Investigation Three: Model the Milky Way Galaxy

(Source: Stars: A Classroom Investigation – Jill Bechtold, University of Arizona) Objective Use salt grains to make a model of the Milky Way and locate our Solar System in one of the spiral arms. Materials *Kosher salt *Blue glitter *Cubic centimeter boxes *Colored dots (yellow) *Scissors *Glue stick Procedure 1. Fill the centimeter cube boxes with kosher salt. (Remember, a centimeter cube box holds 1/200 billionths of the stars in the galaxy. 2. Make a model of the Milky Way (using the diagram on the back of the paper) by drawing the dense center of the galaxy and spiral arms of the Milky Way with the glue stick. 3. Sprinkle salt over drawing. Shake paper to distribute. Press in. 4. Sprinkle small amount of glitter on drawing. 5. Look at the picture of the Milky Way Galaxy. Locate our Solar System and place a yellow dot at the proper location.

Question On a separate sheet of paper (full 8½“ x 11” sheet of paper), each group member should answer the following questions:

Compare the size of the Solar System to the size of the Milky Way. There are 200,000,000,000 stars in the Milky Way. Do you think there are other

planets orbiting stars in the Milky Way? Explain your answer.

(Source: Stars: A Classroom Investigation – Jill Bechtold, University of Arizona)

Select and answer one of the following. Your answer should include, at a minimum, one complete paragraph.

Investigate what is at the center of the galaxy. Describe the Milky Way galaxy. Report on a space probe that has studied the Milky Way.

Imagine Home | Ask an Astrophysicist | Black Holes at the Center of the Galaxy

(Submitted October 28, 1996)

I am an undergraduate in Astrophysics at the University of Calgary. I am doing a smallresearch project on the evidence for and against a black hole at the center of the milky way. Ifound your email address on the StarChild page dealing with this topic. I was wondering if youhad any suggestions of articles or books discussing this subject. Thank you for your time.

It is generally believed that a black hole does exist at the center of the Milky Way galaxy. Thelatest value we have seen is that it has a mass of about 2,000,000 that of the Sun. In fact, it isbelieved that this may be common for most galaxies. Observational evidence supports theseideas more and more. However, you must keep in mind that due to the large absorption andsource confusion when trying to look into the center of a galaxy, it is very, very hard to seewhat's there! So we have to be clever about the observations we make and the interpretationsof these observations. This is one reason that X-rays and gamma-rays are powerful probes intrying to answer such questions; they are much more likely to "get out" of the central region ofthe galaxy than other wavelengths.

Some references you may find useful (and which give many more references) are:

Black Holes at the Center of the Galaxy http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961028b.html

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Imagine Home | Ask an Astrophysicist | Sagittarius A*

(Submitted November 30, 1996)

What is known about Sagittarius A*,the center of our galaxy ?

Sgr A* at the center of Milky Way is probably a massive black hole of about a million solarmasses (the mass of the Sun). The mass is estimated from the motion of gas and stars in theregion. Although Sgr A* is gathering mass from its vicinity at a rate of about 10E-4 solarmasses per year, a rather high rate, it is not as bright as would be expected. Therefore, SgrA* is extremely inefficient (one part in 100,000) in converting the gravitational energy of thegathered material into radiation.

Koji Mukaiwith helps from Drs. Chen, Loewenstein and Snowden

Prev Main Next

If words seem to be missing from the articles, please read this.

Sagittarius A* http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961130b2.html

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The Milky Way galaxy is the spiral galaxy we callhome, as do roughly 100 billion other stars. It looksvery much like other spiral galaxies when viewedfrom above. There are spiral arms and a brightcentral part. The Sun is far from the center of theGalaxy, halfway to the edge of the Galaxy along theOrion spiral arm.

The Sun is revolving around the center of the Galaxyat a speed of half a million miles per hour, yet it willstill take 200 million years for it to go around once.Do you feel like you are moving at that speedthrough space? If you did, you would certainly needa seat belt! When we run, we feel the wind on ourbodies because there are molecules which make upthe air that push against our bodies. But there arevery few molecules in the space between the stars.So there is nothing to push against our planet sothat we "feel" like we are rushing around at half amillion miles per hour.

