GalaxiesAstronomy 115

First, which of the following is a galaxy?Open clusterGlobular clusterNebulaInterstellar medium (gas and dust)Supernova remnantNone of these

First, which of the following is a galaxy?Open clusterGlobular clusterNebulaInterstellar medium (gas and dust)Supernova remnantNone of these

What is a star cluster?stars formed together at same timestars are at least weakly gravitationally bound togethertwo types: open (galactic) and globular (shown to right)Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/2007/18/image/a/format/web/results/50/

Open Clustersdozens to thousands of starsyoung stars! only a few million years oldmay still be surrounded by nebula from which they formedlocated in the spiral arms of a galaxyexample: PleiadesFate: generally, the stars drift apart (not enough gravity)Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/open/2004/20/image/a/results/50/

More open star clusters

Globular Clustersmillions to hundreds of millions of starsold! 6 to 13 billion yearsmostly red giants and dwarfsstars are clumped closely together, especially near the center of the cluster (densely); stars dont drift apartsurround our disk as a halo

Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/1999/26/image/a/results/50/

What is a nebula?A cloud in spaceMade of gas and dust Can have stars inside Most of the ones we see are inside our Milky Way GalaxyDifferent types

Large, massive, bright nebulaeEmission NebulaThe hot gas is emitting light

Colder, darker nebulaeDark dust blocking the hot gas behind it

Leftovers from an ExplosionSupernova remnant (smaller, less gas)

So, what is a galaxy?A large group of stars outside of our own Milky WayMade of billions to trillions of stars, held together by its own gravity, with all different ages of starsAlso may have gas and dust Spiral, or elliptical, or irregular shaped

Galaxy ClassificationSaSbScE0 = SphericalSmall nucleus; loosely wound armsE1E6E0, , E7Large nucleus; tightly wound armsE7 = Highly elliptical

Spiral galaxy--AndromedaNOAO/AURA/NSF Images at http://www.noao.edu/image_gallery/html/im0606.html and http://www.noao.edu/image_gallery/html/im0685.html

Elliptical Galaxies

Irregular GalaxiesNASA and NOAO/AURA/NSF Images at http://hubblesite.org/newscenter/archive/releases/galaxy/irregular/2005/09/results/50/ , http://www.noao.edu/image_gallery/html/im0560.html , and http://www.noao.edu/image_gallery/html/im0993.html

Irregular GalaxiesOften: result of galaxy collisions / mergersOften: Very active star formation (Starburst galaxies)Some: Small (dwarf galaxies) satellites of larger galaxies (e.g., Magellanic Clouds)Large Magellanic CloudNGC 4038/4039The Cocoon Galaxy

Galaxy DiversityThe Hubble Deep Field:10-day exposure on an apparently empty field in the skyEven seemingly empty regions of the sky contain thousands of very faint, very distant galaxies Large variety of galaxy morphologies: Spirals EllipticalsIrregular (some interacting)

Gas and Dust in GalaxiesSpirals are rich in gas and dustEllipticals are almost devoid of gas and dustGalaxies with disk and bulge, but no dust are termed S0

Barred Spirals Some spirals show a pronounced bar structure in the center They are termed barred spiral galaxiesSequence: SBa, , SBc, analogous to regular spirals

Our Galaxy: the Milky Wayhas about 200 billion stars, and lots of gas and dustis a barred-spiral (we think)about 100,000 light-years wideour Sun is halfway to the edge, revolving at half a million miles per hour around the center of the Galaxytakes our Solar System about 200 million years to revolve once around our galaxy

The Milky Way

Mapping the Milky WayWe can see: Stars and star clusters microwaves generated by water from H II regions (called the MASER technique) traces the Milky Ways spiral armsNebulae infrared light (detected by the Spitzer Space Telescope) shows the outline of the heat generated by the barOther galaxies (analogous structure as our galaxy)

How do we know what our Galaxy looks like?

