ASTR EXAM 4
To estimate the age of the Universe we can use the idea that distance = velocity x time, or time = distance/velocity. Using the Hubble law, we can substitute d for the distance to an incoming galaxy and Hd for its velocity and find the travel time t: We have to change the units of H from km/s/Mpc to km/yr/km, or 1/year. It turns out that H = 70 km/s/Mpc = 7.16 x 10^-11 1/year. Use H = 7.16 x 10^-11 to find that the time since the Big Bang is t = 1/H = _____ x 10^10 years, or about 14 billion years. (A more advanced calculation shows that the universe is 13.7 billion years old, so our simpler result was quite good!)
1.4
Active galaxies are galaxies whose centers (nuclei) emit abnormally large amounts of energy from a tiny region in their core. Approximately _____ of all known galaxies are found to be active, but astronomers suspect that all large galaxies may go through active phases during their lives, especially when they are _____ and still actively merging. The centers of these active systems are sometimes called _____ (active galactic nuclei). Active galaxies can be divided into three overlapping classes: Seyfert galaxies, radio galaxies, and quasars.
10% young AGNs
The scale below shows the distance from the Milky Way galaxy (MW) to two other galaxies, G1 and G2. On the second scale, where will the two galaxies be by the time the Universe has expanded to twice its present size (so each galaxy is twice as far from the Milky Way)? During this expansion, galaxy G1 has moved a distance of _____ and galaxy G2 has moved a distance of _____. This means that galaxy G2, which is twice as far away from the Milky Way as galaxy G1, is moving away _____ as fast as galaxy G1. This argument could be made putting the 0 Mpc mark at any other galaxy instead of at the Milky Way. The conclusion is that every observer, no matter where in the Universe he/she/it is, will see the Universe expand ______, obeying _____ Hubble law.
100 Mpc 200 Mpc twice the same way the same
The scale below shows the distance from the Milky Way galaxy (MW) to two other galaxies, G1 and G2. On the second scale, where will the two galaxies be by the time the Universe has expanded to twice its present size (so each galaxy is twice as far from the Milky Way)? After this expansion, galaxy G1 will be at the _____ mark.
200 Mpc
The most distant Cepheid variable star yet observed (in a galaxy called NGC 4603) has a pulsation period of 60 days. Using the period-luminosity law of Homework 11, this star is found to have an absolute magnitude of M = -6.3. This star has an apparent magnitude m = 26.3. Now you can find the star's distance in parsecs. Using the formula: with the apparent magnitude m = 26.3 and the absolute magnitude M = -6.3, the distance to the star is _____ x 10^7 parsecs, over 100 million light years away. (This is considered relatively nearby!)
3.31
Most Type Ia supernovae have about the same absolute magnitude at peak brilliance, M = -19.5. The most distant supernova yet observed has an apparent magnitude m = 23.2. How far away is this supernova? If a supernova at its peak brilliance has apparent magnitude m = 23.2 and absolute magnitude M = -19.5, then the distance d (in parsecs) to the supernova is: In this case, m = -19.5 and M = 23.2, so the distance to the Type Ia supernova is _____ x 10^9 parsecs, over 1 billion light years away.
3.47
Kepler's third law: can be used to determine the mass of the supermassive black hole located at Sgr A*. (The masses m and M are in solar units, the semimajor axis a is in AU, and the orbital period P is in years.) A star named SO-2 (see Figure 16.28) orbits the supermassive black hole with a period of P = 15.6 years. The semimajor axis of the star's orbit is a = 1020 AU. Now you can calculate the value of m + M and find that m + M = _____ x 10^6 solar masses. This is the sum of the masses of SO-2 and the black hole. If we ignore the mass of SO-2 as being negligibly small, then we can conclude that the mass of the supermassive black hole is nearly 5 million solar masses. (Note that 1 million = 1 x 10^6.)
4.36
The scale below shows the distance from the Milky Way galaxy (MW) to two other galaxies, G1 and G2. On the second scale, where will the two galaxies be by the time the Universe has expanded to twice its present size (so each galaxy is twice as far from the Milky Way)? After this expansion, galaxy G2 will be at the _____ mark.
400 Mpc
The diagram shows Hubble's original data from 1929. The Hubble constant H is the slope of the line through Hubble's data. Thus Hubble's own value for H was: H = slope = rise/run = ?/2 = _____ km/s/Mpc Estimate the rise (the vertical dashed line) from the graph (hint: it is between 800 and 1000) and calculate Hubble's value of H. Your result will be much larger than the present value of km/s/Mpc! (Hubble mistakenly used another type of variable star instead of Cepheids.)
466.5
In 1929, Edwin Hubble discovered that almost every galaxy is moving away from us. (The exceptions are a few nearby galaxies, whose motion is influenced by the gravitational attraction of the Milky Way.) This motion is a special one, with galaxies twice as far from us moving away as fast. This motion is described by: where V is the galaxy's recession velocity in kilometers per second, d is the distance to the galaxy in megaparsecs (abbreviated Mpc, 1 Mpc = 1 million parsecs = 3.26 million light years), and H is the Hubble constant in kilometers per second of speed per Mpc of distance. The Hubble constant H tells how fast the distant galaxies are receding. If H = 70 km/s/Mpc, then a galaxy that is 10 Mpc away has a speed of _____ km/s, and a galaxy that is 20 Mpc away has a speed of _____ km/s.
700 1400
Astronomers use Kepler's third law: to measure the mass of our Milky Way galaxy. (The masses m and M are in solar units, the semimajor axis a is in AU, and the orbital period P is in years.) For the Sun orbiting the center of the Milky Way, we have the values a = 8 kpc and P = 220 million years. To use these values in the formula for m + M, we have to convert a into AU and P into years. The values you will use are a = 1.65 x 10^9 AU and P = 2.2 x 10^8 years. Now you can calculate the value of m + M and find that m + M = _____ x 10^10 solar masses. Because the Sun's mass, m, is much, much smaller than the mass of the Milky Way, we can conclude that the Milky Way's mass is about 1 x 10^11 solar masses. It may be more because this calculation includes only the mass that lies within the Sun's orbit around the Milky Way's center. Much more mass may lie beyond the Sun's orbit.
9.3
This is an edge-on view of our Milky Way galaxy. Identify: A _____ B _____ C _____ D _____
A: The position of the Sun (8 kpc from the center) B: the Galactic bulge C: the Galactic disk D: the globular clusters
All living organisms use the the same kinds of atoms for their structure and function. These atoms are not only widely used, but they are also abundant throughout the universe. All life on Earth is based on carbon-based molecules called organic molecules, including _____ acids, proteins, and deoxyribonucleic acid. Deoxyribonucleic acid is better known as _____. Experiments have shown that when molecules like those in Earth's primitive atmosphere - carbon dioxide (CO2), nitrogen (N2), water vapor (H2O), and hydrogen (H2) - are exposed to artificial lightning, they spontaneously assemble into a wide variety of _____ molecules. This shows that the molecular basis of life would have formed naturally on the young Earth. Astronomers believe that life may be abundant in the Universe because primitive life arose quite rapidly on Earth about 3.5 billion years ago, as soon as the era of heavy bombardment by meteoroids ended in the early Solar System. Many interstellar clouds contain a rich mix of _____ molecules, and some meteorites contain _____ acids, so organic molecules could have been brought to the young Earth or other planets by meteorites.
