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Astronomy

25.4 The Center of the Galaxy

Astronomy25.4 The Center of the Galaxy
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  1. Preface
  2. 1 Science and the Universe: A Brief Tour
    1. Introduction
    2. 1.1 The Nature of Astronomy
    3. 1.2 The Nature of Science
    4. 1.3 The Laws of Nature
    5. 1.4 Numbers in Astronomy
    6. 1.5 Consequences of Light Travel Time
    7. 1.6 A Tour of the Universe
    8. 1.7 The Universe on the Large Scale
    9. 1.8 The Universe of the Very Small
    10. 1.9 A Conclusion and a Beginning
    11. For Further Exploration
  3. 2 Observing the Sky: The Birth of Astronomy
    1. Thinking Ahead
    2. 2.1 The Sky Above
    3. 2.2 Ancient Astronomy
    4. 2.3 Astrology and Astronomy
    5. 2.4 The Birth of Modern Astronomy
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  4. 3 Orbits and Gravity
    1. Thinking Ahead
    2. 3.1 The Laws of Planetary Motion
    3. 3.2 Newton’s Great Synthesis
    4. 3.3 Newton’s Universal Law of Gravitation
    5. 3.4 Orbits in the Solar System
    6. 3.5 Motions of Satellites and Spacecraft
    7. 3.6 Gravity with More Than Two Bodies
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  5. 4 Earth, Moon, and Sky
    1. Thinking Ahead
    2. 4.1 Earth and Sky
    3. 4.2 The Seasons
    4. 4.3 Keeping Time
    5. 4.4 The Calendar
    6. 4.5 Phases and Motions of the Moon
    7. 4.6 Ocean Tides and the Moon
    8. 4.7 Eclipses of the Sun and Moon
    9. Key Terms
    10. Summary
    11. For Further Exploration
    12. Collaborative Group Activities
    13. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  6. 5 Radiation and Spectra
    1. Thinking Ahead
    2. 5.1 The Behavior of Light
    3. 5.2 The Electromagnetic Spectrum
    4. 5.3 Spectroscopy in Astronomy
    5. 5.4 The Structure of the Atom
    6. 5.5 Formation of Spectral Lines
    7. 5.6 The Doppler Effect
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  7. 6 Astronomical Instruments
    1. Thinking Ahead
    2. 6.1 Telescopes
    3. 6.2 Telescopes Today
    4. 6.3 Visible-Light Detectors and Instruments
    5. 6.4 Radio Telescopes
    6. 6.5 Observations outside Earth’s Atmosphere
    7. 6.6 The Future of Large Telescopes
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  8. 7 Other Worlds: An Introduction to the Solar System
    1. Thinking Ahead
    2. 7.1 Overview of Our Planetary System
    3. 7.2 Composition and Structure of Planets
    4. 7.3 Dating Planetary Surfaces
    5. 7.4 Origin of the Solar System
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  9. 8 Earth as a Planet
    1. Thinking Ahead
    2. 8.1 The Global Perspective
    3. 8.2 Earth’s Crust
    4. 8.3 Earth’s Atmosphere
    5. 8.4 Life, Chemical Evolution, and Climate Change
    6. 8.5 Cosmic Influences on the Evolution of Earth
    7. Key Terms
    8. Summary
    9. For Further Exploration
    10. Collaborative Group Activities
    11. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  10. 9 Cratered Worlds
    1. Thinking Ahead
    2. 9.1 General Properties of the Moon
    3. 9.2 The Lunar Surface
    4. 9.3 Impact Craters
    5. 9.4 The Origin of the Moon
    6. 9.5 Mercury
    7. Key Terms
    8. Summary
    9. For Further Exploration
    10. Collaborative Group Activities
    11. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  11. 10 Earthlike Planets: Venus and Mars
    1. Thinking Ahead
    2. 10.1 The Nearest Planets: An Overview
    3. 10.2 The Geology of Venus
    4. 10.3 The Massive Atmosphere of Venus
    5. 10.4 The Geology of Mars
    6. 10.5 Water and Life on Mars
    7. 10.6 Divergent Planetary Evolution
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  12. 11 The Giant Planets
    1. Thinking Ahead
    2. 11.1 Exploring the Outer Planets
    3. 11.2 The Giant Planets
    4. 11.3 Atmospheres of the Giant Planets
    5. Key Terms
    6. Summary
    7. For Further Exploration
