Optional Supplementary Chapter: Galaxies

The solar neighborhood

The solar neighborhood is a small volume of space centered on the Sun. Leaving the solar system, our closest neighbor is the triple star system, alpha Centauri, at a distance of about 4 light years. Spurred by Silicon Valley entrepreneur Yuri Milner’s Starshot initiative, scientists are seriously tackling the engineering challenges for sending a probe to alpha Centauri.

Figure 1 below shows a 3-D diagram for stars that are closer than 15 light years – stars in our immediate solar neighborhood. As humanity travels out into the galaxy, these are the places that we will visit first.  It is possible that school children of the future will memorize the names of these destinations in the same way that school children of the past memorized the names of planets in our solar system.

One might guess that bright stars are closest and faint stars are farther away.  If all stars were the same intrinsic brightness, that would be true.  However, for “normal” stars, the brightness of stars is correlated with the mass of the star.  And when stars evolve to become “red giants” or “supernovae” they also brighten.  With a telescope, we measure the apparent brightness of a star. We can translate this to an absolute scale to learn the true luminosity of a star only if we know the distance to the star.  Think for a moment about how you would measure the distances to stars.  It’s not easy – this has been a major effort in astronomy over the past few decades.

Beyond the solar neighborhood: our galaxy

How big is the Milky Way galaxy compared to the solar neighborhood? The scale of the Milky Way galaxy is nothing short of staggering. Figure 2 below sketches the diameter of the flattened disk of our galaxy, which stretches 100,000 light years across. The Sun is located about two thirds of the way between the center and the outer edge of the Milky Way.

The 15 light year scale of our local neighborhood is puny in comparison and is not even resolvable in the full scale sketch of our galaxy shown in Figure 3. It is impossible for us to take a picture of the Milky Way because we have not traveled far enough through space to gain a full view of our galaxy. Instead, we have images looking toward the center of the galactic plane. The ensemble of images that we have made of the Milky Way probe the physical extent, including the thickness of the galactic disk. Considering the ratio of the diameter of the galaxy to the height of the galactic disk, the dimensions of the Milky Way are thinner than a dime.

The Milky Way (MW) galaxy contains a stack of two disks: a thin disk with stars, gas, and dust, and a thicker disk that is comprised of older stars. We think that accumulated gravitational interactions cause the population of younger stars in the thin disk to wander out into what we call the thick disk. There is a central bulge in the MW galaxy that contains mostly young and massive stars, and a supermassive black hole (we now know that supermassive black holes reside at the centers of almost every galaxy and that the mass of the black holes scales with the mass of the host galaxy).

In addition to ~400 billion stars, our galaxy contains collections of stars called globular clusters that are randomly distributed in the spherical volume that surrounds the flattened disk of the MW galaxy. A globular cluster looks like a spherical cloud of light from the great distances that we view them. However, telescopes have resolved these clouds into dense collections of millions of stars. The stars in globular clusters are uniformly old – they are the most ancient relics in our MW galaxy. The stars in globular clusters are gravitationally bound and orbit each other, while the globular cluster as a whole moves in an orbital path in the galaxy. Messier object 107 is an example of one of the ~150 globular clusters in our galaxy. The globular clusters in the Milky Way are a self-contained, gravitationally bound ensemble of stars that orbit the galaxy. 

 

In addition to the old, spherically distributed globular clusters, there are smaller clusters of stars that are called “open clusters.” Open clusters are found in the disk of the galaxy; like the globular clusters, all of the stars have about the same age; however, open clusters are associations of young stars.  Examples of open clusters include the Pleiades cluster shown in Figure 5.

 Other Galaxies

In detail, galaxies are as unique as snowflakes. However, Hubble noticed that galaxies could be broadly classified by a few — and only a few — different large-scale morphologies. His first hypothesis was that he might be looking at an evolutionary sequence. There are beautiful spiral galaxies like the nearby Andromeda Galaxy, with bright blue arms where massive young stars are forming. There are also distinct giant elliptical galaxies like M87 that astronomers call “red and dead” because they no longer harbor regions of active star formation. However, the evolution of galaxies is not a simple linear transition from spiral to elliptical. We now know that galaxies grow by mergers, like the dwarf irregular galaxy, NGC 4214. Galaxy mergers trigger new cycles of star formation.

Mergers can be subtle, with large galaxies sweeping up smaller ones, and the Milky Way galaxy is not innocent of this galactic canabalism. We have evidence for more than a dozen streams of stars that our galaxy has swallowed and is gravitationally digesting. New research suggests that more than half the mass of our galaxy may have come from the accretion of other galaxies, as shown in the galaxy merger simulation below. The Large Magellanic and Small Magellanic clouds that can be clearly seen in the southern hemispheres are satellite galaxies that are being gravitationally lured into the Milky Way.  Mergers are part of the circle of life for galaxies.

Recall that our galaxy contains about 150 globular clusters – spherical aggregates of light from many thousands of stars. In 1918, Harlow Shapley estimated the distances to globular clusters. He made the simplifying assumption that all clusters had nearly the same brightness, and reasoned that some clusters were fainter because they were farther away. Under this assumption (which turned out to be reasonable) the globular clusters appeared to be distributed in a spherical volume that was centered on a point in the Sagittarius constellation. Shapley argued that this point was also the center of the galaxy and estimated that the Sun was 15,000 pc from the center of the galaxy. Copernicus unseated the Earth as the center of the solar system, and Shapley showed that the Sun was not at the center of the galaxy. Subsequent studies have shown that the Sun is actually about 8 kpc from the center of the galaxy, so Shapley overestimated the physical extent of the Milky Way, but only by a factor of two.