All living organisms use the the same kinds of atoms for their structure and function. These atoms are not only widely used, but they are also abundant throughout the universe. All life on Earth is based on carbon-based molecules called organic molecules, including AMINO acids, proteins, and deoxyribonucleic acid. Deoxyribonucleic acid is better known as DNA. Experiments have shown that when molecules like those in Earth's primitive atmosphere - carbon dioxide (CO2), nitrogen (N2), water vapor (H2O), and hydrogen (H2) - are exposed to artificial lightning, they spontaneously assemble into a wide variety of ORGANIC molecules. This shows that the molecular basis of life would have formed naturally on the young Earth. Astronomers believe that life may be abundant in the Universe because primitive life arose quite rapidly on Earth about 3.5 billion years ago, as soon as the era of heavy bombardment by meteoroids ended in the early Solar System. Many interstellar clouds contain a rich mix of ORGANIC molecules, and some meteorites contain AMINO acids, so organic molecules could have been brought to the young Earth or other planets by meteorites.
American astronomer Harlow Shapley studied the globular star clusters - dense groupings of up to a _____ stars - that orbit the central bulge of the Milky Way. Shapley noticed that the globular clusters _____ scattered uniformly across the whole sky, but are concentrated in the same place that the stars are concentrated, toward the constellation _____. Shapley realized that when looking toward that constellation, he was looking toward the center of our Galaxy. To make his map of the clusters, Shapley needed to know their distance from the Sun. He found the distances to the clusters by observing the variable _____ stars within them using the same method we used in Homework 11. Shapley found that the Sun _____ at the center of the Milky Way galaxy. Today we know that the Sun is about _____ kpc from the Milky Way's center.
American astronomer Harlow Shapley studied the globular star clusters - dense groupings of up to a MILLION stars - that orbit the central bulge of the Milky Way. Shapley noticed that the globular clusters ARE NOT scattered uniformly across the whole sky, but are concentrated in the same place that the stars are concentrated, toward the constellation SAGITTARIUS. Shapley realized that when looking toward that constellation, he was looking toward the center of our Galaxy. To make his map of the clusters, Shapley needed to know their distance from the Sun. He found the distances to the clusters by observing the variable RR LYRAE stars within them using the same method we used in Homework 11. Shapley found that the Sun IS NOT at the center of the Milky Way galaxy. Today we know that the Sun is about EIGHT kpc from the Milky Way's center.
An active galaxy has at its core a supermassive _____ that contains anywhere from millions to up to several billions of solar masses of matter. Around the black hole an _____ as large as the entire Solar System swirls furiously under the intense _____ pull of the black hole. Orbiting under the black hole's fatal attraction, the gas in the accretion disk is heated to incandescence by _____ forces as it collides with other gas in the disk at a significant fraction of the speed of light. Although most of the mass ultimately falls into the black hole, some material from the accretion disk "boils off" at enormously high temperature and escapes. This outflow is channeled into two oppositely directed _____ by the black hole's _____ field. These jets form the radio _____ of radio galaxies.
An active galaxy has at its core a supermassive BLACK HOLE that contains anywhere from millions to up to several billions of solar masses of matter. Around the black hole an ACCRETION DISK as large as the entire Solar System swirls furiously under the intense GRAVITATIONAL pull of the black hole. Orbiting under the black hole's fatal attraction, the gas in the accretion disk is heated to incandescence by FRICTIONAL forces as it collides with other gas in the disk at a significant fraction of the speed of light. Although most of the mass ultimately falls into the black hole, some material from the accretion disk "boils off" at enormously high temperature and escapes. This outflow is channeled into two oppositely directed JETS by the black hole's MAGNETIC field. These jets form the radio LOBES of radio galaxies.
When starlight strikes an interstellar cloud, the cloud may appear blue or red. An emission nebula is a huge gas cloud that emits visible light. Nearby hot, blue stars emit large amounts of _____ radiation that can be absorbed by hydrogen atoms. Ultraviolet photons ionize the hydrogen gas by tearing the _____ free of the nucleus. When the electron recombines with the ionized hydrogen atom, the atom emits a characteristic _____ photon. For this reason, an emission nebula is also called an _____ region. (The H stands for _____, and the Roman numeral II means it is ionized.) HII regions are frequently seen surrounding sites of active star formation, in the _____ of galaxies.
An emission nebula is a huge gas cloud that emits visible light. Nearby hot, blue stars emit large amounts of ULTRAVIOLET radiation that can be absorbed by hydrogen atoms. Ultraviolet photons ionize the hydrogen gas by tearing the ELECTRON free of the nucleus. When the electron recombines with the ionized hydrogen atom, the atom emits a characteristic REDDISH photon. For this reason, an emission nebula is also called an HII region. (The H stands for HYDROGEN, and the Roman numeral II means it is ionized.) HII regions are frequently seen surrounding sites of active star formation, in the SPIRAL ARMS of galaxies.
Astronomers measure the rotation speed of stars as they orbit the the Galaxy at different distances from the center and make a graph called a _____ curve, as shown below. The curve shows that the stars in the disk are moving _____ than would be expected from the observed mass of detectable stars and gas. This discrepancy has led astronomers to hypothesize that our Galaxy is embedded in a huge, massive halo of _____ matter. The _____ matter exerts a gravitational force of the detectable stars and gas, speeding them up, but it emits no as-yet observed radiation of its own. Moreover, observations of its gravitational effects suggest that this immense halo has a radius of at least 100 kiloparsecs, so that it is much _____ than the visible Milky Way. What _____ matter is made of is one of the greatest mysteries in astronomy today.
Astronomers measure the rotation speed of stars as they orbit the the Galaxy at different distances from the center and make a graph called a ROTATION curve, as shown below. The curve shows that the stars in the disk are moving FASTER than would be expected from the observed mass of detectable stars and gas. This discrepancy has led astronomers to hypothesize that our Galaxy is embedded in a huge, massive halo of DARK matter. The DARK matter exerts a gravitational force of the detectable stars and gas, speeding them up, but it emits no as-yet observed radiation of its own. Moreover, observations of its gravitational effects suggest that this immense halo has a radius of at least 100 kiloparsecs, so that it is much LARGER than the visible Milky Way. What DARK matter is made of is one of the greatest mysteries in astronomy today.
Astronomers wanted to know how the speed of the expansion of the Universe changed as the universe aged. To do this, they observed Type Ia _____ blasts in a number of remote galaxies, from which they can find both the distance and the speed of these systems. They found that the expansion in the past was _____ than it is now. That is, the expansion of the Universe seems to be speeding up! Einstein's equations of general relativity can explain this accelerating expansion by using a term call the _____ constant. Einstein himself set this constant to zero, but now it seems that a non-zero value is needed for a correct description of the universal expansion. A non-zero cosmological constant describes an additional energy component of the Universe that is called _____ energy. This energy fills all of space, causing a repulsive force between any two points in space. For two points that are close together, the effect is too small to notice. However, if the two points are twice as far apart, the repulsive force between them is twice as great. As a result, the repulsive force can be very strong over the large distances between galaxies. In this way, dark energy is causing the expansion of the Universe to _____!
