# The Milky Way Galaxy

## The Formation of the Galaxy


### Learning Objectives

By the end of this section, you will be able to:
1. Describe the roles played by the collapse of a single cloud and mergers with other galaxies in building the Milky Way Galaxy we see today
2. Provide examples of globular clusters and satellite galaxies affected by the Milky Way’s strong gravity.
Information about stellar populations holds vital clues to how our Galaxy was built up over time. The flattened disk shape of the Galaxy suggests that it formed through a process similar to the one that leads to the formation of a protostar (see The Birth of Stars and the Discovery of Planets outside the Solar System). Building on this idea, astronomers first developed models that assumed the Galaxy formed from a single rotating cloud. But, as we shall see, this turns out to be only part of the story.

### The Protogalactic Cloud and the Monolithic Collapse Model

Because the oldest stars—those in the halo and in globular clusters—are distributed in a sphere centered on the nucleus of the Galaxy, it makes sense to assume that the protogalactic cloud that gave birth to our Galaxy was roughly spherical. The oldest stars in the halo have ages of 12 to 13 billion years, so we estimate that the formation of the Galaxy began about that long ago. (See the chapter on The Big Bang for other evidence that galaxies in general began forming a little more than 13 billion years ago.) Then, just as in the case of star formation, the protogalactic cloud collapsed and formed a thin rotating disk. Stars born before the cloud collapsed did not participate in the collapse, but have continued to orbit in the halo to the present day ().
Gravitational forces caused the gas in the thin disk to fragment into clouds or clumps with masses like those of star clusters. These individual clouds then fragmented further to form stars. Since the oldest stars in the disk are nearly as old as the youngest stars in the halo, the collapse must have been rapid (astronomically speaking), requiring perhaps no more than a few hundred million years.

### Collision Victims and the Multiple Merger Model

In past decades, astronomers have learned that the evolution of the Galaxy has not been quite as peaceful as this monolithic collapse model suggests. In 1994, astronomers discovered a small new galaxy in the direction of the constellation of Sagittarius. The Sagittarius dwarf galaxy is currently about 70,000 light-years away from Earth and 50,000 light-years from the center of the Galaxy. It is the closest galaxy known (). It is very elongated, and its shape indicates that it is being torn apart by our Galaxy’s gravitational tides—just as Comet Shoemaker-Levy 9 was torn apart when it passed too close to Jupiter in 1992.
The Sagittarius galaxy is much smaller than the Milky Way and is about 10,000 times less massive than our Galaxy. All of the stars in the Sagittarius dwarf galaxy seem destined to end up in the bulge and halo of the Milky Way. But don’t sound the funeral bells for the little galaxy quite yet; the ingestion of the Sagittarius dwarf will take another 100 million years or so, and the stars themselves will survive.
Since that discovery, evidence has been found for many more close encounters between our Galaxy and other neighbor galaxies. When a small galaxy ventures too close, the force of gravity exerted by our Galaxy tugs harder on the near side than on the far side. The net effect is that the stars that originally belonged to the small galaxy are spread out into a long stream that orbits through the halo of the Milky Way ().
Such a tidal stream can maintain its identity for billions of years. To date, astronomers have now identified streams originating from 12 small galaxies that ventured too close to the much larger Milky Way. Six more streams are associated with globular clusters. It has been suggested that large globular clusters, like Omega Centauri, are actually dense nuclei of cannibalized dwarf galaxies. The globular cluster M54 is now thought to be the nucleus of the Sagittarius dwarf we discussed earlier, which is currently merging with the Milky Way (). The stars in the outer regions of such galaxies are stripped off by the gravitational pull of the Milky Way, but the central dense regions may survive.
Calculations indicate that the Galaxy’s thick disk may be a product of one or more such collisions with other galaxies. Accretion of a satellite galaxy would stir up the orbits of the stars and gas clouds originally in the thin disk and cause them to move higher above and below the mid-plane of the Galaxy. Meanwhile, the Galaxy’s stars would add to the fluffed-up mix. If such a collision happened about 10 billion years ago, then any gas in the two galaxies that had not yet formed into stars would have had plenty of time to settle back down into the thin disk. The gas could then have begun forming subsequent generations of population I stars. This timing is also consistent with the typical ages of stars in the thick disk.
The Milky Way has more collisions in store. An example is the Canis Major dwarf galaxy, which has a mass of about 1% of the mass of the Milky Way. Already long tidal tails have been stripped from this galaxy, which have wrapped themselves around the Milky Way three times. Several of the globular clusters found in the Milky Way may also have come from the Canis Major dwarf, which is expected to merge gradually with the Milky Way over about the next billion years.
In about 4 billion years, the Milky Way itself will be swallowed up, since it and the Andromeda galaxy are on a collision course. Our computer models show that after a complex interaction, the two will merge to form a larger, more rounded galaxy ().
We are thus coming to realize that “environmental influences” (and not just a galaxy’s original characteristics) play an important role in determining the properties and development of our Galaxy. In future chapters we will see that collisions and mergers are a major factor in the evolution of many other galaxies as well.

