Astronomy Test 1

The charge of a typical nucleus:

always positive

Which of the choices is formed by an electron orbiting around a proton?

atom

A discovery in 2002 about the neutrino show that neutrinos:

can change from one type of neutrino to another

In the wave model of light, the color of light is determined by the ____ such that blue light has _____ compared to yellow light.

distance between the waves; smaller wavelength

A neutrino is an example of a/an:

elementary particle

Thermal equilibrium of a star requires that the ___ and the ___ must be equal.

energy production, luminosity

What type of reactions occurs in the core of the sun?

fussion reactions

Which of the following parts of the spectrum has the smallest wavelength?

gamma ray

The conditions for nuclear reactions to occur in the sun are:

high temperature, high density

With respect to the proton, the electron has a ___ charge and ___ mass:

negative, much less

Which force is the cause for light?

Electromagnetic (EM) force

Which force(s) has/have an infinite range?

Electromagnetic (EM), Gravitational

The three temperature scales (Fahrenheit, Celsius, Kelvin) are defined such that when the scales are compared to each other, ___ is the coldest overall.

0 degrees K

How is a neutrino different from a neutron? List all the ways you can think of.

1. The neutrino's mass is much, much smaller than a neutron's. 2. neutrinos hardly interact with matter at all, where as a neutron interacts with other particles. 3. Neutrinos are not one of the particles to make up an atom. 4. Neutrinos can "oscillate" (change from one type of neutrino to another as they travels between the Sun's core and Earth). 5. Neutrinos are not affected by the strong (nuclear) force; neutrons are affected. 6. Neutrinos are elementary particles; neutrons are made of smaller quarks.

Give some everyday examples of the transport of heat by convection and by radiation.

A floor heater spreads heat throughout a room by convection. Ironically, the style of heater called a radiator doesn't really radiate much energy. Most of its heat is transported by convection of the air which it heats (you can test this easily by comparing how well it warms your hand if you place it 12 inches off to the side to how well it warms your hand if it is 12 inches above it). Absorption of sunlight and subsequent radiation of infrared energy by Earth in its daily cycle is an example of radiation. (Heat from a light bulb or from a fire also are examples of radiation.)

What is a wave? Use the terms wavelength and frequency in your definition.

A wave is a repeating disturbance that travels outward, like ripples moving through water. The distance between crests (or troughs) of the wave is the wavelength, and the number of crests that move past a given location each second (or other unit of time) corresponds to the frequency of the wave.

Explain how we can deduce the temperature of a star by determining its color.

According to Wien's law, the peak wavelength of a blackbody spectrum is inversely related to its temperature. The color of a star is indicative of where the peak wavelength of its spectrum is located, so color can indicate to us the temperature of a star. Bluer stars are hotter, whereas red stars are cooler.

Where in an atom would you expect to find electrons? Protons? Neutrons?

An atom is a structure that is composed of the nucleus of an element with electrons in orbit around the nucleus. The nucleus at the center of an atom is where we can find the protons and neutrons.

Why is an iron atom a different element from a sodium atom? a. A sodium atom has fewer neutrons in its nucleus than an iron atom has b. An iron atom has more protons in its nucleus than a sodium has c. A sodium atom is bigger than an iron atom d. An iron atom has more electrons than a sodium atom

Answer: b. Checking a periodic table, sodium is element number 11 (meaning it has 11 protons in the nucleus) and iron is element number 26 (with 26 protons). Choices a and d are true, but it is the number of protons that determines the element.

If the Sun instantaneously stopped giving off light, what would happen on Earth? a. Earth would immediately get dark. a. Earth would get dark about 8 minutes later. b. Earth would get dark about 1 hour later. c. Earth would get dark about 24 hours later. d. Earth would get dark about 1 year later.

Answer: b. The Earth is about 8 light minutes away from the Sun. The light that is already on its way to the Earth would still arrive. We would not notice a change until the light ends 8 minutes later

Which method of heat transport occurs within the sun?