    8. Collaborative Group Activities
    9. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  13. 12 Rings, Moons, and Pluto
    1. Thinking Ahead
    2. 12.1 Ring and Moon Systems Introduced
    3. 12.2 The Galilean Moons of Jupiter
    4. 12.3 Titan and Triton
    5. 12.4 Pluto and Charon
    6. 12.5 Planetary Rings
    7. Key Terms
    8. Summary
    9. For Further Exploration
    10. Collaborative Group Activities
    11. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  14. 13 Comets and Asteroids: Debris of the Solar System
    1. Thinking Ahead
    2. 13.1 Asteroids
    3. 13.2 Asteroids and Planetary Defense
    4. 13.3 The “Long-Haired” Comets
    5. 13.4 The Origin and Fate of Comets and Related Objects
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  15. 14 Cosmic Samples and the Origin of the Solar System
    1. Thinking Ahead
    2. 14.1 Meteors
    3. 14.2 Meteorites: Stones from Heaven
    4. 14.3 Formation of the Solar System
    5. 14.4 Comparison with Other Planetary Systems
    6. 14.5 Planetary Evolution
    7. Key Terms
    8. Summary
    9. For Further Exploration
    10. Collaborative Group Activities
    11. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  16. 15 The Sun: A Garden-Variety Star
    1. Thinking Ahead
    2. 15.1 The Structure and Composition of the Sun
    3. 15.2 The Solar Cycle
    4. 15.3 Solar Activity above the Photosphere
    5. 15.4 Space Weather
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  17. 16 The Sun: A Nuclear Powerhouse
    1. Thinking Ahead
    2. 16.1 Sources of Sunshine: Thermal and Gravitational Energy
    3. 16.2 Mass, Energy, and the Theory of Relativity
    4. 16.3 The Solar Interior: Theory
    5. 16.4 The Solar Interior: Observations
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  18. 17 Analyzing Starlight
    1. Thinking Ahead
    2. 17.1 The Brightness of Stars
    3. 17.2 Colors of Stars
    4. 17.3 The Spectra of Stars (and Brown Dwarfs)
    5. 17.4 Using Spectra to Measure Stellar Radius, Composition, and Motion
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  19. 18 The Stars: A Celestial Census
    1. Thinking Ahead
    2. 18.1 A Stellar Census
    3. 18.2 Measuring Stellar Masses
    4. 18.3 Diameters of Stars
    5. 18.4 The H–R Diagram
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  20. 19 Celestial Distances
    1. Thinking Ahead
    2. 19.1 Fundamental Units of Distance
    3. 19.2 Surveying the Stars
    4. 19.3 Variable Stars: One Key to Cosmic Distances
    5. 19.4 The H–R Diagram and Cosmic Distances
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  21. 20 Between the Stars: Gas and Dust in Space
    1. Thinking Ahead
    2. 20.1 The Interstellar Medium
    3. 20.2 Interstellar Gas
    4. 20.3 Cosmic Dust
    5. 20.4 Cosmic Rays
    6. 20.5 The Life Cycle of Cosmic Material
    7. 20.6 Interstellar Matter around the Sun
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  22. 21 The Birth of Stars and the Discovery of Planets outside the Solar System
    1. Thinking Ahead
    2. 21.1 Star Formation
    3. 21.2 The H–R Diagram and the Study of Stellar Evolution
    4. 21.3 Evidence That Planets Form around Other Stars
    5. 21.4 Planets beyond the Solar System: Search and Discovery
    6. 21.5 Exoplanets Everywhere: What We Are Learning
    7. 21.6 New Perspectives on Planet Formation
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  23. 22 Stars from Adolescence to Old Age
    1. Thinking Ahead
    2. 22.1 Evolution from the Main Sequence to Red Giants
    3. 22.2 Star Clusters
    4. 22.3 Checking Out the Theory
    5. 22.4 Further Evolution of Stars
    6. 22.5 The Evolution of More Massive Stars
    7. Key Terms
    8. Summary
    9. For Further Exploration
    10. Collaborative Group Activities
    11. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  24. 23 The Death of Stars
    1. Thinking Ahead
    2. 23.1 The Death of Low-Mass Stars
