san diego astronomy association · outreach committee report. received a nice presentation from the...

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

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.



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:

Heise KQ Ranch 5/19 5/26 6/23 6/9 7/14 7/28 8/11 8/25 9/8 9/22 10/6 10/20



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.



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



SDAA ContactsClub Officers and Directors

President Michael Vander Vorst [email protected] (858) 755-5846Vice-President Bill Carlson [email protected] (425) 736-8485Recording Secretary Brian McFarland [email protected] (619) 462-4483Treasurer Ed Rumsey [email protected] (858) 722-3846Corresponding Secretary Jeff Herman [email protected] (619) 846-4898Director Alpha Bob Austin [email protected] (760) 787-1174Director Beta Paul “Moose” Pountney [email protected] (619) 465-7014Director Gamma Michael Finch [email protected] (760) 440-9650Director Delta Kin Searcy [email protected] (858) 586-0974

CommitteesSite Maintenance Bill Quackenbush [email protected] (858) 395-1007Observatory Director Jim Traweek [email protected] (619) 207-7542Private Pads Mark Smith [email protected] (858) 484-0540Outreach Kin Searcy [email protected] (858) 586-0974N. County Star Parties Doug McFarland [email protected] (760) 583-5436S. County Star Parties -Vacant- [email protected] E. County Star Parties -Vacant- [email protected] Central County Star Parties Kin Searcy [email protected] (858) 586-0974Camp with the Stars Doug McFarland [email protected] (760) 583-5436K.Q. Ranch Coordinator Michael Vander Vorst [email protected] (858) 755-5846Newsletter Andrea Kuhl [email protected] (858) 547-9887New Member Mentor Jerry Hilburn [email protected] (858) 565-4059Webmaster Bob Austin [email protected] (760) 787-1174AISIG Kin Searcy [email protected] (858) 586-0974Site Acquisition Jerry Hilburn [email protected] (858) 565-4059Field Trips Bill Carlson [email protected] (425) 736-8485Grants/Fund Raising Jerry Hilburn [email protected] (858) 565-4059Merchandising Paul “Moose” Pountney [email protected] (619) 465-7014Publicity Jerry Hilburn [email protected] (858) 565-4059Roboscope Director Jerry Hilburn [email protected] (858) 565-4059Governing Documents Scott Baker [email protected] (858) 442-7513TDS Network Bill Carlson [email protected] (425) 736-8485Amateur Telescope Making Peter De Baan [email protected] (760) 745-0925

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].

SDAA Editorial StaffEditor - Andrea [email protected]

Assistant Editor: Rick ImbraContributing Writers

Trudy Bell Dr. Tony Phillips



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.



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.



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.



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 .



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.




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.
