P2151 Final Flashcards
Describe the sun
star; glowing ball of gas held by gravity and powered by nuclear fusion at the centre
Photosphere
region at sun’s surface from which all visible light is emitted
Core
interior of sun; nuclear rxns generate E
Radiation Zone
Interior of sun; E travels outward in the form of EM radiation
Convection zone
interior of sun; Sun’s matter is in constant convective motion
Solar constant
the amount of solar E reaching the top of Earth’s atmosphere each second
Luminosity
total amount of E radiated from the surface per second.
Where does much of our knowledge of the solar interior come from?
Mathematical models
What model best fits the observed properties of the sun?
The standard solar model
Helioseismology
The study of vibrations of the solar surface caused by P waves in the interior; provides further insight into the Sun’s structure
Granulation
shows the effect of solar convection zone; in the photosphere
Supergranulation
large transient patterns on the photosphere
Chromosphere
Sun’s lower atmosphere. Most absorption lines are produced in upper photosphere and chromosphere
Transition zone
Above the chromosphere of sun; T increases from a few thousand to a million K.
Corona
Above transition zone of sun; sun’s thin, hot upper atmosphere.
Solar wind
The corona begins to flow outward when it escapes the sun’s gravity (about 15 solar radii)
Sunspots
Earth-sized regions on solar surface that are a little cooler than the surrounding region. Produce intense magnetism
How often do the number and locations of sun spots vary?
11 years
Why does the sunspot cycle happen?
The sun’s magnetic field rises and falls. The overall direction reverses from one sunspot cycle to the next. This is a 22-year cycle that results when the direction of the field is taken into account, called the solar cycle
Active regions of Sun
Concentrated solar activity; associated with groups of sunspots
Prominences
Looplike or sheetlike structures produced when hot gas ejected by activity on the solar surface interacts with the Sun’s magnetic field
Flares
intense, violaent surface explosions that blast particles and radiation into interplanetary space
Coronal mass ejections
huge blobs of magnetized gas escaping into interplanetary space.
Coronal Holes
Low density regions of corona where most of solar wind escapes
Describe the nuclear fusion process
hydrogen is converted to helium in the core of the sun
How are nuclei held together?
Strong nuclear force
Proton-proton chain
4 protons are converted to a helium nucleus, and some mass is lost
Neutrinos
Massless particles that are produced in the proton-proton chain; escape from the Sun
How do neutrinos interact?
Weak nuclear force
Can we detect neutrinos?
A small fraction of them are detectable
Solar neutrino problem
Substantially fewer neutrinos are observed than are predicted by theory. The accepted explanation is that neutrino oscillations convert some neutrinos to other particles between the sun and earth.
Trigonometric parallax
method of measuring distances, specifically to the nearest stars.
A star with a parallax of 1 arc second is how far from earth?
1 parasec, which is about 3.3 light years
Proper motion
True motion of a star across the sky. Measures the star’s velocity perpendicular to our line of sight.
How is the star’s radial velocity measured?
By the Doppler shift of spectral lines emitted by the star, along the line of sight
Apparent brightness
Rate at which E from the star reaches a detector.
Magnitude scale
optical astronomers use this to express and compare stellar brightnesses. Greater magnitude, fainter the star.
Apparent magnitude
measure of apparent brightness
Absolute magnitude
apparent magnitude it would have if placed at a std distance of 10 pc from the viewer. Measure of star’s luminosity.
How do astronomers measure T of stars?
By measuring their brightness through 2 or more optical filters and then fitting a blackbody curve
Photometry
Measurement of the amt of starlight received through each member of a set of filters
Spectroscopic observations of stars provide an accurate means of determining what?
Stellar T and composition
How do astronomers classify stars?
According to the absorption lines in their spectra
Standard stellar spectral classes in order of decreasing T:
O, B, A, F, G, K, M
Radius-Luminosity T relationship
estimates the size of stars.
Dwarfs
stars comparable in size or smaller than the sun
Giants
Stars up to 100 times larger than the sun
Supergiants
stars more than 100 times larger than the sun
Red supergiants
large, cool, and luminous
White dwarfs
small, hot, and faint
H-R Diagram
Plot of stellar luminosity vs. stellar spectral classes or T.
Main sequence
about 90% of all stars plotted on a H-R diagram lie here, which stretches from hot, bright blue supergiants to cool, faint red dwarfs (diagnol on the plot)
Are blue or red dwarfs more common?
Red
About how many starts are in the white-dwarf region?