    3. 23.2 Evolution of Massive Stars: An Explosive Finish
    4. 23.3 Supernova Observations
    5. 23.4 Pulsars and the Discovery of Neutron Stars
    6. 23.5 The Evolution of Binary Star Systems
    7. 23.6 The Mystery of the Gamma-Ray Bursts
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  25. 24 Black Holes and Curved Spacetime
    1. Thinking Ahead
    2. 24.1 Introducing General Relativity
    3. 24.2 Spacetime and Gravity
    4. 24.3 Tests of General Relativity
    5. 24.4 Time in General Relativity
    6. 24.5 Black Holes
    7. 24.6 Evidence for Black Holes
    8. 24.7 Gravitational Wave Astronomy
    9. Key Terms
    10. Summary
    11. For Further Exploration
    12. Collaborative Group Activities
    13. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  26. 25 The Milky Way Galaxy
    1. Thinking Ahead
    2. 25.1 The Architecture of the Galaxy
    3. 25.2 Spiral Structure
    4. 25.3 The Mass of the Galaxy
    5. 25.4 The Center of the Galaxy
    6. 25.5 Stellar Populations in the Galaxy
    7. 25.6 The Formation of the Galaxy
    8. Key Terms
    9. Summary
    10. For Further Exploration
    11. Collaborative Group Activities
    12. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  27. 26 Galaxies
    1. Thinking Ahead
    2. 26.1 The Discovery of Galaxies
    3. 26.2 Types of Galaxies
    4. 26.3 Properties of Galaxies
    5. 26.4 The Extragalactic Distance Scale
    6. 26.5 The Expanding Universe
    7. Key Terms
    8. Summary
    9. For Further Exploration
    10. Collaborative Group Activities
    11. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  28. 27 Active Galaxies, Quasars, and Supermassive Black Holes
    1. Thinking Ahead
    2. 27.1 Quasars
    3. 27.2 Supermassive Black Holes: What Quasars Really Are
    4. 27.3 Quasars as Probes of Evolution in the Universe
    5. Key Terms
    6. Summary
    7. For Further Exploration
    8. Collaborative Group Activities
    9. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  29. 28 The Evolution and Distribution of Galaxies
    1. Thinking Ahead
    2. 28.1 Observations of Distant Galaxies
    3. 28.2 Galaxy Mergers and Active Galactic Nuclei
    4. 28.3 The Distribution of Galaxies in Space
    5. 28.4 The Challenge of Dark Matter
    6. 28.5 The Formation and Evolution of Galaxies and Structure in the Universe
    7. Key Terms
    8. Summary
    9. For Further Exploration
    10. Collaborative Group Activities
    11. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  30. 29 The Big Bang
    1. Thinking Ahead
    2. 29.1 The Age of the Universe
    3. 29.2 A Model of the Universe
    4. 29.3 The Beginning of the Universe
    5. 29.4 The Cosmic Microwave Background
    6. 29.5 What Is the Universe Really Made Of?
    7. 29.6 The Inflationary Universe
    8. 29.7 The Anthropic Principle
    9. Key Terms
    10. Summary
    11. For Further Exploration
    12. Collaborative Group Activities
    13. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  31. 30 Life in the Universe
    1. Thinking Ahead
    2. 30.1 The Cosmic Context for Life
    3. 30.2 Astrobiology
    4. 30.3 Searching for Life beyond Earth
    5. 30.4 The Search for Extraterrestrial Intelligence
    6. Key Terms
    7. Summary
    8. For Further Exploration
    9. Collaborative Group Activities
    10. Exercises
      1. Review Questions
      2. Thought Questions
      3. Figuring for Yourself
  32. A | How to Study for an Introductory Astronomy Class
  33. B | Astronomy Websites, Images, and Apps
  34. C | Scientific Notation
  35. D | Units Used in Science
  36. E | Some Useful Constants for Astronomy
  37. F | Physical and Orbital Data for the Planets
  38. G | Selected Moons of the Planets
  39. H | Future Total Eclipses
  40. I | The Nearest Stars, Brown Dwarfs, and White Dwarfs
  41. J | The Brightest Twenty Stars
  42. K | The Chemical Elements
  43. L | The Constellations
  44. M | Star Chart and Sky Event Resources
  45. Index

