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Up to this point, we have concentrated on gaining the fundamental knowledge needed to understand the stars in the Galaxy and some of the ways they may end their lives. Stars give material back to the interstellar medium whence they came. The "stuff between the stars" was mentioned during the birth of stars, this lesson pursues the topic in greater depth. How do we know the shape and size of the Milky Way? What lies at the very center of our galactic home? The lab exercise assigned to this lesson introduces you to a special type of variable star, called an RR Lyrae variable, that has been instrumental in our measurements of distances within the Galaxy. At the turn of the 20th century, around 1920, astronomers believed that the solar system was close to the center of the Milky Way, and that our galaxy was the entire universe! By using the special characteristics of the RR Lyrae stars and observations of them in dozens of globular clusters, we learned that we were, in fact, about two-thirds of the way to the edge of our galaxy. The Earth was not the center of the solar system, and now the solar system was not the center of the Galaxy.
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After completing this lesson, you should be able to
Our discussion of the Milky Way Galaxy will focus on the four regions: nucleus, bulge, disk, and halo. Each region has its own special characteristics, dynamics, substructure, and stellar populations. We start here with the galactic nucleus, the site (it seems) of a massive black hole. We will then "work our way out" from the center and end with how we know about the extent and structure of the Galaxy.
A special kind of variable star, the RR Lyrae variable, have played an important role in our knowledge of the Galaxy. We learn how astronomers use these stars to determine distances to globular clusters in our galaxy, and how the knowledge of the distribution of these globular clusters led to the realization that not only was the Earth not the center of the solar system, the Sun was not the center of the Galaxy. Our egocentric attitude took another hit.
From an abstract of a scientific paper concerning the possibility of a massive black hole at the heart of the Milky Way:
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[Bulletin of the American Astronomical Society 97.07] The Galactic
Center Black Hole A. Eckart, R. Genzel (MPE Garching Germany) |
Since we have just finished our discussion of black holes and violent stellar explosions, we shall start our study of the galaxy within which we reside at its center and the supermassive black hole that is residing there. You will probably note some similarities between supermassive black holes and stellar black holes. The biggest difference, however, is not hard to guess: stellar black holes are 5-10 solar masses or so, supermassive black holes are 5-10 million solar masses or so! Let's take a closer look at the beast.
Start with Fig. 16.1 of your text that shows a full-sky view of the Milky Way from Mount Graham, Arizona (which is actually only half of the total extent of the Galaxy). The first image shown here is a small section of the lower-left hand part of that picture, and identifies the location of the galactic center, the constellations Sagittarius and Scorpio, and a few other objects within that region of the sky. What we are going to do over the next series of images is steadily zoom in. As we do, note two thing: the scale of the images shown at the bottom, and the wavelength region of the electromagnetic spectrum, shown at the top. Be sure to take the time to follow the links provided with these images to learn even more about the nucleus of the Galaxy. (Images from A Galactic Center Mystery, and links provided through the Chandra X-Ray Observatory.)
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The
Galactic Center: A Wide-Field, Low-Frequency Image |
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Galactic
Center (Survey) Multiwavelength Close-Up |
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Sagittarius A*:
Chandra Catches Milky Way Monster Snacking |
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This series of images approach the nucleus of the Galaxy, and isolate the region known as Sagittarius A: Sgr A West, East, and A*. We do not yet have the technology to actually see the black hole or even come within a distance where we can see the material actually spiralling in. We can detect the X-ray and radio signals, however, and view the huge number of stars present there at infrared wavelengths.
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An artist's idea of what the environment around a supermassive black hole might look like. |
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The center of the Galaxy is approximately 25,000 light years away from the Sun. This image is a time-lapse movie in infrared light and shows how stars in the central 1 light year of our galaxy have moved over the seven-year period from 1992-1999. To be able to detect this kind of motion over such a short period of time, these stars must be moving extremely fast! Plus, if these stars are moving this fast, there must be something extremely massive that is holding them in, otherwise they would fly right out of the Galaxy. It is through the analysis of the proper motion of these stars that astronomers have calculated a mass of about 1.5 million times our Sun's, confined to an extremely small region. The location of Sagittarius A* is marked by the yellow cross. (Credit: A. Eckart (U. Koeln) & R. Genzel (MPE-Garching), SHARP I, NTT, La Silla Obs., ESO)
Your text has two of the best images showing just how crowded the bulge of our galaxy is. The first one opens the chapter and shows the center in infrared light. You should look ahead to Fig. 16.16 for a zoomed-in version of the nucleus. The other figure showing the astonishing density of stars in the bulge is Fig. 16.7, which shows the bulge in visible light. The density of stars in the bulge of our galaxy is about 50,000 per cubic parsec. How does this compare with the density of stars in our neighborhood? (Hint: how far away is the nearest star?)
