Cepheid Variables in M100

Cepheid Variable Star in M100

Cepheid Brightness Changes

Color Image of M100 Galaxy

Animation Animations

M100 Cepheids

Cepheid Variables

Distance Scale

Press Release Text

HST Measures Precise Distance to the Most Remote Galaxy Yet (PR94-49 October 26, 1994)

Background Information

Cosmic Yardsticks

Hubble Constant

Cepheid Variables in M100

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Links to public HST pictures

Zolt Levay -- levay@stsci.eduOffice of Public Outreach -- outreach@stsci.edu

June 14, 1995

Copyright© 1990-1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.

Cepheid Variables in M100

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PHOTO RELEASE NO.: STScI-PRC94-49a EMBARGOED UNTIL: 2:00 P.M. EDT Wednesday, October 26, 1994

CEPHEID VARIABLE STAR IN GALAXY M100

This NASA Hubble Space Telescope image of a region of the galaxyM100 shows a class of pulsating star called a Cepheid Variable. Thoughrare, these stars are reliable distance indicators to galaxies. Based on theHubble observation, the distance to M100 has been measured accurately as56 million light-years (+/- 6 million light-years), making it the farthestobject where intergalactic distances have been determined precisely. Hubble's high resolution pinpoints a Cepheid, which is located in astarbirth region in one of the galaxy's spiral arms (bottom frame). The topthree frames were taken on (from left to right) May 9, May 4, May 31, andthey reveal that the star (in center of each box) changes brightness. Cepheids go through these changes rhythmically over a few weeks. Theinterval it takes for the Cepheid to complete one pulsation is a directindication of the stars's intrinsic brightness. This value can be used tomake a precise measurement of the galaxy's distance.

Only Hubble Space Telescope has the required sensitivity and resolution todetect these "cosmic milepost" type stars out to great distances from Earth,according to astronomers. Typically, Cepheids in a crowded region of adistant galaxy are too faint and the resolution too poor, as seen fromground-based telescopes, to be detected clearly. Hubble was used to maketwelve one-hour exposures, timed carefully in a two-month observingwindow, to discover 20 Cepheid variable stars in the M100 galaxy.Though M100 is the most distant galaxy in which Cepheid variables havebeen discovered, HST must find Cepheids in even more distant galaxiesbefore accurate distances can be used to calculate a definitive size and agefor the universe.

Technical Information:The Hubble Space Telescope image was taken with the Wide FieldPlanetary Camera 2 (WFPC 2). This black and white picture was taken atvisible light wavelengths.

Target Information:M100 (100th object in the Messier catalog of non-stellar objects) is amember of the huge Virgo cluster of an estimated 2,500 galaxies. Thegalaxy can be seen by amateur astronomers as a faint, pinwheel-shapedobject in the spring constellation Coma Berenices.

Credit: Dr. Wendy L. Freedman, Observatories of the Carnegie Institution of Washington, and NASA

The Wide Field and Planetary Camera 2 was developed by the JetPropulsion Laboratory (JPL) and managed by the Goddard Space FlightCenter for NASA's Office of Space Science.

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PHOTO RELEASE NO.: STScI-PRC94-49b EMBARGOED UNTIL: 2:00 P.M. EDT Wednesday, October 26, 1994

HUBBLE SNAPSHOTS CAPTURE PULSATION RATE OF A "STELLAR MILEPOST"

This sequence of images taken with NASA's Hubble Space Telescopechronicles the rhythmic changes in a rare class of variable star (located inthe center of each image) in the spiral galaxy M100. This class ofpulsating star is called a Cepheid Variable. The Cepheid in this Hubblepicture doubles in brightness (24.5 to 25.3 apparent magnitude) over aperiod of 51.3 days.

The interval it takes for the Cepheid to complete one pulsation is a directindication of the stars's intrinsic brightness. This value can be used tomake a precise measurement of the galaxy's distance, which turns out to be56 million light-years from Earth.

Cosmic distance measurements as accurate as this are needed to calculatethe rate at which the universe is expanding. This value, called the HubbleConstant, is used to estimate the age and size of the universe.

Though M100 is the most distant galaxy in which Cepheid variables havebeen discovered, HST must find Cepheids in a larger sample of galaxiesbefore a definitive number can be agreed upon for the size and age of theuniverse

Hubble Space Telescope was used to image repeatedly a region of M100 inorder to pick out the flickering Cepheid candidates from normal stars. Twelve one-hour exposures, timed carefully in a two-month observingwindow, resulted in the discovery of 20 Cepheid variable stars.

