Dari Galaksi Kita Hingga Kepulauan Semesta |
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Di awal abad 20, para astronom tidak dapat memastikan ukuran dari galaksi kita. Secara umum, mereka percaya ukurannya kira-kira beberapa puluh ribu tahun cahaya, barangkali kurang dari itu juga. (A light year, nearly six trillion
miles, is the distance traveled in a year moving at the speed of light
in a vacuum.) Also, observations early in the 20th century made it seem
that our solar system was near the center of the galaxy. The
way astronomers were misled is explained here.
 It is believed that the great mass of the stars … are arranged in the form of a lens- or bun-shaped system … considerably flattened towards one plane … the Sun occupies a fairly central position. —English astronomer Arthur Eddington, 1914 [Full Quote]  |
Who Was George Ellery Hale? 
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This
vision of the universe was soon replaced with a revolutionary new conception,
based largely on the observations of the American astronomer Harlow Shapley
at the Mount Wilson Observatory. The astronomer and scientific entrepreneur
George Ellery Hale had founded the observatory on a mountain peak overlooking
Los Angeles in 1904, and four years later master instrument-builder George
Ritchey completed a 60-inch reflecting telescope designed specifically
for astronomical photography.
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 M22, a globular cluster of many thousands of stars. By assuming that certain types of stars here were as bright as similar nearby stars whose distances had been measured, Shapley could estimate the distance to this far object. (You can EXIT this site to see NASA's picture of M22.) |
The
first hint of a drastically revised understanding
of our galaxy came in 1916. Studying a "globular cluster,"—a
group of hundreds of thousands of stars—Shapley noticed faint
blue stars. If they were similar to bright blue stars near the Sun, they
must be about 50,000 light years away to explain why they looked so faint.
He pushed ahead to establish distances more conclusively using a new and
ingenious method of measuring the universe.
Shapley
built a new understanding of the universe by measuring distances to stars
based on properties of a type of variable stars called "Cepheids"
(named after the constellation Cepheus, in which a typical such star was
first noticed). They are giant stars, and thus visible to great distances.
Each Cepheid varies in brightness over time.
It is worthy of notice...that the brighter variables have longer periods. —Henrietta Swan Leavitt
In
1908
the American astronomer Henrietta Leavitt had pointed to a remarkable
rule that Cepheids obey. During routine comparisons of photographs she
discovered variable stars, brighter on some photographs and fainter on
other photographs taken at different times. Leavitt noticed that the brighter
the variable star, the longer its period.
The
16 variable stars
Leavitt measured were all in the same group of stars, the Small Magellanic
Cloud. Thus they were all approximately the same distance from the Earth.
Therefore their apparent magnitudes (observed brightness) were directly
related to their absolute magnitudes (intrinsic brightness, as it would
be seen at some arbitrary standard distance). The conclusion was a remarkable
"period-luminosity relation"—the
longer the period, or time, from a Cepheid's maximum brightness to minimum
and back to maximum, the greater the intrinsic luminosity of the star.
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Who Was Henrietta Leavitt? More About Women in Astronomy |
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Assuming
that the system
of globular clusters was sort of a galactic skeleton, Shapley had the
galactic outline, its size, and the place of the solar system within it.
The Sun was far toward one edge of the galactic plane, not near the middle.
He showed that the system of stars was ten or even a hundred times larger
than previous estimates, and that the Sun is many tens of thousands of
light years away from the center of the galaxy.
The system of globular clusters, which is coincident in general, if not in detail, with the sidereal arrangement as a whole, appears to be somewhat ellipsoidal.… The center of the sidereal system is distant from the Earth … —Shapley [Full quote] |
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Shapley
used the period-luminosity relation to estimate distances. First, he collected
all the available data on Cepheid stars, from his own observations and
from other astronomers including Leavitt. The distance to some of the
nearer Cepheids had been measured, and thus Shapley could figure out their
absolute magnitudes. The only physics he needed was the simple rule that
brightness decreases with the square of the distance. Then Shapley graphed
period versus absolute magnitude.
