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THE SPIRAL NEBULAE ARE STELLAR SYSTEMS

In the 1780s and 1790s, British astronomer William Herschel cataloged large numbers of nebulae and speculated that some of these might be comparable in size and nature to the Milky Way. In his conjectures, Herschel was following an earlier suggestion by German philosopher Immanuel Kant that nebulae might be large disks of stars—“island universes” independent of the Milky Way and separated from it by vast distances. In the 19th century, using improved telescopes, British astronomer Lord Rosse discovered that some nebulae had “arms” arranged in a spiral, while his compatriot William Huggins found that many nebulae consisted of a mass of stars. However,
aside from the fact that they might contain stars, nebulae were still little understood by the turn of the 20th century, when a young scientist from Indiana named Vesto Slipher began to study them.

“It seems to me that with this discovery, the great question, if the spirals belong to the system of the Milky Way or not, is answered with great certainty: they do not.” Ejnar Hertzsprung

The Lowell Observatory

From 1901, Slipher worked at the Lowell Observatory in Flagstaff, Arizona. The observatory had been founded by American astronomer Percival Lowell in 1894. Lowell had selected the site because its high altitude, at over 6,900 ft (2,100 m), with few clouds, and its distance from city lights, meant it guaranteed good visibility almost every night. Lowell’s venture marked the first time an observatory had been built deliberately in a remote, high place for optimal observations.

Lowell initially hired him for a short-term position, but Slipher would remain for his entire career. Lowell and Slipher worked well together, with the unassuming new recruit content to leave the limelight to his flamboyant employer. Slipher was a talented mathematician and had practical mechanical skills, which he put to use installing new spectrographic equipment. He set to work developing improved techniques in spectrography—the separation of light coming from celestial objects into its constituent wavelengths, and the measurement and analysis of those wavelengths.

Slipher used the 24-in (61-cm) Alvan Clark telescope at the Lowell Observatory to observe the spiral nebulae. Today, people can use the original telescope at the observatory’s visitor center.

Studying nebulae

Slipher’s initial work and research were directed at the planets, but from 1912, at Lowell’s request, he began to study the mysterious spiral nebulae. Lowell had a theory that they were spirals of gas that were coalescing into new solar systems. He asked Slipher to record the spectra of the light from the outer edges of the nebulae, to determine if their chemical makeup resembled that of the solar system’s gas giant planets.

Making small adjustments to its mechanism, Slipher managed to increase the sensitivity of Lowell’s spectrograph, a complex 450-lb (200-kg) instrument attached to the eyepiece of the Observatory’s 24-inch (61-cm) refractor telescope. During the fall and winter of 1912, he obtained a series of spectrograms from the largest of the spiral nebulae, which was located in the constellation of Andromeda and known at the time as the Andromeda nebula.

The pattern of spectral lines in the nebula’s spectrum (like a fingerprint of its composition) indicated a “blueshift”—they were unexpectedly displaced toward the short-wavelength/high-frequency blue end of the spectrum by what is known as a Doppler shift. That could only mean that the light waves coming from the Andromeda nebula were being shortened, or compressed, and their frequency raised, because the nebula was rushing toward Earth at a considerable speed. Slipher’s calculations revealed that the nebula was approaching at 200 miles per second (300 km per second). Doppler shifts had been measured for astronomical bodies before, but shifts of this size were unprecedented. Slipher asserted that “we have at the present no other interpretation for it. We may conclude that the Andromeda nebula is approaching the solar system.”

The spectra of galaxies moving toward Earth exhibit “blueshifts” and those receding from Earth exhibit “redshifts” because the light waves are squashed or stretched when viewed from Earth. These are called Doppler shifts after the Austrian physicist Christian Doppler, who first explained such phenomena.

Discovering Doppler shifts

Over the next few years, Slipher studied 14 more spiral nebulae and found that nearly all were traveling at incredible speeds relative to Earth. Most remarkably, whereas some were moving toward Earth, most were showing redshifted spectra, where the wavelengths had stretched, meaning they were moving away from Earth. The nebula known as M104 (also called NGC 4594), for example, was flying away at an astonishing speed of nearly 600 miles per second (1,000 km per second). Another called M77, or NGC 1068, was receding at 680 miles per second (1,100 km per second). Altogether, out of the 15 galaxies observed, 11 were significantly redshifted. In 1914, Slipher presented his results to the American Astronomical Society and received a standing ovation.

