Term
| The two globular cluster stars have the same age |
|
Definition
| because all stars in a cluster form at about the same time. |
|
|
Term
| These are also the oldest stars |
|
Definition
| because globular clusters usually contain the oldest stars in the universe. |
|
|
Term
|
Definition
|
|
Term
| Hot, blue main-sequence stars must be no more than a few million years old |
|
Definition
| because these stars do not live much longer than that. |
|
|
Term
| A star’s abundance of heavy elements (heavier than helium) depends |
|
Definition
| primarily on when it was born. |
|
|
Term
| Very old stars lacked earlier generations of stars manufacturing heavy elements when they were born, |
|
Definition
| and thus have very low levels. |
|
|
Term
| Globular cluster stars (very old stars) therefore have a very small abundance of heavy elements — |
|
Definition
| typically less than 0.1 percent. |
|
|
Term
| Open clusters in the disk are generally quite young, and their stars therefore have more |
|
Definition
| heavy elements — typically about 2 percent. |
|
|
Term
| Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest. |
|
Definition
|
|
Term
| The central bulge extends out about |
|
Definition
| 5,000 light-years from the galactic center. |
|
|
Term
| Our solar system is about 28,000 light-years from the center, |
|
Definition
| which is about halfway through the disk. |
|
|
Term
| The halo extends to much greater distances than the 50,000 light-year diameter of the disk, |
|
Definition
| which is why this choice is ranked first. |
|
|
Term
| Imagine a photon of light traveling the different paths in the Milky Way described in the following list. Rank the paths based on how much time the photon takes to complete each journey, from longest to shortest. |
|
Definition
|
|
Term
| The light-travel time converts directly to a |
|
Definition
|
|
Term
| Each item below belongs either with the population of disk stars or the population of halo stars of the Milky Way Galaxy. Match each item to the appropriate population. |
|
Definition
|
|
Term
| Where are large dust clouds predominantly located in the galaxy M51? |
|
Definition
| within or on the edges of the spiral arms. |
|
|
Term
| Spiral galaxies, large, star-forming dust clouds are located primarily within or on the edges of |
|
Definition
|
|
Term
| Where are the ionization nebulae predominantly located in the galaxy M51? |
|
Definition
| within or on the edges of the spiral arms of the galaxy. |
|
|
Term
| Like the dust clouds, ionization nebulae are located primarily within or on the edges of |
|
Definition
|
|
Term
| Spiral arms are regions of |
|
Definition
|
|
Term
| Why are the ionization nebulae so bright? |
|
Definition
| They are regions where gas is ionized by hot, young stars. |
|
|
Term
| The gas in ionization nebulae is ionized |
|
Definition
|
|
Term
|
Definition
| are found in star-formation regions of the galaxy. |
|
|
Term
|
Definition
|
|
Term
| Spiral arms are found within the pancake-shaped disk, which surrounds the roughly spherical, |
|
Definition
|
|
Term
| The central bulge is surrounded by the relatively flat disk and the much larger, spherical region called the |
|
Definition
|
|
Term
| Most globular clusters are found in the |
|
Definition
|
|
Term
| Active star formation occurs only in the |
|
Definition
|
|
Term
| The halo therefore contains only old stars that formed long ago, while the disk contains stars of |
|
Definition
|
|
Term
| Which of the following statements correctly summarize key differences between the disk and the halo? |
|
Definition
|
|
Term
| Although dust blocks visible light and most ultraviolet light, it does not block most |
|
Definition
| infrared, radio, or X-ray wavelengths. |
|
|
Term
| There is a great deal of gas and many stars orbiting around the tiny central source known as Sgr A*, which must be a very massive object to hold so much |
|
Definition
| material in orbit around it. |
|
|
Term
| Sgr A* was originally discovered (and named) because it is a source of a great deal of |
|
Definition
| radio emission located in the constellation Sagittarius. |
|
|
Term
| The key to concluding that a object is a black hole is to |
|
Definition
|
|
Term
|
Definition
| measure the orbital period and distance of gas clouds or stars that orbit the object, which then allows us to learn the object's mass by applying Newton's version of Kepler's third law. |
|
|
Term
| Kepler's first law, tells us that bound orbits |
|
Definition
|
|
Term
The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass ([image]) of a central object, in solar masses, from an orbiting object's period ([image]) in years and semimajor axis ([image]) in astronomical units:
[image]
Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object?
