"Yardsticks" in Neighbor Galaxy Double Universe's Size
The primary message of this article is that a miscalibration of the
Cepheid distance scale, stemming from the existence of two populations
of Cepheids with two different period-luminosity relationships, resulted
in an underestimation of distances in the Universe. When this
miscalibration was corrected, the size of the Universe doubled
overnight.
Before we discuss the problem with the Cepheid variable distance scale
that was discovered in the 1940s, researchers had already seen evidence
of a problem with the distance scale. Astronomers are always looking
for different types of "standard candles" – objects of known
brightness for which a distance can then be calculated using the
observed brightness. One such object that astronomers tried to use in
the early days of extra-galactic observations was the globular clusters
of a galaxy. Globular clusters are a type of star cluster that tend to
lie in the halo of a galaxy. In 1931, Hubble compared the brightest
globular cluster in our galaxy to the brightest one in Andromeda, and
found that the one in Andromeda was intrinsically much dimmer
than the one in ours.
Later, he compared the globular clusters in our galaxy to those in
Messier 33 (M 33), another spiral galaxy that is further than Andromeda.
He found that the globular clusters in M 33 were still fainter than
those in Andromeda, based on the distances he had earlier found. Hubble
recognized that it was possible for globular clusters to vary from
galaxy to galaxy, but it seemed unlikely that they would preferentially
get dimmer the further they were from our galaxy. This pointed to a
problem with the distance scale, but Hubble did not have an explanation
for the origin of the problem.
When Baade discovered that there were, in fact, two different
populations of stars, it did not take researchers long to connect the
dots between the discrepancy that Hubble was seeing in the globular
clusters of distant galaxies and the Cepheid distance scale. Baade's
images of Andromeda and Andromeda's two companion galaxies, Messier 32
and NGC 205, showed that there were two populations of stars, which he
called "Type I" and "Type II" – Type I stars were bluer and
brighter whereas the Type II were redder and fainter. He recognized
that the globular clusters were rife with Type II stars, whereas the
disk of the galaxies tended to have both Type I and II stars.
As it happens, the Large Magellanic Cloud, the location of the Cepheid
variables that Henrietta Leavitt observed, is populated with Type I
stars. However, when Shapley calibrated the Cepheid period-luminosity
relationship, he was using observations of Cepheid variables in globular
clusters – where Type II stars reign. The Cepheid variables that
Hubble observed in the Andromeda nebula were Type I, but he unknowingly
used Shapley's calibration that were calibrated for Type IIs. The problem lies
in the fact that there are two different calibrations of the
period-luminosity for the two different populations of Cepheid
variables.
The re-calibration of the Cepheid scale for the Type I Cepheids showed
that the distances to the Type I Cepheids was off by about a factor of
2. In other words, astronomers were finding distances that were half as
far as they should have been. Andromeda suddenly went from being a
"mere" 800,000 light-years away to being about 1,800,000 light-years
away. With this new distance, the brightest globular clusters in
Andromeda were now about the same intrinsic brightness as those found
in our own galaxy.
To bring this discussion into a more modern note, astronomers have
studied the differences between Type I and Type II stars in more detail.
Typically, these are now referred to as Population I and Population II
instead of Type I and Type II. Astronomers have found that one
difference between Population I and II stars it the amount of heavy
elements in the stars. Population II stars show very little heavy
elements where Population I have more. It is thought that Population II
stars are, therefore, older stars, formed earlier in our Universe's
lifetime. This is because the Universe started as mainly hydrogen and
helium. This is still true, but the abundances of the elements heaver
that hydrogen and helium have slowly increased as stars fuse hydrogen
into heavier and heavier elements. Population I stars were likely
formed out of the remains of earlier, Population II stars.
Other resources
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