The name alone, “magnetar” elicits a magnificent, powerful and strong astronomical object, and most of these “magnetic stars” are whirling, X-ray blasting dynamos, shooting out strong bursts of energy. But there are some magnetars which seem to have a softer, quieter side, and are called soft gamma repeaters and anomalous X-ray pulsars. However, they might not be as soft as they appear. A team of astronomers using the several different space- and Earth-based observatories have found a supposed ‘weakling’ was only masking its superpowers. The new findings indicate the presence of a huge internal magnetic field in these seemingly less powerful pulsars, which is not matched by their surface magnetic field.
Magnetars are a type of neutron stars, which are the collapsed remains of massive, rapidly rotating stars. They collapses down to tiny cores, with the hot neutron liquid rising and falling from the center to the crust setting up a dynamo effect, creating that incredible magnetic field. Although they are on average only about 30km in diameter, a magnetar can have a magnetic field billions of times that of our Sun.
It was thought that dramatic flares and bursts of energy came from only the strong class of magnetars, but these same features have been observed emanating from a weakly magnetized, slowly rotating pulsar.
“We have now discovered bursts and flares, i.e. magnetar-like activity, from a new pulsar whose magnetic field is very low,” said Dr Silvia Zane, from UCL’s (University College London) Mullard Space Science Laboratory, and an author of the research.
The neutron star, SGR 0418+5729, was discovered on June 5, 2009 when the Fermi Gamma-ray Space Telescope detected bursts of gamma-rays from this object. Follow-up observations four days later with the Rossi X-Ray Timing Explorer (RXTE) showed that, in addition to sporadic X-ray bursts, the neutron star exhibits persistent X-ray emission with regular pulsations that indicate that the star has a rotational period of 9.1 seconds.
What makes SGR 0418 different from similar neutron stars is that, unlike those stars that are observed to be gradually rotating more slowly, continued monitoring of SGR 0418 over a span of 490 days has revealed no evidence that its rotation is decreasing.
“It is the very first time this has been observed and the discovery poses the question of where the powering mechanism is in this case. At this point, we are also interested in how many of the other normal, low field neutron stars that populate the galaxy can at some point wake up and manifest themselves as a flaring source,” said Zane.
The team of astronomers, led by Dr. Nanda Rea of Institut de Ciencies de l’Espai (ICE-CSIC, IEEC) in Barcelona, wonder how large an imbalance can be maintained between the surface and interior magnetic fields. SGR 0418 represents an important test case.
“If further observations by Chandra and other satellites push the surface magnetic field limit lower, then theorists may have to dig deeper for an explanation of this enigmatic object,” said Rea.
The upper crust of a neutron star is thought to be composed of crystallized iron, may have centimeter high mountains and experiences occasional ‘star quakes’ which may precede what is technically known as a glitch. These glitches and the subsequent post-glitch recovery period may offer some insight into the nature and behavior of the superfluid core of neutron stars.
The events leading up to a neutron star quake go something like this. All neutron stars tend to ‘spin down’ during their life cycle, as their magnetic field applies the brakes to the star’s spin. Magnetars, having particularly powerful magnetic fields, experience more powerful braking.
During this dynamic process, two conflicting forces operate on the geometry of the star. The very rapid spin tends to push out the star’s equator, making it an oblate spheroid. However, the star’s powerful gravity is also working to make the star conform to hydrostatic equilibrium (i.e. a sphere).
Thus, as the star spins down, its crust – which is reportedly 10 billion times the strength of steel – tends to buckle but not break. There may be a process like a tectonic shifting of crustal plates – which create ‘mountains’ only centimeters high, although from a base extending for several kilometres over the star’s surface. This buckling may relieve some of stresses the crust is experiencing – but, as the process continues, the tension builds up and up until it ‘gives’ suddenly.
The sudden collapse of a 10 centimeter high mountain on the surface of a neutron star is considered to be a possible candidate event for the generation of detectable gravitational waves – although this is yet to be detected. But, even more dramatically, the quake event may be either coupled with – or perhaps even triggered by – a readjustment in the neutron’s stars magnetic field.
It may be that the tectonic shifting of crustal segments works to ‘wind ‘up’ the magnetic lines of force sticking out past the neutron star’s surface. Then, in a star quake event, there is a sudden and powerful energy release – which may be a result of the star’s magnetic field dropping to a lower energy level, as the star’s geometry readjusts itself. This energy release involves a huge flash of x and gamma rays.
