- 4623 -
HISTORY OF THE
MIKY WAY – discoveries by
Hubble, - It's been 100 years since we learned the
Milky Way is not the only galaxy. On
Sunday November 23, 1924, 100 years ago this month, readers perusing page six
of the New York Times would have found an intriguing article, amid several
large adverts for fur coats.
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------------ 4623
- HISTORY OF THE MIKY
WAY – discoveries by Hubble.
-
- The headline read: “Finds Spiral Nebulae
are Stellar Systems”. Confirms View
That They Are 'Island Universes'; Similar to Our Own. The story detailed a groundbreaking
discovery: James Hubble had found that two spiral-shaped nebulae, objects made
up of gas and stars, which were previously thought to reside within our Milky
Way galaxy, were located outside it.
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- These objects were actually the Andromeda
and Messier 33 galaxies, the closest large galaxies to our Milky Way. Today, up
to several trillion galaxies are estimated to fill the universe, based on
observations of tens of millions of galaxies.
-
- Four years before Hubble's announcement, an
event called "the great debate" had taken place in Washington DC
between the American astronomers Harlow Shapley and Heber Curtis. Shapley had
recently shown the Milky Way to be larger than previously measured. Shapley
argued that it could accommodate spiral nebulae within it. Curtis, on the other
hand, advocated for the existence of galaxies beyond the Milky Way.
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- In hindsight, and ignoring certain details,
Curtis won the debate. However, the method Shapley used to measure distances
across the Milky Way was critical to Hubble's discovery, and was inherited from
the work of a pioneering US astronomer: Henrietta Swan Leavitt.
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- In 1893, a young Leavitt was hired as a
"computer" to analyze images from telescope observations at Harvard
College Observatory, Massachusetts. Leavitt studied photographic plates from
telescope observations of another galaxy called the Small Magellanic Cloud.
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- Leavitt was searching for stars whose
brightness changed over time. From over a thousand variable (changing) stars,
she identified 25 were of a type known as “Cepheids”, publishing the results in
1912.
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- The brightness of Cepheid stars changes
with time, so they appear to pulse. Leavitt found a consistent relationship:
Cepheids that pulsed more slowly were intrinsically brighter (more luminous)
than those pulsing more quickly. This was called the "period-luminosity
relationship."
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- Other astronomers realized the significance
of Leavitt's work: the relationship could be used to work out distances to
stars. While a student at Princeton University, Shapley used the
period-luminosity relationship to estimate distances to other Cepheids across
the Milky Way. This is how Shapley reached his estimate for our galaxy's size.
-
- But, in order for astronomers to be sure
about distances within our galaxy, they needed a more direct way to measure
distances to Cepheids. The stellar parallax method is another way to measure
cosmic distances, but it only works for nearby stars. As the Earth orbits the
sun, a nearby star appears to move relative to more distant background stars.
This apparent motion is known as “stellar parallax”. Through the angle of this
parallax, astronomers can work out a star's distance from Earth.
-
- The Danish researcher Ejnar Hertzsprung used
stellar parallax to obtain the distances to a handful of nearby Cepheid stars,
helping calibrate Leavitt's work. The
New York Times article emphasized the "great" telescopes at the Mount
Wilson Observatory near Los Angeles, where Hubble was working. Telescope size
is generally assessed by the diameter of the primary mirror. With a 100-inch
(2.5-meter) diameter mirror for collecting light, the Hooker telescope at Mount
Wilson was the largest telescope at the time.
-
- Large telescopes are not only more
sensitive to resolving galaxies, but also create sharper images. Edwin Hubble
was therefore well placed to make his discovery. When Hubble compared his
photographic plates taken using the 100 inch telescope with those taken on
previous nights by other astronomers, he was thrilled to see one bright star
appear to change in brightness over time, as expected for a Cepheid.
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- Using Leavitt's calculations, Hubble found
that the distance to his Cepheid exceeded Shapley's size for the Milky Way.
Over subsequent months, Hubble examined other spiral nebulae as he searched for
more Cepheids with which to measure distances.
-
- Word of Hubble's observations was spreading
among astronomers. At Harvard, Shapley received a letter from Hubble detailing
the discovery. He handed it to fellow astronomer Cecilia Payne-Gaposchkin,
remarking: "Here is the letter that has destroyed my universe."
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- Besides estimating the distance to a
galaxy, telescopes can also measure the speed at which a galaxy moves towards
or away from Earth. In order to do this, astronomers measure a galaxy's
spectrum: the different wavelengths of light coming from it. They also
calculate an effect known as the “Doppler shift” and apply it to that spectrum.
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- The Doppler shift occurs for both light and
sound waves; it is responsible for the pitch of a siren increasing as an
emergency vehicle approaches, then decreasing as it passes you. When a galaxy
is moving away from Earth, features of the spectrum known as absorption lines
have longer measured wavelengths than they would if they were not moving. This
is due to the Doppler shift, and we say that these galaxies have been
"redshifted."
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- Beginning in 1904, the American astronomer
Vesto Slipher used the Doppler technique with a 24-inch telescope at the Lowell
Observatory in Flagstaff, Arizona. He found that nebulae, including Andromeda,
were all redshifted. Slipher found they were moving away from Earth at speeds
as high as a thousand kilometers a second.
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- Hubble combined Slipher's measurements with
his distance estimates for each galaxy and discovered a relationship: the
further a galaxy is from us, the faster it is moving away from us. This can be
explained by the expansion of the universe from a common origin, which would
become known derisively as the “Big Bang”.
-
- The announcement 100 years ago cemented
Hubble's place in the history of astronomy. His name would later be used for
one of the most powerful scientific instruments ever created: the Hubble space
telescope. It seems incredible how, over the course of just five years, our
understanding of the universe was brought into focus.
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-
November 25, 2024 HISTORY OF
THE MILKY WAY 4623
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“Jim Detrick” -----------
--------------------- --- Monday, November 25,
2024
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