Astronomy Exam Questions and Answers: LSU ASTR 1102 Hynes, Exams of Engineering

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LSU ASTR 1102 Hynes

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ASTR 1102 Hynes

An astronomer says that a globular cluster contains about 7x10^5 stars (seven times ten to the power five, 7e5 on a calculator). How many stars is this in non-scientific notation? Correct Answers 700, Light has a particle nature, and these particles are called photons. Which region of the electromagnetic spectrum has the lowest energy photons? Correct Answers Infrared Which of the following tools does a typical professional astronomer spend most time using? Correct Answers Computers Which of the following does NOT differ between different kind of electromagnetic radiation? Correct Answers The speed at which the radiation travels The star Betelgeuse is about 500 light years away from us in the constellation Orion. If this star underwent a supernova explosion right now, approximately how long would it be until we found out about it? Correct Answers 500 years

At what wavelength would you expect to see the peak in the spectrum of a star of temperature 2900K? Correct Answers 1000 nm Wien's Law says that w=0.0029/T, where T is the temperature and w is the wavelength. So w=0.0029/2900=0.000001m=1000nm. At what wavelength would you expect to see the peak in the spectrum of a star of temperature 2 9000 K? Correct Answers 100 nm Wien's Law says that w=0.0029/T, where T is the temperature and w is the wavelength. So w=0.0029/29000=0.000001m=100nm. When an astronomer calls an object a black body, what do they mean? Correct Answers It absorbs all the light that falls on it A blackbody absorbs all of the light that falls on it and reflects none. It may still be very hot, and emit radiation with a color dependent on its temperature.

A hot star has a temperature of 23,000 K (about 4x hotter than the Sun). How much more energy does it emit per square meter of its surface than the Sun? Correct Answers 256x more energy? Correct Answers The Stefan-Boltzmann Law says that the energy emitted per square meter depends on the fourth power of the temperature. If the star is 4x hotter then it emits 4x4x4x4=256x more energy. A hot, glowing, solid, object surrounded by cool gas emits which type of spectrum? Correct Answers Absorption line Cool gases in front of a hotter background source of light produce absorption line spectra. The spectrum of an astronomical object contains only emission lines. What are you most likely to be seeing? Correct Answers A hot, thin, transparent gas

Correct Answers 81x more energy The Stefan-Boltzmann Law says that the energy emitted per square meter depends on the fourth power of the temperature. If the star is 3x hotter then it emits 3x3x3x3=81x more energy. Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star HALVES, what will happen to the peak wavelength of the spectrum? Correct Answers It will double in wavelength Wien's Law is an inverse relation - higher temperature means shorter (smaller) wavelength. There are no powers, so half the temperature means double the wavelength. Blackbody A is at 20000 degrees K while blackbody B is at 5000 degrees K. Therefore blackbody A gives off how many times more total energy than blackbody B? Correct Answers 256

The total energy given off depends on the fourth power of the temperature (Stefan-Boltzmann Law). If the temperature is 4x higher, the energy output is 4x4x4x4=64x higher. Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star INCREASES by a factor of four, what will happen to the peak wavelength of the spectrum? Correct Answers It will be a quarter of the previous wavelength? Correct Answers Wien's Law is an inverse relation - higher temperature means shorter (smaller) wavelength. There are no powers, so four times the temperature means one quarter the wavelength. Which of the following types of astronomical observations CAN EASILY be performed by a telescope on the ground? Correct Answers Radio The atmosphere blocks all kinds of electromagnetic radiation except radio, optical, and some infrared wavelengths. Of the options presented only radio is possible from the ground How does the nuclear energy production in the core of the Sun compare to the energy radiated from surface of the Sun? Correct Answers

the speed of light, or 300 km/s (300,000 m/s). In math, v/c=0.6/600, so v=0.6/600c = 0.001c = 300, m/s. How is the Sun's luminosity produced? Correct Answers* Nuclear fusion Only nuclear fusion is important in producing the Sun's current luminosity. Gravitational contraction did play a role in getting the process started 5 billion years ago. Chemical burning has never been relevant to the Sun. Which of the following correctly describes the structure of the Sun's interior from inside to outside Core, radiative zone, convective zone A star is moving away from the Earth at 200 km/s. The hydrogen-alpha absorption line normally has a wavelength of 656.3nm. What can you say about the wavelength at which it will appear in the spectrum of the star? Correct Answers It would be more than 656.3 nm The star is moving away from us so its spectrum is red-shifted. Red-shifts correspond to longer wavelengths, so the wavelength of the line would become longer than 656.3nm. How was the chemical composition of the Sun 3 billion years ago different from today?

