Transcript Activity : Milky Way

Swinburne Online Education Exploring Galaxies and the Cosmos
The Milky Way - Detailed Structure
Count the arms in this
superb image of
NGC1232 by ANTU - the
new 8.2 metre telescope
of ESO’s VLT in Chile.
Loading image . . .
Activity:
Galactic Spiral Arms
© Swinburne University of Technology
Summary
This Activity aims to provide examples, theories and
understanding in the following areas:
• Spiral arms in the Milky
Way and other galaxies.
• Star forming regions
highlighting the spiral
features.
• Mechanisms for formation
and persistence of spiral
arms.
ANTU image showing how dark rifts
and bright clusters of stars delineate
what we see as ‘spiral arms’.
Spiral shapes in Nature
Regular shapes and patterns in nature invite a search
for a mechanism for their formation.
Your author for this section has tried to find Planet Earth
examples to introduce each Activity. (It will get harder as
we journey further out in the Universe!)
In this case, this ‘one-armed’
spiral shape, both beautiful and
mysterious, does not suggest
any ‘rotational motion’ to us. It
challenges life scientists to
understand its formation and its
change to . . .
Australian tree fern, backyardus caldwellii
Regularities provide clues to understanding
In science, organised structures such as occur in biology,
in atomic structure, and in the solar system, lead to
theories - enabling predictions.
Amongst the seeming chaos of
stars and dust, the spiral arms of
galaxies stand out, both in beauty
and in inviting explanation.
Ironically we probably know
more about the dynamics of
distant galaxies than we know
about the mystery of life, as in
this simple tree fern frond.
More familiar spirals
Rotating fireworks and rotating garden water sprinklers
also produce ‘spiral arm’ effects.
In these cases the ‘mechanism’ is the outward directed
jet effect of the gunpowder or water, respectively.
The jet effect certainly implies such arms trail the
rotation direction. Will we find galaxy arms trail or might
they at times lead the rotation direction of the galaxy?
We will be searching for a ‘mechanism’ for the generation
and sustaining of galaxy spiral arms.
Spirality and rotation
We will see that symmetric and spiral galaxy features do
not imply rotation in the way we might at first expect.
We have seen that, early in our own
century, and due to telescope limitations
of the day, we had no evidence that the
spiral nebulae were not part of the entire
Milky Way ‘universe’.
In fact, the spiral nebulae were suspected, by some, as
being embrionic solar systems according to the ‘nebula
hypothesis’ - and the suggestion of rotation of the spiral
shape must have added to the incorrect idea.
The Winding Problem
If the spiral arms rotated with the stars that comprise
them, how would their appearance change with time?
In the previous Activity we saw how the V-R velocity curve
was generally flat away from the central region. Hence the
period of a star orbit, P=2pR/V, increases with Radius.
Consider two orbits of the inner star of a sample of four:
After 1 orbit
After 2 orbits
Within a few rotations, the arms
would wind up tightly - and could
not persist as observed.
Radii and periods are in the ratios 1:2:3:4
Do spiral arms (appear to) trail
or lead
?
Establishing the rotation direction of
a galaxy depends on knowing the orientation of the plane
of the galaxy.
The previous Activity showed that the right side of NGC253
is receding from us (faster than the centre of the galaxy).
But . . .
This tilt would mean the galaxy is
rotating anti clockwise.
The dark rifts on the near side of the
galaxy establishes that clockwise is
the correct interpretation.
In most cases, where the tilt of the
plane can be determined, arms
AAT 023 appear to trail.
Leading or trailing spiral arms?
There may be some exceptions to
the rule.
For example, where, in the one
image (eg NGC4622 at right), we
appear to have arms in both
directions, leading arms must, at
least, be possible.
NOAO
Really, a lot of astronomy involves applying logic and
puzzle solving to the problem at hand and can be great
fun. Certainly a little physics and maths helps (and we
can still get it wrong!) . . .
m
The winding problem demonstrates that the spiral
arms do not rotate with the stars comprising the
arms. (Indeed, do the arms rotate at all! What
observations could we make to decide if they do?)
As we now proceed to review the actual nature of
spiral arms and the phenomena associated with
them, bear the winding problem in mind to
anticipate a solution to it.
What do we see as ‘arms’?
To investigate spiral arms in our own Galaxy, what do
we search for and what difficulties are involved?
Taking our clues from other
galaxies, what objects (in
visible light first) seem to
indicate the positions of spiral
arms?
• Dark dust rift lanes
• Bright, clustering, stars
• Pink (hydrogen) nebulae
NGC5238
M83
AAT 008
We can detect such markers in the Milky Way (with one
proviso - what is it?). So far, so good . . .
Not so easy
Not all galaxies show prominent spiral arms.
This image shows the same
star,
dust
and gas
spiral arm markers as in the
NGC5238 image in the
previous frame, but the arms
are not as well delineated.
NGC300
AAT 057
The task of identifying arms
in our own Galaxy will be difficult if it is a galaxy similar
to NGC300.
And there’s one further problem . . .
We have to try to identify spiral
arm markers from our viewpoint
within the Galactic disk itself!
This image of an edge-on spiral
galaxy shows the extent of
absorbing dust and gas that may
also hamper our observations
within our own Galaxy.
It also highlights the fact that
spiral arms are not evident when
‘viewed from the side’ - the
problem will be same as we try
to find arms in our own Galaxy.
AAT 101
The distance problem
Locating the spiral arm markers from within the plane of
our own Galaxy:
Though we can identify the star, gas and dust spiral
arm markers, when they are all superimposed in our
side-on view, how do we:
a) identify each marker
along with its distance,
b) view through to arm
markers at large distances.
This image toward the centre of our
Galaxy shows stars, hydrogen nebulae
and dust lanes. But how can we
identify spiral arms and view through
the confusion to large distances?
NGC Milky Way!
AAT 028
Markers to spiral arms: i) dust
The following is to remind us of the value and difficulty
in the various markers to spiral arms:
This image of the Orion Nebula
clearly indicates the dense
clouds of dust on this side and
the far side of the ‘trapezium’
cluster of young stars.
And recall that, even away from
such clouds, the average
interstellar extinction is about
one magnitude per kiloparsec
toward the centre of the Galaxy.
For our Galaxy therefore, dust serves only to hamper
visual observations to greater distances.
HST
Markers to spiral arms: ii) stars
NGC1232
ESO 8.2m ANTU
Young open clusters of stars, including very luminous O
and B main-sequence stars, are the main visible features
that dominate spiral arms.
Since massive OB stars are short lived objects, spiral
arms must correspond to regions of active star formation.
This also suggests that even the spiral arm may be a
transient phenomena as far as its location is concerned.
Markers to spiral arms: iii) hydrogen gas
Emission nebulae result from excitation by intense UV
radiation from hot young O and B stars.
Amongst many radiation
mechanisms, the pink colour of
the Hydrogen alpha emission is
usually predominant (HII regions).
Though visible in nearby galaxies,
the difficulty of observation to
large distances in our Galactic
plane limits their usefulness as
spiral markers in our own Galaxy.
AAT 012
M20
Trifid Nebula - distance about 2Kpc
21cm emission from HI clouds
intensity
As described in the last Activity, 21cm emissions can be
received from large distances - from throughout our Galaxy.
Galactic
centre
C
C
-100 -50
0
50 100
radial velocity km/sec
21 cm observations are made in a given direction.
Sun
A radial velocity plot is made from Doppler shifted signals. Positive
(recession) velocities should come from regions interior to the Sun’s
orbit. The highest velocity peak identifies cloud C at Rmin.
21cm spiral arm tracing
intensity
Identifying gas clouds from 21cm radiation enables
spiral arms to be ‘traced’.
A
B
D
C
A
Galactic
centre
B
C
-100 -50
0
50 100
radial velocity km/sec
D
Sun
The negative (approach) velocity at point A identifies a cloud exterior
to the Sun’s orbit.
Cloud B shares the Sun’s velocity.
Cloud at D orbits between the Sun and the cloud at C.
Giant Molecular Clouds
The best current spiral tracers.
Giant molecular clouds (GMC’s), from which massive
stars are born, can be located in a way similar to that of
HI regions.
Different wavelengths and molecules (such as CO) are
used to produce the Doppler shifted radial velocity
diagrams.
Survey’s of GMC’s strongly delineate some of the
Galaxy’s spiral arms shown in the next frame . . .
The spiral features of our Galaxy
What is the outcome from the various forms of arm
tracing for our Galaxy?
180o
Sun
90o
Galactic
Centre
Longitude
- based on various
types of tracers, at
points (not shown)
along the arms.
Evidence is not
available for regions
inside the sector
masked by the Galactic
centre.
270o
0o
Galactic
What causes spiral arms?
What do regions of gas and
dust and young stars imply?
They are regions where young
stars are forming from the gas
and dust.
The collapse of gas and dust to
form stars requires a trigger either slow contraction under
gravity or compression by factors
such as supernovae* explosions.
But if it is compression, why does
it seem to trace out spiral arms?
M16
Eagle Nebula
AAT 047
*Click here to be reminded about supernovae
Density Waves
Recall that spiral arms do not share the rotation rate of
the stars comprising them. (The winding problem).
Link the factors a) that star forming regions require a
cause for compression to increased densities and b) that
the arms do not share the stellar rotation about the
galactic centre.
Could the arms themselves be providing
the increased density? Are they ‘density
waves’ moving through a uniform gas,
dust and star field, producing the starforming regions which in turn delineate
the arms?
Arm velocity
Star velocity
Various Models
A number of density wave models lead to the generation
of spiral patterns.
In the mid-1960’s the Lin and Shu model proposed that in
the inner disk the stars overtake the spiral arm; in the
outer disk the spiral pattern moves faster than the stars.
Stars in the central region also drift closer to the centre,
pulled backward by the stars further out in the spiral - if,
as is generally observed, the spiral trails the galactic
rotation.
Two questions arise - and remain under investigation:
1. Will the spiral pattern eventually die out?
2. What causes the density wave initially?
Computer Models
The advances in computer speed has enabled galaxies
of tens of thousands of stars to be simulated.
Resulting galaxies bear remarkable similarity to real
galaxies - with arms of varying looseness and even
including barred spirals that are included in Hubble’s
tuning fork diagram classification of galaxies.
Elliptical orbits
Material orbiting a galaxy in slowly precessing elliptical
orbits also produces regions of increased density.
The mathematics of the model cannot be included here
but a visual simulation can be achieved as follows:
1. Take an ellipse.
2. Enclose an ellipse reduced
(here 90%) and rotated (here 10o)
from the previous ellipse.
3. Repeat.
The spiral effect beginning to appear would be an area of
increased density if the ellipses represented the orbits of
stars, gas and dust. This is shown more completely in the
next frame.
Here we show 46 orbits (all with same eccentricity) nested so
each is 95% of the size of, and rotated by 10o from, the next
larger orbit.
The two dark spirals
have not been added they are an effect of line
closeness (density).
NGC2997
AAT 017
NGC2997 is superimposed, showing remarkable
coincidence, especially along the inner arms, and inviting a
reason for the ‘extra arms’ (dotted).
.
Inclination of galaxies
The match in the previous frame also involved the
deliberate tilting of the resulting nested ellipses (shrinking
the vertical size) to match NGC2997’s spiral arms.
This suggests we should be very careful when assessing
the true inclination of galaxies to the ‘plane of the sky’.
For example, the ‘faceon’ spiral galaxy shown
at left is a deliberately
distorted image of . . .
the real M81 galaxy at
right!
NOAO
Summary
• Spiral tracers that are evident in external galaxies can be
used within our own Galaxy (apart from the sector masked by
the Galactic Center) to trace spiral arms.
These include:
Young galactic clusters Regions of absorbing gas & dust
HII regions
HI regions
Giant Molecular Clouds
• Spiral arms do not rotate at the same rate as the stars that
comprise them, but can be explained as density waves moving
relative to the background material.
• Our Galaxy appears to be a two (plus two) armed spiral of
intermediate pitch angle or winding looseness (type Sb) perhaps similar to NGC 2997 in the earlier frame. The brightest
part of the Milky Way is our view of the Sagittarius arm
between the Sun and the Galactic centre.
The Galactic Centre.
In any spiral galaxy, two features stand out:
• the spiral arms, and
• the brighter, bulging centre.
What lies in the innermost region is obscured by gas, dust
and the increased star density - and this is especially so
for our own Galaxy from our viewpoint out in the disk.
The observing tools for overcoming
these difficulties, and what has been
found to lie in the inner regions, are
the subjects of the next Activity.
Portion of NRAO VLA Radio Image of the Galactic Centre
Image Credits
AAT/IAC/RGO/UKS images © David Malin (used with permission):
http://www.aao.gov.au/local/www/dfm
Individual Malin images (© David Malin (used with permission)), shown with
a 6 character code - such as AAT028, - are found at the website ending with
that code; eg:
http://www.aao.gov.au/local/www/dfm/aat028.html
NOAO (National Optical Astronomy Observatories) image © Association of
Universities for Research in Astronomy Inc. (AURA), all rights reserved.
Credit AURA/NOAO/National Science Foundation.
http://www.noao.edu/image_gallery/galaxies.html
Hubble Space Telescope images indexed by subject:
http://oposite.stsci.edu/pubinfo/subject.html
ESO (European Southern Observatory) VLT images:
http://www.eso.org/outreach/info-events/ut1fl/astroimages.html
Hit the Esc key (escape)
to return to the Index Page
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Introducing supernovae
A supernova is the catastrophic collapse of a star to form
a black hole or a neutron star.
Left: The Crab
Nebula is the
remains of a
supernova
observed by
astronomers in
1054AD.
We learnt that
supernovae occur when
massive stars reach the
end of their life. Once a
massive star has
exhausted its nuclear
fuel, it collapses in a
brilliant explosion,
releasing vast amounts
of energy and violently
ejecting its outer layers.
background
More amazing facts
If a supernova occurred within
50 light years of Earth, the
gamma rays and high energy
particles emitted during the
explosion would kill most lifeforms and cause severe
genetic damage in others. A
local supernova explosion is a
possible explanation for the
extinction of the dinosaurs.
The supermassive star Eta Carinae. 150 years ago, Eta Carinae suffered a violent
outburst which gave the star its present strange appearance. Miraculously, the star
survived the outburst - for now. Astronomers believe that Eta Carinae may soon explode
in a supernova.
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