Space Telescope Science Institute, Baltimore, MD

January 6, 2000


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.

The principal investigators are: John MacKenty, Jesus Maiz Apellaniz (Space Telescope Science Institute), Colin Norman (Johns Hopkins University), Nolan Walborn (Space Telescope Science Institute), Richard Burg (Johns Hopkins University), Richard Griffiths (Carnegie Mellon University), and Rosemary Wyse (Johns Hopkins University).

Image Credit: NASA and The Hubble Heritage Team (STScI)

Image files are available on the Internet at:
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Space Telescope Science Institute, Baltimore, MD

October 7, 1999


To mark the first anniversary of the Hubble Heritage Project, we present four NASA Hubble Space Telescope images of nebulae surrounding stars in our own Milky Way galaxy. Two of these Wide Field Planetary Camera 2 images show interstellar gas and dust around young stars at the beginning of their lives, and two more show gas ejected from old stars that are nearing the end of theirs. Remarkably, in spite of the completely different evolutionary stages, the nebulae have some striking features in common, including evidence of diametrically opposed gas ejections from both the young and old stars.

Image Credits: NASA and The Hubble Heritage Team (AURA/STScI)


HH 32

HH 32 is an example of a "Herbig-Haro object," which is formed when young stars eject jets of material back into interstellar space. This object, about 1,000 light-years from Earth, is somewhat older than Hubble's variable nebula, and radiation from the bright central star has already cleared much of the dust out of the central region, thus exposing the star to direct view. Many young stars, like the central object in HH 32, are surrounded by disks of gas and dust that form as additional material is attracted gravitationally from the surrounding nebula. Material in the disk gradually spirals in toward the star and eventually some of it accretes onto the star, increasing its mass. A fraction of the gas, however, is ejected perpendicularly to the disk at speeds near 200 miles per second, and forms two oppositely directed jets. These jets plow into the surrounding nebula, producing strong shock waves that heat the gas and cause it to glow in the light of hydrogen atoms (red) and sulfur ions (blue); this glow is called a Herbig-Haro object, in honor of astronomers George Herbig and Guillermo Haro, who did much of the early work in this area in the 1950's. The jet on the top side, whose furthest extent is about 0.2 light-year from the star, is pointed more nearly in our direction, while the opposite jet on the bottom lies on the far side of the star and is fainter because it is partially obscured by dust surrounding the star.

The Hubble Heritage team made this image from observations of HH 32 acquired by Salvador Curiel (UNAM) and collaborators.

Image Credit: NASA and The Hubble Heritage Team (AURA/STScI)


Hubble's variable nebula is named (like the Hubble telescope itself) after the American astronomer Edwin P. Hubble, who carried out some of the early studies of this object. It is a fan-shaped cloud of gas and dust which is illuminated by R Monocerotis (R Mon), the bright star at the bottom end of the nebula. Dense condensations of dust near the star cast shadows out into the nebula, and as they move the illumination changes, giving rise to the variations first noted by Hubble. The star itself, lying about 2,500 light-years from Earth, cannot be seen directly, but only through light scattered off of dust particles in the surrounding nebula. R Mon is believed to have a mass of about 10 times that of the Sun, and to have an age of only 300,000 years. There is probably a symmetrical counterpart of the fan-shaped nebula on the southern side of the star, but it is heavily obscured from view by dust lying between this lobe and our line of sight.

The Hubble Heritage team made this image from observations of R Mon acquired by William Sparks (STScI), Sylvia Baggett (STScI) and collaborators.

Image Credit: NASA and The Hubble Heritage Team (AURA/STScI)

NGC 2346

NGC 2346, in contrast to the first two young objects, is a so-called "planetary nebula," which is ejected from Sun-like stars which are near the ends of their lives. NGC 2346 is remarkable because its central star is known to be actually a very close pair of stars, orbiting each other every 16 days. It is believed that the binary star was originally more widely separated. However, when one component of the binary evolved, expanded in size, and became a red-giant star, it literally swallowed its companion star. The companion star then spiralled downwards inside the red giant, and in the process spewed out gas into a ring around the binary system. Later on, when the hot core of the red giant was exposed, it developed a faster stellar wind, which emerged perpendicularly to the ring and inflated two huge "bubbles." This two-stage process is believed to have resulted in the butterfly-like shape of the nebula. NGC 2346 lies about 2,000 light-years away from us, and is about one-third of a light-year in size.

The Hubble Heritage team made this image from observations of NGC 2346 acquired by Massimo Stiavelli (STScI), Inge Heyer (STScI), and collaborators.

Image Credit: NASA and The Hubble Heritage Team (AURA/STScI)

NGC 2440

NGC 2440 is another planetary nebula ejected by a dying star, but it has a much more chaotic structure than NGC 2346. The central star of NGC 2440 is one of the hottest known, with a surface temperature near 200,000 degrees Celsius. The complex structure of the surrounding nebula suggests to some astronomers that there have been periodic oppositely directed outflows from the central star, somewhat similar to that in NGC 2346, but in the case of NGC 2440 these outflows have been episodic, and in different directions during each episode. The nebula is also rich in clouds of dust, some of which form long, dark streaks pointing away from the central star. In addition to the bright nebula, which glows because of fluorescence due to ultraviolet radiation from the hot star, NGC 2440 is surrounded by a much larger cloud of cooler gas which is invisible in ordinary light but can be detected with infrared telescopes. NGC 2440 lies about 4,000 light-years from Earth in the direction of the constellation Puppis.

The Hubble Heritage team made this image from observations of NGC 2440 acquired by Howard Bond (STScI) and Robin Ciardullo (Penn State).

Image Credit: NASA and The Hubble Heritage Team (AURA/STScI)

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