Astronomers using NASA's Hubble Space Telescope have identified what may be the most luminous star known – a celestial mammoth which releases up to 10 million times the power of the Sun and is big enough to fill the diameter of Earth's orbit. The star unleashes as much energy in six seconds as our Sun does in one year.
The image, taken by a University of California, Los Angeles (UCLA)-led team with the recently installed Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard Hubble, also reveals a bright nebula, created by extremely massive stellar eruptions. The nebula is so big (four light-years) that it would nearly span the distance from the Sun to Alpha Centauri, the nearest star to Earth's solar system.
The astronomers estimate that when the titanic star was formed one to three million years ago, it may have weighed up to 200 times the mass of the Sun before shedding much of its mass in violent eruptions.
"This star may have been more massive than any other star, and now it is without question still among the most massive - even at the low end of our estimates," says Don F. Figer of UCLA. "Its formation and life stages will provide important tests for new theories about star birth and evolution."
Violent Eruptions Produce Nebula
The UCLA astronomers estimate that the star, called the "Pistol Star" (for the pistol shaped nebula surrounding it), is approximately 25,000 light-years from Earth near the center of our Milky Way galaxy. The Pistol Star is not visible to the eye, but is located in the direction of the constellation Sagittarius, hidden behind the great dust clouds along the Milky Way.
The Pistol Star was first noted in the early 1990s, but its relationship to the nebula was not realized until 1995, when Figer proposed in his Ph.D. thesis that the "past eruptive stages of the star" might have created the nebula. The Hubble spectrometer results confirm this conclusion.
The astronomers believe that the Pistol nebula was created by eruptions in the outer layers of the star which ejected up to 10 solar masses of material in giant outbursts about 4,000 and 6,000 years ago. The star will continue to lose more material, eventually revealing its bare hot core, sizzling at 100,000 degrees.
Burning at such a dramatic rate, the Pistol Star is destined for certain death in a brilliant supernova in 1-3 million years. "Massive stars are burning their candles at both ends; they are so luminous that they consume their fuel at an outrageous rate, burning out quickly and often creating dramatic events, such as exploding as supernovae," said Mark Morris, a UCLA professor of astronomy and co-investigator. "As these stars evolve, they can eject substantial portions of their atmospheres - in the case of the Pistol Star, producing the nebula and an extreme stellar wind (outflow of charged particles) that is 10 billion times stronger than our Sun's." The Pistol Star would be visible to the naked eye as a fourth magnitude star in the sky (which is quite impressive given its distance of 25,000 light-years) if it were not for interstellar dust clouds of tiny particles between the Earth and the center of the Milky Way that absorb the star's light. The most powerful telescopes cannot see the Pistol Star in visible wavelengths. However, ten percent of the infrared light leaving the Pistol Star reaches Earth, putting it within reach of infrared telescopes, which have seen rapid technological advances in recent years - spurred by projects such as NICMOS. The Pistol Star was so massive when it was born that it brings into question current thinking about how stars are formed, say the UCLA astronomers. In the current view, stars form within large dust clouds which contract under their own gravity, eventually forming hot clumps that ignite the hydrogen fusion process.
The star may radiate enough energy to halt the inward fall of material, thus limiting its maximum mass. The initial mass of the Pistol Star may have exceeded this theoretical upper limit. "It is perhaps no accident that this extreme-mass star is found near the center of the Galaxy," says Morris. "Current evidence leads us to believe that the star formation process there may favor stars much more massive than our modest Sun."
Over the coming year, the team will be using the new near-infrared spectrometer that Ian S. McLean's team is building at UCLA for the giant 10-meter Keck II telescope in Hawaii. The new instrument will be used to measure the velocities of the expanding gas shells.
In addition to Figer, Morris, and McLean, the team also includes Caltech physicist Gene Serabyn and Columbia University astronomer R. Michael Rich.
BACKGROUND INFORMATION: HUBBLE FINDS AN ERUPTING, MASSIVE STAR IN THE HEART OF THE MILKY WAY
Astronomers using the recently installed infrared camera on NASA's Hubble Space Telescope have identified one of the most massive stars known, emitting as much as 10 million times the power of the Sun and with a radius larger than the distance between our Sun and the Earth. The star appears to be expelling its outer layers in violent eruptions, producing a brilliant nebula which now surrounds it.
The UCLA-led team, headed by Don F. Figer, will now have the opportunity to investigate the evolution of one of the most luminous stars known in the Galaxy. "This star may have been born more massive than any other star," said Figer. "Its formation and life stages will provide important tests for new theories."
What Was Seen?
The image, obtained with the recently installed Near-Infrared Camera and Multi-Object Spectrometer (NICMOS), reveals an erupting star and the gas it expelled. The nebula surrounds the star and shows up prominently in this image taken in the light of ionized hydrogen gas. Its internal structure suggests that the star ejected matter in two major eruptions, about 6,000 and 4,000 years ago. The nebula is four light-years in size, roughly the distance between the Sun and the nearest star, Alpha Centauri.