Astronomers wanted to know how the speed of the expansion of the Universe changed as the universe aged. To do this, they observed Type Ia SUPERNOVA blasts in a number of remote galaxies, from which they can find both the distance and the speed of these systems. They found that the expansion in the past was SLOWER than it is now. That is, the expansion of the Universe seems to be speeding up! Einstein's equations of general relativity can explain this accelerating expansion by using a term call the COSMOLOGICAL constant. Einstein himself set this constant to zero, but now it seems that a non-zero value is needed for a correct description of the universal expansion. A non-zero cosmological constant describes an additional energy component of the Universe that is called DARK energy. This energy fills all of space, causing a repulsive force between any two points in space. For two points that are close together, the effect is too small to notice. However, if the two points are twice as far apart, the repulsive force between them is twice as great. As a result, the repulsive force can be very strong over the large distances between galaxies. In this way, dark energy is causing the expansion of the Universe to ACCELERATE!
During the first few hundred thousand years of the Universe, when the Universe was opaque and light and matter were constantly interacting, normal matter _____ begin to clump together. However, dark-matter clumps _____ begin to draw together, forming high-density regions, because it does not interact interact with light or normal matter except via _____. These dark-matter clumps provided gravitational "seeds" to attract normal matter as soon as light stopped interacting with matter, after the era of _____ when the CMB was produced. These dark-matter "seeds" left an imprint on the CMB in the form of tiny fluctuations in the _____ of the CMB, at most just a few hundred micro-Kelvin, compared to the average temperature of 2.725 K. (See Figure 18.9 on page 490 of your text.)
During the first few hundred thousand years of the Universe, when the Universe was opaque and light and matter were constantly interacting, normal matter COULD NOT begin to clump together. However, dark-matter clumps COULD begin to draw together, forming high-density regions, because it does not interact interact with light or normal matter except via GRAVITY. These dark-matter clumps provided gravitational "seeds" to attract normal matter as soon as light stopped interacting with matter, after the era of RECOMBINATION when the CMB was produced. These dark-matter "seeds" left an imprint on the CMB in the form of tiny fluctuations in the TEMPERATURE of the CMB, at most just a few hundred micro-Kelvin, compared to the average temperature of 2.725 K. (See Figure 18.9 on page 490 of your text.)
Galaxies are not "island universes." Galaxies are often found in groupings called galaxy _____. The largest groups of galaxies are called _____ clusters because they contain hundreds to thousands of member galaxies. The many collisions in the crowded inner part of rich clusters create giant _____ galaxies as galaxies merge together. In contrast, some galaxy clusters, called _____ clusters, contain relatively few members (sometimes only a dozen or so). Our Milky Way galaxy belongs to a poor cluster of about 40 members called the _____. The great mass of galaxy clusters creates gravitational interactions between them and draws them together into larger structures called _____. A supercluster contains a half dozen to several dozen galaxy clusters spread throughout a region of space tens to hundreds of millions of light-years across. Clusters and superclusters often appear to form chains or shells surrounding regions nearly empty of galaxies. These empty spaces are called _____ and are around 100 - 400 million light-years across. Computer simulations of the development of the large-scale structure of the universe show that without _____ matter, the large-scale distributions of galaxies fails to match what we observe. This provides more evidence for the existence of dark matter.
Galaxies are not "island universes." Galaxies are often found in groupings called galaxy CLUSTERS. The largest groups of galaxies are called RICH clusters because they contain hundreds to thousands of member galaxies. The many collisions in the crowded inner part of rich clusters create giant ELLIPTICAL galaxies as galaxies merge together. In contrast, some galaxy clusters, called POOR clusters, contain relatively few members (sometimes only a dozen or so). Our Milky Way galaxy belongs to a poor cluster of about 40 members called the LOCAL GROUP. The great mass of galaxy clusters creates gravitational interactions between them and draws them together into larger structures called SUPERCLUSTERS. A supercluster contains a half dozen to several dozen galaxy clusters spread throughout a region of space tens to hundreds of millions of light-years across. Clusters and superclusters often appear to form chains or shells surrounding regions nearly empty of galaxies. These empty spaces are called VOIDS and are around 100 - 400 million light-years across. Computer simulations of the development of the large-scale structure of the universe show that without DARK matter, the large-scale distributions of galaxies fails to match what we observe. This provides more evidence for the existence of dark matter.
Neutral hydrogen gas (made of hydrogen atoms with their electrons attached) forms cold _____ clouds. Clouds more than a few tens of light-years from hot stars may be only a few degrees above absolute zero. Such cold material emits no visible light because it has too _____ energy to generate visible photons. However it can emit _____ of very low energy because radio waves carry much _____ energy than visible light does.
HI little radio waves less
In 1965, Arno Penzias and Robert Wilson (shown below in front of their microwave receiver) of Bell Telephone Laboratories accidentally discovered the _____ (CMB). This is the radiation that was released when the Universe became transparent, and arrived at Earth in the form of _____. The CMB has a nearly perfect _____ spectrum. Using Wien's law, astronomers calculate that the temperature that describes the empty space in the Universe is 2.725 K, only a little _____ than absolute zero. We see the same microwave radiation coming from every direction, so it _____ be caused by stars or clouds of gas and dust. It is light that started its journey to Earth when the Universe was a few hundred thousand years old, during the _____ era. The discovery of this radiation, which was predicted, is one of the cornerstones of the Big Bang theory, that the universe was born in a small, hot, dense state and expanded rapidly.
In 1965, Arno Penzias and Robert Wilson (shown below in front of their microwave receiver) of Bell Telephone Laboratories accidentally discovered the COSMIC MICROWAVE BACKGROUND (CMB). This is the radiation that was released when the Universe became transparent, and arrived at Earth in the form of MICROWAVES. The CMB has a nearly perfect BLACKBODY spectrum. Using Wien's law, astronomers calculate that the temperature that describes the empty space in the Universe is 2.725 K, only a little WARMER than absolute zero. We see the same microwave radiation coming from every direction, so it CANNOT be caused by stars or clouds of gas and dust. It is light that started its journey to Earth when the Universe was a few hundred thousand years old, during the RECOMBINATION era. The discovery of this radiation, which was predicted, is one of the cornerstones of the Big Bang theory, that the universe was born in a small, hot, dense state and expanded rapidly.
The object located at the center of our Milky Way galaxy is called Sagittarius A*. What evidence do astronomers have that Sgr A* is a supermassive black hole having several million solar masses? Measurements of this source show that it is less than 10 AU in diameter (about the size of _____ orbit.
Jupiter's
There are three main types of galaxies: spiral galaxies, elliptical galaxies, and irregular galaxies. The spiral galaxies are further divided into (normal) spirals and barred spirals. Identify each photo as a spiral, barred spiral, elliptical, or irregular galaxy. M87 _____ NGC 6822 _____ M 101 _____ NGC 1300 _____
M 87: elliptical galaxy NGC 6822: irregular galaxy M101: spiral galaxy NGC 1300: barred spiral galaxy
There are two candidates for dark matter: One candidate for dark matter is large numbers of black holes and other stellar remnants in the haloes of galaxies, an idea called the _____ hypothesis (massive compact halo object). However, careful observations have shown that MACHOs _____ account for more than abut 10% of the Milky Way's halo mass.
MACHO cannot
Many astronomers think that the presence of two different populations of stars show that star formation _____ occurred continuously in the Milky Way. _____ stars probably formed in a major burst at the time of the Galaxy's birth during its initial _____, whereas _____ stars formed later and continue forming even today.
