X-Ray Transients

The universe is a harsh place, and that becomes obvious when you start detecting all the forms of radiation that comes from various sources. Space, in itself, is not very conducive to life, but when you have huge amounts of X-rays randomly traveling through the solar systems, you do sometimes wonder how life developed anywhere. X-rays transients are just one reason why space travel is a lot harder than many people think.

What Are X-rays Transients?

They are X-rays that periodically pass through the solar system, and then they just disappear.

Detecting these has only recently been possible, thanks to space travel. The atmosphere absorbs X-rays, so we can’t detect them on the surface of the Earth.

Fortunately, the use of essentially ICBMs with an X-ray detector strapped to them in the 1960s became a cheap method of trying to detect X-rays and getting a baseline reading of the background X-ray readings. This method detected the first X-ray transient source: Cen X-2.

Nobody knew what it was, and because there was a four-year gap between all three readings, the source remains undiscovered. What this did do was pave the way for the discovery of other X-ray transients, including Cen X-4.

These ICBM launches eventually gave way to Vela satellites, which were ostensibly for nuclear test monitoring. However, they also carried X-ray detectors, which meant that they could be used for X-ray transient detection.

Eventually, sufficient X-ray transient sources were discovered that they were put in their own class. But what were they?

Binary Stars Hold the Answer

As the data was released and declassified — albeit very slowly, as the satellites held militarily sensitive information — astrophysicists were able to identify the areas where X-ray transients occurred: They all appeared to be binary stars.

It was clear that the binary stars were producing these X-rays, but why?

It turns out that these binary systems typically involve a functioning star, such as a main-sequence star or a red giant, and either a neutron star or a black hole. In some cases, it can involve a nova and a black hole.

It seems likely that the two stars have elliptical orbits, where one star passes through the accretion disc of the other at regular intervals. The denser star gobbles up hydrogen and helium, creating a burst of X-rays and gamma rays from the sudden increase in fusible elements. The other star passes through and resumes its journey orbiting around the denser star, albeit with significantly less mass.

In this system, you get low-mass and high-mass X-ray bursters, depending on the mass of the star that is being scavenged by the neutron star or black hole. You also get soft X-ray transients, which exhibit less violent X-ray outputs compared to the bursters. There are also supergiant X-ray transients, which exhibit their own particular set of characteristics.

Other Sources of X-ray Transients

Our own sun can produce X-rays periodically, and this can lead to X-ray transients. The sun’s overall output is factored into the background radiation level, so when it produces a major mass ejection, the X-rays within the solar system suddenly increases. Because it is so close, it can have a significant impact on the earth, creating geomagnetic storms.

Jupiter can also create X-ray transients, and these can be detected quite easily from Earth. However, these are a particular sort of X-rays, so they can be differentiated from ones produced by binary stars.

X-ray transients are a particular space hazard, but they do not generally affect intra-solar-system travel. Should we wish to go further, it’s essential that we understand how these X-rays are produced to ensure we can avoid or compensate for them.

What is a Falling Star?

Poets talk about falling stars; astronomers talk about meteors. They’re the same thing, but the phenomenon resonates differently for scientists and regular people. For many of us, pretty much the most exciting thing about watching the night sky is catching sight of a falling or shooting star.

But What Is It Really?

A falling or shooting star is the visible path of a meteoroid (or debris from combusting meteoroids of any size) entering the Earth’s atmosphere. Upon entry it is referred to as a meteor; if it hits the Earth intact it is referred to as a meteorite. The bright light that even dust-sized falling stars emit as they heat up and disintegrate during their journey through the atmosphere can make them appear to be much larger than they are. If an actual star were ever to fall to earth, the planet would be instantly obliterated. Real stars (such as Earth’s sun) are enormous.

On a normal night, depending on location and the level of light pollution, most stargazers can expect to see a falling or shooting star every 20 minutes or so. They appear abruptly and arc briefly across the dome of the night sky. During a meteor shower, when meteors, debris and dust move in a cluster, the sky can seem full of falling or shooting stars.

