- Read Lab 15: It is hard to understand software/hardware tools without first seeing and playing with them, but insofar as possible you should be prepared to use the tools. You could also fill out any parts of the lab that can be done ahead of time.
- Startup Presentation
- Read a sufficient amount of the articles linked to the following terms etc. so that you can define and/or understand the terms etc. at the level of our class: Doppler effect, Galactic disk, Galactic spiral arms, H I region, hydrogen 21-centimeter line (AKA 21-cm radiation), Leiden Observatory (Leiden Observatory History (in Dutch)), Milky Way (AKA the Galaxy), Orion Arm, radial velocity, radio telescope, spiral arms, spiral galaxy
- Write out the definitions required in Part A of Lab 15.
Supplementary Lab Preparation: The items are often alternatives to the required preparation and to each other.
- Bennett (2008 edition): p. 611--615 on Milky Way or the corresponding pages in similar books.
- Many other intro astro books give similar presentations on nature of the Milky Way.
-
For labs with instructor DavidJ, you should always be prepared for a pre-lab quiz based on the
preparation for the lab.
The quiz might be omitted if it's not feasible or convenient. The students may or may not be informed ahead of time of quiz omission depending on the circumstances.
The quizzes in total are 40 % of the course grade. However, only the top five quiz marks are counted.
In preparing for a quiz, go over the Required Lab Preparation.
The Supplementary Lab Preparation (see above) could help, but is only suggested if you feel you need more than the required Required Lab Preparation.
There is no end to the studying you can do, but it is only a short quiz.
One to two hours prep should suffice.
There will be 10 or so questions and the time will be 10 or so minutes.
The questions will range from quite easy to challenging.
There may or may not be a prep quiz to test yourself with ahead of the lab period.
The solutions might be posted at Constelllations: Quiz Solutions after the quiz is given. Whether they are or not depends on the circumstances of each individual semester.
- Check as needed:
- The
Galactic Structure lab
is an inside lab.
It is actually quite short with help for the instructor to manipulate the Excel spreadsheet.
It may be an appropriate last lab for the semester.
It is a bit of an ease-off at the end.
- If it is the last lab of the semester, there are several things to do.
- Usually the
course evaluations
will be done.
- The instructor can haul in a
C8
and use the power-transfer box (heavy black box) from the storeroom and power it up.
The students could then do the lab final practice in their spare moments.
There is a makeups/lab final practice week, but one has to expect, that even if they need practice, most students who are NOT doing makeups won't come.
- Get the names of people who are going to do makeups and what makeups they are doing.
Tell them they have to prep for that makeup.
Students who have missed more than one lab are entitled to a makeup.
Students who have missed only one lab can be allowed to do them with instructor permission.
The instructor should be fair with permissions: none at all, first come first serve for a few, everyone who wants one.
- You will need to set out the handouts on
Galactic spiral arms
(available in the storeroom) ,
protractors,
and rulers.
- Usually the
course evaluations
will be done.
- Do course evaluations
if this is the night to do them.
The procedure is to distribute them and leave the room.
Arrange for a student to collect them after 10--15 minutes.
The student can meet you in the hall and you can escort the student to the office to leave them on Natasa's desk.
Then return and carry on.
- If this is the last lab, get names of those who are doing makeups and the
labs they are making up.
People with more than one lab missing are entitled to a makeup.
People who have missed only one are NOT entitled.
They can do them with instructor permission.
The instructor has to be fair about permissions.
In a one-missing-lab case, doing the lab will only change a student's grade by a small amount.
But we want them to have the experience of the lab---it's NOT about grades.
- If there are
C8's
for practice for the
lab final
point them out
and point that here is the
lab final description.
- See Usual Startup for the
usual startup for the Startup Presentation.
Get students into new groups and at a convenient moment get the student names in their groups.
