Arts & Sciences

Newsletter
Fall 1995 Vol. 17 No. 1

Neutron Stars: New Frontiers around Old Stars

James Cordes

The combination of fast spin and intense magnetic field acts as an electric generator, producing voltages in excess of a trillion volts across the surface of a neutron star. These sources of energy cause the radiation from neutron stars, which is often seen as precisely spaced pulses. Objects that generate such radiation are called pulsars. Pulses are seen because the radiation emanates in a beam, similar to that of the lamp in a lighthouse, and is swept across our direction, once for each rotation period of the star (each pulsar "day").

My purpose here is to describe what the future may reveal, but more importantly, I hope to explain why this kind of science is important and why it is important for our society to support basic research.

First, let me give you some background information on neutron stars and pulsars. Pulsars were discovered serendipitously in 1967 by Jocelyn Bell, Anthony Hewish, and others at Cambridge University. The lighthouse analogy mentioned above is an apt one because, when the signals were first deduced to be celestial in origin, it was speculated that they might be the beacons of advanced extraterrestrial civilizations. Within a few months, however, the pulses were demonstrated to be consistent with natural source s of radiation, and discussions over the next year or so led to the firm conclusion that the particular objects in question were neutron stars, as Thomas Gold of Cornell University had suggested.

So . . . what is a neutron star, anyway? A neutron star is both a giant nucleus and a tiny, hyperdense star. It is formed during the collapse of a star as gravity squeezes ordinary atoms together, thereby melding the electrons and protons into neutrons. Neutron stars are common in our Milky Way galaxy and elsewhere because they are a natural endpoint of stars that are about ten times more massive than the sun. They are "born" in the most violent events we know of -- supernova explosions. We have measured t he speeds of neutron stars and have found an extraordinary one, moving 1,600 kilometers per second (New York to L.A. in three seconds!). This object is leaving behind a wake, like a boat moving on a lake, as it moves through the galaxy. The shape of this wake is such that we have dubbed it the "Guitar Nebula." With its observed speed, it will eventually escape the gravity of the Milky Way and enter intergalactic space.

What else makes neutron stars interesting? They may underlie the mystery of gamma-ray bursts, which are bright flashes of gamma rays (particles similar to light particlesÑphotons -- but with much more energy than light). The flashes last about one second and are seen about once per day. They occur in all directions. Discovered in 1967 by spy satellites looking for gamma-rays from nuclear explosions on Earth, the bursts were later thought to arise from events happening on relatively nearby neutron stars.

It is now thought that bursts originate from sources much farther away than the neutron stars in our neighborhood of the galaxy. Rather, they may be from neutron stars in a "halo" that surrounds the Milky Way. Or they may be in the furthest known galaxies, all the way across the universe. In the latter case, known as the "cosmological" interpretation, the total power radiated in a burst is enormous. And the easiest way to explain the radiation in this case is through the collision of two neutron stars that have spiralled into each other as an endpoint of their binary partnership. Neutron stars can have companions other than neutron stars. In fact, we have identified pulsars that have any companion that you can think of -- normal "main sequence" stars; white-dwarf stars; neutron stars; even planets, such as the three planets now identified around the millisecond pulsar 1257+12, discovered and analyzed at Arecibo by Alex Wolszczan, now at Penn State. This spectacular system lends credence to the idea that pl anet formation is a common, natural process that does not rely on improbable accidents. At Cornell, graduate student Joe Lazio and I are using sophisticated computer methods to analyze data on pulsars to find additional planets. One of our approaches is based on "genetic algorithms." These algorithms use genetic-like manipulations of mathematical variables to explore the possible orbits of planets around neutron stars. We have found genetic algorithms to be fascinating in themselves and expect that they wi ll find additional applications in astronomy.

There is one kind of pulsar companion that we have not found . . . yet: the black hole. Very massive ordinary stars can form black holes directly when they explode as supernovae. Our best estimates suggest that there should be about as many binary systems containing a black hole and a neutron star as there are double-neutron-star binaries. So . . . why don't we know of any black-hole binaries? Well, they may be as "common" as double-neutron-star binaries, but that is still fairly rare. Moreover, they may b e harder to find, for technical reasons. But, finding such binaries is part of the Òholy grailÓ that several research groups around the world are seeking.

Indeed, what are we doing in neutron-star science and how and why are we doing it? It is truly exciting to examine a pulsar signal as it appears on a computer screen or oscilloscope. One sees a blip of radio energy, a fraction of a second in duration, that typically has traveled for thousands of years through the galaxy. And it has originated from an object that is stranger than anything we can ever experience in the laboratory. But it is even more exciting to find new pulsars! We now know of about 700 pul sars in the galaxy out of an estimated 100,000 active radio pulsars and about a billion older, inactive neutron stars. When we search for new pulsars, we do not aim at merely increasing the size of our collection, in baseball-card fashion. Rather, we are trying to find the rare star in an unusual situation, such as one having a black-hole companion. Or others with neutron-star companions that have very short orbital periods, such as one hour or less. These would be extraordinary laboratories of space-time!

A project now under way in my pulsar group is the construction of a new digital hardware device that will allow much more sensitive searches for pulsars. A masters-degree student in electrical engineering, Dirk Koechner, is working on some of the design. We have incorporated copies of some of the hardware in our junior-senior laboratory course (Experimental Astronomy), which uses the dish antenna on the roof of the Space Sciences Building.

A question on everyone's mind, especially in the current state of the world, is "What good is this?" where "this" applies to anything in industry or academia or government. When applied to astronomy, a discipline that is enjoyed by many but does not immediately suggest "necessary," I would begin by pointing out that astronomy is a perfect venue for teaching students a wide range of skills and disciplines. These range from electronics, high-performance computing, and fundamental physics, to important social aspects of science, such as networking, cooperation, and scientific ethics.

Another way of answering the question is by describing what some of my former Ph.D. students are doing. Two are teaching at colleges or universities and are active in research. Another is working at Brookhaven on particle accelerators. Still another has worked for seven years at Bell Labs on underwater sonar systems. Two are working at the Naval Research Laboratory, one on mostly astronomical research, another on remote sensing of the Earth. Astronomy at the graduate level is excellent training for a numbe r of postgraduate disciplines. I believe that astronomy is special among the sciences because physics and engineering learned and applied in the context of astronomy stretch one's imagination and understanding. And, therefore, students trained in astronomy are well prepared to contribute to our society in a host of ways, whether in a technical field in industry, government, academia, or in fields such as space law, which has technical components. Navigation is a traditional mainstay of navies that have rel ied on astronomy for millennia. It is therefore not surprising that the U.S. Navy is still a significant employer of astronomers. But instead of using sextants and optical observations of stars, the navy and others now use radio observations of quasars to track and analyze the rotation of the Earth.

The third part of my answer to "What good is this?" recognizes that, while astronomy does not bring food to the table in any direct way, it certainly delivers food for thought; and I think it is in the question of our origins and our destiny where lies the appeal of the stars to virtually every living person.