Technical Information:The Hubble Space Telescope images were taken on (from left to right)April 23, May 4, 9, 16, 20, 31, 1994 with the Wide Field Planetary Camera2 (WFPC 2) This black and white picture was take at visible lightwavelengths.

Credit: Dr. Wendy L. Freedman, Observatories of the Carnegie Institution of Washington, and NASA

The Wide Field and Planetary Camera 2 was developed by the JetPropulsion Laboratory (JPL) and managed by the Goddard Space FlightCenter for NASA's Office of Space Science.

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PHOTO RELEASE NO.: STScI-PRC94-49c EMBARGOED UNTIL: 2:00 P.M. EDT Wednesday, October 26, 1994

THE SPIRAL GALAXY M100

An image of the grand design of spiral galaxy M100 obtained with NASA'sHubble Space Telescope resolves individual stars within the majestic spiralarms. (These stars typically appeared blurred together when viewed withground-based telescopes.)

Hubble has the ability to resolve individual stars in other galaxies andmeasure accurately the light from very faint stars. This makes spacetelescope invaluable for identifying a rare class of pulsating stars, calledCepheid Variable stars embedded within M100's spiral arms.

Cepheids are reliable cosmic distance mileposts. The interval it takes forthe Cepheid to complete one pulsation is a direct indication of the stars'sintrinsic brightness. This value can be used to make a precisemeasurement of the galaxy's distance, which turns out to be 56 millionlight-years.

M100 (100th object in the Messier catalog of non-stellar objects) is amajestic face-on spiral galaxy. It is a rotating system of gas and stars,similar to our own galaxy, the Milky Way. Hubble routinely can viewM100 with a level of clarity and sensitivity previously possible only for thevery few nearby galaxies that compose our "Local Group.''

M100 is a member of the huge Virgo cluster of an estimated 2,500galaxies. The galaxy can be seen by amateur astronomers as a faint,pinwheel-shaped object in the spring constellation Coma Berenices.

Technical Information:The Hubble Space Telescope image was taken on December 31, 1993 withthe Wide Field Planetary Camera 2 (WFPC 2). This color picture is acomposite of several images taken in different colors of light. Bluecorresponds to regions containing hot newborn stars.

The Wide Field and Planetary Camera 2 was developed by the JetPropulsion Laboratory (JPL) and managed by the Goddard Space FlightCenter for NASA's Office of Space Science.

Credit: J. Trauger, JPL and NASA

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CONTACT: Ray Villard, STScI EMBARGOED UNTIL: 2:00 P.M. EDT (410) 338-4514 Wednesday, October 26, 1994 Dr. Wendy L. Freedman PRESS RELEASE NO.: STScI-PR94-49 Carnegie Observatories (818) 304-0204

HUBBLE SPACE TELESCOPE MEASURES PRECISE DISTANCE TO THE MOST REMOTE GALAXY YET

An international team of astronomers using NASA's Hubble SpaceTelescope announced today the most accurate measurement yet of thedistance of the remote galaxy M100, located in the Virgo cluster ofgalaxies.

This measurement will help provide a precise calculation of theexpansion rate of the universe, called the Hubble Constant, which iscrucial to determining the age and size of the universe.

"Although this is only the first step in a major systematic program tomeasure accurately the scale, size, and age of the universe," notedDr. Wendy L. Freedman, of the Observatories of the CarnegieInstitution of Washington, "a firm distance to the Virgo cluster is acritical milestone for the extragalactic distance scale, and it hasmajor implications for the Hubble Constant."

HST's detection of Cepheid variable stars in the spiral galaxy M100, amember of the Virgo cluster, establishes the distance to the cluster as56 million light-years (with an uncertainty of +/- 6 millionlight-years). M100 is now the most distant galaxy in which Cepheidvariables have been measured accurately.

The precise measurement of this distance allows astronomers tocalculate that the universe is expanding at the rate of 80 km/sec permegaparsec (+/- 17 km/sec). For example, a galaxy one millionlight-years away will appear to be moving away from us at approximately60,000 miles per hour. If it is twice that distance, it will be seento be moving at twice the speed, and so on. This rate of expansion isthe Hubble Constant.

