- 4626 - HUBBLE - discoveries other galaxies? 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. The headline read: Finds Spiral Nebulae are Stellar Systems: "Dr. Hubbell Confirms View That They Are 'Island Universes'; Similar to Our Own."
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- HUBBLE -
discoveries other galaxies?
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- The American astronomer at the center of the
article, Dr. Edwin Powell Hubble, was probably bemused by the misspelling of
his name. But the story detailed a groundbreaking discovery: 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.
-
- 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
carried out by other observatory researchers.
-
- 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.
-
- 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 named the "period-luminosity
relationship."
-
- 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.
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.
-
- 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."
-
- 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.
-
- 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."
-
- 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.
-
- 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 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 29, 2024 HUBBLE -
discoveries other galaxies? 4626
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--------------------- --- Friday, November 29,
2024
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