Astronomy Final

Below is a list of 5 protostars. Which will become a Brown Dwarf?0.01 Msun0.5 Msun5 Msun100 Msun500 Msun

0.01Msun

Understand the evolution of a protostar for low and high mass stars. Relate to the graph

1. A protostar assembles from a collapsing cloud fragment. It is concealed beneath a shroud of dusty gas. 2. A protostar shrinks and heats as gravitational potential energy is converted into thermal energy. 3. Surface temperature rises when radiation becomes the dominant mode of energy flow within the protostar. 4. The fusion rate increases until it balances the energy radiated from the star's surface.

What is the Big Bang?

1. The Cosmic Microwave Background (CMB), thermal blackbody radiation. A blackbody spectrum occurs when radiation is constantly scattered by matter, which are exactly the conditions during the "Era of Nuclei". This was predicted by George Gamow in the 1940's.2. The CMB is highly uniform (1 part in 10^5) difference from one spot to another. This prediction simply comes from running gravitational instability backwards; by the time of the CMB, the matter fluctuations (and hence temperature fluctuations) should be very small. This prediction came much later, in the 1980's, as large-scale structure was becoming understood.

Below is a list of 5 protostars. Which takes the shortest time to become a star?0.01 Msun0.5 Msun5 Msun100 Msun500 Msun

100Msun

What is the mass-luminosity relationship?

A star's radius can be calculated if we know its luminosity and surface temperature using the Stefan Boltzmann Law. We can determine the radius of a star from its luminosity and surface temperature. For a given luminosity, the greater the surface temperature, the smaller the radius must be. For a given surface temperature, the greater the luminosity the larger the radius must be. Main-sequence stars are stars like the Sun but with different masses. The mass-luminosity relation expresses a direct correlation between mass and luminosity for main-sequence stars. The greater the mass of a main-sequence star, the greater its luminosity (and also the greater its radius and surface temperature). The greater the mass of a main sequence star, the greater its luminosity, surface temperature, and its radius.

What is the Planck time?

A very short time after the Big Bang, space and time began to behave in the way we think of them today. This short time interval is called planck time. 10^-43 seconds after the big bang. The planck time represents a limit to our knowledge of conditions at the very beginning of the universe.

How does studying open and globular clusters help us understand stellar evolution?

All stars have: same distance, same composition, same age; only difference is the mass they were born with

Know how temperature of an object relates to the energy it emits.

As an object is heated, it glows more brightly and its peak color shifts to shorter wavelengths. The thermal energy of an object comes from the kinetic energy of its internal particlesm and the hotter the material, the faster the partcles, and the hotter the material, the faster the particles move around. For hotter solids and liquids, particles vibrate faster and collide more intensely with their neighbors for hotter gases the particles streaming around at higher average speeds. Black body radiation: The higher the temperature, the greater the intensity of light at all wavelengths. The moss important feature in the spectrum- the dominant wavelength- is called the wavelength of maxiumum emission, at which the curve has its peak and the intensity of emitted energy is strongest. The higher the temperature the shorter the wavelength of maximum emission. Imagery: Ice water: Slower vibrations of particles and lower thermal energy Hot coffee: Faster vibrations of particles nad higher thermal energy

How do we know our galaxy has spiral arms? How do they form?

Astronomers believe that galaxies have spiral arms because galaxies rotate - or spin around a central axis - and because of something called "density waves." ... A spiral galaxy's rotation, or spin, bends the waves into spirals. Stars pass through the wave as they orbit the galaxy center. Theories: There are two leading theories of spiral structure in galaxies. According to the density-wave theory, spiral arms are created by density waves that sweep around the Galaxy. The gravitational field of this spiral pattern compresses the interstellar clouds through which it passes, thereby triggering the formation of the OB associations and H II regions that illuminate the spiral arms. According to the theory of self-propagating star formation, spiral arms are caused by the birth of stars over an extended region in a galaxy. Differential rotation of the galaxy stretches the star-forming region into an elongated arch of stars and nebulae. The presence of spiral arms is traced by hot, young stars and the interstellar gas & dust from which they form.

What are the apparent magnitude and absolute magnitude of a star?

Astronomers define star brightness in terms of apparent magnitude — how bright the star appears from Earth — and absolute magnitude — how bright the star appears at a standard distance of 32.6 light-years, or 10 parsecs. As you go farther away, it seems dimmer. By figuring out the distance to the star, we can find it's real luminosity. So if we know distance we know the real brightness. Apparent magnitude measure a star's brightness; absolute magnitude measures its luminosity.

Galaxy A is moving away from us at 6 x 104 km/s. Galaxy B is moving away from us at 3 x 105 km/s. Which galaxy is farthest from us and how much farther?

B, 5 times

The Universe expands: why, how, and how do we know?

Because of the general expansion of space, all distant galaxies appear to be moving away from us, with speeds that increase with distance from our galaxy. An observer in one of these distant galaxies would apparently see all galaxies moving away from her, the more distant galaxies moving faster.

How do we make an H-R Diagram and what can we use it for?

By looking at the main sequence, we see that stars are fusing H into HE. Mass increases as you go up the graph. O stars are 100 M sub sun. M stars are 0.08 M sub sun. We learn the mass which leads us to info about Lifetimes Mass/Luminosity ratio of star. If the mass is larger then more luminous which means its burning up fuel in core faster so it has a shorter life. O V stars are bright, Hot, Large, massive, with short lives. M V stars are Dim Cool Small and Low mass with long lives.

How do electron degeneracy pressure and neutron degeneracy pressure affect the evolution of a star?

Electron degeneracy is a stellar application of the Pauli Exclusion Principle, as is neutron degeneracy. No two electrons can occupy identical states, even under the pressure of a collapsing star of several solar masses. ... Electron degeneracy halts the collapse of this star at the white dwarf stage.

What happened in the Universe when it was about 380,000 years old? Why, at that time, did it change from opaque to transparent?

Electrons and nuclei combined to form neutral atoms, allowing photons to fly freely through the Universe. Before then, during the first 380,000 years, the universe was a hot opaque plasma. When the temperature was 3000K, background photons had engeries great enough to prevent electrons and protons from binding to form hydrogen atoms. Only since 380,000 have the energies of photons been low enough to permit hydrogen atoms to exist. era of recombination= electrons recombining to form atoms. BEFORE RECOMBINATION: Temperatures were so high that electrons and protons could not combine to form hydrogen atoms. The universe was opaque: photons underwent frequent collisions with electrons. AFTER RECOMBINATION: Temperatures necame low enough for hydrogen atoms to form. The universe became transparent; collision between photos and atoms became infrequent. Matter and radiation were no longer at the same temperature. Priot to t=380,000 the universe was filled with photons colliding vigorously with protons and electrons. This state of plasma is opaque. After t=380,000 the photons no longer had enough energy to keep protons and electrons apart. As soon as the temperature of the radiation field fell below 3000k, protons and electrons began combining to form hydrogen atoms. These atoms do not absorb low-energy photons, so space became transparent. Today these photons constitute the microwave background.

