The distance from the Sun to Mars is about 220,000,000 km. What is this distance in scientific notation?
Count the number of digits (both zero and nonzero) that are to the right of the first digit (but to the left of the decimal point, if there is one present); there are eight digits in 20,000,000. This number, eight, is then the exponent of 10.
Which of the following fundamental requirements, if shown to be True, establishes that a particular theory is a scientific theory?
The theory predicts new observations, even if they prove the theory wrong.
A scientific theory must have the property of predicting new phenomena that have not yet been observed. If these phenomena turn out not to occur, then the theory is wrong and has to be either modified or discarded.
Which of the following statements best represents the overall rationale for scientific investigation?
Reality is comprehensible, and a limited number of fundamental principles govern the nature and behavior of the universe.
This sentiment is the basis for scientific study, and it is borne out by observation.
What are constellations?
Groupings of stars that cover various areas of sky from large to small.
Constellations are groupings of stars that (for the most part) stand out against the surrounding stars, e.g., Orion and the Big Dipper.
A Perfect Blackbody
absorbs all energy falling upon it and emits a characteristic spectrum of radiation whose intensity as a function of wavelength depends only on its temperature.
Longest to Shortest Wavelength
Radio, Microwave, visible, ultraviolet, gamma-ray
A piece of steel is heated by a blacksmith until the wavelength of maximum emission of radiation is measured to be 1 μm, in the infrared part of the spectrum. How would the blacksmith have to change its temperature in order that this peak wavelength would move to 0.5 μm, or 500 nm in the visible spectral range?
Double its temperature.
Peak Wavelength changes inversely with temperature.
In a vacuum, blue light differs from red light in that...
Blue light has a higher frequency than red light.
Twice the size
Suppose a light source is emitting red light at a wavelength of 700 nm and another light source is emitting ultraviolet light at a wavelength of 350 nm. Each photon of the ultraviolet light has
twice the energy of each photon of the red light.
Photon energy varies inversely with the wavelength. Each ultraviolet photon has half the wavelength of a red-light photon, so it has 1/(½) = 2 times the energy.
The presence of dark lines in the solar spectrum, the so-called Fraunhofer lines, means that
a cooler layer of gas overlies the deeper, hotter layers of the solar atmosphere.
This simple observation tells us that the temperature falls (for a short distance) as height increases.
When a source of light is moving away from the observer, the wavelength of the detected light is shifted with respect to the emitted or rest wavelength. The size of this shift in wavelength is described by the Doppler effect and is equal to the
value of the rest wavelength times the ratio of the speed if the source to the speed of light.
The shift is dependent upon wavelength and upon the ratio of the speed of the source to that of light.
The Sun has a temperature of 5800K, and the peak of its radiation is around a wavelength of 500nm. At what wavelength would you expect to see the peak of the radiation from an extremely hot star of temperature 58,000K?
Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star doubles, what will happen to the peak wavelength of the spectrum?
It will be half of the previous wavelength.
Wien's Law is an inverse relation - higher temperature means shorter (smaller) wavelength. There are no powers, so double the temperature means half the wavelength.
Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star DECREASES by a factor four, what will happen to the peak wavelength of the spectrum?
It will increase by a factor of four in wavelength.
We deduce that the Sun does not obtain its energy solely from gravitational contraction because:
this could not explain the age of the sun.
Gravitational contraction can release a lot of energy, so could explain the current temperature and age of the Sun. It is very gradual, so would not be obvious on human timescales. The main problem is that it only provides energy for a short time (on astronomical timescales), and so could not explain the age of the Sun.
How does the Sun generate energy?
Converting H to He
Comes from thermonuclear fusion of H to He
After the Sun's core hydrogen is depleted by nuclear fusion, the core will consist primarily of:
The bulk of the Sun's life is spent fusing hydrogen to helium, so after the hydrogen is depleted, helium will remain. Later, helium will be fused to carbon and oxygen, but that was not the question that was asked.
How is the Sun's mass changing?