Do galaxies evolve over time?Edwin Hubble (whom well hear more about next lecture) in 1926 classified known galaxies according to shape, and suggested an evolution of galaxies from elliptical to spiral as they aged. The diagram was called a tuning fork due to its shape

No one evolutionary path for galaxiesAs detection methods grew more sophisticated, using the infrared (Spitzer telescope), radio (Very Long Baseline Array) and gamma ray (Compton telescope) portions of the EM spectrum, the tuning fork is no longer regarded as containing an evolutionary sequence its simply a way of classifying galaxies.

It is true that irregular galaxies seem to form from galactic collisions, and that some spiral galaxies lose their arms to become elliptical (Milky Way + Andromeda fate), there is no good model to describe galactic evolution.

From http://hubblesite.org/newscenter/archive/releases/star-cluster/globular/2007/18/full/results/50/

Astronomers have long thought that globular star clusters had a single "baby boom" of stars early in their lives and then settled into a quiet existence. New observations by NASA's Hubble Space Telescope, however, are showing that this idea may be too simple. The Hubble analysis of the massive globular cluster NGC 2808 provides evidence that star birth went "boom, boom, boom," with three generations of stars forming very early in the cluster's life. "We had never imagined that anything like this could happen," said Giampaolo Piotto of the University of Padova in Italy and leader of the team that made the discovery. "This is a complete shock."Globular clusters are the homesteaders of our Milky Way Galaxy, born during our galaxy's formation. They are compact swarms of typically hundreds of thousands of stars held together by gravity. "The standard picture of a globular cluster is that all of its stars formed at the same time, in the same place, and from the same material, and they have co-evolved for billions of years," said team member Luigi Bedin of the European Space Agency, the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in Garching, Germany, and the Space Telescope Science Institute in Baltimore, Md. "This is the cornerstone on which much of the study of stellar populations has been built. So we were very surprised to find several distinct populations of stars in NGC 2808. All of the stars were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster." Finding multiple stellar populations in a globular cluster so close to home has deep cosmological implications, the researchers said."We need to do our best to solve the enigma of these multiple generations of stars found in these Hubble observations so that we can understand how stars formed in distant galaxies in our early universe," Piotto explained.The astronomers used Hubble's Advanced Camera for Surveys to measure the brightness and color of the cluster stars. Hubble's exquisite resolution allowed the astronomers to sort out the different stellar populations. The Hubble measurements showed three distinct populations, with each successive generation appearing slightly bluer. This color difference suggests that successive generations contain a slightly different mix of some chemical elements. "One assumption, although we have no direct proof," said team member Ivan King of the University of Washington in Seattle, "is that the successively bluer color of the stellar populations indicates that the amount of helium increases with each generation of stars. Perhaps massive star clusters like NGC 2808 hold onto enough gas to ignite a rapid succession of stars." The star birth would be driven by shock waves from supernovae and stellar winds from giant stars, which compress the gas and make new stars, King explained. The gas would be increasingly enriched in helium from previous generations of stars more massive than the Sun.Astronomers commonly assume that globular clusters produce only one stellar generation, because the energy radiating from the first batch of stars would clear out most of the residual gas needed to make more stars. But a hefty cluster like NGC 2808, which is two to three times more massive than a typical globular cluster, may have enough gravity to hang onto that gas, which has been enriched by helium from the first stars. Of the about 150 known globular clusters in our Milky Way Galaxy, NGC 2808 is one of the most massive, containing more than 1 million stars. Another possible explanation for the multiple stellar populations is that NGC 2808 may only be masquerading as a globular cluster. The stellar grouping may have been a dwarf galaxy that was stripped of most of its material due to gravitational capture by our galaxy. Omega Centauri, the only other stellar system Piotto's group found to have multiple generations of stars, is suspected to be the remnant core of a dwarf galaxy, Bedin said. Although the astronomers' search is only in its infancy, they say multiple stellar populations may be a typical occurrence in other massive clusters."No one would make the radical step of suggesting that previous work on other clusters is no longer valid," King said. "But this discovery shows that the study of stellar populations in globular clusters now opens up in a new direction." From http://hubblesite.org/newscenter/archive/releases/star-cluster/open/2004/20/text/results/50/

