MA #1
mostly X rays.
A gas heated to millions of degrees would emit
about half way from the center of the galaxy to the outskirts of the galactic disk
About where is our solar system located within the Milky Way Galaxy?
Its light has taken 7 billion years to reach us.
Because we live in an expanding universe, distant galaxies are farther away from us today than they were when the light we see from them started on its journey to us. So what do we mean when we say that a galaxy is 7 billion light-years away?
shorter, more
Compared to a cold object, a hotter object of the same size emits most of its light at __________ wavelengths and emits _____ light overall.
Every chemical element produces a unique spectral fingerprint.
Comte was proven wrong in his claim that we could never learn the composition of stars. What do we know today that Comte did not know when making his claim, and that makes it possible for us to learn the chemical compositions of stars?
The star's spectrum peaks in visible light. The star emits much more total light than the planet. The planet emits virtually all its light as infrared light.
Consider a planet orbiting another star that is very similar to our Sun. Assume the planet is about the size of Earth and has an Earth-like orbit around its star. Which of the following statements are true about the light coming from the star and planet?
Star A is moving away from us and Star B is moving toward us.
Consider a spectral line with a rest wavelength of 750 nm. In the spectrum of Star A, the line is shifted to 751 nm. In the spectrum of Star B, the line is shifted to 749 nm. Which statement is true?
a peak intensity located at shorter wavelength This is one of the two laws of thermal radiation, also called Wien's law: Hotter objects produce photons with higher average energy, which means the peak intensity is located at shorter average wavelength.
For an object producing a thermal spectrum, a higher temperature causes the spectrum to have ___________.
The star is moving toward us.
From laboratory measurements, we know that a particular spectral line formed by hydrogen appears at a wavelength of 486.1 nanometers (nm). The spectrum of a particular star shows the same hydrogen line appearing at a wavelength of 485.9 nm. What can we conclude?
it reflects green light and absorbs other colors.
Grass appears green because
100,000 light-years
How big is the Milky Way Galaxy?
the light-travel time to the object
How do you determine how much an object has aged since its light left? If you want to calculate how much an object has aged since its light left on its way to Earth, the one thing you need to know is __________.
2.5 million light-years
How far away is the Andromeda Galaxy?
28,000 light-years
How far away is the center of the Milky Way Galaxy?
the distance to the object
How long does it take light to travel great distances? If you want to calculate how long it takes light to travel from some distant object to Earth, the one thing you need to know is __________.
81 times more energy.
If two objects are the same size but one object is 3 times hotter than the other object, the hotter object emits
The star is rotating.
If we observe one edge of a star to be redshifted and the opposite edge to be blueshifted, what can we conclude about the star?
100 meters
If we use 1 millimeter to represent 1 light-year, how large in diameter is the Milky Way Galaxy?
radio waves
If you have a telescope that is observing light with wavelengths of a few meters, you are observing __________.
It would be very similar to the spectrum of the human.
Imagine being on the Moon and looking at the thermal radiation spectrum of Earth. How would it compare to the spectra shown on the graph in the video?
lowest frequency
In the illustration of the solar spectrum, the upper left portion of the spectrum shows the __________ visible light.
They allow us to see distant objects as they were long in the past.
In what sense are telescopes like time machines?
Incandescent bulbs emit most of their energy as infrared light.
Incandescent light bulbs emit thermal radiation because their filaments are heated to about 2500 Kelvin. LED light bulbs emit only at particular visible wavelengths. Why do incandescent bulbs require more energy to shine with the same amount of visible light as LED bulbs?
Because the peak wavelength of the spectrum depends on the temperature of the object producing the spectrum.
Most continuous spectra are examples of what we also call thermal radiation spectra. Why do we call them "thermal" spectra?
Because the strongest visible emission lines from hydrogen are red.
Most interstellar clouds are made mostly of hydrogen (because hydrogen is the most common element in the universe). Why are these clouds usually dominated by the color red?
the approximate temperature of the Sun's surface
Notice that the Sun's spectrum appears brightest (or most intense) in the yellow-green region. This fact tells us __________.