- Everyone go the computers and launch Mozilla Firefox
and click on the bookmark jeffery astlab,
Caption: "Red foxes(Vulpes vulpes) at the British Wildlife Centre, Surrey, England." Photo 2008 Aug17, Sunday.
Credit/Permission: © Keven Law, 2008 / Creative Commons CC BY-SA 2.0.
Image linked to Wikipedia.
- Then click down the chain
Lab Schedule,
Lab 15: Milky Way,
Startup Presentation,
and scroll past the foxes
to here.
- Objectives:
- Our first objective is to see in part
how the Galactic spiral arm structure
was first determined using
radiation from the
cold neutral atomic hydrogen gas
(temperature
usually of order 50--100 K and density of order
20--50 atoms/cm**3 of
H and
He with of order 2 % or less of
metals:
Wikipedia: Interstellar medium:
interstellar matter)
that is concentrated in the
spiral arms.
The part we actually do is just to use distance measurement to the spiral arms.
- Along the way, we will learn something about the
hydrogen atom
how 21-cm radiation
is generated, and
why radio radiation
can penetrate the
cosmic dust clouds
to enable radio astronomers
to see to the far side of the
Milky Way.
- And also learn how
radio astronomers used the rotation of the
Milky Way
and the Doppler effect
to separate out the various spiral arms.
- Our first objective is to see in part
how the Galactic spiral arm structure
was first determined using
radiation from the
cold neutral atomic hydrogen gas
(temperature
usually of order 50--100 K and density of order
20--50 atoms/cm**3 of
H and
He with of order 2 % or less of
metals:
Wikipedia: Interstellar medium:
interstellar matter)
that is concentrated in the
spiral arms.
- First, we note that the
Galactic disk
is laced with interstellar dust
that is opaque to visible light
over distances of order a few
kiloparsecs.
The Galactic disk is about 30 kpc in diameter.
Now the Solar System is in the Galactic disk
So our view of the Milky Way in visible light is limited to only a fraction.
In particular, we cannot see in the visible light to the Galactic center and beyond.
Before the 1930s, astronomers were in denial that the interstellar dust caused this problem despite the fact that the dust lanes are evident in the long exposure images of the Milky Way and other spiral galaxies.
Caption: "360 degree panorama of Racetrack Playa in Death Valley at night. The Milky Way is visible as the arc in the center. A sailing stone is also seen below along with the tracks of other stones."
Because this is a 360 degree panorama, the Milky Way looks like a top view a ring. The image has made the horizon crossings look very close together relative to the maximum altitude above the horizon.
You never see Milky Way like this with the naked eye. It really follows a great circle on the celestial sphere. But the Earth blocks half of it from our view.
I think this picture has to be long exposure to make the Milky Way look so bright---but maybe not.
Double click for the high resolution image.
Credit/Permission: Dan Durisco for US National Park Service, 2005 / Public domain.
Image linked to Wikipedia.
But in the 1930s, being in deniable was no longer tenable.
We have a classic can't see-the-forest-for-the-trees situation.
But interstellar dust is much more transparent in the radio spectrum.
We are still in the forest, but at least in parts of the radio spectrum, the trees are transparent.
Conveniently, radio astronomy also started in the 1930s (see Wikipedia: Radio astronomy: History).
Caption: The first parabolic antenna radio telescope used for radio astronomy built in 1937 radio astronomy pioneer Grote Reber (1911--2002) (see Wikipedia: Parabolic antenna: History).
The parabolic antenna is 9 meters in diameter---a record at the time.
This was the second radio telescope after the non-parabolic antenna radio telescope of Karl Jansky (1905--2002).
Grote Reber was an amateur astronomer initially.
He built his parabolic antenna in his backyard in his hometown of Wheaton, Illinois.
This is where obsession leads.
Credit/Permission: Grote Reber (1911--2002), 1937 (uploaded to by David Kennedy (AKA User:Gothic2), 2008) / Public domain.
Image linked to Wikipedia.
