AST104 Homework 2
(Select all answers that apply.) The star Bellatrix has an apparent magnitude m = 1.64 and absolute magnitude M = -2.78. Nearby on the sky, Rigel has an apparent magnitude m = 0.13 and absolute magnitude M = -7.84. Which of the following statements is/are true about these two stars?
- Rigel is more luminous than Bellatrix. - Rigel is brighter in Earth's sky than Bellatrix.
The range of apparent magnitudes of stars (besides the Sun) that you can see with the unaided eye from Earth is about
-1 to +6
Suppose you stand 10 meters away from a light bulb and observe its apparent brightness (intensity) as 5 W/m2. What would be the intensity you observe if you do so from 20 meters away?
1.25 W/m^2
The farthest that the dwarf planet Pluto gets from the Sun during its orbit is about 50 AU. How much fainter is the Sun as viewed from Pluto compared to when it is viewed from Earth?
1/2500 as bright
TRAPPIST-1 is a red dwarf star hosting three Earth-sized planets. Its spectrum is peaked at about 1200 nm. What is this star's temperature?
2500 K
What happens when an electron transitions from a higher energy level to a lower energy level in an atom?
A photon is given off.
We shine the light of an incandescent lamp through a cloud of cooler gas. What is the form of the spectrum we observe?
Almost all wavelengths are present in the spectrum, but a few special wavelengths are missing.
Using a good pair of binoculars, you observe a section of the sky where there are stars of many different apparent brightnesses. You find one star this appears especially dim. This star looks dim because it is
It could be more than one answer. But, there is no way to tell which answer is right just by its apparent brightness.
Why is an absorption spectrum especially useful for astronomers?
It has dark lines in it that allow astronomers to determine what elements are in the star.
Star A has an apparent magnitude of 1.5 while Star B has an apparent magnitude of 3.5. How do they compare in brightness?
Star A is 6.25 times brighter.
Two stars have the same diameter and are at the same distance from us (but in different parts of the sky.) Star A has a temperature of 4000 K, while star B has a temperature of 8000K. Which of the following statements is true?
Star B has 16 times the luminosity that Star A has.
Two stars have the exact same luminosity, but Star Y is four times dimmer looking than Star X. This means that
Star Y is twice as far away as Star X.
An incandescent light bulb (the old-fashioned kind with a hot wire inside) glows yellow-white. The burner on an electric stove glows red hot. Which has the higher temperature?
The light bulb.
Why are astronomers much more interested in the luminosity of a star than its apparent brightness?
The luminosity tells us how bright a star really is, while apparent brightness only tells us how bright it happens to look from Earth.
Colliding neutron stars emit an enormous flash of electromagnetic radiation across all wavelengths. Which type of electromagnetic radiation reaches Earth first?
You can't fool me. All of these travel through space at the same speed.
The star Vega in the constellation Lyra has a temperature of about 9600 K. What is the peak wavelength of its spectrum?
about 300 nm
An artist who likes working with sources of light decides to make a modern sculpture out of electrified glass tubes that contain very thin (rarified) neon gas. When the sculpture is finished, and the electricity is turned on, the tubes glow with a rich red color. What we are seeing is
an emission spectrum.
Which of the following stars would be the brightest, as seen in our sky?
apparent magnitude m = -1
After a nice dinner around the campfire on a camping trip, you and a friend decide to get away from the fire to observe the stars. As you get farther and farther away, you see the brightness of the fire
decrease as the distance squared.
dimmest to brightest in the sky.
higher number is dimmer and lower number is brighter
When an astronomer rambles on and on about the luminosity of a star she is studying, she is talking about
how much energy the star gives off each second.
Absolute magnitude is a measure of
luminosity
An astronomer discovers a new star and wants to measure its temperature. She would typically do this by
making a blackbody curve and finding the wavelength of the peak.
Two stars have the same luminosity, but Star B is three times farther away from us than Star A. Compared to Star A, Star B will look
nine times fainter.
(Select all answers that apply.) A star would become more luminous (emit more total light) if we could
raise its temperature. make it bigger.
The Stefan-Boltzmann Law relates the energy flux coming from a blackbody (such as a star) to its
temperature
Wien's Law relates the wavelength at which a star gives off the greatest amount of energy to the star's
temperature
(Select all answers that apply.) A star's luminosity is determined by its
temperature size.
(Select all answers that apply.) A star's overall color is determined by its
temperature.
