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 ruptions, 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."
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.)