final exam Flashcards
A star is 230 light-years away. The light we see tonight from that star left it
Because light travels a distance of one light-year in one year of time, it will travel a distance of 230 light-years in 230 years of time. This means that if we are receiving the light tonight, it has been traveling for 230 years from the source in order to reach us.
The point in the sky directly above your head at any given time is called the
zenith
The path that the Sun appears to make in the sky over the course of a year is called the celestial equator
FALSE
The great astronomer of ancient times who summarized and improved a system of circles upon circles to explain the complicated motions of the planets (and published the system in a book now called The Almagest) was
Ptolemy
The scientist who first devised experimental tests to demonstrate the validity of the heliocentric model of the solar system was
Galileo
The celestial sphere turns around once each day because
Our position on a rotating Earth causes the stars, planets, and Sun to appear to revolve around us.
Which ancient Greek thinker suggested (long before Copernicus) that the Earth is moving around the Sun?
Aristarchus i
In this diagram of the celestial sphere, there are five lines that point to different parts of the picture. Use the drag-and-drop environment to label the indicated parts of the figure.
The location of the figure defines which direction the zenith is located: always directly overhead of the observer. This point travels with the observer, in a sense, and the horizon with it since the horizon is 90° from the zenith in all directions.
The North Celestial Pole, on the other hand, is fixed against the background stars based on the direction our planet’s rotational axis points, so its position in one’s local sky may vary as a result of one’s location on the Earth’s surface, and the Celestial Equator with it since the Celestial Equator is 90° from the North Celestial Pole in all directions.
At the Earth’s equator, you would see the celestial poles on your horizon.
true
How did Eratosthenes measure the size of the Earth?
The memory aid “altitude = latitude” applies to the north celestial pole’s altitude as seen from a particular latitude on Earth, but the idea translates to observing the Sun as well. Although the Sun’s altitude from a particular latitude depends on the time of year, the difference between the observed altitude of the Sun from two latitudes at local solar noon, tells you the difference in latitude between those two locations.
Eratosthenes took advantage of this idea by comparing the altitude of the Sun h1
at noon as seen from his town (measured by observing the shadow cast by a pillar or post in the ground) with the altitude of the Sun h2
at noon as seen from a different town to the south. He recognized that the difference between the two observed altitudes compared to the full 360 degrees in a circle is equal to the distance between the two towns d
compared to the full circumference of the Earth C
:
h1−h2/360∘=dC
From this expression he was able to calculate the circumference of the Earth in the same physical units as the distance between the two towns.
In an ellipse, the ratio of the distance between the foci and the length of the major axis is called
eccentricity
remember newtons laws
ok
Consider an ellipse with a major axis of l = 6 cm and an eccentricity of e = 0.71.
Part (a) What is the length of the semimajor axis a
, in cm?
The semimajor axis a
of an ellipse is half the major axis; the eccentricity plays no role in this calculation.
a=l/2 =6 cm / 2
a=3.000 cm
Kepler’s third law relates a planet’s orbital period to the semi-major axis of the orbit.
TRUE
The diagram below shows the seasons that our planet Earth undergoes for the Northern Hemisphere. The position of the Earth is labeled for one of the key seasonal dates, the Autumnal (Fall) Equinox. Label the other three positions correctly. Note the blue arrow on the circle of the Earth’s orbit, which shows which way the Earth moves.
just google it
Which of the following statements about electromagnetic radiation is FALSE?
Not only are photons (the particles of light) massless, but they are also electrically neutral.
Which, from among the following options, has the longest wavelength?
The lowest-energy forms of light have the longest wavelengths. These forms of light include infrared waves, microwaves, and (at the absolute longest) radio waves.
A star has a surface temperature of 8800 K. At what wavelength (in nanometers) will it give off maximum light?
This form is useful when we are given a temperature in kelvins (K) or a wavelength in nanometers (nm). Since this is the case, we can solve for the temperature with a little algebra and plugging in the given value of the temperature:
λmax=2.9×10^6 nm⋅K/T
= 2.9×10^6 nm⋅K/ 8800 K
T=329.5 nm
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Understanding Blackbody Radiation: It turns out that stars behave like an idealized object that scientists call a blackbody. Thus, understanding blackbodies and how they give off energy helps us to understand how stars shine. For a blackbody, the higher its temperature, the smaller the wavelength at which it gives off the peak amount of radiation. This is described by wien’s law. Thus, really hot stars will shine most intensely with ultraviolet radiation. Also, the higher a star’s temperature, the greater the flux of radiation coming from it. This last statement is called stefan-Boltzman law.
