Hubble Space Telescope Establishes Accurate New Distance Measurement To Neighbouring Galaxy
16 January 1991
Recent observations of the remnants of Supernova 1987A, conducted with NASA's Hubble Space Telescope have provided an unexpected bonus - an accurate determination of the absolute distance to the Large Magellanic Cloud, a satellite galaxy of the Milky Way, located in the southern hemisphere.The refined value of 169 000 light-years is accurate to within five percent say the researchers. Previous estimates range from 343 000 to 179 000 light-years. This new measurement is a cornerstone for the determination of the size and the age of the Universe.
These findings were reported today at the meeting of the American Astronomical Society in Philadelphia, Pennsylvania by an international team of astronomers: Nino Panagia, Roberto Gilmozzi and F. Duccio Macchetto, ESA astronomers at the Space Telescope Science Institute in Baltimore, Dr. Hans-Martin Adorf at the Space Telescope European Coordinating Facility in Garching, Germany, and Dr. Robert P. Kirshner of the Harvard Smithsonian Center for Astrophysics in Cambridge.
"This is an achievement of great importance because the distance to the Large Magellanic Cloud is an essential step to the calibration of the cosmological distance scale," says Panagia. "Therefore, being able to determine this distance with an accuracy of 5% means that after a few more comparable steps, we will be able to estimate the Hubble constant (the distance scale of the Universe) to an accuracy of 10-15% and thus, the age of the Universe with that high an accuracy as well."
These results are based on observations made with the European Space Agency's Faint Object Camera (FOC) last August. The FOC revealed for the first time the detailed structure of an eerie gaseous ring surrounding the remnants of supernova 1987A, which exploded on 23 February 1987 in the Large Magellanic Cloud.
This ring is a relic of the nitrogen enriched stellar envelope that was ejected in the form of a stellar wind by the progenitor star when it was in the phase of red supergiant. The material was later compressed by a fast wind from the star when it had evolved back to a blue supergiant state, just a few thousand years before the explosion occurred. The stellar wind was apparently more efficient at compressing gas along an equatorial belt to produce a ring-like rather than shell-like structure.
"Thanks to the FOC observations," says Nino Panagia, "we know that we are dealing with a real ring which is an equatorial structure." HST's ability to resolve the ring structure has allowed astronomers to accurately measure its angular diameter which is the first step toward estimating the supernova's true distance. HST's high resolution shows that the ring has an angular diameter of 1.66 arcseconds, which is equivalent to the apparent separation between a pair of car headlights located 100 miles away.
Once aware of the ring structure, the astronomers were able to estimate the ring's physical diameter, which is the second step in calculating its true distance. This required analyzing three years worth of 5N1987A ultraviolet data gathered by NASA/ESA's International Ultraviolet Explorer (IUE) satellite. The data is a record of how ultraviolet emissions from the ring brightened and dimmed following the 1987 explosion.
Since the ring is inclined by 43 degrees along the line-of-sight, light from the full circumference of the ring did not reach Earth at the same instant. Light from the edge of the ring tilted toward Earth reached the IUE 80 days after the supernova explosion. Light from the edge of the ring tilted farthest away from Earth did not arrive until 420 days after the explosion. This time difference allows the researchers to accurately calculate the ring's physical diameter of 1.37 light-years.
With precise values for angular size and physical diameter in hand, determination of the true distance of 169 000 light-years is a simple and straightforward trigonometric calculation.
The supernova ring has also provided new insights into the evolution of a massive star. By comparing the present diameter of the ring and its expansion velocity, (as determined from observations made with IUE as well as ground-based optical measurements) the researchers infer that about 5000 years ago the star turned back to a blue supergiant after being a red supergiant for more than half a million years. "These are direct measurements of evolutionary times for an individual star," Panagia emphasizes, "This result has never been obtained before in any other case on time scales so diverse."