Here are some additional images, complements of Christopher Pickering. As you take a closer look at them, imagine what the night sky would look like if our Sun were located close to the bulge. How many additional stars would you have to study in Astronomy 101? Actually, there would be no astronomy classes if the Sun were located in the bulge. The density of stars there is so great that planets do not stay attached to their parent star. There are too many close encounters that gravitationally strip planets away from stars.
Let's get familiar with just how the Milky Way looks to us: a fuzzy, wide swath of light going across the sky. If our galaxy were spherical, with stars distributed uniformly throughout the sphere, the density of stars we would see would be the same everywhere. But, what we notice is that the Milky Way looks like a band: thick in some regions, and thin and barely noticeable in others. This shows that the Galaxy is relatively flat. Farther along in these notes, under The Structure of the Milky Way, we cover how our view changed with the work of Harlow Shapley. Take a look at the linked Gallery of Milky Way Images from astrophotographers. Think if you have seen similar views yourself.
The evidence is mounting and becoming more and more convincing (although controversy may prevail for a while) that the Milky Way is, in fact, a barred spiral galaxy. Research that has sought to explain "expanding arms," carbon monoxide gas kinematics (dealing with motion), large-scale asymmetries in neutral hydrogen distribution, asymptotic giant branch star populations, radial distribution of carbon monoxide, near-infrared counts of carbon stars, and populations of bulge red clump stars all point to the same thing: the Galaxy has a bar that is pointed just off of our line-of-sight to the center.
The 2MASS survey's view of the entire Milky Way, seen from our vantage point of Earth.
("Atlas Image obtained as part of the Two Micron All Sky
Survey (2MASS), a joint project of the University of Massachusetts and the Infrared
Processing and Analysis Center/California Institute of Technology, funded by the
National Aeronautics and Space Administration and the National Science Foundation.")
Image Credit:
2MASS/J. Carpenter, M. Skrutskie, R. Hurt
Near-infrared lightwhich is just beyond the red edge of the visible spectrumcan't penetrate dust as well as X rays can. But observing the Milky Way at these longer wavelengths reveals the more common, cooler stars that make up the bulk of the galaxy's stellar population. Using a select group of 30,000 stars among the 300 million or so cataloged in a huge near-infrared survey called 2MASS (Two Micron All Sky Survey), researchers have produced the first bird's-eye view of our galaxy's spiral disk and the cigar-shape structure, called a bar, at its center.
Of all the stars recorded by 2MASS, the astronomers chose to analyze carbon stars first because of three remarkable properties. First, the stars are so bright that telescopes on Earth can detect them throughout the galaxy. Second, although the Milky Way's dust dims the light from carbon stars, the stars remain easy to recognize and they can't be confused with other stars in the survey. Best of all, carbon stars act as standard candlesall these stars have about the same luminosity, as if they were light bulbs of the same wattage. That property turns them into veritable mileage posts marking off distances throughout the galaxy.
By comparing the luminosity of a carbon star to how bright it appears in the sky, Skrutskie, Weinberg, and their colleagues determined the distance to each of the 30,000 carbon stars in 2MASS. Their preliminary map reveals the outer limits of the Milky Way's stellar disk. At the center of the galaxy, the map shows that the galactic bar measures about 15,000 light-years across.
A comparison of
the galactic center in optical and at infrared wavelengths from the
2 micron all-sky survey at IPAC.
After comparing these two images, it becomes a bit more obvious how important it is
that we observe our universe across the entire electromagnetic spectrum.
So, if we could get a bird's eye view of the Galaxy, what would it look like?
Some think it resembles the barred galaxy NGC 253, shown at the right (click on the
image for a larger view). Observations
have indicated that rather than just two main arms coming off each end of the bar,
the Galaxy has 4 arms. This is not unusual for a barred spiral. Take a look at the
following sketch that has used data to support the artist's rendition:
Why were our ideas regarding the overall structure of the Milky Way Galaxy wrong for so long?
The information and images given above should give you a huge hint. Compare the following two images. The first one is of the galaxy NGC 891, a galaxy thought to closely resemble our Milky Way; the second, another image of the center of the Milky Way, showing the Sagittarius-Scorpio region. Hint: the dark stuff is NOT a hole or an empty lane in the galaxies. What do you think would be the problem? [Another hint: it's a four-letter word that starts with "d"]
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We have gradually through the weeks of this course expanded our horizons from our view of the night sky on Earth, to the Sun, to the nearby stars, to the stuff in between the stars, and now to the complete galaxy. Stop for just a moment and take inventory of the objects you have studied so far. Be as specific or as general as you like. How many different things can you come up with? How much do you remember about each one and its place in our galaxy?