For all electromagnetic waves, the frequency multiplied by the wavelength will be the same constant number.
True
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When someone who has never thought much about astronomy looks up at the sky, it’s easy to believe that everything turns around the Earth and that we are in the middle of things; such a view is called the geocentric model. Astronomers call the point in the sky above our heads our Zenith, and where the dome of the sky meets the Earth our Horizon. In the 20th century, astronomers divided the sky into 88 boxes, and each box is now called a Constellation. The belt of the sky through which the Sun, Moon, and planets are seen to move in the course of the day and the course of a year is called the Zodiac.
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If you were standing at Earth’s north pole, and you looked up to the zenith (the point directly above your head), you would be looking at the point where the North Celestial pole is located. At the Earth’s North Pole, the celestial equator would be at your Horizon . If, on the other hand, you were at the Earth’s equator, you would see one point of the celestial equator pass through your Zenith If you are looking at the sky from the continental United States, the north celestial pole would have an angular height (an altitude) equal to your Latitude. Right now, the star located very close to our north celestial pole is Polaris.
A light-year is
Equal to approximately 9.5 trillion km, or 5.9 trillion miles, the distance that light travels in one Earth year is a commonly-used unit of length or distance in astronomy.
A star is 230 light-years away. The light we see tonight from that star left it
Because light travels a distance of one light-year in one year of time, it will travel a distance of 230 light-years in 230 years of time. This means that if we are receiving the light tonight, it has been traveling for 230 years from the source in order to reach us.
The Astronomical Unit (AU) as defined by astronomers is
Astronomers defined this as a distance unit because the actual distance in any physical unit in use at the time was not known.
If a star is 900 light-years away, that means the light we see tonight left that star 900 years ago.
TRUE
By looking billions of light-years out into space, astronomers are actually seeing billions of years into the past.
TRUE
At the Earth’s equator, you would see the celestial poles on your horizon.
true
The Sun revolves around (orbits) the Earth in about 365 days (what we call one year.)
FALSE
We now know that the orbit of a stable planet around a star like the Sun is always in the shape of
ellipses.
When a planet, in its orbit, is closer to the Sun, it
moves faster (perihelion)
According to Kepler’s third law, there is a relationship between the time a planet takes to revolve around the Sun and the planet’s
Kepler’s 3rd law is written as an equation:
p^2=a^3
In this equation, p
corresponds to the orbital period, the time a planet takes to revolve around the Sun. The quantity a
corresponds to the average orbital distance from the Sun (the semimajor axis).
In words, this means that planets which orbit farther from the Sun on average have longer orbital periods.
Newton showed that to change the direction in which an object is moving, one needs to apply
Newton’s 2nd law of motion describes the means by which the motion of an object can be changed through the action of an outside force. Such a force, when applied to an object, produces an acceleration in that object that is inversely related to its mass (more-massive objects accelerate less under a given force than less-massive objects). Quantitatively, this takes on the equation form
F=ma
This acceleration can be a simple change in speed, a change in the direction of motion, or both.
Which of the following statements about the force of gravity is FALSE?
The force of gravity between two masses M
and m
whose centers are separated by some distance d
takes on the mathematical form
FG=GMm/d^2
By this description, increasing either term in the numerator (M
or m
) increases the gravitational force between the two objects. Increasing the denominator, on the other hand, decreases the strength of the force.
The figure to the right shows a set of ellipses. The ellipse on the top has a semimajor axis of 1 (in some arbitrary unit). The four choices below it in the grey region have their sizes shown (in the same arbitrary units). Based on the given semi-major axis for the orbit in the white region above, which of the lettered orbits would have the same orbital period?
Kepler’s 3rd law is stated most simply that there is a direct relationship between the orbital period p
of an object and the orbit’s semimajor axis a
:
p^2=a^3
The semimajor axis is half of the full length of the major axis, which means in this case that the orbit in question has a major axis of 2 units. An orbit that will have the same period, then, will also have a major axis of 2 units. Among the four choices shown, this corresponds to orbit d.
The number of degrees of arc that your location is north or south of the Earth’s equator is called your
Lines of latitude run east-west in parallel circles around the Earth, measuring an angle north or south from the Earth’s equator.
The “prime meridian” (where longitude equals zero) passes through
This city is the site of the Royal Observatory, in Greenwich, England.
To locate objects on Earth, we call the number of degrees east or west of the Greenwich Meridian its:
longitude
A solar day is slightly longer than a sidereal day.
true
How fast do electromagnetic waves travel?