Astronomers using NASA's Hubble Space Telescope have helped settle a mystery that has puzzled scientists concerning the exact distance to the famous nearby star cluster known as the Pleiades, or the Seven Sisters.The Pleiades cluster, named by the ancient Greeks, is easily seen as a small grouping of stars lying near the shoulder of Taurus, the Bull, in the winter sky. Although it might be expected that the distance to this well-studied cluster would be well established, there has been an ongoing controversy among astronomers about its distance for the past seven years.The mystery began in 1997, when the European Space Agency's satellite Hipparcos measured the distance to the Pleiades and found it is 10 percent closer to Earth than traditional estimates, which were based on comparing the Pleiades to nearby stars. If the Hipparcos measurements were correct, then the stars in the Pleiades are peculiar because they are fainter than Sun-like stars would be at that distance. This finding, if substantiated, would challenge our basic understanding of the structure of stars.But measurements made by the Hubble telescope's Fine Guidance Sensors show that the distance to the Pleiades is about 440 light-years from Earth, essentially the same as past distance estimates and differing from the Hipparcos results by more than 40 light-years. The Hubble results will be presented June 1 at the American Astronomical Society meeting in Denver, Colo.The new results agree with recent measurements made by astronomers at the California Institute of Technology and NASA's Jet Propulsion Laboratory, both in Pasadena, Calif. Those astronomers used interferometer measurements from Mt. Wilson and Palomar observatories in California, reporting that the star cluster is between 434 and 446 light-years from Earth.The discrepancy in the distance to the Pleiades is more than an arcane argument over details. Astronomers have only one direct means for gauging distances to stars, called the parallax method. With current telescopes, this method gives accurate results only for distances up to about 500 light-years. Distances beyond that limit must be determined by indirect methods, based on comparing the brightness of distant stars with those of nearer ones of the same type, and making the assumption that both objects have the same intrinsic, or true, brightness. Astronomers can thus build up a distance ladder, based on ever more-distant objects, ultimately leading to the use of supernovae as "standard candles" for the most distant reaches of the universe."Reliance on the accuracy of the measurements of nearby objects is crucial to getting the distance ladder of the universe correct," said David Soderblom of the Space Telescope Science Institute in Baltimore, Md., and lead astronomer on the Hubble study. "The new Hubble result shows that the measurements made by Hipparcos contain a small, but significant, source of error that requires further exploration. New space missions are now being planned to carry out even more precise distance measurements out to greater distances."Soderblom and his team used Hubble's Fine Guidance Sensors to measure slight changes in the apparent positions of three stars within the cluster when viewed from different sides of Earth's orbit. Due to the motion of the Earth around the Sun, the position of a star in the Pleiades, will appear to shift relative to stars farther away. This effect, called parallax, can be used to calculate the distance to the star with simple geometry; a similar method of triangulation is used by surveyors to measure distances on Earth. Soderblom's team took its measurements six months apart over a 2 1/2-year period.Making these kinds of measurements of a star's movement is very difficult. The Fine Guidance Sensors are so precise that if the human eye had the same ability to measure small angles, it would be able to see a quarter 16,000 miles away.Hipparcos was the first space observatory to make precise measurements of the positions and motions of celestial objects. Before Hipparcos, astronomers determined the distances to stars like the Pleiades by measuring parallax with ground-based telescopes. Those observations were less precise because Earth's atmosphere distorts light from stars, limiting the telescopes' resolution.From http://hubblesite.org/newscenter/archive/releases/star%20cluster/open/2006/17/image/a/results/50/