Many astronomers think that the presence of two different populations of stars show that star formation HAS NOT occurred continuously in the Milky Way. POPULATION II stars probably formed in a major burst at the time of the Galaxy's birth during its initial COLLAPSE, whereas POPULATION I stars formed later and continue forming even today.
Olbers' paradox asks, "Why is the sky dark at night?" If the Universe is infinite in extent and has existed forever, then you should see a star in any direction you look in the sky. This means that the night sky should be _____ instead of _____! The solution to Olbers' paradox is that the Universe has not existed forever; it was created about _____ billion years ago in the Big Bang. The light from any object more than 13.7 billion light years away _____ had time to reach us, and so the sky looks _____ at night!
Olbers' paradox asks, "Why is the sky dark at night?" If the Universe is infinite in extent and has existed forever, then you should see a star in any direction you look in the sky. This means that the night sky should be BRIGHT instead of DARK! The solution to Olbers' paradox is that the Universe has not existed forever; it was created about 13.7 billion years ago in the Big Bang. The light from any object more than 13.7 billion light years away HAS NOT had time to reach us, and so the sky looks DARK at night!
Some of the Milky Way's stars are bound together gravitationally into groups called star clusters. There are two types of star clusters. Open clusters contain up to a few _____ stars that are scattered loosely in space in a volume with a radius of typically 7 to 20 light years. They are sometimes called "Galactic clusters" because most of them lie in the _____ of the Galaxy. The stars in open clusters are usually _____ stars, and they trace the Galaxy's spiral arms. Astronomers think that open clusters form when giant, cold, interstellar gas clouds move into the Galaxy's spiral arms. The clouds are compressed by the stronger _____ field with the arm and collapse, breaking up into hundreds of stars whose mutual gravity binds them into the cluster. Once formed, an open cluster continues to orbit the Galaxy in the _____, but over hundreds of millions of years, stars gradually escape from it and so the cluster eventually dissolves. Our own Sun probably once was a member of such an open star cluster, but its companion stars have long since scattered across space.
Open clusters contain up to a few HUNDRED stars that are scattered loosely in space in a volume with a radius of typically 7 to 20 light years. They are sometimes called "Galactic clusters" because most of them lie in the DISK of the Galaxy. The stars in open clusters are usually POPULATION I stars, and they trace the Galaxy's spiral arms. Astronomers think that open clusters form when giant, cold, interstellar gas clouds move into the Galaxy's spiral arms. The clouds are compressed by the stronger GRAVITATIONAL field with the arm and collapse, breaking up into hundreds of stars whose mutual gravity binds them into the cluster. Once formed, an open cluster continues to orbit the Galaxy in the DISK, but over hundreds of millions of years, stars gradually escape from it and so the cluster eventually dissolves. Our own Sun probably once was a member of such an open star cluster, but its companion stars have long since scattered across space.
Our Milky Way galaxy is a huge collection of several hundred billion stars. The Milky Way is about _____ kpc in diameter, and the disk is about _____ kpc thick. [1 kiloparsec (kpc) = 1000 parsecs.] The Sun moves around the center of the Milky Way with a speed of about _____ kilometers per second, and it takes about _____ million years to complete one trip around it.
Our Milky Way galaxy is a huge collection of several hundred billion stars. The Milky Way is about 30 kpc in diameter, and the disk is about 1 kpc thick. [1 kiloparsec (kpc) = 1000 parsecs.] The Sun moves around the center of the Milky Way with a speed of about 220 kilometers per second, and it takes about 220 million years to complete one trip around it.
In principle, physical laws can be applied back to a time as early as about 10^-43 second after the Big Bang, knows as the _____ time. The theories of science cannot yet handle times earlier than the Planck time. When the Universe was 10^-35 second old, everything in our visible Universe today was packed into a volume smaller than a proton, and the radiation and matter had a temperature of about 10^27 K. Under these extraordinary conditions, something happened that drove the Universe's expansion at an ever accelerating rate, which cosmologists call _____. Space may have been filled everywhere with a constant energy until inflation ended, when the Universe was 10^-32 second old. During inflation the universe expanded incredibly fast, becoming 10^25 times larger, from the size of a proton to a few centimeters in size.
Planck inflation
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 10(-43) second and T = 10^(32) K: _____
Planck time
Do the following statements about stars refer to Population I stars or Population II stars? These stars are generally blue.
Population I
Do the following statements about stars refer to Population I stars or Population II stars? These stars are in the Milky Way's disk and concentrated in its spiral arms.
Population I
Do the following statements about stars refer to Population I stars or Population II stars? These stars can be young or old.
Population I
Do the following statements about stars refer to Population I stars or Population II stars? These stars have a high heavy-element content, similar to our Sun's.
Population I
Do the following statements about stars refer to Population I stars or Population II stars? These stars have approximately circular orbits in the Milky Way's disk.
Population I
Do the following statements about stars refer to Population I stars or Population II stars? These stars are found in the Milky Way's halo and bulge.
Population II
Do the following statements about stars refer to Population I stars or Population II stars? These stars are found plunging through the Milky Way's disk.
Population II
Do the following statements about stars refer to Population I stars or Population II stars? These stars are generally red.
Population II
Do the following statements about stars refer to Population I stars or Population II stars? These stars are old.
Population II
Do the following statements about stars refer to Population I stars or Population II stars? These stars have a low heavy-element content, less than 1 percent of our Sun's.
Population II
Quasars, like Seyfert galaxies, fluctuate in brightness. Large changes in brightness can occur within hours. Rapid changes of this sort give clues to the _____ of the emitting object. The brightness from an emitting object _____ change instantaneously because light from its far side takes longer to reach Earth then light from its near side. The change in brightness therefore _____ be faster than the time it takes light to travel across the emitting object, from one side to the other. If a quasar varies in brightness in a time of a few days, then the emitting region can be no more than a few _____ across. An emitting core a few light-days across is _____ compared to the size of a typical galaxy (100,00 light-years across). In fact, such a core is about 10 million times smaller than a galaxy, or abut 200 AU in diameter, about 5 times the size of Pluto's orbit around the Sun. These findings tell us that some quasars have the power output of _____ of stars packed into a volume smaller than the Solar System!
Quasars, like Seyfert galaxies, fluctuate in brightness. Large changes in brightness can occur within hours. Rapid changes of this sort give clues to the SIZE of the emitting object. The brightness from an emitting object CANNOT change instantaneously because light from its far side takes longer to reach Earth then light from its near side. The change in brightness therefore CANNOT be faster than the time it takes light to travel across the emitting object, from one side to the other. If a quasar varies in brightness in a time of a few days, then the emitting region can be no more than a few LIGHT-DAYS across. An emitting core a few light-days across is TINY compared to the size of a typical galaxy (100,00 light-years across). In fact, such a core is about 10 million times smaller than a galaxy, or abut 200 AU in diameter, about 5 times the size of Pluto's orbit around the Sun. These findings tell us that some quasars have the power output of TRILLIONS of stars packed into a volume smaller than the Solar System!
Even if life is abundant in the universe, intelligent life could be either common or rare. One of the biggest uncertainties is the lifetime of a technologically advanced civilization. How long would such a civilization last before they destroyed themselves by their lack of wisdom and foresight? The _____ (insert the acronym that stands for Search for Extraterrestrial Intelligence) project has been searching for radio signals from an extraterrestrial civilization since 1992, but so far none has been found.