A Brief History

People have always been excited, and often scared, by this celestial phenomenon. Ancient Greek philosophers such as Aristotle posited theories (incorrectly as it turned out) about what caused stars to fall and blaze so brightly, and historians and writers the world over have tracked and described these events — long before astronomy was even considered a science.

Arab historical accounts refer to the year of 902 AD as the Year of the Stars. At this time the Leonid meteors (the Leonids are a spectacular meteor shower event that occurs every 30 years or so) were especially active over Northern Africa and the Mediterranean.

In 1833, another Leonids event occurred over much of eastern North America, sparking renewed interest, both public and scientific, in falling stars and meteor showers. American astronomer Denison Olmstead correctly noted that the falling stars seemed to originate from the same part of the sky (which he called the radiant) and that gravity played a part in their fall. His work, which included many first hand accounts from non-scientists, ushered in a new era of observation and scientific research.

How to See Falling Stars Clearly

Some shooting stars can be seen as part of a seasonal phenomenon. The Perseid meteor shower, for example, derives from the debris stream of the periodic comet Swift-Tuttle, which takes 133 years to orbit the Earth. The Perseids appear in the skies across the Northern Hemisphere late each summer, and have since the mid-1800s. At the peak of this annual event, up to 60 falling stars can be seen between midnight and dawn, depending on light conditions. Generally, the less ambient light from homes or structures, or the moon, the better the chances of spotting a falling star. Some countries are establishing Dark Sky Preserves, to help amateur astronomers get the best possible falling star experience.

Whether we call them stars or simple comet debris, falling or shooting, the phenomenon triggers a special visual experience for children and adults around the world. So much so that a sweet ritual usually accompanies the sighting of a falling star. While in ancient Greece shooting or falling stars were believed to mark the passage of souls, in many modern societies the tradition is to make a wish when you see one.

What is Comet?

Hidden in the outer reaches of our solar system are chunks of frozen gas, rocks and dust — the nuclei of comets. As these unassuming, icy lumps travel toward the sun, they’re transformed into brilliant celestial objects.

In ancient civilizations, the unexpected presence of these glowing objects in the heavens inspired fear, but we now know that comets are billions of years old and an intricate part of our solar system.

How Comets are Formed

Comets were created when the solar nebula collapsed to form the sun and planets. The debris froze together in clumps in distant regions of the solar system, creating a comet’s nucleus.

As a comet orbits near the sun, the ice heats and vaporizes, generating a thin atmosphere around the nucleus called a coma. Loose bits of rock and dust are blown by solar winds to form two tails illuminated by the sun. One tail contains dust particles and shines yellow and white. The second tail consists of ionized molecules and glows blue.

Where Do Comets Come From?

There are two types of comets. Short-period comets have an orbit of less than 200 years and are believed to travel from the Kuiper Belt outside of Neptune’s orbit. Long-period comets take more than 200 years for a return visit and originate in the Oort cloud, the most distant part of our solar system.

Comets have been cited as early as 1000 BC and were named kometes by Greek philosophers, which means a head of long hair. Individual comets are generally named for the persons who discovered them.

Notable Comets in Recent History

These recent comets generated scientific and public interest:

  • Halley: Astronomer Edmond Halley proposed that a comet appearing in 1531, 1607 and 1682 was the same comet with a 75-year orbit. He accurately predicted its return in 1758. Halley’s Comet appeared in 1986 and will return in 2061.
  • Iyeka-Seki: The brightest comet of the 20th century, Iyeka-Seki is a Kreutz Sungrazer — a comet whose orbit takes it close to the sun. In 1965, Iyeka-Seki approached to within 744,000 miles of the sun’s center. It’s next expected in 3000.
  • Hyakutake: In 1996, a spacecraft called Ulysses unexpectedly crossed the tail of long-period comet Hyakutake. Scientists realized the accidental interaction was due to the surprising length of the comet’s tail. Approximately 350 million miles long, it was twice the length of any other known comet. Hyakutake is not expected for another 14,000 years.
  • Hale-Bopp: In 1997, Hale-Bopp became the most widely-viewed comet, visible to the naked eye for 18 months. Its nucleus measured 60 miles, compared to an average nucleus which ranges from 300 feet to 30 miles. Hale-Bopp released dust streams more than eight times that of previously observed comets. Its next approach is in 4385.
  • McNaught: After McNaught appeared in 2007, scientists examined the space disturbed by its presence and realized the comet’s scale. Ulysses took 2.5 days to cross the shocked solar winds around Hyakutake, but it took 18 days for it to cross those of McNaught.

