ASTRONOMY

LESSON 4
Panoramic view of the night sky over Death Valley.
Mauna Kea Milky Way Panorama
The Milky Way Over Mauna Kea
Moonlight, Mars, and Milky Way
 Explanation: This panorama view of the sky is really a drawing. It was made in the 1950s under the
s u p e r v i s i o n o f a s t r o n o m e r K n u t L u n d m a r k a t t h e L u n d O b s e r v a t o ry i n S w e d e n . T o c r e a t e t h e p i c t u r e ,
draftsmen used a mathematical distortion to map the entire sky onto an oval shaped image with the plane
of our Milky Way Galaxy along the center and the north galactic pole at the top. 7,000 individual stars
are shown as white dots, size indicating brightness. The "Milky Way" clouds, actually the combined light
o f d i m , u n r e s o l v e d s t a r s i n t h e d e n s e l y p o p u l a t e d g a l a c t i c p l a n e , a r e a c c u r a t e l y p a i n t e d o n , i n t e r r u p t ed b y
d r a m a t i c d a r k d u s t l a n e s . T h e o v e r a l l e f f e c t i s p h o t o g r a p hi c i n q u a l i t y a n d r e p r e se nt s t h e v i s i b l e s k y .
Explanation: Here lie 526,230,881 of the brightest
stars known. The US Naval Observatory has
deployed their monster Precision Measuring
Machine to digitize photographic plates covering
the whole sky and creating the above map.

Yellow corresponds to 150,000 stars per square degree, while

dark blue corresponds to only 500 stars per square degree. (For
comparison, the Full Moon takes up about 1/4 of a square degree.)
The most striking feature on this whole sky projection is the
central disk of our Milky Way Galaxy, which stretches across the
middle. Dark dust lanes are evident there by the great number of
USNO-A2.0 Catalog: stars they obscure. The two bright spots seen south of the Milky
A Digital Sky Way's disk are the neighboring Magellanic Cloud galaxies.
 Explanation: This quite stunning panorama of the entire sky is a
mosaic of 51 wide-angle photographs. Made over a three year
period from locations in California (USA), South Africa, and
Germany, the individual pictures were digitized and stitched
together to create an apparently seamless 360 by 180 degree
view. Using a mathematical prescription like one often used to
map the whole Earth's surface onto a single flat image, the
complete digital mosaic was distorted and projected onto an
oval shape. The image is oriented so the plane of our Milky Way
Galaxy runs horizontally through the middle with the Galactic
center at image center and Galactic north at the top.

Most striking are the "milky" bands of starlight from the multitude
of stars in the galactic plane cut by the dark, obscuring dust clouds
strewn through the local spiral arms. In fact, almost everything
visible here is within our own Milky Way Galaxy. Two fuzzy
patches in the lower right quadrant of the mosaic do correspond to
external galaxies, though. Known as the Magellanic Clouds, these
are small, nearby satellite galaxies of the magnificent Milky Way.
 Explanation: Radio telescope tuned to
408MHz (408 million cycles per second)
Near this frequency, cosmic radio waves
a r e g e n e r at e d b y h i g h e n e r g y e l e c t r o n s
s p i r a l i n g a l o n g m a g n e t ic f i e l d s .

In the resulting false color image, the galactic plane

runs horizontally through the center, but no stars are
visible. Instead, many of the bright sources near the
plane are distant pulsars, star forming regions, and
supernova remnants, while the grand looping structures
The Radio Sky: a r e p i e c e s o f b u b b l e s b l o w n b y l o c a l s t e l l ar a c t i v i t y .
Tuned to 408MHz External galaxies like Centaurus A, located above the
plane to the right of center, and the LMC (below and
right) also shine in the Radio Sky.
 The Infrared Sky
This is a panoramic view of the enire
sky (rendered in an Aitoff projection).
Not produced directly from the 2MASS
sky images, this view has been
compiled from star counts in the
2MASS point source database. Each
color represents the local density of
stars seen in each of the 3 infrared
bands in the survey. Almost 100 million
stars appear here, going down to Ks
magnitudes as faint as 13.5.
 H e r s c h e l ' s m a p o f t h e M i l k y W a y f r o m H e r s c he l ' s 1 7 8 5
p u b l i c a t i o n s h ow i n g h i s m a p o f t h e M i l k y W a y a s d e r i v e d
from his counting of stars in various parts of the sky.
 K a p t ey n ' s U n i v e r s e f r o m h i s
1922 publication of his
p h o t o g r a ph i c s t a r c o u n t s u r v e y .
 Hubble image of Bok
Globules in IC 2944,
which show how these
The combined effects of dark nebulae block the
interstellar extinction and light of background
reddening by dust makes it objects from reaching
Earth.
impossible for us to observe stars
that are located behind too much
dust. Because of this, our view of
the Milky Way is blocked in many
directions. Because of the dust in
our galaxy, in optical light we
can only see nearby stars well.
This means that we are not able
to see all the way to the edges of
the distribution of stars in space.
Because we can see roughly the
same distance in each direction
we look in the sky because of the
dust, this gives the illusion that
the Sun is in the center of the
distribution of stars.
 In 1917, Harlow Shapley used the
globular clusters in the Milky Way to
gain a better understanding of the
Milky Way Galaxy.
He measured their positions and
distances and plotted their locations
on a two-dimensional chart.