[The period-luminosity curve is] based upon more than 230 stars, and, except for zero-point uncertainty [the uncertainty in the distance measured by other methods to the nearer Cepheids], is probably correct within one or two hundredths of a magnitude. —Harlow Shapley
Shapley
made the reasonable assumption
that Cepheids in distant globular clusters obey the same physics as nearby
Cepheids. He observed the periods of distant Cepheids, read off their
presumed absolute magnitudes from his graph of period versus luminosity,
and compared that absolute magnitude with the observed apparent magnitude.
This produced distances to many far-away Cepheids—and to the globular
clusters in which they resided. (Some globular clusters did not have Cepheids
he could measure, and he used other, cruder methods to estimate their
distances.)
Shapley
found that the globular clusters are arranged symmetrically around the
galaxy, about as many above the plane of the galaxy as below. The clusters
seemed to avoid the plane itself, the Milky Way. Shapley wrote that "this
great mid-galactic region, which is peculiarly rich in all types of stars,
planetary nebulae, and open clusters, is unquestionably a region unoccupied
by globular clusters." Shapley acknowledged that there was an alternative
explanation. Maybe globular clusters were not, as he believed, actually
missing from the region, but instead were hidden by clouds of absorbing
matter along the spine of the Milky Way.
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The
dwindling significance
of humans and their particular planet had dwindled further still. Shapley
noted a historical progression from belief in a small universe, with humankind
at its center, to a larger universe with the Earth further from the center.
The geometry had been transformed from geocentric to heliocentric to a-centric.
The psychological change was no less, he insisted, from homocentric to
a-centric. Some astronomers had long doubted that the solar system was
near the center of the galaxy and that people enjoyed a privileged place
in the universe. They felt that the odds, given a random distribution,
were small. Now Shapley gave this philosophical position scientific substance.
The physical universe was anthropocentric to primitive man.... the significance of man and the Earth in the sidereal scheme has dwindled with advancing knowledge of the physical world... —Shapley [Full quote] |
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The
Great Debate
Shapley's
galaxy
was far larger than any previous estimate (aside from earlier guesses
of an infinite stratum of stars). It might indeed be the entire universe.
For Shapley had showed that globular clusters were clearly part of the
galaxy, not independent island universes. Other nebulae (concentrations
of stars and dust), especially spiral-shaped ones, might still lie outside
our galaxy. But if they were similar in size to our now enormous galaxy,
they seemed implausibly large. Separate island universes were not impossible,
but they seemed less likely now that Shapley had multiplied the size of
our galaxy many fold.
From the new point of view our galactic universe appears as a single, enormous, all-comprehending unit... The adoption of such an arrangement leaves us with no evidence of a plurality of stellar 'universes'. —Shapley (Full quote)
Shapley
defended his conclusions
in the so-called "Great Debate" before the National Academy
of Sciences on 26 April 1920. His major concern was the size of the galaxy.
His model of a drastically larger galaxy, with the solar system far from
its center, was largely correct. But he was on less solid ground when
he argued that the spiral nebulae, which seemed to be much smaller, were
part of our galaxy. His opponent, Heber Curtis, argued that the galaxy
could be as large as Shapley said, yet still be only one of many island
universes, if it happened by chance to be several times larger than the
average. Ultimately observations would prove Curtis correct, but in 1920
Shapley had the stronger position. You can read more about
the Great Debate here.
The
centuries-old debate was resolved only by new scientific evidence, produced
using larger telescopes and new observational techniques, including photography
and spectroscopy. The key proponent of island universes was Edwin Hubble,
who like Shapley did his revolutionary work at the Mount Wilson Observatory.