Understand Newton's Universal Law of Gravitation, and Newton's version of Kepler's 3rd Law, and their relevance to the last half of the course.

F= gravitational force between two objects. m1 the mass of the first object, m2 mass of the second object. d or r = distance between the two objects. G= universal constant for gravitation. Newton's Law of Universal Gravitation. any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Newton proved that the force that causes, for example, an apple to fall toward the ground is the same force that causes the moon to fall around, or orbit, the Earth. This universal force also acts between the Earth and the Sun, or any other star and its satellites. Newton realized that gravity shapes the orbit of planets. Newton's version of K's 3rd Law: Newton demonstrated that Kepler's third law follows logically from his law of gravity. If two objects with masses M1 and M2 orbit each other the Period P of their orbit and teh semimajor axis a of their orbit (the average distance) are related by an equation that we call Newton's form of Kepler's third law. The law is valid whenever two objects orbit each other because of their mutual gravitational attraction. It is valuabel for the study of binary stars. If the orbital period O and smimajor axis a of the two stars in a system are nown astronmer can use this formula to calculate the sum m1+ m2 of the masses of the two stars.

Know about spherical (closed), flat, hyperbolic (open) and accelerating universes and their properties. How do their sizes change with time?

Flat: expansion and gravity are perfectly balanced: Critical density: Density of flat U. If Po is greater than the critical density pc, the universe has a positive curvature and is closed. If Po is less than Pc it has negative curvature and is open. If po is equal to pc, the universe is flat. A closed spherical universe: Positive curvature=spherical. Gravity beats expansion and it collapses and repeats. Flat Universe: Zero curvature=flat. Gravity=expansion. Balanced Universe. An open Universe: negative curvature=hyperbolic. Gravity beats expansion and it expands forever.

Know about the four forces in the Universe, how and when they separate, and the properties of Grand Unified Theories.

Four basic forces— gravity, electromagnetism, the strong force, and the weak force— explain all the interactions observed in the universe. Grand unified theories (GUTs) are attempts to explain three of the forces in terms of a single consistent set of physical laws. A supergrand unified theory would explain all four forces. GUTs suggest that all four fundamental forces were equivalent just after the Big Bang. However, because we have no satisfactory supergrand unified theory, we can as yet say nothing about the nature of the universe during this period before the Planck time (t =10-43 s after the Big Bang). At the Planck time, gravity froze out to become a distinctive force in a spontaneous symmetry breaking. During a second spontaneous symmetry breaking, the strong nuclear force became a distinct force; this transition triggered the rapid inflation of the universe. A final spontaneous symmetry breaking separated the electromagnetic force from the weak nuclear force; from that moment on, the universe behaved as it does today.

How do galaxies interact with one another?

Galaxies need not actually collide to exert strong forces on each other. Tidal forces between colliding galaxies can deform the galaxies from their original shapes, just as the tidal forces of the moon on Earth deform the oceans and helf give rise to the tides. The galactic deformation is so great that thousands of stars can be hurled into intergalactic space. As two galaxies pass through each toher, they are severly distorted by gravitational interactions and throw out a pair of extended tails. The interactions also prevent galaxies from continuing on their original paths and instead gall back together for a second encounter. The Milky Way is going to collide with Andromeda in 3.75 billion years. When two galaxies merge, the result is a bigger galaxy and if it is located in a rich cluster, it may capture and devour additional galaxies, growing to enormous dimensions by galactic cannibalism. Close encounters between galaxies provide a third way of forming spiral arms (in addition to density waves and propogating star formation) and its when two galaxies collide by drawing out long streamers of stars or by compressing clouds of interstellar gas. The very fact of our existence may be intimately related to interactions between galaxies. Spiral arms compress the interstellar mdeium in the Milky Way's disk to form Population I stars which have enough heavy elements to form planets like earth.

How can we represent the changing age, structure, and energy source of an evolving star in the H-R D?What are the evolutionary stages of a low mass star? Of a high mass star?

Google docs

Protostars: A protostar's energy mainly comes from

Gravitational contraction! Star formation begins in dense, cold nebulae, where gravitational attraction causes a clump of material to condense into a protestor. As a protostar grows by the gravitational accretion of gases, Kelvin-Helmholtz contraction causes it to heat and begin glowing. Its relatively low temperature and high luminosity place it in the upper-right region on an H-R diagram. Further evolution of a protostar causes it to move toward the main sequence on the H-R diagram. When its core temperatures become high enough to ignite steady hydrogen burning, it becomes a main-sequence star. The more massive the protostar, the more rapidly it evolves. Not all protostellar clouds reach star status. Compared to the star it will form, the H-R Diagrams shows that a protostar has low temperature and high luminosity.

What are pair production and annihilation?

Heisenberg's uncertainty principle states that the amount of uncertainty in the mass of a subatomic particle increases as it is observed for shorter and shorter time periods. Because of the uncertainty principle, particle-antiparticle pairs can spontaneously form and disappear within a fraction of a second. These pairs, whose presence can be detected only indirectly, are called virtual pairs. A virtual pair can become a real particle-antiparticle pair when high-energy photons collide. In this process, called pair production, the photons disappear, and their energy is replaced by the mass of the particle-antiparticle pair. In the process of annihilation, a colliding particle-antiparticle pair disappears and high energy photons appear.

How do Hubble's constant, the critical density of the flat universe, and the density parameter help us understand those properties of the Universe?

In cosmology, the phrase "critical density" refers to the density needed to produce precisely flat space on average throughout the universe. If the density of matter in the universe turned out to be greater than the critical density, it would imply that the universe is closed. The degree of "flatness" of the universe, which determines whether we live in an open or a closed universe, has been determined recently by measuring the typical size of the "hot spots" in the structure of the cosmic microwave background. The parameter of the average density of matter in the present universe, more than any other, is considered to be critical in determining the ultimate fate of the universe. FF

What are protostar properties and how does a protostar make energy?