These results are being published in the October 27 issue of thejournal Nature. The team of astronomers is jointly led by Freedman,Dr. Robert Kennicutt (Steward Observatory, University of Arizona), andDr. Jeremy Mould (Mount Stromlo and Siding Spring Observatories,Australian National University).

Dr. Mould noted, "Those who pioneered the development of the HubbleSpace Telescope in the 1960s and 1970s recognized its unique potentialfor finding the value of the Hubble Constant. Their foresight has beenrewarded by the marvelous data that we have obtained for M100."

Using Hubble's Wide-Field and Planetary Camera (WFPC2), the team ofastronomers repeatedly imaged a field where much star formationrecently had taken place, and was, therefore, expected to be rich inCepheids -- a class of pulsating stars used for determiningdistances. Twelve one-hour exposures, strategically placed in atwo-month observing window, resulted in the discovery of 20 Cepheids.About 40,000 stars were measured in the search for these rare, butbright, variables. Once the periods and intrinsic brightness of thesestars were established from the careful measurement of their pulsation

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rates, the researchers calculated a distance of 56 million light-yearsto the galaxy. (The team allowed for the dimming effects of distanceas well as that due to dust and gas between Earth and M100.)

Many complementary projects are currently being carried out from theground with the goal of providing values for the Hubble Constant.However, they are subject to many uncertainties which HST was designedand built to circumvent. For example, a team of astronomers using theCanada-France-Hawaii telescope at Mauna Kea recently have arrived at adistance to another galaxy in Virgo that is similar to that found forM100 using HST -- but their result is tentative because it is based ononly three Cepheids in crowded star fields.

"Only Space Telescope can make these types of observations routinely,"Freedman explained. "Typically, Cepheids are too faint and theresolution too poor, as seen from ground-based telescopes, to detectCepheids clearly in a crowded region of a distant galaxy."

Although M100 is now the most distant galaxy in which Cepheid variableshave been discovered, the Hubble team emphasized that the HST projectmust link into even more distant galaxies before a definitive numbercan be agreed on for the age and size of the universe. This is becausethe galaxies around the Virgo Cluster are perturbed by the large massconcentration of galaxies near the cluster. This influences their rateof expansion.

REFINING THE HUBBLE CONSTANT

These first HST results are a critical step in converging on the truevalue of the Hubble Constant, first developed by the Americanastronomer Edwin Hubble in 1929. Hubble found that the farther agalaxy is, the faster it is receding away from us. This "uniformexpansion" effect is strong evidence the universe began in an eventcalled the "Big Bang" and that it has been expanding ever since.

To calculate accurately the Hubble Constant, astronomers must have twokey numbers: the recession velocities of galaxies and their distancesas estimated by one or more cosmic "mileposts," such as Cepheids. Theage of the universe can be estimated from the value of the HubbleConstant, but it is only as reliable as the accuracy of the distancemeasurements.

The Hubble constant is only one of several key numbers needed toestimate the universe's age. For example, the age also depends on theaverage density of matter in the universe, though to a lesser extent.

A simple interpretation of the large value of the Hubble Constant, ascalculated from HST observations, implies an age of about 12 billionyears for a low-density universe, and 8 billion years for ahigh-density universe. However, either value highlights along-standing dilemma. These age estimates for the universe areshorter than the estimated ages of some of the oldest stars found inthe Milky Way and in globular star clusters orbiting our Milky Way.Furthermore, small age values pose problems for current theories aboutthe formation and development of the observed large-scale structure ofthe universe.

COSMIC MILEPOSTS

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Cepheid variable stars rhythmically change in brightness over intervalsof days (the prototype is the fourth brightest star in the circumpolarconstellation Cepheus). For more than half a century, from the earlywork of the renowned astronomers Edwin Hubble, Henrietta Leavitt, AllanSandage, and Walter Baade, it has been known that there is a directlink between a Cepheid's pulsation rate and its intrinsic brightness.Once a star's true brightness is known, its distance is a relativelystraightforward calculation because the apparent intensity of lightdrops off at a geometrically predictable rate with distance. AlthoughCepheids are rare, once found, they provide a very reliable "standardcandle" for estimating intergalactic distances, according toastronomers.

