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:
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