Stars And Galaxies final exam Indiana university

What is an astronomical unit (AU)?

Distance between the earth and the sun

What is the zodiac?

The band of sky where the sun, moon, and planets are found

What is the reason we see different constellations at different times of the year?

The earth orbits the sun

Describe how the pointer stars in the Big Dipper are used to find Polaris.

The pointer stars are the two stars at the far end of the bowl from the handle. Motion along the line defined by these stars, out of the bowl, leads to Polaris—the North Star.

Explain what actually moves outward with a water wave that is generated by dropping a pebble in a pond.

Wave crests and energy move outward with the wave. However, the water itself does not move outward with the wave. It just moves in a small circle about its original position.

What is electromagnetic radiation? What effect does it have on electrons?

Electromagnetic radiation (light) is a means by which energy is transported through space as a wave. This wave causes electrons to oscillate back and forth as it passes, in a direction that is perpendicular to the wave travel direction.

What is the physical significance of the speed of light?

The speed of light (in a vacuum) is the absolute speed limit in the universe. Nothing can go faster than this limit.

Define the terms wavelength, frequency, and amplitude.

Wavelength is the distance between successive wavecrests. Frequency is the number of wavecrests passing a point per time. Amplitude is the height of the wave above the undisturbed level.

What is a nanometer (nm)?

A nanometer is one-billionth (10^9) of a meter.

What is the approximate range of wavelength for visible light?

Visible light ranges in wavelength from about 700 nm (red) to 400 nm (violet).

What is meant by "near infrared" and "near ultraviolet"?

The term "near" indicates location relative to the visible spectrum. The near infrared region of the electromagnetic spectrum is just to the long wavelength side of red. The near ultra-violet region is just to the short wavelength side of violet.

What is a photon? What are its properties?

All forms of electromagnetic radiation consist of individual energy packets called photons, which have some of the characteristics of particles. A photon is the smallest unit of electro-magnetic radiation and cannot be divided. The energy of a photon is proportional to its frequency and thus inversely proportional to its wavelength.

For what type of radiation is the particle-like nature least apparent? For what type is it most apparent?

The higher the photon energy, the more the particle-like nature of photons is apparent. Thus, radio photons have the least particle-like nature and gamma-ray photons have the most particle-like nature.

Describe the difference between a transverse wave and a longitudinal wave. Give examples of both types of wave.

Transverse Wave: Direction of oscillation is perpendicular to direction of wave travel. Examples: waves on a string, sideways Slinky wave, electromagnetic radiation.
Longitudinal Wave: Direction of oscillation is parallel to direction of wave travel. Examples: compression wave on a Slinky, sound wave.

Is a water wave transverse or longitudinal?

A water wave combines both transverse (vertical) and longitudinal (horizontal) motion. The latter motion is a back and forth oscillation parallel to direction of wave travel. The combination of these two motions causes the water to move in a small circle about its equilibrium position.

List the types of electromagnetic radiation in order of increasing wavelength.

Electromagnetic radiation from shortest to longest wavelength: gamma-ray, X-ray, ultraviolet, visible, infrared, microwave, radio

Explain what black body radiation is.

Black body radiation is produced by an ideal absorber and emitter of electromagnetic radiation. Real-world objects, such as kilns and stars, provide approximations to a perfect black body. An ideal black body produces a continuous spectrum, i.e. there are no sharp dips or peaks.

What is a spectrum? Why are spectra useful to astronomers?

A spectrum is a separation of light (electromagnetic radiation) into its constituent wavelengths, using a prism or grating. Obtaining a spectrum allows us to determine how much light is received at each wavelength. This allows astronomers to determine the physical nature of the object that emits the light, e.g. a gas cloud, star, or galaxy.

What is a spectral line? Describe the difference between emission and absorption lines.

A spectral line is a sharp peak or dip in a spectrum at a particular wavelength. A peak is called an emission line, while a dip is called an absorption line.

What is the relation between the size of an electron orbit in an atom and the energy of the orbit?

The larger the orbit, the higher its energy. Thus, the smallest possible orbit corresponds to the lowest energy level (i.e. the ground state).

What causes photons to be emitted and absorbed by atoms?

Photons are emitted by atoms when electrons drop to lower energy levels, releasing energy. Photons are absorbed by atoms when electrons move to higher energy levels, which requires energy input.

How does the visible series of spectral lines of hydrogen (the Balmer series) arise? What are the wavelengths and colors of the first three lines in the Balmer series?

Balmer photons are emitted when electrons drop from higher levels into the second
energy level of a hydrogen atom, i.e. the second smallest orbit. Balmer photons are absorbed when an electron in a hydrogen atom moves from the second energy level into a higher level.

What is an absorption-line spectrum?

Normal stars show an absorption-line spectrum. It is a continuous spectrum (rainbow of colors) with absorption lines (dips) at certain wavelengths. The dips are caused by absorption of energy by atoms and (for cool stars) molecules.

What is an emission-line spectrum?

Some astronomical objects show an emission-line spectrum. Rather than a continuous spectrum, there are emission lines (sharp peaks) at certain wavelengths, with little or no radiation between the peaks. The peaks are caused by emission of energy by atoms.

What is the Doppler effect?

The Doppler effect is caused by the motion of a light source either toward or away from an observer. When a light source moves toward or away from an observer, the observed wavelength is shifted from the emitted wavelength by an amount proportional to the speed of the object. Motion toward the observer results in a blue shift (shift to shorter wavelengths). Motion away from the observer results in a red shift (shift to longer wavelengths).

How much larger is the Sun than the Earth in diameter? In volume?

The diameter of Sun is about 100× the diameter of the Earth. Thus, the volume of the Sun is about 10^6× the diameter of the Earth, i.e. if the Sun were hollow it would take 10^6 Earth-sized objects to fill it.

What is the average density of the Sun? How does this compare with water?

The average density of the Sun is 1400 kg/m^3. This is similar to that of water (1000 kg/m^3).

What is hydrostatic equilibrium for the Sun?

Hydrostatic equilibrium is a balance between the outward gas pressure force in the Sun and the inward pull of gravity. The Sun supports its own weight by its outward gas pressure force, so that it is neither contracting nor expanding.

What is thermal equilibrium for the Sun?

Thermal equilibrium is a state in which the energy radiated by the photosphere of the Sun is replaced by an internal energy source, nuclear fusion in the core.

What are the central and surface temperatures of the Sun?

The central temperature of the Sun is about 1.6 × 10^7 K and the temperature in the photosphere is about 5800 K.

What are the two most abundant elements in the Sun?

Hydrogen and helium are the two most abundant elements in the Sun (as well as in the universe as a whole).

What is the physical state of the material in the Sun?

Below the photosphere of the Sun, the hydrogen and helium in the Sun are in an ionized state, i.e. the electrons move freely rather than being bound to a particular nucleus.

What type of particle is a normal hydrogen nucleus?

A normal hydrogen nucleus is a proton.

About how long has the Sun been in existence and about how much longer will the Sun's present nuclear energy reserve last?

The Sun has been in existence for 5 × 10^9 years (5 billion). The Sun's present hydrogen energy reserve will last for another 5 × 10^9 years.

What mechanisms transport energy in the Sun? Describe each process. Where does each mechanism dominate?

Energy is transported in the Sun by both radiation and convection. In radiation, photons random walk outward in the Sun, carrying energy outward. In convection, hot gas rises and cool gas sinks, also carrying energy outward. Radiation dominates in the core and the inner envelope of the Sun. Convection dominates in the outer envelope.

What is the evidence that convection occurs in the Sun?

The observation of granulation in the Sun's photosphere provides direct evidence of convection. The bright areas in the granulation pattern are the tops of rising columns of hotter gas. The dark areas are sinking columns of cooler gas.

List the layers of the solar atmosphere, in order of increasing distance from the Sun.

layers: photosphere, chromosphere, transition zone, corona

Define the following structures/events observed in the Sun's atmosphere: spicule, prominence, flare, coronal mass ejection.

spicule - a jet of hot gas shooting upward in the chromosphere
• prominence - a looping region of hot gas, supported by magnetic field, that extends into the corona
• flare - a violent outburst of radiation and particles from a small area in the solar atmosphere
• corona mass ejection - a release of a huge balloon-shaped region of high-energy gas from the corona into the solar wind

Describe the properties of a sunspot.

Sunspots are regions in the photosphere that are somewhat cooler than the surrounding area and thus emit less light. They are regions of much stronger magnetic field than the surround-ing area, which inhibits heat flow. Sunspots tend to be found in groups of at least two. They can last for about two months.

How often does a sunspot maximum occur? What is the underlying cause of the solar cycle?

A sunspot maximum occurs once every eleven years (11 yr). The cycle of solar activity is a result of interaction between the Sun's magnetic field and its rotation.

A sunspot maximum occurs once every eleven years (11 yr). The cycle of solar activity is a result of interaction between the Sun's magnetic field and its rotation.

The solar wind is a stream of charged particles, mostly protons and electrons, escaping from the solar corona. Solar wind particles that become trapped by the Earth's magnetic field cause aurora. At periods of high solar activity, the resulting changes in the Earth's ionosphere can affect radio communications on Earth. In extreme cases, power grids on Earth have shorted out as a result of changes in the ionosphere brought about by interaction with the solar wind.

What is a neutrino? What produces neutrinos in the Sun? What happens to the neutrinos next? Why are astrophysicists interested in detecting neutrinos from the Sun?

A neutrino is a very low mass particle that interacts very weakly with other matter. Neutrinos are produced by nuclear reactions in the Sun, such as the fusion of two protons to form a deuterium nucleus. Neutrinos that are created in the Sun escape directly. Thus, the solar neutrinos that we detect on Earth give us a direct measure of the nuclear reaction rate deep in the core of the Sun.

What is a "neutrino telescope"? Where are these located and why?

Neutrino telescopes use large amounts of matter—often large water tanks—which capture a very small fraction of the total number of neutrinos that pass through. Neutrino telescopes are located deep underground, to shield out other subatomic particles that, unlike neutrinos, can-not pass directly through the Earth.

Compare the luminosity and surface temperature of a red giant with those for the Sun?

A red giant has a higher luminosity and a lower surface temperature than the Sun.

Where do we find low-mass stars on the main sequence? High-mass stars?

Low-mass stars are found in the lower-right part of the main sequence. These stars have low luminosity and low surface temperature. High-mass stars are found in the upper-left part of the main sequence. These stars have high luminosity and high surface temperature.

In what types of regions do stars form?

Stars form in cold, dense interstellar clouds. This type of region is known as a dark nebula. The large quantities of dust in a dark nebula absorb the visible light of the stars that lie behind it, blocking them from our view.

How is the energy lost by radiation from the surface of a protostar replaced?

Energy radiated from the surface of a protostar is replaced by energy released through gravitational contraction. This keeps a protostar in thermal equilibrium.

Why can more be learned about protostars by observing them in infrared rather than visible light?

Protostars are surrounded by thick dust clouds that absorb more visible than infrared light. Thus more of the infrared radiation from the protostars is able to escape from these clouds, providing us with more information on the nature of the processes within the clouds.

What are the approximate mass limits for main-sequence stars? What is the reason for these limits.

The lower mass limit for main-sequence stars is 0.08 M . Below this mass, stars do not undergo hydrogen fusion in their cores and are called brown dwarfs. The upper mass limit for main-sequence stars is about is about 100 M . Above this mass, stars are blown apart by radiation pressure.

What type of fusion occurs in the core of a horizontal branch star?

The core of a horizontal branch star undergoes fusion of helium to carbon. Some of the carbon fuses with helium to produce oxygen.

What is a planetary nebula?

Planetary nebulae are produced in the later stages of life of stars with masses below about 8 times the mass of the Sun. These stars begin unstable pulsations which eject the outer layers of the stars into space where they form expanding gas shells. The stellar remnant left behind will become a small, hot, white dwarf. A star may lose more than half its mass in the planetary nebula stage.

What is the final state of the Sun?

After the Sun ejects its envelope during the planetary nebula phase, the remaining core of the Sun will become a white dwarf. Thus, the final state of the Sun is a white dwarf that gradually becomes cooler and fainter.

What is the maximum possible mass for a white dwarf? For a neutron star?

For a white dwarf, Mmax = 1.4 M . For a neutron star, Mmax ≈ 3 M .

What is the state of the core of a massive star immediately prior to the star undergoing a supernova explosion? What event triggers the explosion?

Just prior to the supernova explosion, the core of a massive star is composed of iron and it does not undergo nuclear fusion. As the mass of the iron core increases, due to nuclear fusion in the surrounding shells, it eventually exceeds 1.4 M . At this point, the core becomes unstable and collapses. This core collapse is the event that triggers the explosion.

Describe the events that lead to the ejection of the envelope of a massive star in a supernova explosion.

As the iron core of a massive star collapses, the inner part of the envelope follows it inwards. When the core collapses to a radius of about 10 km, it reaches nuclear density and bounces back slightly. This bounce drives a powerful shock wave into the envelope, which provides enough energy to completely eject the envelope into the space around the star.

What is left of a massive star following a supernova explosion?

The collapsed core of a massive star that began with between 8 and 25 M becomes a neutron star, with a mass of about 1.4 M and a radius of about 10 km. This is surrounded by a supernova remnant, a rapidly expanding cloud of gas that once was the envelope of the doomed star. If the initial mass of the star was more than 25 M the final remnant of the star is a black hole.

Where do the elements heavier than helium originate?

