Astronomy Chapter 10
The star Lee has an apparent magnitude of 0.1 and is located about 250 parsecs away from Earth. Which of the following is most likely the absolute magnitude for Lee? a.) -6.9 b). 0.1 c.) 7.1
-6.9 The farhter away a star is, the dimmer it gets. Absolute magnitude is calculated for a distance of 10 parsecs, so Lee must be very bright to shine so much from far away.
If the parallax angle for Star A is 1 arcsecond, what is the distance from the Sun to Star A?
1 parsec
Explain the difference between a star's apparent brightness and its luminosity.
A star's apparent brightness is how bright it appears from Earth while it's luminosity is a more objective quanitity in that it measures the amount of energy a star actually emits per second.
Which appears brighter, a small handheld flashlight 1 ft (30 cm) away or a large spotlight atop a lighthouse 10 mi (16 km) away?
A very nearby flashlight will appear much brighter than a very distant spotlight. Although a larger spotlight emits more light than a small flashlight, which of them appears brighter depends on distance.
How many times less light falls on a newspaper illuminated by a lightbulb if the newspaper is moved a distance of 3 times away?
According to the inverse-square law nature of light, the intensity of the light received decreases with the square of the distance. In this instance, increasing the distance 3 times means the newspaper receives on 1/(3)^2 or one-ninth the amount of light originally received.
The brightest of the main sequence stars have what other relatively large characteristics?
According to the mass-luminosity relation, the most luminous main-sequence stars are also the most massive, have the largest radii, and have the greatest temperatures.
If we were observing our Sun from a distance of 10 pc, what would be its apparent and absolute magnitudes?
Because a star's apparent magnitude when viewed from a distance of 10 pc is the same of its absolute magnitude, both magnitudes would be the same.
Which is hotter, a green star or a yellow star?
Because green light has a shorter wavelength than yellow light, the star that is primarily emitting green light must be at a higher temperature than a star emitting mostly yellow light.
If two stars have the same surface temperature and are the same size, which star, if either, is more luminous? Explain your reasoning.
Both are the same. Luminosity is affected only by size and temperature
Which two pairs of labeled stars have the same luminosity?
C, F
Which two pairs of labeled stars in the diagram have the same temperature?
D and E
Describe how binary star systems are vital figuring out the masses of stars.
Each of the two stars in a binary star system moves in an elliptical orbit about the center of mass of each system. The center of mass is located by plotting the separate orbits of the two stars. It lies on the common focus of the two elliptical orbits. Comparing the relative sizes of the two orbits around the center of mass yields the ratio of the two stars' masses, M1/M2. The sum of the two masses would already be known from Kepler's Third Law, so you can than calculate the individual masses
The star Tau Ceti has an apparent magnitude of about +3 and an absolute magnitude of about +6. Is it much closer or much farther from Earth than 10 pc?
From Earth, Tau Ceti appears to be relatively bright at +3. However, if it were viewed from a distance of 10 pc, it would appear much dimmer, at +6. The only way this can be is if Tau Ceti is much closer to Earth than 10 pc.
How many light years away is Alpha Centauri if it has a parallax angle of 0.772 arcsec?
Given that the distance in parsecs is the inverse of the parallax angle, d=1/0.7772, then d=1.35 pc. Because 1 pc is 3.26 ly, we find that 1.35 pc x (3.26 ly/1 pc) = 4.4 ly
Which brown dwarf is hotter, an L or a T?
Given that the modern spectral sequence of stars and brown dwarfs from hottest to coolest is OBAFGKMLT, L-spectral class brown dwarfs are hotter than T-spectral class brown dwarfs
If two stars are at the same temperature, but one is 3 times larger, how many times more luminous is it?
If luminosity is given by the Stefan-Boltzmann law where L = 4(pi)R^2(sigma)(T^4), then if radius R is tripled, then L must increase by the R^2, which is, in this instance, 3^2 or 9 times, for a star 3 times larger but at the same temperature
If two stars in a binary system were moved farther apart, how would masses and orbital periods change?
If two stars orbiting a common center of mass were moved farther apart, their masses would not change, but the period would increase following Kepler's third law.
What about the nature of atoms in Sunlike stars makes the notion that the "larger stars have the broadest absorption lines" incorrect?
In low-density atmospheres of the largest stars, atoms tend to be widely separated and have relatively fewer collisions with other atoms that result in light being emitted at highly specific wavelengths that are observed as narrower lines.
