Measuring our galaxy

There is a yardstick to measure our galaxy.

Cepheus is a constellation near the north pole of the sky, meaning it should be visible on clear nights the year around in Maryland. The constellation's outline looks a bit like a house, nestled between Cassiopeia (which looks like a "W" or "M") and Draco, the dragon. The tip of Cepheus' "roof" points generally toward the north star, and below its "foundation" is Cygnus, the swan.

One of the constellation's lower left "foundation" stars is delta Cephei. In the 1780s, European amateur astronomer John Goodricke discovered that delta Cephei varied in brightness. In the 1910s, while studying large numbers of variable stars similar to delta Cephei, American computer Henrietta Swan Leavitt determined that "there is a simple relation between the brightness of the variable and their periods." [1, 13]

Knowing this tight relationship between a Cepheid variable star's luminosity and the variation period was a key finding. Early 20th century astronomers could use a triangulation method, stellar parallax, to find the distances to nearby Cepheid variable stars. Comparing these distances with the period-luminosity relation gave a calibration for a yardstick to find any similar Cepheid's distance from us. [2]

Harlow Shapley used Cepheid variables in the 1910s to measure distances to globular clusters and thereby reveal the location of the center of our galaxy. [8, 9, 10] By 1929, Edwin Hubble estimated the Andromeda "spiral nebula" to be 900,000 light years away, which was too far to be part of our Milky Way. The Andromeda spiral nebula had to be another galaxy, and thus became the most distant object known at that time in the entire universe. [3, 11, 12]

Cepheid variables continue as cosmic yardsticks today, helping to find  distances to galaxies up to 56 million light years away. [4, 5]

Notes

1. "Delta Cephei", by Kerri Malatesta, American Association of Variable Star Observers, [ http://www.aavso.org/vsots_delcep ], accessed 2013-08-07.

2. "Delta Cephei is a famous variable star", by Bruce McClure and Deborah Byrd, 2012-09-08, [ earthsky.org/brightest-stars/delta-cephei-the-kings-famous-variable-star ], accessed 2013-08-07.

3. "Andromeda Nebula lies outside Milky Way Galaxy", Cosmic Times, 1929, Goddard Space Flight Center, [ http://cosmictimes.gsfc.nasa.gov/online_edition/1929Cosmic/andromeda.html ], accessed 2013-08-07.

4. "Hubble Space Telescope measures precise distance to the most remote galaxy yet", 1994-10-26, news release STScI-1994-49, [ hubblesite.org/newscenter/archive/releases/1994/49/ ], accessed 2013-08-07.

5. "Hubble discovers Cepheid variables in M100", [ http://messier.seds.org/more/m100_hst2.html ], accessed 2013-08-07.

6. "Measurement of galaxy distances: Cepheid variables", Jeffrey A. Willick, [ http://ned.ipac.caltech.edu/level5/Willick/Willick2.html ], accessed 2013-08-07.

7. "Period-luminosity relation for variable stars", Nick Strobel, [ http://www.astronomynotes.com/ismnotes/s5.htm ], accessed 2013-08-07.

8. "1920: Harlow Shapley finds our place in the Milky Way", [ http://cosmology.carnegiescience.edu/timeline/1920 ], accessed 2013-08-11.

9. "Globular clusters and the structure of the galactic system", Harlow Shapley, Publications of the Astronomical Society of the Pacific, vol. 30, no. 173, February, 1918, pages 42 to 54, [ http://www.jstor.org/stable/40710119 ], accessed 2013-08-11.

10. "Studies based on the colors and magnitudes in stellar clusters. VII. The distances, distribution in space, and dimensions of 69 globular clusters", Harlow Shapley, Astrophysical Journal, 48, 154-181 (October 1918), DOI 10.1086/142423, [ http://articles.adsabs.harvard.edu//full/1918ApJ....48..154S/0000154.000.html ], accessed 2013-08-11.

11. "1929: Edwin Hubble discovers the universe is expanding", [ http://cosmology.carnegiescience.edu/timeline/1929 ], accessed 2013-08-11.

12. "A spiral nebula as a stellar system, Messier 31", Edwin P. Hubble, Astrophysical Journal, 69, 103-158 (March 1929), DOI 10.1086/143167, [ http://articles.adsabs.harvard.edu/full/1929ApJ....69..103H ], accessed 2013-08-11.

13. "1912: Henrietta Leavitt discovers the distance key", [ http://cosmology.carnegiescience.edu/timeline/1912 ], accessed 2013-08-11.