San Diego Astronomy Association Celebrating Over 40 Years of Astronomical Outreach Observatory (619) 766-9118 http://www.sdaa.org A Non-Profit Educational Association P.O. Box 23215, San Diego, CA 92193-3215 May 2012 SDAA Business Meeting Next meeting will be held at: 3838 Camino del Rio North Suite 300 San Diego, CA 92108 May 8th at 7pm Next Program Meeting May 16, 2012 at 7pm Mission Trails Regional Park Visitor and Interpretive Center 1 Father Junipero Serra Trail CONTENTS Date: 16 May 2012 Speaker: Jerry Hilburn Topic: Mars Mission’s Update Please join Jerry Hilburn for a presentation on the current status of the Mars missions and more info on the upcoming Curiosity landing scheduled to arrive at the Red Planet on Aug. 6, 2012 (EDT). We will cover the current rover mis- sion and MRO as well. San Diego Astronomy Association (SDAA) sponsors speakers on a wide range of astronomy topics on the third Wednesday of every month at the Mis- sion Trails Regional Park Visitors Center. The Program meeting begins at 7:00 PM. Each attendee receives one free door prize ticket. After announcements and a small amount of business, the audience is treated to the featured presentation. At the close of the meeting the door prizes are presented. The event is open to the public. The Mission Trails Regional Park Visitors Center is at One Fr. Juni- pero Serra Trail, San Diego CA 92119. Call the park at 619-668-3281 for more information or visit http://www.mtrp.org. Please contact Bill Carlson ([email protected] or 425-736-8485) if you have any questions, comments, or ideas for the Program Meetings. May 2012, Vol XLX, Issue 5 Published Monthly by the San Diego Astronomy Association $2.50 an issue/$30.00 year Incorporated in California in 1963 May Program Meeting ............... 1 April Minutes ......................... 2 Annular Solar Eclipse ................... 4 Venus Transit .......................... 4 May Calendar ............................ 5 SDAA Contacts ....................... 6 ASIG Gallery.............................. 7 Web Only---------------------- Julian Starfest ................. 9 NASA Hepls Europe Study a Comet....10 Simulating the Universe ............ 11 Catching Stars that go BANG! ....... 12 How the Milky Way Became Spiral.....13 Newsletter Deadline The deadline to submit articles for publication is the 15th of each month. May Program Meeting
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San Diego Astronomy Association · Outreach Committee Report. Received a nice presentation from the Girl Scouts in appreciation of the star party at Heise. Program Report. The IDA
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San DiegoAstronomy AssociationCelebrating Over 40 Years of Astronomical Outreach
Observatory (619) 766-9118http://www.sdaa.org
A Non-Profit Educational AssociationP.O. Box 23215, San Diego, CA 92193-3215
May 2012
SDAA Business MeetingNext meeting will be held at:3838 Camino del Rio North
Suite 300San Diego, CA 92108
May 8th at 7pmNext Program Meeting
May 16, 2012 at 7pmMission Trails Regional Park
Visitor and Interpretive Center1 Father Junipero Serra Trail
CONTENTS
Date: 16 May 2012Speaker: Jerry HilburnTopic: Mars Mission’s Update
Please join Jerry Hilburn for a presentation on the current status of the Mars missions and more info on the upcoming Curiosity landing scheduled to arrive at the Red Planet on Aug. 6, 2012 (EDT). We will cover the current rover mis-sion and MRO as well.
San Diego Astronomy Association (SDAA) sponsors speakers on a wide range of astronomy topics on the third Wednesday of every month at the Mis-sion Trails Regional Park Visitors Center. The Program meeting begins at 7:00 PM. Each attendee receives one free door prize ticket. After announcements and a small amount of business, the audience is treated to the featured presentation. At the close of the meeting the door prizes are presented. The event is open to the public. The Mission Trails Regional Park Visitors Center is at One Fr. Juni-pero Serra Trail, San Diego CA 92119. Call the park at 619-668-3281 for more information or visit http://www.mtrp.org. Please contact Bill Carlson ([email protected] or 425-736-8485) if you have any questions, comments, or ideas for the Program Meetings.
