Adi1008 wrote:Starting this up for next year's topic:
Suppose an astronomer discovers an elliptical galaxy whose major axis is 2 times longer than its minor axis.
a. What would the classification of this galaxy be?
b. What assumption do you make when classifying the galaxy about its orientation when viewed from Earth?
a. The Hubble classification of elliptical galaxies is En where n = 10(1 - b/a). Using this formula, you get that the galaxy is classified as E5.
b.You would assume that the galaxy is perpendicular to your line of sight from Earth, essentially flat from your point of view.
Adi1008 wrote:Starting this up for next year's topic:
Suppose an astronomer discovers an elliptical galaxy whose major axis is 2 times longer than its minor axis.
a. What would the classification of this galaxy be?
b. What assumption do you make when classifying the galaxy about its orientation when viewed from Earth?
a. The Hubble classification of elliptical galaxies is En where n = 10(1 - b/a). Using this formula, you get that the galaxy is classified as E5.
b.You would assume that the galaxy is perpendicular to your line of sight from Earth, essentially flat from your point of view.
Good answer! A few quick comments on the second part: the assumption is indeed that the galactic plane is perpendicular to the line of sight from Earth. I think the part saying it appears "flat" could be a bit ambiguous though and a proctor/grader could interpret it as meaning edge-on, instead of face-on. When you're viewing a (circular/disc-like) object at an angle, you're seeing the projection of the circle as an ellipse. To classify the galaxy, we need to know if we're viewing the galaxy at an angle or if it's actually that shape. Your turn
edit: After doing more reading, I've actually found that the Hubble scheme classifies galaxies entirely by how they appear to us on the Earth. The galaxy could be spherical in reality, but if it appears very elliptical, it'd be given a classification that says it's very elliptical. Bad question on my part
Stanford University
University of Texas at Austin '22
Seven Lakes High School '18
Beckendorff Junior High '14
Adi1008 wrote:Starting this up for next year's topic:
Suppose an astronomer discovers an elliptical galaxy whose major axis is 2 times longer than its minor axis.
a. What would the classification of this galaxy be?
b. What assumption do you make when classifying the galaxy about its orientation when viewed from Earth?
a. The Hubble classification of elliptical galaxies is En where n = 10(1 - b/a). Using this formula, you get that the galaxy is classified as E5.
b.You would assume that the galaxy is perpendicular to your line of sight from Earth, essentially flat from your point of view.
Good answer! A few quick comments on the second part: the assumption is indeed that the galactic plane is perpendicular to the line of sight from Earth. I think the part saying it appears "flat" could be a bit ambiguous though and a proctor/grader could interpret it as meaning edge-on, instead of face-on. When you're viewing a (circular/disc-like) object at an angle, you're seeing the projection of the circle as an ellipse. To classify the galaxy, we need to know if we're viewing the galaxy at an angle or if it's actually that shape. Your turn
Suppose that you observe a galaxy edge-on, and it appears to resemble a lens. You observe two slight outgrowths that seem to be bars.
a. What is the name of this type of galaxy, and which two types of galaxies does it serve as an intermediate between?
b. Which type of stars are most common in this type of galaxy?
c. Provide two theories regarding the origin of this type of galaxy.
Knyte_Xjn wrote:
a. The Hubble classification of elliptical galaxies is En where n = 10(1 - b/a). Using this formula, you get that the galaxy is classified as E5.
b.You would assume that the galaxy is perpendicular to your line of sight from Earth, essentially flat from your point of view.
Good answer! A few quick comments on the second part: the assumption is indeed that the galactic plane is perpendicular to the line of sight from Earth. I think the part saying it appears "flat" could be a bit ambiguous though and a proctor/grader could interpret it as meaning edge-on, instead of face-on. When you're viewing a (circular/disc-like) object at an angle, you're seeing the projection of the circle as an ellipse. To classify the galaxy, we need to know if we're viewing the galaxy at an angle or if it's actually that shape. Your turn
Suppose that you observe a galaxy edge-on, and it appears to resemble a lens. You observe two slight outgrowths that seem to be bars.
a. What is the name of this type of galaxy, and which two galaxies does it serve as an intermediate between?
b. Which type of stars are most common in this type of galaxy?
c. Provide two theories regarding the origin of this type of galaxy.
a. Lenticular galaxy (classification: SB0, because of the appearance of bars). It serves as an intermediate between spiral/barred spiral galaxies and elliptical galaxies.
b. Lenticular galaxies are old and don't have a lot of star formation. They generally contain older stars (e.g. Population II stars).
c. The first theory is that lenticular galaxies are "faded" spiral galaxies. As the spiral galaxy gets older, it "uses up" gas and dust and eventually loses its spiral structure, resulting in the lenticular galaxy. The second theory is that lenticular galaxies are the result of galaxies merging together because many lenticular galaxies have higher surface brightnesses than spiral galaxies. The resulting, newly merged galaxy is more massive and loses its "arms" and ends up looking like a disk.