In the case of a magnetar-type neutron star, this flash can outshine most other x-ray sources in the universe. Magnetar flashes also pump out substantial gamma rays – although these are referred to as soft gamma ray (SGR) emissions to distinguish them from more energetic gamma ray bursts (GRB) resulting from a range of other phenomena in the universe.
However, ‘soft’ is a bit of a misnomer as either burst type will kill you just as effectively if you are close enough. The magnetar SGR 1806-20 had one of largest (SGR) events on record in December 2004.
Along with the quake and the radiation burst, neutron stars may also experience a glitch – which is a sudden and temporary increase in the neutron star’s spin. This is partly a result of conservation of angular momentum as the star’s equator sucks itself in a bit (the old ‘skater pulls arms in’ analogy), but mathematical modeling suggests that this may not be sufficient to fully account for the temporary ‘spin up’ associated with a neutron star glitch.
González-Romero and Blázquez-Salcedo have proposed that an internal readjustment in the thermodynamics of the superfluid core may also play a role here, where the initial glitch heats the core and the post-glitch period involves the core and the crust achieving a new thermal equilibrium – at least until the next glitch.
The spectacular gravity of neutron stars offers great opportunities for thought experiments. For example, if you dropped an object from a height of 1 meter above a neutron star’s surface, it would hit the surface within a millionth of a second having been accelerated to over 7 million kilometers an hour.
But these days you should first be clear what kind of neutron star you are talking about. With ever more x-ray sensitive equipment scanning the skies, notably the ten year old Chandra space telescope, a surprising diversity of neutron star types are emerging.
The traditional radio pulsar now has a number of diverse cousins, notably magnetars which broadcast huge outbursts of high energy gamma and x-rays. The extraordinary magnetic fields of magnetars invoke a whole new set of thought experiments. If you were within 1000 kilometres of a magnetar, its intense magnetic field would tear you to pieces just from violent perturbation of your water molecules. Even at a safe distance of 200,000 kilometres, it will still wipe all the information off your credit card – which is pretty scary too.
Neutron stars are the compressed remnant of a star left behind after it went supernova. They retain much of that stars angular momentum, but within a highly compressed object only 10 to 20 kilometers in diameter. So, like ice skaters when they pull their arms in – neutron stars spin pretty fast.
Furthermore, compressing a star’s magnetic field into the smaller volume of the neutron star, increases the strength of that magnetic field substantially. However, these strong magnetic fields create drag against the stars’ own stellar wind of charged particles, meaning that all neutron stars are in the process of ‘spinning down’.
This spin down correlates with an increase in luminosity, albeit much of it is in x-ray wavelengths. This is presumably because a fast spin expands the star outwards, while a slower spin lets stellar material compress inwards – so like a bicycle pump it heats up. Hence the name rotation powered pulsar (RPP) for your ‘standard’ neutron stars, where that beam of energy flashing at you once every rotation is a result of the braking action of the magnetic field on the star’s spin.
It’s been suggested that magnetars may just be a higher order of this same RPP effect. Victoria Kaspi has suggested it may be time to consider a ‘grand unified theory’ of neutron stars where all the various species might be explained by their initial conditions, particularly their initial magnetic field strength, as well as their age.
It’s likely that the progenitor star of a magnetar was a particularly big star which left behind a particularly big stellar remnant. Thus, these rarer ‘big’ neutron stars might all begin their lives as a magnetar, radiating huge energies as its powerful magnetic field puts the brakes on its spin. But this dynamic activity means these big stars lose energy quickly, perhaps taking on the appearance of a very x ray luminous, though otherwise unremarkable, RPP later in their life.
Other neutron stars might begin life in less dramatic fashion, as the much more common and just averagely luminous RPPs, which spin down at a more leisurely rate – never achieving the extraordinary luminosities that magnetars are capable of, but managing to remain luminous for longer time periods.
The relatively quiet Central Compact Objects, which don’t seem to even pulse in radio anymore, could represent the end stage in the neutron star life cycle, beyond which the stars hit the dead line, where a highly degraded magnetic field is no longer able to apply the brakes to the stars’ spin. This removes the main cause of their characteristic luminosity and pulsar behaviour – so they just fade quietly away.
For now, this grand unification scheme remains a compelling idea – perhaps awaiting another ten years of Chandra observations to confirm or modify it further.
A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A. Credit: NASA/CXC
Supernova remnant Cassiopeia A (Cas A) has always been an enigma. While the explosion that created this supernova was obviously a powerful event, the visual brightness of the outburst that occurred over 300 years ago was much less than a normal supernova, — and in fact, was overlooked in the 1600’s — and astronomers don’t know why. Another mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. But in 1999, astronomers discovered an unknown bright object at the core of Cas A. Now, new observations with the Chandra X-Ray Observatory show this object is a neutron star. But the enigmas don’t end there: this neutron star has a carbon atmosphere. This is the first time this type of atmosphere has been detected around such a small, dense object.