NASA's Hubble Space Telescope has captured the most detailed images to date of the open star clusters NGC 265 and NGC 290 in the Small Magellanic Cloud two sparkling sets of gemstones in the southern sky.These images, taken with Hubble's Advanced Camera for Surveys, show a myriad of stars in crystal clear detail. The brilliant open star clusters are located about 200,000 light-years away and are roughly 65 light-years across.Star clusters can be held together tightly by gravity, as is the case with densely packed crowds of hundreds of thousands of stars, called globular clusters. Or, they can be more loosely bound, irregularly shaped groupings of up to several thousands of stars, like the open clusters shown in this image.The stars in these open clusters are all relatively young and were born from the same cloud of interstellar gas. Just as old school-friends drift apart after graduation, the stars in an open cluster will only remain together for a limited time and gradually disperse into space, pulled away by the gravitational tugs of other passing clusters and clouds of gas. Most open clusters dissolve within a few hundred million years, whereas the more tightly bound globular clusters can exist for many billions of years.Open star clusters make excellent astronomical laboratories. The stars may have different masses, but all are at about the same distance, move in the same general direction, and have approximately the same age and chemical composition. They can be studied and compared to find out more about stellar evolution, the ages of such clusters, and much more.The Small Magellanic Cloud, which hosts the two star clusters, is one of the small satellite galaxies of the Milky Way. It can be seen with the unaided eye as a hazy patch in the constellation Tucana (the Toucan) in the Southern Hemisphere. The Small Magellanic Cloud is rich in gas nebulae and star clusters. It is most likely that this irregular galaxy has been disrupted through repeated interactions with the Milky Way, resulting in the vigorous star-forming activity seen throughout the cloud. NGC 265 and NGC 290 may very well owe their existence to these close encounters with the Milky Way.The images were taken in October and November 2004 through F435W, F555W, and F814W filters (shown in blue, green, and red, respectively).From http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/1999/26/image/a/results/50/

This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 15 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives. By analyzing the Wide Field and Planetary Camera 2 (WFPC2) images, including images taken through an ultraviolet filter, astronomers have found a large population of "blue stragglers" in the core of the cluster. These stars appear to be unusually young and more massive than the other stars in a globular cluster. However, stellar collisions can occur in dense stellar regions like the core of M80 and, in some cases, the collisions can result in the merger of two stars. This produces an unusually massive single star, which mimics a normal, young star. M80 was previously unknown to contain blue stragglers, but is now known to contain more than twice as many as any other globular cluster surveyed with NASA's Hubble Space Telescope (HST). Based on the number of blue stragglers, the stellar collision rate in the core of M80 appears to be exceptionally high. M80 is also unusual because it was the site of a nova explosion in the year 1860. Nova outbursts occur when a close companion star transfers fresh hydrogen fuel to a burned-out white dwarf. Eventually the hydrogen ignites a thermonuclear explosion on the surface of the white dwarf, giving rise to the nova outburst. The ultraviolet Hubble observations have revealed the hot, faint remnant of this exploding star, which was named T Scorpii in the 19th century. Curiously, however, the WFPC2 observations have revealed only two other nova-like close binary stars in M80, far fewer than expected theoretically based on the stellar collision rate. So the blue stragglers in M80 seem to indicate that there are lots of collisions, yet the nova-like stars suggest only a few. Sometimes life for astronomers isn't so simple, but it is from exploring discrepancies like this that our understanding eventually deepens. Rising from a sea of dust and gas like a giant seahorse, the Horsehead nebula is one of the most photographed objects in the sky. The Hubble telescope took a close-up look at this heavenly icon, revealing the cloud's intricate structure. This detailed view of the horse's head is being released to celebrate the orbiting observatory's eleventh anniversary. 1. WHAT DOES THE PICTURE SHOW? The Horsehead is a cold, dark cloud of gas and dust, silhouetted against the bright nebula, IC 434. The top of the nebula also is being sculpted by radiation from a massive star located out of Hubble's field of view.

Information at http://hubblesite.org/newscenter/archive/releases/galaxy/spiral/2007/41/results/50/ NOVEMBER 29, 2007: Resembling festive lights on a holiday wreath, this NASA/ESA Hubble Space Telescope image of the nearby spiral galaxy M74 is an iconic reminder of the impending season. Bright knots of glowing gas light up the spiral arms, indicating a rich environment of star formation. M74 is located roughly 32 million light-years away in the direction of the constellation Pisces, the Fish. The image is a composite of Advanced Camera for Surveys data taken in 2003 and 2005.Located in the constellation of Andromeda, the Princess, the Andromeda Galaxy is a large spiral galaxy very similar to our own Galaxy, the Milky Way. It is over 65,000 light-years in diameter and approximately 2.2 million light-years in distance. The area shown in this image is quite large on the sky, covering about five times the area of the full Moon.Information on the galaxy on the left at http://hubblesite.org/newscenter/archive/releases/galaxy/elliptical/2007/08/image/a/results/50/

