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How to Calculate the Expansion Rate of the Universe (in Ten Easy Steps)...

1) Witness a supernova (SN) explosion in a nearby galaxy
2) With ground-based telescope, measure lightcurve of SN over several weeks and its spectral output (Needs to be a Type 1A supernova)
3) Propose for Hubble Space Telescope time using Advanced Camera for Surveys to observe host galaxy
4) Win awarded time; observe over many months in several filters
5) Measure light fluctuations from Cepheid variables in the host galaxy
6) Compare host galaxy's Cepheids to Cepheids in the Milky Way Galaxy
7) Determine an accurate distance to the host galaxy
8) Compare SN light curve in nearby host galaxy to other galaxies much farther away
9) Get advanced degree in cosmology
10) Write peer-reviewed journal article with results

Supernova SN2002fk in NGC 1309

In 2002, NGC 1309 hosted a supernova, a star that blew off its outer layers in a massive explosion. This ground-based image of NGC 1309 was taken in 2002 when SN2002fk was still visible. It was observed with the Lick Observatory KAIT telescope. The Hubble ACS image at right shows the location of the supernova, well after its light has died down.


Click to enlarge
Lick Observatory image: W. Li and A. V. Filippenko (University of California, Berkeley)
HST image: NASA, ESA, The Hubble Heritage Team (STScI/AURA), and A. Riess (STScI)

Cepheid Variables in NGC 1309


NASA, ESA, The Hubble Heritage Team (STScI/AURA), and A. Riess (STScI)

Supernovae, such as SN2002fk, can be used to calibrate distance measurements in the universe, because other, fainter stars of known brightness can be observed in the same galaxy. These stellar "standard candles" are the Cepheid variable stars, which vary regularly in brightness with periods that are directly related to their intrinsic brightness, and thus allow the distance to the galaxy--and the supernova--to be determined directly. However, only the Hubble Space Telescope, equipped with the Advanced Camera for Surveys, has the capability to resolve these individual Cepheids in NGC 1309.

The below image shows a Hubble ACS observationsof NGC 3370 which was also taken for its resolvable Cepheid variables. It hosted a type Ia supernova in 1994.


Click to visit Heritage release of NGC 3370
NASA, ESA, The Hubble Heritage Team (STScI/AURA), and A. Riess (STScI)