Defining the Milky Way

Prior to the twentieth century, astronomers did not know the large-scale distribution of stars and other matter in the universe. Throughout history, most people, including many astronomers, believed that the Milky Way contains all of the stars in the cosmos. In other words, they thought that the “Galaxy” and the “universe” were the same thing. We begin the study of galaxies by learning how that belief changed.

15-1 Studies of Cepheid variable stars revealed that the Milky Way is only one of many galaxies

The concept that our Galaxy is but one of many was put forth in 1755, when the German philosopher Immanuel Kant (1724–1804) suggested that vast collections of stars lie far beyond the confines of the Milky Way. Less than a century later, the Irish astronomer William Parsons (1800–1867) observed the structure of some of those “island universes” proposed by Kant. Parsons was the third earl of Rosse in Ireland. He was rich, he liked machines, and he was fascinated by astronomy. Accordingly, he set about building gigantic telescopes. In February 1845, his pièce de résistance was finished. This telescope’s massive mirror measured 1.8 m (6 ft) in diameter and was mounted at one end of an 18-m (60-ft) tube controlled by cables, straps, pulleys, and cranes (Figure 15-2a). For many years, this triumph of nineteenth-century engineering enjoyed the distinction of being the largest telescope in the world.

Figure 15-2: RIVUXG High-Tech Telescope of the Mid-Nineteenth Century (a) Built in 1845, this structure housed a 1.8-m-diameter telescope, the largest of its day. The improved resolution it provided over other telescopes was similar to the improvement that the Hubble Space Telescope provided over Earthbound optical instruments when it was launched. The telescope, as shown here, was restored to its original state in 1996–1998. (b) Using his telescope, Lord Rosse made this sketch of the spiral structure of the galaxy M51 and its companion galaxy NGC 5195. (c) A modern photograph of M51 (also called NGC 5194) and NGC 5195. The spiral galaxy M51 in the constellation of Canes Venatici is known as the Whirlpool Galaxy because of its distinctive appearance. The two galaxies are about 20 million ly from Earth.

With this new telescope, Lord Rosse examined many of the glowing interstellar clouds previously discovered and catalogued by the Herschels. William Herschel, his sister Caroline, and his son John, among others, discovered and recorded details of many fuzzy-looking astronomical objects, called nebulae (singular, nebula). With the high resolution provided by his telescope, Lord Rosse observed that some of these nebulae have a distinct spiral structure. A particularly good example is M51, also called the Whirlpool Galaxy or NGC 5194.

464

Lord Rosse had no photographic equipment in 1845, so he made drawings of what he observed. Figure 15-2b is his drawing of M51. Views like this inspired him to echo Kant’s proposal of island universes. Figure 15-2c shows a modern photograph of M51. It is interesting to note the differences between the perception and interpretation of astronomical objects and a camera’s recording of them.

Most astronomers of Rosse’s day did not agree with the notion of island universes outside of our Galaxy. They thought that the Milky Way contained all stars in the universe—that the Milky Way was the universe. The nebulae in the shapes of disks seen by the Herschels were explained away as newly forming stars and their nascent planets. In April 1920, a formal discussion, now known as the Shapley–Curtis debate, was held at the National Academy of Sciences in Washington, D.C. Harlow Shapley (1885–1972) argued that the spiral nebulae are relatively small, nearby objects scattered around our Galaxy. Heber D. Curtis (1872–1942) championed the island universe theory, arguing that each of these spiral nebulae is a separate rotating system of stars, much like our own Galaxy. While the Shapley–Curtis debate focused scientific attention on the size of the universe, nothing was decided, because no one had any firm evidence to demonstrate exactly how far away the spiral nebulae were. Astronomers desperately needed to devise a way to measure the distances to the nebulae. A young teacher and former basketball player originally from Missouri, who moved to Chicago to study astronomy, finally achieved this goal. His name was Edwin Hubble.