4,400 kilometers
On the 1-to-10-billion scale, about how far is it to the nearest stars besides the Sun?
longer, lower
Red light has a _____ wavelength and a _____ frequency than does blue light.
100 billion
Roughly how many stars are in the Milky Way Galaxy?
4 billion years
Suppose a galaxy is located 10 billion light-years away. Given that the universe is now about 14 billion years old, how old was the universe at the time when the galaxy emitted the light we see today?
nothing
Suppose that you had "X-ray vision" that allowed you to see X rays. What would you notice when you looked at a friend standing near you that you could not notice with your visible light vision alone?
the size of a typical campus
Suppose we imagine the Sun to be about the size of a grapefruit. How big an area would the orbits of the eight planets of the solar system cover?
The galaxy formed before the universe was 1 billion years old. Bc universe is 14b years old
Suppose we observe a galaxy that is 13 billion light-years away. Which of the following can we conclude?
Compare the wavelengths of lines in the Sun's spectrum to the wavelengths of lines produced by chemical elements in the laboratory.
Suppose we want to know what the Sun is made of. What should we do?
its wavelength and energy
Suppose you know the frequency of a photon and the speed of light. What else can you determine about the photon?
The blue star has a hotter surface temperature than the red star.
Suppose you see two stars: a blue star and a red star. Which of the following can you conclude about the two stars? Assume that no Doppler shifts are involved. (Hint: Think about the laws of thermal radiation.)
They have a frequency of 2 hertz.
Suppose you watch a leaf bobbing up and down as ripples pass it by in a pond. You notice that it does two full up and down bobs each second. Which statement is true of the ripples on the pond?
The advance of science and technology may someday provide ways to answer questions that seem unanswerable today.
The "extraordinary" part of Comte's claim was his statement that we could never learn the composition of stars. Which of the following best summarizes the key lesson we should learn from the fact that his claim was ultimately proven wrong?
its spectrum depends on the temperature of the object emitting it The spectrum from an object that emits idealized thermal radiation depends only on the object's temperature.
Thermal radiation gets its name because __________.
reflect visible light.
We can see each other in the classroom right now because we
the part of the universe that could be observed in principle, including things that may require future technologies
What do we mean by the observable universe?
the nearest star (or star system) besides our Sun, about 4 light-years away
What is Alpha Centauri?
a star-forming cloud located far outside our solar system
What is the Orion Nebula?
14 billion years
What is the best estimate for the age of the universe?
A spectrum shows a blueshift when the object is moving toward us and a redshift when it is moving away from us.
When on object's spectral lines are shifted from their rest wavelengths to longer wavelengths, we say that the object's spectrum shows a redshift. When the lines are shifted to shorter wavelengths, we say that the object's spectrum shows a blueshift. Which statement about a blueshift and a redshift is true?
an absorption line spectrum.
When white light passes through a cool cloud of gas, we see
gamma rays radio waves ultraviolet infrared visible light X rays microwaves
Which of the following are forms of light (electromagnetic radiation)?
The Sun's hot interior produces a continuous rainbow of color, but cooler gas at the surface absorbs light at particular wavelengths.
Which of the following best describes why the Sun's spectrum contains black lines over an underlying rainbow?
you, Earth, solar system, Milky Way, Local Group, Local Supercluster
Which of the following has your "address" in the correct order?
hot, thin gas
Which of the following objects is not a close approximation of a thermal emitter?
Pass a narrow beam of sunlight through a prism
Which of the following procedures would allow you to make a spectrum of the Sun similar to the one shown, though with less detail?
X rays travel through space faster than radio waves.
Which of the following statements about X rays and radio waves is not true?
A hot object emits more total radiation per unit surface area than a cool object.
Which of the following statements about thermal radiation is always true?
It will take me light-years to complete this homework assignment.
Which of the following statements does not use the term light-year in an appropriate way?
shorter, higher, higher
X-ray photons have a _____ wavelength, _____ frequency, and _____ energy than do ultraviolet photons.
