STS-67 Day 4 Highlights

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On Sunday, March 5, 1995 at 6 a.m. CST, STS-67 Payload Status Report # 07 reports: (3/5:22 MET) Commander Steve Oswald maneuvered the Space Shuttle Endeavour to enable the Hopkins Ultraviolet Telescope (HUT) to lead an observation of a mysterious star in the constellation Cassiopeia. This star, known to astronomers as KPD0005, belongs to a class of planetary nebulae, or clouds of gas with bright centers, that includes some of the hottest stars known. So hot are these stars' surfaces, in fact, that their elements are completely ionized, making their chemical composition difficult to analyze. Only ultraviolet astronomy, such as the Astro-2 instruments employ, can yield answers to questions about the chemistry and evolution of this type of star.

Following this, the HUT science team then turned their attention back to the search for signs of interstellar helium, using light from a distant quasar, known as 1700+64. Since it is believed that helium was formed in the Big Bang at the beginning of the universe, the actual detection of intergalactic helium would probably be the most significant scientific result that the HUT observations might produce. Because of the enormous distances of these objects, the targets are necessarily faint, and such observations are the most difficult that HUT scientists will attempt. They are also the most sensitive measurements ever made for interstellar helium.

Another priority for the HUT scientists involves the study of Type II Seyfert galaxies. These galaxies, such as NGC1068 which was observed last night, have bright centers that emit energy over a broad range of frequencies. It is believed that the core of NGC #1068 is ionized by shock waves of gas moving at hundreds of miles per second, and the ultraviolet observations by the Astro-2 instruments are testing this hypothesis.

The Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) team has also taken great interest in NGC 1068 and made some very difficult indirect observations of the galaxy's core using the electron cloud above the galaxy as a polarization mirror. "By observing the diffuse light reflected from the electron cloud, we are able to take measurements of the otherwise obscure nucleus of the galaxy," said WUPPE co-investigator Karen Bjorkman. On Astro-1, observations by WUPPE helped confirm that a thick torus, or doughnut-shaped cloud, exists around the nucleus of NGC #1068, while HUT observations showed unexpectedly high temperatures near the torus.

Early in the morning, WUPPE scientists also took the opportunity to view a couple of interactive binary star systems, V356 Sagittarii and Vela X-1. A binary star system consists of a pair of stars that orbit each other. In the case of an interactive binary, the stars stay so close to each other that they actually exchange gases.

Astronomers believe that in an X-ray binary such as Vela X-1, a neutron star (or pulsar) gravitationally strips material off of its companion star. A lot of the stripped material settles into orbit to form an oval disc shape. Polarization measurements by WUPPE allow scientists to measure the size and shape of this disc, and it also provides information about the stars involved. As WUPPE Principal Investigator Arthur Code observed, "the unique feature of interactive binary stars' polarization is that we can use that information to calculate the mass exchanged between the stars." Last night's observations were enhanced by the fact that the Astro-2 instruments could view both members of each binary side by side as they interact.

The Ultraviolet Imaging Telescope (UIT) made observations of an open cluster called NGC #3532. This is a diffuse cluster of stars and is believed to be less than ten million years old. Because UIT's field of view is well matched to the sizes of most clusters in our Milky Way Galaxy, UIT scientists are able to perform ultraviolet observations of many stars at the same time. UIT observers are searching for ultraviolet counterparts to X-ray sources as well as for white dwarfs and other faint, hot stars. These ultraviolet-bright objects are of great interest for several reasons: they can be used as probes of the interstellar medium; they can be used to provide highly accurate distance measurements to the clusters; and they will enhance understanding of the chemical evolution of the Milky Way Galaxy.

UIT also imaged several ancient globular clusters in our Milky Way galaxy as well as other nearby spiral galaxies. The latter observations will be used to further our understanding of star-formation in the spiral arms of these galaxies.

On Sunday, March 5, 1995 at 1 p.m. CST, STS-67 MCC Status Report # 07 stated that there has been some indications that the crew's stationary bicycle exercise is imparting some vibration to the shuttle that is interfering with the Instrument Pointing System's ability to precisely direct the Astro-2 telescopes at their targets. The crew is being asked to schedule exercise a little earlier so that the vibration has stopped by the time Endeavour moves into darkness and celestial observations must resume.

On Sunday, March 5, 1995 at 6 p.m. CST, STS-67 Payload Status Report # 08 reports: (3/17:22 MET) Both the Hopkins telescope and the Wisconsin Ultraviolet Photo- Polarimeter Experiment (WUPPE) examined two Seyfert galaxies, Markarian 279 and Fairall 9, in a study that could provide insights into some of the most powerful events in the universe. Named for Vanderbilt University astronomer Carl Seyfert, who identified this type of galaxy in 1943, Seyferts have extremely bright, compact cores containing clouds of gas moving at thousands of miles per second. Astronomers theorize that supermassive black holes would be the only source capable of generating the immense power given off by these objects.

The HUT team also practiced acquiring quasar Q 1542+54 to refine procedures for locating it efficiently in future observations. Their success gives them an additional illumination source to use in their search for intergalactic helium. Yesterday, they successfully locked onto the quasar 1700+64, which will likely remain their primary target for the study.

Several observations today examined the dust and gas from which stars form. HUT selected a reflection nebula and the star which illuminates it. The star, AE-Aurigae, is a very hot, massive blue star with surface temperatures from 50,000 to 70,000 degrees Fahrenheit (28,000 to 40,000 Kelvin). Light from the star illuminates a cloud of dust surrounding it, creating a bright, colorful nebula. Observation by the Astro telescopes may help determine the composition of the dust and the sizes and shapes of the grains.

The first observation of the day with WUPPE as the lead instrument was HD 50138, viewed after Mission Specialist John Grunsfeld and Payload Specialist Ron Parise took over science crew duties. This pre-main-sequence star is thought to be a young object, several times more massive than our sun, that has just formed out of the interstellar gas and dust. Polarization measurements should help show the shape of the nebula which surrounds the star and indicate where it is coming from. The study should help astronomers learn more about the ways in which young stars form and how their birth clouds eventually disperse.

The WUPPE team also observed the star HD 197770, an interstellar polarization probe. The star serves much like a flashlight, illuminating the vast and normally invisible clouds of dust and gas in the space between stars. The materials in these swirling clouds are, in essence, the stuff of past and future stars. Over eons, as stars grow old, they kick out vast amounts of very hot material -- mostly gas and dust -- into space. These materials form into clouds, and over many thousands of years the clouds collapse and form new stars. By determining how much the ultraviolet light passing through the clouds is polarized, or vibrating in a preferred direction rather than randomly, WUPPE can provide further clues as to the nature of this interstellar dust.

Phi Persei, a hot, rapidly spinning star, was another subject of study for WUPPE. The star exhibits an unusual ultraviolet spectrum that may be caused by a shell of gas around the star, possibly the remains of a layer that has been spun off the star by its rapid rotation. This shell is thought to be in the form of a disk around the equator of the star, and the scattering of starlight by the disk polarizes the light.

Celestial objects chosen for observation by the Ultraviolet Imaging Telescope (UIT) included M31, the famous Andromeda galaxy in our "local group," the cluster of galaxies which includes our own Milky Way. UIT researchers are conducting ultraviolet studies of the structure of local group galaxies. They will also examine the photographs to study the physics of star formation contained in these galaxies.

The UIT team also selected various kinds of open and globular star clusters, groups of stars formed from the same basic material at the same time. By comparing clusters of different ages and by looking at different types of stars of the same age, astronomers can learn a great deal about how each type of star changes in its lifetime.

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