Both Convection and Radiation

Burning your hand on the handle of a pan is an example of which method of heat transport?

Conduction

Which force causes electrons to stay in orbit around a nucleus?

Electromagnetic (EM) force

Which of the following statements is not correct about the modern (quantum) model of the atom.

Electrons prefer higher energy states.

Describe the two main ways that energy travels through the Sun.

Energy is created through fusion of hydrogen into helium at the center of the Sun. This energy first enters the radiative zone of the Sun. This is where photons run into gas particles and get reemitted in random directions. The radiative zone goes to about 2/3 of the way to the surface. Once the energy has traveled 2/3 of the way to the surface, it reaches the convection zone where the hot gas flows up and down (like water boiling in a pot) carrying the energy to the solar surface.

Which is more dangerous to living things, gamma rays or X-rays? Explain.

Gamma ray photons have more energy since they have a shorter wavelength (higher frequency) than x-rays. So, they can potentially do more damage to biological organisms than x-rays. This does not imply that X-rays are necessarily safe. X-ray photons have more energy than any other form of light, except for gamma rays. X-rays can also damage biological organisms.

Define and explain "hydrostatic equilibrium" in reference to the Sun.

Hydrostatic equilibrium means that the sun is stable in size. This means that the inward gravitational force is exactly balanced by the outward gas pressure. An inflated balloon is in hydrostatic equilibrium as the outward air pressure is balanced by the elastic force of the rubber that makes up the balloon.

Why does a hot piece of metal glow red?

It emits red light

Why are molecules so rare in the universe?

Molecules are composed of atoms and ions that are bound together. For a molecule to form, there needs to be many atoms or ions close enough together that there is a higher chance for a collision to make a bound molecule. In space, most material is far too spread out. Also, if the temperature are too high, the molecule will break apart into separate atoms. The two conditions, lots of mass and low temperatures, are rare in the universe.

Appendix J lists the stars that appear brightest in our sky. Are most of these hotter or cooler than the Sun? Can you suggest a reason for the difference between this answer and the answer to the previous question? (Hint: Look at the luminosities.) Is there any tendency for a correlation between temperature and luminosity? Are there exceptions to the correlation?

Most (but not all) of the brightest stars are hotter than the Sun. The majority of these stars are not bright because they are at small distances; they are bright because they emit a lot of energy. So, the very bright stars in our sky also tend to have very large luminosities compared to the Sun. When we look in the region around the Sun, the cooler, less luminous stars are much more common. If this were not the case, stars would be in both lists. There does tend to be a correlation between temperature and luminosities. Hotter stars tend to be more luminous. There are some exceptions where a star is cool, but is still very luminous. For example, Arcturus and Betelgeuse are cooler than the Sun, but have much higher luminosities. This is because they are giant or supergiant stars.

Is your textbook the kind of idealized object (described in section on radiation laws) that absorbs all the radiation falling on it? Explain. How about the black sweater worn by one of your classmates?

No, the textbook reflects light of different colors unlike a blackbody, which reflects no light. A black sweater is a somewhat better approximation of a blackbody since it absorbs almost all light and reflects very little light. But since it reflects enough light so that everyone can see your sweater, it is not a true blackbody.

Which method of heat transport moves heat by photons?

Radiation

An astronomer discovers a type-M star with a large luminosity. How is this possible? What kind of star is it?

Since M stars are cool and emit very little energy per square meter of the surface, the only way that an M star can have a high luminosity is if it is very large (i.e., has a lot of square meters of surface area). This star is either a giant or a supergiant.

What is the ultimate source of energy that makes the Sun shine?

The Sun generates energy by the fusion of hydrogen into helium. The resulting helium weigh slightly less than the four hydrogen that go into the reaction. The difference in mass has been converted to energy. The amount of energy is described by Einstein's equation E = mc2.