Learning Objectives

By the end of this section, you will be able to:

  • Describe the radio and X-ray observations that indicate energetic phenomena are occurring at the galactic center
  • Explain what has been revealed by high-resolution near-infrared imaging of the galactic center
  • Discuss how these near-infrared images, when combined with Kepler’s third law of motion, can be used to derive the mass of the central gravitating object

At the beginning of this chapter, we hinted that the core of our Galaxy contains a large concentration of mass. In fact, we now have evidence that the very center contains a black hole with a mass equivalent to 4.6 million Suns and that all this mass fits within a sphere that has less than the diameter of Mercury’s orbit. Such monster black holes are called supermassive black holes by astronomers, to indicate that the mass they contain is far greater than that of the typical black hole created by the death of a single star. It is amazing that we have very convincing evidence that this black hole really does exist. After all, recall from the chapter on Black Holes and Curved Spacetime that we cannot see a black hole directly because by definition it radiates no energy. And we cannot even see into the center of the Galaxy in visible light because of absorption by the interstellar dust that lies between us and the galactic center. Light from the central region of the Galaxy is dimmed by a factor of a trillion (1012) by all this dust.

Fortunately, we are not so blind at other wavelengths. Infrared and radio radiation, which have long wavelengths compared to the sizes of the interstellar dust grains, flow unimpeded past the dust particles and so reach our telescopes with hardly any dimming. In fact, the very bright radio source in the nucleus of the Galaxy, now known as Sagittarius A* (pronounced “Sagittarius A-star” and abbreviated Sgr A*), was the first cosmic radio source astronomers discovered.

A Journey toward the Center

Let’s take a voyage to the mysterious heart of our Galaxy and see what’s there. Figure 25.14 is a radio image of a region about 1500 light-years across, centered on Sagittarius A, a bright radio source that contains the smaller Sagittarius A*. Much of the radio emission comes from hot gas heated either by clusters of hot stars (the stars themselves do not produce radio emission and can’t be seen in the image) or by supernova blast waves. Most of the hollow circles visible on the radio image are supernova remnants. The other main source of radio emission is from electrons moving at high speed in regions with strong magnetic fields. The bright thin arcs and “threads” on the figure show us where this type of emission is produced.

Radio Image of Galactic Center Region. Many features are identified in this complex radio image. The scale at lower left (defined by a double headed horizontal arrow) reads: “~0.5O ~75 pc ~240 LY”. The objects listed, from upper left to lower right, are: “Sgr D HII”, “Sgr D SNR”, “SNR 0.9+0.1”, “Sgr B2”, “Sgr B1”, “New SNR 0.3+0.0”, “Arc”, “Threads”, “Sgr A*”, “New feature: The Cane”, “Background Galaxy”, “Threads”, “New thread: The Pelican”, “Sgr C”, “Coherent structure?”, “Snake” and “Sgr E”. Below center, three more features are labeled (from top to bottom): “SNR 359.1-00.5”, “Mouse” and “SNR 359.0=00.9”.
Figure 25.14 Radio Image of Galactic Center Region. This radio map of the center of the Galaxy (at a wavelength of 90 centimeters) was constructed from data obtained with the Very Large Array (VLA) of radio telescopes in Socorro, New Mexico. Brighter regions are more intense in radio waves. The galactic center is inside the region labeled Sagittarius A. Sagittarius B1 and B2 are regions of active star formation. Many filaments or threadlike features are seen, as well as a number of shells (labeled SNR), which are supernova remnants. The scale bar at the bottom left is about 240 light-years long. Notice that radio astronomers also give fanciful animal names to some of the structures, much as visible-light nebulae are sometimes given the names of animals they resemble. (credit: modification of work by N. E. Kassim, D. S. Briggs, T. J. W. Lazio, T. N. LaRosa, and J. Imamura (NRL/RSD))