Like other spiral galaxies, the Milky Way has a bulge,a disk, and a halo. Although all are parts of the samegalaxy, each contains different objects. The halo andcentral bulge contain old stars and the disk is filledwith gas, dust, and young stars. Our Sun is itself afairly young star at only 5 billion years old. The MilkyWay galaxy is at least 5 billion years older than that.

The Milky Way Galaxy - Our Home http://www.windows.ucar.edu/tour/link=/the_universe/Milkyway.html

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This false-color image shows the near-infrared sky as seenby the COBE satellite. The dominant source of light atthis wavelength is stars in our galaxy. So, you end up withan image that shows the thin disk and the central bulge ofthe Milky Way Galaxy. Our Sun lies in the plane of the disk,28,000 light years from the center which is why the MilkyWay disk appears edge-on to us.Click on image for full size (25K JPEG)Image courtesy of NASA

How far across is the Milky Way? At a given time how can one locate the center of our galaxy as we orbitthe Sun and the Sun orbits the galaxy? Is our solar system moving away from the center of the galaxy?How many years does it take the Sun to orbit the center of the galaxy? Specifically, where in the universecan our solar system be found?

The Milky Way galaxy is the home of the Sun and our solar system. Thereare 200 billion other stars in the Milky Way galaxy too. Our galaxy is aspiral galaxy, with a bulged center and arms that start in the center andform a flat pinwheel shape. The galaxy is about 90,000 light-years across.The Sun is located about two-thirds of the way out from the center in theOrion Arm.

The Sun (and our solar system) is revolving around the center of theGalaxy at a speed of half a million miles per hour, but it still takes 200million years for it to go around once. As far as we can tell, the Sun andthe solar system are not moving away from the center of the galaxy. TheSun has made less than 25 trips around the galaxy in its lifetime.

The center of our galaxy is located about 28,000 light-years away, beyondthe constellation Sagittarius (actually just beyond the border of Sagittariusand Scorpio). So, if you can locate these two constellations in the sky,you'll be looking toward the center of our galaxy!

The Milky Way is part of a set of galaxies known as the Local Group,which includes several dozen different galaxies within 3 million light-years.Only one of these, the Andromeda galaxy, is close to the size of the MilkyWay. This Local Group is part of a supercluster, known as the Virgosupercluster, which has at least 5,000 member galaxies and is roughly

100 million light-years across. Beyond this level of organization, not much is known about our position in the universe.

Submitted by Jim (age 61, Tennessee, USA), Amy (age 19, Massachusetts, USA), Stephanie (age 14, Colorado, USA), Jo (age 21),Bharat (age 52, India)(July 17, 2001)

How far across is the Milky Way? At a given time how can one locate the center of our galaxy a... http://www.windows.ucar.edu/tour/link=/kids_space/milky_way_ask.html

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Kepler: NASA’s First Mission Capable of Finding Earth-Size and Smaller PlanetsKepler will be the first space mission to search for Earth-size and smaller planets in the habitable zone of other stars in our neighborhood of the galaxy. Kepler is a special-purpose spacecraft that precisely measures the light variations from thousands of distant stars, looking for planetary transits. When a planet passes in front of its parent star, as seen from our solar system, it blocks a small fraction of the light from that star—this is known as a transit. Searching for transits of distant “Earths” is like looking for the drop in brightness when a moth flies across a searchlight. Measuring repeated transits, all with a regular period, duration and change in brightness, provides a method for discovering and confirming planets and their orbits—planets the size of Earth and smaller in the habitable zone around other stars similar to our Sun.

The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery in the past decade of more than 250 giant planets orbiting

stars beyond our solar system. With the exception of the pulsar planets, most of the extrasolar planets detected to date are gas giants. The challenge is to find terrestrial planets, which are 30-600 times less massive than Jupiter. Kepler is specifically designed to search for Earth-size and smaller planets in the habitable zone of solar-like stars out to distances of about three thousand light years.

