Cosmic Times

Andromeda Nebula Lies Outside Milky Way Galaxy

The primary message of this article is that until 1924, astronomers did not know that the Milky Way Galaxy was just one of many galaxies in our Universe. A secondary message is that determining distances to the stars in other galaxies was only possible due to improvements in technology and due to the discovery, by Henrietta Leavitt, of the period-luminosity relationship of Cepheid variable stars.

During the time of the 1919 Cosmic Times, astronomers did not have definitive proof that galaxies existed outside our Milky Way Galaxy. Observers had seen fuzzy patches in the sky, but they could not resolve them into anything more than fuzzy patches. Without further evidence, astronomers could only call these regions "nebulae", which was generally associated with a cloud of gas and dust in space. The notes on "Great Debate" Resolved talk a little more about these nebulae and what astronomers thought they were. Here we'll talk more about how Hubble revealed the true nature of some of these nebulae.

Standard Candles

Since astronomers cannot simply pull out giant measuring sticks to determine distances to objects in the Universe, "standard candles" have become a holy grail of sorts for studying the Universe. A standard candle is any object that has a known brightness. With a known brightness, astronomers can then infer a distance based on the observed brightness of the object and the 1/r2 law (see the Just how far is that star? lesson plan).

Cepheid variables

In the early 1900s, Harvard astronomers employed women as "computers". They took on tedious tasks such as determining the brightness of stars on photographic plates and searching photographic plates for variable stars.

Henrietta Leavitt was tasked with searching for variable stars in photographic plates taken between 1893 and 1906 at Harvard College's observatory in Peru. In 1908 she published a catalog of over 1777 variable stars. For 16 of these stars, she was able to measure a period for their variability, and she noticed that the brighter stars of this sample had longer periods.

By 1912, Leavitt had determined the period for a total of 25 variable stars. She saw that the relationship she had observed in her 1908 catalog held up with these 25 stars. In fact, since all of the stars were in the Small Magellanic Cloud, they could be considered to all be at nearly the same distance. Given that, the relationship between their period and luminosity was intrinsic to the stars, and not just a trick of viewing stars at different distances.

These stars showed a similarity to another known variable star called Delta Cepheid (the fourth brightest star in the constellation of Cepheus), so they are collectively called Cepheid variables.

The one thing that Leavitt was missing was a distance to these Cepheid variables in her sample. She had a period-luminosity relationship, but no "zero point" for that relationship. It needed to be calibrated against something with a known distance.

Calibrating the Cepheid scale

Ejnar Hertzsprung was the first person to try and calibrate the period-luminosity scale for Cepheid variables. Using his calibration, he came up with a distance to the Small Magellanic Cloud of 30,000 light years (actually, the published value was 3,000 light years, but is believed to be a typo which Hertzsprung either did not catch or did not correct for some reason).

The next attempt to calibrate the Cepheid scale came with Harlow Shapley in 1918. Shapley included "Cepheids" from globular clusters in his calibration of the period-luminosity relationship, which will become important in the next issue of the Cosmic Times. It was this calibration of the relationship that Hubble used when he observed Cepheid variables in the Andromeda Nebula.

Observing Andromeda

In 1924, Hubble was able to resolve the spiral arms of the Andromeda Nebula into stars. Some of the stars that Hubble resolved were Cepheid variables, for which Hubble was able to determine periods. Armed with these periods and the calibrated period-luminosity relationship from Shapely, Hubble was able to estimate the distance to Andromeda, and found that it was 860,000 light years away – further than the furthest measured stars. At the time, the Milky Way was estimated to be about 300,000 light years, which we now know to be a huge overestimate, but even with such an overestimate of the Milky Way's size, Andromeda was far beyond the boundary.

Interestingly, even though Andromeda is generally thought to be the first definitive observation of a star system far outside the Milky Way, Hubble actually published results on NGC 6822 first. NGC 6822 is an irregular galaxy that Hubble estimated to be about 700,000 light years away, also very well outside Shapley's estimated size of the Milky Way. It is thought that this result got very little attention because Hubble had already published preliminary results for the spiral nebulae, so the astronomical community generally agreed that there were other galaxies in the Universe.

Other resources

The following web pages have more detailed information on the women computers and the Cepheid distance scale:

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC