EAS BUSINESS…
August Meeting Recap
August's EAS speaker was UW geologist
Tony Irving talking about Northwest meteorite hunting and meteorites. We also got an update on the Battle Point
group's Ritchie telescope and some solar prominence videos.
Last months
star party was a great success with perfect weather. Thanks to Lani and Paul Schoenberg for hosting the star party.
The Camp
Delaney star party also had near perfect conditions, with darker skies than
Table Mountain, very pleasant temperatures, and singing astronomers. "Thanks" to the Olympic
Astronomical Society for inviting us to this most relaxing of star
parties. The Oregon Star Party also was September 9th –12th
Saturday September 25th 1999 (Next) Meeting
The Everett Astronomical Society's next
meeting will be at 7:00 PM, Saturday September 25th, in the PROVIDENCE Monte
Cristo Room at Providence Hospital PACIFIC Clinic on Pacific Avenue in
Everett. September's speaker at the EAS meeting will be Dr.
Andrew Vanture from Everett Community College astronomy department
Future Activities
The annual October Orion Star Party at Table
Mountain is set for October 8-10 at the usual Lion Rock
area. Contact Jim Bielaga at Anacortes
Telescope and Wild Bird if you need additional details.
October's EAS meeting speaker will be Jim Bielaga
giving a show tour of many of the major star parties across the US that he
attended in the past year.
November speaker is scheduled to be John Rudolph
giving us an update on archeoastronomy investigations.
Member News
Financial Health
The club maintains a safe $950+ balance. We try to
keep approximately a $500 balance to allow for contingencies.
Club Star Party Info
This
month’s scheduled star party is scheduled for Saturday evening October 9,
weather permitting, The site is scheduled to be Ken and Judy Ward's home north
of Monroe, east of Snohomish. Please
call and RSVP to (360) 784-7757
We try to hold informal close-in
star parties each month during the spring and summer months on a weekend near
the New moon at a member’s property or a local park. (call Dave Mullen at (425)
347-3151 or club officers for info.)
During the winter, phone tree is used to arrange spur-of-the –moment
events during clear weather spells when there are significant celestial
happenings.
Club Scopes’ Status
Scope Loan
Status Waiting
10-inch
Dobsonian On loan No wait list
8-inch Dobsonian On
Loan No wait
list
60 mm Refractor On
Loan No wait
list
Astro Calendar
September 1999
Sep 10-11 -2nd Annual Northern Arizona Star Party, Rock And
Cross Ranch, Arizona
Sep 18 - Deep Space 1, End Of Primary Mission
Sep 20 - Moon Occults Neptune
Sep 21 - Moon Occults Uranus
Sep 23 - Autumnal Equinox, 11:23 UT
Sep 25 - Full moon "Harvest Moon"
Sep 25 -
EAS Meeting - 7:00 PM - Providence Pacific Clinic
Sep 26 - Venus at greatest
brilliancy in morning sky
October 1999
Oct 05 - Moon 5 degrees north of Venus
Oct 06 - Mercury at aphelion
Oct 08 - Zodiacal light visible next 2 weeks (in dark skies)
Oct 8-10 - Orion Star Party at Table Mountain
Oct 09 - Draconid meteor shower peak
Oct 09 - EAS Star Party - Ken & Judy Ward's
Oct 22 - Orionid meteor shower peak
Oct 23 - Jupiter at opposition
Oct 24 - Full moon "Hunter's Moon"
Oct 30 -
EAS Meeting - 7:00 PM - Providence Pacific Clinic
Oct 31 - All Hallow's eve
Oct 31 - End of Daylight Savings time, set clocks back
Over The Airwaves
E.A.S.
members, Jim Ehrmin and Pat Lewis present the astronomy radio show, "It's
Over Your Head", on radio station KSER.
The show is broadcast every Wednesday morning at 7:20 AM to KSER FM
90.7. The six minute astronomy segment
gives a weekly look of what's up in the night sky over Snohomish County. Pat
would appreciate your suggestions about subjects for scripts that you would
find interesting. If you have
information on a good subject, send her a copy. If you think of a good subject but don't have the information,
call her; she may be able to research it.
Send to Pat Lewis, 5307 30th N.E., Seattle WA 98105, or call (206)
524-2006. If you are a listener
of the program show your support by giving the program director of KSER a
call! KPLU 88.5 FM National Public
Radio has daily broadcasts of "Star Date" by the McDonald Observatory
of the University of Texas at Austin, Monday through Friday at 8:58 A.M. and
5:58 P.M. Saturday and Sunday). The
short 2 minute radio show deals with current topics of interest in astronomy.
The
University of Washington TV broadcasts programs from NASA at 12:00 AM Monday
through Friday, 12:30 AM Saturday, and 1:30 AM Sunday on the Channel 27 cable
station.
EAS Library – Book & Video List
The EAS has a library of books, videotapes, and software
for members to borrow. We always value
any items you would like to donate to this library. You can contact Ray Shere to borrow or donate any materials.
MEMBERSHIP BENEFITS & INFORMATION
Membership in the Everett Astronomical Society (EAS) will
give you access to all the material in the lending library. The library, which
is maintained by Ray Shere, consists of several VCR tapes, over 40 books,
magazines, and software titles.
Membership includes invitations to all of the club meetings and star
parties, plus the monthly newsletter, The Stargazer. In addition you will be able subscribe to Sky
and Telescope for $29.95 that
is $7 off the normal subscription rate, contact the treasurer for more
information. When renewing your subscription to Sky
& Telescope you should send your S&T renewal form along with a
check made out to Everett Astronomical Society to the EAS address. The EAS treasurer will renew your Sky and Telescope subscription for
you. Astronomy magazine ($24) offers a similar opportunity to club
members once a year in September.
EAS is a member of the Astronomical League and you will
receive the Astronomical League's newsletter, The Reflector. Being a member also allows you the use of
the club's telescopes, an award winning 10 inch Dobsonian mount reflector,
built as a club project or the 60mm refractor.
Contact Dave Mullens (425-347-3151) to borrow a telescope. EAS dues are $25. Send your annual dues to
the Everett Astronomical Society,
P.O. Box 12746, Everett, WA 98206.
Funds obtained from membership dues allows the Society to publish the
newsletter, pay Astronomical League dues and maintain our library.
OBSERVER’S INFORMATION…
Lunar Facts
Sep 02 Last
Quarter Moon
Sep 09 New
Moon
Sep
17 First Quarter Moon
Sep
25 Full Moon
Oct 02 Last
Quarter Moon
Oct 09 New
Moon
Oct
17 First Quarter Moon
Oct
24 Full Moon
Oct 31 Last
Quarter Moon
Up In The Sky -- The Planets
MERCURY is not visible until late November.
VENUS rises 4 hours before the sun in early October, reaching greatest elongation on October 31.
MARS is extremely low in the southwest just after sunset.
JUPITER rises at the end of evening twilight, and is visible all night.
SATURN rises just after Jupiter, visible to Jupiter's lower left, and is visible all the rest of the night.
URANUS and NEPTUNE, dim at
magnitudes 6 and 8, respectively, are in Capricornus, which is up in the
southeast by late evening.
PLUTO, is in
Ophiuchus, visible just after sunset, but is magnitude 13.8.
Constellation(s) of the Month
TUCANA: “The Toucan”,
for which this constellation is named, borders on the constellations of Grus,
Hydrus, Indus, Octans, and Phoenix in the Southern Hemisphere. It ranks 74th in overall
brightness among the constellations, containing 15 stars brighter than
magnitude 5.5. Its central point is
located at RA=23h43m and Dec.= -66.5 degrees.
It is completely visible from latitudes South of +14 degrees, and
completely invisible from latitudes North of +33 degrees. This constellation ranks 48th in
overall size, taking up 294.56 square degrees, or 0.714% of the sky. Tucana has no known meteor showers, and no
associated Messier objects; it also has no associated asterisms. Its midnight culmination date is September
17th, and its solar conjunction date is March 18th. Tucana is one of 11 constellations invented
by Pieter Keyser and Frederick de Houtman, during the years 1595-1597. The Small Magellanic Cloud (SMC) is located
within the region of Tucana; this object was discussed in last month’s column
of “Mirror Images”. The mass of the SMC
is about 2,000 million times that of the sun, and it lies at a distance of
approximately 200,000 light years.
Roughly 1 degree from the western edge of the SMC lies one of the best
globular clusters in the sky, 47 Tucanae.
Optically this globular appears near to the SMC, but it is actually
located within the outer confines of our Milky Way. 47 Tucanae is second only to Omega Centauri as the finest
globular cluster in the entire sky.
Finally, the constellation of Tucana has the distinction of being the
discovery point of one of the best comets of the 20th Century: Comet
Bennett. This comet reached a magnitude
of very nearly zero in March, 1970.
Young Astronomer’s Corner
The Young
Astronomer’s Corner is in the middle of a new series; this time we are talking
about the planets. Last month the
subject of this column was Mercury; this month it will be Venus. We are listing the astronomical facts about
each planet; for the month of September, our
guest is Venus, and these are the facts:
Rotation around the Sun:
approximately every 225 days (earth = 365 days).
Orbit: from 0.72 to 0.73 Astronomical Units;
this is an orbit that varies between approximately 67 and 68 million miles from
the sun.
Inclination of Orbit: 3.4 degrees.
Diameter at Equator: 12,104 kilometers (or 7,565
miles).
Mass: 0.82 that of earth (about 8-tenths
that of earth).
Density: 5.2 times that of water
Period of Rotation on its own axis: 243 days,
zero hours, and 14.4 minutes (earth = 24 hours).
Satellites (moons): none
Gravity: about nine-tenths (0.91) that of earth.
Special Notes: Venus is never
very far from the sun in the sky. It reaches
its greatest elongation of 45 to 47 degrees approximately 72 days before and
after inferior conjunction. At its
greatest brightness, Venus is close to magnitude –4.4, and is then brighter
than everything in the sky except the sun and the moon. Venus has very hostile surface and
atmospheric conditions; it has been determined that its atmosphere rotates
almost 60 times faster than the solid planet itself. In a telescope, the disc of Venus appears a brilliant yellowish-white
in color, and, like the moon, exhibits phases.