Initial energy source is Gravitational Contraction (aka, the Kelvin-Helmholz Mechanism): The Protostar shrinks slowly, releasing gravitational energy. 50% goes into photons, and is radiated away as starlight. other 50% goes into heating the Protostar interior. The energy generated from ordinary stars comes from the nuclear fusion occurring at their centers. Protostars also generate energy, but it comes from the radiation liberated at the shocks on its surface and on the surface of its surrounding disk.

How do we think galaxies formed?

Initiated by Dark Matter, galaxies formed from the merger of smaller objects. Gravity leads to the clumping of dark matter before the regular matter starts being drawn together. Galaxies form "from the bottom up"- by the merger of smaller objects to form full size-galaxies. Once a subgalactic unit combine, they make an object called a protogalaxy. The rate at which stars form within a protogalaxy may determine whether it becomes a spiral or an elliptical. Spiral galaxies: Forms relatively slow as the gas surrounding them has enough time to settle by collisions to form a flattened disk, star formation continues because the disk contains an ample amount of hydrogen from which to make new stars.If there is enough tork and if everything is spinning in one direction then spirals will form. Ellipticals: They have an overall motion, but not fast and not all of them. They form in a protogalaxy at a rapid rate, therefore all of the available gas is used up to make stars before a disk can form. Because it is becoming dense quickly, the denser it is, gas is getting squished and squeezed triggering star formation. it shrinks down except it cannot shrink perpendicular to the spin axis bc physics doesn't allow it. There is early star formation in the center where the gas is squeezed in tightly. Protogalaxies are thought to have eben composed of hydrogen and helium gas, so the first stars were Population II star with hardly any metals. As stars die and form planetary nevulae or supernovae, they eject into the interstellar medium gases enriched in metals. In a spiral galaxy, there is ongoing star formation in the disk so these metals are incorporated into new generations of stars, making relatively metal-rich Population I stars. By contrast, an elliptical galaxy has a single flurry of star formation when it si young, after which star formation ceases. Elliptical galaxies therefore contain only metal-poor Population II stars.

What are globular clusters and how do they help us understand our galaxy? How were they determined? How can we measure its mass?

Interstellar extinction obscures our view within the plane and makes distant objects appear dim. To find our location in the galaxy, we use bright object that are oart of the galaxy but lie outside its plane in unobscured regions of the sky. Those bright objects are globular clusters, which are a class of star clusters associated with the galaxy but which lie in the halo. They composed of old, metal-poor, Population II stars. They contain only 1% of the total number of stars in the halo; most halo stars are single Population II stars in isolation. It is a spherical distribution of roughly 10^6 stars packef in a volume only a few hundred light years across. In order to use globular clusters to determine our location in the galaxy, we must determine the distances from Earth to these clusters. Pulsating variable stars in globular clusters provide distances, giving astronomers the key to the dimensions of our galaxy. RR Lyrae are found in globular clusters and by using the period-luminosity relationship for these stars, shapley determined the distances to the 93 globular clusters then known.

What is the Hertzsprung-Russell Diagram?

It is a graph of luminosity (or absolute magnitude) and spectral type (Temperature) It holds lots of info about stars like its lifetime, stellar radius, and stellar mass, and specific groupings. Its a graph of luminosity versus surface temperature.

Spectra and Kirchoff's laws

Law 1: A hot opaque body, such as a perfect blackbody, or a hot , dense gas, produces a continuous spectrum, a complete eainbow of colors without any specrtal lines. Viewing a blackbody through a cool gas cloud creates dark spectral lines. Law 2: A hot, transparent gas produces an emission line spectrum- a series of bright spectral lines against a dark background. Law 3: A cool, transparent gas in fromt of a source of a continuous spectrum (like a distant star) produces an absorption line spectrum- a series of dark spectral lines among the colors of the continuous spectrum. Furthermore the dark lines in the absorption pectrum of a particular gas occur at exactly the same wavelength as the bright lines in the emission spectrum of that same gas. We see absorption spectra when we see stars because their atmospheres are fluffier than their regions. In general, a gas cloud can both emit and absorb light. Whether an emission line spectrum or an absorption line spectrum is observed from an intervening gas ckoud depends on the relative tmeperatures of the gas cloud and its background. Absorption lines are seen if the background is hotter than the gas, and emission lins are seen if the background is cooler. Spectral lines are produced when one electorn jumps from one energy level to another one. Absorption: Incoming photon. Atom absorbs a 656.3 nm photon; absorbed energy causes electron to jump from the n=2 orbit up to the n=3 orbit. Emission: Emitted photon; electron falls from teh n=3 orbit to teh n=2 orbit; energy lost by atom goes into emitting a 656.3 nm photon. An atom must absorb energy for the electron to go from an inner to an outer orbit; an atom must release energy for the electron to go from an outer to an inner orbit. The atom can show the same spectral lines as either emission or absorption because when an electron jumps from one orbit to another, the energy of the phootn that is emitted or absorbed equals the difference, and hence the photon energy, is teh same whether the jump is from a low orbit to a high orbit. To produce an absorption line spectrum begin with a relatively cool gas so that the electron in most of the atoms are inner, low-energy orbits. If a beam of light with a continuous spectrum is shone through the gas, most wavelengths will pass through undisturbed. Only those photons will be absorped whose energies are just right to excite an electron to an allowed outer orbit. Hence, only certain wavelengths will be absorbed and dark lines will appear in the spectrum at those wavelengths. The properties of energy levels explain the emission line spectra and absorption line spectra of gases. Continuous spectra: In a liquid or a solid, atoms are so close that they almost touch, and thus these atoms interact strongly with each other. In addition, each of the many possible interactinos ina. liquid ort solid have their own energy levels. The pattern of distinctive bright spectral lines that the atoms would emit in isolation becomes crowded with additional lines forming a continuous spectrum.

What is Luminosity Class? How do you find it?

Luminosity classes tell us the size of a star with Supergiants being the largest, Main Sequence smallest, absorption lines in a star spectrum are affected by the density and pressure of the stars atmosphere, with denser atmospheres. Luminosity class represents different stages of evolution: Our sun is a G2 V. I= supergiants. II= Bright Giants. III= Giants. IV= Subgiants. V= main sequence

how to use magnitudes to compare brightnesses of stars.

Magnitude system was devised by Hipparchus around 150 BC. It is another way to represent brightness. Small numbers are brighter. Big numbers are dimmer. Apparent magnitude (m) is the way we see the stars, similar to b. Absolute magnitude (M) is the way the star would look if it were at 10 parsecs, similar to L. To figure out luminosity from absolute magnitude, one must calculate that a difference of five on the absolute magnitude scale is equivalent to a factor of 100 on the luminosity scale — for instance, a star with an absolute magnitude of 1 is 100 times as luminous as a star with an absolute magnitude of 6.