SETI
Some spiral galaxies have very ragged-appearing arms. These features may be caused by self-propagating _____ formation. In this theory, star formation starts at some random point in the _____ of a galaxy. As these stars heat the gas around them and explode as _____, they generate a disturbance that makes the surrounding gas clouds collapse and turn into _____. The region of star formation spreads across the galaxy's disk. Because stars nearer the center of a galaxy orbit in _____ time than those farther out, the region of star formation is drawn out into a spiral.
Some spiral galaxies have very ragged-appearing arms. These features may be caused by self-propagating STAR formation. In this theory, star formation starts at some random point in the DISK of a galaxy. As these stars heat the gas around them and explode as SUPERNOVAS, they generate a disturbance that makes the surrounding gas clouds collapse and turn into STARS. The region of star formation spreads across the galaxy's disk. Because stars nearer the center of a galaxy orbit in LESS time than those farther out, the region of star formation is drawn out into a spiral.
Spiral and elliptical galaxies range enormously in size; some are large, and some are small. However, spiral and elliptical galaxies have different contents. Spiral galaxies contain a mix of young and old stars (Population I and II), but elliptical galaxies contain mostly _____ (Population II) stars. This difference in the kinds of stars composing spiral and elliptical galaxies is understandable in terms of their different gas content. To make _____ (Population I) stars, a galaxy must contain dense clouds of gas and dust. Spiral galaxies have about _____ of the mass of their disk in the form of such interstellar clouds, and so they can _____ make the young stars of their spiral arms. On the other hand, ellipticals contain much _____ interstellar matter. Many elliptical galaxies contain very low density but very hot (10^7 K) gas. Such hot gas _____ readily form stars, and so it is no surprise that elliptical galaxies _____ contain young, blue stars like the ones we see in the disks of spiral galaxies.
Spiral and elliptical galaxies range enormously in size; some are large, and some are small. However, spiral and elliptical galaxies have different contents. Spiral galaxies contain a mix of young and old stars (Population I and II), but elliptical galaxies contain mostly OLD (Population II) stars. This difference in the kinds of stars composing spiral and elliptical galaxies is understandable in terms of their different gas content. To make YOUNG (Population I) stars, a galaxy must contain dense clouds of gas and dust. Spiral galaxies have about 15% of the mass of their disk in the form of such interstellar clouds, and so they can EASILY make the young stars of their spiral arms. On the other hand, ellipticals contain much LESS interstellar matter. Many elliptical galaxies contain very low density but very hot (10^7 K) gas. Such hot gas DOES NOT readily form stars, and so it is no surprise that elliptical galaxies RARELY contain young, blue stars like the ones we see in the disks of spiral galaxies.
The categorization of spiral galaxies into barred and unbarred spirals is now seen as a _____-term difference. Computer models of galaxies show that most disk-shaped systems will form a bar if they undergo a _____ disturbance, perhaps as the result of a close encounter with a neighboring galaxy. Our own Milky Way has a _____ bar. Some computer models also suggest that over hundreds of millions of years, the orbits spread out again and the bar _____. Thus, bars may appear and disappear, and so an ordinary spiral galaxy _____ change into a barred spiral and then back again
The categorization of spiral galaxies into barred and unbarred spirals is now seen as a SHORT-term difference. Computer models of galaxies show that most disk-shaped systems will form a bar if they undergo a GRAVITATIONAL disturbance, perhaps as the result of a close encounter with a neighboring galaxy. Our own Milky Way has a WEAK bar. Some computer models also suggest that over hundreds of millions of years, the orbits spread out again and the bar DISSOLVES. Thus, bars may appear and disappear, and so an ordinary spiral galaxy MAY change into a barred spiral and then back again
The motion of galaxies away from us is caused by the expansion of the Universe. This expansion is the result of _____ itself expanding. It _____ like a bomb burst flinging fragments in all directions from a single central location. It is like a giant loaf of raisin bread expanding as it rises. As the dough rises, the raisins embedded in the dough all move farther away from each other. The raisins _____ moving through the dough; they are being carried along with the dough as the bread expands and rises. In the same way, distant receding galaxies _____ moving away from each other through space. They are being carried along with the space as the space expands. In the raisin bread, each raisin sees every other raisin moving away from it, with raisins twice as far away moving away twice as fast. However, no raisin can say it is at the center of the expansion of the dough because every raisin is moving away from every other raisin. In the same way, astronomers on Earth see every galaxy moving away from us, with galaxies twice as far from us moving away twice as fast, as described by the Hubble law:
The motion of galaxies away from us is caused by the expansion of the Universe. This expansion is the result of SPACE itself expanding. It IS NOT like a bomb burst flinging fragments in all directions from a single central location. It is like a giant loaf of raisin bread expanding as it rises. As the dough rises, the raisins embedded in the dough all move farther away from each other. The raisins ARE NOT moving through the dough; they are being carried along with the dough as the bread expands and rises. In the same way, distant receding galaxies ARE NOT moving away from each other through space. They are being carried along with the space as the space expands. In the raisin bread, each raisin sees every other raisin moving away from it, with raisins twice as far away moving away twice as fast. However, no raisin can say it is at the center of the expansion of the dough because every raisin is moving away from every other raisin. In the same way, astronomers on Earth see every galaxy moving away from us, with galaxies twice as far from us moving away twice as fast, as described by the Hubble law:
The oldest stars in our galaxy have an age of approximately _____ billion years, which astronomers take as the age of the Milky Way itself. Our Galaxy's stars _____ all created at a single time in the distant past. On average, approximately 7 or 8 stars are born in the Milky Way each year, although stars probably were born at a much _____ rate when our galaxy first formed. As we learned in Module 5, the age of the Solar System is about _____ billion years old.
The oldest stars in our galaxy have an age of approximately 13 billion years, which astronomers take as the age of the Milky Way itself. Our Galaxy's stars WERE NOT all created at a single time in the distant past. On average, approximately 7 or 8 stars are born in the Milky Way each year, although stars probably were born at a much HIGHER rate when our galaxy first formed. As we learned in Module 5, the age of the Solar System is about 4.5 billion years old.
The space between stars is not empty; it contains interstellar matter composed of _____ and _____ particles. On average, each cubic centimeter of space contains only a few atoms of gas, compared with the air we breathe, which contains about 10^19 atoms per cubic centimeter. Gravity has pulled most of the interstellar matter into a thin layer in the _____ of our Galaxy. Within this layer, gravity and gas pressure have further clumped the dust and gas into clouds whose sizes range from a few light years to a few hundred light years. The gas in interstellar clouds has a composition similar to that of the Sun and other stars, namely about 71% _____ and 27% _____. The tiny dust particles are made of silicates and _____ compounds, coated with a thin layer of _____ made of water, carbon monoxide, and methyl alcohol. If the cloud is extremely thick or dense, it may dim the light of background stars so the stars cannot be seen through the cloud. This is called a _____ nebula.
The space between stars is not empty; it contains interstellar matter composed of GAS and DUST particles. On average, each cubic centimeter of space contains only a few atoms of gas, compared with the air we breathe, which contains about 10^19 atoms per cubic centimeter. Gravity has pulled most of the interstellar matter into a thin layer in the DISK of our Galaxy. Within this layer, gravity and gas pressure have further clumped the dust and gas into clouds whose sizes range from a few light years to a few hundred light years. The gas in interstellar clouds has a composition similar to that of the Sun and other stars, namely about 71% HYDROGEN and 27% HELIUM. The tiny dust particles are made of silicates and CARBON compounds, coated with a thin layer of ICES made of water, carbon monoxide, and methyl alcohol. If the cloud is extremely thick or dense, it may dim the light of background stars so the stars cannot be seen through the cloud. This is called a DARK nebula.