In the 18^th and 19^th centuries, natural scientists had observed fuzzy, extended objects in the night sky that they called “nebulae.”  In 1755, Immanuel Kant interpreted these nebulae as “Island Universes” – large collections of gravitationally bound stars. Shapley believed that Kant’s island universes were part of our galaxy. Indeed, he had no reason to believe that there was a physical edge to our galaxy. In 1920, when physicists were still struggling to determine the speed of light, a famous debate about the nature of island universes took place between two prominent astronomers, Harlow Shapley and Heber Curtis, at a meeting of the National Academy of Science. Shapley, who had overestimated the size of the Milky Way, maintained that the nebulae were part of our galaxy, and Curtis argued that they were outside of our galaxy. This debate was a good example of how humans struggle to piece together scientific theory with imprecise or inaccurate data.

The distances to stars

A breakthrough in understanding the distances to stars came about when Henrietta Swan Leavitt measured the brightness variations for more than 2400 stars in the Magellanic Clouds, satellite galaxies of the Milky Way that are visible from the southern hemisphere of Earth. A certain class of these stars, Cepheid variables, showed a regular periodicity in their cycle of brightness and dimming. Leavitt pointed out that the repetition timescales, or periodicity in the brightness variation cycles, were longer for the brighter variable stars. By correlating the periodicity of the variation brightness with the intrinsic brightness (the period-luminosity relation), Leavitt discovered an important “standard candle.”   Measure the period of variability in a Cepheid star and you know the intrinsic brightness. Then, compare the apparent brightness that you observe with the intrinsic brightness; because more distant stars appear fainter, the distance to the star can be calculated this way.

The debate about whether nebulae were part of our galaxy was resolved a few years later in 1923 when Edwin Hubble used the 100-inch Mount Wilson telescope to resolve individual stars in M31 and M33, two examples of “spiral nebulae.” Hubble measured the brightness variations for a Cepheid variable, and from the period-luminosity relation (above) he knew the true luminosity of the stars. Knowing the luminosity of the star allowed Hubble to estimate a distance to M31 and M33 that was even larger than Shapley’s over-estimated size of the Milky Way galaxy. The conclusion was that M31 and M33 were not part of our galaxy.

The realization that we were looking at other galaxies well outside of our own Milky Way meant that the universe was much bigger that anyone imagined. The true nature and the distances to the “island universes” was a mystery for almost two centuries. Once this mystery was solved, humans began to realize that the Milky Way galaxy is only a tiny speck in a vast universe. How big is the universe?  That’s a great question – current estimates put the “size” of the universe at more than 90 billion light years.

We now know that there are hundreds of billions of galaxies in the universe. The Hubble Space Telescope (HST) has made a dramatic change in our understanding of what’s out there. HST orbits the Earth once every ~90 minutes. In 1994, the HST photographed a specific patch of the sky near the Big Dipper. This patch was selected because it did not have very many stars – as far as our observations with ground-based telescope could tell, this was just a boring, dark, empty part of the sky. Astronomers stacked more than 340 images to construct a breath-taking view of the universe. The Hubble deep field – an area that is about one tenth the size of the full moon – contains about 1500 galaxies. There is nothing special about this direction in the sky (now we have Hubble deep field images taken in several different directions); the celestial sphere is wall-papered with hundreds of billions of galaxies, each with hundreds  of billions of stars. Read that last sentence again; it is a staggering result that re-ordered our place in the universe.

What’s beyond galaxies? Relatively recently, we have learned that a tenuous gas surrounds galaxies. This gas is visible only at ultraviolet wavelengths, so it couldn’t be observed from the surface of Earth (where the atmosphere blocks out UV light).

Olbers Paradox

One of the most important ways to make progress in science (and life) is to rule out things that cannot be true. Those humans who can extract information from what seems like a vacuum to everyone else have changed the course of science. As Sherlock Holmes more eloquently stated: “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

Here is Olbers’ thought experiment: if you move the Sun twice as far away, it is fainter by a factor of four. However, its angular size has also decreased by a factor of four, so that the number of photons per unit area has stayed constant. In an infinite universe, every bit of the sky would have a star somewhere along our line of site. Given an infinite age and a static universe, the light from even the most distant stars would have reached us. If all other stars were as bright as the Sun, then the night sky should be as bright as the Sun. Many possible resolutions have been offered for Olbers’ paradox. Perhaps:

• our Sun is special (humanity’s favorite go-to explanation)
• there is obscuring dust in the universe
• the universe is finite in size or the distribution of stars is not uniform
• the universe is finite in age and the light from distant stars has not yet reached us
• the universe is expanding so that the light is red-shifted and no longer visible to us.

Each of these explanations has profound implications for the universe. In particular, if the universe is finite in age, this implies that it had a beginning!  Let’s put the resolution to Olbers’ paradox on hold for a moment, and look at the evidence that emerged in the 20^th century.