May 2012, Vol XLX, Issue 5Published Monthly by the San Diego Astronomy Association$2.50 an issue/$30.00 yearIncorporated in California in 1963May Prog ram Meet ing. . . . . . . . . . . . . . .1Apr i l M inu t e s. . . . . . . . . . . . . . . . . . . . . . . . . 2Annular Solar Eclipse.... . . . . . . . . . . . . . . .4Venus Trans i t . . . . . . . . . . . . . . . . . . . . . . . . . . 4May Calendar. . . . . . . . . . . . . . . . . . . . . . . . . . . .5SDAA Contac t s. . . . . . . . . . . . . . . . . . . . . . . 6ASIG Galler y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7We b O n l y - - - - - - - - - - - - - - - - - - - - - -J u l i a n S t a r f e s t . . . . . . . . . . . . . . . . . 9NASA Hepls Europe Study a Comet....10Simulating the Universe. . . . . . . . . . . .11Catching Stars that go BANG!.......12How the Milky Way Became Spiral.....13
Newsletter DeadlineThe deadline to submit articles
for publication is the15th of each month.
May Program Meeting
San Diego Astronomy Association
Page 2 SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
SDAA Board of Directors Monthly Business Meeting Minutes10 April 2012-Unapproved and Subject to Revision
1. Call to order. The meeting was called to order at 7:05 pm with the following board members in attendance: Michael Vander Vorst, President; Bill Carlson, Vice President; Ed Rumsey, Treasurer; Jeff Herman, Corresponding Secretary; Brian McFarland, Recording Secretary; Mike Finch, Director; Kin Searcy, Director; Paul “Moose” Pountney, Director; Bob Austin, Director.
2. Approval of Last Meeting Minutes. Approved
3. Priority / Member Business. None
4. Standard Reports.
Treasurer’s Report. • Treasurer’sreportapproved• Forinsurancepurposesweneedariskmanagementplanthatidentifiesrisksandmitigationsforsuchthingsasstarparties,TDSproperty & assets, website & newsletter copyright issues, etc. We need someone to chair this effort.• Replacedouroldcreditcardwithanewonewithahigherlimit.• Openedtwonewsavingsaccounts(oneforJSFthatisindependentof PayPal).
Membership Report. The membership total is now 538.
Site Maintenance Report.BillQuackenbushandBillCarlsonaregoingtoupgradethewifinodesonSaturdayApril14thif theweather cooperates. $450 maximum has been approved for this upgrade.
Observatory Report. Still waiting to hear back on paint. A few star parties cancelled due to dew.
Private Pad Report. No report.
Outreach Committee Report. Received a nice presentation from the Girl Scouts in appreciation of the star party at Heise.
Program Report. The IDA is presenting at the next program meeting.
NASA Robotic Observatory. No report.
AISIG Report. Scott Dixon hosted the last meeting. The April meeting will be held at MTRP and the topic will be equipment set up.
Governing Documents Report. No report.
Newsletter Report. No report.
Website Report. Need to begin to actively look for a replacement Webmaster. Bill Carlson will request volunteers at the next pro-gram meeting, and Michael Vander Vorst will pen an article for the newsletter.
Site Master Plan Committee. No report.
San Diego Astronomy Association
Page 3SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
Merchandise Report. Need to stock up for JSF (hates, shirts, etc). Moose will solicit estimates.
5. Old Business. • LightningBreweryAnniversaryandStarParty–LightningisnowlookingatAugustorSeptember.BrianMwillcontactJim Crute and try and get agreement for September.
6. New Business. • April28isAstronomyDay,butRHFleetisnotsupportingit.TheSDAAwilllookintomovingittoMTRPinstead.• TheSDAAwilltryandhostVenustransiteventsonJune5thatbothTDSandSycamoreCanyon.• TheSDAAwilltryandhostannulareclipseeventsonMay20thatbothTDSandSycamoreCanyon.• BobAneeds$50fora12mmeyepiecefortheclub’sPST(12mmistheoptimumeyepieceforthisscope).• WewillhaveaboothatOPT’sastronomyeventinJuly.
7. Adjournment. Meeting adjourned at 8:30 pm.
Dates set for KQ Ranch and Wm Heise County Park. They are as follows:
Page 4 SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
Public Outreach Announcement: May 20, 2012 Annular Solar Eclipse
The SDAA will be making viewing of the May 20th Annular Solar Eclipse available to the public. We are planning two loca-tions, Sycamore Canyon Open Space Preserve and the SDAA Observing site at Tierra Del Sol.