Part C: 13 yr
Part D: 800 AU |
|
Definition
3 million solar masses
From Part D you know that the semimajor axis distance is [image], and from Part C you know that the orbital period is [image]. Plugging these values into the given formula gives a mass of 3.0 million solar masses. |
|
|
Term
| You've now found that the central object has a mass of about 3 million solar masses but is no more than about 70 [image] in diameter—which means it cannot be much larger than the size of our planetary system. Why do these facts lead astronomers to conclude that the central object is a black hole? |
|
Definition
| There is no known way to pack so much mass into such a small volume without it collapsing into a black hole. |
|
|
Term
| If we could see our own galaxy from 2 million light-years away, it would appear _________. |
|
Definition
| as a flattened disk with a central bulge and spiral arms |
|
|
Term
| How does the interstellar medium affect our view of most of the galaxy? |
|
Definition
| It prevents us from seeing most of the galactic disk with visible and ultraviolet light. |
|
|
Term
| Interstellar dust absorbs |
|
Definition
|
|
Term
| Applying the Newton's version of Kepler's third law (or the orbital velocity law) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine ______. |
|
Definition
| the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center. |
|
|
Term
| How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk? |
|
Definition
| It should have a higher fraction of elements heavier than hydrogen and helium. |
|
|
Term
| A recently formed star in the disk will have a higher fraction of elements heavier than hydrogen and helium result of |
|
Definition
| the ongoing chemical enrichment of interstellar gas. |
|
|
Term
| Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not? |
|
Definition
| The 10% figure is too high because there are not enough heavy elements to make that much dust. |
|
|
Term
|
Definition
|
|
Term
| Dust grains are made of heavier elements, not from the hydrogen and helium that make up |
|
Definition
| 98% of the chemical content of the galaxy. |
|
|
Term
| In fact, dust represents only about |
|
Definition
| 1% of the mass in molecular clouds. |
|
|
Term
| The most common form of gas in the disk of the Milky Way galaxy is _________. |
|
Definition
|
|
Term
| Atomic hydrogen gas is gas that is too warm for molecules to form, but cool enough so the |
|
Definition
|
|
Term
| How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today? |
|
Definition
| The total amount of gas will be much less than it is today. |
|
|
Term
| With each subsequent generation of stars, some material is |
|
Definition
| "locked away" permanently in brown dwarfs and stellar corpses. |
|
|
Term
| The amount of gas available for recycling gradually |
|
Definition
|
|
Term
| Over time, the star-gas-star cycle leads the gas in the Milky Way to _________. |
|
Definition
| have a greater abundance of heavy elements. |
|
|
Term
| With every cycle, new stars have died and produced heavy elements that are |
|
Definition
|
|
Term
| Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful? |
|
Definition
| the Chandra X-ray Observatory. |
|
|
Term
| Why would the Chandra X-ray Observatory be best at observing the radiation from gas inside an interstellar bubble created by a supernova? |
|
Definition
| Because the gas is very hot and glows in X rays. |
|
|
Term
| f you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see? |
|
Definition
| Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles. |
|
|
Term
| The interstellar medium is quite |
|
Definition
|
|
Term
| What observational evidence supports the galactic fountain model (which describes how gas cycles between the disk of the galaxy and regions high above the disk)? |
|
Definition
| We see hot gas high above the region of the disk near our solar system, along with cool gas that appears to be raining down from the halo. |
|
|
Term
| The gas going upward from the galaxy comes from superbubbles that form from the combined shock waves of many |
|
Definition
|
|
Term
| As the gas rises from the galaxy it cools and |
|
Definition
| gravity eventually brings it back down to the disk. |
|
|
Term
| All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________. |
|
Definition
|
|
Term
| Globular clusters are distributed throughout the halo, and are found in the |
|
Definition
| disk only if they are currently passing through it on their orbits. |
|
|
Term
| Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found _________. |
|
Definition
| only in or near star-forming clouds |
|
|
Term
| Why are blue stars only in or near star-forming cloud? |
|
Definition
| Because blue stars are hot and therefore massive and therefore short-lived, which means they never have time to venture far from the places where they were born. |
|
|
Term
| Which of the following statements comparing halo stars to our Sun is not true? |
|
Definition
| Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime. |
|
|
Term
| Most stars are less massive than the Sun and therefore progress through their stages of life at a |
|
Definition
| much slower rate than the Sun. |
|
|
Term
| Very low-mass stars have more of their main-sequence lives remaining than the Sun, despite their |
|
Definition
|
|
Term
| Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed? |