Expected Results

Kepler will continuously monitor over 100,000 stars similar to our Sun for brightness changes produced by planetary transits. At the beginning of the mission, planets of all sizes orbiting very close to their stars will be found. After three years, we will be able to discover planets with orbits of one year, that is those in the habitable zone of stars like the Sun. If Earth-size planets in the habitable zone are common, then life may be ubiquitous in our galaxy. On the other hand, if no terrestrial planets are found, then “Earths” may be rare. N

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Three or more transits of a given star all with a consistent period, brightness change and duration provide a rigorous method of detection and confirmation. The data will reveal the planet’s:

Size from the brightness change and size of the star;•Orbital period from the time between transits;•Orbital size from the mass of the star and the period;•Temperature from the planet’s orbit and the temperature •of the star.

From the data we can calculate the fraction of stars that have planets, and the distributions of planetary sizes and orbits for many different types of stars.

The results will tell us how often planets occur in the habitable zone of other stars. If common, then hundreds of Earth-size planets in the habitable zone and thousands outside the habitable zone will be detected.

The Spacecraft

The Kepler spacecraft consists of a spacecraft bus and a single instrument called a photometer, that is, a light meter, which can simultaneously measure the brightness variations of over 100,000 stars with a precision of about 20 parts per million (ppm). This precision allows detection of Earth-like transits, which cause a change in brightness of 84 ppm of a solar-like star that lasts for a few hours to about half of a day. The photometer is so sensitive that planets as small as Mars can be detected when they occur in short-period orbits like many of the giant planets already discovered. So as not to miss any transits, Kepler will stare at the same star field in the Cygnus-Lyra region for the entire mission.

Kepler’s aperture is nearly one meter in diameter and Kepler will be the largest Schmidt-type telescope ever launched. Schmidt optics have an unusually large field of view. Kepler’s will be bigger than an open hand held at arm’s length. The detectors used are charged coupled devices (CCDs) similar to those found in consumer digital cameras. However, unlike an ordinary digital camera with a few megapixels, Kepler has an array of 95 megapixels.

Scientific Community Involvement

There are three ways for the broader scientific community to participate in the mission via NASA Research Opportunities. Scientists will be invited to propose to:

Conduct complementary investigations that support the •planetary detection science of Kepler;Use • Kepler to observe other types of astrophysically interesting objects in its field of view, such as variable stars, quasars and galaxies; andAnalyze the unique • Kepler data archive for phenomena relating to stellar activity.

The archive will contain three and one-half or more years of continuous observations with unprecedented photometric precision of stars. For example, such data are useful for estimating how often stars like our Sun could cause a climate change like that which brought on the mini-ice age in the 17th century.

Education and Public Outreach Program

The EPO program leverages pre-existing collaborations, networks, and team experience to maximize the development and impact of EPO products and activities. It includes:

Formal Programs—• Hands On Universe for grades 9-12; KeplerNET, an undergraduate consortium; and Great Explorations in Math and Science (GEMS) Space Science Sequence for grades 3-5 and 6-8. GEMS reaches thousands of teachers through over 80 GEMS sites/centers nationwide and worldwide;Informal Programs—Exhibits and programs for science •and technology museums and planetaria; andPublic Outreach Programs—Kits for amateur astronomers •via the Night Sky Network; nationally broadcast science documentaries; and StarDate radio programs.

Mission Organization and Status

The Kepler Mission was competitively selected in December 2001 as NASA’s tenth Discovery mission. NASA Ames Research Center is responsible for the data analysis and scientific interpretation of the data, the development of the ground system and management of the operations phase. NASA’s Jet Propulsion Laboratory is responsible for managing the development phase. Ball Aerospace and Technologies Corporation is responsible for developing the photometer and spacecraft and supporting mission operations.

As of fall 2007, all of the flight hardware has been built. The Assembly Test and Launch Operations Readiness Review was held in September 2007. The Photometer environmental and performance testing was completed in April 2008. Integration and testing with the spacecraft was begun in May 2008. Launch is planned for March 2009.

Kepler Discovery Mission

Science Principal Investigator Project ManagerWilliam Borucki James FansonNASA Ames Research Center Jet Propulsion Laboratory

Kepler will find planets by looking for tiny dips in the brightness of a star caused by planetary transits.

