Study Set for Astronomy Exam 1

Find direction (toward or away from us) of a star in which this line appears at wavelength 121.9 nm.

away from us

Find direction (toward or away from us) of a star in which this line appears at wavelength 122.7 nmnm.

away from us

The star Sirius is 8.6 light-years from Earth (in our Earth-based reference frame). Suppose you traveled from Earth to Sirius at 0.92 c. During your trip, how far would you measure the distance from Earth to Sirius to be?

d′ =6.7ly

How much greater is the light-collecting area of a 6-meter telescope than a 3-meter telescope?

four times

Compared to red light, blue light has higher frequency and

higher energy and shorter wavelength

Where should you put a telescope designed for ultraviolet observations?

in earth orbit

If you see the color red in an X-ray image from the Chandra X-Ray Observatory, it means:

it depends: the colors are chosen arbitrarily to represent something about the X rays recorded by the telesc

Carla is traveling past you at a speed close to the speed of light. According to her, how much time passes for you while 1 minute passes for her?

less than one minute

Carla is traveling past you at a speed close to the speed of light. According to you, how much time passes for Carla while one minute passes for you?

less than one minute

In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find direction (toward or away from us) of a star in which this line appears at wavelength 120.5 nm

toward us

Which transition represents an electron breaking free from the atom?

transition E (look to lc)

The International Space Station orbits Earth at a speed of about 30,000 km/hrkm/hr. While 1 hour passes on Earth, how much less time passes on the Station? Assume that both the Station and Earth are in free-float frames, although in reality they are not. (Hint: Start by converting the Station's speed to km/skm/s.)

t−t′ =1.39 μs

Marta has a spaceship that measures 60 meters from end to end when at rest. If she flies past you at 0.75 c, how long will you measure it to be?

L′ =40 mm

Represent 1 light-day in miles.

1 light-day = 1.61e10 mi

Represent 1 light-day in kilometers.

1 light-day = 2.59e10 km

Represent 1 light-hour in kilometers.

1 light-hour = 1.08e9 km

Represent 1 light-hour in miles.

1 light-hour = 6.71e8 mi

Represent 1 light-minute in miles.

1 light-minute = 1.12e7 mi

Represent 1 light-minute in kilometers.

1 light-minute = 1.80e7 km

Represent 1 light-second in miles.

1 light-second = 1.86e5 mi

Represent 1 light-second in kilometers.

1 light-second = 3.00e5

A typical adult uses about 2,500 CaloriesCalories of energy each day. (Hint: 1 CalorieCalorie = 4184 JJ) Use this fact to calculate the typical adult's average power requirement, in watts.

120W

Your electric utility bill probably shows your energy use for the month in units of kilowatt-hours. A kilowatt-hour is defined as the energy used in 1 hour at a rate of 1 kilowatt (1,000 watts); that is, 1 kilowatt-hour = 1 kilowatt x 1 hour. If your bill says you used 700 kilowatt-hours, how much energy did you use in joules?

2.52e9 J

What is its frequency?

2.7e18 Hz

How many significant digits does the following number have: 5.67 x 10-8

3

The speed of light is 299,792,458 m/s. Write the speed of light in scientific notation with three significant digits.

3.00e8m/s

Your electric utility bill probably shows your energy use for the month in units of kilowatt-hours. A kilowatt-hour is defined as the energy used in 1 hour at a rate of 1 kilowatt (1,000 watts); that is, 1 kilowatt-hour = 1 kilowatt x 1 hour. Use this fact to convert 1 kilowatt-hour into joules.

3.60e6 J

The diffraction limit is a limit on:

A telescope's angular resolution.

What is the energy of a radio photon from the same station? (relates to other radio one)

E =8.75e-28J

According to general relativity, Earth goes around the Sun rather than flying straight off into space because

Earth is going as straight as possible, but the shape of spacetime makes this path go round and round.

Which of the following correctly lists our "cosmic address" from small to large?

Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe;

Put the size and number of stars in the Milky Way Galaxy and the number of stars in the observable universe into perspective.

If the Milky Way were the size of a football field, then the distance to the nearest star would be about a few millimeters. A single star would be a point-like particle on this scale. It would take thousands of years to count all stars in the Galaxy aloud. The number of stars in the observable universe is comparable to the number of grains of sand on all of the beaches on Earth.