If hydrogen is the most abundant atom in a star, why do the spectra of hot stars show no prominent hydrogen lines?
In the hottest stars, the electrons have been removed from hydrogen atoms and, as reuslt, the existing hydrogen atoms do not have an electron available to create absorption spectra
Is a spectral class F2 star more similar to to an A-spectral class star or a G-spectral class star?
In the sequence OBAFGKM, F-spectral class stars with designations F1, F2, F3, and F4 are more similar to A-spectral class stars and F6, F7, F8, F9 are more similar to G-spectral class stars.
Look at the H-R Diagram on page 57 of the Lecture Tutorials. Stars U and V have the same surface temperature. Given that Star U is actually much more luminous than Star B, what can you conclude about the size of Star U compared to Star V? Explain your reasoning.
It is bigger because increasing surface area means increasing luminosity
Star U has a greater surface temperature than Star X. Given that Star X is actually just as luminous as Star U, what can you conclude about the size of Star X compared to Star U? Explain your reasoning.
It must be smaller than star U because increasing surface area means increasing luminosity
How does a K5 V star compare to a K5 II star in terms of temperature, luminosity and radius?
Main sequence stars are luminosity class "V" whereas giant stars are luminosity class "II". In this instance, the two stars have the same temperatures because they are in the same spectral class, but a luminosity class II star is larger and has a greater luminosity than a much smaller main sequence star.
The Full Moon has an apparent magnitude of -12.6, and when Mars is at its brightest in the night sky, its apparent magnitude is +2.0. Which of the two objects has the bigger apparent magnitude number? Which object will look brighter from Earth, the full Moon or Mars? How do you know?
Mars; Full Moon; the lower the number, the brighter the object is
For stars, what is the difference between proper motion and motion due to parallax?
Motion due to parallax is an object's displacement because of the change in an observer's point of view.
If two stars have the same absolute magnitude, do they necessarily have the same temperature?
No
If you use two hot plates of different sizes, can you assume that the hot plate can cook a large pot of spaghetti first is at a higher temperature?
No
As spectral type numbers increase within the G-spectral class of star, do the larger numbers represent higher temperature stars?
No. Within a particular spectral class, the larger numbers correspond to cooler stars. For example, a G2 star is hotter than a G8 star because a G2 is closer to the hotter F spectral class stars in the sequence OBAFGKM.
Why can't the distances to most stars in our Galaxy be measured using parallax?
Parallax only works for stars that are relatively nearby, and the majority of stars in our Galaxy are much too far away to exhibit any apparent shift in position as Earth orbits the Sun.
The star Pleione in the constellation Taurus is 190 times as luminous as the Sun but appears only 3.19 x 10^(-13) as bright as the sun. How far is Pleione from Earth?
Pleione's luminosity to that of the Sun (L/luminosity of the sun= 190) and the ratio of their apparent brightnesses (b/brightness of sun = 3.19 x 10^(-13)) when rearranged in the equation relating luminosity, distance, and brightness, yields the distance from Earth to Pleione as 2.44 x 10^7 times greater than the distance from Earth to the Sun.
Menkalinan is an A2 star in the constellation Auriga. What is its spectral class? What is its spectral type? Which gives a more precise description of the spectrum of Menkalinan?
Spectral class: A spectral type: absorption line the class gives a more precise description
For Star A-D, state whether the star is closer than, farther than, or exactly 10 parsecs away from Earth. Explain your reasoning.
Star A and D are exacrly 10 parsecs away from Earth because the apparent and absolute magnitudes are the same Star B is closer because its apparent magnitude is brighter than its absolute magintude Star C is farther because its apparent magnitude is dimmer than its absolute
Which object is actually brighter: Star A or Star D? Explain your reasoning.
Star A. It has a lower value for absolute magnitude.
Look at the table on page 34 of the lecture tutorials. Which object appears brighter from Earth: Star C or D? Explain your reasoning.
Star D because it has a lower number for apparent magnitude
Star Y appears much brighter than Star Z when viewed from Earth, but is found to actually give off much less light. Assign a set of possible values for the apparent and absolute magnitudes of these stars that would be consistent with the information given in the previous statement. Explain your reasoning.
Star Y: 2 (apparent magnitude), 10 (absolute magnitude) Star Z: 5(apparent), 3 (absolute) Star Y must be closer than star Z.