National Aeronautics and Space Administration

Ames Research CenterMoffett Field, California 94035 - 1000

www.nasa.gov

2 Kepler: Finding Earth-Size and Smaller Planets NASA Facts

Learn more at the Kepler web site: http://kepler.nasa.gov

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Marshall Space Flight Center Fact Sheet

Quick Facts: The Chandra X-ray Observatory

NASA’s newest space telescope, the Chandra X-ray Observatory, will allow scientists fromaround the world to obtain unprecedented X-ray images and spectra of violent, high-temperatureevents and objects to help us better understand the structure and evolution of our universe.

It will also serve as a unique tool to study detailed physics in a unique laboratory -- the universeitself – one that cannot be replicated here on Earth.

Managed by NASA’s Marshall Space Flight Center in Huntsville, Ala., Chandra is a sophisticated,state-of-the-art instrument that represents a tremendous technological advance in X-rayastronomy.

Did you know?

The Chandra X-ray Observatory is the world’s most powerful X-ray telescope. It haseight-times greater resolution and will be able to detect sources more than 20-times fainterthan any previous X-ray telescope. The Chandra X-ray Observatory, with its Inertial Upper Stage and support equipment, is

the largest and heaviest payload ever launched by the Space Shuttle. The Chandra X-ray Observatory’s operating orbit takes it 200-times higher than the HubbleSpace Telescope. During each orbit of the Earth, Chandra travels one-third of the way tothe Moon. The Chandra X-ray Observatory’s resolving power is – 0.5 arc-seconds -- equal to theability to read the letters of a stop sign at a distance of 12 miles. Put another way,Chandra’s resolving power is equivalent to the ability to read a 1-centimeter newspaperheadline at the distance of a half-mile. If the State of Colorado were as smooth as the surface of the Chandra X-ray Observatory

mirrors, Pike’s Peak would be less than an inch tall. Another of NASA’s incredible time machines, the Chandra X-ray Observatory will be able

to study some quasars as they were 10 billion years ago. The Chandra X-ray Observatory will observe X-rays from clouds of gas so vast that it

takes light more than five-million years to go from one side to the other. Although nothing can escape the incredible gravity of a black hole, not even light, theChandra X-ray Observatory will be able to study particles up to the last millisecond beforethey are sucked inside. It took almost four centuries to advance from Galileo’s first telescope to NASA’s HubbleSpace Telescope — an increase in observing power of about a half-billion times. NASA’sChandra X-ray Observatory is about one-billion times more powerful than the first X-raytelescope, and we have made that leap in slightly more than three decades.

Chandra Mission at a Glance:

Chandra X-ray Observatory Mission Duration

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The Gamma-ray Large Area Space Telescope (GLAST) is an inter-national and multi-agency space mission that will study the cosmos in the energy range 10 keV to > 300 GeV. The main instrument, the Large Area Telescope (LAT), has superior area, angular resolution, field of view, and deadtime that together provide a factor of 30 or more advance in sensitivity relative to the EGRET instrument on the Compton Observatory, as well as provide capability for study of tran-sient phenomena. The GLAST Burst Monitor (GBM) has a field of view several times larger than the LAT and provides spectral cover-age of gamma-ray bursts that extends from the lower limit of the LAT down to 8 keV. With the LAT and GBM, GLAST is a flexible observa-tory for investigating the great range of astrophysical phenomena best studied in high-energy gamma rays.

The anticipated advances in astronomy and physics with GLAST are among the central subjects of NASA’s Beyond Einstein program and the Department of Energy’s particle physics research program. The GLAST mission is also supported by the physics and astrophysics programs in the partner countries of France, Germany, Italy, Japan, and Sweden. The mission is supported by a vigorous, multidisci-plinary guest investigator program to maximize the discovery poten-tial. GLAST was successfully launched June 11, 2008.

The Whole Sky: With its very large field of view, the LAT sees ~20% of the sky at any time. In sky survey mode, which is the primary observing mode, the LAT covers the entire sky every three hours. The observatory can also be pointed as needed and can slew au-tonomously when sufficiently bright gamma-ray bursts are detected onboard by either instrument.

Guest Investigator Program: The Guest Investigator and GLAST Fellows Program are planned to start in 2008. For further infor-mation, see http://glast.gsfc.nasa.gov/ssc/. Information about the GLAST Users’ Group can be found at: http://glast.gsfc.nasa.gov/ssc/resources/gug/.