Future Observations

There have been a dozen international space missions studying short-period comets. The European Space Agency is planning a future three-spacecraft mission to visit a long-period comet as it enters our inner solar system.

Astronomer Alan Hale, co-discoverer of Hale-Bopp, regularly documents comets. He shares information about observable comets and incoming ones.

Comets have been roaming our solar system for billions of years. Whether they’re quietly travelling distant regions or making a spectacular splash in our sky, they’re a beautiful reminder of the vastness of our galaxy and our ancient origins.

Cataclysmic Variables

The majority of stars are not solo stars like our Sun. Instead, they are binary or trinary systems, and in some cases, there can be many more. The stars don’t have to be equal, either. You might get a small dwarf star circulating a red giant, or perhaps a main-sequence star gradually being ripped apart by a black hole. Or they could be equal. But cataclysmic variable stars are essentially stars that go boom on a regular basis. What’s up with that?

The Discovery of Cataclysmic Variable Stars

Cataclysmic variable stars have long been observed, with the first known observation by Pére Dom Anthelme in 1670. Others carried on that work, particularly during the 19th century with bigger and better telescopes. More importantly, spectroscopy became useful to identify anomalies, such as those often found with novae. These stars were oddities that periodically brightened and dimmed, and spectroscopic measurements also varied.

During the 1940s, spectroscopic data showed that there were two distinct sets of emission spectra, which suggest that a known dwarf nova was actually two stars: a binary system. Further data backed this up.

In the 1960s, a series of missile launches were used to scan the heavens. Essentially, it was an X-ray detector strapped to an ICBM. The launches were programmed to scan a particular sector for 3 to 5 minutes, and then they would fall back to Earth with the data. One launch detected an X-ray transient, which is an X-ray burst that momentarily appeared and then disappeared when it was next scanned.

This provided the first inkling that the universe was even more violent than we had thought.

This system was replaced by the Vela series of satellites, and they detected even more X-ray bursts, as well as more than a few from their official job: nuclear weapon test detection. Other satellites supplanted them.

All of this data came in handy when eventually declassified. Scientists in the ’70s started to work out what these X-ray bursts were.

Stars That Go Boom

The answer is surprisingly simple. Take one main-sequence (usually dwarf) star, and then pair it with a white dwarf. Because the stars are orbiting very closely, they are very hard to tell apart. The main sequence star must also pass near or through the dwarf star’s accretion disc. This strips out a load of the main sequence star’s fusible material, which immediately reacts violently on the surface of the white dwarf. The result is a series of huge and controlled thermonuclear explosions that are many orders of magnitude bigger than the largest thermonuclear weapon humanity has ever created.

It’s this that creates a huge burst of X-rays and gamma rays. This has the effect of sterilizing all life in the system and perhaps further.

There are several types of CV system:

  • Novae are extremely bright. This creates a new, incredibly bright light in the sky that can last for several months. These include M31N 2008-12a.
  • Dwarf nova is not as bright, and their output can perhaps last a few hours or as long as a couple of weeks. These are the most common type of CV. These include U Geminorum and SS Cygni.
  • Polars are magnetic cataclysmic variables. They have strong magnetic fields that force the material to go to the north and south poles, where they continuously burn. They glow, but they do not produce outbursts. Polars include AM Herculis.

Binary systems are some of the most common in the Milky Way, and it’s almost inevitable that a good portion of them are some sort of cataclysmic variable star or will be. Despite the lack of mainstream news interest, understanding them is important if we are to understand our place in the galaxy.