Fireworks of Star Formation Light up a Galaxy:
Newly released images obtained with NASA's Hubble Space Telescope in July 1997 reveal episodes of star formation that are occurring across the face of the nearby galaxy NGC 4214.
Located some 13 million light-years from Earth, NGC 4214 is currently forming clusters of new stars from its interstellar gas and dust. In the Hubble image, we can see a sequence of steps in the formation and evolution of stars and star clusters. The picture was created from exposures taken in several color filters with Hubble's Wide Field Planetary Camera 2.
NGC 4214 contains a multitude of faint stars covering most of the frame, but the picture is dominated by filigreed clouds of glowing gas surrounding bright stellar clusters.
The youngest of these star clusters are located at the lower right of the picture, where they appear as about half a dozen bright clumps of glowing gas. Each cloud fluoresces because of the strong ultraviolet light emitted from the embedded young stars, which have formed within them due to gravitational collapse of the gas.
Young, hot stars have a whitish to bluish color in the Hubble image, because of their high surface temperatures, ranging from 10,000 up to about 50,000 degrees Celsius. In addition to pouring out ultraviolet light, these hot stars eject fast "stellar winds," moving at thousands of kilometers per second, which plow out into the surrounding gas. The radiation and wind forces from the young stars literally blow bubbles in the gas. Over millions of years, the bubbles increase in size as the stars inside them grow older.
Moving to the lower left from the youngest clusters, we find an older star cluster, around which a gas bubble has inflated to the point that there is an obvious cavity around the central cluster. The most spectacular feature in the Hubble picture lies near the center of NGC 4214. This object is a cluster of hundreds of massive blue stars, each of them more than 10,000 times brighter than our own Sun. A vast heart-shaped bubble, inflated by the combined stellar winds and radiation pressure, surrounds the cluster. The expansion of the bubble is augmented as the most massive stars in the center reach the ends of their lives and explode as supernovae.
Deprived of gas, the cluster at the center of NGC 4214 will be unable to form further new stars, and its luminous stars will continue to go supernova and disappear. Elsewhere in the galaxy, however, gas will start to collapse and form yet another new generation of stars, even as the clusters visible today gradually fade away.
The faint stars covering most of the picture are much older than the bright blue supergiants, and show us that episodes of star birth have been occurring in NGC 4214 for billions of years.
1. Where are the young star clusters?
The youngest clusters are at the lower right of the picture, where they appear as about half a dozen bright clumps of glowing gas. Each cloud glows because of the strong ultraviolet light emitted from the embedded young stars, which have formed within them due to the gravitational collapse of the gas. These hot stars also eject fast "stellar winds" moving at millions of miles per hour (thousands of kilometers per second), which plow into the surrounding gas. The radiation and wind from the young stars literally blow bubbles in the gas.
2. What is the blue and white blob in the center of the galaxy?
This object is a cluster of hundreds of massive blue stars, each more than 10,000 times brighter that our Sun. A vast heart-shaped bubble, inflated by the combined stellar winds and radiation pressure, surrounds the cluster. The bubble will increase in size as the most massive stars in the center reach the ends of their lives and explode as supernovae.
An Expanding Bubble in Space:
Astronomers, using the Wide Field Planetary Camera 2 on board NASA's Hubble Space Telescope in October and November 1997 and April 1999, imaged the Bubble Nebula (NGC 7635) with unprecedented clarity. For the first time, they are able to understand the geometry and dynamics of this very complicated system. Earlier pictures taken of the nebula with the Wide Field Planetary Camera 1 left many issues unanswered, as the data could not be fully calibrated for scientific use. In addition, those data never imaged the enigmatic inner structure presented here.
The remarkably spherical "Bubble" marks the boundary between an intense wind of particles from the star and the more quiescent interior of the nebula. The central star of the nebula is 40 times more massive than the Sun and is responsible for a stellar wind moving at 2,000 kilometers per second (4 million miles per hour or 7 million kilometers per hour) which propels particles off the surface of the star. The bubble surface actually marks the leading edge of this wind's gust front, which is slowing as it plows into the denser surrounding material. The surface of the bubble is not uniform because as the shell expands outward it encounters regions of the cold gas, which are of different density and therefore arrest the expansion by differing amounts, resulting in the rippled appearance. It is this gradient of background material that the wind is encountering that places the central star off center in the bubble. There is more material to the northeast of the nebula than to the southwest, so that the wind progresses less in that direction, offsetting the central star from the geometric center of the bubble. At a distance of 7,100 light-years from Earth, the Bubble Nebula is located in the constellation Cassiopeia and has a diameter of 6 light-years.