The object at the core is very small – only about 20 km wide, which was key to identifying it as a neutron star, said Craig Heinke from the University of Alberta. Heinke is co-author with Wynn Ho of the University of Southampton, UK on a paper which appears in the Nov. 5 edition of Nature.
“The only two kinds of stars that we know of that are this small are neutron stars and black holes,” Heinke told Universe Today. “We can rule out that this is a black hole, because no light can escape from black holes, so any X-rays we see from black holes are actually from material falling down into the black hole. Such X-rays would be highly variable, since you never see the same material twice, but we don’t see any fluctuations in the brightness of this object.”
Heinke said the Chandra X-ray Observatory is the only telescope that has sharp enough vision to observe this object inside such a bright supernova remnant.
But the most unusual aspect of this neutron star is its carbon atmosphere. Neutron stars are mostly made of neutrons, but they have a thin layer of normal matter on the surface, including a thin–10 cm–very hot atmosphere. Previously studied neutron stars all have hydrogen atmospheres, which is expected, as the intense gravity of the neutron star stratifies the atmosphere, putting the lightest element, hydrogen, on top.
But not so with this object in Cas A.
“We were able to produce models for the X-ray radiation of a neutron star with several different possible atmospheres,” Heinke said in an email interview. “Only the carbon atmosphere can explain all the data we see, so we are pretty sure this neutron star has a carbon atmosphere, the first time we’ve seen a different atmosphere on a neutron star.”
An artist’s impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth’s atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss
So how does Heinke and his team explain the lack of hydrogen and helium on this neutron star? Think of Cas A as being a baby.
“We think we understand that as due to the really young age of this object–we see it at the tender age of only 330 years old, compared to other neutron stars that are thousands of years old,” he said. “During the supernova explosion that created this neutron star (as the core of the star collapses down to a city-sized object, with an incredibly high density higher than atomic nuclei), the neutron star was heated to high temperatures, up to a billion degrees. It’s now cooled down to a few million degrees, but we think its high temperatures were sufficient to produce nuclear fusion on the neutron star surface, fusing the hydrogen and helium to carbon.”
Because of this discovery, researchers now have access to the complete life cycle of a supernova, and will learn more about the role exploding stars play in the makeup of the universe. For example, most minerals found on Earth are the products of supernovae.
“This discovery helps us understand how neutron stars are born in violent supernova explosions,” said Heinke.
A star is a star, right? Sure there are some difference in terms of color when you look up at the night sky. But they are all basically the same, big balls of gas burning up to billions of light years away, right? Well, not exactly. In truth, stars are about as diverse as anything else in our Universe, falling into one of many different classifications based on its defining characteristics.
All in all, there are many different types of stars, ranging from tiny brown dwarfs to red and blue supergiants. There are even more bizarre kinds of stars, like neutron stars and Wolf-Rayet stars. And as our exploration of the Universe continues, we continue to learn things about stars that force us to expand on the way we think of them. Let’s take a look at all the different types of stars there are.
A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy release by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.
T Tauri Star:
A T Tauri star is stage in a star’s formation and evolution right before it becomes a main sequence star. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main sequence stars; they’re about the same temperature but brighter because they’re a larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.
Main Sequence Star:
The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass and brightness, but they’re all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy.
A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.
The lower mass limit for a main sequence star is about 0.08 times the mass of the Sun, or 80 times the mass of Jupiter. This is the minimum amount of gravitational pressure you need to ignite fusion in the core. Stars can theoretically grow to more than 100 times the mass of the Sun.
Red Giant Star:
When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically. The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The red giant phase of a star’s life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.
White Dwarf Star:
When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity. A white dwarf shines because it was a hot star once, but there’s no fusion reactions happening any more. A white dwarf will just cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.
Red Dwarf Star:
Red dwarf stars are the most common kind of stars in the Universe. These are main sequence stars but they have such low mass that they’re much cooler than stars like our Sun. They have another advantage. Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars. Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years. The smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.
If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn’t form a white dwarf when it dies. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star. As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons. This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.
The largest stars in the Universe are supergiant stars. These are monsters with dozens of times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.
As you can see, stars come in many sizes, colors and varieties. Knowing what accounts for this, and what their various life stages look like, are all important when it comes to understanding our Universe. It also helps when it comes to our ongoing efforts to explore our local stellar neighborhood, not to mention in the hunt for extra-terrestrial life!