About 9% of stars
Where do the remaining 1% of stars fall on the H-R diagram?
Red-giant region
Spectroscopic parallax
method of determining distance by measuring spectral type and luminosity to estimate the distance of a star on the main sequence. Valid for stars up to several thousand parsecs from Earth
Luminosity class
allows astronomers to distinguish main-sequence stars from ginats and supergiants of the same spectral type
Many stars are not isolated in space, but orbit other stars in _____
Binary-star systems
Visual binary
both stars can be seen and their orbits charted
Spectroscopic binary
stars cannot be resolved, but their orbital motion can be detected spectroscopically
Eclipsing binary
orbit is oriented st one star periodically passes in front of the other, as seen from Earth. This dims the light we receive. This can allow mass determination.
What stars exhaust their fuel rapidly and have much shorter lifetimes than the SUn?
High-mass stars
Low-mass stars
consume their fuel slowly and may remain on the main sequence for trillions of years
Interstellar medium
occupies the space among the stars. Made up of cold (<100K) gas, atomic or molecular hydrogen and helium, and dust grains.
Extinction
Diminution of starlight by dust
Reddening of light
Dust preferentially absorbs short wavelength radiation, which reddens light passing through interstellar clouds.
What is interstellar dust composed of?
Silicates, graphite, iron, ‘dirty ice’
What are interstellar dust particles like?
Elongated/rodlike
Polarization of starlight
Provides a means of studying interstellar dust particles
Nebula
Fuzzy bright or dark patch on the sky
Emission nebulae
extended clouds of hot, glowing interstellar gas. They are associated with star formation, when hot O- and B-type stars heat and ionize their surroundings
What does studying the emission lines produced by excited nebular atoms allow astronomers to measure?
Properties of nebulae
Dark dust clouds
cold, irregularly shaped regions in the interstellar medium whose constituent dust diminishes or obscures the light from background stars
Molecular clouds
in interstellar medium; cold, dark, cool and dense enough that much of the gas exists in molecular form. Dust protects molecules and acts as catalyst for their formation.
Molecular cloud complex
millions of times more massive than the sun; several molecular clouds found close together
What length of radiation allows cold, dark regions of interstellar space containing atomic hydrogen to be observed in the radio spectrum? How is it produced?
21 cm radiation
Produced when the electron in an atom of H reverses its spin
What are molecular clouds observed by?
Radio radiation emitted by molecules they contain; radio waves are not absorbed by interstellar medium
What is the most common constituent of molecular clouds?
Hydrogen; molecular hydrogen is hard to observe
How do astronomers study the composition of molecular clouds?
Other ‘tracer’ molecules that are less common but easier to detect. Many complex molecules have been identified in interstellar clouds
Star formation
When an interstellar cloud collapses under its own gravity and breaks up into pieces comparable in mass to our sun.
Evolutionary track
evolution of contracting cloud; seen on H-R diagram.
Protostar
as a collapsing prestellar fragment heats up and becomes denser, it eventually becomes a protostar. Very warm, luminous object that emits mainly IR. Protostars central T becomes high E to fuse hydrogen, becoming a star.
How long does the star formation process take for a star like the sun? What about more massive/less massive stars?
About 50 million years. More massive stars pass through formation stages more rapidly. Less massive stars take much longer to form
Zero-age main sequence
region in the H-R diagram where stars lie when the formation process is over
What is the main property in determining a star’s characteristics and lifespan?
Mass
More massive stars have shortest lifespans
Brown dwarfs
Low-mass fragments that never reach the point of nuclear ignition
What is used in studying early phases of cloud contraction and fragmentation?
Radio telescopes
What IR observations allow us to see?
Later stages of star formation process.
Protostellar winds
powerful; produced by protostars. Encounter less resistance in directions perp to a star’s protostellar disk and expel two jets of matter in direction of protostar’s poles in bipolar flow
Shock waves
produced as young hot stars ionize surrounding gas forming emission nebulae. Can compress other interstellar clouds and trigger more star formation
Star Cluster
hundreds or thousands of stars
Open clusters
few hundred to few thousand stars, found mostly in plane of Milky Way. typically contain bright blue stars; formed recently
Globular clusters
found mainly away from Milky Way plane, may contain millions of stars. Include no main-sequence stars larger than sun, indicating they formed long ago.
Core hydrogen-burning phase
stably fusing hydrogen into helium at their centers. Stars leave the main sequence when H in core is exhausted.