Now let’s focus in on the central region using a more energetic form of electromagnetic radiation. Figure 25.15 shows the X-ray emission from a smaller region 400 light-years wide and 900 light-years across centered in Sagittarius A*. Seen in this picture are hundreds of hot white dwarfs, neutron stars, and stellar black holes with accretion disks glowing with X-rays. The diffuse haze in the picture is emission from gas that lies among the stars and is at a temperature of 10 million K.

Galactic Center in X-rays. The galactic center is seen in this image as an irregular white source at center, surrounded by clouds of hot gas. Bright supernova remnants are seen to the left of center and at far right as blue-green blobs. The colors in this false-color image indicate X-ray energy bands: red (low energy), green (medium energy), and blue (high energy).
Figure 25.15 Galactic Center in X-Rays. This artificial-color mosaic of 30 images taken with the Chandra X-ray satellite shows a region 400 × 900 light-years in extent and centered on Sagittarius A*, the bright white source in the center of the picture. The X-ray-emitting point sources are white dwarfs, neutron stars, and stellar black holes. The diffuse “haze” is emission from gas at a temperature of 10 million K. This hot gas is flowing away from the center out into the rest of the Galaxy. The colors indicate X-ray energy bands: red (low energy), green (medium energy), and blue (high energy). (credit: modification of work by NASA/CXC/ UMass/D. Wang et al.)

As we approach the center of the Galaxy, we find the supermassive black hole Sagittarius A*. There are also thousands of stars within a parsec of Sagittarius A*. Most of these are old, reddish main-sequence stars. But there are also about a hundred hot OB stars that must have formed within the last few million years. There is as yet no good explanation for how stars could have formed recently so close to a supermassive black hole. Perhaps they formed in a dense cluster of stars that was originally at a larger distance from the black hole and subsequently migrated closer.

There is currently no star formation at the galactic center, but there is lots of dust and molecular gas that is revolving around the black hole, along with some ionized gas streamers that are heated by the hot stars. Figure 25.16 is a radio map that shows these gas streamers.

Central 10 Light-Years of the Galaxy. The bright region at lower right is Sagittarius A* and the straight filaments running from lower left to upper right are collectively known as “The Arc”. See [Figure 25_04_RImage] to see these features in context with other objects near the galactic center.
Figure 25.16 Sagittarius A. This image, taken with the Very Large Array of radio telescopes, shows the radio emission from hot, ionized gas in the center of the Milky Way. The lines slanting across the top of the image are gas streamers. Sagittarius A* is the bright spot in the lower right. (credit: modification of work by Farhad Zadeh et al. (Northwestern), VLA, NRAO)

Finding the Heart of the Galaxy

Just what is Sagittarius A*, which lies right at the center our Galaxy? To establish that there really is a black hole there, we must show that there is a very large amount of mass crammed into a very tiny volume. As we saw in Black Holes and Curved Spacetime, proving that a black hole exists is a challenge because the black hole itself emits no radiation. What astronomers must do is prove that a black hole is the only possible explanation for our observations—that a small region contains far more mass than could be accounted for by a very dense cluster of stars or something else made of ordinary matter.

To put some numbers with this discussion, the radius of the event horizon of a galactic black hole with a mass of about 4 million MSun would be only about 17 times the size of the Sun—the equivalent of a single red giant star. The corresponding density within this region of space would be much higher than that of any star cluster or any other ordinary astronomical object. Therefore, we must measure both the diameter of Sagittarius A* and its mass. Both radio and infrared observations are required to give us the necessary evidence.

