ASTR 1102 : Arto Study Guide Final
Homework #2
1. Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star
doubles, what will happen to the peak wavelength of the spectrum?
Wien's Law is an inverse relation - higher temperature means shorter (smaller) wavelength. There are
no powers, so double the temperature means half the wavelength.
The correct answer is: It will be a half of the previous wavelength
2. 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?
A light year is the distance light travels in a year. It takes light 500 years to travel 500 light years.
The correct answer is: 500 years
3. Which of the following best describes how a modern astronomer views a constellation?
Modern astronomers divide the whole sky into constellations so that any point can be placed in a
constellation. Often they are more interested in the faint objects that are invisible to the naked eye and
that lie in between or beyond the brighter stars that make up the well-known figures. Although a
constellation does define a region of space, it includes objects out to the limits of visibility, well outside
our own Galaxy. The correct answer is: A well-defined region of the sky
4. All electromagnetic radiation travels at a constant speed of light. X-rays have a shorter
wavelength, HIGHER frequency, and X-ray photons are MORE energetic than visible light
photons. The correct answer is: X-rays have a shorter wavelength than visible light
5. Which type of electromagnetic radiation has a wavelength adjacent to, but longer than
ultraviolet light?
The sequence here from shortest wavelength to longest is Gamma-ray, X-ray, Ultraviolet,
Visible, Infrared, Radio, so Visible is adjacent to and longer than Ultraviolet.The correct answer
is: Visible
6. Compared to visible light, ultraviolet light has:
Ultraviolet photons are shorter wavelength than visible light. Shorter wavelengths mean higher
frequencies and photon energies.
The correct answer is: Shorter wavelength and higher energy per photon
7. All electromagnetic radiation travels at a constant speed of light. Radio photons have less
energy than infrared photons, a LONGER wavelength, and a lower frequency.
The correct answer is: Radio waves have a shorter wavelength than infrared photons
8. Light has a particle nature, and these particles are called photons. Which region of the
electromagnetic spectrum has the highest energy photons?
The sequence here from lowest energy to highest energy is infrared, visible, ultraviolet, X-ray,
gamma-ray. The correct answer is: Gamma-rays
9. At what wavelength would you expect to see the peak in the spectrum of a star of temperature
5800K?
Wien's Law says that w=0.0029/T, where T is the temperature and w is the wavelength. So
w=0.0029/5800=0.000001m=500nm. The correct answer is: 500nm
10. The Sun has a temperature of 5800K, and the peak of its radiation is around a wavelength of
500nm. At what wavelength would you expect to see the peak of the radiation from an
extremely hot star of temperature 58,000K?
Wien's Law shows that the peak wavelength is inversely proportional to the temperature (i.e.
w1/w2=T2/T1, where w1 and w2 are the wavelengths of stars of temperature T1 and T2). So
w1/500=5800/58,000 and w1=0.1*500=50nm.
The correct answer is: 50nm
11. An astronomer says that a galaxy contains about 5x10^10 stars (five times ten to the power ten,
5E10 on a calculator). How many stars is this in non-scientific notation?
This is a five followed by ten zeroes. Another way to think of it is that a billion is 10^9, so 10^10
is ten billion, and 5x10^10 is fifty billion. The correct answer is: 50 billion
12. Assume the spectrum of a star is exactly a blackbody. If the surface temperature of the star
DECREASES by a factor four, what will happen to the peak wavelength of the spectrum?
Wien's Law is an inverse relation - higher temperature means shorter (smaller) wavelength.
There are no powers, so a quarter of the temperature means four times the wavelength.
The correct answer is: It will increase by a factor of four in wavelength
13. When an astronomer says a star approximately has a black-body spectrum, what do they mean?
Stars have spectra with approximately the same shape as black bodies. They still have some
absorption lines. Some emit most of their light at visible wavelengths, and they have a wide
range of temperatures. The correct answer is: It has a spectrum with a similar shape to a black-
body of the same temperature
14. Sort the kinds of light below into order from longest to shortest wavelength.
The correct answer is: Very long wavelength
– Radio, Long wavelength
Document Summary
Homework #2: assume the spectrum of a star is exactly a blackbody. There are no powers, so double the temperature means half the wavelength. The correct answer is: it will be a half of the previous wavelength: the star betelgeuse is about 500 light years away from us in the constellation orion. A light year is the distance light travels in a year. It takes light 500 years to travel 500 light years. Modern astronomers divide the whole sky into constellations so that any point can be placed in a constellation. Often they are more interested in the faint objects that are invisible to the naked eye and that lie in between or beyond the brighter stars that make up the well-known figures. Although a constellation does define a region of space, it includes objects out to the limits of visibility, well outside our own galaxy.