To the right of the central star is a ridge of much denser gas. The lower left portion of this ridge is closest to the star and so is brightest. It is experiencing the most intense ultraviolet radiation as well as the strong wind and is therefore being photoevaporated the fastest. The ridge forms a V-shape in the image, with two segments that are aligned at the brightest edge. The upper of these two segments is viewed quite obliquely as it trails off into the back of the nebula. The lower segment comes both toward the observer and off to the side. This lower ridge appears to lie within the sphere described by the bubble but is not actually "inside" the shocked region of gas. Instead it is being pushed up against the bubble like a hand being pushed against the outside of a party balloon. While the edge of the hand appears to be inside the balloon, it is not. As the bubble moves up but not through the ridge, bright blue arcs form where the supersonic wind strikes the ridge to form an apparent series of nested shock fronts.
The region between the star and ridge reveals several loops and arcs which have never been seen before. The high resolution capabilities of Hubble make it possible to examine these features in detail in a way that is not possible from the ground. The origin of this bubble-within-a-bubble" is unknown at this time. It may be due to a collision of two distinct winds. The stellar wind may be colliding with material streaming off the ridge as it is photoevaporated by the star's radiation.
Located at the top of the picture are dense clumps or fingers of molecular gas which have not yet encountered the expanding shell. These structures are similar in form to the columns in the Eagle Nebula, except that they are not being eroded as energetically as they are in that nebula. As in the Eagle, the clumps are seen to emit light because they are being illuminated by the strong ultraviolet radiation from the central star, which travels much faster than the shell and has reached the outer knots long before the expanding rim will.
Object Names: Bubble Nebula, NGC 7635
1. What are the yellow-colored "clouds" to the right of the star?
These "clouds" are a ridge of much denser gas. The lower left portion of this ridge is the brightest because it is closest to the star. But the star's intense ultraviolet light and its strong "wind" of material is heating and eroding this area the fastest. The region between the star and the ridge reveals several loops and arcs that have never been seen before. Hubble's sharp resolution allows astronomers to examine these features in greater detail. Astronomers are uncertain about the origin of this "bubble-within-a-bubble." It may be due to a collision of two distinct winds of material. The star's intense wind may be colliding with material streaming off the ridge of gas, which the star's intense radiation is heating and eroding.
2. What are the blobs of gas in the picture's upper left corner?
Those blobs are dense clumps or fingers of gas that are being illuminated by the star's light. The blobs have not yet encountered the expanding bubble.
Lone Black Holes Discovered Adrift in the Galaxy:
Two international teams of astronomers using NASA's Hubble Space Telescope and ground-based telescopes in Australia and Chile have discovered the first examples of isolated stellar-mass black holes adrift among the stars in our galaxy.
All previously known stellar black holes have been found in orbit around normal stars, with their presence determined by their effect on the companion star. The two isolated black holes were detected indirectly by the way their extreme gravity bends the light of a more distant star behind them.
"These results suggest that black holes are common and that many massive but normal stars may end their lives as black holes instead of neutron stars," said David Bennett of the University of Notre Dame, South Bend, Ind. Bennett presented his team's results today in Atlanta at the 195th meeting of the American Astronomical Society.
The findings also suggest that stellar-mass black holes do not require some sort of interaction in a double-star system to form but may also be produced in the collapse of isolated massive stars, as has long been proposed by stellar theorists.
The black hole's gravity acts like a powerful lens, bending the light of a background star so that it appears as two separate images when the black hole slowly drifts in front of it. The bending angle is about 100 times smaller than the angular resolution of Hubble, so the two distorted images of the background star cannot be separated, even in high-resolution Hubble images.
However, the black hole's gravity also magnifies these stellar images, causing them to brighten as the black hole passes in front. Bennett's team was searching for these passages, called gravitational microlensing events.