What determines a star's lifetime on the Main Sequence? What are binary stars and what can we learn from them?

Main sequence stars gain their energy through conversion of hydrogen to helium in their cores. Main Sequence stars evolve into Giants and Supergiants (ch 19, 20) Binary stars help learn Stellar Masses. A star's mass control's its life but its hard to measure, If we find two stars in binary systems that orbit each other then use Newton's version if Kepler's 3rd law. This only tells us the total mass, we need to find the center of mass of the binary system and do a little geometry. Then we can determine individual masses for each star. Binary star systems provide the best means for scientists to determine the mass of a star. As the pair pulls on each other, astronomers can calculate the size, and from there determine characteristics such as temperature and radius. These factors help characterize single main sequence stars in the universe. Binary stars, in which two stars are held in orbit around each other by their mutual gravitational attraction, are surprisingly common. Those that can be resolved into two distinct star images by an Earth-based telescope are called visual binaries. Each of the two stars in a binary system moves in an elliptical orbit about the center of mass of the system. Binary stars are important because they allow astronomers to determine the masses of the two stars in a binary system. The masses can be computed from measurements of the orbital period and orbital dimensions of the system.

What are elliptical, spiral, and irregular galaxies?

Millions of glalaxies are visible across every unobscured part of the sky. Galaxies come in different sizes and shapes. Hubble classified galaxies into four broad categories based on their appearance. Spirals, barred spirals, ellipticals, and irregulars. Rich clusters tend to be dominated by ellipticals while galaxies in poorer clusters tend to be spiral galaxies

What is in the halo? How does it move? Whats in it?

Most stars in the halo are Population II stars in isolation. The other 1% is globular clusters, old, metal-poor, Population II stars. These ancient stars orbit the galaxy along paths tilted at random angles to the dick of the Milky Way, as do the globular clusters. No hot O or B stars in the halo, therefore there is no star formation, it ceased long ago.

Below is a list of 5 protostars. Which takes the longest time to become a star?0.01 Msun0.5 Msun5 Msun100 Msun500 Msun

Msun0.5

Why do we think dark matter exists? What might it be?

One way: Rotation curves! By Vera Rubin. She noticed that the stars are moving at great speeds and they are so far away from the center so how are they not flying off? If there is dark matter then the gravity will be strong because there is enough mass for gravity to have an effect. Rotation curves were flat out to great distances from cetner; the orbital speeds remain the same to the edges of these galaxies. IF most of the mass were concentrated near the center, these rotation curves would fall off at large distances but these and many other galaxies have flat rotation curves that do not fall off. Second way: Gravitational lensing. A gravitational lens is a distribution of matter between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. The light is bent by a massive cluster of galaxies and when it comes into your eyeball, you get a bunch of different distorted images of the galaxy behind the massive object. You can measure how distorted they are, and how much dark matter is there. When light from a distant object passes close to a very massive object, such as a cluster of galaxies, on its way to Earth, it can be deflected. As a result, the image of the background object distorts and brightens. The gravitational lensing technique allows astronomers to observe very faint background objects and to study the distribution of dark matter inside clusters. Vast assemblages of dark matter reveal their presence by bending passing rays of light. For gravitational lensing to work, the alignment must be very good between Earth, the massive galaxy is to faint to be noticeable, leaving a regular view of the background galaxy. PROVEN WAY WITH PROOF: Direct proof was found when two clusters of galaxies collided. In august 26 th 2006. They traced what the galaxies in each of the clusters were doing, observed the gas, and from that observed that matter was following the galaxies. During a collision the galaxies do bash into each other, they just go through each other and come out the other side. However, gas takes up a lot of space inbetween and in galaxies. Gas bashes into other gas and they interact with each other and distort each other. BUT there was an amount of dark matter was way less interactive and followed the collision-less galaxies and therefore we think dark matter is something that is collision-less, WIMPS. Weaking Interacting Massive Particles.

What is our galaxy? What is its size and structure?

Our Galaxy has a disk about 50 kpc (160,000 ly) in diameter and about 600 pc (2000 ly) thick, with a high concentration of interstellar dust and gas in the disk. The galactic center is surrounded by a large distribution of stars called the central bulge. This bulge is not perfectly symmetrical, but may have a bar or peanut shape. The disk of the Galaxy is surrounded by a spherical distribution of globular clusters and old stars, called the galactic halo. There are about 200 billion (2 ´ 1011) stars in the Galaxy's disk, central bulge, and halo. The Milky Way Galaxy is a spiral galaxy consisting of a thin disk about 100,000 light years in diameter with a central bulge and a spherical halo that surrounds the disk. ... Our galaxy, the Milky Way, is a spiral galaxy so named because of the "spiral arms". The milky way appears as a band of light that stretches all away around the sky. The solar system is nowhere near the galactic center. The total mass of galaxy could exceed 10^12M of which only about 10% is regular matter mostly in the form of stars. There is a supermassive black hole at the center of our milky way. The Milky way is part of a poor cluster (the Local Group, with a few dozen members).

What and where are Pop I and Pop II stars?

Population I stars: Younger, metal rich, more heavy elements. Population II stars: Older, metal poor. Population I are particularly found in the disk, moving along nearly circular orbits around the galactic center. Population II stars tend to be found in the bulge and halo, they have more randomly oriented orbits around the galactic center. The difference between Pop I and II suggest different ages of star formation as the galaxy formed from a collapsing cloud of gas, with early-forming stars (and clusters) having a spheroidal distribution, while the gas eventually settled into a disk, where newer stars continue to form from material enriched in metals by earlier supernovas.

How does a protostar become a star? Know how to relate the evolution on a graph

Protostars start out as big protostar clouds. Gravity makes the luminous low temperature cloud of gas cruch down into a hotter, smaller star. Gravitational contraction is what protostar makes energy from, it squeezes it down and as it does, it creates more and more energy and its heating up. Because it is denser, its harder for the heat to escape. When the protostar is still forming, its a fluffly cloud whose heat is pouring out pretty well and as it gets denser the heat gets stuck inside which is why it gets hotter on the inside. Eventually, the core will be hot enough to start fusion on the inside once it is a MAIN SEQUENCE.

What are reflection, dark, and emission nebulae?