With its large leap in capabilities, GLAST is addressing many impor-tant science topics including: Active Galactic Nuclei and their jets, Gamma-ray bursts, pulsars, the origin of cosmic rays, probing the era of galaxy formation and the optical-UV Extragalactic Background Light (EBL), searches for signals of new phenomena, including par-ticle dark matter annihilations and other topics in particle astrophys-ics, EGRET unidentified gamma-ray sources, and solar flares. We expect that, with its capabilities, GLAST will also yield important un-anticipated findings.

Project Management: Goddard Space Flight Center Project Scientist: Steve Ritz, GSFC Project Manager: Kevin Grady, GSFCLAT Management: Stanford Linear Accelerator Center LAT PI: Peter Michelson, StanfordGBM Management: Marshall Space Flight Center GBM PI: Charles Meegan, MSFC GBM Co-PI: Jochen Greiner, MPEInternational Partners: France, Germany, Italy, Japan, and SwedenGLAST Science Support Center: Goddard Space Flight CenterGLAST Users’ Group: Chair: Josh Grindlay, HarvardEducation and Outreach: Lynn Cominsky, Sonoma State UniversitySpacecraft Contractor: General Dynamics Advanced Information SystemsFor More Information: Mission: http://glast.gsfc.nasa.gov/ LAT: http://glast.stanford.edu/ GBM: http://gammaray.nsstc.nasa.gov/gbm/ GSSC: http://glast.gsfc.nasa.gov/ssc/ EPO: http://glast.sonoma.edu/

Large Area Telescope(LAT) Requirements*

EGRET

Energy Range 20 MeV to > 300 GeV 20 MeV to 30 GeV

Peak Effective Area1 > 8000 cm2 1500 cm2

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Source Location Determination4 < 0.5' 15'

Point source Sensitivity5 < 6 x 10-9 cm-2 s-1 ~ 10-7 cm-2 s-1

* For more info see: http://www-glast.slac.stanford.edu/software/IS/glast_lat_performance.htm1 After background rejection and all selections, on-axis, 1 - 10 GeV2 Single photon, 68% containment, on-axis 3 1σ, on-axis, 100 MeV - 10 GeV4 1σ radius, flux 10-7 cm-2 s-1, > 100 MeV, high |b|, 1-year all sky survey, photon spectral index -25 > 100 MeV, high |b|, 1-year all sky survey, photon spectral index -2, 5σ detection

GLAST Burst Monitor(GBM) Requirements

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Energy Range 8 keV to > 25 MeV 25 keV to 10 MeV

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Burst Sensitivity2 < 0.5 cm-2 s-1 0.2 cm-2 s-1

Alert GRB Location3 ~15° (goal) ~ 25°

Burst Sensitivity On-board Trigger4 < 1.0 cm-2 s-1 0.3 cm-2 s-1

1 1σ, 0.1 - 1 MeV 2 50 - 300 keV, 5σ detection, ground analysis3 Calculated on-board; > 1 sec burst of 10 photons cm-2 s-1 , 50 - 300 keV 4 50% efficiency level for bursts within GBM FOV, excluding observational inefficiencies (e.g., SAA and Earth occultations), 50 - 300 keV

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Instrument Design

The instruments on the GLAST mission are the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The LAT has four subsystems: a solid state detector (sili-con strip) pair conversion tracker for gam-ma-ray detection and direction measure-ment, a CsI calorimeter for measurement of the energies, a plastic scintillator anticoinci-dence system to provide rejection of the in-tense background of charged particles, and a flexible trigger and dataflow system. The LAT is modular, consisting of a 4 × 4 ar-ray of identical towers with 880,000 silicon-strip detector channels. The GBM has 12 NaI scintillators and two BGO scintillators mounted on the sides of the spacecraft. The GBM will view the entire sky not occulted by Earth, with energy coverage from a few keV to 30 MeV, overlapping with the lower energy limit of the LAT and with the range of GRB detectors on previous missions.