The spiral arms of other galaxies are demarcated by their Population I stars, especially the hot blue ones of spectral classes O and B. These stars have very _____ lifetimes; they do not live long enough to travel very far from their birthplace. Therefore the locations we observe for these stars show where they _____. This tells us that most of a spiral galaxy's stars form in its _____. By measuring the location of O and B stars near the Sun, astronomers can make a crude picture of the spiral arms near us, even though they cannot view the arms directly. Such maps were the first evidence that we live in a _____ Galaxy.
The spiral arms of other galaxies are demarcated by their Population I stars, especially the hot blue ones of spectral classes O and B. These stars have very SHORT lifetimes; they do not live long enough to travel very far from their birthplace. Therefore the locations we observe for these stars show where they WERE BORN. This tells us that most of a spiral galaxy's stars form in its SPIRAL ARMS. By measuring the location of O and B stars near the Sun, astronomers can make a crude picture of the spiral arms near us, even though they cannot view the arms directly. Such maps were the first evidence that we live in a SPIRAL Galaxy.
The study of other galaxies began in the eighteenth century, when the French astronomer Charles _____ accidentally discovered many galaxies during his searches for new _____. He noticed a large number of faint, diffuse patch of light, and to avoid confusing them with comets, he assigned them numbers and made a catalog of their positions. Although many of Messier's objects have since been identified as star clusters or glowing gas clouds in the Milky Way, several dozen are _____. These and other objects in Messier's catalog are still known by their Messier, or _____ numbers, such as M31, the _____ galaxy. In the nineteenth century, other astronomers, such as Sir William Herschel, began systematically to map the heavens. Herschel's work is now known as the New _____ Catalog (or NGC for short), and it gives names to many galaxies, such as NGC 1275.
The study of other galaxies began in the eighteenth century, when the French astronomer Charles MESSIER accidentally discovered many galaxies during his searches for new COMETS. He noticed a large number of faint, diffuse patch of light, and to avoid confusing them with comets, he assigned them numbers and made a catalog of their positions. Although many of Messier's objects have since been identified as star clusters or glowing gas clouds in the Milky Way, several dozen are GALAXIES. These and other objects in Messier's catalog are still known by their Messier, or M numbers, such as M31, the ANDROMEDA galaxy. In the nineteenth century, other astronomers, such as Sir William Herschel, began systematically to map the heavens. Herschel's work is now known as the New GENERAL Catalog (or NGC for short), and it gives names to many galaxies, such as NGC 1275.
To better understand the geometry of the Universe, astronomers compared the observed average density of the Universe with a theoretically calculated critical density. If the observed density is greater than the critical density, the Universe has positive curvature and is _____; if it is less, the Universe has negative curvature and is _____. If the observed density equals the critical density, the universe has zero curvature and is _____. If astronomers include only normal matter and dark matter, the actual density of matter is approximately 30% of the critical density. However, using the relation E = mc^2, astronomers find that dark energy has the equivalent of more than twice as much mass as that of all the other mass in the Universe, dark and observable, combined. When dark energy is included, the observed average density of the universe is _____ the critical density, and is a further confirmation that the Universe is _____.
To better understand the geometry of the Universe, astronomers compared the observed average density of the Universe with a theoretically calculated critical density. If the observed density is greater than the critical density, the Universe has positive curvature and is CLOSED; if it is less, the Universe has negative curvature and is OPEN. If the observed density equals the critical density, the universe has zero curvature and is FLAT. If astronomers include only normal matter and dark matter, the actual density of matter is approximately 30% of the critical density. However, using the relation E = mc^2, astronomers find that dark energy has the equivalent of more than twice as much mass as that of all the other mass in the Universe, dark and observable, combined. When dark energy is included, the observed average density of the universe is EQUAL TO the critical density, and is a further confirmation that the Universe is FLAT.
There are two candidates for dark matter: The other candidate for dark matter is a kind of matter unlike the matter we know. This is supported by particle physicists who suspect that there may be other forms of matter not yet observed in the laboratory, such as "weakly interacting massive particles," or _____. These are promising candidates because they do not interact with normal matter except through their _____ pull. It is beginning to look as though galaxies are actually large self-gravitating clouds of WIMPs in which a sprinkling of "normal" matter has formed the stars and clouds that we can see.
WIMPs gravitational
When galaxies collide, the _____ force of the galaxies alters the orbits of their stars and thereby changes the shape of each galaxy. Although interactions of this sort are disruptive to galaxies, they _____ harm the individual stars. Stars are so _____ within a galaxy that most stars move harmlessly past each other. Thus, galaxy collisions are something like tossing two handfuls of sand together: most of the grains simply pass by one another without hitting. Clouds of gas and dust in the galaxies are not so lucky. Being much _____ than stars, clouds _____ collide, and their impact may compress them enough to trigger a burst of star formation within them. Such " _____ galaxies," as these collisionally stimulated galaxies are called, are some of the _____ luminous galaxies known.
When galaxies collide, the GRAVITATIONAL force of the galaxies alters the orbits of their stars and thereby changes the shape of each galaxy. Although interactions of this sort are disruptive to galaxies, they RARELY harm the individual stars. Stars are so FAR APART within a galaxy that most stars move harmlessly past each other. Thus, galaxy collisions are something like tossing two handfuls of sand together: most of the grains simply pass by one another without hitting. Clouds of gas and dust in the galaxies are not so lucky. Being much LARGER than stars, clouds DO collide, and their impact may compress them enough to trigger a burst of star formation within them. Such " STARBURST galaxies," as these collisionally stimulated galaxies are called, are some of the MOST luminous galaxies known.
The 21-centimeter radiation has proved extremely valuable for studying the Milky Way and other galaxies. Hydrogen is _____ in space, and so the signal of the 21-centimeter radiation is strong. From the strength of the signal, astronomers can deduce the amount of hydrogen in an area. If the gas is moving, the wavelength will be Doppler-shifted, from which the gas's _____ along our line of sight can be found. Radio observations using radio telescopes thereby allow astronomers to map not only where the gas is concentrated in our Galaxy, but also how it is moving.
abundant speed
Today astronomers have evidence that many (perhaps even all) large galaxies have a massive _____ at their core. However, _____ of the galaxies close to us are active as quasars. Because quasars are so distant, we see them the way they were in the distant past, not the way they are "now." For example, when we look at a quasar that is 10 billion light-years away, we are seeing light that has taken 10 billion years to reach us. Thus, we are seeing the quasar as it was 10 billion years ago. Galaxies whose cores were once active quasars probably surround us, but their activity has long since died away. They _____ harbor huge black holes, but they have used up the gas that powered their activity.
black hole none still
Astronomers call the process of a large galaxy capturing and absorbing a small galaxy galactic _____. Galactic cannibalism is one of the main mechanisms by which galaxies grow. Most galaxies start out as small, gas-rich systems, which actively form stars and are therefore _____ in color. Subsequently, when these galaxies collide and grow into larger systems, they undergo bursts of star formation, and perhaps consume much of their gas. If enough stars form at once, the _____ explosions of many massive stars may drive gas out of the galaxy.
