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“What’s Up”
“Springtime: Galaxies!”
April 24 - Mid May
actually a bit bigger than our Milky Way, and is moving towards us; in three
billion years, the two will collide! Many of the familiar galaxies in Messier’s
list are contained in vast clusters lying in the direction of the constellations
Ursa Major, Leo, Virgo, and others such as Coma and Hercules. Let’s have a
look.
Some History: Ancient peoples never
had to contend with artificial light pollution,
and on moonless nights a somewhat mottled
band of ghostly light arcing across the sky was
always prominent. To the unaided eye this
appeared quite distinct from the brilliant stars. The
Kung Bushmen of the Kalahari Desert in Africa thought
it looked like a spinal column and called it the “Backbone of Night”; the
ancient Chinese called it “The Silver River”. We refer to it as “The Milky
Way”, from the Greek legend whereby the goddess Hera squirted breast milk
across the Heavens. “Gala” (
) is in fact Greek for “milk”, and is the root
of the word for “galaxy”.
Observing Galaxies: For observing other than M31, you’ll need at least
10x50 binoculars or a small telescope. A good place to start spring time galaxy
hunting is the familiar Big Dipper (part of the constellation Ursa Major),
prominent in the northern sky year-‘round but poised almost overhead at this
time of year after dark. A bright pair of galaxies, M81 and M82, can be found
by drawing a diagonal line across the “bowl” of the Dipper, from the star
Phecda to Dubhe (see chart), and extending this line to the same distance
beyond the bowl. M81 is another spiral galaxy, the largest of its own small
group, and is nicknamed Bode’s Galaxy after its discoverer, Johann Elert Bode
(1747-1826). M82, an irregular galaxy, is undergoing a tremendous burst of
star formation and is also called a starburst galaxy. It’s also sometimes called
The Cigar Galaxy because it’s seen almost edge-on. They’re both about 12
million ly away, and easily glimpsed together in 10x50 binoculars. M51 lies
under the end of the “handle” of the Big Dipper. To find it, imagine a 30-60-90
triangle, placed upside down between the handle’s end star (Alkaid) and
second-last star (the double star Mizar and Alcor), with the short side extending
down from Alkaid; M51 will be near the tip. Visualizing an equilateral triangle
between these two handle stars, pointing upwards instead, brings us to M101 (at
the triangle’s apex), a large and faint spiral galaxy. With a small telescope you
can find fainter M108 and M109 near the bowl stars Merak and Phecda,
respectively.
Classifying Galaxies: Galaxies come in many shapes and sizes, but can be
arranged into three large groups. The smallest are scruffy little dwarf galaxies
comprising “only” millions of stars in a rough blob. Dwarf galaxies are often
satellites of larger galaxies, the way moons orbit planets. The rest are broadly
divided into elliptical and spiral galaxies. Our own Milky Way is a pretty
typical spiral containing ~200 billion stars in spiral arms making up a flat disk
100,000 light years (ly) across and 2500 ly thick, and a central bulge of older
stars perhaps 16,000 ly thick. The Sun is 28,000 ly from the centre, on the
inside edge of a spiral arm. (A “light year” is the distance light travels in a year,
or about 9500 billion km). Many spirals have bars protruding from the bulge at
the ends of which spiral arms begin; we now know the Milky Way to also be a
barred spiral galaxy. It has a family of about a dozen dwarfs (most of which
are difficult to see clearly). Unlike spirals, elliptical galaxies are relatively
featureless balls of stars flattened to various degrees, but they cover an
enormous range of sizes, from dwarfs to titanic giants formed from galaxy
mergers with over a trillion stars!
Galaxies are typically spread throughout the universe in groups and clusters.
Our own neck of the woods in the cosmos is called the Local Group, and in it
our Milky Way is joined by two other spiral galaxies that are visible even with
the unaided eye in a rural, moonless sky (making them the largest objects you
can see without aid!). They’re both in the same part of the sky, in or near the
constellation Andromeda; the great Andromeda Galaxy (M31) and
the Triangulum Galaxy (M33). A pair of binoculars will easily reveal
M31 to be a smooth, flat oval of light that appears to be eight times
wider than the Full Moon (which appears to be ? degree wide)! In fact,
the Andromeda Galaxy was first noted by the ancient Persian
astronomer Al Sufi in 905 A.D., well before the telescope was
invented. M31 is the nearest large galaxy, 2.3 million ly away, is
W
E
M81
The constellation Leo, rising in the east after dark in the Spring, is another
rich hunting ground for galaxies; the Leo Group resides in space behind it,
about 40 million ly distant. Two groups of galaxies are visible under the belly
and haunches of the Lion; a star chart will help you. M65 and M66 lie together
under the haunches (“below” the star theta-Leo, also called Chertan), and M95
(a picturesque barred spiral), M96 and M105 (ellipticals) lie below the belly,
about a third of the way towards Leo’s brightest star, Regulus. M105 is
notable. Many galaxies, both spiral and elliptical, appear to have giant black
holes at their centres; M105 has one with a mass of about 50 million solar
masses! (The Milky Way is thought to contain a 3 million solar mass black
hole).
The Virgo Cluster, however, has to be the richest and best-known galaxy
cluster. It’s further than the Leo Group (60 million ly) so its members are
fainter and you’ll need a telescope. Amongst the cluster’s 2000 members are
over a dozen Messier galaxies, located about 10 degrees (about the width of
your outstretched fist) east of the “hind quarters” star in Leo, bright Denebola.
