• Light-year
• Cosmic distance ladder
• Hubble constant
• Redshift

References

• "IAU Units". www.iau.org. International Astronomical Union. Retrieved 2025-01-31.

• Peebles, P. J. E. (1993). Principles of physical cosmology. Princeton series in physics. Princeton, N.J: Princeton Univ. Press. ISBN 978-0-691-01933-8.

External Links

• NASA – Resources on cosmic measurements.
• Sloan Digital Sky Survey – Galaxy mapping projects.