Based on their colors, which of the following stars is hottest? Which is coolest? Archenar (blue), Betelgeuse (red), Capella (yellow).

The color is determined by the temperature. According to Wien's law, a hot object will have the peak emission in shorter wavelengths compared to cooler object. The star that appears most blue would be the hottest: Archenar. The star that appears most red would be the coolest: Betelgeuse.

What distinguishes one type of electromagnetic radiation from another? What are the main categories (or bands) of the electromagnetic spectrum?

The different bands of the spectrum each have a characteristic wavelength or frequency range. In order from lowest to highest energy, the main bands are: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma ray.

Describe in your own words what is meant by the statement that the Sun is in hydrostatic equilibrium.

The gas pressure and gravity are in balance everywhere throughout the Sun, from the very center to the surface. This means the gas pressure at any depth within the Sun pushes outward and can support the weight of all of the gas pressing down upon it, due to gravity. So, the Sun neither expands nor contracts but remains as it is; this is true at every point within the Sun as well as for the Sun overall.

What two factors determine how bright a star appears to be in the sky?

The luminosity and distance of a star determine its apparent brightness in the sky.

The star Antares has an apparent magnitude of 1.0, whereas the star Procyon has an apparent magnitude of 0.4. Which star appears brighter in the sky?

The magnitude scale goes backwards. So, the star that we get the most energy from is the one with the lower magnitude. Procyon appears brighter in the sky.

Appendix I lists some of the nearest stars. Are most of these stars hotter or cooler than the Sun? Do any of them emit more energy than the Sun? If so, which ones?

The majority of the nearest stars are cooler (type M) than the sun (type G0). Looking at the Luminosity column, only Alpha Centauri A, Sirius A, and Procyon A have luminosities greater than 1 solar luminosity. So, they emit more energy than the Sun. All of the other stars have luminosities less than 1 solar luminosity. (By the way, spectral type T refer to objects called brown dwarfs, with are not true stars)

What do measurements of the number of neutrinos emitted by the Sun tell us about conditions deep in the solar interior?

The neutrinos are being produced in the solar core by fusion reactions, and measuring their number gives us a direct probe into what is happening in the Sun's core. It helps confirm that there are enough hydrogen fusion reactions (each of which produces a pair of neutrinos) going on in the Sun's core to explain the energy output of the Sun.

Suppose the proton-proton cycle in the Sun were to slow down suddenly and generate energy at only 95% of its current rate. Would an observer on Earth see an immediate decrease in the Sun's brightness? Would she immediately see a decrease in the number of neutrinos emitted by the Sun?

The observer would see a decrease in the number of neutrinos almost immediately since neutrinos take only about 2 seconds to travel from the center of the Sun to its surface and another 8 minutes to reach Earth. Light (photons), on the other hand, takes about a hundred thousand to a million years to traverse the distance from the center of the Sun to its surface, so 105 to 106 years would elapse before an observer on Earth saw a decrease in the brightness of the Sun.

Explain how parallax measurements can be used to determine distances to stars. Why can we not make accurate measurements of parallax beyond a certain distance?

The parallax of a star is its apparent shift in position as the Earth moves around the Sun. Images from six months apart will show small shift in position of nearby stars compared to more distant objects (like other galaxies). As a star gets more distant, the shift decreases, making the angle of distant stars more difficult to measure. We are limited in ability to measure extremely small angles.

What are the ingredients and the end results of the proton-proton chain in the Sun? Why is energy released in the proton-proton chain?

The proton-proton chain is a set of three reactions that, in the end, change 4 protons (Hydrogen) into 1 Helium. There are also two neutrinos created during the formation of Helium. Energy is released because the reaction converts a tiny bit of matter into energy through E=mc2. One thousand grams of Hydrogen will only produce 993 grams of Helium.

Which of the following can you determine about a star without knowing its distance, and which can you not determine: radial velocity, temperature, apparent brightness, or luminosity? Explain.