It decreases because matter is converted to energy and escapes as radiation.
The energy radiated from the Sun ultimately originates from the conversion of mass into energy in nuclear reactions. Consequently the Sun's mass is gradually decreasing.
Which of the following best describes all the processes important in energy getting out of the Sun?
Radiation and convection.
Energy from the core first escapes by radiation through the radiative zone, then by convection through the convective zone, and ultimately leaves the surface as radiation.
One of the processes carrying energy from the core of the Sun to its surface is convection. What does the process of convection involve?
the motion of hot gasses.
In convection, circulating currents of hot gas carry heat upwards. On a day to day basis, we know convection as 'Hot air rises'
Which of the following statements most accurately summarises what is meant by hydrostatic equilibrium in the Sun?
On average, gas neither moves up nor down because pressure from below balances the force of gravity.
Hydrostatic equilibrium is about pressure balancing gravity. Exactly the same thing happens when you float in water. If convection is occurring, then some gas rises and some sinks, but on average there is no overall motion of a layer up or down.
The surface layers of the Sun are very massive. What stops the Sun from collapsing under its own weight?
The pressure of the very high-temperature gas within the Sun supports the outer layers.
The layers of the Sun are supported by pressure from the layers below them. This is called hydrostatic equilibrium - pressure balancing gravity. Exactly the same thing happens when you float in water.
Which of the following best describes the spectrum of the Sun?
A bright continuous spectrum, containing numerous absorption lines created by the atmosphere of the Sun.
The Sun is a ball of opaque, hot gas, surrounded by layers of cooler transparent gas. By Kirchhoff's 3rd Law, this produces an absorption line spectrum. The absorption lines are known as Fraunhofer lines.
Are unusually cool areas of the photosphere
Are associated with magnetic fields
Come in pairs
Can sometimes be seen with the naked eye.
Which of the following correctly describes the structure of the Sun's atmosphere from inside to outside
Photosphere, chromosphere, corona.
Which of the following events on the surface of the Sun is MOST likely to damage satellites, endanger astronauts, and disrupt communications?
A coronal mass ejection.
A coronal mass ejection is the most violent event on the Sun's surface and produces the most dramatic Solar storms. If one is directed at the Earth, satellites must be put into safe modes, astronauts will be evacuated from the space station or must take cover in shielded compartments, satellite communications may be disrupted, and power outages can occur on Earth.
What is the parallax of a star at 500 parsecs?
Why do astronomers not use annual parallax (i.e. motion of stars relative to background objects) to measure the distances to bright stars far away in our Galaxy?
Annual parallax can only be measured for nearby stars
The maximum distance at which we can measure reliable parallaxes is a few hundred parsecs. The Galaxy is thousands of parsecs across.
Star B appears FAINTER to us than star A. Why could this be?
Star B is behind more dust, or star B is smaller than star A, or star B is further away than A.
If a star is further away, coolor, or behind more dust, we expect it to be dimmer. We would need more observations to tell which is the correct explanation.
Star A has the same luminosity as star B, but is 4x farther away. How much dimmer does it appear?
Brightness varies following the inverse square law. If the star is 4x farther away it is 4x4=16x dimmer.
Star A appears 3x as bright as star B even though it is 2x as far away. How many times more luminous is star A than star B?
12x more luminous
An A star has a temperature of 10,000K and a G star 5,000K. If they both have the same radius, which of the following statements are true?
The A star is 16x as luminous and emits most of its energy at shorter wavelengths
A blue supergiant has a radius 10x that of the Sun, and temperature 10x that of the Sun. How many times more luminous than the Sun is it?
The star Sirius is noticeably bluer than the Sun because
it is hotter.
Only a hotter star will be noticeably bluer. A larger star would be the same color, dust will make things redder, and if the star is moving towards us it will only be very slightly bluer, too little to notice with the eye.
Which are the hottest stars?
O then B.
Where in the Hertzsprung-Russell diagram would you expect to find a 50 solar mass main-sequence star?