This image from NASA's Hubble Space Telescope shows the diverse collection of galaxies in the cluster Abell S0740 that is over 450 million light-years away in the direction of the constellation Centaurus.The giant elliptical ESO 325-G004 looms large at the cluster's center. The galaxy is as massive as 100 billion of our suns. Hubble resolves thousands of globular star clusters orbiting ESO 325-G004. Globular clusters are compact groups of hundreds of thousands of stars that are gravitationally bound together. At the galaxy's distance they appear as pinpoints of light contained within the diffuse halo.Other fuzzy elliptical galaxies dot the image. Some have evidence of a disk or ring structure that gives them a bow-tie shape. Several spiral galaxies are also present. The starlight in these galaxies is mainly contained in a disk and follows along spiral arms.This image was created by combining Hubble science observations taken in January 2005 with Hubble Heritage observations taken a year later to form a 3-color composite. The filters that isolate blue, red and infrared light were used with the Advanced Camera for Surveys aboard Hubble.

Information on the galaxy on the right at http://hubblesite.org/newscenter/archive/releases/galaxy/elliptical/1995/07/results/50/ JUNE 14, 1995: This Hubble telescope photo mosaic shows a field of distant galaxies. The brightest object in this picture is NGC 4881 [just above center], an elliptical galaxy in the outskirts of the Coma Cluster, a great cluster of galaxies more than five times farther away than the Virgo Cluster. The distance to the Coma Cluster is an important cosmic yardstick for scaling the overall size of the universe. About Image on Left:MARCH 3, 2005: What happens when a galaxy falls in with the wrong crowd? The irregular galaxy NGC 1427A is a spectacular example of the resulting stellar rumble. Under the gravitational grasp of a large gang of galaxies, called the Fornax cluster, the small bluish galaxy is plunging headlong into the group at 600 kilometers per second or nearly 400 miles per second.

1. IS NGC 1427A ACTIVELY FORMING STARS? NGC 1427A shows numerous hot, blue stars that have been formed very recently, showing that star formation is occurring extensively throughout the galaxy. Within the Fornax cluster, there is a considerable amount of gas lying between the galaxies. When the gas within NGC 1427A collides with the Fornax gas, it is compressed to the point that it starts to collapse under its own gravity. This leads to formation of the myriad of new stars seen across NGC 1427A. The tidal forces of nearby galaxies in the cluster may also play a role in triggering star formation on such a massive scale.2. WHAT IS THE EVENTUAL FATE FOR NGC 1427A? NGC 1427A will not survive long as an identifiable galaxy passing through the cluster. Within the next billion years, it will be completely disrupted, spilling its stars and remaining gas into intergalactic space within the Fornax cluster.

About image on right, From http://www.noao.edu/image_gallery/html/im0993.htmlThe glowing gas of the interstellar medium (ISM) is the breeding ground for the formation of new stars, and the cemetery where the ashes of dead stars ultimately return. A team led by astronomers from the National Optical Astronomy Observatory (NOAO) has conducted a new study called the Magellanic Cloud Emission Line Survey (MCELS) that focused expressly on the ISM in the Large Magellanic Cloud and Small Magellanic Cloudthe nearest major galaxies to the Milky Way.

From http://news.nationalgeographic.com/news/2008/06/080603-milky-way.html

Astronomers unveiled today what they are calling the best map ever produced of the Milky Way galaxy. The new view shows our spiral galaxy as it would look face-on to a very distant observer. The map is based on findings about the structural evolution of the Milky Way. The researchers determined that the Milky Way actually has two fewer major arms than previously believed. In barred spiral galaxies like our own, major arms have a high density of stars, produce lots of new stars, and are clearly connected to the long bar of stars at the galactic center. By contrast, minor arms have high gas density and presumably less star formation. Scientists had long thought that the Milky Way has four major arms. But the new images show that the spirals are actually made of two major arms and two minor ones. "These major arms plus the bar could be the things that really stand out if you were looking at the Milky Way galaxy from, say, [our nearest galactic neighbor] Andromeda," Benjamin said.