We are looking for volunteers that can provide solar scopes, white light or hydrogen-alpha, for both locations. If you can help out with this public outreach event, please contact Bob Austin at [email protected] or 760-787-1174. Please include which location you can attend so we know we have both locations covered.
Venus Transit, 06-05-2012
A transit of Venus occurs when Venus passes directly between the sun and earth. The alignment is extremely rare coming in pairs of eight years apart and then not happening again for over 100 years. The most recent transit of Venus was in 2004 and the upcoming transit on June 5th, it will be the last chance to witness the event in your lifetime (the next chance will occur in 2117). The San Diego Astronomy Association (SDAA) is planning to give the public an opportunity to view this rare event through spe-ciallyfilteredtelescopes.Thesefiltersdecreasethebrightnessof thesun’slightthroughthetelescopetosafelevels.TheSDAAwillbeofferingtwolocationstoviewthetransit.ThefirstlocationwillbeatSycamoreCanyonOpenSpacePreserve at the Hwy 67 staging area. Entry to the Preserve is located ½ mile south of Scripps Poway Pkwy on Hwy 67, just east of Poway. This location could be prone to the coverage of San Diego’s June marine layer and the transit may not be visible if the marine layer clouds roll in. Since this is our last chance to view such an alignment, the SDAA will have a second location for viewing that should not be affected by the marine layer. This second location will be the SDAA’s own observing site located in Boulevard in the far-east coun-ty of San Diego. The address is 961 Tierra del Luna Rd, Boulevard, CA, 91905 and directions can be found at http://docs.sdaa.org/_tds/TDS_Directions.pdf. Venus will begin to transit across the face of the Sun around 3:30 PM and will still be progressing across the Sun as it sets. John Kuhl will be coordinating our efforts to bring this to the public at TDS and Bob Austin at Sycamore Canyon. We’ve created an email address to let us know if you can help with this public outreach event, [email protected]. Please specify which location you’ll be attending because the email goes to both John and Bob. We look forward to hearing from you.
San Diego Astronomy Association
Page 5SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
May 2012 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
1
2 Stars in the Park
3
4 Fletcher
Elementary Alpine Academy
Julian Charter
5
6 Full Moon
7
8 SDAA Business
Meeting
9 Pack 295
10
11 Stars at Mission
Trails
12 Space Day
13
14
15 Advanced Training
Associates
16 SDAA Program
Meeting
17
18 Stars At
Sycamore Canyon
19 Heise Camp with
the Stars
20 Annular Eclipse
of the Sun
New Moon
21
22
23 AISIG Meeting
24
25 Pazaaz Star Party
26
27
28
29
30
San Diego Astronomy Association
Page 6 SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
Have a great new piece of gear? Read an astronomy-related book that you think others should know about? How about a photograph of an SDAA Member in action? Or are you simply tired of seeing these Boxes in the Newsletter rather than something, well, interesting?
Join the campaign to rid the Newsletter of little boxes by sharing them with the membership. In return for your efforts, you will get your very own by line or pho-tograph credit in addition to the undying gratitude of the Newsletter Editor. Just send your article or picture to [email protected].
Page 7SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
AISIG Gallery
Scott Dixon captured this image of the Jelly Fish Nebula from his home observatory in Poway. The AISIG group had a great meeting at his home in March and toured his home observatory.
San Diego Astronomy Association
Page 8 SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
MEMBERSHIP INFORMATIONSend dues and renewals to P.O. Box 23215, San Diego, CA 92193-3215. Include any renewal cards from Sky & Telescope or Astronomy magazine in which you wish to continue your subscription. The expiration date shown on your newsletter’s mailing label is the only notice that your membership in SDAA will expire. Dues are $60 for Contributing Memberships; $35 for Basic Membership; $60.00 for Private Pads; $5 for each Family membership. In addition to the club dues the annual rates for magazines available at the club discount are: Sky & Telescope $32.95 and Astronomy $34. Make checks payable to S.D. Astronomy Assn. PLEASE DO NOT send renewals directly to Sky Publishing. They return them to us for processing.