To explore this cluster a star chart in magazines like “SkyNews”, “Astronomy”
or “Sky and Telescope” will get you started, and a book like Terry Dickinson’s
recommended “Nightwatch” will dig a little deeper; the region is so rich that
dedicated star atlases usually include charts of just the Virgo Cluster. It is so
big that its gravity is pulling our Local Group towards it! At the heart of this
rich cluster is the giant elliptical galaxy M87, with ten times the stars of our
home galaxy! Rich galaxy clusters generally have giant ellipticals at their
centres, probably the result of many mergers of normal galaxies throughout the
cluster’s history. Their cannibalized blackholes also merge; the central black
hole in M87 is a 3 billion solar mass monster!
Chris Stevenson RASC, St. John’s Centre
ACTIVITIES
1. Galaxies are described in this article as large grouping of stars,
not unlike a community. Can you find other examples of groups
/communities within The Telegram?
2. In Astronomy visual descriptions are used to describe what an
object looks like; the above article contains many examples of this.
Can you find more of these visual descriptions throughout The
Telegram?
For more activities go to
www.thetelegram.com and click on
Brought to you by
Point this
astronomical chart
toward North and
match the stars with
those in the real sky.
S
Planets
Viewable in a pair of Binoculars or small telescope
Mercury - (magnitude –1.5) is emerging from the glow of sunset.
Venus - (magnitude –4.2) is low in the dawn. Look for it above the eastern
horizon about 20 or 30 minutes before sunrise.
Mars - (magnitude +1.2) is both dim and very low in the sunrise glow.
Jupiter - (magnitude –2.1, in Capricornus) shines low in the southeast during
early dawn.
Saturn - (magnitude +0.6, near the hind foot of Leo) shines in the southeast at
dusk. Saturn's rings are 3 1/2 ° from edge on and will open to a maximum of 4°
in May.
Uranus - (6th magnitude) is hidden low in the sunrise glow.
Neptune - (8th magnitude) is also hidden in the glow of dawn.
Pluto (dwarf planet) - (14th magnitude, in northwestern Sagittarius) is located
in the southeast before the first light of dawn.
Magnitude: A number denoting the brightness of a star or other celestial object. The
higher the magnitude, the fainter the object. For example, a 1st-magnitude star is
100 times brighter than a 6th-magnitude star.
Moon
Waxing
First Quarter
Full
Last Quarter
Waning
New
April 30
May 1
May 9
May 17
April 19
April 25
New Moon - If the Moon is on the same side of the Earth as the Sun, then the
face of the Moon that would ordinarily be seen from the Earth is no longer
illuminated by the Sun’s rays and therefore cannot be seen.
Comets
Viewable in a pair of Binoculars or small telescope
C/2008 T2 (Cardinal)
(Magnitude 9.2) is moving through the constellation Auriga, and can be
glimpsed with a small telescope or binoculars. It is expected to reach a visual
magnitude of around 8 in mid June.
Comet C/2009 F6 (Yi-SWAN)
A small 8th magnitude comet is now making its way slowly across
Cassiopeia toward Perseus and should be within reach of small telescopes for
most of April and May 2009.
For more detailed information on finding these objects log onto www.rasc.ca/stjohns/
under Observing on the lefthand side of the web page click bright comets.
WARNING!“ When using a telescope or binoculars, always be sure NEVER TO LOOK
AT THE SUN! This can cause serious and permanent eye damage. To be safe, always make
sure the Sun is fully set below the horizon before going outside with your telescope or
binoculars.”
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news
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Newspaper In Education
THE TELEGRAM
0-3636427
Lord Rosse’s drawing of the Whirlpool Galaxy, M51, next
to a contemporary photograph.
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In the mid-19th century the French comet-hunter Charles Messier (17301817) compiled his famous list of 110 “faint fuzzies”, non-comet “nuisances”
for him but popular targets for astronomers ever since. Sir William Herschel
(1738-1822) and other astronomers would turn larger and larger telescopes
towards these catalogued “nebulosities” and discover many to have a generally
circular, and often spiral-shaped, structure; they called these spiral nebulae,
and presumed them objects within our own “island universe”. A famous
example would be M51, nicknamed The Whirlpool Galaxy today, which was
studied in 1845 by William
Parsons, the 3rd Earl or
Rosse, with the giant
telescope he had just built.
His careful painting clearly
shows M51’s spiral nature. It
wasn’t until 1925 when Sir
Edwin Hubble (after whom
the space telescope is named)
that it was realized these
“spiral nebulae” were a
separate “island universes”
like our own – suddenly the
cosmos got a whole lot larger!
g
rin
C
tar
M82
Illustration by: Shawn Martin
After the astronomical telescope was refined and put to use first by Galileo
in 1609, and by many others in the decades and centuries that followed, it was
seen that, under some magnification, much of the visible Milky Way resolves
into thousands and thousands of stars; not milk at all! You can demonstrate this
to yourself with a simple pair of binoculars under a dark sky. Towards the
constellation Sagittarius (relatively low in the south during summer from
Canada) the Milky Way is brightest and most complex; in the opposite
direction, the winter Milky Way is relatively diffuse. This hinted to the German
philosopher Immanuel Kant (1724-1804), that maybe our Sun is just one star
amongst many, together forming a large disk or island universe, and that we
lived within it towards its edge. This idea turned out to be essentially correct.
Shawn Martin Observing Director RASC, St. John’s Centre
Sp
Spring in the Northern Hemisphere is a
great time to get out and observe galaxies.
Let’s learn a bit about what galaxies
actually are, how they’re classified, and
where to find them in the Springtime Sky.