A Super-Massive Star
The "Pistol Star" (named for the "Pistol"-shaped nebula which surrounds it) was first noticed in the early 90's; however, its relationship to the surrounding nebula was not realized until 1995. It that year, Figer proposed in his PhD thesis that "past eruptive stages of the star" might have created the nebula. The Pistol Star may have started with as much as 200 solar masses of material, but it is currently shedding much of its mass in violent eruptions. In the most recent eruptions, it might have expelled 10 solar masses of material!
Perhaps the most luminous star known, the Pistol Star emits almost 10 million times the power generated by the Sun, and it would engulf the Earth if it were placed at the Sun. Burning at such a dramatic rate, the star is destined to a short life and an abrupt end. It is currently one to three million years old, and could die in a spectacular supernova at any time in the next three million years; in comparison, the Sun is half way through its 10 billion years lifetime. The star may represent the short-lived "missing link" between normal hot stars and the exotic Wolf- Rayet stars which have lost their outer layers. This transitional phase is so rare that the Pistol Star would be only the seventh of its kind in the Galaxy. Weighing in at 100 to 200 times the mass of the Sun when it began life, the Pistol Star may have less than 10 solar masses when the eruptions finally stop!
Why Has It Taken So Long To Find?
Such a luminous star should be very bright in the sky, but clouds of tiny dust grains between us and the Galactic Center strongly absorb most of the light being emitted by the star. In fact, without interstellar dust absorption, this star would be visible with the naked eye, even at its distance of 25,000 light-years. Instead, however, only about one in every trillion visible-light photons survives the long journey to the Earth.
"The key to finding the star was the advent of sensitive infrared array cameras, analogous to hand-held video cameras, but operating at longer wavelengths," said Ian S. McLean, who is responsible for developing infrared instruments at UCLA. Dust absorbs much less at longer wavelengths, so as much as one in ten infrared photons leaving the star can survive the long journey. A similar effect can be seen during a sunset – the blue light
from the Sun is scattered by the Earth's atmosphere while the red light continues relatively unimpeded, leading to reddish sunsets. Astronomers have studied this star at ground-based telescopes by using infrared imaging arrays that were initially developed in anticipation of the NICMOS camera.
What Are the Implications tor Star Formation, Stellar Evolution, and the Galactic Center?
Stars that are much more massive than the Sun pose serious problems for our theories of star formation and evolution. All stars form out of dense collapsing condensations in large dust clouds. When an individual cloud core has amassed enough material to begin nuclear hydrogen burning, it may emit enough energy to halt the inward flow of material. The ignition of the new star will then limit the mass that a star can gather. The small sample of very massive stars tells us just how effective this self-regulation can be. This picture is complicated by the fact that clouds in the turbulent environment at the center of the Galaxy, where this star was born, are more likely to be "triggered" into forming stars by collisions with other clouds.
Even after forming, massive stars continue to live in a finely-tuned balance between the inward pull of gravity and the outward pressure of radiation from the emitted energy. Massive stars are just stable enough to withstand the relentless radiation pressure emanating from their cores and pushing out their tenuous outer layers. One small change in that balance, such as a temporary flare-up of the nuclear burning rate in the stellar interior, could produce devastating results in powerful eruptions that push away the star's outer layers, as seen in the image.
Mark Morris of UCLA explains, "It is perhaps no accident that this extreme-mass star is found near the center of the Galaxy. Current evidence leads us to believe that the star formation process there may favor stars much more massive than our modest Sun, and it appears that the Pistol Star is at the top of the heap among its ponderous peers."
R. Michael Rich (Columbia Univ.) comments, "The center of the Milky Way has long been known as a location of great activity, including, likely, a massive black hole. Using the capability of NICMOS and the clear vision of HST, we now find it also a place where some of the most massive stars known have formed only just yesterday, on a cosmic timescale, and where only tomorrow, a violent supernova could occur."
Future Questions/Work
The team is looking forward to an exciting observational program in the coming year. McLean says, "We expect the nebula to be expanding, so we will be using NIRSPEC, a new near-infrared spectrometer being built at UCLA for the giant 10-meter Keck II telescope, to measure the velocities of the expanding shells."
Gene Serabyn (Caltech) adds, "With the new infrared array technology, we will make a census of all the young and old stars found in the vicinity of the center of the Milky Way, something that has not heretofore been possible. In particular, the entire star formation history of our Galactic nucleus can now begin to be decoded." Besides making new observations, the team will also explore more computer models to determine more accurate physical parameters for the star and the chemical composition of the ejected material.
The Research Team
Don F. Figer, Mark Morris, and Ian S. McLean (UCLA), Gene Serabyn (Caltech), and R. Michael Rich (Columbia Univ.)
