Stars can live for billions of years, shining brightly in a dark universe until they exhaust the nuclear fuel at their cores.
But for many stars, the trade-off for such longevity is a spectacularly violent end: They may be torn apart as supernovae, or they may collapse in upon themselves, forming a black hole so dense and powerful that not even light can escape its gravity. This is how such an extreme phenomenon in the universe influences its surroundings. If matter falls into a black hole, all evidence of it will disappear. But first it sends a swan song of radiation into the universe.
This is the radiation that the Nuclear Spectroscopic Telescope Array (NuSTAR), which was launched into orbit in June, is to detect. With sensitivity up to 100 times greater and resolution capability 10 times greater than its predecessors, NuSTAR will provide much sharper images than previously seen. As the first telescope ever to use true focusing optics in surveying the high-energy X-ray region of the electromagnetic spectrum, NuSTAR’s mission will be to map those heavenly bodies that emit hard or high-energy X-ray radiation.
Astronomers hope to find out which galaxies contain a supermassive black hole, how fast the holes rotate and how they grow so big. They also plan to take a closer look at the radiation from neutron stars and supernovae.
Black holes can’t be seen and don’t emit radiation or even let it escape — they only reveal themselves by the extreme effect they exert on their surroundings: In some cases, the temperature around a black hole is so high that matter releases X-ray radiation, which scientists can detect via an X-ray telescope.
NuSTAR is not the first X-ray telescope to be launched, but its sensitivity and extremely high resolution set it apart. Its optics are larger than any previous X-ray telescope optics. They focus the X-rays before they strike the newly developed detectors that function as X-ray cameras. This sharpens the images and allows astronomers to detect even weak X-ray sources, which they otherwise couldn’t see.
However, because X-rays tend to pass through objects, they can be absorbed, which makes them difficult to capture. But they can be reflected and imaged if they are slightly bent with a specially designed mirror system.
NuSTAR’s optics consist of two units of 133 concentric tubes made of a very thin layer of flexible glass, somewhat like that used in laptop computer displays. The glass has a special coating that reflects X-rays and sends them to the detectors. There, the X-rays are converted into electrical signals, which astronomers see as ultrasharp images of the universe.
In these images, astronomers will be able to see the largest black holes hidden at galaxies’ centers, which can weigh as much as billions of suns. When they swallow clouds of gas and even entire stars, one of the most violent energy discharges in the entire universe occurs.
Astrophysicists have been chasing the secrets of massive black holes for years. These ultradense enigmas undoubtedly played a vital role in the evolution of the universe: Black holes likely grew along with galaxies, and so their presence influenced star formation within galaxies. If scientists can understand how galaxies developed, they may also come to understand more about the evolution of the universe.
One of the primary goals for NuSTAR is to provide scientists with information about what happens in galactic cores with supermassive black holes. A tremendous amount of electromagnetic radiation is emitted from these cores, the theory goes, because the matter becomes superheated as it orbits ever closer to the black hole. But astrophysicists don’t fully understand the details of this process.
Jets of charged particles are also emitted from the area surrounding super-massive black holes, moving away from the black hole nearly at the speed of light. Scientists hope NuSTAR will also shed light on how these jets originate.
One of the places NuSTAR will examine is our own galaxy, the Milky Way, which is thought to contain a supermassive black hole of relatively modest dimensions, about the mass of 4 million suns. Astrophysicists are certain that the black hole exists, because they can see stars orbiting it, but so far information about it has come from indirect observation of those stars. NuSTAR’s direct observations should help astronomers understand the black hole’s behavior and how it affects the Milky Way.
The telescope will not only be able to take a look at the supermassive black holes at galactic centers, but at smaller black holes as well. Superheavy stars explode as supernovae at the end of their lives, with the lightest ones becoming neutron stars and the heavier ones turning into black holes. Scientists hope to use data from NuSTAR to discover how a supernova explodes and is subsequently converted into a black hole.
We already know that supernovae occur when the heaviest stars run out of matter to fuse in their cores, but we don’t know much about how these explosions take place. A supernova emits an extreme amount of light and ejects lots of heavy elements into the universe before it finally ends up as a very compact object: either a black hole or a neutron star.
The elements that are spewed out are later reused in new star systems; our own solar system originated in this way. Because NuSTAR can detect the radiation from the radioactive substances generated in the explosion, astronomers intend to use this data to reconstruct the process of a supernova explosion.
The ideal scenario is for NuSTAR to observe a supernova in action in the Milky Way, but this is unlikely. Around 50 years pass between such events in our galaxy, so it would take a phenomenal amount of luck for the telescope to spot one during its two-year primary mission.
There will be no shortage of tasks for the new telescope, and if everything goes according to plan, NuSTAR will collect an extraordinary amount of data in just a few years’ time. And it isn’t alone; other X-ray telescopes soon will be looking into extreme phenomena. Russia’s Space Research Institute and Germany’s Max Planck Institute for Extraterrestrial Physics are collaborating with other space agencies on the launch of the Spectrum-X-Gamma telescope in early 2013; however, the project has already been delayed several times. Even if that launch is canceled, the Japan Aerospace Exploration Agency is poised to deploy the Astro-H X-ray telescope, which was developed in conjunction with NASA, in 2014.
Before long, the new generation of X-ray telescopes—their ambitious missions fueled by their superior optics — may well answer some of the central questions about black holes, the matter they affect and the galaxies at their mercy.
In search of… black holes
Ever since the 1960s, satellites equipped with X-ray cameras have orbited Earth, providing astronomers with a wealth of information about the universe. In fact, such observations of the extreme radiation immediately surrounding black holes has helped confirm their existence.
Both NASA’s Chandra telescope and the XMM-Newton launched by the European Space Agency have been functioning excellently since they were launched in 1999, producing impressive X-ray images of the universe and generating thousands of scientific articles. But these telescopes are only able to detect soft X-rays, which do not penetrate the gas clouds of the universe. Thanks to advancements in X-ray mirrors and detectors in recent years, the NuSTAR telescope can now detect more violent hard X-ray radiation much more precisely, making it extremely useful in the search for black holes.
SUPERNOVAE: the death throes of stars
Some massive stars end their lives in spectacular explosions as supernovae. Gas and dust are ejected into space, leaving behind a small, very dense object: a small black hole or a neutron star.
If either of these pairs up with” a star, it can attract matter from the star. Just as happens in supermassive black holes at galactic centers, the matter forms a disk. When it is heated, the matter emits X-rays that the NuSTAR telescope can detect. Measuring these X-rays can help give astronomers an idea of the number of black holes and neutron stars that exist in our galaxy.
NuSTAR will also survey the remains of several supernovae that have exploded in the Milky Way during the past 500 years. The radiation from such remnants may tell scientists how the explosion happened and provide them with a new understanding of these stellar phenomena.
BLACK HOLES: the gluttons of galaxies
Some astronomers are convinced that most large galaxies have a supermassive black hole at their centers that attracts gas, dust and stars with an extraordinarily powerful gravitational pull. This process generates intense radiation seen only in conjunction with black holes. NuSTAR will allow scientists to analyze this radiation and estimate the size of many black holes as well as how they attract matter.