In the top left.
Where in the Hertzsprung-Russell diagram would you expect to find a white-dwarf star?
In the bottom-left
If you can place a star in a Hertzsprung-Russell diagram, it is possible to estimate its radius. Which law or other relationship do you use to do this?
the Stefan Boltzmann Law
The Stefan-Boltzmann Law allows you to relate the temperature, luminosity, and radius of the star.
A dark cloud scattering light from foreground stars.
A dark cloud of dust can either appear as a reflection nebula when a star is in the foreground, or as a dark nebula when there are stars in the background.
a cloud of dust that blocks light from background stars.
A dark cloud of dust can either appear as a reflection nebula when a star is in the foreground, or as a dark nebula when there are stars in the background.
We see an emission nebula via
Mainly red light emitted by hot hydrogen atoms
Emission nebulae are caused by hot, glowing gas. Since most of the gas in the universe is hydrogen, hydrogen emission usually dominates.
Why do distant stars in our Galaxy appear redder than they should?
Their blue light is scattered by intervening dust
Intervening dust scatters the blue light allowing most of the red light to pass through. Reddening is caused by scattering by dust, not absorption by gas. The Doppler effect does not produce noticeable color changes in stars in our galaxy.
How can we best see protostars buried inside dusty nebulae?
With infrared telescopes
Infrared penetrates dust well and is produced in large amounts when a protostar is still cooler than it will be on the main-sequence.
In which of the following nebulae will it be easiest to form stars?
A cool, high density nebula
Gravity must overcome pressure to form a star. High gravity requires high density. Low pressure requires low temperature.
If a protostar is contracting which of the following is true?
Gravity is overcoming pressure
If a star is shrinking then gravity is overcoming pressure. If it is expanding pressure is overcoming gravity.
Which of the following is occurring before a protostar joins the main-sequence?
It is releasing energy by Kelvin-Helmholtz contraction
It is collapsing
core is getting hotter
How do protostars get their energy?
Protostars obtain their energy from gravitational collapse (like brown dwarfs)
Which of the following can help to stimulate star formation?
galactic spiral arms
collisions between gas clouds
Anything that works to compress gas clouds can trigger star formation. This can include collisions between gas clouds, and supernovae, and spiral arms in a galaxy are really waves of compression moving through the galaxy.
Which of the following statements about the history of the Sun (within the last few billion years) are correct?
The young Sun was a little less luminous than it is now
The Sun's luminosity has increased a little over its main-sequence lifetime. This is because as hydrogen in the core is converted into helium, the core becomes a little hotter and denser and nuclear reactions proceed faster
The total lifetime of the Sun is estimated to be 10 billion years. If a star had twice the mass of the Sun, but a luminosity 10x larger, what could you say about its expected lifetime, assuming it otherwise behaves like the Sun?
The star will live 2 billion years
Why do massive stars have shorter lifetimes than the Sun?
They have more fuel than the Sun, but burn it at a much higher rate, so it still does not last so long
All stars initially burn hydrogen for most of their lifetimes. Massive stars are larger than the Sun, so have more hydrogen fuel. However they burn it MUCH faster (producing a higher luminosity) so they have a shorter lifetime.
Globular clusters contain red giant stars. How do they mainly generate energy?
Fusion of hydrogen to helium in a shell around the core
Red giants are burning hydrogen to helium in a shell around the core. Usually, if a star is a giant it is undergoing shell burning. If it was burning helium in a shell, it would be called an asymptotic giant branch star (AGB) to differentiate it from red giants.
Globular clusters contain red dwarf stars. How do they mainly generate energy?
Fusion of hydrogen to helium in the core
Red dwarfs are main-sequence stars. All main-sequence stars are fusing hydrogen to helium in the core.
If a red giant star is expanding, which of the following is true?
Pressure is overcoming gravity
Expanding means that pressure is dominating over gravity, contracting would mean gravity is dominating.
Where in the H-R diagram would you expect to find stars which are burning hydrogen in a shell around the core?