Prepared by N. Gehrels, J.E. McEnery, J.D. Myers, and S. Ritz for the GLAST mission team.Revised: June 26, 2008

LAT

GBM - HE BGO(1 of 2)

GBM - LE NaI(3 of 12)

The Spitzer Space Telescope is the fourth andfinal element in the National Aeronautics and SpaceAdministration's family of orbiting "GreatObservatories,"which includes theHubble SpaceTelescope, theCompton Gamma-Ray Observatoryand the ChandraX-RayObservatory. Thisnew infraredobservatory is alsoan important sci-entific and techni-cal bridge toNASA's OriginsProgram – anattempt to addresssuch fundamentalquestions as"Where did wecome from? Are we alone?"

Turning Up the HeatAll objects in the universe with temperatures

above absolute zero – corresponding to zero degreesKelvin, which is about -273 Celsius, or -460Fahrenheit – emit some infrared radiation, or heat.Infrared wavelengths lie beyond the red portion of thevisible spectrum, and are invisible to the human eye.Trying to detect weak infrared light from astronomi-cal objects is a tricky business because telescopesproduce their own heat, referred to as infrared noise.

So astronomers have to chill their telescopes andinstruments to temperatures hovering just slightlyabove absolute zero to enable detection of any incom-

ing infraredradiation. Andsince mostinfrared lightemitted bycelestialobjects isabsorbed byEarth's atmos-phere, scien-tists look toorbiting tele-scopes to pro-vide the high-est sensitivityat infraredwavelengths.

The SpitzerSpace

Telescope will capture those celestial objects and phe-nomena that are too dim, distant or cool to study byother astronomical techniques.

The Infrared 'Great Observatory'NASA's Great Observatories give astronomers an

orbiting "toolbox" that provides multi-wavelengthstudies of the universe. The Hubble Space Telescopehas delivered a wealth of spectacular images and hasfundamentally changed perceptions of the cosmos.Observing at optical, ultraviolet and near-infraredwavelengths, Hubble will continue operating through

Spitzer Space Telescope

NASA FactsNational Aeronautics andSpace Administration

Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena, CA 91109

the year 2010.

The Compton Gamma-Ray Observatory, launchedin 1991, studied extremely high-energy phenomena inthe universe such as solar flares, quasars and cosmic-ray interactions, before its mission ended in 1999.

The Chandra X-Ray Observatory, launched in1999, currently examines energetic phenomena suchas supernovas and black holes, and will operatethrough at least 2009.

In the summer of 2003, the Spitzer SpaceTelescope joined this spectacular suite of space instru-ments and provide complementary and synergisticobservations of some of the many cosmic dramas thathave caught the attention of the astronomical commu-nity. The observatory brings a fresh vantage point onprocesses that have until now remained mostly in thedark, such as the formation of galaxies, stars andplanets.

The spacecraft consists of an 85-centimeter-diam-eter (33-inch) telescope and three cryogenicallycooled science instruments incorporating state-of-the-art, large-format infrared detector arrays. The obser-vatory is capable of studying the cosmos at infraredwavelengths from 3 to 180 microns (a human hair isabout 50 microns in diameter).

Within our Milky Way galaxy, the infrared tele-scope will discover and characterize dust discs aroundnearby stars, thought to be signposts of planetary sys-tem formation. It will also provide new insights intothe birth process of stars, an event normally hiddenbehind veils of cosmic dust. Outside our galaxy, thenew infrared facility will help astronomers uncoverthe engines – thought to be galaxy collisions or blackholes – powering ultra-luminous infrared galaxies.The observatory will also probe the birth and evolu-tion of galaxies in the early and distant universe. Thetelescope is designed for a lifetime of five years,which should provide substantial improvements incapability over previous orbiting infrared telescopes.

Drifting into Deep SpaceWhen the National Academy of Sciences declared

the Spitzer Space Telescope (formerly named theSpace Infrared Telescope Facility) the highest-prioritynew initiative in astronomy and astrophysics for the1990s, the mission's total development cost totaledroughly $2.2 billion. However, the observatory has

since undergone radical changes in design to meetevolving cost constraints. Moreover, ongoingadvances in the technology of infrared detectors, cou-pled with innovative choices in orbit and the designof cryogenic systems, have maintained the scientificvitality of the telescope at roughly one-third the origi-nal cost estimate.