The total inventory of the Galaxy probably includes at least 200 billion other stars (maybe one trillion solar masses) and their planets, thousands of open and globular clusters and nebulae, the gas and dust of the interstellar medium, and perhaps any number of exotic objects like stellar black holes and a supermassive black hole at the Galaxy's center. Its diameter is about 30,000 parsecs (100,000 light years). The galactic disk is separated into complex, and sometimes indistinguishable, layers. The thin disk is about 2000 parsecs thick and contains the dust, gas, and hottest and youngest stars. The thick disk adds about another 700 pc onto the disk, and contains somewhat older (and therefore lower mass, lower metallicity) stars.
Before we venture out into the rest of our Galaxy, let's pause to look around at the Sun's nearest neighbors. The majority of our neighbors are low-mass, main-sequence stars that are too dim to be seen with the naked eye. Check out this list of the 60 stars nearer than 5 pc from the Observer's Handbook of the Royal Astronomical Society of Canada. Only four of these stars are more massive than our Sun. Since it is impossible to tell how far away the stars are just by looking at them, we have measured the parallaxes of these stars and thousands of others. For stars that are too far away to have a detectable parallax, we use the method of spectroscopic parallax or main-sequence fitting to determine their distances. From these measurements and the location of the stars in the sky, we are able to put together images of where we live. Click on each of the images for a larger view. The final image shows the Milky Way and our nearest neighbors. (The following images A-D, and F reproduced with permission from Richard Powell and his Atlas of the Universe. The image of M83 (a galaxy the Milky Way is believed to resemble) is from the archives of the Sloan Digital Sky Survey.)
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One of the many perplexing questions about spiral galaxies such as our galaxy, is the formation and persistence of spiral arms. How do they get started in the first place, and why don't they disappear? Galaxies rotate. The stars, their planets, the interstellar medium, all revolve around the center of our galaxy. At one time it was thought that the spiral arms of the Milky Way and other spiral galaxies were the result of originally straight arms "winding up" due to this rotation. The Sun orbits the center every 250 million years and has gone around about 60 times. We know that the stars inward of the Sun orbit faster, and stars outward from the Sun orbit slower. As the cartoon at the right demonstrates, over the course of the Milky Way's lifetime, 15 billion years or so, the spiral arm structure would be destroyed (by the way, this cat rotates only six times). If this were the galaxy M 83, shown on the left below, then after about 50 rotations, it would look more like the image on the right that is the same galaxy "rotated" digitally about 10 times.
Why aren't spiral galaxies all wound up? Be sure to review the section in your text that discusses spiral structure and the density wave theory. It's the best explanation we have to date for the overall spiral structure seen, even though it does not explain everything, especially the complicated or chaotic patterns. In physics, we try to simplify our models in order to understand the mechanisms involved. Galaxies are objects that have resisted most attempts at simple categories and simple explanations.
A leading theory is that the spiral arms represent density waves, coiled waves of gas compression moving through the disk. These density waves, although revolving around the center of the Galaxy, do so much more slowly than the individual stars. As the interstellar medium is compressed, star formation, with its subsequent luminous O and B star formation, is initiated. In this case, the density waves drive star formation.
An alternative explanation has star formation driving the density waves. We've talked about the effect massive stars have on their surroundings. These stars compress the interstellar medium around them, perhaps initiating additional star formation. Massive stars live for a very short time before going supernova. The subsequent shock waves could also create even more star formation. This theory is known as self-propagating star formation.
An analogy of the density wave theory (befitting the problems we are having these days) is that of traffic on the freeway in the presence of an accident off to the side of the road. Drivers all slow down as a caution and to "look." The density of cars increases momentarily, but all cars eventually move through the area and resume normal speed beyond the accident. From above, one would see the area where there was the slow-down—even more so if all cars turned on their headlights. Think of this the next time you get trapped in traffic!
Where do these spirals come from? What was responsible for generating the density wave in the first place or for creating the line of newborn stars whose evolution drives the advancing spiral arm? We don't really know. Scientists speculate that (1) instabilities in the gas near the Galactic bulge, (2) the gravitational effects of nearby galaxies, or (3) the possible barlike asymmetry within the bulge itself may have had a big enough influence on the disk to get the process going.
How do we know we live in a spiral galaxy? We have to look at data from observations taken at wavelengths throughout the electromagnetic spectrum. We just have to look just beyond our neighborhood to notice that young stellar objects, emission nebulae, O and B stars, and recently formed open clusters are distributed in a spiral pattern. From this evidence, we conclude that the spiral arms trace young, massive star formation.