What does it mean if you see the color red in an x-ray image from the Chandra X-Ray Observatory?

It depends; the colors are chosen arbitrarily to represent something about the x-rays recorded by the telescope.

What does the famous E = mc2 equation have to do with special relativity?

It is a direct consequence of the theory.

Why do we say that light is an electromagnetic wave? What is the relationship among wavelength, frequency, and speed for light?

Light is a vibration of electric and magnetic fields. f=cλ

A spaceship has a rest mass of 540,000 tons. If you could measure its mass when it was traveling at half the speed of light, what would the value be?

M′ =6.24e5tons

Put the size of our solar system and distance to nearby stars into perspective on the 1-to-10-billion scale.

On the 1-to-10-billion scale, the Sun is about the size of a grapefruit, Jupiter is about the size of a marble, and Earth is about the size of a ball point in a pen. The distances between planets are about ten meters in the inner solar system and about hundred of meters in the outer solar system. The nearest stars are about thousands of kilometers from the Sun.

Choose the correct statement describing, what will you see if you look at them with a telescope that has an angular resolution of 0.5 arcsecond.

One point of light that is the blurred image of both stars.Nothing at all.

Which of the following best describes why your rocket could never reach the speed of light?

The absoluteness of the speed of light means you could never keep up with the light coming from you and your rocket.

Compare this to the diffraction limit of the Hubble Space Telescope for visible light. Why, to be useful, radio telescopes must be much larger than optical telescopes?

The angular resolution of the radio telescope is much poorer than such of HST. Radio telescopes are much larger than optical telescopes because the wavelengths of radio waves are much larger than such of visible light.

Define astronomical unit.

The average distance between Earth and the Sun, which is approximately 149.6 million kilometers

When we say the universe is expanding, we mean that:

The average distance between galaxies is growing with time.

How deeply does each portion of the electromagnetic spectrum penetrate Earth's atmosphere? Why are space telescopes so important?

The only wavelengths that make it all the way to the surface are visible light and radio wavelengths along with small parts of the infrared and ultraviolet spectrum. Most of other wavelengths, including gamma ray and x-ray, can be observed only from very high altitudes or space. As a result, without space-based observatories we could not study light from much of the electromagnetic spectrum.

Which of the following must be true of a person who shares the same reference frame as you?

The person must not be moving relative to you.

Would the order of the ranking for the different forms of light listed in Part A be different if you were ranking the frequency of the different forms of light (lowest to highest, left to right)? What if the ranking is done for the wavelengths of the different forms of light (shortest to longest, left to right)?

The ranking by frequency would have the same order, since energy is directly proportional to frequency. The ranking by wavelength would have the opposite order, because frequency and wavelength are inversely related.

The fact that nearly all galaxies are moving away from us, with more distant ones moving faster, helped us to conclude that:

The universe is expanding.

Match the types of spectra with the conditions that would cause us to see each of them.

We see an absorption line spectrum when we look through a cloud of gas at a dense hot object emitting light. We see an emission line spectrum when we look at a cloud of thin and hot gas We see a continuous spectrum when we look a dense hot object emitting light.

If you observe people moving by you at very high speed, you will say that their time runs slowly, their lengths are contracted in the direction of motion, and their masses are increased from their rest masses. What will they say about you?

Your time runs slowly, your length is contracted in the direction of motion, and your mass is increased from your rest mass.

Suppose you had a spaceship so fast that you could make a roundtrip journey of 1 million light-years (in Earth's reference frame) in just 50 years of ship time. If you left in the year 2030, you would return to Earth

a million years from now

The Hubble Space Telescope obtains higher-resolution images than most ground-based telescopes because it is:

above earth's atmosphere

Find direction (toward or away from us) of a star in which this line appears at wavelength 121.1 nm.

toward us

List the different forms of light in order from highest to lowest energy.

lowest to highest: radio, infrared, visible, ultraviolet, x-rays, gamma rays

A subatomic particle that normally decays in 1 microsecond is created in a particle accelerator and is traveling at close to the speed of light. If you measure its lifetime in that case, you'll find that it is

more than 1 microsecond.