Reflection Nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. ... Besides the luminous bright nebulae, there are dark or absorption nebulae which can be seen because they obscure, or absorb, the light coming from stars or bright nebulae behind them. Reflection Nebulae: Light from stars encounter small dust grains. Note that blue light is scattered more effectively than red light. It appears streaky because interstellar magnetic fields align dust grains. (polarization) If light strikes an interface so that there is a 90o angle between the reflected and refracted rays, the reflected light will be linearly polarized. The direction of polarization (the way the electric field vectors point)is parallel to the plane of the interface. An emission nebulae is a nebula that shines with its own light. High energy UV photons ionise the hydrogen in the interstellar gas cloud. As the hydrogen recombines and returns to its neutral state, optical photons are emitted. Emission nebulae are clouds of ionised gas that, as the name suggests, emit their own light at optical wavelengths.

What are Sa, Sb, Sc, and E0, E1,... E7 galaxies?

Sa are spirals with smooth, broad spiral arms and a fat central bulge are called. Sb have well defined spiral arms and a moderate sized central bulge. Sc are narrow, well defined spiral arms and have a tiny central bulge. Sa has 4% mass made up of gas and dust, Sb has 8%, and Sc has 25%. Interstellar gas and dust is the material from which stars are formed, so Sc has a greater proportion of its mass involved in star formation than an Sb or Sa galaxy. So Sc has a larger disk (where star formation happens) and a little central bulge 9where little star formation happens). Sa has a small disk and a large central bulge. E0 galaxies are the roundest elliptical galaxies and E7 are the flattest elliptical galaxies. In E0 galaxies, stars motions are isotropic (equal in all directions). In E7 the randomness of star motion is anisotropic which means that the range of star speeds is different in different directions.

Why are variable stars not stable?

Some of them vary in brightness by as much as 100 times, and some have cycles that repeat as often as every few days, while others vary over months or years. In most cases these stars pulsate because they are at the end of their lives and have become unstable.

Spectral Types?

Spectral Types are based on relative strength of absorption lines in stars as seen to the right. Pattern if dark lines tells us spectral type for each star. Hottest O have very few lines because of Hydrogen and Helium are ionized. coolest M have many lines because even molelules can make absorption lines. OBAFGKM subdivided into 0-9 parts O0 O1 O2 O3 O4 O5 O6 O7 O8 O9 A0 A1 A2 A3 A4... A9 B0.... M9

What are trigonometric (stellar) and spectroscopic parallax?

Spectroscopic Parallax Method measures apparent magnitude (m) of star. From the spectrum, determine Spectral Thpe (OBAFGKM) Luminosity Class (I, II, III, IV, V)

How do elliptical, spiral, and irregular galaxies differ? How did the differences arise?

Spiral galaxies are characterized by their arched lanes of stars just as is our own Galaxy. The arms contain young hot blue stars and their associated H II regions, indicating ongoing star formation. Thermonuclear reactions within stars create metals (elements heavier than hydrogen or helium) These metals are dispersed into space as the stars evolve and die. So, if new star are being formed from the interstellar matter in spiral galaxies, they will incorporate these metals and be Population I stars. Stars in the disk of a spiral galaxy are Pop I, but relatively little star formatino in the central bulge of spiral galaxies and tehse regions are dominated by old Population II stars that have a low metal content. Elliptical galaxies have elliptical shapes, no spiral arms and Hubble subdivided into how round or flattened they look. The reason that ellipticals look so different from spirals is because they are practically devoid of interstellar gas and dust. So, there is little material from which star formation could have recently occurred and there is no evidence of young stars in elliptical galaxies. They are composed of old, red, Population II stars with only small amounts of metals. Galaxies that do not fit into the scheme of spirals barred spirals and ellipticals are usually reffered to as irregular galaxies. They are rich in interstellar gas and dust and have both young and old stars. Contain substantial interstellar gas and dust. Tidal forces on these irregular galaxies by the Milky Way help to compress the gas which triggers star formation.

What are the sources of energy for protostars and stars?

Star formation begins in dense cold nebulae, where gravitational attraction causes a clump of material to condense into a protostar. As a protostar grows by the gravitational accretion of gases, gravitational contraction causes it to heat and begin glowing. Its relatively low temperature and high luminosity place it in the upper right region on an H-R diagram. Further evolution of a protostar causes it to move toward the main sequence on the H-R diagram. When its core temperatures become high enough to ignite steady hydrogen burning, it becomes a main sequence star Fusion of H to HE + energy in core means A star is born.

What is the disk in the milky way? How does it move? What stars are in it?

Stars in the disk travel along orbits that remain in the plane of the disk. The stars in the disk are young, metal-rich, Population I stars like the Sun. The disk of a galaxy appears bluish because its light dominated by radiation frmo hot O and B main sequence stars. Such stars have very short main-sequence lifetimes. There is active star formation in the galactic disk.

Describe the large-scale structure of the Universe

Stars throughout the universe are found in clusters. Clusters is either poor or rich, depending on how many galaxies it contains. Poor clusters far outnumber rich ones and are called groups. One of these such groups are called local groups which contains 40 galaxies. Clusters are grouped into superclusters Astronomers map out the large scale structure of superclusters through initially mapping the distribution of galaxies out to great distances (by measuring positions in the sky and the distances to galaxies (these can be estimated by measuring the redshifts). The large-scale structure of the universe arose from primordial density fluctuations. The first stars were much more massive and luminous than stars in the present-day universe. The material that they ejected into space seeded the cosmos for all later generations of stars. Galaxies are generally located on the surfaces of roughly spherical voids. Models based on dark energy and cold dark matter give good agreement with details of this large-scale structure.

How do we measure distances to stars?

Stellar Parallax Method to determine distance. It is the most direct method but it only works on the nearest of stars. Defines a parallax angle p (arsecs) d=1/p. It measure distances out to 1,000 pc. Not the best way bc that distance is tiny compared to size of galaxy where disk is 50,000 pc across

What is dark matter, how and where is it found?

The Galaxy's Rotation Curve is a graph of the speeds of galactic rotation measured outward from the galactic center. We would expect that for gas clouds beyond the confines of most of the galaxy's mass, the orbital speed should decrease with increasing distance from the galaxy's center, just as the orbital speeds of the planets decrease with increasing distance from the Sun. EXCEPT we know the roation curve is quite flat, indicating unifrom orbital speeds well beyond the visible edge of the galactic disk. We conclude that there is a large amount of matter outside the Sun's orbit. Unlike the galaxy's stars and dust, its dark matter forms a roughly spherical halo. 90% of the remaining mass that we do not know of must be dark matter. WE SENSE ITS PRESENCE IN OUR GALAXY ONLY THROUGH ITS GRAVITATIONAL INFLUENCE ON THE ORBITS OF STARS AND GAS CLOUDS. The density of dark matter halo decreases with increasing distance from the center of the galaxy.