### Key Concepts and Summary

The Galaxy began forming a little more than 13 billion years ago. Models suggest that the stars in the halo and globular clusters formed first, while the Galaxy was spherical. The gas, somewhat enriched in heavy elements by the first generation of stars, then collapsed from a spherical distribution to a rotating disk-shaped distribution. Stars are still forming today from the gas and dust that remain in the disk. Star formation occurs most rapidly in the spiral arms, where the density of interstellar matter is highest. The Galaxy captured (and still is capturing) additional stars and globular clusters from small galaxies that ventured too close to the Milky Way. In 3 to 4 billion years, the Galaxy will begin to collide with the Andromeda galaxy, and after about 7 billion years, the two galaxies will merge to form a giant elliptical galaxy.

### For Further Exploration


### Articles

Blitz, L. “The Dark Side of the Milky Way.” Scientific American (October 2011): 36–43. How we find dark matter and what it tells us about our Galaxy, its warped disk, and its satellite galaxies.
Dvorak, J. “Journey to the Heart of the Milky Way.” Astronomy (February 2008): 28. Measuring nearby stars to determine the properties of the black hole at the center.
Gallagher, J., Wyse, R., & Benjamin, R. “The New Milky Way.” Astronomy (September 2011): 26. Highlights all aspects of the Milky Way based on recent observations.
Goldstein, A. “Finding our Place in the Milky Way.” Astronomy (August 2015): 50. On the history of observations that pinpointed the Sun’s location in the Galaxy.
Haggard, D., & Bower, G. “In the Heart of the Milky Way.” Sky & Telescope (February 2016): 16. On observations of the Galaxy’s nucleus and the supermassive black hole and magnetar there.
Ibata, R., & Gibson, B. “The Ghosts of Galaxies Past.” Scientific American (April 2007): 40. About star streams in the Galaxy that are evidence of past mergers and collisions.
Irion, R. “A Crushing End for Our Galaxy.” Science (January 7, 2000): 62. On the role of mergers in the evolution of the Milky Way.
Irion, R. “Homing in on Black Holes.” Smithsonian (April 2008). On how astronomers probe the large black hole at the center of the Milky Way Galaxy.
Kruesi, L. “How We Mapped the Milky Way.” Astronomy (October 2009): 28.
Kruesi, L. “What Lurks in the Monstrous Heart of the Milky Way?” Astronomy (October 2015): 30. On the center of the Galaxy and the black hole there.
Laughlin, G., & Adams, F. “Celebrating the Galactic Millennium.” Astronomy (November 2001): 39. The long-term future of the Milky Way in the next 90 billion years.
Loeb, A., & Cox, T.J. “Our Galaxy’s Date with Destruction.” Astronomy (June 2008): 28. Describes the upcoming merger of Milky Way and Andromeda.
Szpir, M. “Passing the Bar Exam.” Astronomy (March 1999): 46. On evidence that our Galaxy is a barred spiral.
Tanner, A. “A Trip to the Galactic Center.” Sky & Telescope (April 2003): 44. Nice introduction, with observations pointing to the presence of a black hole.
Trimble, V., & Parker, S. “Meet the Milky Way.” Sky & Telescope (January 1995): 26. Overview of our Galaxy.
Wakker, B., & Richter, P. “Our Growing, Breathing Galaxy.” Scientific American (January 2004): 38. Evidence that our Galaxy is still being built up by the addition of gas and smaller neighbors.
Waller, W. “Redesigning the Milky Way.” Sky & Telescope (September 2004): 50. On recent multi-wavelength surveys of the Galaxy.
Whitt, K. “The Milky Way from the Inside.” Astronomy (November 2001): 58. Fantastic panorama image of the Galaxy, with finder charts and explanations.