cannibalism blue supernova
Further evidence of _____ matter comes from observations of massive galaxies and clusters of galaxies. These galaxies and clusters may form a _____ lens. If the light from a distant object pass by an extremely massive galaxy or cluster on their way to Earth, the light will bend as it passes through the curved space around the massive galaxy or cluster. The light will then produce images of the distant object. From the positions of the images, the total mass of the massive galaxy or cluster can be determined. Astronomers have found that massive galaxies and clusters contain large amounts of _____ matter.
dark gravitational dark
The spiral arms of most spiral galaxies are caused by spiral _____-waves. The stars in the Galaxy's _____ orbit in the same direction and in nearly the same plane, traveling in (almost) elliptical orbits. However, the axis of each orbit is precessed; each one has a slightly different direction. This produces an overall spiral pattern (shown below). The spiral pattern is where stars and gas happen to come closer together in a spiral _____-wave. This compression causes interstellar clouds to collapse and form _____.
density disk density stars
On the other hand, if the speed of a galaxy is known (from its Doppler-shifted spectral lines), then Hubble's Law in the form: tells the _____ to that galaxy. If a recession velocity of V = 57,400 km/s is measured for a galaxy and H = 70 km/s/Mpc, then its distance is _____ Mpc. This is how the distances to the farthest objects in the universe are measured.
distance 820
Although space expands, the objects themselves - galaxies, stars, planets, and so forth - _____ expand. Gravity and other forces maintain such material bodies at a relative constant size. On the other hand, light waves _____ altered by the expansion of space, and that expansion creates the _____ of the galaxies that we observe. As a light wave from a galaxy moves through space, it is _____ as the space expands, just as the coils in a spring are stretched if you pull on the ends.
do not are redshift stretched
However, the Milky Way galaxy did not form in isolation. It grew to its present size by merging with smaller galaxies. A number of dim, _____ galaxies orbit very close to the Milky Way. Several of them are so close that the Milky Way's _____ is tearing them apart, leaving long faint streams of stars within the Milky Way's _____. These dwarf galaxies and streams of stars are evidence that the Milky Way swallowed smaller galaxies as it grew to its present size. In fact, in about 5 billion years, the Milky Way itself will be "eaten" by its nearest large neighboring galaxy _____ (also called the Andromeda galaxy because it is found in the constellation of Andromeda).
dwarf gravity halo M31
The cold HI clouds emit radio wave with a wavelength of 21 centimeters. This is called 21-centimeter radiation, and is emitted when the _____ in the neutral hydrogen atom flips over as it orbits the central proton.
electron
Through repeated cannibalism, a galaxy may grow into a giant, which perhaps explains why abnormally large _____ galaxies are found at the centers of groups and clusters of galaxies. Such interactions potentially allow some ellipticals to turn back into spirals. This can happen if an elliptical captures a gas-rich system. The gas then becomes the _____ of a "new" spiral, while the elliptical becomes its _____.
elliptical disk bulge
If the collision/merger leaves little gas remaining in the system, an _____ galaxy results. Without gas to form new stars, the elliptical galaxy grows _____ with time.
elliptical redder
Astronomers observe that the stars in the outer part of the disk of spiral galaxies (including our Milky Way galaxy) orbit the galactic nucleus _____ than expected from Kepler's laws (see Figure 17.40 on page 473). The rotation curves of spiral galaxies _____ drop to low velocities as expected. This is evidence that our Milky Way galaxy and other galaxies must contain large amounts of unseen mass in their outer parts. This unseen mass is known as _____ matter.
faster do not dark
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 10^8 years and T = 100 K: _____
first stars and galaxies form
The theory of inflation solves two problems in cosmology: The _____ problem: why is the universe flat to such a high precision? Inflation's answer is that the rapid inflation of the early Universe flattened any initial curvature of _____ the Universe may have started with.
flatness space
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 10^(14) years and T = 0.01 K: _____
fuel runs out, leaving only dark stars and galaxies
When the Universe was very young, it consisted of protons, neutrons, and electrons. The Universe was hot and dense enough for nuclear reactions to _____ some of the protons and neutrons into helium. Current calculations show that the amount of helium and other light elements formed depends on how hot and dense the Universe was and how long those conditions lasted. The conditions in this early time can be related to the density of matter in the Universe today. If the Universe was lower in density, _____ 4He would have been produced, because the particles would not have collided and fused as often. This would also leave _____ amounts of some partial fusion products like deuterium (2H) and 3He. By about 10 minutes after the expansion began, the Universe had cooled to about 1 billion K and spread apart so much that fusion stopped, leaving about _____ of the matter transformed into 4He, in excellent agreement with the observed amount of helium in the Universe. The amounts of other light elements and isotopes also agree well with predictions based on the Big Bang theory. This provides further strong confirmation of the Big Bang theory.
fuse less higher 24%
Astronomers have concluded that a large galaxy like the Milky way began as a vast, slowly rotating _____, perhaps a million light years in diameter and containing about 1011 solar masses of gas. The cloud was composed of almost pure _____ because no stars yet existed to make the heavy elements. Clumps of gas within the cloud gradually formed massive, short-lived stars that blew up as _____, adding the first _____ elements to the gas cloud even before it had finished collapsing.
gas cloud hydrogen and helium supernovas heavy
Some of the Milky Way's stars are bound together _____ into groups called star clusters. There are two types of star clusters: open clusters and globular clusters
gravitationally
As the collapse continued, more stars formed, now with at least some _____ elements. Because these stars were formed from infalling gas, they follow elongated "plunging" orbits - becoming the Population II stars that we see today. Some of the gas survived to form a rotating _____. That gas disk, now rich in _____ elements from the death of Population II stars, then began to form the first Population I stars that orbit the center of the Galaxy.
heavy disk heavy
The theory of inflation solves two problems in cosmology: The _____ problem: why is the temperature of the CMB so uniform? If we look at the radiation coming from one spot in the sky and then look at radiation coming from exactly the opposite directing in the sky, the two locations are separated by the diameter of the observable Universe - over 27 billion light years. But the Universe is less than 14 billion years old, so there seems to have been no time when these regions could have "communicated" to balance their temperatures. Inflation solves this in the period before inflation began. The entire visible Universe today was packed together in such a _____ volume during the first 10-35 second that light could travel across it thousands of times, smoothing out any differences that may have existed. That in turn makes the temperature of the cosmic microwave background almost exactly the same in every direction.
horizon small
Because the Universe is expanding, as we look farther away we are also seeing the Universe when it was younger and more dense. The enormous compression of the early Universe suggests that it was extremely _____, as hot and dense as the interior of a star. Such hot conditions mean that electrons had not yet combined with the numerous hydrogen and helium nuclei, and so the Universe was very opaque to light. After the young Universe had expanded for several hundred thousand years, it had cooled and expanded enough that the Universe became transparent. Astronomers often call this the _____ era because the ionized atoms would then have combined with their electrons to make a transparent neutral gas. From this time onward, light was able to travel large distances and reach us today, although with its wavelength greatly _____ by the expansion of the Universe so it arrives at Earth in for form of microwaves..