The atmosphere of Venus consists primarily of carbon dioxide (98%), 1-3%
nitrogen, and smaller percentages of helium, neon, krypton, and argon. The atmospheric pressure on the surface of
Venus is about 90 times greater than that of earth, and the surface temperature
is extremely high, much higher than that of earth’s average surface
temperature. In fact, the surface
temperature of Venus is higher than that of any other planet. This is the result of the planet’s
“greenhouse effect” involving the
layered clouds of Venus, and the large amount of carbon dioxide in its
atmosphere. Venus has a nickel-iron
core, which rotates slowly; as a
result, Venus has little of its own magnetic field. The surface of Venus shows much evidence of
past volcanic activity. Venus has had
several man-made probes visit it: these include the Russian Venera probes, as
well as the Mariner 2, 5, and 10 probes; the Pioneer Venus probes, and
Magellan.
Astronomy and
Telescope “Lingo”
Astronomy lingo: JUPITER-CROSSER: A very rare type of asteroid whose orbit crosses the orbit
of Jupiter; the gravitational influence of Jupiter makes this type of orbit
very short-lived.
Telescope lingo: BASELINE: The straight line
between two observational points; for example, the line between the elements of
an interferometer. The longer the
baseline between two radio telescopes, for instance, the finer the detail that
can be resolved in a radio source.
Astronomy Fun
Facts
** Saturn
revolves around the sun once every 29.5 Earth years; a Saturn year is thus
about three decades long. Saturn thus
orbits around the Sun only about 2.5 times during the average human life span!
** The
greatest distance between any two planets in the solar system is between
Neptune and Pluto; this distance, which occurs about once every 500 years,
yields a maximum separation of almost 7,500 billion miles!!
** The oldest
moon rock found thus far was dated at 4.6 billion years old. It was found and returned during the Apollo
missions. This age dates back to the
time that the Earth and Moon were formed.
This rock was one billion years old before the first single-celled
organisms appeared on earth, and it is now the oldest rock on earth!!
“MIRROR” IMAGES
The
“MIRROR” IMAGES” column is taking a break this month, and will return in October to
address the type of object known as emission nebulae. See you then!
Astronomical Notes --
On & Off the Net...
The
formation of the Solar System was hurried along by a nearby gamma- ray burst,
two astrophysicists in Ireland suspect. Rather than aborting the birth of
planets, the flood of energy may have melted primordial dust grains, seeded the
formation of meteorites and helped form the rocky planets, including Earth.
For
over a century, astronomers have tried to understand what made clumps of dust
circling the young Sun melt into chondrules-rocky beads rich in iron and
silicon minerals that make up the bulk of stony meteorites. Suggestions
included shock waves and gigantic flashes of lightning.
Now
Brian McBreen and Lorraine Hanlon of University College Dublin suggest that all
the chondrules in the Solar System formed in a matter of minutes 4.5 billion
years ago when a gamma-ray burst-one of the most powerful explosions in the
Universe-seared the dust and gas circling the Sun with intense X-rays and gamma
rays. Astronomers aren't sure what causes gamma-ray bursts, but they may occur
when supermassive stars explode at the end of their lives (New Scientist, 3 April,
p 5).
In
a paper that will appear in a future issue of Astronomy and Astrophysics,
McBreen and Hanlon calculate that a gamma-ray burst within 300 light years
would have flooded the dusty disc circling the young Sun with enough energy to
fuse up to 100 Earth masses of material into droplets that cooled into
chondrules. These, and the dust from which they formed, are rich in iron, which
would have soaked up X-rays and gamma rays very efficiently. "It
explains the key role played by iron, which dominates the X-ray and gamma-ray
absorption," says McBreen.
If
the theory is right, it makes the Solar System more unique than many scientists
would like. McBreen and Hanlon believe that only one Sun-like star in a
thousand would have been close enough to a gamma-ray burst to form chondrules.
Because they also think that the dense chondrules settle quickly into the plane
of a protoplanetary disc and speed the formation of planets, their theory
implies that solar systems such as ours are rare.
"Forming
chondrules really is a long-standing problem, so if this mechanism accounts for
them, that would be pretty fantastic," says Alan Boss, an
astrophysicist at the Carnegie Institution in Washington DC. Still, he is
reluctant to rely on an unlikely event as a crucial factor in the formation of
the Solar System, and wonders whether the idea can explain other features of
chondrules, such as their size and abundance. "I don't think you'd want
to invoke it unless it takes care of everything," he says.
Specialists
in meteorites are intrigued by McBreen's idea. "Chondrule formation
remains a thorny subject, so it's good to see a new idea in the area,"
says Ian Wright, a meteoriticist at the Open University in London. He notes
that most of the researchers studying meteorites believe chondrules did not
form all at once, although the case is not closed. "It will certainly
cause debate, and it's an interesting idea that can be tested in our labs."
From:
Robert Adler, New Scientist issue 11th September 99
http://www.newscientist.com
Massive
black holes, long-thought to make only a modest contribution to the universe's
total energy output compared with ordinary stars, may actually be responsible
for up to half of all the radiation produced in the universe since the Big
Bang. Details of this theory, based on measurements of background X-radiation
and the growth of massive black holes obscured by gas, are being presented
today at the X-ray Astronomy 1999 meeting in Bologna, Italy, by Professor
Andrew Fabian, a Royal Society Research Professor at the Institute of
Astronomy, University of Cambridge.
The
black holes responsible for this energy production are probably present in the
centers of most galaxies and contain the mass of millions, or even billions, of
suns compressed into a region smaller than the solar system. Distant galaxies
with suspected massive black holes within their exceptionally bright cores are
commonly known as quasars. They produce energy in an accretion process, when
gas approaching the black hole swirls inwards, attaining very high velocities
and temperatures in the extremely strong gravitational field.
This
hot, fast-moving gas is very luminous, emitting radiation across a broad
spectrum -- from visible light, through the ultraviolet to X-rays -- before
disappearing within the event horizon of a black hole. The event horizon is the
black hole's point of no return, beyond which gravity is so intense that
nothing, not even light, can escape. Outside the event horizon, there is a
larger region where the black hole exerts a powerful gravitational influence,
but not so powerful that nothing can escape. This is the region where accretion
occurs and the intense radiation is emitted.
"The
background of X-radiation found in space cannot be explained by stars or by
ordinary quasars," says Professor Fabian. "What is required,
following earlier ideas, is a population of obscured quasars. For every
ordinary quasar about ten more obscured ones are needed, meaning that most
massive black holes growing by accretion are hidden from the view of observers
looking at visible light, or in the ultraviolet and near infrared wavebands."
Visible
light and ultraviolet radiation from the accreting gas try to escape from the
black hole region but are absorbed by nearby dust and gas. The X-rays are not
absorbed and therefore provide a true measure of the total amount of energy
being emitted. The absorbed energy, however, can also provide a useful measure
of black hole power. This absorbed energy is re-emitted in the form of far
infrared radiation (alternatively known as sub-millimeter radiation), which
also penetrates the dust and gas.
Energy
emitted from the regions close to massive black holes has been underestimated,
Fabian says, because orbiting X-ray observatories have so far not been able to
detect the dust-penetrating X-rays and also because there are no superior far
infrared telescopes to observe the re-emitted black hole radiation. Stars, on
the other hand, are well documented because they radiate their energy largely
as visible and ultraviolet light. This radiation is measured by world-class
telescopes both in orbit and on Earth.
"Recent
ground-based observations in the submillimeter band made with the United
Kingdom's Submillimeter Common User Bolometer Array (SCUBA) on the James Clerk
Maxwell Telescope in Hawaii, and the Diffuse Infrared Background Experiment
(DIRBE) on NASA's Cosmic Background Explorer (COBE) satellite do support the idea
that much of the energy in the universe has been absorbed and re-radiated at
much longer wavelengths. However, the process of absorption and re-radiation
conceals whether the energy is from stars or black holes," said
Fabian.
Observations
with NASA's Chandra X-ray Observatory, launched in July, and ESA's XMM X-ray
satellite, scheduled for a December launch are expected to provide the first
accurate cosmic census of the power of black holes over the age of the
Universe, and detect gas-obscured X-ray sources.
Penetrating
25,000 light-years of obscuring dust and myriad stars, NASA's Hubble Space
Telescope has provided the clearest view yet of a pair of the largest young
clusters of stars inside our Milky Way galaxy.
Located
less than 100 light-years from the very center of the Galaxy, these giant
clusters have a remarkable excess of massive stars and offer new clues as to
how such monumental clusters form.
The
Hubble images reinforce the emerging view that the galactic core is a unique
place in the Galaxy, where conditions under which stars form are completely
different from elsewhere in the Galaxy. The core is a site of ongoing violent
star formation, as clouds of molecular hydrogen laced with dust zip around the
center of our galaxy like wayward comets. A fireworks show of star birth
ignites when the clouds collide.
Called
the Arches and Quintuplet clusters, they are 2 and 4 million years old,
respectively. The older cluster is more dispersed, and it has
stars
on the verge of blowing up as supernovae, such as the Pistol Star. The Pistol Star is the brightest star in the
Galaxy and was also imaged by Hubble in 1997. Both clusters are destined to be
ripped apart in just a few million years by gravitational tidal forces in the
Galaxy's core. But in the brief time they are around, they shine more brightly
than any other star cluster in the Galaxy.
Having
an equivalent mass greater than 10,000 stars like our sun, the monster clusters
are 10 times heavier (or more massive) than typical young star clusters
scattered throughout our Milky Way. The more compact
Arches
cluster is so dense, over 100,000 of its stars would fill a spherical region in
space whose radius is the distance between the Sun and its nearest neighbor,
the star Alpha Centauri, 4.3 light-years away.
Only
1 out of every 10,000,000 stars in the Galaxy is as luminous as the bright Arches cluster stars. This suggests
that conditions are so extreme at the hot and dynamic hub of our galaxy, massive
stars are favored to form. At least a dozen of the stars weigh about 100 times
more than our sun.
Both
clusters might have formed when two giant clouds, containing dust and molecular
hydrogen, had a head-on-collision. This precipitated the birth of thousands of
stars. However, warm interstellar temperatures, powerful magnetic fields, and
turbulence inside the interstellar gas may
have
inhibited smaller clumps of hydrogen from falling together to create many stars
lower in mass than our sun.
The
observations were made by Don Figer of the Space Telescope Science Institute,
using Hubble's NICMOS infrared camera.
Figer
next plans to analyze data obtained with a new near-infrared spectrometer,
which he helped build for the Keck telescope. The analysis will help determine just how quickly the
Arches cluster will evaporate due to tidal forces. Hubble's successor, the Next
Generation Space Telescope, scheduled for launch in 2008, will be able to
clearly see the fainter stars in the cluster's core, giving astronomers better
insight into star-forming conditions in the heart of our galaxy and into
whether conditions there allow stars like our sun to form.
Image files are available at:
http://oposite.stsci.edu/pubinfo/pr/1999/30
Higher resolution versions are available at:
http://oposite.stsci.edu/pubinfo/pr/1999/30/pr-photos.html
http://oposite.stsci.edu/pubinfo/pr/1999/30/extra-photos.html
When
Voyager flew by Jupiter's moon Europa in 1979, it photographed geological
surface features unlike any others ever seen in the solar system. Near Europa's
south pole, chains of scalloped lines joined arc-to-arc at the cusp ran for
hundreds of miles across the frozen, fractured surface.
Until
now, there have been no good ideas as to what formed these bizarre
"cycloidal" features, or "flexi," as they were officially
dubbed by the International Astronomical Union.
Now,
planetary scientists at the University of Arizona in Tucson provide a model for
how these features are created. It is perhaps the most convincing evidence yet
for a global ocean. They report on it in today's issue of Science (Sept. 17).
Scientists
know that Europa has a 100-mile-thick layer of water -- 20 times thicker than
the Earth's oceans -- but the visible top layer is frozen. This new strong
evidence for a liquid global ocean below the surface makes Europa a prime
target in the search for life beyond Earth.
Gregory
V. Hoppa and others at the UA Lunar and Planetary Laboratory theorize that
cycloidal cracks form in Europa's solid-ice surface with the daily rise and
fall of tides in the subsurface ocean. They painstaking modeled and scrutinized
images of Europa taken by the Galileo spacecraft between 1996 and 1999. The new
images show that cycloidal cracks and ridges are widely distributed over all of
the moon.
Hoppa
has posted images and explanatory animation of cycloidal crack formation on the
web site:
http://pirlwww.lpl.arizona.edu/~hoppa/science.html
Europa
is about the size of our moon. Tidal stresses on its ice-covered ocean ebb and
flow as it orbits Jupiter, which is 300 times as massive as Earth. According to
the UA researchers' model, Europa's ocean tides rise and fall a distance of 30
meters. By comparison, tides at most ocean beaches on Earth rise and fall 1 to
2 meters, or 4 to 6 feet. "What
causes the cycloid to form is that Europa is in a slightly eccentric orbit
because of Io and Ganymede (other Jovian moons). Sometimes Europa is a little
closer, other times a little farther from Jupiter. When Europa is closer to Jupiter,
the tides are higher because Jupiter is pulling on it more. When Europa is
farther, the tides fall because Jupiter's force falls. This causes Europa's ice
shell to flex."
The
UA model shows that when tidal stress reaches the tensile strength of ice, the
ice begins to crack. It takes very little stress to form the initial crack --
something like the force it takes to break a saltine cracker -- because
Europa's surface ice is weakened by countless linear fractures.
The
crack propagates relatively slowly across the ever-changing stress field. It
moves following a curving path until stress drops below the tensile strength of
the ice, when it halts. A few hours later, when tidal stress again exceeds the
tensile strength of ice, the crack begins a new curve in another direction.
"You
could probably walk along with the advancing tip of a crack as it was forming
-- if you could survive Europa's radiation environment," Hoppa said.
"And while there's not enough air to carry sound, you would definitely
feel vibrations as it formed."
One
of their most striking conclusions is that each arc segment forms in 3.5 days
-- the time it takes Europa to make one complete orbit around Jupiter. The
cycloids faithfully record the 85-hour daily flexing of Europa's ice shell just
as trees faithfully record each growing season in annual rings.
"We
can look at a crack that has 4 or 5 cusps, each formed every 3.5 days, and know
that the entire chain formed in about 2.5 weeks," Hoppa said.
Arc
segments in the cycloid, each ranging from 75 km to 200 km long, form cracks
stretching a thousand kilometers over the ice in a fraction of an instant in
geological time. Eventually, cracks evolve into ridges, typically double
ridges, according to the UA model. The
scientists also can determine which direction the cracks traveled as they
formed based on the orientation of the arcs and the hemisphere in which they
are found.
"What
amazes me about this is just how long these features have been a mystery,"
Hoppa said. "We've been staring at pictures of them for 20 years, since
Voyager. We didn't know what made them. And it seems what they've been telling
us all along is that an ocean was there when these things formed."
LINKS:
http://pirlwww.lpl.arizona.edu/~hoppa/science.html
The
discovery of a new type of low-surface-brightness reflection nebula around
aging stars has provided important clues about how stars lose mass and form
planetary nebulae.
"The
results from a recently completed optical imaging survey of proto-planetary
nebula candidates has shown us that stars don't lose mass in a spherically
symmetric way at the ends of their lives," said Margaret Meixner, a
professor of astronomy at the University of Illinois. "Some other
process of mass loss -- such as an axisymmetric superwind -- is occurring."
As
certain types of stars age, their stellar winds create glowing envelopes of gas
and dust called planetary nebulae. Intermediate-mass stars -- like the sun --
move through a transitional proto-planetary nebula stage on their way to
becoming planetary nebulae.
"One
of the most significant changes that occurs during the transition is the
emergence of axisymmetry in the circumstellar shell of gas and dust,"
Meixner said. "While most stars show a high degree of spherical
symmetry, most planetary nebulae display either bipolar or elliptical symmetry.
Therefore, the departure from spherical symmetry must take place somewhere
along the evolutionary sequence between the two phases."
To
investigate potential morphological (structure) trends during the transition,
Meixner and colleagues studied 27 candidate proto-planetary nebulae with the
Hubble Space Telescope.
"The
Hubble's high-resolution imaging capabilities allowed us to identify
low-surface-brightness reflection nebulosities around 21 of the candidate
objects," Meixner said. "All 21 nebulae showed varying degrees
of asphericity, and we clearly recognized two basic types of structure."
In
the first type of structure -- never before observed in proto-planetary nebulae
-- a bright, central star is embedded in a faint, elliptically elongated shell
of gas and dust. In the second type, the central star is partially or completely
obscured by a bipolar structure.
"The
fact that we see elliptical structures in addition to bipolar structures helps
to constrain the time scale of when the shaping process occurred,"
Meixner said. "The intrinsic axisymmetry of these reflection
nebulosities demonstrates that the axisymmetry frequently found in planetary
nebulae predates the proto-planetary nebula phase."
Meixner
and her colleagues suggest that the axisymmetry found in proto-planetary
nebulae could be created by an equatorially enhanced stellar superwind. The
onset of the superwind would initiate the morphological shift from spherical to
axial symmetry, becoming more pronounced in planetary nebula.
Images are available at http://www.astro.uiuc.edu/~ueta/project/hstsnap/images/17436+5003s.jpg
and
http://www.astro.uiuc.edu/~ueta/project/hstsnap/images/slide17436.gif
Observations
by a University of Illinois astronomer have shown that magnetic fields are a
critical component controlling when and how stars form.
"Understanding
the physics governing the structure and evolution of dense interstellar
clouds is a necessary part of understanding the fundamental astrophysical
process of star formation," said Richard Crutcher, a professor of
astronomy at the U. of I. "Theoretical studies have suggested that
magnetic fields play a vital role in the evolution of interstellar clouds and
in the formation of stars, but those studies needed to be compared with
observational data."
Two
basic problems have persisted in our understanding of star formation, Crutcher
said. First, in a fully formed star, the outward pressure of thermonuclear
reactions in the core will balance the inward pull of gravity. In a molecular
cloud, however, some other force must be supporting the cloud against its own
gravity. Otherwise, all the clouds would have collapsed into stars long
ago. The second problem involves
transferring excess angular momentum from a developing star. As a molecular
cloud coalesces into stars, the material rotates faster and faster -- like a
spinning ice skater who tucks in her arms. Unless the excess angular momentum
is removed, the star will fly apart.
"Theorists
have performed extensive simulations that show how an interstellar cloud might
collapse in the presence of a magnetic field," Crutcher said. "In
those studies, the researchers could prevent the clouds from quickly collapsing
and forming stars, and they could get rid of the extra angular momentum, by
making the magnetic fields sufficiently strong."
To
test theory against data, Crutcher measured the strengths of magnetic fields in
27 interstellar clouds of varying molecular density. By comparing each cloud's
magnetic energy with its gravitational energy, he found that magnetic fields
were strong enough to control the rate of collapse and to assist in the star-formation
process by providing a means of shedding excess angular momentum.
"The
magnetic field strength -- which does indeed scale with the square root of the
gas density as theory predicts -- is nearly large enough to keep the cloud from
collapsing," Crutcher said. "The gravitational energy is still
about twice as strong as the static magnetic energy, but the magnetic field
also supports the cloud indirectly by allowing magnetic turbulence and waves to
be present. "
The
turbulence and waves supply an additional force that opposes the pull of
gravity and provide a mechanism for transferring angular momentum from the
developing star into the surrounding envelope of gas and dust. "By flinging a small amount of
matter outward along the magnetic field lines, the magnetic waves can remove a
huge amount of angular momentum, making star formation possible," said
Crutcher.
When
the universe was about one-tenth of its current age, more than 10 billion years
ago, the precursors of massive galaxies such as our own Milky Way were being
built from smaller galaxies that collided and merged, triggering violent bursts
of star formation. This scenario, one of two competing views of young galaxies
proposed by cosmologists, is supported by a recent analysis of supercomputer
data led by researchers at the University of California, Santa Cruz.
The
new study sheds light on the nature of the most distant galaxies observed by
modern telescopes. These galaxies, called "high-redshift galaxies"
because their light has been strongly shifted toward longer (redder)
wavelengths by the expansion of the universe, have presented a challenge to
cosmologists since they were first detected in the early 1990s.
The
high-redshift galaxies are very faint because of their great distance from the
earth, but astronomers have analyzed the light from them and determined that
they actually shine very brightly and are dominated by young, hot stars. They
are also small compared to similarly bright galaxies in the nearby universe.
These observations indicate a very high rate of star formation, and the
challenge is to explain how these young galaxies accumulated enough mass to
burn so brightly.
"The
mystery is that these very luminous galaxies are so far away that the light we
see today left them when the universe was only 10 to 15 percent of its current
age, and one wonders how so many bright galaxies had already formed so early in
the evolution of the universe," said Tsafrir Kolatt, a postdoctoral
researcher at UCSC and lead author of a paper describing the new findings.
Their
results suggest that the high-redshift galaxies offer a glimpse of the universe
at a stage when the large-scale structure of the cosmos was still forming.
Coauthors
Joel Primack and Sandra Faber, University Professor of astronomy and
astrophysics, both at UCSC, have been collaborating since the 1980s on efforts
to understand the formation of galaxies, clusters of galaxies, and other
large-scale structures in the universe.
The
paper, entitled "Young galaxies: What turns them on?," evaluates two
competing scenarios that have been proposed to explain the origins of
high-redshift galaxies. Both scenarios incorporate the standard features of
modern cosmology, including a universe dominated by dark matter, mysterious
particles that make up at least 90 percent of the mass of the universe. In this
standard model of the cosmos, galaxies form within large halos of dark matter.
According
to one explanation of high-redshift galaxies, they sit in the middle of very
massive dark-matter halos. The gravitational pull of the dark matter feeds gas
into the center, where it condenses into stars.
Kolatt
calls this the "central quiescent" scenario, because it describes a
steady rate of star formation proceeding over an extended period of time.
For
this scenario to work, however, the galaxy must have a huge supply of gas to
maintain a high rate of star formation.
The
alternative view, championed by Primack and his collaborators, is the
"collisional starburst" scenario, in which collisions between small
galaxies trigger intense but relatively short-lived bursts of star formation.
When two galaxies collide, clouds of gas get funneled toward the center of the
larger galaxy, and the gas condenses to form new stars.
Through
this mechanism, relatively small galaxies can generate very high rates of star
formation.
To
test these two scenarios, the researchers used supercomputers to simulate the
behavior of matter over billions of years in a representative chunk of the
universe. The results of the simulation provided strong support for the
collisional starburst scenario.
Cosmologists
rely heavily on such supercomputer simulations to test their theories of how
the universe evolved. These simulations track the interactions of millions of
particles of matter, starting with the initial conditions shortly after the Big
Bang and proceeding according to the standard laws of physics and the latest
theories regarding the nature of dark matter and other critical factors. Each
simulation generates a representation of the universe that can be compared with
current observations.
The
simulation used in this study, developed by Kravtsov and Klypin at NMSU,
provided an unprecedented level of resolution, enabling the researchers to
analyze the interactions of relatively small clumps of matter and to detect
collisions and mergers. Supercomputers were needed not only for the simulation
itself but also for the analysis of the output from the simulation. The
researchers used computers at the Naval Research Laboratory in Washington,
D.C., and the National Center for Supercomputing Applications at the University
of Illinois.
"This
project is right at the edge of what's possible with the biggest, fastest
computers available," Primack said.
Both
the simulation and its analysis, led by Kolatt and Bullock, were innovative,
Primack added. "The simulation provided such high resolution that it
was possible to find every lump of dark matter in the simulation volume and to
tell whether one dark matter halo merged with another; nobody has ever done
this before halo by halo, looking at every particle in every halo," he
said.
The
researchers then used data from earlier simulations, performed at UCSC, that
showed what happens when two galaxies collide. Those results enabled them to
calculate the star formation rates that would result from the mergers of dark
matter halos and their associated galaxies. "For each individual
collision in the simulation, we had to calculate how many stars would form, how
long they would shine, and what colors would be observed in the light from them,"
Kolatt said.
The
results showed that the observed properties of the high-redshift galaxies are
best explained by the collisional starburst scenario. "The simulation
gave us the right numbers [of high-redshift galaxies], the right clustering
properties, and the right brightnesses to explain what we actually see with the
telescopes," Bullock said.
Most
of the collisions in the simulation occurred in or near relatively massive dark
matter halos, resulting in strong clustering of the starbursts in space. "We
believe that what we're seeing in these high-redshift galaxies is the build-up
of structure in the universe, and that this process generates stars with high
efficiency," Bullock said.
The
central quiescent scenario did not fare so well, because the simulation did not
generate enough galaxies of sufficient mass to account for the evolution of the
number of high-redshift galaxies observed, Kolatt said.
In
addition to presenting results that match current observations, the authors
make predictions that can be tested by further observations of high-redshift
galaxies. A key test that can differentiate between the two scenarios is how
the number of galaxies changes with increasing distance.
At
higher and higher redshifts, the number of galaxies observed should fall off
much faster for the central quiescent model than for collisional starbursts.
Another important test will be precise measurements of the masses of
high-redshift galaxies, which Faber hopes to obtain within the next year.
"We
suspect that we will find the galaxies are small, and that will confirm that
these objects are bright but low mass, meaning they must be undergoing a
starburst," Faber said.
A
tempestuous relationship between an unlikely pair of stars may have created an
oddly shaped, gaseous nebula that resembles an hourglass nestled within an
hourglass.
Images
taken with Earth-based telescopes have shown the larger, hourglass-shaped
nebula. But a picture taken with NASA's Hubble Space Telescope, reveals a
small, bright nebula embedded in the center of the larger one. Astronomers have
dubbed the entire nebula the "Southern Crab Nebula" (He2-104),
because, from ground-based telescopes, it looks like the body and legs of a
crab. The nebula is several light-years long.
The
possible creators of these shapes cannot be seen at all in this Wide Field and
Planetary Camera 2 image. It's a pair of aging stars buried in the glow of the
tiny, central nebula. One of them is a red giant, a bloated star that is
exhausting its nuclear fuel and is shedding its outer layers in a powerful
stellar wind. Its companion is a hot, white dwarf, a stellar zombie of a
burned-out star. This odd duo of a red giant and a white dwarf is called a
symbiotic system. The red giant is also a Mira Variable, a pulsating red giant,
that is far away from its partner. It could take as much as 100 years for the
two to orbit around each other.