Let's take a look at a side view of the Milky Way Galaxy, using the COBE satellite image (NASA) as shown in Fig. 16.8 of your text. Surrounding the disk and bulge of the Galaxy is the halo. The halo is thought to extend many galactic radii away from the center. The halo is made up of very old stars, stars that have been around since the Milky Way Galaxy formed 12-15 billion years ago. As such, they are all reddish in color due to old age and low temperatures. Some of these stars are lone, isolated stars, but many of them are gravitationally bound into the spectacular and fascinating globular clusters (Ch. 13). There is little or no free gas or dust, and no evidence of star formation for billions and billions of years. The objects in the halo are free to orbit in random directions. White dwarfs abound, as do scattered planetary nebulae.
The major component of the halo, however, is not luminous at all. It is called dark matter. Dark matter plays an important role in the Universe, seemingly holding together galaxies, galaxy groups, clusters, and superclusters. Astronomers have been trying for decades to succeed in measuring the amount of dark matter in the Universe and see if there is a sufficient amount to cause the expansion of the Universe to halt and reverse. (Note: when we say that the Universe is expanding, this means it is expanding on large scales, of order 60 million light years or so. Our galaxy is not expanding, nor are the galaxies in the Local Group moving away from each other consistently. Gravity has taken hold across local distances.) We will not discuss dark matter in these notes, but be sure to read about it in the text.
What key discovery opened up the true vastness of our galaxy? The variable stars known as RR Lyrae. RR Lyrae stars are old, low-mass stars that lie on or near the horizontal branch of the H-R Diagram. They are passing through a phase where their atmospheres are unstable. During this stage, however, they pulsate--changing their luminosity in a regular and predictable manner. Astronomers can identify these particular stars, and because their variability is fairly well understood, determine their luminosity. Once we have the luminosity of a star and have measured its apparent magnitude, we have its distance through the now familiar magnitude equation: m – M = 5 log(d) – 5. Here are two actual light curves of RR Lyrae stars. The one of the left is of the prototype star: RR Lyrae itself, with a period of 0.5668 days. The one shown on the right is a newly discovered one (2000), the discovery made by Dr. Scott Anderson and then graduate student Armin Rest of the University of Washington using imaging data from the Sloan Digital Sky Survey (an observational program that mapped the sky). The plot was made from data taken by undergraduates in astronomy using the Monashtash Observatory in Ellensburg, Washington. The period for this star is 0.48588 days, an extremely precise determination. Note that the light curves measure the change in the apparent magnitude of the star and note also the similarity in the way the light curves look. (The data are not shown over time; rather, astronomers do a bit of an "accordion" calculation that essentially folds all of the data so that 1.5 periods are emphasized.)
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Cepheid stars are massive stars that are transversing the top of the H-R diagram as they evolve. They are usually found in the disks of galaxies, in regions of active star formation. Cepheid stars are arguably the most important standard candle (bulb) in the distance ladder used for measuring the size of the Universe. Although Cepheids are intrinsically much more luminous than RR Lyrae stars, usually 100 times as luminous or more, they have the drawback of being found in the crowded regions of galaxies. RR Lyrae stars can be found in uncrowded regions and thus are more ideal for distance determinations, as long as they are not farther than 1-2 million or so light years away. On the other hand, Cepheids have been observed in a galaxy over 50 million light years away!
Once the secrets of the RR Lyrae stars were revealed and that information used to calculate the distances to the globular clusters of our galaxy, we found out that the Sun, most definitely, was not at the center. In the lab, RR Lyrae and the Distance to M4, you used an RR Lyrae variable, Star No. 42 in the globular cluster M4 to find the distance to that cluster. M4 is one of the nearest globular clusters in the sky.
| Property | Disk | Halo | Bulge |
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| Shape | highly flattened | roughly spherical; mildly flattened | football shaped |
| Star Types | young and old | only old stars | young and old stars; more old stars at greater distances from center |
| ISM | gas and dust | no gas or dust | gas and dust, especially in inner regions |
| New Stars? | ongoing star formation | no star formation for past 10 billion years | ongoing star formation in inner regions |
| Dynamics | contents move in circular orbits in the Galactic plane | stars have random orbits in three dimensions | largely random orbits with some net rotation about the center |
| Substructure | spiral arms | no obvious substructure | nucleus; ring of gas and dust near center; bar? |
| Color | overall white color with blue spiral arms | stars reddish in color due to old age and cool temperatures | yellow-white due to mix of stars |
From the textbook: