title>Spiral Nature of the Milky Way
Milky Way Galaxy: A Spiral Galaxy
It is difficult to deduce the appearance of the Milky Way because we
live inside of it. We do, however, believe that
the Milky Way is a spiral galaxy similar in form to, e.g., M51,
despite the fact that, to us, the Milky Way appears to us like
a band of light across the sky in visible light.
Our inside-out and poor view of the Milky Way galaxy in visible light
is due to the obscuration caused by the dust contained in the disk of our
Galaxy. In light of this, how do we go about studying
the structure of the Milky Way galaxy?
- Firstly, how do we see to the ends
of our Galaxy? That is, how do we
penetrate the veil of obscuring
dust in our Galaxy?
Answer: In general, objects emit
electromagnetic radiation (light)
over a wide range of wavelengths (colors)
and materials do not block
electromagnetic radiation of different
wavelengths equally (e.g., why do garage door openers work, why
you can pick up radio signals indoors, why x-rays reveal skeletons inside
of people and other animals).
Electromagnetic radiation with long wavelengths
such as infrared, micro-waves
and radio emission, and electromagnetic radiation
with short wavelengths such as
x-rays and gamma rays, are able to penetrate
the veil of dust in our Galaxy and allow us to see to the ends of
the Galaxy.
Comment--Visible light has wavelengths of roughly 3x10-5
cm to 10-4 cm in length. This is tiny. Radio emission has much
longer wavelengths, on the order of a cm (or so) and larger.
- Since we know how to see through the dust (we oberve at short or
long wavelengths), we
ask, What kinds of objects in the Galaxy produce
large amounts of the appropriate radiation?
This is important because even if
our Galaxy is transparent to gamma rays, this is of no use unlesss the
objects in our Galaxy produce gamma rays.
If they do not, then the fact that the disk is transparent
to gamma rays is not relevant. Additionally, we must also ask
the following. Are the objects which produce the appropriate radiation
also the kind of object which we should study in
order to enable ourselves to map the structure of the spiral arms?
Spiral Structure
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SPIRAL DENSITY WAVE THEORY
(spiral arms ---> star formation)
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SELF-PROPAGATING STAR FORMATION
(star formation ---> spiral arms)
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In Grand-design spirals, the spiral arms are not likely to
be material arms. Because the disks rotate differentially, any material
arm would wind-up. Spiral arms are compression waves that move through
the gas,
The compression wave induces star formation ===> concentration of OB stars,
HII regions, Giant Molecular clouds near the arms. The waves may be induced
by companion galaxies, e.g.,
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In flocculent spirals, the spiral arms may be material
spiral arms. In
SPSF, one imagines that some perturbation leads to star formation in
localized sites. These sites of star formation then excite star formation in
neighboring sites, which excite star formation in neighboring sites, and so on.
This leads to chains of star formation. The linear chains are wound-up by
differential rotation creating arcs of spiral arms. |
So, for both scenarios, Young stars and star forming
regions are prime candidates as spiral arm tracers
Structure Tracers
- Massive, bright stars
(O & B stars) are hot with very short lifetimes for a star
(only millions of years
as compared to the Sun's lifetime of 10 billion years).
Because O & B stars are hot (temperatures greater than 10-20 thousand Kelvin),
they produce large amounts of high energy EM radiation (they are strong
sources of ultraviolet radiation) and so are capable of ionizing the hydrogen
gas in their vicinity thus forming
HII (ionized hydrogen) regions.
Ionized hydrogen atoms
(HII) produce strong visible radiation (giving them their
reddish color) but they also produce strong radio emission.
This is important because it allows us to see the HII regions
throughout the disk of the Milky Way Galaxy.
Because HII regions are produced by
O & B stars with their short lifetimes, HII regions concentrate to
their birth sites near the spiral arms
of our Galaxy
(e.g., M51 and its HII regions). HII
regions and O & B stars are nice
ways to trace the spiral arms of our Galaxy.
- HI (neutral hydrogen) regions.
An important fact is that neutral hydrogen can produce
radio emission. In particular, HI atoms produce radio emission with
a wavelength of 21 cm. This is good because hydrogen is the most
abundant element in the Universe.
Although HI is pervasive in the disk of our Galaxy, it tends to clump
in clouds which concentrate toward the spiral arms of our Galaxy. The HI
clouds are thus useful ways to map out the structure of our Galaxy.
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Giant Molecular Clouds (star formation sites) generate micro-waves
and infrared emission. Shorter in wavelength (when compared to
radio emission) but longer in wavelength than visible light, micro-waves and
infrared emission are useful probes of the structure of our Galaxy.
It is found that Giant Molecular Clouds contain
carbon monoxide (CO) molecules which emit radiation with a wavelength of
2.6 mm.
Rotation Curve of the Milky Way Galaxy
Beyond showing us what the
Milky Way Galaxy looks like, we can also infer another interesting
feature of the disk of our Galaxy from the above structure tracers;
we can map out how material moves in the disk of our Galaxy. Using the
Doppler shift, we are able to determine how fast objects move around
the disk of our Galaxy. We find the following rotation curve for the
Milky Way Galaxy:
The point where
the visible disk of the Galaxy ends (the disk as
defined by the stars) is around 40,000-50,000 light years
from the center of the galaxy. Interestingly,
we see HI gas beyond the end of the star-disk of our Galaxy.
We use this information
in the next section to infer an interesting fact about the mass and
make-up of the
Milky Way galaxy.