Electromagnetic waves are the formal term for what we call “light” - which travels at the speed of light regardless of its energy.
The fastest speed in the universe is
Einstein’s theory of relativity says it the speed of light
Wien’s law relates the wavelength at which a star gives off the greatest amount of energy to the star’s
Wien’s law describes the relationship between the temperature of an object like a star and the wavelength at which it gives off the greatest amount of energy:
λmaxT=2.9×10^6 nm K
The Stefan-Boltzmann law relates the energy flux coming from a blackbody (such as a star) to its
The Stefan-Boltzmann law describes the relationship between the star’s temperature and its energy flux F
(power emitted per unit area):
F=P/A=σT^4
The larger spectrum at the top of the figure shown illustrates an absorption spectrum from a source that is stationary with respect to the observer (you). The black lines are the absorption lines in the spectrum, at specific wavelengths. Suppose instead that the source was actually moving away from you at a high speed. Which of the (smaller) lettered spectra would you expect the observed spectrum to resemble? (In this scenario, the large spectrum at the top is the emitted spectrum.)
The large spectrum that is shown is what is being emitted by the source. It would be the spectrum you would observe if there was no motion occurring. However, the scenario describes motion away from you, and such recessional motion introduces a redshift. The red end of the spectrum is the one where wavelengths are longer, so the spectrum you will observe is the one where the absorption lines have been shifted to the longer wavelength end - in this case, to the right.
The larger spectrum at the top of the figure shown illustrates an absorption spectrum from a source that is stationary with respect to the observer (you). The black lines are the absorption lines in the spectrum, at specific wavelengths. Suppose instead that the source was actually moving away from you at a high speed. Which of the (smaller) lettered spectra would you expect the observed spectrum to resemble? (In this scenario, the large spectrum at the top is the emitted spectrum.)
The large spectrum that is shown is what is being emitted by the source. It would be the spectrum you would observe if there was no motion occurring. However, the scenario describes motion toward you, and such approaching motion introduces a blueshift. The blue end of the spectrum is the one where wavelengths are shorter, so the spectrum you will observe is the one where the absorption lines have been shifted to the shorter wavelength end - in this case, to the left.
The larger spectrum at the top of the figure shown illustrates an absorption spectrum from a source that is moving with respect to the observer (you). The black lines are the absorption lines in the spectrum, at specific wavelengths.
Part (a) Suppose you were told that the source was moving toward you at a high speed. Which of the spectra below would you expect the actual emitted (rest-frame) spectrum to resemble? (In this scenario, the large spectrum at the top is the observed spectrum.)
The large spectrum that is shown is what is being observed from the source, and you know that motion is happening so you know it has been Doppler-shifted one way or the other. The scenario describes motion toward you, and such approaching motion introduces a blueshift. The blue end of the spectrum is the one where wavelengths are shorter, so the spectrum you are observing is one where the absorption lines have been shifted toward the shorter wavelength end - in this case, to the left. This means the original emitted spectrum was toward the right.
Not all wavelengths of electromagnetic radiation can penetrate the Earth’s atmosphere. Of the following types of waves that come from space, which one are you likely to be able to detect most easily from our planet’s surface?
Although our atmosphere is very effective at absorbing most forms of light, visible light and most radio wave light can penetrate to the surface - which is why radio telescopes do not need to be launched into space.
A carrier wave on a campus radio is broadcasting at a frequency f = 87.5 MHz. What is the wavelength λ
of the carrier wave of this radio, in meters?
The relationship between a light wave’s speed c
, its wavelength λ
, and its frequency f
, is given by
λf=c
so the wavelength can be found by using some algebra and plugging in the given values
λ=c/f. = (3×10^8 m/s)/87.5×10^6 Hz
The emitted infrared radiation from a dwarf planet has a wavelength of maximum intensity at λmax
= 72000 nm. What is the temperature T
, in kelvins, assuming it follows Wien’s law?
Wien’s law describes the relationship between the temperature T
of an emitting body and the peak wavelength λmax
of its spectrum
λmaxT=2.9×10^6 nm⋅K
This form is useful when we are given a temperature in kelvins (K) or a wavelength in nanometers (nm). Since this is the case, we can solve for the temperature with a little algebra and plugging in the given value of the peak wavelength:
T=2.9×10^6 nm⋅K/λmax =2.9×10^6 nm⋅K/ 72000 nm
T=40.28 K
To break up light into the component colors that it contains, astronomers use a device called
This device can use either a prism or a diffraction grating to reveal the spectrum, and goes by many names: spectrometer, spectrograph, or spectroscope.