Where on the H-R diagram are the stars with the greatest luminosity and highest temperatures?
The H-R diagram is plotted as increasing luminosity on the vertical axis and decreasing temperature along the horizontal axis. The stars with the greatest luminosity and the highest temperature are found in the upper left-hand corner of the H-R diagram.
How much more luminous than the Sun are the brightest stars? How much less l uminous are the dimmest? Which stars are more common?
The brightest stars are 10,000 times more luminous than the sun while the dimmest stars are 10,000 times less luminous. Dimmer stars are more common.
What principal characteristic of a star's spectrum most dominates which spectral class letter of the alphabet it is assigned?
The characteristic that most dominates is the prominence of certain hydrogen absorption lines visible in the star, with A class being the greatest and O class being the least.
If an astronomer inadvertently measures a parallax angle for a star to be smaller than it actually is, is the star incorrectly assumed to be farther away or closer to Earth than it actually is?
The parallax angle is smallest for the most distant stars, so if a parallax angle was measured to be too small, then the astronomer would assume the star is farther away than it actually is.
If a red star and a blue star both appear equally bright and both are the same distance from Earth, which one has the larger radius?
The red star because the radius needs to balance out the temperature if both stars have the same luminosity.
Why does it take at least six months to make a measurement of a star's parallax?
To measure the distance to a star, astronomers measure the parallax shift of the star using two points of view that are at opposite sides of Earth's orbit. It takes six months to get from one point to the other
Which star is larger, X or Y?
X because it is more luminous even though both have the same temperature
Which star is larger, Y or V?
Y because it has the same luminosity as V but has a smaller temperature
Do stars with the same luminosity have the same absolute magnitude number?
Yes
If you use two hot plates at the same temperature, can you assume that the hot plate that can cook a large pot of spaghetti first is larger?
Yes
If you use two hot plates of the same size, can you assume that the hot plate that can cook a large pot of spaghetti first is at the higher temperature?
Yes
If I see strong Hydrogen absorption lines in the spectrum of a star, what type of star is it? What is the temperature? What is the luminosity?
You are looking at a Class A star. It's temperature is around 9000 K. It is a main sequence star of luminosity close to the Sun.
If our Sun is many times hotter, many times larger, and has more active nuclear reactions than a brown dwarf does, what do they have in common?
a chemical composition of three-quarters hydrogen, one-quarter helium, and 1% or less of metals
For each of the following star descriptions, state whether the star would be a red giant, white dwarf, or main sequence star. a) very bright (high luminosity) and very hot (high temperature) b) very dim and cool c) very dim and very hot d) very bright and cool
a) main sequence b) main sequence c) white dwarf d) red giant
What is the difference between apparent magnitude and absolute magnitude?
apparent magnitude is how bright a star appears to an Earth-based observer while absolute magnitude is how bright a star appears from a distance of 10 pc
If a red star and a blue star both have the same radius and both are the same distance from Earth, which one looks brighter in the night sky? Why?
blue because it has a higher temperature
Look at the H-R Diagram on page 117. What are the spectral type, temperature, absolute magnitude number, and luminosity of Star A?
spectral type: B Temperature: 18,000 K Absolute magnitude: 0 Luminosity: 100
If two stars are the same size, but one has a higher surface temperature, which star, if either, is more luminous. Explain your reasoning.
the hotter one
Why is the magnitude scale called a "backward" scale?
the lower the number, the brighter the star
What information about stars do astronomers learn from binary star systems that cannot be learned in any other way? What measurements do they make of binary systems to garner this information?
the masses of stars semimajor axis and orbital period
If two stars have the same surface temperature and one is more luminous, what can you conclude about the sizes of the stars?
the more luminous one is larger
If two hot plates have the same temperature and one cooks the pot of spaghetti more quickly, what can you conclude about the sizes of the stars?
the one that cooks spaghetti more quickly is larger
What is the inverse square law? Use it to explain why an ordinary lightbulb can appear brighter than a star, even though the lightbulb emits far less light energy per second.
the relationship between how bright something appears and how close it is apparent brightness of a star's light= star's luminosity divided by 4pi(distance to star)^2 b=L/(4pid^2) Since you are significantly closer to the lightbulb than you are to the star, the lightbulb will appear brighter
Do stars of the same temperature have the same spectral type?
yes