First, let’s look at how the mass can be measured. If we zero in on the inner few light-days of the Galaxy with an infrared telescope equipped with adaptive optics, we see a region crowded with individual stars (Figure 25.17). These stars have now been observed for almost two decades, and astronomers have detected their rapid orbital motions around the very center of the Galaxy.

Near-infrared View of the Galactic Center. The measured orbits of eight stars are plotted orbiting the galactic center, shown as ellipses of different colors. The dots that lie along each ellipse are the observed data points. The scale at upper left, indicated with a short double headed arrow, reads: 0.1”. At upper right the orientation of the image is indicated with arrows, north is up and east is to the left.
Figure 25.17 Near-Infrared View of the Galactic Center. This image shows the inner 1 arcsecond, or 0.13 light-year, at the center of the Galaxy, as observed with the giant Keck Telescope. Tracks of the orbiting stars measured from 1995 to 2014 have been added to this “snapshot.” The stars are moving around the center very fast, and their tracks are all consistent with a single massive “gravitator” that resides in the very center of this image. (credit: modification of work by Andrea Ghez, UCLA Galactic Center Group, W.M. Keck Observatory Laser Team)

If we combine observations of their periods and the size of their orbits with Kepler’s third law, we can estimate the mass of the object that keeps them in their orbits. One of the stars has been observed for its full orbit of 15.6 years. Its closest approach takes it to a distance of only 124 AU or about 17 light-hours from the black hole. This orbit, when combined with observations of other stars close to the galactic center, indicates that a mass of 4.6 million MSun must be concentrated inside the orbit—that is, within 17 light-hours of the center of the Galaxy.

Even tighter limits on the size of the concentration of mass at the center of the Galaxy come from radio astronomy, which provided the first clue that a black hole might lie at the center of the Galaxy. As matter spirals inward toward the event horizon of a black hole, it is heated in a whirling accretion disk and produces radio radiation. (Such accretion disks were explained in Black Holes and Curved Spacetime.) Measurements of the size of the accretion disk with the Very Long Baseline Array, which provides very high spatial resolution, show that the diameter of the radio source Sagittarius A* is no larger than about 0.3 AU, or about the size of Mercury’s orbit. (In light units, that’s only 2.5 light-minutes!)

The observations thus show that 4.6 million solar masses are crammed into a volume that has a diameter that is no larger than the orbit of Mercury. If this were anything other than a supermassive black hole—low-mass stars that emit very little light or neutron stars or a very large number of small black holes— calculations show that these objects would be so densely packed that they would collapse to a single black hole within a hundred thousand years. That is a very short time compared with the age of the Galaxy, which probably began forming more than 13 billion years ago. Since it seems very unlikely that we would have caught such a complex cluster of objects just before it collapsed, the evidence for a supermassive black hole at the center of the Galaxy is convincing indeed.

Finding the Source

Where did our galactic black hole come from? The origin of supermassive black holes in galaxies like ours is currently an active field of research. One possibility is that a large cloud of gas near the center of the Milky Way collapsed directly to form a black hole. Since we find large black holes at the centers of most other large galaxies (see Active Galaxies, Quasars, and Supermassive Black Holes)—even ones that are very young—this collapse probably would have taken place when the Milky Way was just beginning to take shape. The initial mass of this black hole might have been only a few tens of solar masses. Another way it could have started is that a massive star might have exploded to leave behind a seed black hole, or a dense cluster of stars might have collapsed into a black hole.

Once a black hole exists at the center of a galaxy, it can grow over the next several billion years by devouring nearby stars and gas clouds in the crowded central regions. It can also grow by merging with other black holes.