Besides being an ideal hunting ground for the Cepheids, M100 alsocontains other distance indicators that can in turn be calibrated withthe Cepheid result. This majestic, face-on, spiral galaxy has beenhost to several supernovae, which are also excellent distanceindicators. Individual supernovae (called Type II, massive explodingstars) can be seen to great distances, and, so, can be used to extendthe cosmic distance scale well beyond Virgo.

As a crosscheck on the HST results, the distance to M100 has beenestimated using the Tully-Fisher relation (a means of estimatingdistances to spiral galaxies using the maximum rate of rotation topredict the intrinsic brightness) and this independent measurement alsoagrees with both the Cepheid and supernova "yardsticks."

HST Key Projects are scientific programs that have been widelyrecognized as being of the highest priority for the Hubble SpaceTelescope and have been designated to receive a substantial amount ofobserving time on the telescope. The Extragalactic Distance Scale KeyProject involves discovering Cepheids in a variety of importantcalibrating galaxies to determine their individual distances. Thesedistances then will be used to establish an accurate value of theHubble Constant.

----------------------------------------------------------------------

The Key Project Team on the Extragalactic Distance Scale consists ofSandra Faber, Garth Illingworth & Dan Kelson (Univ. of California,Santa Cruz), Laura Ferrarese & Holland Ford (Space Telescope ScienceInstitute), Wendy Freedman, John Graham, Robert Hill & Randy Phelps(Carnegie Institution of Washington), James Gunn (PrincetonUniversity), John Hoessel & Mingsheng Han (University of Wisconsin),John Huchra (Harvard-Smithsonian Center for Astrophysics), Shaun Hughes(Royal Greenwich Observatory), Robert Kennicutt, Paul Harding, AnneTurner & Fabio Bresolin (Univ. of Arizona), Barry Madore & NancySilbermann (JPL, Caltech), Jeremy Mould (Mt. Stromlo, AustralianNational University), Abhijit Saha (Space Telescope Science Institute),and Peter Stetson (Dominion Astrophysical Observatory).

* * * * * * * * * * * *

The Space Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. (AURA) for NASA, undercontract with the Goddard Space Flight Center, Greenbelt, MD. TheHubble Space Telescope is a project of international cooperationbetween NASA and the European Space Agency (ESA).

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Space Science Short

National Aeronautics andSpace Administration

NASA HeadquartersWashington, D.C. October 1994______________________________________________________________________________

Cosmic Yardsticks

Astronomers gauge the dimensions of space by using "distance indicators"-- celestial objects with unique properties that allow for their distances tobe deduced. Reliable distance measurements are a crucial factor indetermining a precise value for the universe's expansion rate (called theHubble Constant) which is needed to estimate the size and age of theuniverse. (To calculate the Hubble Constant, astronomers also need toknow how fast a galaxy is moving away from us, measured by spectralredshift.)

Measuring the distance to a faraway galaxy involves a complicated set ofclosely-linked steps. First, distance indicators within our galaxy are usedas a stepping stone to calibrate other distance indicators in nearby galaxies,which in turn creates yet another stepping stone to calibrate distances toeven more faraway galaxies.

The first rung in the "distance scale ladder" can be found in our MilkyWay neighborhood, in nearby open star clusters such as Hyades and theUrsa Major cluster. An open cluster is a collection of young stars with acommon motion in space. Because the Hyades and the Ursa Major clusterare close to us, their distances can be derived using radial velocity (motiontoward or away from us) and proper motion measurements of memberstars. This allows astronomers to obtain the intrinsic brightness, orluminosity, of different types of stars in these open clusters.

Astronomers then measure the brightness of stars with similar properties inmore distant open clusters. By assuming that these stars would have thesame intrinsic brightness as their nearby counterparts, a distance to theremote open clusters is calculated by comparing the apparent and intrinsicbrightness of their member stars.

To obtain distances to nearby galaxies, astronomers use "primary distanceindicators." These are objects that can be observed within our galaxy orhave characteristics that can be theoretically modeled. Examples includeCepheid variable stars, novae, supernovae, and RR Lyrae stars.

Two well-defined primary distance indicators, or "standard candles," arethe Cepheids and fainter RR Lyrae stars. They have a regular variation inbrightness, and the period of this pulsation is closely linked to the star'sintrinsic brightness. So, if the pulsation period of a star is known, its truebrightness can be deduced. The distance to the star can then be calculatedby comparing its true brightness with its apparent brightness.