|
Definition
| It is probably a halo star that is currently passing through the disk. |
|
|
Term
| All the disk stars in our vicinity should be orbiting the galactic center at about the same speed, which is why their speeds relative to the Sun |
|
Definition
|
|
Term
| Halo stars have very different orbits, so when they pass through the disk they have high speeds relative to the |
|
Definition
|
|
Term
| Why do we believe that most of the mass of the Milky Way is in the form of dark matter? |
|
Definition
| The orbital speeds of stars far from the galactic center are surprisingly high. |
|
|
Term
| Orbital speeds of stars far from the galactic center are surprisingly high which suggests |
|
Definition
| that these stars are feeling gravitational effects from unseen matter in the halo. |
|
|
Term
| Spiral arms appear bright because ________. |
|
Definition
| they contain more hot young stars than other parts of the disk |
|
|
Term
| Spiral arms have enhanced density that leads to more |
|
Definition
|
|
Term
| Young hot stars don't live long enough to move far from the |
|
Definition
| places where they are born. |
|
|
Term
| How did star formation likely proceed in the protogalactic cloud that formed the Milky Way? |
|
Definition
| The stars that formed first could orbit the center of the galaxy in any direction at any inclination. |
|
|
Term
| If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen? |
|
Definition
| Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out. |
|
|
Term
| Young hot stars don't live long enough to survive until the |
|
Definition
| enhanced density of gas in a spiral arm (which leads to star formation) passes by. |
|
|
Term
| What is the best evidence for an extremely massive black hole in the center of the Milky Way? |
|
Definition
| The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System. |
|
|
Term
| Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy? |
|
Definition
| It is by far the brightest source of visible light lying in the direction of the galactic center. |
|
|
Term
| We cannot see to the center of the galaxy with |
|
Definition
|
|
Term
| How does the diameter of the disk of Milky Way Galaxy compare to its thickness? |
|
Definition
| The diameter is about 100 times as great as the thickness. |
|
|
Term
| The diameter of the Milky Way is about 100,000 light-years and the thickness about |
|
Definition
| 1,000 light-years that is why the disk appears so thin. |
|
|
Term
| What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy? |
|
Definition
|
|
Term
| The central bulge is even visible to the naked eye, since it makes the Milky Way in the night sky wider in the direction |
|
Definition
| of the galactic center (toward Sagittarius). |
|
|
Term
| The Sun's location in the Milky Way Galaxy is _________. |
|
Definition
| in the galactic disk, roughly halfway between the center and the outer edge of the disk |
|
|
Term
| The Sun lies about 28,000 light-years from the center of the galaxy, which is just over half the roughly |
|
Definition
| 50,000 light-year radius of the disk. |
|
|
Term
| What do we mean by the interstellar medium? |
|
Definition
| the gas and dust that lies in between the stars in the Milky Way galaxy |
|
|
Term
| Interstellar medium is the stuff that |
|
Definition
| occupies the spaces between stars. |
|
|
Term
| What are the Magellanic Clouds? |
|
Definition
| two small galaxies that probably orbit the Milky Way Galaxy |
|
|
Term
| The two Magellanic Clouds are called the |
|
Definition
| Large Magellanic Cloud and the Small Magellanic Cloud. |
|
|
Term
| How do disk stars orbit the center of the galaxy? |
|
Definition
| They all orbit in roughly the same plane and in the same direction. |
|
|
Term
| How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path? |
|
Definition
| by applying Newton's version of Kepler's third law to the orbits of the Sun or other nearby stars around the center of the Galaxy |
|
|
Term
| We use a "simplified" version of Newton's version of Kepler's third law this law that allows us to calculate mass from orbital velocity and distance; this is the "orbital velocity law" derived in Mathematical Insight 19.1 of The Cosmic Perspective. (Perform an example equation.) |
|
Definition
|
|
Term
| Elements heavier than hydrogen and helium constitute about _________ of the mass of the interstellar medium. |
|
Definition
|
|
Term
| The overall chemical composition is about |
|
Definition
| 70% hydrogen, 28% helium, and 2% everything else. |
|
|
Term
| What do we mean by the star-gas-star cycle? |
|
Definition
| It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium. |
|
|
Term
|
Definition
| subatomic particles that travel close the speed of light |
|
|
Term
| Cosmic rays precise origin is still mysterious, but they are likely produced by |
|
Definition
|
|
Term
| The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with _________. |
|
Definition
| radio telescopes observing at a wavelength of 21 centimeters |
|
|
Term
| Radio emission in the 21 cm line is the only significant emission from most |
|
Definition
|
|
Term
| Which of the following analogies best describes how the structure of the galaxy's spiral arms is maintained? |
|
Definition
| Like cars slowing in traffic to look at an accident, stars slow as they pass through the spiral arms. |
|
|