Correct Answers It had more hydrogen The Sun generates energy by turning hydrogen into helium by thermonuclear fusion. That means that 3 billion years ago it must have had more hydrogen and less helium than it has today. Carbon and iron have not significantly changed One of the processes carrying energy from the core of the Sun to its surface is convection. What does the process of conviction involve? Correct Answers The motion of hot gases In convection, circulating currents of hot gas carry heat upwards. On a day to day basis, we know convection as 'Hot air rises' The Sun contains 74 percent hydrogen

Deuterium is a heavier isotope of hydrogen containing the same number of protons (one), but one neutron where normal hydrogen has no neutrons. Its mass is about double that of hydrogen. What can you learn about a star from its spectrum? Correct Answers Its velocity towards or away from us, its chemical composition, and its temperature. While we have not studied stars in detail yet, we have discussed the fact that temperatures, chemical composition and velocity can all be studied with the spectrum The Sun's lifetime is about 10 billion years. If another star had the same mass as the Sun but was ten times LESS luminous, how long would it be expected to live? Correct Answers 100 billion years If the star had the same mass as the Sun it would have the same amount of fuel. If it is ten times less luminous, then ten times less energy must be produced in the core and so it is burning its fuel ten times slower. If it is burning fuel ten times slower, then it will last ten times longer, or 100 billion years.

• ASTR 1102 Hynes Exam

A star has a temperature of 11,600K (exactly twice that of the Sun), but the same size of the Sun. The peak of the Sun's spectrum is at 500nm. At what wavelength is the peak of the stars spectrum? Correct Answers 250nm What is an emission nebula? Correct Answers A region containing hot excited gas How do protostars get their energy? Correct Answers Gravitational collapse Beta Fictoris is 3X as hot as the Sun, but has the same radius and mass. How many times more energy does it radiate? Correct Answers 81X Which of the following types of star would you expect to have the LONGEST main-sequence lifetime? Correct Answers K stars Why have astronomers not used parallax (motion of stars relative to background objects) to measure the distances to bright stars far away in our Galaxy until very recently? Correct Answers Until very recently, annual parallax could only be measured for nearby stars

A star has a radius twice that of the Sun (2 Rsun) and temperature three times that of the Sun (3 Tsun = 17,400K). How many times more luminous than the Sun is it? Correct Answers 324x What is the parallax of a star at 4 parsecs? Correct Answers 0.25 arcsec What is the parallax of a star at 200 parsecs? Correct Answers 0.005 arcsec At what wavelength would you expect to see the peak in the spectrum of a star temperature 2900K? Correct Answers 1000nm A very hot star has a temperature of 58,000K (10x hotter than the Sun). How much more energy does it emit per square meter of its surface than the Sun? Correct Answers 10,000x more energy The star Eta Carinae is about 8000 light years away from us. If this star underwent a supernova explosion right now, approximately how long would it be until we found out about it? Correct Answers 8,000 years Where would a gas cloud contracting into a protostar enter the Hertzsprung Russell diagram? Correct Answers

A distant nebula has a reddish color. Why could this be? Correct Answers Either you are seeing glowing hydrogen gas, or the nebula is reddened by dust How can we best see protostars buried inside dusty nebulae? Correct Answers With infrared telescopes Which of the following best describes how a modern astronomer views a constellation? Correct Answers A well-defined region of the sky Star A has the same luminosity as star B, but is 4x farther away. How much dinner does it appear? Correct Answers 16x dimmer The star Betelguse is cooler than the Sun. Its spectrum is Redder than the sun Star 1 has a temperature of 10,000K and radius 5 solar radii. Star 2 has temperature 5,000 K and radius 10 Solar radii. What is the ratio of their luminosities, L1/L2? Correct Answers 4 Two stars have the same temperature and luminosity, but one is 10 light years away from the Earth, and the other is 1,000 light years away. The more distant star appears to be

10,000X fainter Which of the following make up a constellation as defined by modern astronomers? Correct Answers Bright stars, faint stars and distant galaxies What are parsecs used to measure? Correct Answers Distance In which of the following nebular will it be easiest to form stars? Correct Answers A cool, high density nebula A star has a parallax of 0.1 arcseconds. How far away is it? Correct Answers 10 parsecs The star Alnilan is about 400 parsecs away from us. How long does its light take to reach us? Correct Answers 1,300 years A star has a parallax of 0.02 arcseconds. How far away is it? Correct Answers 50 parsecs Where in the Hertzsprung-Russell diagram would you expect to find the largest stars? Correct Answers