Be able to use Hubble's Law in simple calculations

The Hubble Law relates the redshifts of remote galaxies to their distances from Earth. The redshifts of spiral nebulae reveal a basic law of our expanding universe. The more distant a galaxy the greater its redshift and the more rapidly it is receding from us. Nearby galaxies are moving away from us slowly, and more distant galaxies are rushing away from us much more rapidly. Redshift of a receding object: Z= redshift of an object. lambda sub zero= ordinary, unshifted wavelength of a spectral line. Lambda= wavelength of that spectral line that is actually observed from the object. Hubble law: v= recessional velocity of a galaxy Hsub zero=Hubble constant. d= distance to the galaxy. Math shortcut: Both on top do the same. If the Velocity of one galaxies is 100,000 km/s and the secon id 300,000 is km/s away, the first galaxy is closer because the velocity and the distance are dependant on each other, so its closer bc that velocity is smaller. The distance for the second is 300,000 so its farther. The bigger the velocity the farther the distance.

The Rotation of the Galaxy and Dark Matter:

The Sun orbits around the center of the Galaxy at a speed of about 220 km/s. It takes about 220 million years to complete one orbit. From studies of the rotation of the Galaxy, astronomers estimate that the total mass of the Galaxy is about a trillion Msun. Only about 10% of this mass is in the form of visible stars, gas, and dust. The remaining 90% is in some nonvisible form, called dark matter, that extends beyond the edge of the luminous material in the Galaxy. Our Galaxy's dark matter may be a combination of MACHOs (dim, star-sized objects), massive neutrinos, and WIMPs (relatively massive subatomic particles).

What is the physical reason that astronomers can find the luminosity class (I, II, III, IV, or V) of a star using the star's spectrum?

The absorption lines in the spectrum are affected by the density and pressure of the star's atmosphere.

How can the color of a star indicate its temperature?

The color of a star indicates the surface temperature of a star. The blue stars are the stars with the hottest surface, the yellow are the mild temperature, and the orange and red are the ones with the coolest surface.

How to you determine the color of the star?

The color of stars tells us the surface temperature as well. Hot star=blue. Cool star=reddish. There are 7 Stellar Spectral Types that represent temperatures of stars: O B A F G K M. Discovered by women!! A star's color depends on its surface temperature. A cool star with surgace temperature 3000 K emits much more red light than blue light, and so apprears red/ A warmer star with surface temperature 5800 K emits similar amounts of all visible wavelengths and so appears yellow-white. A hit star with surface temperature 10,000 K emits much more blue light than red light, and so appears blue.

What is the shape of space and how is it determined?

The distribution of matter in the early universe was not perfectly uniform, and these temperatures variations reveal the nonuniformities- the clumps and voids- in the mass distribution. Clumps in the distribution of matter produce a gravitational redshift. Redshifted CMB photons not only have longer wavelengths and lower energy, but they also correspond to lower CMB temperatures. The cooler bluer spots correspond to denser regions. Clumps of matter seen at early times are the "seeds" of structures that continued to grow into today's superclusters through gravitational contraction. The shape of our universe indicates its matter and energy content. The degree of curvature depends on the combined average mass density of all forms of matter and energy. The curvature of the universe is actually determined by the combined mass density (Po) and the critical density (Pc)

galactic nucleus

The innermost part of the Galaxy, or galactic nucleus, has been studied through its radio, infrared, and X-ray emissions (which are able to pass through interstellar dust). A strong radio source called Sagittarius A* is located at the galactic center. This marks the position of a supermassive black hole with a mass of about 4 million Msun.

Know a little about the Local Group, nearby clusters of galaxies, superclusters of galaxies, bubbles, voids, and walls in the large-scale distribution of galaxies in space.

The local group has 40 galaxies most of which are dwarf ellipticals. The Milky Way Galaxy, the Andromeda Galaxy, and the large and small magellanic clouds belong to the poor cluster called the Local Group. Voids are large concentrations of galaxies surround large regions of space with very few galaxies. Superclusters of galaxies are not spread uniformly across the universe, but are found in vast sheets separated by immense voids.

Know in simple terms the characteristics of stars in each of the following major regions of the H-R D: Main Sequence (= mass sequence), Pre-Main Sequence (for protostars), Giant Branch, Supergiant Branch, Horizontal Branch, White Dwarf region

The main sequence stretching from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) dominates the HR diagram. It is here that stars spend about 90% of their lives burning hydrogen into helium in their cores. Main sequence stars have a class labelled V. red giant and supergiant stars (luminosity classes I through III) occupy the region above the main sequence. They have low surface temperatures and high luminosities which, according to the Stefan-Boltzmann law, means they also have large radii. Stars enter this evolutionary stage once they have exhausted the hydrogen fuel in their cores and have started to burn helium and other heavier elements. White dwarf stars (luminosity class D) are the final evolutionary stage of low to intermediate mass stars, and are found in the bottom left of the HR diagram. These stars are very hot but have low luminosities due to their small size. Giants are both luminous and cool. In order for giants to be luminous, thye must be huge. They 10 to 100 times more luminous than the sun and have temperatures of 3000 to 6000 K. Red giants have 3000 to 4000 K. Both giants and supergiants have thermonuclear reactions happening. White dwarfs, 9% of stars on diagram, Hot, low luminosities, small, no thermonuclear reactions, not true stars, mere remnants of stars, burning embers.

What happens to wavelength when you increase or decrease frequency and vice versa?

The number of complete wavelengths in a given unit of time is called frequency (f). As a wavelength increases in size, its frequency and energy (E) decrease. From these equations you may realize that as the frequency increases, the wavelength gets shorter. As the frequency decreases, the wavelength gets longer.

What is cosmological redshift?

The redshifts of distant galaxies are not doppler shifts; they are caused by the expansion of space itself. The photons have traveled across space that has been expanding and their wavelengths have expanded with it, becoming redder. If the lookback time for a distant galaxy is 1 billion years, that means the light from the galaxy took 1 billion years to reach us so we are seeing it as it was 1 billion years ago.