Figure 15-3: RIVUXG A Cepheid Variable Star in Galaxy M100 One of the most reliable ways to determine the distance to moderately remote galaxies is to locate Cepheid variable stars in them, as discussed in the text. The distance of 50 million ly (15.2 Mpc) from Earth to the galaxy M100 in the constellation Coma Berenices was determined using Cepheids. Insets: The Cepheid in this view, one of 20 located to date in M100, is shown at different stages in its brightness cycle, which recurs over several weeks.

In 1923, Hubble (1889–1953) took a historic photograph of M31, then called the Andromeda Nebula. It was one of the spiral nebulae around which controversy raged. On the photographic plate he discovered what first appeared to be a nova. Referring to previous plates of that region, he soon realized that the object was actually a Cepheid variable star. As we saw in Section 12-12, these pulsating stars vary in brightness periodically. Further scrutiny over the next several months revealed many other Cepheids. Figure 15-3 shows a Cepheid in the galaxy M100, at different stages of brightness.

465

Figure 15-4: The Period–Luminosity Relation This graph shows the relationship between the periods and average luminosities of classical (Type I) Cepheid variables and the closely related RR Lyrae stars (discussed in Chapter 12). Each dot represents a Cepheid or RR Lyrae whose luminosity and period have been measured.

Margin Question 15-1

Question

Referring to Figure 15-4, how would the brightness of a Cepheid variable with peak luminosity of 1000 L change if it were observed every 5 days?

Only a decade before, in 1912, the American astronomer Henrietta Leavitt (1868–1921) had published an important study of Cepheid variables. Leavitt studied many Cepheids in the Small Magellanic Cloud, then also believed to be a nebula, but now known to be a galaxy passing very close to the Milky Way. Leavitt’s study led her to the period–luminosity relation for Type I Cepheids (see Section 12-13). Leavitt established that a direct relationship exists between a Cepheid’s luminosity (or absolute magnitude) and its period of oscillation, which we saw in Figure 12-28 and which is presented in more detail in Figure 15-4. By observing the star’s period and apparent magnitude, its distance can be calculated, as described in An Astronomer’s Toolbox 15-1: Cepheids and Supernovae as Indicators of Distance.

AN ASTRONOMER’S TOOLBOX 15-1: Cepheids and Supernovae as Indicators of Distance

Because their periods are directly linked to their luminosities, Cepheid variables are among the most reliable tools astronomers have for determining the distances to relatively nearby galaxies. To this day, astronomers use this link—much as Hubble did back in the 1920s—to measure intergalactic distances. More recently, they have begun to use Type Ia supernovae, which are far more luminous and thus can be seen much farther away, to determine the distances to more remote galaxies.

Example: In 1992, a team of astronomers used Cepheid variables in a galaxy called IC 4182 to deduce that galaxy’s distance from Earth. They used the Hubble Space Telescope on 20 separate occasions to record images of the stars in IC 4182. By comparing these images, the astronomers could pick out which stars vary in brightness. In this way, they discovered 27 Cepheids in IC 4182. Using their observations, they determined the (changing) apparent magnitudes of the Cepheid variables and plotted their light curves. The Hubble Space Telescope is particularly well-suited for studies of this kind, because its extraordinary angular resolution makes it possible to pick out individual stars at great distances. One such Cepheid has a period of 42.0 days and an average apparent magnitude (m) of +22.0. (See Section 11-2 for an explanation of the apparent magnitude scale.) By comparison, the dimmest star you can see with the naked eye has m = +6; this Cepheid in IC 4182 appears less than one-millionth as bright. The star’s spectrum shows that it is a metal-rich Type I Cepheid variable.

