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  • Published on: 15th October 2019
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Chandra X-ray Observatory

NASA's Chandra X-ray Observatory was launched and deployed by Space Shuttle Columbia on July 23, 1999, is the most sophisticated X-ray observatory built to date. The observatory was first proposed to NASA in 1976, they began funding in 1977, and after twenty years it was launched into space. Chandra was created to observe X-rays from high-energy regions of the universe, such as the remnants of exploded stars.

The Observatory has three major parts, the first is the X-ray telescope, whose mirrors focus X-rays from celestial objects; second is the science instruments which record the X-rays so that X-ray images can be produced and analyzed; and third is the spacecraft, which provides the environment necessary for the telescope and the instruments to work.

Chandra's unusual orbit was achieved after deployment by a built-in propulsion system which boosted the observatory to a high Earth orbit. This orbit, which has the shape of an ellipse, takes the spacecraft more than a third of the way to the moon before returning to its closest approach to the Earth of 16,000 kilometers (9,942 miles). The time to complete an orbit is 64 hours and 18 minutes. The spacecraft spends 85% of its orbit above the belts of charged particles that surround the Earth. Uninterrupted observations as long as 55 hours are possible and the overall percentage of useful observing time is much greater than for the low Earth orbit of a few hundred kilometers used by most satellites.

Extraordinary commitment and precision is required to plan and build telescopes that will be placed in space where they are operated by remote control in a hostile environment of wild temperature swings and hard vacuum, after withstanding the controlled fury of launch. The entire process typically takes many years and creativity is demanded when unexpected changes are imposed. The Chandra observatory was first proposed to NASA in 1976 and funding began in 1977 when NASA's Marshall Space Flight Center started the definition studies of the telescope. In 1992, there was a major restructuring of the observatory. NASA decided that in order to reduce cost, the number of mirrors would be decreased from twelve to eight and only four of the six scientific instruments would be used. At this point the planned orbit was changed from low to high Earth orbit to preserve the scientific capability of Chandra. Teams of scientists, engineers, technicians and managers who work at numerous government centers, Universities and corporations have been building and assembling Chandra over the past twenty years. Many of these dedicated men and women have been involved in the project from its inception.

Originally, it was called the Advanced X-ray Astrophysics Facility, but in 1998 it was changed to Chandra after Subrahmanyan Chandrasekhar. Chandra immigrated in 1937 from India to the United States, where he joined the faculty of the University of Chicago, a position he remained at until his death. He and his wife became American citizens in 1953.

Trained as a physicist at Presidency College, in Madras, India and at the University of Cambridge, in England, he was one of the first scientists to combine the disciplines of physics and astronomy. Early in his career he demonstrated that there is an upper limit to the mass of a white dwarf star, which is now called the Chandrasekhar limit. A white dwarf is the last stage in the evolution of a star such as the Sun. When the nuclear energy source in the center of a star such as the Sun is exhausted, it collapses to form a white dwarf. This discovery is basic to much of modern astrophysics, since it shows that stars much more massive than the Sun must either explode or form black holes.

Chandra was a popular teacher who guided over fifty students to their Ph.D.’s. His research explored nearly all branches of theoretical astrophysics and he published ten books, each covering a different topic, including one on the relationship between art and science. For 19 years, he served as editor of the Astrophysical Journal and turned it into a world-class publication. In 1983, Chandra was awarded the Nobel prize for his theoretical studies of the physical processes important to the structure and evolution of stars.

Cassiopeia A is a well-known supernova remnant located about 11,000 light years from Earth. Chandra's sharp X-ray vision allows scientists to determine both the amount and location of these crucial elements objects like Cas A produce. Due to its unique evolutionary status, Cassiopeia A (Cas A) is one of the most intensely studied of these supernova remnants. A new image from NASA's Chandra X-ray Observatory shows the location of different elements in the remains of the explosion: silicon (red), sulfur (yellow), calcium (green) and iron (purple). Each of these elements produces X-rays within narrow energy ranges, allowing maps of their location to be created. The blast wave from the explosion is seen as the blue outer ring.

X-ray telescopes such as Chandra are important to study supernova remnants and the elements they produce because these events generate extremely high temperatures — millions of degrees — even thousands of years after the explosion. This means that many supernova remnants, including Cas A, glow most strongly at X-ray wavelengths that are undetectable with other types of telescopes.

Chandra's sharp X-ray vision allows astronomers to gather detailed information about the elements that objects like Cas A produce. For example, they are not only able to identify many of the elements that are present, but how much of each are being expelled into interstellar space.

The Chandra data indicate that the supernova that produced Cas A has churned out prodigious amounts of key cosmic ingredients. Cas A has dispersed about 10,000 Earth masses worth of sulfur alone, and about 20,000 Earth masses of silicon. The iron in Cas A has the mass of about 70,000 times that of the Earth, and astronomers detect a whopping one million Earth masses worth of oxygen being ejected into space from Cas A, equivalent to about three times the mass of the Sun. (Even though oxygen is the most abundant element in Cas A, its X-ray emission is spread across a wide range of energies and cannot be isolated in this image, unlike with the other elements that are shown.)

Astronomers have found other elements in Cas A in addition to the ones shown in this new Chandra image. Carbon, nitrogen, phosphorus and hydrogen have also been detected using various telescopes that observe different parts of the electromagnetic spectrum. Combined with the detection of oxygen, this means all of the elements needed to make DNA, the molecule that carries genetic information, are found in Cas A.

Oxygen is the most abundant element in the human body (about 65% by mass), calcium helps form and maintain healthy bones and teeth, and iron is a vital part of red blood cells that carry oxygen through the body. All of the oxygen in the Solar System comes from exploding massive stars. About half of the calcium and about 40% of the iron also come from these explosions, with the balance of these elements being supplied by explosions of smaller mass, white dwarf stars.

While the exact date is not confirmed, many experts think that the stellar explosion that created Cas A occurred around the year 1680 in Earth's timeframe. Astronomers estimate that the doomed star was about five times the mass of the Sun just before it exploded. The star is estimated to have started its life with a mass about 16 times that of the Sun, and lost roughly two-thirds of this mass in a vigorous wind blowing off the star several hundred thousand years before the explosion.

Earlier in its lifetime, the star began fusing hydrogen and helium in its core into heavier elements through the process known as "nucleosynthesis." The energy made by the fusion of heavier and heavier elements balanced the star against the force of gravity. These reactions continued until they formed iron in the core of the star. At this point, further nucleosynthesis would consume rather than produce energy, so gravity then caused the star to implode and form a dense stellar core known as a neutron star. The exact means by which a massive explosion is produced after the implosion is complicated, and a subject of intense study, but eventually the infalling material outside the neutron star was transformed by further nuclear reactions as it was expelled outward by the supernova explosion. Chandra has repeatedly observed Cas A since the telescope was launched into space in 1999. The different datasets have revealed new information about the neutron star in Cas A, the details of the explosion, and specifics of how the debris is ejected into space.

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