Sky & Telescope - October 2022 - 16

Climbing the Ladder
Step 3. Galaxies
Just looking at a remote galaxy doesn't reveal its distance. In fact,
in the early 20th century many astronomers assumed that " spiral
nebulae " were part of our Milky Way. Others correctly believed
they were huge collections of stars similar to and way beyond
the Milky Way. If so, rough guesstimates of their distances could
be made by simply assuming that they all have the same size
and luminosity as our home galaxy - just like Huygens assumed
that other stars were similar to the Sun. In fact, after American
cosmology pioneer Edwin Hubble established the true nature of
galaxies by measuring distances to the nearest ones (see next
section), he made similar assumptions about more distant galaxies
- for instance, that the brightest star-forming region in any
galaxy always emits more or less the same amount of light. That
enabled him to conclude that a galaxy's distance is proportional
to its observed recession velocity and helped him to discover that
the universe is expanding.
Galaxies' Motion Away from Us
1,000
500
3.26 million
Distance (light-years)
6.52 million
Today, to gauge a galaxy's distance, astronomers use
standard candles - objects whose true luminosity is
known. Extragalactic eclipsing-binary stars are generally
too faint to study in much detail, but Cepheids are
bright stars. In 1923, Hubble was the first to discover a
Cepheid in the outer regions of the Andromeda " Nebula, "
enabling him to convincingly prove that the fuzzy
spiral lies well outside our Milky Way Galaxy. (We now
know it's 2½ million light-years away.) Using the Hubble
Space Telescope, astronomers have studied Cepheids
in galaxies tens of millions of light-years away. According
to Adam Riess (Space Telescope Science Institute),
the current best calibration of the Cepheid period-luminosity
law comes from Gaia parallaxes and accurate
Hubble photometry. " The result is a 1% accuracy in
Cepheid distances, " he says.
More recently, Wendy Freedman (University of Chicago)
has pioneered the use of red giant stars as standard
candles. Red giants are highly evolved stars that have
used up the hydrogen fuel in their cores, converting it
into helium. It turns out that red giants can only be so
luminous: The star reaches a maximum value when
the helium core becomes dense enough to generate
a runaway thermonuclear explosion. So if you plot the
Hertzsprung-Russell diagram for a large enough collection
of red giant stars in a particular galaxy, the tip
of the red giant branch (TRGB) is always located at the
same true luminosity. " There are many advantages of
the TRGB [method] over Cepheids, " says Freedman. " In
the future, I think that it will prove more accurate than
the Cepheids. "
104
106
Main
102
1
10-4
10-2
40,000
20,000
10,000
5,000
Temperature (kelvin)
2,500
sequence
Tip of the
red giant
branch
Red giant
branch
16 OCTOBER 2022 * SKY & TELESCOPE
HUBBLE DIAGRAM SOURCE: E. HUBBLE / PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES 1929; TRGB SOURCE:
RICHARD POGGE; SNIA LIGHTCURVE SOURCES: C. R. NAVE / HYPERPHYSICS AND JOHN LUCEY / DURHAM UNIV.
Velocity (km/s)
Luminosity (solar)

Sky & Telescope - October 2022

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