Stars burning hydrogen in a shell around the core are either red giants (if low mass) or red supergiants (if high mass). Both groups are towards the top-right of the H-R diagram, cool but luminous.
Which of the following correctly describes the first part of the lifecycle of the Sun?
Protostar, main-sequence, sub-giant, red-giant
The full sequence is 1. protostar, 2. main-sequence, 3. sub-giant, 4. red-giant, 5. helium flash, 6. horizontal branch star, 7. asymptotic giant branch star, 8. white dwarf.
Which of the following phases in the life of a star like the Sun lasts longest?
Stars spend 90% of their lifetime on the main-sequence.
Which type of star would you NOT expect to find in a globular cluster?
Globular clusters are old, so they only contain old stars that have had long lifetimes. Blue supergiants are massive and very short-lived stars. All the blue supergiants that were born in globular clusters died long ago.
Star clusters are useful because many quantities are approximately the same for all the stars. Which of the following are not even approximately the same?
Star clusters are born with a wide range of star masses. The other quantities are all, at least approximately, the same.
Which of the following best describes an open cluster?
A loose collection of young, hot stars
Globular clusters are dense and old. Open clusters are loose and young.
Many pulsating stars are seen for which the luminosity can be estimated if the pulsation period is known. Why is this so important to astronomers?
Stars of known luminosity can be used to measure distances
Any time you know the luminosity of an object, you can combine it with its apparent brightness to deduce the distance.
What is found at the center of a planetary nebula?.
Black holes and neutron stars are only produced by supernova. Brown dwarfs are protostars that never ignited fusion of hydrogen to helium and are not stellar remnants.
Which of the following is not produced by the death of a star?
A brown dwarf
Brown dwarfs are protostars that never ignited fusion of hydrogen to helium and are not stellar remnants.
Why are there no 3 solar mass white dwarfs?
Electron degeneracy pressure cannot support a 3 solar mass white dwarf
Electron degeneracy pressure is well understood. The maximum mass it can support is 1.4 solar masses, so this is the maximum mass a white dwarf can have.
What is meant by the Chandrasekhar mass?
The maximum mass possible for a white dwarf
Chandrasekhar calculated the maximum mass that electron degeneracy could support against gravitational collapse. This is the maximum mass that a white dwarf can have.
Which of the following statements about stellar evolution IS true
Massive stars are unable to fuse iron into heavier elements in their core
Supernovae leave behind supernova remnants, not planetary nebulae. The Sun is not massive to end its life in a supernova. Low mass stars cannot produce elements heavier than carbon and oxygen. Massive stars end their lives in supernovae and do not leave behind white dwarfs. The true statement is that massive stars cannot produced elements heavier than iron in their core - these elements are only produced during supernovae.
Which of the following sequence of nuclear fuels is in the order in which a massive star would burn them?
Hydrogen, helium, carbon, silicon
What is the heaviest element that can be produced by normal nuclear fusion in the core of a massive star?
Iron has the lowest mass per nucleon (i.e. proton or electron). Fusing iron into heavier elements requires adding energy to make up the extra mass necessary.
Which of the following elements can ONLY be produced in a supernova?
Hydrogen has been present in the universe since the big bang, helium was primordial too, but all stars can fuse hydrogen to helium. Carbon is produced by fusion in the cores of most stars eventually, iron is only produced in the cores of massive stars. Only gold requires a supernova for production.
What is most likely to be left behind by a Type II supernova?
A neutron star
Type II supernovae are caused by core collapse. The collapse only stops once the pressure from neutrons balances gravity. This produces a neutron star. The only other possible option is a black hole (which is not a choice for this question).
What is left behind by a type Ia supernova?
A type Ia supernova occurs when carbon burning is ignited in a white dwarf. The burning is explosive and destroys the white dwarf.
What is the cause of a Type Ia supernova?