Scott also imaged the colorful Seagull Nebula, IC 2177, from Poway using narrowband techniques.
San Diego Astronomy Association
Page 9SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
San Diego Astronomy Association
Page 10 SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
NASA Helps Europe Study a Comet–Up Close and Personal
By Dr. Tony Phillips
Europe’s Rosetta spacecraft is on its way to intercept comet 67P/Churyumov-Gerasimenko. Comets have been intercepted before, but this mission is different. Rosetta aims to make history by landing a probe on the comet’s surface while the mother ship orbits over-head.“RosettaistheEuropeanequivalentof aNASAflagshipmission,”explainsClaudiaAlexander,projectscientistfortheU.S.RosettaProject at NASA's Jet Propulsion Laboratory. “It will conduct the most comprehensive study of a comet ever performed.” Rosetta’s payload contains 21 instruments (11 on the orbiter, 10 on the lander) designed to study almost every aspect of the comet’s chemistry, structure, and dynamics. Three of the sensors were contributed by the U.S.: Alice (an ultraviolet spectrometer), IES (an ion and electron sensor), and MIRO (a microwave sounder). The main event of the mission will likely be the landing. The 100-kg lander, which looks a bit like a cross between NASA’s old Vi-king Mars landers and a modern microsatellite, will spend two weeks fastened to the comet’s icy surface. The European-built probe will collect samples for analysis by onboard microscopes and take stunning panoramic images from ground level. “First the lander will study the surface from close range to establish a baseline before the comet becomes active,” explains Alexan-der.“Thentheorbiterwillinvestigatetheflowof gasanddustaroundthecomet'sactive,ventingnucleus.” Rosetta’s sensors will perform the experiments that reveal how the chemicals present interact with one another and with the solar wind. Alice and MIRO detect uncharged atoms and molecules, while IES detects the ions and electrons as the solar wind buffets the nucleus. One problem that often vexes astronomers when they try to study comets is visibility. It’s hard to see through the dusty veil of gas billowing away from the heated nucleus. The microwaves MIRO detects can penetrate the dust, so MIRO can see and measure its target molecules even when other instruments can’t. MIRO is one of several experiments focused on the comet’s structural properties. It will determine the comet’s dielectric constant, emissivity, and thermal conductivity to determine whether it is made of a powdery loose material, has a detectable layer of loose mate-rial, or is hard as rock. “Wewanttofindoutwhethercometshaveretainedmaterialfromwhen the solar system formed,” says Alexander. “If the ancient materi-als are still there, we can get an idea of what conditions were like at the dawn of the solar system.” Rosetta enters orbit in 2014. Stay tuned for updates! Check out “Comet Quest,” the new, free iPhone/iPad game that has you operating the Rosetta spacecraft yourself. Get the link at space-place.nasa.gov/comet-quest.
This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronau-tics and Space Administration.
Rosetta’s lander Philae will eject from the spacecraft, touch down on the comet’s nucleus, and immediately fireaharpoonintothesurfacetoanchoritself soitwon’t drift off in the weak gravity.
San Diego Astronomy Association
Page 11SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
Astronomy has historically been an observational
rather than a laboratory or experimental science. Ex-
cept for lunar scientists or meteorite collectors (who
can touch rocks from other solar system bodies), most
astronomers can only point large telescopes, collect
electromagnetic radiation, focus it onto detectors, and
analyze what they observe.
But now supercomputers and powerful computa-
tional techniques increasingly allow astrophysicists to
experiment with the initial conditions and physical
laws for astronomical processes, including the forma-
tion of the universe. Indeed, large cosmological simu-
lations—computational working models—are the ba-
sis for much current astrophysical research.
Latest and greatest: Bolshoi The most accurate cosmological simulation yet
made of the evolution of the large-scale structure of
the early universe is being described in a series of re-
search papers that began to be published in Astro-
physical Journal and other journals in October 2011.
Named “Bolshoi”—the Russian word for “great” or
“grand”—the simulation models the evolution of a
representative volume
of the universe about 1
billion light-years on a
side, a volume that
would contain over a
million galaxies. The
computer code took 6
million CPU hours to
run on the Pleiades
supercomputer at
NASA Ames Research
Center. The calculated
results—spectacular
visualizations of what
the universe was like
at 180 different times
from the Big Bang to
the present epoch—
were saved for later
analysis. Some of the raw data plus detailed summa-
ries and analyses of the outputs are now publicly
available to the world’s astrophysicists.