Stanford University
University of Texas at Austin '22
Seven Lakes High School '18
Beckendorff Junior High '14
Adi1008 wrote:
Good answer! A few quick comments on the second part: the assumption is indeed that the galactic plane is perpendicular to the line of sight from Earth. I think the part saying it appears "flat" could be a bit ambiguous though and a proctor/grader could interpret it as meaning edge-on, instead of face-on. When you're viewing a (circular/disc-like) object at an angle, you're seeing the projection of the circle as an ellipse. To classify the galaxy, we need to know if we're viewing the galaxy at an angle or if it's actually that shape. Your turn
Suppose that you observe a galaxy edge-on, and it appears to resemble a lens. You observe two slight outgrowths that seem to be bars.
a. What is the name of this type of galaxy, and which two galaxies does it serve as an intermediate between?
b. Which type of stars are most common in this type of galaxy?
c. Provide two theories regarding the origin of this type of galaxy.
a. Lenticular galaxy (classification: SB0, because of the appearance of bars). It serves as an intermediate between spiral/barred spiral galaxies and elliptical galaxies.
b. Lenticular galaxies are old and don't have a lot of star formation. They generally contain older stars (e.g. Population II stars).
c. The first theory is that lenticular galaxies are "faded" spiral galaxies. As the spiral galaxy gets older, it "uses up" gas and dust and eventually loses its spiral structure, resulting in the lenticular galaxy. The second theory is that lenticular galaxies are the result of galaxies merging together because many lenticular galaxies have higher surface brightnesses than spiral galaxies. The resulting, newly merged galaxy is more massive and loses its "arms" and ends up looking like a disk.
Knyte_Xjn wrote:
Suppose that you observe a galaxy edge-on, and it appears to resemble a lens. You observe two slight outgrowths that seem to be bars.
a. What is the name of this type of galaxy, and which two galaxies does it serve as an intermediate between?
b. Which type of stars are most common in this type of galaxy?
c. Provide two theories regarding the origin of this type of galaxy.
a. Lenticular galaxy (classification: SB0, because of the appearance of bars). It serves as an intermediate between spiral/barred spiral galaxies and elliptical galaxies.
b. Lenticular galaxies are old and don't have a lot of star formation. They generally contain older stars (e.g. Population II stars).
c. The first theory is that lenticular galaxies are "faded" spiral galaxies. As the spiral galaxy gets older, it "uses up" gas and dust and eventually loses its spiral structure, resulting in the lenticular galaxy. The second theory is that lenticular galaxies are the result of galaxies merging together because many lenticular galaxies have higher surface brightnesses than spiral galaxies. The resulting, newly merged galaxy is more massive and loses its "arms" and ends up looking like a disk.
Correct, your turn!
The following is an H I profile (left) and column density radial profile (right) of the galaxy NGC 4559, taken from Barbieri et al. (2005):
a. What is "H I", and what does the "I" mean?
b. Why does the H I profile have two peaks?
c. Estimate the average radial velocity (in km/s) of the galaxy from the profile to one significant figure.
d. What is the wavelength of the spectral line associated with H I regions?
e. How much does that spectral line get shifted at the average radial velocity found in part (c), in cm?
Stanford University
University of Texas at Austin '22
Seven Lakes High School '18
Beckendorff Junior High '14
Adi1008 wrote:
a. What is "H I", and what does the "I" mean?
H I means hydrogen lines, the one specifies that they they are unionized
b. Why does the H I profile have two peaks?
The galaxy is spinning, thus the side with a lower radial velocity is spinning towards us, while the other side is spinning away from us. (ignoring the total recessional velocity)
c. Estimate the average radial velocity (in km/s) of the galaxy from the profile to one significant figure.
800 km/s
d. What is the wavelength of the spectral line associated with H I regions?
In my notes it says 21.1 cm, but that seems extremely large... IDK, I'll go with it
e. How much does that spectral line get shifted at the average radial velocity found in part (c), in cm?
v=zc --> z=v/c = (800 km/s)/(300000km/s) = 0.002667. Multiplying this by the wavelength, 21.1cm, yields a redshift by 0.0563 cm
EDIT: Didn't use the profile on the right, so I must have gone wrong somewhere. I think that problem e probably had something to do with the profile on the right...
Adi1008 wrote:
a. What is "H I", and what does the "I" mean?
H I means hydrogen lines, the one specifies that they they are unionized
b. Why does the H I profile have two peaks?
The galaxy is spinning, thus the side with a lower radial velocity is spinning towards us, while the other side is spinning away from us. (ignoring the total recessional velocity)
c. Estimate the average radial velocity (in km/s) of the galaxy from the profile to one significant figure.
800 km/s
d. What is the wavelength of the spectral line associated with H I regions?
In my notes it says 21.1 cm, but that seems extremely large... IDK, I'll go with it
e. How much does that spectral line get shifted at the average radial velocity found in part (c), in cm?
v=zc --> z=v/c = (800 km/s)/(300000km/s) = 0.002667. Multiplying this by the wavelength, 21.1cm, yields a redshift by 0.0563 cm
EDIT: Didn't use the profile on the right, so I must have gone wrong somewhere. I think that problem e probably had something to do with the profile on the right...
Looks good to me. I don't think you needed the figure on the right to do anything. Here's some more information about the 21.1 cm line. Your turn!
Stanford University
University of Texas at Austin '22
Seven Lakes High School '18
Beckendorff Junior High '14
PM2017 wrote:Sorry guys, I've been ridiculously busy recently, but I haven't forgotten about this.
Since I've been failing epically to find the actual equation for the T-F relationship that also give the necessary units, I'll go with a basic radial velocity problem:
A binary system involves a star identical to the sun, and another star with exactly three times the mass of the sun. The period of this system is 243 days. Given that it is 50,000 pc away, and the only recessional velocity comes from the expansion of space, draw the radial velocity chart of this system. Assume that the system is one the same plane as the viewer and that the orbit is perfectly circular.