Explosion of a white dwarf
Type Ia supernova are supernovae showing no hydrogen in their spectrum, but with silicon lines present. Unlike other kinds of supernova they do not occur when the core of a star collapses, but instead when a white dwarf exceeds the Chandrasekhar mass.
If you see a nebula with a radio pulsar at its center, what could it be?
A remnant of a type II supernova
Radio pulsars are neutron stars. Planetary nebulae leave white dwarfs behind. Type Ia supernova leaving no compact star behind (the white dwarf is destroyed). Only a type II supernova (of the options given) leaves behind a neutron star.
Which of the following is NOT left behind by the death of a star?
Deaths of low mass stars leave a planetary nebula with a white dwarf at the center. The death of a high mass star leaves a neutron star or black hole at the center of a supernova remnant. Dark nebulae are associated with regions of star formation.
Which of the following scenarios best describes what we believe will happen at the end of the lifetime of a 5 Solar mass star?
The envelope will be ejected leaving a planetary nebula surrounding a white dwarf
Deaths of low mass stars (below about 8 Solar masses) leave a planetary nebula with a white dwarf at the center. The death of a high mass star leaves a neutron star or black hole after either a type II supernova or a gamma-ray burst. Type Ia supernovae happen much later when a white dwarf accretes matter from a companion star.
The speed of light in a vacuum is always the same independent of motion of the light source or the observer
The speed of light (in a vacuum) is finite, and a fundamental constant of physics.
If light is emitted by an object in the intense gravity field just outside the event horizon of a black hole, how will it appear different to a distant observer?
Its frequency is decreased and light is red-shifted because time passes more slowly in strong gravity
Light passes slower in strong gravitional fields than it does in empty space. This means the frequency of light is lower and so the light is red-shifted.
According to the General Theory of Relativity, a beam of light bends when it passes close to a massive object because
The photons follow the curvature of space-time
The speed of light is a constant, and it does not even make sense to define a force acting on a photon that has no mass. In relativity, the effect of gravity is to curve space time, so that light no longer travels in straight lines.
What is the event horizon of a black hole?
The event horizon is the point beyond which no information of any kind can escape, at which gravity is strong enough that light cannot escape, and at which space curves in on itself.
I am flying towards you in a spaceship at 100,000 km/s. I see light from my headlamps (!) moving towards you at 300,000 km/s. How fast will the light seem to be moving to you?
Light (in the vacuum of space at least) is always measured to move at 300,000 km/s regardless of how you are moving.
Which of the following correctly describes what we would see if we watched an astronaut fall into an extremely massive black hole?
He would become redder as his light is red-shifted
Objects in a strong gravitational field experience time dilation, so that time appears to pass slower for them. This means that any light from the astronaut is shifted to lower frequencies, and longer wavelengths (i.e it is red-shifted) It also means that any radio signals he gives out would be shifted to lower frequencies (not higher) and that he would appear to us to fall slower than he actually does (not faster).
You have a black hole, a white dwarf, and a neutron star all of which have the same mass as the Sun. Which of the following is in order of size from SMALLEST TO LARGEST?
Black hole, neutron star, white dwarf
Black holes are the most compact, then neutron stars, then white dwarfs. You must compare objects of the same mass, however. A one million Solar mass black hole is much larger than a one Solar mass white dwarf.
A beam of light is now known to bend as it passes a massive object such as the Sun. What is the correct explanation for this "bending"?
Space is curved around a massive object, and everything including light must follow this curvature.
Space is curved around a massive object, and everything including light must follow this curvature.
Can X rays escape from inside the event horizon of a black hole? Why, or why not?
No. Nothing (not X rays or anything else) can escape through the event horizon.
The X rays that we see from black holes are believed to be emitted by matter that is in the process of spiraling into the black hole but has not yet passed through the event horizon. This matter is compressed and heated as it approaches the event horizon, and it emits X rays as a result. If X rays are emitted anywhere inside the black hole, they cannot escape.
Jill is a navigator on a spaceship that is passing Mars at 99% of the speed of light. What does she see when she looks at a spaceport building on Mars that has a clock on it?
