THE SOUTHWORTH PLANETARIUM
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Founded January 1970
2022-2023: XII
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Sunset: 6:53 p.m.
Civil twilight ends: 7:22 p.m.
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Moon phase: Waning gibbous (81% illuminated)
Moonrise: 9:02 p.m.
Moonset: 12:08 p.m. (9/15/2022)
Julian date: 2459837.16
"I find that a duck's opinion of me is very much influenced over whether or
not I have bread." -Mitch Hedberg


THE DAILY ASTRONOMER Wednesday, September 14, 2022
About Those Galactic Distances...


For the first time this school year, we return to the woefully neglected
Pandora's Jar,* the vessel in which we store subscribers' questions:
questions that we strive to answer in a timely fashion. And often fail to
do so. :-(

The first query of the 2022-23 school year was written by subscriber Theo
K, who, after having seen and been amazed by the Webb Space Telescope
images, wondered how astronomers were able to determine the distances to
other galaxies. Brilliant question. The distance to even the closest major
galaxy, Andromeda, equals 2.2 million light years. Many galaxies are
located billions of light years from us. How on Earth -and, yes, we phrased
it that way on purpose- can we humans possibly measure such unfathomably
vast distances?

To explain the method, we turn our attention to the "Cosmic Distance
Ladder."


*Cepheid review*
 First, we need to review the Cepheid variable method that enables
astronomers to determine the distances to the closest galaxies.
Cepheids are variable stars whose variability is directly related to
luminosity.  The longer the variability period, the more luminous the
star.    By observing the amount of time separating successive maxima
(times when the brightness is at maximum), an astronomer can measure the
Cepheid's true brightness . Through use of the distance modulus equation,
one can determine the Cepheid variable star's distance if its intrinsic
brightness (absolute magnitude M) and apparent brightness (apparent
magnitude m) are both known.

[image: download.png]
The distance modulus formula:   m = apparent magnitude, M = absolute
magnitude; d = distance (in parsecs).   A Cepheid variable's period is
directly related to its absolute magnitude.   By comparing the true to
apparent brightness, the star's distance is known. By extension, the
distance to the star's host galaxy will also be determined.

The problem with the Cepheid variable method is distance limitation.
 The method is valid out to 20 million light years.  While a considerable
distance, the cosmos is immensely larger.      In order to measure the
distances of more remote galaxies, astronomers generally rely on a
technique related to the Universal expansion.

In 1929, Edwin Hubble (1889-1953)  announced his discovery that the
Universe is expanding in all directions.    This expansion is a consequence
of the Big Bang, the primordial event that created the cosmos.  Ever since
its inception around 13.8 billion years ago, the cosmos has been growing
steadily larger.   Consequently, the galaxies within it appear to move away
from each other unless they happen to be so close that gravitational
attraction overwhelms the expansion.**      Hubble devised a law relating a
galaxy's recession speed and distance:

                                         * v  =  H x d*

where   v = a galaxy's velocity;  H = Hubble constant; d = distance in
megaparsecs.  (One megaparsec equals 3.26 million light years.)  The Hubble
Constant is measured in units of kilometers per second per Megaparsec.
 One of the most recent Hubble constant estimates, published by a research
team led by a University of Oregon astronomer, cites a value of 71.5 for
the Hubble Constant.        Any galaxy moving at 71.5 kilometers per second
away*** from the Milky Way would therefore be located 1 Megaparsec away
from the Milky Way. A galaxy receding from the Milky Way at 143 kilometers
per second would be 2 Megaparsecs away from us and so forth.

The *Hubble Law *can tell us a galaxy's distance "simply" by measuring the
galaxy's speed.      How can astronomers measure this speed?   By measuring
the galaxy's "light shift."   A galaxy consists of innumerable stars, all
of which produce light.   If the galaxy is moving away from us, that light
will become elongated as a consequence of the recession.

[image: 500px-Redshift_blueshift.png]
At the top of this simplified diagram a star moves away from the observer.
The star's emitted light is "stretched out" as a result of this motion.
 The elongation of light increases its wavelength, which decreases its
frequency.      The light is shifted toward the red end of the spectrum, as
red light has a lower frequency than blue light,   At the diagram's bottom,
the star moves toward the observer and its light is compressed.  The
wavelength decreases and its frequency increases, shifting the light toward
the "blue end" of the spectrum.

[image: main-qimg-1882da28ef34be15ceca6073a1cdd664.webp]

In reality, the emitted light is separated into its component colors Within
the spectrum one finds a series of absorption lines that occupy specific
"rest" wavelengths.    The lines within a spectrum of a receding galaxy
shift toward the red end of the spectrum.  The greater the shift, the
faster the recession velocity and the more distant the galaxy.

Application of the Hubble Law enables astronomers to measure the distances
to remote galaxies throughout the Virgo Super Cluster and throughout the
Universe.




*Now, wait a second! Didn't Pandora possess a box, not a jar? Well, in
Hesiod's "Works and Days," he mentions that Pandora opened a jar, which in
Greek is the word "pithos." Desiderius Erasmus Roterodamus (1466-1536), was
believed to have changed the word to "pxyis," meaning "box" in this 1508
publication Adagia.   Consequently, the container encapsulating all the
ills of humankind became known as a box, not a jar.

**The Andromeda and Milky Way Galaxies are currently moving toward each
other and will collide within 4 - 6 billion years.

***One notable consequence of this revised value is that the Universe's age
would be lowered to approximately 12.8 billion years



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