Co-principal investigators Joel R. Primack
(University of California, Santa Cruz) and Anatoly
Klypin (New Mexico State University) based the Bol-
shoi simulation on both the most precisely known ob-
servational data and the most robust physical theory.
For observation, Primack and Klypin based the
Bolshoi simulation on a meticulous data set combining
ground-based observations with an extended run from
the highly successful NASA Explorer mission WMAP
(the Wilkinson Microwave Anisotropy Probe).
WMAP measured the detailed anisotropy (unevenness
of temperature and other characteristics) over the
whole sky of the cosmic microwave background radia-
tion left over from the Big Bang that formed the uni-
verse 13.7 billion years ago. The anisotropy reveals a
wealth of information about the history and composi-
tion of the early universe.
For theory, the Bolshoi simulation is based on the
Lambda Cold Dark Matter cosmogony (CDM for
short), now accepted as the standard modern theoreti-
cal framework for understanding the formation of the
large-scale structure in the universe. Ordinary atomic
matter makes up less than 5 percent of the universe;
only about half a percent is visible as stars, nebulae,
dust, and planets. Some 23 percent of the universe is
made of invisible, transparent “cold dark matter,” felt
only through its gravitational influence. CDM pre-
dicts that repeated mergers of smaller clumps of dark
matter end up creating bigger dark matter “halos,”
within which galaxies and clusters of galaxies form
and congregate. The Greek letter lambda () in
CDM represents the fact that 72 percent of the uni-
verse is “dark energy,” causing the universe’s expan-
sion to accelerate. Since CDM says the universe is
mostly made of invisible dark matter and dark energy,
it might better be called the Double Dark theory.
Revealing the invisible Thus, the Bolshoi simulation models not just how
the minority of the visible universe of stars, gas, and
dust evolved, but also how the vast majority of the
invisible universe evolved—rendering the invisible
visible for astronomers to study, and to predict struc-
tures that astronomers can seek to observe.
- Trudy E. Bell, M.A.
Further reading: Details about the Bolshoi simulation
appear at http://hipacc.ucsc.edu/Bolshoi/ .
Simulating the Universe
AstroShort
Snapshot from the Bolshoi simula-
tion at a red shift z=0 (meaning at
the present time), showing filaments
of dark matter along which galaxies
are predicted to form. CREDIT: Anatoly Klypin (New Mexico
State University), Joel R. Primack
(University of California, Santa Cruz),
and Stefan Gottloeber (AIP, Germany).
The University of California High-Performance AstroComputing Center (UC-HIPACC), based at the University of California, Santa Cruz, is a consortium of nine University of California cam-puses and three Department of Energy laboratories (Lawrence Berkeley Laboratory, Lawrence Livermore Laboratory, and Los Alamos National Laboratory). UC-HiPACC fosters collabora-tions among researchers at the various sites by offering travel and other grants, co-sponsoring conferences, and drawing attention to the world-class resources for computational astronomy within the University of California system. More information appears at http://hipacc.ucsc.edu .
San Diego Astronomy Association
Page 12 SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
About once a century in any galaxy, a star spontaneously
explodes—so brilliant that for a few days it can outshine all
other stars in a small home galaxy. Although frequent by
cosmic standards, supernovae are rare in human terms:
since the invention of the telescope, none has been seen to
explode in our Milky Way.
So how can astronomers study such catastrophic stellar
suicides, especially the hours immediately after ignition?
Answer: Partner two automated telescopes with real-
time supercomputing to monitor tens of thousands of galax-
ies every night, so that statistically there’s a high chance of
spotting a star exploding in some galaxy.
“Just since April 2009, we’ve discovered over 1,300
supernovae!” exclaimed Peter Nugent, senior staff scientist
at Lawrence Berkeley National Laboratory and principal
investigator of the Palomar Transient Factory (PTF) Type Ia
supernova program (supernovae come in different types).