The twinkling of stars is caused by

motion of air in our atmosphere

If you wanted a radio telescope to achieve the same angular resolution as a visible-light telescope, it would need to be:

much larger

Suppose that two stars in a binary star system are separated by a distance of 80 million kilometers and are located at a distance of 160 light-years from Earth. Can the Hubble Space Telescope resolve the two stars?

no

Suppose you look at two stars that are separated in the sky by 0.1 arcsec, using a telescope with an angular resolution of 0.5 arcsec. What will you see?

one point of light that is the blurred image of both stars

What provides the strongest evidence that everyone always measures the same speed of light?

precise measurements of the speed of light under different circumstances.

A spectral line that appears at a wavelength of 321 nm in the laboratory appears at a wavelength of 328 nm in the spectrum of a distant object. We say that the object's spectrum is:

redshifted

What does it mean when a light beam is moving at the speed of light, for someone standing still and the person is coming at you and you see the speef of light go from the floor to the ceiling?

the person coming at you measures a shorter amount of time

What does it mean when a light beam is moving at the speed of light, for someone standing still is measuring the time and you see the speed of light at the same speed, but taking a longer, slanted path?

the person standing standing still measures a longer amount of time

In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the speed for a star in which this line appears at wavelength 121.1 nm..

v=1200 km/s

In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the speed for a star in which this line appears at wavelength 120.5 nm.

v=2700 km/s

In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the speed for a star in which this line appears at wavelength 122.7 nm

v=2700 km/s

In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find direction (toward or away from us) of a star in which this line appears at wavelength 121.1 nm.

v=740 km/s

A fly has a mass of 1 gram at rest. How fast would it have to be traveling to have a mass of 3000 kilograms (about that of a large SUV)?

very close to the speed of light

For the game of interstellar baseball, teams have used robotic enhancements to allow superhuman feats. Consider a pitcher who can throw a baseball at 65 % of the speed of light and is practicing in a spaceship traveling away from Earth at 95 %% of the speed of light (on the way to the team's next game). Assuming that he throws the baseball in the same direction the spacecraft is traveling, how fast will those of us on Earth measure the baseball to be going?

v′ =0.9892c

The star Betelgeuse is about 600 light-years away. If it explodes tonight,

we won't know about it until about 600 years from now.

Suppose that two stars in a binary star system are separated by a distance of 80 million kilometers and are located at a distance of 160 light-years from Earth. What is the angular separation of the two stars? Give your answer in degrees.

θ =3.0e-6∘

Now remember that humans have two eyes that are approximately 7 centimeters apart. Estimate the diffraction limit for human vision, assuming that your "optical interferometer" is just as good as one eyeball as large as the separation of two regular eyeballs.

θ2−eyes =1.8″

Suppose that two stars in a binary star system are separated by a distance of 80 million kilometers and are located at a distance of 160 light-years from Earth. What is the angular separation of the two stars? Give your answer in arcseconds.

θ=1.1e-2′′

What is the diffraction limit of a 120-meter radio telescope observing radio waves with a wavelength of 21 centimeters?

θ=440″

Calculate the diffraction limit of the human eye, assuming a wide-open pupil so that your eye acts like a lens with diameter 0.8 centimeter, for visible light of 500-nanometer wavelength.

θeye =16″

How does this compare to the diffraction limit of a 10-meter telescope?

θeye/θtelescope =1300

What is the wavelength of an X-ray photon with energy 11 keV (1.1×104 eV)? (1 eVeV = 1.60 ×× 10−19−19 joule.)

λ =0.11 nm

What is the wavelength of a radio photon from an AM radio station that broadcasts at 1320 kilohertz?

λ =227 mm

The Sun radiates a total power of about 4×10264×1026 watts into space. The Sun's radius is about 7×1087×108 meters. Using your answer from part A and the Stefan-Boltzmann law, calculate the average surface temperature of the Sun. (Note: The temperature calculated this way is called the Sun's effective temperature.)

≈6,000K

The Sun radiates a total power of about 4×10264×1026 watts into space. The Sun's radius is about 7×1087×108 meters. Calculate the average power radiated by each square meter of the Sun's surface. (Hint: The formula for the surface area of a sphere is A=4πr^2.)

≈6e7 W/m^2