How far is the sun through its main sequence lifetime?
About halfway.
Hydrogen shell-burning phase
Nonburning He core surrounded by a layer of burning H
Subgiant branch and red-giant branch
Where a star like the sun moves off the main sequence
Electron degeneracy pressure
Makes the core unable to react to new E source, and He burning begins violently in a He flash.
Horizontal branch
after He flash expands the core and reduces star’s luminosity, the star moves here, now having a core of burning He surrounded by a shell of burning H
Asymptotic gian branch
as He burns in core, it forms an inner core of nonburning carbon. The carbon shrinks and heats the overlying burning layers, and the star becomes a red giant, even more luminous than before
Planetary nebula
When a stars envelope is ejected into space. Core becomes visible as hot, faint, dense white dwarf, and diffuses into space.
Black dwarf
white dwarf cools and fades into a cold black dwarf
Do high or low mass stars evolve more rapidly?
High
Which stars never initiate a He flash?
High mass
Which stars die explosively?
High mass
Main Sequence-turnoff mass
no stars above this mass remain on the main sequence. Stars below this mass have not yet evolved into giants and lie on MS. This can be used to measure a cluster’s age
Roche lobe
tear-drop shaped which defines the region of space within which matter ‘belongs’ to the star
Nova
star that suddenly increases greatly in brightness, then slowly fades back to its normal appearance (in a few months)
What makes a Nova?
the result of a white dwarf in a binary system drawing H-rich material from its companion
Accretion disk
gas spirals inward and builds up on the white-dwarfs surface, eventually becoming hot and dense enough for H to burn explosively, causing a large increase in dwarf’s luminosity.
How big do stars have to be to form heavier elements in their cores?
More than 8 solar masses
Which heavy element stops the fusion process?
Iron
When does a star’s core begin to collapse?
As it grows in mass, it becomes unable to support itself against gravity
Core Collapse Supernova
Iron nuclei are broken down into protons and neutrons. Protons combine with electrons to form more neutrons. When the core becomes so dense that neutrons come into physical contact, the collapse stops and core rebounds, sending a violent shock wave out through the rest of the star
Type 1 supernovae
hydrogen poor and have a light curve similar in shape to that of a nova
Type 2 supernovae
hydrogen rich and have a plateau in light curve a few months after maximum. This is a core-collapse supernova.
Carbon detonation supernova
Carbon oxygen white dwarf in a binary system gains mass, collapses, and explodes as its carbon ignites
Supernova Remnant
shell of exploded debris surrounding the site of the explosion and expanding into space at a speed of thousands of km / s
Stellar nucleosynthesis
production of new elements by nuclear rxns in the cores of evolved stars
Helium capture
elements heavier than C tend to form this way. At high core T, photodisintegration breaks apart heavy nuclei, providing He-4 nuclei for synthesis of more massive elements
Neutron Capture
how elements beyond iron form in the cores of evolved stars.
Neutron Star
ultracompressed ball of material. Extremely dense, very hot at formation, magnetized, and rapidly rotating. Cool down and lose their magnetism as they age
Lighthouse model
Neutron stars, because they are magnetized and rotating, send regular bursts of EM E into space. Beams are produced by charged particles confined by strong magnetic fields
Pulsar
Beams of neutron stars we can see from earth.
X-Ray Burster
When hydrogen burning starts explosively on star’s surface
Rapid rotation of inner part of accretion disk causes neutron star to spin faster as new gas arrives on surface. The rapidly rotating neutron star produces what?
A millisecond pulsar
Gamma-Ray Bursts
Energetic flashes of gamma rays, observed about once a day, distributed uniformly over the sky. Can measure distances, implying extreme luminosity. Theories that these are produced from the violent merging of neutron stars
Einstein’s special theory of relativity
Behaviour of particles moving at speeds comparable to the speed of light. Agrees with Newton’s theory at low v.
Einstein’s general theory of relativity
Describes gravity in terms of warping, or bending, of spacetime by the presence of mass. More mass, greater warping.
Upper limit on mass of neutron star
3 solar masses
Black hole
region of space from which nothing can escape.
Schwarschild radius
Radius at which escape speed from a collapsing star equals the speed of light. The sphere of this is called the event horizon
What concepts occur around black holes?
Gravitational redshift and time dilation.
Galaxy
huge collection of stellar/interstellar matter isolated in space and bound together by its own gravity
Galactic disk
appears as a broad band of light across the sky, called the Milky Way.