More spiral arms

The spiral arms of the milky way divide the disk. Spiral arms contain all young massive bright stars because of star formation. Spiral arms are caused by density waves that sweep around the galaxy. The density wave model is a theory for spiral arms. These waves make matter pile up in the spiral arms which one can imagine as the crest of a wave. Individual parts of the galaxy's material are compressed only temporarily when they pass through a spiral arm. The pattern of spiral arms persists, just as the waves made by a stone dropped in the water can persist for awhile after the stone has sunk. In a galaxy, stars play the role of water molecules as they exert forces on each other because they are affected by each other's gracity. If a region of above average density should form, its gravitational attraction will draw nearby matterial into it. The displacement of this material will change the gravitational force that it exerts on other parts of the galaxy, causing additional displacements. Imagery: A density wave in a spiral galaxy is analogous to a crew of painters moving slowly along the highway, creating a moving traffic jam. Like such a traffic jam, a density wave in a spiral galaxy is a slow moving region where stars, gas, and dust are more densely packed than in teh rest of the galaxy. As the material of the galaxy passes through the density wave, it is compressed. This triggers star formation. When interstellar gas and dust moves through a spiral arms it is compressed into new nebulae. This compression begins the process by which new stars form. The most luminous among these are the hot massive blue O and B stars which may have emission nebulae (H II regions) associated with them. The density wave model explains why the disk of our galaxy is domiated by metal rich Population I stars. Because the material left over from ancient stars ie enrich in heavy elements, new generations of stars formed in spiral arms are likely to be more metal rich than their ancestors. The asymmetrical gravitational field of of the central bulge's bar pulls on the stars and interstellar matter of galaxy to generate density waves.

What happens to temperature and density, matter and radiation in the Universe during its evolution?

The temperature of background radiation has declined over the eons thanks to the expansion of the universe. the universe was radiaiton dominated 13.7 billion years ago when the unvierse was compressed and the density of matter increased. Photons were more crowded together and their wavelengths were shorter and less redshifted and higher energy than they do today. Because of this added energy, the mass density of radiation increases more quickly as we go back in time than does the average density of radiation increases more quickly as we go back in time. There was a time in the ancient past when Pm( average density of matter) equaled prad (mass density of radiation), and even further back, prad was greater than pm so that radiation dominated, but then pm became greater than prad so that matter prevailed over radiation. This transition from a radiation dominated universe to a matter dominated universe occured about 24.,000 years after the Big Bang. Approximately 380,000 years after the Big Bang, when the temperature fell below 3000 K, hydrogen atoms formed and the radiation field "decoupled" from the matter in the universe. After that point, the temperature of matter in the universe was not the same as the temperature of radiation. The time when the first atoms formed is called the era of recombination.

What is the Main Sequence Turnoff Point and how do you find it?

The turnoff point for a star refers to the point on the Hertzsprung-Russell diagram where it leaves the main sequence after the exhaustion of its main fuel. It is often referred to as the main sequence turnoff. By plotting the turnoff point of the stars in star clusters, one can estimate the cluster's age. We will now determine the main sequence turn off age of M67. The main sequence turn off age tells us how old the cluster is. The mass of a star sets the luminosity, the temperature, the size, and how fast it will evolve off of the main sequence. This is fortunate because stars of the same mass will evolve at the same rate. Spectral Class Luminosity Time on Main Sequence O 104 Lsun < 107 years B 103 Lsun 108 < t < 109 years A 101 Lsun 109 years G 100 Lsun 1010 years K 10-1 Lsun 1011 years

What methods can you use to find extragalactic distances?

The various methods of distribution determination are interrelated because one is used to calibrate the other. Standard candles are not very "standard" and sitances measured in this way are somewhat uncertain. Another is the Tully-Fisher relation; the width of the hydrogen 21-cm emission line of a spiral galaxy which is related to the galaxy's luminosity. The broader the line the more luminous the galaxy. Such a relationship exists because radiation from the approaching side of a rotating galaxy is blueshifted while that from the galaxy's receding side is redshifted. Thus, the 21-cm line is doppler broadened by an amount directly related to how fast a galaxy is rotating/ Elliptical galaxies do not rotate so the Tully Fisher relation cannot be used to determine their distances. The next is the distance ladder: One technique can be used to calibrate another. Cepheids provide distances to nearby galaxies, making it possible to determine the peak luminsoity of each supernova using its maximum apparent brightness and the inverse-square law. once the peak luminosity is known, it can be used to determine the distance to Type IA supernovae in more distant galaxies. Because one measureing technique leads us to the next one like rungs on a laddder, the technique is called the distance ladder.

Why does the central bulge of a spiral galaxy appear reddish yellow when compared to the color of the spiral arms?

There is no star formation there, and the star population is dominated by old, long-lived, low-mass red stars.

Where do all of the elements heavier than Helium come from?

They were produced inside stars! It is generally believed that most of the elements in the universe heavier than helium were created in stars when lighter nuclei fuse to make heavier nuclei. The process is called nucleosynthesis. Nucleosynthesis requires a high-speed collision, which can only be achieved with very high temperature.

What is a standard candle?

To determine the distance to a remote galaxy, astronomers look for a standard candle- an object for which we are likely to know its true luminosity (absolute magnitude) By measuring how bright the standard candle appears, astronomers can calculate its distance-and hence the distance to the galaxy of which it is part-using the inverse-square law. Standard Candles must: Be luminous enough to be seen from great distances, we should be fairly certain of its luminosities, should be easily identifiable by light curve. Cepheid variables make reliable standard candles and their luminosities can be determined from their periods through the period-luminosity relation. RR Lyrae stars (whichare Population II stars often found in globular stars and helped find the size of the galaxy)

What are Type II Supernovae How do they form?

Type 2 Supernova Events When a collapsing star implodes due extreme pressure during fusion being unable to stop gravity (crushing the star) the loss of inner core pressure causes an extreme explosion or release of heat and light and the formation of very dense heavy materials (iron elements with atomic #'s greater than 26). Depending on the mass of the star either a pulsar, white dwarf, neutron star or black hole is created. Type II Supernovae: Type II supernovae are produced by the collapse of the core of a massive star and most likely to be found in the disk of a galaxy. Type II supernovae are produced by massive stars whose cores collapse following the exhaustion of their fusion processes. ... A type II supernova does not involve a white dwarf but instead requires a massive star to reach the end of its ability to generate energy in its core. Type II supernovae result from the explosion of a massive star after its iron core collapses and rebounds. Since massive stars have lots of hydrogen in their outer layers when they explode, Type II supernovae have hydrogen emission lines in their spectra which can be used to identify them. Requirements for a Type 11 supernovae: 1. Must be 8 times the mass of the sun2. Must be less than 40 to 50 times the mass of the sun3. Core must collapse first4. 1 solar mass units (Smau)=1.989× 1030 kg.