How it works
Atop Palomar Mountain, the Samuel Oschin telescope—
a 48-inch (1.2-meter) Schmidt—acts as an automated wide-
field survey camera, snapping sequential exposures of 8
square degrees across the night sky. Each minute or so, its
sensitive CCD 101-megapixel sensor array records stars and
galaxies as faint as 20th magnitude.
Each digital image is instantly beamed to the San Diego
Supercomputing Center at the University of California, San
Diego, and then 400+ miles north to the National Energy
Research Scientific Computing Center (NERSC) at Law-
rence Berkeley National Laboratory. Within minutes, super-
computers subtract each incoming image from reference
images, comparing new sources of light to the Sloan Digital
Sky Survey and other databases.
“We collect about 50 gigabyes of raw data per night,”
Nugent says, “and typically discover about a million things
that vary. The vast majority of them are ‘garbage’—known
variable stars, asteroids, etc. But one or two per night are
young supernovae!”
Coordinates of suspected supernovae are forwarded 500
miles back down to Palomar to a 60-inch photometric tele-
scope for detailed brightness measurements that same
night—and possibly also to 15 other telescopes around the
world for spectroscopic observation.
Brilliant discovery
The PTF’s most spectacular find so far made newspaper
headlines last summer: on August 24, 2011, a supernova
(SN 2011fe) brightening in the Pinwheel Galaxy in Ursa
Major only 21 million light-years away. The nearest and
brightest Type Ia supernova to be spotted by the PTF, on
September 10th, it peaked at visual magnitude 9.9.
In a paper published in Nature on December 15th, 2011,
Nugent and coauthors conclude that SN 2011fe was a white
dwarf star 1.4 times as massive as the sun, but only the di-
ameter of Earth. It was stealing gas from a close sun-like
companion until a runaway thermonuclear explosion ig-
nited. Found only 11 hours (plus 21 million years!) after it
exploded, it was the youngest supernova ever detected.
–Trudy E. Bell, M.A.
Catching stars that go BANG!
AstroShort
The beautiful Pinwheel Galaxy in the constellation Ursa Major
(the Big Dipper) is shown the night before supernova SN 2011fe
exploded on August 22, 2011 (left), half a day after it exploded
(middle) on August 23, and a day later (right) on August 24 (green
arrows). The supernova reached maximum brightness on Septem-
ber 10, 2011, and then began fading. It was both the nearest and
the youngest supernova discovered by the Palomar Transient
Factory, being discovered only 11 hours after it detonated.
The Carver IBM iDataPlex supercomputer at NERSC at Lawrence
Berkeley National Laboratory does much of the real-time analysis
of images for the PTF, comparing digital images taken with the 48
-inch Samuel Oschin telescope on Mount Palomar with reference
images to identify supernovae. It found SN 2011fe.
The University of California High-Performance AstroComputing Center (UC-HIPACC), based at the University of California, Santa Cruz, is a consortium of nine University of California cam-puses and three Department of Energy laboratories (Lawrence Berkeley Laboratory, Lawrence Livermore Laboratory, and Los Alamos National Laboratory). UC-HiPACC fosters collabora-tions among researchers at the various sites by offering travel and other grants, co-sponsoring conferences, and drawing attention to the world-class resources for computational astronomy within the University of California system. More information appears at http://hipacc.ucsc.edu .
San Diego Astronomy Association
Page 13SAN DIEGO ASTRONOMY ASSOCIATION NEWS AND NOTES, MAY 2012
Astronomers have long known that the Milky Way is a
spiral galaxy. But how did our home galaxy get its beautiful
spiral arms?
A simulation run on the GreenPlanet supercomputer
cluster at the University of California, Irvine suggests its
spiral structure may have been triggered by an act of cosmic
violence: a series of collisions with a dwarf galaxy.
Dwarf galaxy, big impact
Since 1994, it’s been known that the Sagittarius Dwarf
galaxy—named after the constellation in which it appears
from Earth—is in a polar orbit around the Milky Way and
in the process of
merging with our
galaxy. In 2003,
infrared tele-
scopes and su-
percomputers
that traced the
orbital motions
of its stars re-
vealed that the
Sagittarius
Dwarf had actu-
ally collided with
the Milky Way
twice—once 1.9
billion years ago
and again 0.9
billion years
ago—and that it
is now coming in
for a third colli-
sion in just an-
other 10 million
years.