It appears that the monster black hole at the center of our Galaxy is not finished “eating.” At the present time, we observe clouds of gas and dust falling into the galactic center at the rate of about 1 MSun per thousand years. Stars are also on the black hole’s menu. The density of stars near the galactic center is high enough that we would expect a star to pass near the black hole and be swallowed by it every ten thousand years or so. As this happens, some of the energy of infall is released as radiation. As a result, the center of the Galaxy might flare up and even briefly outshine all the stars in the Milky Way. Other objects might also venture too close to the black hole and be pulled in. How great a flare we observe would depend on the mass of the object falling in.

In 2013, the Chandra X-ray satellite detected a flare from the center of our Galaxy that was 400 times brighter than the usual output from Sagittarius A*. A year later, a second flare, only half as bright, was also detected. This is much less energy than swallowing a whole star would produce. There are two theories to account for the flares. First, an asteroid might have ventured too close to the black hole and been heated to a very high temperature before being swallowed up. Alternatively, the flares might have involved interactions of the magnetic fields near the galactic center in a process similar to the one described for solar flares (see The Sun: A Garden-Variety Star). Astronomers continue to monitor the galactic center area for flares or other activity. Although the monster in the center of the Galaxy is not close enough to us to represent any danger, we still want to keep our eyes on it.

Voyagers in Astronomy

Andrea Ghez

A lover of puzzles, Andrea Ghez has been pursuing one of the greatest mysteries in astronomy: what strange entity lurks within the center of our Milky Way Galaxy?

Photograph of Andrea Ghez.
Figure 25.18 Andrea Ghez. Research by Ghez and her team has helped shape our understanding of supermassive black holes. (credit: modification of work by John D. and Catherine T. MacArthur Foundation)

As a child living in Chicago during the late 1960s, Andrea Ghez (Figure 25.18) was fascinated by the Apollo Moon landings. But she was also drawn to ballet and to solving all sorts of puzzles. By high school, she had lost the ballet bug in favor of competing in field hockey, playing the flute, and digging deeper into academics. Her undergraduate years at MIT were punctuated by a number of changes in her major—from mathematics to chemistry, mechanical engineering, aerospace engineering, and finally physics—where she felt her options were most open. As a physics major, she became involved in astronomical research under the guidance of one of her instructors. Once she got to do some actual observing at Kitt Peak National Observatory in Arizona, and later at Cerro Tololo Inter-American Observatory in Chile, Ghez had found her calling.

Pursuing her graduate studies at Caltech, she stuck with physics but oriented her efforts toward observational astrophysics, an area where Caltech had access to cutting-edge facilities. Though initially attracted to studying the black holes that were suspected of dwelling inside most massive galaxies, Ghez ended up spending most of her graduate study and later postdoctoral research at the University of Arizona studying stars in formation. By taking very high-resolution (detailed) imaging of regions where new stars are born, she discovered that most stars form as members of binary systems. As technologies advanced, she was able to track the orbits danced by these stellar pairings and thereby could ascertain their respective masses.

Now an astronomy professor at UCLA, Ghez has since used similar high-resolution imaging techniques to study the orbits of stars in the innermost core of the Milky Way. These orbits take years to delineate, so Ghez and her science team have logged more than 20 years of taking super-resolution infrared images with the giant Keck telescopes in Hawaii. Based on the resulting stellar orbits, the UCLA Galactic Center Group has settled (as we saw) on a gravitational solution that requires the presence of a supermassive black hole with a mass equivalent to 4.6 million Suns—all nestled within a space smaller than that occupied by our solar system. Ghez’s achievements have been recognized with one of the “genius” awards given by the MacArthur Foundation. More recently, her team discovered glowing clouds of warm ionized gas that co-orbit with the stars but may be more vulnerable to the disruptive effects of the central black hole. By monitoring these clouds, the team hopes to better understand the evolution of supermassive black holes and their immediate environs. They also hope to test Einstein’s theory of general relativity by carefully scrutinizing the orbits of stars that careen closest to the intensely gravitating black hole.

Besides her pioneering work as an astronomer, Ghez competes as a master swimmer, enjoys family life as a mother of two children, and actively encourages other women to pursue scientific careers.

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