How Far Is Far?
To get an idea of the vastness of space
consider
an excellent analogy by Robert Jastrow,
a well-known astro-physicist: If you imagine the Sun as an orange
placed in Central Park,
NY, then the Earth would be a grain of sand 30 feet away.
Jupiter,
our largest planet would
be a cherry pit one city block away. Pluto, our tiny outermost
planet, would be 10 city blocks away. Alpha Centauri, the star
nearest
to our Sun, would be yet another orange sitting on the Golden Gate
Bridge
in San Francisco. And, the editors of Astronomy magazine add that
"... at a typical shuttle speed of 17,600 mph it would take 168,000
years to get there."
It boggles the mind, doesn't it?
Are supernovae common in our galaxy?
Supernovae, cataclysmic stellar explosions,
are common in the universe but only four are positively known to have
occurred
in our own galaxy. They are:
SN 1006 in Lupus
A bright star, two to three times the size of Venus
and as bright as the quarter Moon, appeared in the area of Beta Lupi on
the evening of May 1, 1006. It was recorded in the logs of an
Islamic
astronomer in Cairo and a monk in Switzerland; as well as by the royal
astronomers of China and Japan. The star remained brightly
visible
for months although it is not explicit that it was visible during
daylight.
From their descriptions, there is no doubt that it was a supernova;
and,
that it was near enough to be in our own galaxy. Earlier
supernovae
(in 185 and 393 A.D.) were recorded by the Chinese but little is known
about these events. A faint, shell-type distorted nebula with no
core pulsar, PKS 1459-41, is generally accepted as the remnant of
this great supernova of 1006.
SN 1054 in Taurus
The nebulous remnants of the second supernova
ever recorded in our galaxy were discovered by an English amateur in
1731.
Twenty-seven years later, Charles Messier labeled it "M1" -- the first
entry in his famous catalog. But Chinese astronomers had seen the
appearance of this "guest star" on July 4, 1054. It was seen
during
daylight for 23 days; and, remained visible to the naked eye at night
for
over a year. Today, the famous Crab Nebula and its tiny, central
pulsar occupy the site. It has been, and will continue to be,
studied
to death by astrophysicists because of its truly astounding and
intriguing
nature.
SN 1572 in Cassiopeia
The third supernova ever recorded in our own
galaxy occurred on November 6, 1572, when a bright star appeared not
far
north of Chi Cassiopeiae. The royal Danish astronomer, Tycho Brahe,
immediately
recognized its unusual nature and studied it intensely. The star
was so bright that it could be seen in broad daylight for two
weeks.
At night, it was visible to the naked eye for sixteen months before
finally
fading from view. He became the authority on this star and, not
surprisingly,
it was dubbed "Tycho's Star". A faint, nebula exists at the site.
SN 1604 in Ophiuchus
Six years before the invention of the
telescope,
the fourth (and as yet the last) supernova to occur in our galaxy
appeared
on October 9, 1604, and remained visible to the naked eye at night
until
March, 1606. The star became known as Kepler's Star because
Kepler
studied it intensely. From his observations it is known today to
have been a Type I supernova; that is, one in which the white dwarf of
a close binary pair explodes with great violence. The site is
about
one degree northwest of the star 52 Ophiuchi where a faint nebula can
be
found. No other local supernovae have occurred in the Milky Way
since
Kepler's Star; but in 1987 a Type II supernova (wherein a star is
simply
blown to bits) occurred in the Large Magellanic Cloud, one of our
nearby
satellite galaxies. This star was a massive, blue variable that
disappeared
in a huge, cataclysmic explosion.
Where's the biggest star in the sky?
No one knows. To get an accurate diameter
of a distant object one must have an accurate distance to that
object.
And the distance to stars beyond our immediate neighborhood is
notoriously
inaccurate. To further complicate matters, some stars huff and
puff
-- thus changing their size and light output. So astronomers are
understandably loathe to give distances to stars. They prefer,
for
example, to say that ... a star's diameter is hundreds of times the
size
of our Sun, or that the star's diameter is eleven times the distance
from
Earth to the Sun, or that the star's diameter must be "around a billion
miles", or simply that the star is "one of the biggest known".
They
hedge, because they really have to. But here are comments
regarding
a dozen of the biggest of the biggies among the stars visible to the
naked
eye. To appreciate the size of these stars imagine our Sun as a
grain
of sand and then all these stars would be on the order of a basketball.
Epsilon Aurigae may be "... vastly larger
than any other star known."
Zeta Aurigae "... some estimates have ranged
up to 300 solar diameters."
Y Canis Venaticorum (aka La Superba) has
a "... diameter nearly one billion miles."
Eta Carinae has recently been determined
to a be a luminous blue variable -- a new class
of stars characterized
by violent instability brought on by their monstrous size.
In other words, the
star is so big that it will soon blow up.
Mu Cephei (Herschel's Garnet Star) is "...
possibly as much as a billion miles across."
VV Cephei is considered the most colossal
binary star system known and its primary
component "... is
often said to be among the largest known stars."
Omicron Ceti (aka Mira the Wonderful) is
nearby and "... is one of the 10 largest stars
... some 400 times
the diameter of the Sun." But it may be even larger at maximum.
R Coronae Borealis is a mystery. It
is believed to be very distant and yet still visible
despite its
cloudy nature; so the inference is that it must be truly a monster
star.
Alpha Cygni (aka Deneb) is a prominent
member
of the Summer Triangle. It is "... one of
the greatest
supergiant stars known."
