White Dwarf Pulsar
Published on Jan 7, 2008 at 11:07 pm.
2 Comments.
Filed under neutron stars, stars, white dwarfs.
In introductory astronomy classes, we talk about white dwarfs and pulsars. They are generally thought of as completely separate end products of stellar evolution. A star of less that 8 solar masses (initial mass) will shed its outer layers into space and leave behind a degenerate object known as a white dwarf. The white dwarf is like the burnt out core of the star. It has no nuclear fusion, and unless it is part of a binary star system, it generally doesn’t do much other than just cool down. White dwarf stars do, however, have extremely strong magnetic fields, high gravity at their surfaces, and rapid rotation rates. These properties give rise to all sorts of interesting observable features that I don’t want to go into in this post. Most white dwarf stars have a mass of about a solar mass, or so. Under no circumstances can a white dwarf have more than 1.4 times the mass of the Sun.
Stars that initially are larger than 8 solar masses die as their core collapses in a violent event that throws the outer parts of the star off in a gigantic explosion called a supernova. What is left after the supernova is a giant cloud of expanding gas and a tiny collapsed object that is what is left of the core of the star. Usually, this tiny remnant is a body compressed to such an extent that most of its atoms are destroyed and it consists of a collection of subatomic particles, mostly neutrons. We call this a neutron star. Interestingly, except for very large stars, most neutron stars are not much more massive than white dwarf stars at their formation. If the star has exceedingly high mass, then the remnant may have just a bit too much mass. If a neutron star has too much mass, then it collapses further to the point that it becomes a black hole. Neutron stars also have powerful magnetic fields, even more powerful than those of white dwarfs. They also rotate much faster than white dwarfs. The intense magnetic fields give rise to beams of radiation that emanate from near the magnetic poles of the neutron star. As on Earth, the magnetic poles tend to not align exactly with the rotational poles. So, these beams of radiation sweep through space like giant search lights. If the rotation of the neutron star causes the beam to sweep past the direction of Earth (as seem from the neutron star), then we here on Earth observe a pulse of radiation coming from the neutron star. The neutron star seems to have very regular pulses of radiation as it rotates. In that case, we call it a pulsar.
I could do entire blog entries on the process of stellar death and stellar remnants. I spend over a week on the topic in my introductory astronomy class. But, what I wanted to talk about today is something different that I saw in a recent news note from NASA.
Yukikatsu Terada, a researcher at Saitama University, in Japan, using the Suzaku X-Ray observing satellite (a joint NASA and JAXA mission) has found a white dwarf star that is acting a lot like a neutron star pulsar. The white dwarf that is the subject of his study is AE Aquarii. It was already known to be an odd sort of beast. AE Aquarii (AE Aqr) is a binary star consisting of a white dwarf and an ordinary star. Material is pulled from the ordinary star onto the white dwarf. This material heats up, flares, and does all sorts of interesting things that makes the binary star change in brightness. It is thus a type of variable star called a cataclysmic variable. But, AE Aqr was known to be strange in that the white dwarf was known to be far more magnetically active than most other white dwarf stars in cataclysmic variable pairs. Terada was looking to see if the magnetic field was capable of accelerating particles to high speeds, in much the way that the magnetic field of neutron stars can do. What he found, using Suzaku, was that AE Aqr actually pulses in X-rays, much like a neutron star pulsar. That means that AE Aqr is acting like a white dwarf pulsar.
So, the universe is full of all sorts of interesting things out there. It never ceases to amaze me at how many new interesting things there are to find.
-Astroprof
Image courtesy of NASA, Casey Reed
thirst4knowledge on May 19, 2008 at 8:16 am: 1
does these pulses have any effect on earth or electronic devices?
Astroprof on May 19, 2008 at 12:57 pm: 2
No, they are far too weak at this distance.