By the time Slipher had presented his next paper on spiral nebulae in 1917, the ratio of redshifted to blueshifted nebulae had risen to 21:4. In this paper, Slipher noted that the average velocity at which they were approaching or receding—scientifically known as “radial velocity”—was 450 miles per second (700 km per second). This was much faster than any star had ever been measured moving relative to Earth. Slipher found it almost inconceivable that the spiral nebulae could be passing through the Milky Way at such speeds, and he began to suspect that they were not moving through the Milky Way at all, stating: “It has for a long time been suggested that the spiral nebulae are stellar systems seen at great distances … This theory, it seems to me, gains favor in the present observations.” Slipher was echoing Kant’s suggestion that some nebulae, in particular the spiral ones, could be separate galaxies from the Milky Way.

In 1920, partly prompted by Slipher’s findings, a formal debate took place in Washington, D.C., to discuss whether the spiral nebulae were separate galaxies outside the Milky Way. Now referred to as the “Great Debate,” two eminent American astronomers advocated opposing positions—Harlow Shapley that the spiral nebulae were part of the Milky Way; and Heber D. Curtis that they were far beyond it. Neither astronomer changed his position as a result of the debate, but many perceptive figures were concluding by this time that the spirals had to be outside the Milky Way.

The galaxy NGC 4565, which Slipher established to be receding at 700 miles/s (1,100 km/s), is also known as the Needle galaxy because of its thin shape when viewed from Earth.

Slipher’s legacy

Despite an enthusiastic response from many in the astronomical community, some still questioned Slipher’s findings. For more than a decade, until others began to believe Slipher’s ideas and understand the implications arising from them, he was virtually the only person investigating the Doppler shifts of spiral nebulae.

In 1924, a new paper by American astronomer Edwin Hubble put a decisive end to the debate about the nature of spiral nebulae. Hubble had observed a class of stars called Cepheid variables in some nebulae, including the Andromeda nebula. As a result of his observations, Hubble was able to announce that the Andromeda “nebula” and others like it were far too distant to be part of the Milky Way and so must be galaxies outside it. Slipher’s suspicions dating back to 1917 had been proved right. By the time of Hubble’s paper, Slipher had measured the radial velocities of 39 spiral nebulae, the majority of which showed high velocities of recession—as much as 775 miles per second (1,125 km per second). Hubble used Slipher’s measurements of redshifts in galaxies that he had proved were outside the Milky Way to find a relationship between galaxy redshifts and distances.

By the late 1920s, Hubble had used this result to confirm that the universe is expanding. Thus, Slipher’s work in the years 1912–25 played a crucial role in what today is often considered the greatest astronomical discovery of the 20th century, paving the way for further investigations into the motions of galaxies and cosmological theories based on an expanding universe. As for the Andromeda galaxy, it is expected to collide with the Milky Way in about 4 billion years, and together the two are likely to form a new elliptical galaxy.

“In the great majority of cases the nebula is receding; the largest velocities are positive. The striking preponderance of [these positive velocities] indicates a general fleeing from us or the Milky Way.” Vesto Slipher

Some 4 billion years into the future, the night sky will look like this, as the Andromeda galaxy collides with the Milky Way.

VESTO SLIPHER

Vesto Slipher was born on a farm in Mulberry, Indiana, in 1875. Soon after graduation, he started working at the Lowell Observatory in Arizona, where he would remain for more than half a century. Most of Slipher’s major discoveries occurred in the earlier part of his career. He began by investigating the rotational periods of planets, finding evidence, for example, that Venus’s rotation is very slow. Between 1912 and 1914, he made his most significant discovery—that some spiral nebulae are moving at high speed. In 1914, Slipher discovered the rotation of spiral galaxies, measuring spin rates of hundreds of miles per second. He also demonstrated that gas and dust exist in interstellar space. Slipher was director of the Lowell Observatory from 1926 to 1952. During this time, he supervised a search for trans-Neptunian planets, which ledin 1930 to Clyde Tombaugh’s discovery of Pluto.

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