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Physics
Edwin Hubble: Discoverer of the Expanding Universe
Galaxies beyond the Milky Way and cosmic redshift
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Edwin Hubble helped transform astronomy from a study of stars in the Milky Way into a study of a vast, expanding universe. In the 1920s, he used the 100-inch Hooker Telescope at Mount Wilson Observatory to show that many fuzzy spiral nebulae were actually separate galaxies far beyond our own. This discovery greatly expanded the known scale of the cosmos and changed humanity's place in it. His work became one of the foundations of modern cosmology.
Key Facts
- Hubble showed that Andromeda is a separate galaxy, not a nebula inside the Milky Way.
- Hubble's law relates recession speed to distance: v = H0d.
- A galaxy's redshift is defined by z = (lambda observed - lambda emitted) / lambda emitted.
- For small redshifts, recession speed can be estimated by v = cz.
- Cepheid variable stars let astronomers estimate galaxy distances using their period-luminosity relation.
- The Hubble Space Telescope was named in Edwin Hubble's honor and has measured cosmic expansion with great precision.
Vocabulary
- Galaxy
- A galaxy is a huge gravitationally bound system of stars, gas, dust, and dark matter.
- Redshift
- Redshift is the stretching of light to longer wavelengths, often showing that a distant galaxy is moving away.
- Hubble's law
- Hubble's law states that more distant galaxies recede faster, expressed as v = H0d.
- Cepheid variable
- A Cepheid variable is a star whose regular brightness changes reveal its true luminosity and help measure distance.
- Cosmology
- Cosmology is the scientific study of the origin, structure, evolution, and large-scale behavior of the universe.
Common Mistakes to Avoid
- Thinking Hubble discovered the Big Bang itself, which is wrong because he discovered evidence for expansion that later supported Big Bang models.
- Confusing redshift with a galaxy simply being red in color, which is wrong because redshift is a measured change in wavelength caused by motion or cosmic expansion.
- Using Hubble's law for nearby objects like planets or stars in the Milky Way, which is wrong because local gravity dominates over cosmic expansion on small scales.
- Treating H0 as an exact constant with no uncertainty, which is wrong because measurements of the Hubble constant depend on methods and still have scientific tension.
Practice Questions
- 1 A galaxy is 50 megaparsecs away. Using H0 = 70 km/s/Mpc, calculate its recession speed with v = H0d.
- 2 A spectral line normally has wavelength 500 nm but is observed from a galaxy at 525 nm. Calculate the redshift z.
- 3 Explain why discovering Cepheid variables in Andromeda showed that Andromeda is outside the Milky Way.