Galactic bulge
Disk thickens into this near the centre of the galaxy
Galactic halo
old stars and star clusters
Spiral galaxy
Milky Way
Variable stars
study the halo; luminosity changes with time
Pulsating variable stars
Vary in brightness in a repetitive and predictable way
RR Lyrae Variables / Cepheid variables
have same luminosity; can be determined using the period-luminosity relationship. Distance can be determined
What does the Galactic halo lack?
Gas and dust
Are halo stars old?
Yes
Spiral Arms
regions of densest interstellar gas where star formation is taking place.
Spiral density waves
move through disk, triggering star formation as they pass by
Self-propagating star formation
When shock waves produced by the formation and evoln of one generation of stars trigger the formation of the next.
Galactic Rotation curve
plots orbital speed of matter in disk against distance from centre. Can determine mass of galaxy by applying Newton’s laws of motion
Dark Halo
containing far more mass than can be accounted for in the form of luminous matter
Dark Matter
In dark halos; unknown composition. Candidates are low mass stars, exotic subatomic particles.
Gravitational microlensing
Used to study dark matter
Hubble Classification Scheme
Divides galaxies into several classes, depending on their appearance.
Spiral galaxies
flattened disks, central bulges, and spiral arms. Old stars in halos
Barred-spiral galaxies
extended ‘bar’ of material projecting beyond central bulge
Elliptical galaxies
no disk and contain little or no cool gas/dust, although very hot interstellar gas is observed. Old stars.
Irregular galaxies
Do not fit into any other category. Rich in gas/dust and are the sites of vigorous star formation.
Standard Candles
Distance measuring tools. Objects easily identifiable whose luminosities lie within a well-defined range
Tully-Fisher relation
empirical correlation btwn rotational velocity and luminosity in spiral galaxies
Local Group
Milky Way, Andromeda, and several other smaller galaxies; gravitationally bound collection
Galaxy Clusters
consist of many galaxies orbiting one another, bound together by their own gravity
What is the nearest galaxy cluster to the Local Group?
The Virgo Cluster
Hubble’s Law
Distant galaxies are observed to be receding from the Milky way, at speeds proportional to their distances from us
Hubble’s Constant
Proportionality;
70 km/s/Mpc
How do Astronomers use Hubble’s law?
To determine distances to the most remote objects in the universe
Cosmological redshift
redshift associated with the Hubble expansion
Active galaxies
much more luminous; nonstellar spectra, emitting most E outside of visible part of spectrum.
Active galactic nucleus
Nonstellar activity suggests rapid internal motion
Many active galaxies have high-speed, narrow jets of matter shooting out from their central nuclei; the jets transport E from nucleus to where?
Radio lobes lying far beyond the visible portion of the galaxy
Seyfert galaxy
Looks like a normal spiral but has an extremely bright central galactic nucleus. Spectral lines are broad, indicating rapid internal motion
Radio galaxies
emit large amounts of E in radio part of spectrum
Quasars (or quasi-stellar objects)
Most luminous objects known; in vis light, they appear starlike, and their spectra are redshifted. All quasars are very distant
What is the accepted explanation for the observed properties of all active galaxies?
Their E is generated by the accretion of galactic gas onto a supermassive black hole lying in the center
What explains the compact extent of the emitting region and high-speed orbit of gas?
Small size accretion disk
What do typical luminosities of active galaxies require the consumption of?
About 1 solar mass of material every few years
Synchrotron radiation
Charged particles spiraling around magnetic lines produce this, whose spectrum is consistent with radio emission from radio galaxies and jets
What do measurements of galaxy and cluster masses reveal?
Large amounts of dark matter
About what percent of mass in universe is dark matter?
90%
What do most astronomers think of how large galaxies formed?
By the merger of smaller ones and collisions/mergers among galaxies are how galaxies evolve
Starburst galaxy
may result when a galaxy has a close encounter or collision with a neighbor. This compresses galactic gas, resulting in burst of star formation
What do mergers between spirals most likely result in?
Elliptical galaxies
Evolutionary sequence of galaxies
Quasars, active galaxies, normal galaxies
Quasar feedback
may provide a partial explanation of why masses of black holes are correlated with masses of their parent galaxies.
Superclusters
galaxy clusters clump together
Local Supercluster
Virgo cluster, Local group, and several other nearby clusters
Voids
galaxies/clusters are arranged on surfaces of enormous bubbles of matter surrounding low-density regions
What is the origin of voids?
closely related to conditions in the very earliest epochs of the universe
What can be used as probes of the universe along the observer’s line of sight?