Alpha Herculis (aka Rasalgethi) "... seems
to be the largest size known for any star visible
to the naked
eye."
Alpha Orionis (aka Betelgeuse) is another
huffer and puffer. "The star is one of the largest
known, and among
the naked-eye stars probably holds first place."
Beta Orionis (aka Rigel) "... is a true
supergiant
...."
What's all the fuss about "Chandra"?
Putting the Chandra Telescope into orbit is perhaps NASA's
most adventurous and expensive mission ever. All told, the cost
is
somewhere around 4 billion dollars.
Why so expensive? First of all, the orbit required
by Chandra is a lopsided ellipse with an
apogee (highest point) of 87,000 miles and a perigee (lowest point)
of a mere 6,200 miles.
To achieve this orbit Chandra must rely on its own booster rocket after
it gets pushed out of the space shuttle Columbia. Chandra and its
rocket weigh 25 tons, a very big load indeed.
What is Chandra? This is a monster telescope of
the Hubble class; but unlike Hubble which
sees visible light, Chandra will "see" x-rays. The Earth's
atmosphere,
acting like the protective
lead apron worn by x-ray technicians, shields us from all celestial
x-rays. As a result,
astronomers have never clearly seen the sky's x-rays. Yet,
astrophysicists
know that x-rays are the screams of tortured matter. Minor
rocket-borne
x-ray telescopes have revealed tantalizing but only teasing views of
black
holes, supernovae, starburst galaxies, neutron stars and other exotic
members
of the celestial zoo. So Chandra will be the next big plunge into
the unknown universe.
Who is Chandra? Subrahmanyan Chandrasekhar was an
astrophysicist at the University of Chicago who by extending the work
of
Newton and Einstein gave us the theory of how stars
die -- why some of them become white dwarfs or even neutron stars and
yet still others blow
themselves to smithereens. For this he received the Nobel Prize
in 1983.
What is a black hole?
The concept of a black hole was introduced by the English
geologist John Mitchell in 1783.
But it wasn't until a few years ago that the idea of a black hole
became
generally accepted among astronomers. Now they not only recognize
black holes but have come up with many types. Monster black
holes,
with a mass of billions of Suns, lurk in the hearts of the largest
galaxies.
Stellar black holes, the size of New York City, spring from dying
stars.
And teeny black holes theoretically permeate the universe -- according
to Stephen Hawking, an eminent English physicist considered by many as
the father of black holes.
Stellar black holes are currently the best
understood.
They arise with the sudden collapse of a star, a big star of monster
size.
The collapse is so sudden and so forceful that the star's matter is
instantly
compressed into a diminutive ball made up of naked atomic particles
stripped
of their identity by the loss of their integral space. Think of
protons,
neutrons and electrons turned into a "soup" of quarks. No space
at
all, just quarks. To us mere humans, this is a bizarre form of
matter
which our physicists are struggling to reduce to a neat set of
equations.
This ball of matter does not shine; because any captured
light, being composed of energetic photons, is also reduced to captured
energy. Thus the ball cannot be seen directly; and in trying to
explain this idea further, the Princeton physicist John Wheeler coined
a catchy new phrase by saying (in the 1960's) that "... it resembles a
black hole". The name stuck to something that isn't a hole at
all.
Things don't fall into a black hole; rather, they at once
become a part of it as they cross the event horizon. And, the
idea
that a black hole is a wormhole leading into a new dimension is a
figment
of the over-active imagination of the science-fiction buff.
Indeed, the idea that black holes are celestial vacuum
cleaners sucking up everything in sight is
also incorrect. Gravity, a puny force as forces go, does not
exert a vacuum cleaner force until any object is quite close.
That
is why a disk-like girdle of gas and dust is often found rotating ever
faster as it nears a feeding black hole. As a matter of fact,
many
spiral galaxies show a peculiar increase in speed towards the galactic
nucleus when just the opposite is what one would expect in any rotating
disk. Many astronomers interpret this odd phenomenon as a sign of
a galactic black hole.
What is a globular cluster?
"Globular clusters are huge, spherical concentrations
of stars, tens to hundreds of light years
in diameter. The stars they contain are generally much older
than those in open clusters. In a globular cluster the density of
stars is high, about one star per cubic light year, sufficient to
resist
disruption by the tidal forces exerted by the Galaxy. Globulars
are among the oldest known objects in the Universe, with ages of 10
billion
years or more." -- Norton's Star Atlas and Reference Handbook
There are about 200 globular clusters associated with
our Milky Way; most are found in the galactic halo, surrounding the
nucleus
of our galaxy. No one knows how they originate or where they come
from or how they stay together. Open clusters disperse rather
quickly
yet globulars seem to remain tight forever. Some astronomers
postulate
that globular clusters are the remnants of galactic cannibalism -- the
surviving cores of ancient galaxies digested by their hosts, like bones
on a beach. The Great Hercules Globular Cluster, which contains
an
estimated one million stars, is the most prominent globular in our
northern
skies. It is visible to the naked eye on clear, dark nights.
Are those real colors I see in astro pictures?
In most cases, yes. Some pictures however
are taken with filters to highlight otherwise difficult features to see
such as nebulae, elemental gas clouds, interstellar
dust,
etc. These doctored pix are permeated with an obvious
greenish
or reddish glow. Otherwise, "what you see is what you
get."
Not only is the universe a very colorful place but it is very noisy
too.
The universe is not only flooded with visible light, but it is also
alive
with "noise" sources such as X-rays, ultra-violet
light,
radio waves, cosmic rays, solar winds, background
radiation,
and even old-fashioned static from creaking stars.