Click image to open full size
Edwin Hubble, Discoverer of the Expanding Universe
Galaxies, redshift, and Hubble's law
Related Tools
Related Worksheets
Edwin Hubble helped change astronomy from a study of nearby stars into a science of the whole universe. Using the 100-inch Hooker Telescope at Mount Wilson Observatory, he showed that many fuzzy nebulae were actually galaxies far beyond the Milky Way. His work gave strong evidence that the universe is much larger than people had believed. It also led to one of the most important discoveries in modern science, the expansion of the universe.
Hubble compared galaxy distances with their spectra and found that farther galaxies usually have larger redshifts. A redshift means the light from a galaxy is stretched toward longer, redder wavelengths, which is evidence that the galaxy is moving away from us. This relationship became known as Hubble's law, v = H0d, where recession speed increases with distance. The discovery became a foundation for Big Bang cosmology and for measuring the age and scale of the universe.
Key Facts
- Hubble showed that the Andromeda Nebula is a separate galaxy outside the Milky Way.
- Cepheid variable stars were used as standard candles to measure distances to nearby galaxies.
- Hubble's law: v = H0d, where v is recession speed, H0 is the Hubble constant, and d is distance.
- Redshift formula for small speeds: z = Δλ/λ0, where z is redshift and λ0 is the rest wavelength.
- For relatively low speeds, recession speed can be estimated by v ≈ cz, where c = 3.00 x 10^5 km/s.
- A rough estimate for the age of the universe is t ≈ 1/H0, after converting H0 into units of 1/s.
Vocabulary
- Galaxy
- A galaxy is a huge collection of stars, gas, dust, and dark matter held together by gravity.
- Redshift
- Redshift is the stretching of light to longer wavelengths, often caused by an object moving away or by the expansion of space.
- Hubble's law
- Hubble's law states that a galaxy's recession speed is proportional to its distance from us.
- Cepheid variable
- A Cepheid variable is a star whose regular brightness changes can be used to find its true luminosity and distance.
- Hubble constant
- The Hubble constant is the proportionality value that relates a galaxy's distance to its recession speed in the expanding universe.
Common Mistakes to Avoid
- Thinking Hubble discovered galaxies by looking at their shapes alone is wrong because his key evidence came from measuring Cepheid variable stars and distances.
- Treating redshift as only a change in color is wrong because it is a measurable shift in spectral lines across the whole spectrum.
- Using Hubble's law for nearby stars inside the Milky Way is wrong because local gravitational motions dominate over cosmic expansion at small scales.
- Forgetting units in v = H0d is wrong because H0 often uses km/s/Mpc, so distance must be in megaparsecs to get speed in km/s.
Practice Questions
- 1 A galaxy is 50 Mpc away. If H0 = 70 km/s/Mpc, what is its recession speed using v = H0d?
- 2 A spectral line normally has wavelength 500 nm, but it is observed from a galaxy at 510 nm. Find the redshift z = Δλ/λ0, then estimate the recession speed using v ≈ cz with c = 3.00 x 10^5 km/s.
- 3 Explain why Hubble's discovery that distant galaxies are redshifted supports an expanding universe rather than a universe with galaxies sitting still in space.