### Websites

International Dark Sky Sanctuaries: http://darksky.org/idsp/sanctuaries/. A listing of dark-sky sanctuaries, parks, and reserves.
Multiwavelength Milky Way: https://asd.gsfc.nasa.gov/archive/mwmw/mmw_edu.html. This NASA site shows the plane of our Galaxy in a variety of wavelength bands, and includes background material and other resources.
Shapley-Curtis Debate in 1920: https://apod.nasa.gov/debate/debate20.html. In 1920, astronomers Harlow Shapley and Heber Curtis engaged in a historic debate about how large our Galaxy was and whether other galaxies existed. Here you can find historical and educational material about the debate.
UCLA Galactic Center Group: http://www.galacticcenter.astro.ucla.edu/. Learn more about the work of Andrea Ghez and colleagues on the central region of the Milky Way Galaxy.

### Videos

Crash of the Titans: http://www.spacetelescope.org/videos/hubblecast55a/. This Hubblecast from 2012 features Jay Anderson and Roeland van der Marel explaining how Andromeda will collide with the Milky Way in the distant future (5:07).
Diner at the Center of the Galaxy: https://www.youtube.com/watch?v=UP7ig8Gxftw. A short discussion from NASA ScienceCast of NuSTAR observations of flares from our Galaxy’s central black hole (3:23).
Hunt for a Supermassive Black Hole: https://www.ted.com/talks/andrea_ghez_the_hunt_for_a_supermassive_black_hole. 2009 TED talk by Andrea Ghez on searching for supermassive black holes, particularly the one at the center of the Milky Way (16:19).
Journey to the Galactic Center: https://www.youtube.com/watch?v=36xZsgZ0oSo. A brief silent trip into the cluster of stars near the galactic center showing their motions around the center (3:00).
The Nobel Prize Lecture by Dr. Andrea Ghez in 2020: https://www.youtube.com/watch?v=wGw6_CdvGKM.

### Collaborative Group Activities

1. You are captured by space aliens, who take you inside a complex cloud of interstellar gas, dust, and a few newly formed stars. To escape, you need to make a map of the cloud. Luckily, the aliens have a complete astronomical observatory with equipment for measuring all the bands of the electromagnetic spectrum. Using what you have learned in this chapter, have your group discuss what kinds of maps you would make of the cloud to plot your most effective escape route.
2. The diagram that Herschel made of the Milky Way has a very irregular outer boundary (see ). Can your group think of a reason for this? How did Herschel construct his map?
3. Suppose that for your final exam in this course, your group is assigned telescope time to observe a star selected for you by your professor. The professor tells you the position of the star in the sky (its right ascension and declination) but nothing else. You can make any observations you wish. How would you go about determining whether the star is a member of population I or population II?
4. The existence of dark matter comes as a great surprise, and its nature remains a mystery today. Someday astronomers will know a lot more about it (you can learn more about current findings in The Evolution and Distribution of Galaxies). Can your group make a list of earlier astronomical observations that began as a surprise and mystery, but wound up (with more observations) as well-understood parts of introductory textbooks?
5. Physicist Gregory Benford has written a series of science fiction novels that take place near the center of the Milky Way Galaxy in the far future. Suppose your group were writing such a story. Make a list of ways that the environment near the galactic center differs from the environment in the “galactic suburbs,” where the Sun is located. Would life as we know it have an easier or harder time surviving on planets that orbit stars near the center (and why)?
6. These days, in most urban areas, city lights completely swamp the faint light of the Milky Way in our skies. Have each member of your group survey 5 to 10 friends or relatives (you could spread out on campus to investigate or use social media or the phone), explaining what the Milky Way is and then asking if they have seen it. Also ask their age. Report back to your group and discuss your reactions to the survey. Is there any relationship between a person’s age and whether they have seen the Milky Way? How important is it that many kids growing up on Earth today never (or rarely) get to see our home Galaxy in the sky?

### Review Questions


### Thought Questions


### Figuring for Yourself