One major breakthrough was the clever choice oforbit. Instead of orbiting Earth itself, the observatorytrails behind Earth as it orbits the Sun. The spacecraftdrifts slowly away from our planet into deep space,circling the Sun at a distance of about 1 astronomicalunit (the mean Earth-Sun distance of 150 millionkilometers, or 93 million miles). The telescope driftsaway from us at about 1/10th of one astronomical unitper year. This unique orbital trajectory keeps theobservatory away from much of Earth's heat, whichcan reach 250 Kelvin (-23 Celsius, or -10 Fahrenheit)for satellites and spacecraft in more conventionalnear-Earth orbits. The Spitzer Space Telescope willoperate in a more benign thermal environment ofabout 35 Kelvin (-238 Celsius, or -397 Fahrenheit).With this new approach, mission designers allow

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Mother Nature to do much of the work in keeping theobservatory properly chilled.

Moreover, an Earth-trailing orbit will protect theobservatory from Earth's radiation belts, therebyreducing the deleterious effects on the observatory'sextremely sensitive detectors.

A New Way to ChillThe observatory's unique orbit enables a second

major technical innovation: what engineers call"warm-launch architecture." Previous space-boundinfrared observatories enclosed the entire telescopeassembly within a gigantic cryostat – akin to a largethermos bottle – containing a cryogen, or coolant, tomaintain the necessary chill. With this new infraredtelescope, only the science instrument chamber and acompact cryostat will be cold at launch, chilled toabout 1.5 Kelvin (-272 Celsius, or -457 Fahrenheit).Following launch from Cape Canaveral Air ForceStation in Florida, the spacecraft cooled in the deeprecesses of space for about five weeks. The observa-tory uses the vapor from the boil-off of its cryogenfluid to cool the telescope assembly down to its opti-mal operating temperature of 5.5 Kelvin (-268Celsius, or -450 Fahrenheit).

This innovative approach allowed a dramaticreduction in the amount of liquid helium cryogen,with huge savings in launch mass and cost. Carryingonly 360 liters of liquid helium, the observatory isexpected to achieve a working lifetime of about fiveyears, much longer than past infrared space observa-tories. The launch mass is further reduced by usinglightweight optics, including mirrors and supportingstructures made out of the metal beryllium. The entireobservatory, including cryogen, weighs 950 kilo-grams, or about one ton.

New Vision With the help of NASA's sponsorship, industrial

fabricators and university researchers worked togetherto reduce the electronic noise and increase the sensi-tivity and performance of infrared detectors. Thegreat leap in light sensitivity has been matched by adramatic increase in the size of detector arrays. Lessthan two decades ago the first orbiting infrared obser-vatory had only 62 detectors, but the new observatorywill aim nearly 300,000 "eyes" on celestial targets.

Seeing the LightThe infrared energy collected by the observatory

will be studied and recorded by three main scienceinstruments: an infrared array camera, an infraredspectrograph and a multiband imaging photometer.

The infrared array camera enables imaging atnear- and mid-infrared wavelengths. Astronomers usethis general-purpose camera for a wide variety of sci-entific research programs.

The infrared spectrograph allows for both high-and low-resolution spectroscopy at mid-infraredwavelengths. Spectrometers spread light out into itsconstituent wavelengths, called a spectrum.Astronomers then scrutinize spectra for emission andabsorption lines – the fingerprints of atoms and mole-cules. This spectrometer has no moving parts.

The multiband imaging photometer providesimaging and limited spectroscopic data at far-infraredwavelengths. It has three detector arrays, one each fora pre-specified range of wavelengths. The only mov-ing part in the imaging photometer is a scan mirrorfor efficiently mapping large areas of sky.

'The Far, the Cold and the Dusty'The telescope's science team quips that the mis-

sion will seek "the old" (the earliest stars and galax-ies), "the cold" (brown dwarfs and circumstellardiscs) and "the dirty" (dust-obscured processes suchas star and planetary formation). The observatory'spowerful combination of highly sensitive detectorsand long lifetime will allow scientists to view thesetypes of objects and phenomena that have managed toelude astronomers using other astronomical methods.