- One of the must useful
radio spectrum bands
for radio astronomy
is the narrow band of
the hydrogen 21-centimeter line.
Caption: An illustrative cartoon of hydrogen atom with the constituent electron (red) and proton (blue) in ground state energy level.
The cartoon suggests that the electron is orbiting the proton.
This is just NOT true: the electron is in a superposition of positions around the proton.
The probability amplitude (which describes the superposition of positions) is spherically symmetric, in fact.
The ground state energy level is actually two energy levels with a minute energy separation of 5.87433 μeV.
This is the hyperfine splitting due to the interaction of the proton magnetic with magnetic magnetic field of the electron.
Without going into details, the interaction depends on the spin (i.e., intrinsic and invariable angular momentum) of the particles.
The arrows indicate the direction of what in classical physics would be called the axes of rotation. of the electron and proton
-
If your right hand
curls around a rotating object with fingers in the direction of rotation,
your right thumb
points in the direction assigned to the
angular momentum vector
However, you must NOT think of electrons and protons as rotating on an axis in a classical physics sense.
They have angular momentum, but not rotation it seems.
If the electron proton spins are parallel/antiparallel (loosely speaking) the hydrogen atom is in the higher/lower energy hyperfine state.
In most environments, collisional atomic (electron) transitions cause the hydrogen atom to fluctuate between the two hyperfine states with a population distribution based on temperature.
But interstellar space where cold neutral atomic hydrogen gas (temperature usually of order 50--100 K and density of order 20--50 atoms/cm**3 of H and He with of order 2 % or less of metals: Wikipedia: Interstellar medium: interstellar matter) exists radiative atomic transitions from the higher to the lower hyperfine states can occur.
In a radiative atomic transitions photon is emitted.
-
The
wavelength
of the photon
is 21.10611405413 cm vacuum
which is the microwave band.
The emitted radiation is called the hydrogen 21-centimeter line (AKA 21-cm radiation),
The reason why such rarefied medium of neutral atomic hydrogen gas is needed is that the mean lifetime for spontaneous emission is about 1.1*10**7 years.
-
The 21-cm line is a strongly
forbidden transition.
"Forbidden" in this context does not mean it does not happen.
It means is "forbidden" to happen by the strongest mechanism for atomic transitions.
A forbidden transition must happen by a weaker mechanism.
This means the mean lifetimes for forbidden transitions can be very long.
We will probably never observe the 21-cm line in the laboratory---we just can't wait long enough to detect it.
But the vast quantities of cold neutral atomic hydrogen gas in interstellar space radiate 21-cm line copiously.
The 21-cm line was predicted theoretically in 1944 and discovered radio astronomy in 1951.
The 21-cm line is immensely useful in astronomy.
It is a very narrow atomic spectral line.
Doppler effect wavelength shifts of the 21-cm line can be used to measure radial velocity (i.e., line-of-sight velocites from Earth) very exactly.
The 21-cm line was used to map out the Galactic spiral arm structure and it continues to be a vital source of astronomical information.
Credit/Permission: © User:Ken_g6, 2011 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
Caption: An animation showing Doppler effect for sound for a siren from a car at rest and in motion.
Note wavelength is decreased in the direction of motion and increased opposite the direction of motion. The waves are scrunched, the waves are stretched.
Medium frame observer (i.e., one at rest with respect to air and ground) has frequency is high before the car and low after it.
This the sound Doppler effect.
The electromagnetic radiation is similar, but the formulae are different in the two cases.
Credit/Permission: © User:Charly Whisky / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
For velocities much less than the vacuum light speed c (exactly 2.99792458*10**5 km/s ≅ 3*10**5 km/s), one usually only needs the 1st order Doppler effect formula:
v/c=Δλ/λ ,
where λ is the intrinsic
wavelength
(21.1061 ... cm for the 21-cm line),
Δλ is the observed shift in
wavelength
from the instrinsic wavelength,
and
v is the
radial velocity.
The 1st order Doppler effect formula
is valid for v/c<<1.
The 1st order Doppler effect formula is all that is needed to measure Galactic radial velocities which are less than or of order 200 km/s, and so have v/c less than or of order (2/3)*10**(-3).
This allowed much Galactic structure to be mapped out.
There are still gaps in our knowledge.
A major gap is the structure of the Milky Way on the opposite side of the Galactic center from the Solar System.
There matter (including H I regions) is moving largely perpendicular to the line-of-sight direction from Earth, and so exhibits only tiny Doppler effect shifts which do not give insight into the matter's main motions.
- small: U > 200 pc cm**(-2).
- medium: 200 > U > 110 pc cm**(-2).
- large: 110 > U > 70 pc cm**(-2).
The Milky Way as seen from far north in the Galactic coordinate system (which is in constellation Coma Berenices (Berenice's Hair)).
The radial lines start from the Sun (a yellow dot). The radial lines are marked by the constellation they pass through---remember almost all the stars in the constellation are actually very near to us on the scale of the Milky Way. The constellation names are abbreviated: Cas for Cassiopeia, etc.
The Galactic spiral arms are colored differently. The H II regions (which are partly ionized atomic hydrogen gas and are NOT the same as H I region) are marked by dots of the same color as the spiral arm they are in. The size of the dot indicates size of the excitation parameter U:
Credit/Permission: © User:Rursus, 2007 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
Credit/Permission: © User:Rursus, 2007 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
Caption: "Artist's conception of the spiral arm structure of the Milky Way with the two Galactic spiral arms and the Galactic central bar of stars.
Using infrared images from NASA's Spitzer Space Telescope, scientists have discovered that the elegant Galactic spiral arm structure is dominated by just two arms wrapping off the ends of a the Galactic central bar. Previously, the Milky Way was thought to possess four major spiral arms." (Moderately edited.)
The image also shows the Galactic coordinate system which has the Sun (and therefore the Earth) at the origin.
The distances from the which has the Sun are marked in light-years (ly).
Recall that kiloparsec equals approximately 3000 ly.
Credit/Permission: NASA/Jet Propulsion Laboratory (JPL)-Caltech/Robert L. Hurt, 2008 (uploaded to Wikipedia by User:Flame99, 2008) / Public domain.
Image linked to Wikipedia.
Caption: A map of our neighborhood in the Milky Way. The Solar System is the Orion Arm which is a minor Galactic spiral arms---but it's mjor to us.
The image is about 12000 ly by 9000 ly (i.e. 4 kpc by 3 kpc).
Galactic disk has a diameter of about 30 kpc. So in terms of the Milky Way as a whole, the map shows a relatively small region.
Almost all the well known naked eye stars and constellation stars are really close to us in a Galactic sense.
The map shows a small sample of the naked eye stars including Betelgeuse, Polaris, Rigel, and Deneb.
The Perseus Arm is one of the two major Galactic spiral arms. The other is the Scutum.Centaurus Arm.
Carina-Sagittarius Arm is another minor Galactic spiral arm.
Credit/Permission: © Richard Powell, 2006 (uploaded to Wikipedia by User:AndrewRT, 2006) / Creative Commons CC BY-SA 2.5.
Image linked to Wikipedia.
Now I give the brief run through.
The trick to rescale the Excel spreadsheet plot circle without changing its shape is to press control when stretching it with the little bitty circles at the top/bottom/sides.
To make a measurement, the protractor center of rays is centered on the vertex of the angle you wish to measure.
Then the angle you wish to measure is defined by two rays that radiate from the vertex.
The two rays go through whatever two points you are trying to measure the angle between from the vertex you have chosen.
Credit/Permission: © User:Scientif38, 2011 / Creative Commons CC BY-SA 1.0.
Image linked to Wikipedia.