Plot of Globular Cluster positions on the

s ky i n a n X , Y , Z co o r d i n a t e s y s t e m
ce n t e r e d o n t h e G a l a ct i c C e n t e r . I t s h o w s
t h a t g l o b u l a r cl u s t e r s a r e s p h e r i ca l l y
d i s t r i b u t e d a r o u n d t h e G a l a x y ' s ce n t e r .
GALAXY
• refers to a collection of
gravitationally bound stars and
associated material that is above
some minimum size (to differentiate
galaxies from massive star clusters).
 The old stars in the bulge and the halo
are referred to as Population II stars.

The young stars in the disk

population are usually referred
t o a s P o p u l a ti o n I s t a r s .

Schematic of Milky Way with

stellar populations labeled,
s h o wi n g t h e t h i n d i s k , t h e m o r e
spherical central bulge, and the The stars in the bulge
globular clusters which are found are in general older than
the stars in the disk.
above and below the disk.
 Poster image of
the Milky Way in
various
wavelengths of
light including
four different
radio maps, three
different infrared
maps, optical, x-
ray, and gamma-
ray, which
illustrates how
different features
are more
prominent in
different flavors
of light.
 There are three observations that the NASA set of images
makes clear:

1. The disk of the Galaxy is visible most clearly in radio

waves (atomic and molecular hydrogen maps), infrared,
and gamma-rays.
2. The disk is obscured in both the optical and X -ray images.
3. There is a bright point source visible in the very core of
our galaxy in the radio continuum and gamma -ray image.
Radio image of the Galactic Center,
including Sagittarius A, the location
of the exact center of the Galaxy.
 Chandra Image of Sgr A, showing
how it can be resolved into finer
detail, including a small point source
called Sagittarius A*.
 NRAO VLA radio image of
Sgr A West, which shows
that the point source in the
Galactic Center is
surrounded by what appear
to be spiral arms of gas that
orbit this object.
 Diagram of Hubble's
Tuning Fork classification
scheme from Hubblesite.
This is a classical type of
image still used by
astronomers today to
show how galaxies are
classified.

On the left are four images of

elliptical galaxies proceeding from
nearly circular / spheroidal (E0
type) to very elongated elliptical /
ellipsoidal (E7). An image of an
S0 galaxy is next, which is
considered a transition object with
some properties similar to spiral
galaxies and others more like
elliptical galaxies. At this point in
the diagram, the "tuning fork"
shape forks -- with three images
of spiral galaxies on the top and
three images of barred spiral
galaxies on the bottom. As you
progress along the top from Sa to
Sb to Sc or along the bottom from
SBa to SBb to SBc, the arms open
wider and the central bulge
becomes less prominent.
TYPES OF GALAXIES
• Spirals
• Barred Spirals
• Ellipticals
• Irregulars
SPIRALS Classified as an S galaxy with subclassification
a, b, or c;

Hubble image of M104, an Sa

spiral galaxy seen edge-on
Ground-based image of
NGC 4565
BARRED SPIRALS
C l a ss i f i e d a s S B g a l a x i e s
with subclasses a, b, and c,
just like the normal spirals.

The spiral arms in barred

spirals appear to originate at
the ends of the bar, instead of
in the bulge, like they do in
normal spirals.

The properties of barred

spiral galaxies are very
similar to normal spirals.
ELLIPTICALS
Featureless galaxies that
appear to be just a ball of
stars. They do not have
obvious dust lanes and gas
clouds like we see in spiral
galaxies, either.

The roundest ellipticals are

referred to as E0 galaxies,
and the most elliptical-
shaped galaxies that we
have observed are labeled
E7 galaxies.
 Smooth, central brightness

LENTICULARS concentration (bulge similar to E)

surrounded by a large region of
less steeply declining brightness
(similar to a disk).
IRREGULARS
Anything that is so unusual that it can't
be fit into any of the normal categories
is called an irregular galaxy.
• No morphological symmetry
• Lots of young, blue stars and interstellar material
• Smaller than most spirals and elliptical galaxies

Many irregular galaxies are found to contain many young stars and to be
experiencing significant ongoing star formation.