The vast majority of the telescope's observingtime is available to the general scientific communitythrough peer-reviewed proposals. The observatory'sdesign was driven by the goal of making major scien-tific contributions in the following major researchareas:

• Protoplanetary and Planetary Debris Discs:These are flattened discs of dust that surround manystars. Protoplanetary discs include large amounts ofgas, and are presumed to be planetary systems in themaking. Planetary debris discs have most of their gasdepleted, and represent a more mature planetary sys-tem. The remaining dust disc may include gaps

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indicative of fledgling planetary bodies. By observingdust discs around stars at various ages, the newinfrared facility can trace the dynamics and chemicalhistory of evolving planetary systems, and providestatistical evidence of planetary system formation.Such information may also help astronomers chart thehistory of our solar system, while providing targetlists for future planet-searching missions.

• Brown Dwarfs: These curious objects do notpossess enough mass to ignite nuclear fusion reac-tions that power true stars; in fact, astronomers havedescribed them as failed stars. Brown dwarfs are larg-er and warmer than the planets found in our solar sys-tem. At one time just a theory, scientists have begunto detect these long-sought objects. If numerousenough, brown dwarfs may provide an appreciablefraction of the elusive dark matter, or "missing mass,"thought to permeate the universe. Since they aremuch cooler than the Sun, brown dwarfs glow pri-marily in the infrared. The Spitzer Space Telescopewill provide critical data on their abundance and char-acter.

• Ultra-Luminous Infrared Galaxies: Manygalaxies emit more radiation in the infrared than at allother wavelengths combined. These so-called ultra-luminous infrared galaxies might be powered byintense bursts of star formation, stimulated by collid-ing galaxies or by central black holes. The infraredtelescope will trace the origins and evolution of ultra-luminous infrared galaxies out to cosmological dis-tances.

• The Early and Distant Universe: The spaceobservatory will probe galaxies at the cosmic fringe.These objects are so remote that the radiation theyonce emitted has taken billions of years to reachEarth. A consequence of an expanding universe, thesefaraway galaxies are speeding away from us so rapid-ly that most of their optical and ultraviolet light has"red-shifted" into the infrared. The telescope will beable to examine these first stars and galaxies, andthereby clue us into the character of the infant uni-verse.

Apart from these important research areas, theSpitzer Space Telescope's near-infrared instrumentwill peer through obscuring dust that cocoons new-born stars, both in the nearby universe and the centerof our Milky Way Galaxy. The observatory will also

contribute to studies within our solar system, such asinspection of objects in the Kuiper Belt beyond theorbit of Pluto, and the chemical composition ofcomets, asteroids and other small interplanetary bod-ies.

History has repeatedly shown that any giant leapin astronomical capability – as the new infrared tele-scope will offer – is certain to provide serendipitousdiscoveries of unanticipated phenomena.

A Bridge to the FutureThe imaging and spectroscopic results from the

Spitzer Space Telescope will create a solid foundationfor future observatories to build upon. The JamesWebb Space Telescope, a next-generation spaceobservatory envisioned as the successor to the HubbleSpace Telescope, will hold a mirror more than seventimes the area of Hubble's reflector, and will bedesigned to operate at short- and medium-infraredwavelengths. The Spitzer Space Telescope's engineer-ing with cryogenic, lightweight mirrors and radiativecooling are critical to the eventual success of theWebb telescope.

NASA has future plans for the Terrestrial PlanetFinder, a space telescope designed to detect and char-acterize Earth-like planets around nearby stars. Inorder to find individual planets this telescope willrequire advance knowledge about the extent andnature of dust around candidate stars. The SpitzerSpace Telescope's study of planetary debris discs willprovide vital data for eventual Terrestrial PlanetFinder scientists hoping to glimpse an Earth-likeworld.

Heating Up the WebFor more information on the Spitzer Space

Telescope mission visit the mission's home page athttp://www.spitzer.caltech.edu/ .

The Spitzer Space Telescope mission is managedby NASA's Jet Propulsion Laboratory, Pasadena,Calif., for NASA's Office of Space Science,Washington. Science operations are conducted at theSpitzer Science Center at the California Institute ofTechnology in Pasadena. Major partners are GoddardSpace Flight Center, Greenbelt, MD, LockheedMartin Corporation, Denver, Colo., and BallAerospace Corp., Boulder, Colo.

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