She sees the building looks shortened in the direction that she is moving, and the clock on Mars appears to run slower than her own clock.
Motion is always relative to the observer, so she sees herself at rest and Mars as moving. She therefore sees objects on Mars relativistically shortened and clocks running slow
Where in the universe has evidence been found for the existence of supermassive black holes with masses of at least a million solar masses?
in the centers of many galaxies
There is strong evidence, from the measurements of very high-speed motions of material moving around the centers of many galaxies, that a very massive but localized object must exist to provide the requisite curvature of space in these regions.
Why were astronomers misled about our position in the Galaxy until the early 20th century?
They did not believe dust blocked our view of much of the Galaxy
In the first few decades there were quite extensive surveys of the Galaxy with large telescopes. It was not appreciated until 1930 however that these surveys missed many stars that are hidden behind large amounts of dust.
Which of the following best describes how globular clusters are distributed in our Galaxy?
They are distributed spherically about the center of our Galaxy in the halo.
Globular clusters are the most visible part of the halo, although it does contain individual stars as well.
Population I stars are young and metal rich. Where in the Galaxy are most commonly found?
Population I stars are mainly found in the Galactic disk. There are some in the bulge and Galactic center, but this is a more mixed population.
Type II supernovae are produced by the collapse of the core of a massive star. Where in our Galaxy are they most likely to be seen?
In the disk of the Galaxy
Massive stars only live for a short time, so they (and type II supernovae) are only found in regions of recent star formation. The disk of the Galaxy is where star formation is most active in our Galaxy.
Astronomers use 21-cm wavelength radio observations to map the Galaxy. What do these observations actually detect?
Cold H gas
21-cm radio waves come when the electron and proton in a hydrogen atom change their alignment. They are only produced if the hydrogen gas is in its lowest energy ground state, so they originate from cold hydrogen gas.
Which of the following provides evidence for the mysterious dark matter in our Galaxy?
The motion of stars and gas clouds in the outer Galaxy
We infer the presence of dark matter from the speed of rotation of stars and gas clouds in the Galaxy. They rotate too fast to be held in orbit by just the gravity of visible matter. Additional invisible (dark) matter is needed.
In what form is most of the mass of our Galaxy?
We think that most of the mass of our Galaxy is invisible dark matter. It can only be detected by the effect of its gravity.
Which of following types of light is best suited for studying stars in the Galactic center?
Only infrared light from the Galactic center reaches us, optical and ultraviolet is heavily scattered by dust.
A galaxy has a bright linear (line-like) feature in its central region. What kind of galaxy is it most likely to be?
During a galaxy collision the stars that make up the galaxies are most likely to
have their orbits disturbed but otherwise remain intact
The Local Group
contains few (~ 10s) member galaxies.
mostly contain galaxies much smaller than the Milky Way.
is the galaxy cluster to which the Milky Way belongs.
Which is the correct order starting from smallest classification to largest?
Solar System, Milky Way, Local Group, Supercluster
What was the key measurement that showed astronomers that spiral nebulae are galaxies like our own?
How did Hubble determine that spiral nebulae were really galaxies in their own right?
He found Cepheid variables
The type of galaxy least likely to have active star formation is
The kind of galaxy that is most likely the result of galactic collisions is
A lenticular galaxy
is a galaxy that is not purely an elliptical galaxy or a spiral/barred spiral galaxy.
Which two observed parameters of a Cepheid variable star are required in order to use it to determine the distance to a galaxy?
its average brightness or apparent magnitude and its period of variability
Einstein said that there is no way to tell if you are feeling a gravitational force or experiencing an acceleration.
Gravity is an effect of this. Massive bodies create depressions in spacetime and anything moving by the depression is deflected because of it.
Curvature in spacetime so steep that light cannot escape.
the distance around a black hole where the escape velocity is more than the speed of light. Edge of a black hole. Surface.
Center of Black Hole
Small BH, the total force will be strong enough to rip objects apart.