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 The irregular nebula NGC 6822, what would now be called a nearby dwarf galaxy. Thanks to the 100-inch telescope, Hubble was able to detect variable stars here, although it is (by modern measures) 1.5 million light years distant. |
Writing
in his doctoral thesis
in 1917, Hubble noted that catalogs already included some 17,000 small,
faint nebulous objects that could ultimately be resolved into groupings
of stars. Perhaps 150,000 were within the reach of existing telescopes.
Yet, he wrote, "Extremely little is known of the nature of nebulae,
and no significant classification has yet been suggested; not even a precise
definition has been formulated." The way Hubble
discovered to classify nebulae is described here.
After
serving in World War I, Hubble joined the Mount Wilson Observatory staff.
There he took photographs of nebulae with the new 100-inch reflector,
the most powerful telescope in the world. Hubble discovered variable stars
in an irregular nebula (cataloged as NGC 6822). By now Shapley had left
Mount Wilson for the Harvard College Observatory. Hubble wrote to Shapley
in 1923 to tell him of the discovery. Hubble also said he was going to
hunt for more variable stars and to investigate their periods. Shapley
wrote back, "What a powerful instrument the 100-inch is in bringing
out those desperately faint nebulae."
The great spirals … apparently lie outside our stellar system. —Edwin Hubble, 1917 Early in 1924 Hubble wrote to Shapley again. This time Hubble reported, "You will be interested to hear that I have found a Cepheid variable [star] in the Andromeda Nebula [M31]. I have followed the nebula this season as closely as the weather permitted and in the last five months have netted nine novae and two variables." |
 The nearby planetary nebula in Lyra, as photographed in the 1920s. |
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 The central region of the spiral nebula M31. This plate was taken with a 9-hour exposure over two nights in September, 1920, with the Mount Wilson 100-inch reflecting telescope. The Cepheid is in the upper right corner, marked "VAR!" for Variable.  M31: The Andromeda Galaxy. |
When
he found a Cepheid variable, Hubble realized he held the key to distance.
As Shapley had used the period-luminosity relation for Cepheids to find
distances to globular clusters in our galaxy, so Hubble could find the
distance to the spiral nebula M31.
Hubble
found
that "the distance [to M31] comes out something over 300,000 parsecs."
This was roughly a million light years, and several times more distant
than Shapley's estimate of the outer limits of our own galaxy. Hubble
continued: "I have a feeling that more [Cepheid] variables will be
found by careful examination of long exposures."
Here is the letter that has destroyed my universe. —Shapley, 1924
On
reading Hubble's letter,
Shapley remarked to a colleague who happened to be in his office, "Here
is the letter that has destroyed my universe." Shapley admitted that
the large number of photographic plates that Hubble had obtained were
enough to prove that the stars were genuine variables. By August, Hubble
had still more variables to report. Shapley was glad to see this definite
solution to the nebula problem, even if it refuted earlier evidence against
spiral nebulae as island universes. Some of the
evidence against spiral nebulae as island universes was based on a mistake,
as explained here.
Hubble's
discovery of Cepheid variable stars in spiral nebulae, and the distance
determination confirming that spiral nebulae are independent galaxies,
were officially announced on New Year's Day, 1925, at a meeting of the
American Astronomical Society. He followed this preliminary paper by further
work over the next four years, with convincingly voluminous detail. A
good part of Hubble's genius, and much of the acceptance that his revolutionary
conclusions commanded, were due to lots of hard work.
Before
the 1920s ended,
astronomers understood that the spiral nebulae lie outside our own galaxy.
In the previous decade Shapley had multiplied the size of the universe
by about ten times. Hubble multiplied it by another ten - if not more.
Hubble's universe was no longer the one all-comprehending galaxy envisioned
by Shapley. Henceforth the universe was understood to be composed of innumerable
galaxies spread out in space, farther than the largest telescope could
see. Hubble next would show that the universe is not static, as nearly
everyone then believed, but is expanding. What he had made infinite in
space, he would make finite in time.
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