Careful analysis of the two events reveals that the lensing objects are each approximately six times the mass of the Sun. If the objects were ordinary stars with this mass they would be bright enough to outshine the more distant background source star. The masses are also too large to be white dwarfs or neutron stars. This leaves a black hole as the best explanation.
This microlensing detection technique, combined with Hubble's extraordinary resolution to pinpoint the lensed star, opens the possibility for searching for lone black holes and assessing whether they contribute to the galaxy's long-sought "dark matter."
These microlensing events were discovered in 1996 and 1998 by the Massive Compact Halo Object (MACHO) collaboration with the National Science Foundation, using the 1.3-meter telescope at the Mount Stromlo Observatory in Canberra, Australia, while the magnification was still increasing. The prompt discovery and announcement of these events enabled precise follow-up observations by the Global Microlensing Alert Network (GMAN) from the 0.9-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) and by the Microlensing Planet Search (MPS) project, using the 1.9-meter telescope at Mount Stromlo.
The MACHO team surveys tens of millions of stars in the direction of the center of our galaxy, where the starfield is very crowded, increasing the chances for seeing rare gravitational microlensing events. The two events were also of exceptionally long duration, lasting 800 and 500 days respectively, which suggests that the lensing objects have a high mass.
Follow-up observations were done with Hubble on June 15, 1999 to clearly identify the lensed star for the first event and make a precise measurement of its brightness after the lensing event. The Hubble frame indicates that the lensed star was blended with two neighboring stars of similar brightness that could not be separated in the poorer-resolution, ground-based images. Hubble's identification of the lensed star allowed for an accurate estimate of the mass of the black hole.
The 1998 event was brighter, and modeling of the ground-based measurements enabled astronomers to determine the brightness of the lensed star, but this determination awaits confirmation with future Hubble images.
There have been more than 300 instances of gravitational microlensing seen towards the central regions of our galaxy to date. The longest duration microlensing events could be caused by either very massive lenses or slow relative motion between the lens and source, but additional information was needed to determine the parameters of the lensing event. This additional information was obtained by the GMAN and MPS groups using the 0.9-meter telescope at CTIO and the 1.9-meter telescope at Mount Stromlo in the form of an asymmetry in the pattern of magnification with time due to the orbital motion of the Earth. This asymmetry is known as the microlensing parallax effect as it is similar to the parallax effect which yields the distances of nearby stars.
1. What are stellar-mass black holes, and how are they different from supermassive black holes?
Stellar-mass black holes are the compressed remains of giant, exploding stars called supernovas. These compact, gravitational powerhouses keep everything, including light, from escaping their stranglehold. Supermassive black holes, as their name implies, are monsters. They are millions to billions of times more massive than the Sun and are believed to reside at the hearts of most galaxies. Scientists aren't sure how they first formed, but they believe that these massive "eating machines" were created during the early universe. Stellar-mass black holes, on the other hand, can form at any time. Supermassive black holes are easier to detect because scientists know where to look: the centers of galaxies. All black holes, by their very nature, are invisible. But scientists hunting for supermassive black holes probe the centers of galaxies, looking for how the suspected monsters gravitationally influence the stars and dust near the cores.
2. Explain the technique astronomers use to find the "drifting" stellar black holes.
The black hole's gravity acts like a powerful lens, bending the light of a background star, so that it appears as two separate images when the black hole slowly drifts in front of it. However, the black hole's gravity also magnifies these stellar images, causing them to brighten as the black hole passes in front. Astronomers used ground-based telescopes to search for these passages, called gravitational "microlensing" events. The two pictures at left identify the brightening star [center of boxed region] for one of the black holes. They then tapped Hubble and its sharp vision to pinpoint the "lensed" star. The Hubble frame [picture at right] indicates that the lensed star was blended with two neighboring stars of similar brightness that could not be separated in the poorer-resolution, ground-based images. Hubble's identification of the lensed star allowed for an accurate estimate of the mass of the black hole.
3. If astronomers can't see the objects passing in front of the stars, how do they know they're black holes?
Careful analysis reveals that each black hole is approximately six times the mass of the Sun. If they were ordinary stars with this bulk they would be bright enough to outshine the more distant background star. The masses are also too large to be white dwarfs or neutron stars. This leaves a black hole as the most likely explanation.
Photo: Discovering the something in between.
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