Astronomers
speculate that the interaction between these two stars may have sparked
episodic outbursts of material, creating the gaseous bubbles that form the
nebula. They interact by playing a celestial game of "catch": as the
red giant throws off its bulk in a powerful stellar wind, the white dwarf
catches some of it. As a result, an accretion disk of material forms around the
white dwarf and spirals onto its hot surface. Gas continues to build up on the
surface until it sparks an eruption, blowing material into space.
This
explosive event may have happened twice in the "Southern Crab."
Astronomers speculate that the hourglass-shaped nebulae represent two separate
outbursts that occurred several thousand years apart. The jets of material in
the lower left and upper right corners may have been accelerated by the white
dwarf's accretion disk and probably are part of the older eruption.
The
nebula, located in the Southern Hemisphere constellation of Centaurus, is a few
thousand light-years from Earth. The
image taken in May 1999 captures the glow of nitrogen gas energized by the
white dwarf's intense radiation.
Image and illustration files
are available at:
http://oposite.stsci.edu/pubinfo/pr/1999/32
Higher resolution versions are available at:
http://oposite.stsci.edu/pubinfo/pr/1999/32/extra-photos.html
http://oposite.stsci.edu/pubinfo/pr/1999/32/illustration.html
Astronomers
have used the world's two most powerful radar
telescopes to make the most detailed images ever obtained for a large asteroid in a potentially
Earth-threatening orbit.
With
an average diameter of about 3.5 kilometers (2 miles), 1999 JM8 is the largest of the so-called
potentially hazardous asteroids ever
studied in detail. Although this object can pass fairly close to Earth in celestial terms, astronomers concur
that an actual encounter with Earth is
not of concern in the next few
centuries.
The
new images, obtained with NASA's Goldstone Solar System Radar in California and the Arecibo
Observatory in Puerto Rico, reveal that
1999 JM8 is a several-kilometer-wide object with a peculiar shape and an unusually slow and possibly complex spin state, said Dr. Lance Benner of NASA's Jet
Propulsion Laboratory, Pasadena, CA,
who led the team of astronomers. The images are available online at
http://photojournal.jpl.nasa.gov or
http://echo.jpl.nasa.gov/~lance/1999JM8.html.
"It
will take much more data analysis to determine the object's shape and exact rotation state," Benner said.
"But just from looking at the
images we can see that this nearby world is
extremely peculiar. At this point we do not understand what some of the features in the images are, much less
how they originated."
The
asteroid was discovered on May 13, 1999, at a U.S. Air Force telescope in New Mexico that is part
of the Lincoln Near Earth Asteroid
Research Project, managed by the Lincoln
Laboratories of the Massachusetts Institute of Technology. The discovery provided adequate notice for radar
observations to be scheduled at
Goldstone from July 18 to August 8 and at Arecibo from August 1-9 during the asteroid's close approach to 8.5 million kilometers (5.3 million miles), the
equivalent of 22 Earth-Moon
distances.
"Although
Arecibo is the more sensitive telescope, Goldstone is more fully steerable, and we took advantage of the complementary capabilities of the two
antennas," noted Benner.
"The discovery of this object weeks before its closest
approach was a stroke of luck,"
he said. "The asteroid won't come this
close again for more than a thousand years."
Asteroid
1999 JM8 bears a striking resemblance to Toutatis, a similar-sized, slowly rotating object also studied in
detail with radar, said Dr. Scott
Hudson of Washington State University,
who is an expert in using radar images to determine the shapes of asteroids.
"The
fact that both these several-kilometer-wide asteroids are in extremely slow spin states suggests that slow rotators
are fairly common among near-Earth
asteroids," he said. "However, although collisions are thought to be the primary process
that determines asteroid spin states,
we don't know how the slow, complex
states come about."
The
radar imaging technique uses transmissions of
sophisticated coded waveforms and computer determinations of how echoes are distributed in range and
frequency, instead of their angular
distribution, as in normal optical pictures. "Our finest resolution is 15 meters (49 feet) per pixel,
which is finer than that obtained for
any other asteroid, even for spacecraft" said Dr. Jean-Luc Margot, one of the team members
from Arecibo Observatory. "To
get that kind of resolution with an optical
telescope, you'd need a mirror several hundred meters across. Radar certainly is the least expensive way
of imaging Earth- approaching objects."
The
images show impact craters with diameters as small as 100 meters (330 feet) -- about the length of a football field
-- and a few as large as 1 kilometer
(0.6 miles). "The density of
craters suggest that the surface is geologically old, and is not simply a chip off of a parent asteroid,"
said Dr. Michael Nolan, a staff
scientist at the Arecibo Observatory. "We also see a concavity that is about half as wide as the
asteroid itself, but we're not sure yet
whether or not it's an impact crater."
This
is hardly the first time that radar has revealed a near-Earth asteroid with peculiar characteristics, said Dr. Steven Ostro of JPL, who has led dozens of
asteroid radar experiments. Radar studies have revealed a stunning array
of exotically shaped worlds with
compositions ranging from solid metal
to low-density carbonaceous rock and rotation periods ranging from 11 minutes to more than a week. "These are
very, very strange places," he
said. "I really envy the coming
generations of space explorers who will visit them."
FROM THE EDITOR'S TERMINAL
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