Cepheid variable stars are often used as distance calibrators for nearbygalaxies. They are very luminous yellow giant or supergiant stars,regularly varying in brightness with periods ranging from 1 to 70 days. This type of star is in a late evolutionary stage, pulsating due to animbalance between its inward gravitational pull and outward pressure.

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Cepheids are found in remote open clusters whose distances are knownfrom comparison with nearby open clusters. It is, therefore, possible tocalibrate these Cepheids with an independently obtained ruler ot yardstick.

In the past, the best ground-based observations have detected Cepheids innearby galaxies within 12 million light-years. However, all galaxies in thisregion have motions due to gravitational attraction of neighboring galaxies. In order to study the overall expansion of the universe, it is necessary toreach out to Cepheids in galaxies at least 30 million light-years away.

Until the recent Hubble Space Telescope observations of Cepheids inM100, there were no well-calibrated standard candles observable over thisdistance. Therefore, astronomers have been using other kinds of objects,called "secondary distance indicators," to probe even deeper into theuniverse.

Secondary distance indicators, such as planetary nebulae, supernovae, andthe brightest stars are used in galaxies that are so remote that onlyprominent objects can be discerned. (These secondary indicators arecalibrated in nearer galaxies, where distances are known from residentprimary distance indicators, before being applied to more remote galaxies.) The galaxies themselves can also be used as secondary distance indicators. One widely-used strategy, the Tully-Fisher method, uses a correlationbetween the internal motions within galaxies (from radio observations ofcold interstellar gas) with their luminosities. Another method, theFaber-Jackson relation, looks at the random motions of stars in a galaxyobtained from spectroscopic measurements. These relationships are basedon the fact that a more massive galaxy would be more luminous, and wouldrotate faster than a less massive galaxy.

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Space Science Short

National Aeronautics andSpace Administration

NASA HeadquartersWashington, D.C. October 1994______________________________________________________________________________

The Hubble Constant

The Hubble Constant (Ho) is one of the most important numbers incosmology because it is needed to estimate the size and age of theuniverse. This long-sought number indicates the rate at which the universeis expanding, from the primordial "Big Bang."

The Hubble Constant can be used to determine the intrinsic brightness andmasses of stars in nearby galaxies, examine those same properties in moredistant galaxies and galaxy clusters, deduce the amount of dark matterpresent in the universe, obtain the scale size of faraway galaxy clusters,and serve as a test for theoretical cosmological models.

In 1929, American astronomer Edwin Hubble announced his discovery thatgalaxies, from all directions, appeared to be moving away from us. Thisphenomenon was observed as a displacement of known spectral linestowards the red-end of a galaxy's spectrum (when compared to the samespectral lines from a source on Earth). This redshift appeared to have alarger displacement for faint, presumably further, galaxies. Hence, thefarther a galaxy, the faster it is receding from Earth.

The Hubble Constant can be stated as a simple mathematical expression,Ho = v/d, where v is the galaxy's radial outward velocity (in other words,motion along our line-of-sight), d is the galaxy's distance from earth, andHo is the current value of the Hubble Constant.

However, obtaining a true value for Ho is very complicated. Astronomersneed two measurements. First, spectroscopic observations reveal thegalaxy's redshift, indicating its radial velocity. The second measurement,the most difficult value to determine, is the galaxy's precise distance fromearth. Reliable "distance indicators," such as variable stars and supernovae,must be found in galaxies. The value of Ho itself must be cautiouslyderived from a sample of galaxies that are far enough away that motionsdue to local gravitational influences are negligibly small.

The units of the Hubble Constant are "kilometers per second permegaparsec." In other words, for each megaparsec of distance, the velocityof a distant object appears to increase by some value. (A megaparsec is3.26 million light-years.) For example, if the Hubble Constant wasdetermined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshiftcorresponding to a radial velocity of 500 km/s.

The value of the Hubble Constant initially obtained by Edwin Hubble wasaround 500 km/s/Mpc, and has since been radically revised because initialassumptions about stars yielded underestimated distances.

For the past three decades, there have been two major lines of investigationinto the Hubble Constant. One team, associated with Allan Sandage of theCarnegie Institutions, has derived a value for Ho around 50 km/s/Mpc. Theother team, associated with Gerard DeVaucouleurs of the University of

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Texas, has obtained values that indicate Ho to be around 100 km/s/Mpc. Along-term, key program for HST is to refine the value of the HubbleConstant.

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