The historical background to this change in world view has been extensively discussed and whole
books have been devoted to the subject of distance measurement in astronomy [125]. At the
heart of the change was the conclusive proof that what we now know as external galaxies lay
at huge distances, much greater than those between objects in our own Galaxy. The earliest
such distance determinations included those of the galaxies NGC 6822 [61], M33 [62] and
M31 [64].
As well as determining distances, Hubble also considered redshifts of spectral lines in galaxy spectra
which had previously been measured by Slipher in a series of papers [142, 143]. If a spectral
line of emitted wavelength is observed at a wavelength
, the redshift z is defined as
|
Recession velocities are very easy to measure; all we need is an object with an emission line and a spectrograph. Distances are very difficult. This is because in order to measure a distance, we need a standard candle (an object whose luminosity is known) or a standard ruler (an object whose length is known), and we then use apparent brightness or angular size to work out the distance. Good standard candles and standard rulers are in short supply because most such objects require that we understand their astrophysics well enough to work out what their luminosity or size actually is. Neither stars nor galaxies by themselves remotely approach the uniformity needed; even when selected by other, easily measurable properties such as colour, they range over orders of magnitude in luminosity and size for reasons that are astrophysically interesting but frustrating for distance measurement. The ideal H0 object, in fact, is one which involves as little astrophysics as possible.
Hubble originally used a class of stars known as Cepheid variables for his distance determinations. These
are giant blue stars, the best known of which is UMa, or Polaris. In most normal stars, a self-regulating
mechanism exists in which any tendency for the star to expand or contract is quickly damped out. In a small
range of temperature on the Hertzsprung–Russell (H-R) diagram, around 7000 – 8000 K, particularly at high
luminosity1,
this does not happen and pulsations occur. These pulsations, the defining property of Cepheids, have a
characteristic form, a steep rise followed by a gradual fall, and a period which is directly proportional to
luminosity. The period-luminosity relationship was discovered by Leavitt [86] by studying a sample of
Cepheid variables in the Large Magellanic Cloud (LMC). Because these stars were known to be all at
the same distance, their correlation of apparent magnitude with period therefore implied the
P-L relationship.
The Hubble constant was originally measured as 500 km s–1 Mpc–1 [63] and its subsequent
history was a more-or-less uniform revision downwards. In the early days this was caused by
bias2
in the original samples [8], confusion between bright stars and Hii regions
in the original samples [65, 131] and differences between type I and II
Cepheids3 [4].
In the second half of the last century, the subject was dominated by a lengthy dispute between investigators
favouring values around 50 km s–1 Mpc–1 and those preferring higher values of 100 km s–1 Mpc–1. Most
astronomers would now bet large amounts of money on the true value lying between these extremes,
and this review is an attempt to explain why and also to try and evaluate the evidence for
the best-guess (2007) current value. It is not an attempt to review the global history of H0
determinations, as this has been done many times, often by the original protagonists or their close
collaborators. For an overall review of this process see, for example, [161] and [149]; see also data
compilations and reviews by Huchra (http://cfa-www.harvard.edu/~huchra/hubble) and Allen (
http://www.institute-of-brilliant-failures.com/).
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