hot recombination stretched
In an HII region, the ionized _____ atoms have lost their electrons. The freed electrons are a powerful source of _____ emission, emission that helps astronomers see HII regions that might otherwise be hidden from us by dust. Because these HII regions are generally located in _____, radio maps of their location can reveal the spiral structure of our Galaxy. In fact, maps made by combining radio telescope observations and optical telescope observations of HII regions are the best evidence we have that the Milky Way has _____.
hydrogen radio spiral arms spiral arms
Earth _____ at the center of the expansion of the Universe because every galaxy is moving away from every other galaxy. An observer in any other galaxy would see _____ expansion that we see.
is not the same
Some of the Milky Way's stars are bound together gravitationally into groups called star clusters. There are two types of star clusters. Globular clusters are dense balls of up to a few _____ stars. These stars are the _____ stars in the Galaxy and are always _____ stars. Globular clusters are about 40 to 160 light years in radius. Between 150 to 200 of these clusters orbit the central bulge of the Milky Way and outline the galactic halo of stars and tenuous gas.
million oldest Population II
There are three possibilities for the overall geometry of space. Suppose two light beams start out moving parallel to each other. Three things could happen: In a Universe where space has _____ curvature, the light beams will gradually move away from each other. This is called an _____ universe.
negative open
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 3 minutes and T = 10^9 K: _____
neutrons and protons fuse to form helium nuclei
There are three possibilities for the overall geometry of space. Suppose two light beams start out moving parallel to each other. Three things could happen: In a Universe where space has _____ curvature, the light beams will gradually move toward each other until the beams converge. This is called a _____ universe.
positive closed
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 10^(-6) second and T = 10^(13) K: _____
quarks combine to for protons and neutrons
The object located at the center of our Milky Way galaxy is called Sagittarius A*. What evidence do astronomers have that Sgr A* is a supermassive black hole having several million solar masses? An intense source of _____ waves is located at Sgr A*.
radio
Radio galaxies emit large amounts of energy in the _____ part of the electromagnetic spectrum, typically millions of times more than the _____ energy emitted by a normal galaxy. Most are _____ galaxies, and the radio emission comes primarily from immense regions outside the galaxy on either side of it. These regions are called radio _____.
radio radio elliptical lobes
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 10^(-33) second and T = 10^(25) K: _____
rapid expansion called inflation
Although young blue stars are _____ in elliptical systems, such stars are _____ in irregular galaxies. Many of these irregular galaxies also contain large amounts of interstellar matter. In fact, some have more mass in gas than in stars. Accordingly, astronomers think that irregular galaxies, though possibly just as old as other types, have simply _____ used up much of their gas in making stars.
rare common not yet
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 4 x 10^5 years and T = 3000 K: _____
recombination: electrons combine with hydrogen and helium nuclei to form hydrogen and helium atoms
A quasar is an extremely luminous, extremely distant, active galaxy. The first observations of quasars showed only an almost starlike image, and the name is short for "quasi-stellar radio source." Their spectra have the largest _____ known for any kind of astronomical object. On the basis of their _____, the most distant ones are more than 10 billion light-years from Earth. A typical quasar turns out to be about 1000 times more luminous than the Milky Way.
redshift redshift
When starlight strikes an interstellar cloud, the cloud may appear blue or red. A reflection nebula is produced when starlight is _____m from a huge cloud of dust. A reflection nebula has a characteristic _____ color because light is scattered more efficiently for _____ light.
reflected blue blue
In February 2003 the results of a new high-resolution survey of the cosmic microwave background (CMB) were announced. Ten years later in 2013, the results of nine additional years of observing the CMB were released. These results from the Wilkinson Microwave Anisotropy Probe (WMAP), shown below, reveal that the universe is composed of: What we see is that ordinary matter (atoms) is a very _____ portion of the Universe's composition, and even dark matter is outweighed by dark _____. The largest component is dark _____, which drives the Universe to expand faster and faster. Thus the Universe appears to be mostly made of dark _____! The results also show that the Hubble constant is 69.3 km/s/Mpc (accurate to 1 percent), and that the Universe is 13.77 billion years old (accurate to 0.4 percent). Cosmology has become a precise science.
small energy energy energy
Figure 17.18 on page 458 of your text shows images of distant galaxies. Not only are these young galaxies _____, they are _____ numerous than ordinary galaxies today. These remote galaxies show signs of disturbance by neighboring galaxies - the odd shapes, bent arms, and twisted disks caused by collisions (see below) - much _____ frequently than nearby galaxies. Astronomers think that these small young galaxies collide and merge into bigger systems.
small more more
Seyfert galaxy is a _____ galaxy whose nucleus is remarkably luminous. The luminosity of the nucleus of a typical Seyfert galaxy is immense, amounting to the entire radiation output of the Milky Way, but coming from a region less than a light-year across.
spiral
If the collision/merger leaves moderate to large amounts of gas in the system, a large _____ galaxy results. The spiral continues to form stars and remains relatively _____ in color.
spiral blue
The object located at the center of our Milky Way galaxy is called Sagittarius A*. What evidence do astronomers have that Sgr A* is a supermassive black hole having several million solar masses? Hot gas and _____ are observed in rapid orbit about Sgr A*, held by an immense gravitational force.
stars
The object located at the center of our Milky Way galaxy is called Sagittarius A*. What evidence do astronomers have that Sgr A* is a supermassive black hole having several million solar masses? No object is visible there with any kind of _____.
telescope
A Brief History of the Universe: Identify the event that happened at the given time t and temperature T: t = 10^(10) years and T = 3 K: _____
today
If you were to make a video of the expanding Universe and run it backward, you would see all of the galaxies moving toward us at a speed given by the Hubble law, V = Hd. Galaxies twice as far from us would be moving toward us _____ as fast. This means that all the galaxies would arrive here at _____ time! Astronomers therefore conclude that, long ago, the Universe started from an extremely small, hot, and dense beginning called the _____, and has been expanding ever since.
twice the same Big Bang
As the Universe expanded in earliest times, _____ formed in its hot gas. Those waves took the form of fluctuations in its density and temperature. We observe those waves today as variations in the temperature of the CMB radiation across the sky. Because the radiation comes from a time 380,000 years after the Big Bang (the time of _____), regions could have interacted with each other only if they were less than 380,000 light-years apart. Combining this known size with the observation that these fluctuations are about 1o across allows us to determine the Universe's _____.
waves recombination curvature
There are three possibilities for the overall geometry of space. Suppose two light beams start out moving parallel to each other. Three things could happen: In a Universe where space has _____ curvature, the light beams will always remain parallel to each other. This is called a _____ universe.
zero flat
The overall curvature of the Universe can be determined by observing the characteristic size of the fluctuations (slightly hotter and cooler spots) in the cosmic microwave background. If our Universe is flat (zero curvature) then the images of the cosmic microwave background will be dominated by hot and cold spots of around 1o in size (above center). If, on the other hand, the Universe is closed or open, then the bending of light by the curvature of space will distort the images. If the Universe is closed (positive curvature) so parallel lines converge, then the images will be magnified by this curvature, and hot and cold spots will appear larger than 1o on the sky (above left). Conversely, if the Universe is open (negative curvature) so parallel lines diverge, then hot and cold spots will appear smaller than 1o on the sky (above right). A comparison of these computer simulations with the actual observation of the fluctuations in the cosmic microwave background (top) indicates that the Universe has _____ curvature. This shows that the Universe is _____ to very high precision.
zero flat
