Chandra And X-Ray Astronomy

Have you ever thought about how we know that black holes exist? There are ways, for example we can observe the orbits of celestial objects. If it appears that they aren’t revolving around a planet or a star then you can pretty much bet that they are circling a black hole. Another way is when stars of a specific magnitude die they explode or go supernova and when they do they unleash a barrage of cosmic rays and one of those types of rays are x-rays, when this happens the Chandra X-Ray Telescope goes to work. The Chandra X-Ray Observatory is named after the late astrophysicist Subrahmanyan Chandrasekhar. The observatory was released into space from the space shuttle Columbia (STS-93 ) on July 23rd, 1999. Weighing in at over fifty thousand pounds it was, up until that time, the largest payload ever carried by a shuttle. This enormous amount of weight was due largely in part to the Inertial Upper Stage booster rocket that was needed to carry the telescope to its orbit roughly 83, 000 miles above the earth. At this altitude it is safe from radiation emitted from the Van Allen Belt which could be detrimental to the telescope and also clear of the orbits of geostationary satellites. The observatory is operated by the Chandra X-Ray Center at the Harvard-Smithsonian Center For Astrophysics in Cambridge, Mass and is managed by NASA’s Marshall Space Flight Center in Huntsville, AL. You may ask yourself why can’t we just observe these exploding stars from earth? Well the answer is simple the earth’s atmosphere absorbs x-rays therefore x-rays that are transiting the universe have to be collected by space based equipment. However, we can observe the remnants of supernovas from earth but the amount of time that it takes for that light to reach us can range anywhere from hundreds of years to ten’s of thousands of years, or longer. Chandra gives us more of a real-time indication as to when these stars met their fate, now let’s get to the nuts and bolts on how this incredible piece of equipment works!

The telescope has four mirrors, which are coated with the very rare element Iridium (Ir-77) which acts as a reflector, that collect x-ray emissions and the photons (the basic unit of light) that comprise them. The source of these types of high energy x-rays are from extremely hot gases that are usually associated with a major galactic event, such as a supernova. The first known source of this type of x-ray, outside of the sun, was discovered in 1962 emanating from the constellation Scorpius and was named Scorpius X-1 (Sco X-1). The mirrors on Chandra are nested inside one another and are positioned at an angle that is almost perpendicular to the incoming x-rays so that when they strike they hit the 2 upper mirrors and then skip to the 2 bottom mirrors, this double skip is necessary in order for the x-rays to come into focus, they then travel down a 26 foot tube towards the telescopes scientific instruments. As they travel towards these instruments gratings, which have thousands of thin openings are moved into the path of the ray’s. These grating’s separate the x-ray’s by their wavelength, this step is very important because it provides valuable information on each ray’s specific elements as well as the targeted objects density, temperature and motion. The instruments that are used to process the information contained within the x-rays/photons are the Advanced CCD Imaging Spectrometer (ACIS) which provides images and spectral information of the targeted object and a High Resolution Camera (HRC) which uses a Micro Channel Plate (MCP) which is used for the detection of particles such as electrons and ions and radiation from ultraviolet rays and x-rays. These two instruments can be used in unison or independently as needed.

One of the many notable photographs taken by Chandra occurred on September 5th, 2001 when it zeroed in on a highly active area of x-ray activity. The source was later determined to be emanating near the borders of the constellation’s Sagittarius and Scorpius but was hidden from scientists trying to locate it in the optical spectrum, this was due in part to dust from the arm of the Milky Way shielding it. Chandra went to work and captured some of the x-ray’s coming from it and provided a beautiful picture of what has become widely considered among scientists as a supermassive black hole named Sagittarius A, or simply Sag A*residing very close to the center of our own galaxy and it’s devouring anything that is unfortunate enough to get to close to it. It is estimated that this particular black hole is the size of 4. 1 million solar masses, to put this number into perspective the sun is only 1 solar mass and it’s 109 times larger than the earth! This is why Sag A* is classified as supermassive because it could easily swallow up our entire solar system if the two ever met. Scientists at UCLA have also theorized that there are approximately 10, 000 smaller black holes as well as neutron stars, stars composed of neutrons usually due to them going supernova, circling Sag A*. These provide a steady diet for the black hole as roughly every 1 million years one of these smaller black holes or neutron stars gets devoured by Sag A* and is squeezed to an infinite density by the iron grip of its gravitational field. Black holes not only like to crush objects with their gravitational force but they also like to warp things with that same force and one of those things is the fourth dimension, time. Time is so warped within a black hole that Einsteins theory of relativity breaks down and cannot be used instead quantum gravitational calculations have to be applied.

All of this talk of exploding stars and black holes might make you feel uneasy but the likelihood of Sag A* and earth ever crossing paths is very low especially when you consider that Sag A* is roughly 25, 000 light years from earth and 1 light year is equal to 5. 87 trillion miles, of course this distance keeps decreasing as the blackhole continues to expand but as it does the strength of its gravitational field decreases proportionally. Over the years Chandra, using its x-ray gathering vision, has located and shown the world celestial objects that would have otherwise been undetectable. This has been made possible by the brilliant minds that designed and assembled this remarkable piece of equipment, from the people at Northrup Grumman who built Chandra to the people at Raytheon Optical Systems who ground and polished the mirrors to the team at Optical Coating Labs who further polished the lenses to 99. 99% dust free. This was a typical NASA team effort and as usual it was carried out with the professionalism that has become the agency’s calling card. Chandra will eventually be deactivated and left out in space but the contributions that it has made to science will continue to educate future generations for years and years to come.