Elements heavier than helium are produced in massive stars. These stars explode in supernova explosions, ejecting these elements (including carbon, nitrogen, and oxygen, on which life is based) into the interstellar medium and thus enriching it. Stars, planets, and life form from gas ejected by supernovae.

What is the significance of the pulse of neutrinos observed from Supernova 1987a?

The neutrinos detected from Supernova 1987a were direct observational evidence that the core of a massive star collapsed, triggering the supernova. It is a confirmation of our theories of supernova production.

What is a black hole?

A black hole is a highly collapsed object with such strong gravity that nothing can escape from its vicinity, not even light.

What are the basic components of the Milky Way Galaxy?

The basic components of the Milky Way Galaxy are the nucleus, the bulge, the disk, and the halo.

What are the basic components of the Milky Way Galaxy?

The basic components of the Milky Way Galaxy are the nucleus, the bulge, the disk, and the halo.

Where in the Galaxy is the Sun located?

The Sun is located about 25,000 ly from the center of the Galaxy, in the galactic disk, in a spiral arm.

At what wavelengths is the center of the Milky Way Galaxy best observed?

The center of our galaxy is best observed at infrared and radio wavelengths, since these pass through the dust in the galactic disk. The infrared is produced by stars and dust. The radio is produced by gas. We also observe gamma-ray radiation from electron-positron annihilation near the black hole and X-rays from hot gas near the black hole.

What is the best evidence that there is a concentration of 4 × 10^6 M at the center of the Milky Way Galaxy? What is the nature of this concentration?

Careful measurements of the changing positions of stars near the center of the Milky Way Galaxy show that these stars are orbiting a a mass 4 × 106 M at speeds up to thousands of km/s. The mass is so concentrated that it must be in the form of a black hole.

Describe clusters of galaxies

Clusters of galaxies are systems 106−107 ly across, containing 10s to 1000s of galaxies. Clusters are prominent in the large-scale distribution of galaxies in the universe.

Do stars and gas clouds collide when their parent galaxies collide?

Since the distances between stars are so large, stars do not collide with each other in the course of a galaxy collision. However, since gas clouds are large, extended objects, clouds do collide with each other during a galaxy collision.

What is a supercluster? What causes galaxies to cluster in the universe?

A supercluster is a group (or cluster) composed of clusters of galaxies. Gravitational attraction causes galaxies to cluster.

How big is a megaparsec (Mpc) in light years (ly)?

Since each pc is 3.3 ly and a Mpc is 10^6 pc, it follows that a Mpc is 3.3 × 10^6 ly.

What is the Kepler mission? What has it found?

The Kepler mission is a spacecraft telescope that has been detecting planets orbiting about stars other than the Sun using the transit method. To date Kepler has discovered about 4700 candidate planets, some of which are the size of the Earth.

What is an organic molecule? What are two nonterrestrial sites where complex organic molecules are found?

An organic molecule is one involving carbon. Complex organic molecules have been found in the interstellar medium (i.e. in the gas between the stars) and also in newly fallen meteorites. The latter indicates that these molecules are present in the debris that orbits in our solar system.

What is the significance of the dry river channels on Mars?

The presence of these channels indicates that water once flowed on Mars. This may have supported life in the past.

What is the significance of dry river channels on Mars?

The presence of these channels indicates that water once flowed on Mars. This may have supported life in the past.

What method is used by astronomers to search for extraterrestrial civilizations? Why is this method used?

Astronomers use radio telescopes to search for messages that might be broadcast by other civilizations in our galaxy. Radio waves travel long distances through the disk of our galaxy without being absorbed or scattered by the gas and dust. Thus radio provides the best means for interstellar communication. Also, the distances are so large that interstellar travel is impractical.

What gets larger as the universe expands?

The distances between widely separated galaxies (i.e. galaxies that are not in the same cluster) increase as the universe expands. The sizes of planets, stars, and galaxies do not increase as the universe expands, since these objects are held together by their own gravity.

Is there a center to the expanding universe? Explain.

No, the universe expands in such a way that at any location all but the nearest galaxies are seen to move away from that spot. Since this is true for any location in the universe, no location can be considered to be the center of the expansion.

What is the Big Bang? Did it occur at a particular place in the universe?

The Big Bang is the explosive event that began the expansion of the universe. It did not occur at any one point in the universe, but rather occurred everywhere as matter began to rush apart from other matter. The Big Bang occurred about 14 × 10^9 years ago.

What is the cosmological redshift?

As the universe expands, photons traveling through it are stretched and thus are red-shifted. The longer a photon travels, the greater is its redshift. Thus, more distant objects have greater redshifts as given by the Hubble law.