Quasar spectra
Homogeneous
the same everywhere
Isotropic
the same in all directions
Cosmology
study of the universe as a whole
What is one of the main assumptions of cosmology?
The universe is homogeneous and isotopic; known as the principle
What does the principle assumption of cosmology imply?
The universe cannot have a center or an edge
If the universe were homogeneous, isotropic, infinite, and unchanging, what would the night sky look like?
Bright, any line of sight would intercept a star
Olbers’s paradox
The fact that the night sky is dark because we see only a finite part of the universe from Earth
Tracing observed motions of galaxies back in time implies what about 14 billion years ago?
The universe was a hot, dense primeval fireball that expanded rapidly in the Big Band
Did the Big Band happen at a specific location
No; space was compressed to a point in that instant; Big Band happened everywhere at once.
Two possible outcomes of the current expansion of the universe
- Expand forever
- Eventually recollapse
Critical density
density of matter needed for gravity alone to overcome the present expansion and cause the universe to collapse
What do astronomers think the total mass density of the universe is today compared to the critical density/
No more than 30% of the critical density
What is the curvature of spacetime determined by?
The total density of the universe, including that of matter, radiation, and dark E
Closed universe
curvature in a high-density universe is sufficiently large that the universe ‘bends back’ on itself and is finite, somewhat like the surface of a sphere
Open universe
Low density; infinite in extent and has a saddle-shaped geometry
Critical universe
Density precisely equal to the critical value; spatially flat
What do observations of distant supernovae indicate about the expansion of the universe?
The expansion of the universe is accelerating, driven by the effects of dark E
What does data suggest about the shape of the universe?
That it is flat
Cosmic microwave background
Isotropic blackbody radiation field that fills the entire universe. T around 3K. The existence is direct evidence that the universe expanded from a hot, dense state.
What dominates the universe?
Dark energy
When was the universe matter dominated?
When the universe was smaller, a few billion years ago
Radiation Dominated
The early universe
Pair Production
During the first few minutes after the Big Band, matter was formed. Particles and forces froze out of the radiation as T fell below threshold. Unequal amount of matter and antimatter
Four fundamental forces of nature
Gravity
Electromagnetism
Strong nuclear
Weak nuclear
Primordial nucleosynthesis
Formed most of He in universe today
Epoch of Inflation
brief period of rapid expansion of the universe, during which the size of the cosmos increased by a factor of 1050 or more
Horizon problem
according to the standard Big Bang model, there is no good reason for widely separated parts of the universe to be as similar as they are
How does inflation solve the horizon problem?
Takes a small homogeneous patch of the early universe and expands it enormously.
Flatness problem
Why the present density is so close to the critical value
Cold Dark Matter
depends on the T of dark matter at the end of the radiation era; much is cold
“Ripples” in the microwave background
imprint of early density inhomogeneities on the radiation field
Cosmic evolution
continuous process that has led to appearance of galaxies, stars, planets, and life on Earth
How may living organisms be characterized?
By their ability to react to envrmt, grow, reproduce
What is strongly favoured by natural selection
intelligence
What may have led to the formation of amino acids and nucleotide bases?
Reactions between simple molecules in the oceans, powered by natural E sources
What do amino acids build?
Proteins, which control metabolism
What do sequences of nucleotide bases build?
DNA, genetic blueprint of a living organism
What is the best hope for life beyond Earth in the solar system?
Mars
What other outer planet moons may be possibilities for life of some sort?
Europa and Ganymede (Jupiter)
Titan and Enceladus (Saturn)
Extremophiles
thrive in hostile environments
Drake Equation
provides a mean of estimating the probability of intelligent life in the galaxy
Factors of Drake Eqn
galactic star formation rate, likelihood of planets, and number of habitable planets
Chemical and biological factors
probability that life appears and that is subsequently develops intelligence
Cultural and political factors
probability that intelligence leads to technology and lifetime of a civilization in technological state
What is the likely distance to our nearest intelligent neighbor?
Hundreds of parasecs
Is space travel a feasible means of searching for intelligent life?
No
How do we search for extraterrestrial intelligence?
Scanning electromagnetic spectrum for signals
Water Hole
region in the radio range of the electromagnetic spectrum, near the 21-cm line of H and 18-cm line of OH, where natural emissions from the Galaxy happen to be minimized
What region is the best part of the spectrum for communication purposes?
The water hole