According to the period–luminosity relation shown in Figure 15-4, such a Type I Cepheid with a period of 42.0 days has an average luminosity of 33,000 L. In other words, this Cepheid has an average absolute magnitude (M) of −6.5 (compared to M=+4.8 for the Sun). Hence, the difference between the Cepheid’s apparent and absolute magnitudes, called its distance modulus, is

m − M = (+22.0) − (−6.5) = 22.0 + 6.5 = 28.5

From An Astronomer’s Toolbox 11-3, we see that the distance modulus of a star is related to its distance in parsecs (d) by

m − M = 5 log d − 5

where “log” is the base 10 logarithm. This equation can be rewritten as

d = 10(m − M + 5)/5 parsecs

Inserting the value for the distance modulus in this equation, we can obtain the distance to the Cepheid variable and, hence, the distance to the galaxy of which that star is part:

The galaxy is 5 Mpc (1 Mpc = 1 megaparsec = 106 parsecs), or 16 million (1.6 × 107) ly, from Earth.

Astronomers are interested in IC 4182 because a Type Ia supernova was observed there in 1937. Type Ia supernovae are exploding white dwarfs that all reach the same maximum brightness at the peak of their outbursts (see Section 13-4). Once astronomers knew the peak absolute magnitudes of Type Ia supernovae, they could use these supernovae as distance indicators. Because the distance to IC 4182 is known from its Cepheids, the 1937 observations of the supernova in that galaxy allow us to calibrate Type Ia supernovae as distance indicators. At maximum brightness, the 1937 supernova reached an apparent magnitude of m = +8.6. Because the distance modulus of the galaxy (m − M) is 28.5, we see that when a Type Ia supernova is at maximum brightness, its absolute magnitude is

M = m − (m − M) = 8.6 − 28.5 = −19.9

Whenever astronomers find a Type Ia supernova in a remote galaxy, they can combine this absolute magnitude with the observed maximum apparent magnitude to get the galaxy’s distance modulus, from which the galaxy’s distance can be easily calculated (just as we did above for the Cepheids in IC 4182). This technique has been used to determine the distances to galaxies hundreds of millions of parsecs (or light-years) away.

Try these questions: At what distance in parsecs is a star with distance modulus 20? Epsilon (ε) Indi has an apparent magnitude of +4.7 and distance of 3.6 pc. What is its absolute magnitude? What would the Sun’s apparent magnitude and distance modulus be as seen from 100 pc away? Its absolute magnitude is +4.83.

(Answers appear at the end of the book.)

466

Insight Into Science: Room for Debate

Lacking definitive data, competent scientists can develop and believe strikingly different explanations for the same observations. At the time of the Shapley–Curtis debate, the observations allowed both points of view. It was only with the advent of the distance measurement technique used by Hubble that the spiral nebulae were definitely shown to lie outside of the Galaxy.

Thanks to this work, Hubble knew that he could use the characteristics of the fluctuating light of Cepheid variable stars to help him calculate the distance to M31 and put the Shapley–Curtis debate to rest once and for all. His observations of Type I Cepheids led him to determine that M31 is some 2.2 million ly beyond the Milky Way. This distance proves that M31 is not a young star and planet-forming disk of gas and dust, or an open or globular cluster in our Galaxy, but rather an enormous separate stellar system—a separate galaxy. M31, now called the Andromeda Galaxy, is the most distant object in the universe that can be seen with the naked eye. Similar calculations have been done for all galaxies in which Cepheids can be observed. The distances to even more remote galaxies have been determined from observations of Type Ia supernovae in them (see An Astronomer’s Toolbox 15-1).

Hubble’s results, which he presented at the end of 1924, did settle the Shapley–Curtis debate. The universe was recognized to be far larger and populated with far bigger objects than most astronomers had imagined. Hubble had discovered the realm of the galaxies. Today, we know that the universe contains myriad galaxies, of which the Milky Way is just one. Like the Milky Way, each galaxy is a grouping of millions, billions, or even trillions of stars, along with gas, dust, and matter in other forms, all gravitationally bound together.

We now apply Hubble’s method to find Earth’s place in the Milky Way Galaxy. Recall that the Sun’s proximity to us makes it our best-understood star. It might seem that the nearness of the stars and clouds in the Milky Way would make it the best-understood galaxy. However, the clouds of gas and dust that surround the solar system make it very challenging for astronomers to survey completely the distant parts of the Galaxy, which are only now coming into focus.