The radial velocity, temperature, and apparent brightness can all be determined without knowing the distance to the star. The radial velocity is measured by a Doppler shift in its spectrum. Temperature can be determined by its color or its spectrum. The apparent brightness can be directly observed since it is simply how bright the star appears to us. But, in order to estimate a star's luminosity, however, we must know its distance.

Order the seven basic spectral types from hottest to coldest.

The seven basic spectral type are O (hottest), B, A, F, G, K, M (coolest)

What conditions are required before proton-proton chain fusion can start in the Sun?

The two basic conditions for fusion is very high temperature and very high density. Protons naturally repel each other by the electromagnetic (EM) force. The center of the Sun must be hot enough for the motion of the protons to overcome their mutual repulsion and get close enough for the strong nuclear force to turn on and fuse them together. The required temperature is over 10 million K. A very high density is also required to increase the chances of collisions between protons. If there are too few protons in a region of space, there will not be enough collisions to sustain fusion.

Which forces are short range forces? Which forces have an infinite range?

The two nuclear forces, the strong nuclear force and the weak nuclear force are short range forces. This means that if the particles are too far apart, the force becomes zero and disappears. The gravitational force and the electromagnetic force are infinite range forces. This means that while the force gets weaker as the distance between two objects increases, the force does not become exactly zero. These forces will affect objects that are incredibly far apart.

Which forces affect any type or particle? Which forces only affect specific types of particles?

There are two forces will affect any type of particle. These are the gravitational force and the weak nuclear forces. The other two forces will only affect specific types of particles. The electromagnetic force only affects particles that have an electric charge. If a particle has no electric charge (like a neutrino), the electromagnetic force will be equal to zero and not have any influence over it. The strong nuclear force only applies to quarks and particles made of quarks (like protons and neutrons). There is no strong force among electrons or neutrinos.

Define and explain "thermal equilibrium" in reference to the Sun.

Thermal equilibrium means that the sun has a stable temperature. That means that luminosity (energy leaving the surface) is exactly balanced by the energy production in the core.

What is the one fundamental difference between X-rays and radio signals?

They are both forms of light, but their wavelengths are very different

Neutrinos that are created in the center of the sun take roughly __ to reach the Earth; photons take roughly __.

a few minutes, a million years

Which of the following transformations is (are) fusion and which is (are) fission: helium to carbon, carbon to iron, uranium to lead, boron to carbon, oxygen to neon? (See Appendix K for a list of the elements.)

Using Appendix K or a periodic table: helium has a mass of 4; carbon has a mass of 12 = fusion (small nuclei to large nucleus) carbon has a mass of 12; iron has a mass of 56 = fusion; uranium has a mass of 238; lead has a mass of 237 = fission (large nucleus to small nuclei) boron has a mass of 11; carbon has a mass of 12 = fusion oxygen has a mass of 16; neon has a mass of 20 = fusion.

Explain why light is referred to as electromagnetic radiation.

What we perceive as "light" is a traveling wave of oscillating electric and magnetic fields. The term radiation is used here to describe the radiant energy of the fields. In other words, it is the energy that comes from an object and moves, or radiates, through space.

A blackbody at 5500 K has a peak wavelength in the visible part of the spectrum (yellow). If the temperature was made slightly cooler, what part of the spectrum would the peak be in?

infrared (IR)

Cool stars can be very luminous if they are very _____

large

In an atomic nucleus, which type of particle determines what element a nucleus is?

protons

Three stars have different colors. Which of the following shows the colors from coolest to hottest?

red star, yellow star, blue star

To determine the radius (size) of a star, what two properties need to be known?

temperature, luminosity

The energy produced in reactions inside the Sun results from:

the decrease of mass of the particles after the reaction

Scientists know that hydrostatic equilibrium must be true in the sun because:

the sun's mass remains constant