What is the cosmological principle?

We do not occupy a special location in space, because the universe is the same everywhere, on average. The universe is homogenous meaning that every region is the same as every other ergion and isotropic, meaning that teh universe looks the same in every direction. If you were to stand back and look at a very large region of space, any one part of the universe would look basically the same as any other part, with the same kinds of galaxies distributed through space in the same way.

What is the cosmic light horizon and why is it the limit to which we can see out in space?

We know the universe had a definite beginning and thus its age is finite. If the universe is 13.7 billion years old, then most distant objects that we can see are those whose light has traveled 13.7 billion years to reach us. We can only see objects that lie only within an immense sphere centered on Earth. The surface of the sphere is called the cosmic light horizin and our entire observable universe is inside this sphere. We cannot see anything past this sphere because the time required for light to reach us from these incredibly remote distances is greater than the present age of the universe. As time goes by, light from more distant parts of the universe reaches us for the first time and teh size of the cosmic particle horizon (that is our observable universe) increases.

What are the evolutionary stages of a low mass star? Of a high mass star?

Week 11 or google docs

What are the various end states of stars and what determines to which state a star will evolve?

White Dwarf, Black Hole, Neutron star. (Neutron star and black holes were originally supergiants) White Dwarf: Mass has to be less than 1.44 times the mass of the sun. White dwarf started out low down on the main sequence with not too much mass and then pushed out its outer layers at the end of its life and the core turned into a white dwarf. It was being shrunk down so much that electron degeneracy said "stop squeezing me" and the star stops shrinking even though gravity wants it to but electron degeneracy wins. Star with 8M sun can become white dwarfs but the cores must be less than 1.44 M sun. They are very dense. Neutron star= mass is less than 3 times the mass of the sun.

How old/big/fast is the Universe?

about 13.8 billion years old. Recent results from very bright supernovae in very distant galaxies seem to indicate that the expansion of the universe is accelerating (speeding up)

what is the central bulge? (milky way)

barlike shape at center with yellowing stars (somewhat old) The central bulge contains both Population I stars and metal-poor Population II stars. Since Population II stars are though to have formed early in the history of the universe, some stars in the central bulge are ancient, and some are quite young. It looks yellowish/reddish because it contains many red giants and red supergiants, but does not contain luminous, short-lived, blue O and B stars, so there is no ongoing star formation.

After a star runs out of hydrogen in its core, what happens to it?

becomes a red giant

Doppler shift

change in the apparent frequency of a wave as observer and source move toward or away from each other. Measuring the shift of the spectral line allows us to measure the radial velocity of the source (not the total velocity through space). By measuring the blueshift or redshift; we can measure how fast or slow an object is moving towards or away from us.

In which one of the following locations are clumps of gas most likely to be collapsing to form stars?

dark nebulae

Inverse square law

determines how bright a star appears based upon its luminosity (intrinsic brightness) and distance

How long does it take the Sun to orbit in the MWG?

it takes the sun approximately 225-250 million years to complete one journey around the galaxy's center. This amount of time - the time it takes us to orbit the center of the galaxy - is sometimes called a cosmic year. Orbital speeds of stars and gas around hte galactic center is fairly uniform. Stars orbiting between the sun and the galactic center more quickly because the stars have a shorter distance to travel, and stars farther from galactic center than the sun lag behind.

What process makes a red emission nebula glow?

light emitted when electrons jump from high to low orbits.

What is luminosity?

luminosity is an absolute measure of radiated electromagnetic power, the radiant power emitted by a light-emitting object, In astronomy luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical object. A star's luminosity is the amount of energy emitted per second from its entire surface. A cool star (low surface temperature T) for which the energy flux is quite low, can nonetheless be very luminous if it has a large enough radius R. A large star can have a very low luminosity if the star has only a little surface area.

Harlow Shapley concluded that the Sun was not in the center of the Milky Way Galaxy by

mapping the distribution of globular clusters in the galaxy.

To determine the sum of the masses of a visual binary star system, we need to measure for the system

period and semi major axis

What is the Cosmic Microwave Background Radiation?

radiation left over from the Big Bang, after the universe expanded and cooled. The very small detected irregularities in the uniformity of the cosmic microwave background are considered to be very important in the study of the evolution of our universe because they are thought to have led to the development of the present concentrations of matter and energy in clusters of galaxies. An important feature of the microwave background is that its intensity is almost perfectly isotropic, this proves that the universe is isotrpoic. The apparent variation in temperatue is caused by Earth's motion through the cosmos. Because we are moving through this radiation, we see a doppler shift. The existence of such concentrations of mass as well as the existence of super clusters or galaxies shows that the universe is rather "lumpy" and it is homogenous and isotropic on larger scales.

What is differential rotation?

rotation where a body rotates faster or slower at its equator than it does at its poles. Star near the edge takes longer than a star near the center. A star near the center takes less time than a star near the edge.

Which of the following statements concerning spiral galaxies is true and provides a possible reason for the differences between different classes of spiral galaxies, Sa, Sb, and Sc?

the fraction of their total mass which in the form of dust and gas is different; Sa has the lowest fraction whereas Sc has the highest

The spectrum of a star shows an equivalent set of dark absorption lines to those of the us, but with an exception: Every line appears at shorter wavelengrh: each is blueshifted, We can conclude that:

the star is moving rapidly toward Earth.

The smallest mass that a main-sequence star can have is about 0.08 solar mass. The reason for this is that

the temperature in the core of a contracting protostar of less than 0.08 solar masses does not get high enough for nuclear reactions to start.

Does the Universe have a center or an edge?

there is an "edge" for the observable universe because we cannot see any farther than the distance that light has traveled over the lifetime of the universe. An expanding universe does not have a center.

What process makes a blue reflection nebula glow?

tiny dust grains scatter short wavelengths more than long wavelength visible light

What method is used to determine the distances of very remote galaxies?

use of their spectral redshifts and the Hubble law

An astronomer plots the H-R diagram of a star cluster and finds that it contains hot B and A-type stars on the main sequence and cooler K- and M-type stars noticeably above the main sequence. This cluster is

very young, because the G and K stars are still evolving toward the main sequence.

How is Olber's paradox solved?

we cannot see those stars that are farther away from us than the distance that light has traveled since the beginning of the universe. The finite size of the observable universe, with a finite number of stars and galaxies, also helps to resolve Olber's paradox; Galaxies are distributed sparsely enough in our observable universe that there are no stars along most of our lines of sight. This sparse distribution of visible stars is one reason why teh night sky is dark.