Until recently,
most investiga-
tors have been
studying how the
Milky Way’s
tremendous
gravitational field and tidal forces are ripping the Sagittarius
Dwarf into long streamers of stars.
In computations for his dissertation research, however,
former Irvine graduate student Chris Purcell asked a differ-
ent question: What effects did the repeated collisions of the
Sagittarius Dwarf, with its invisible but massive halo of
dark matter, have on the larger Milky Way itself?
Dark matter, visible results
Ordinary matter makes up only 4.6 percent of the cosmic
density; only 0.5 percent is visible. Nearly five times that
much—23 percent—of the universe is made of invisible,
transparent “dark matter,” whose existence is felt through
its gravitational influence. It is now known that every gal-
axy, including the Sagittarius Dwarf (pre-collision) and our
own Milky Way, resides at the center of a giant halo of dark
matter several times larger in radius and many times greater
in mass.
Pre-collision,
the Sagittarius
Dwarf was quite
large—
somewhere in
number of stars
between the
Small and Large
Magellanic
Clouds (the
Milky Way’s two
irregular galaxy
companions visi-
ble to the naked
eye from the
southern hemi-
sphere). But its
dark matter mass
likely exceeded
the mass of all the
visible stars in the
Milky Way.
“When all that
dark matter first
smacked into the
Milky Way like a ghostly belly flop, 80 to 90 percent of it
was stripped off,” Purcell explained. “But the whirling disk
of stars that was the Milky Way at that time was a very
tenuous, chaotic system. That first impact produced insta-
bilities that were amplified and quickly formed spiral arms
and associated ring-like structures in the outskirts of our
Galaxy.”
Purcell’s paper, “The Sagittarius impact as an architect
of spirality and outer rings in the Milky Way,” which he
wrote with four coauthors (including his Irvine dissertation
advisor James S. Bullock), has been published as a Letter in
the September 15, 2011 issue of Nature.
–Trudy E. Bell, M.A.
Two computer-simulation movies of the impact appear in the press
release at http://hipacc.ucsc.edu/MilkyWayImpact.html (scroll
down to the bottom).
How the Milky Way became spiral?
AstroShort
Incoming third impact of the Sagittarius
Dwarf galaxy (blue stream of stars) with our
Milky Way Galaxy (multicolored disk) was
simulated by the GreenPlanet supercomputer
cluster at the University of California, Irvine,
and rendered by co-author Erik J. Tollerud
against a background of galaxies seen in the
Hubble Deep Field. Note the simulated disk’s
ring-like spiral extensions in the outer Milky
Way (upper left), which strongly resemble
actual streams found at low latitudes with
respect to the disk plane, in the nearby region
of the Milky Way viewed from the Earth in
the opposite direction from the center of the
Galaxy. According to a Letter by Chris W.
Purcell and coauthors in the British journal
Nature, those spiral arms began to emerge
after the initial impact of the Sagittarius
Dwarf galaxy nearly two billion years ago.
Computer simulations visualized the disk of
the Milky Way galaxy for three cases: no
impact with a dwarf galaxy, impact with a
Sagittarius Dwarf galaxy of lower mass
(Light Sgr), and impact with a Sagittarius
Dwarf galaxy of higher mass (Heavy Sgr).
Our Milky Way galaxy is shown both edge-
on and face-on in the inset panels; the sun’s
location is marked as a yellow dot and the
present location of the Sagittarius dwarf’s
remnant core is marked as a pink dot, as
shown after more than two billion years of
isolated evolution. Shown in the background
is a global rendering of the ‘Light Sgr’ tidal
debris and the Milky Way disk.
The University of California High-Performance AstroComputing Center (UC-HIPACC), based at the University of California, Santa Cruz, is a consortium of nine University of California cam-puses and three Department of Energy laboratories (Lawrence Berkeley Laboratory, Lawrence Livermore Laboratory, and Los Alamos National Laboratory). UC-HiPACC fosters collabora-tions among researchers at the various sites by offering travel and other grants, co-sponsoring conferences, and drawing attention to the world-class resources for computational astronomy within the University of California system. More information appears at http://hipacc.ucsc.edu .