The Question

I'm wondering what information you have on the X-ray source Cygnus X-1 and on it's nearby star (a blue giant).

The Answer

Cyg X-1 is believed to be a black hole binary, with a 20-35 solar mass black hole and a O9.7Iab companion. The orbital period is 5.6 days. You can learn more about black holes and X-ray binaries in Imagine the Universe! in both the Basic Level and the Advanced Level. In particular, the level 2 "Black Holes" section discusses the Cygnus X-1 binary starry system. It includes some information on the blue giant companion to the unseen compact object. The reading list at the end of the page also gives a number of good books with a great deal of information about black holes: since Cygnus X-1 is still considered to be one of the better black hole candidates, all of these books discuss the binary system in some detail.

We hope you'll take the time to browse through the sites and "read more about it".

The Question

What is the cause of X-ray bursts?

The Answer

Thanks for your question on X-ray bursts. X-ray bursts were discovered by X-ray satellites, and are characterized by a rapid, and very dramatic increase in X-ray flux from an X-ray source, lasting about a second or so. X-ray bursts are seen coming from Globular clusters, which are in the halo of our galaxy, and from sources along the plane of our galaxy. This distribution suggests they come from a population of stars that are common in the disk: X-ray binaries.

An X-ray binary is a pair of stars where one member is a compact object (such as a black hole or neutron star) and the other star is a normal star. The two stars orbit each other at a separation where the gravitational pull of the compact object distorts the normal star and material streams off of it, and onto the compact object. (For more information on X-ray binaries, take a look in the Basic or Advanced High Energy Astrophysics sections of Imagine the Universe!)

In the case of X-ray bursts, we have a very clear hypothesis for what is going on. Researchers have studied many bursts in detail, and from the spectra and light curves we know that a tremendous amount of energy (in about 10 seconds, 10^39 ergs---what the Sun's X-ray corona emits in about 3000 years!) is being released from an area of about 15 km. This is the size that neutron stars are expected to be. hydrogen accreted from the normal star onto the neutron star is continuously fused into helium. A layer of helium is them formed near the surface of the neutron star. When there is enough helium present, an unstable reaction occurs: virtually all of the helium is fused to carbon at once, which we see as the explosive burst. After this, the accretion continues to take place between the two stars. After a while there is again enough material to explosively burn the helium, and another burst is seen.

Studying X-ray bursts give us insight into the accretion process between the two stars, and also the conditions on the surface of the neutron star. For a particular source, X-ray astronomers catalog the bursts, the duration between bursts, and the energy released in bursts to get an idea of what is happening in the helium and hydrogen layers just above the neutron star surface.

X-ray bursts shouldn't be confused with gamma-ray bursts. Gamma-ray bursts are more energetics, and are distributed isotropically on the sky. No optical counterparts have been conclusively identified with any gamma-ray burst. Neutron stars may also be involved in the production of gamma-ray bursts, but no one knows for sure what causes these.

Padi Boyd and Jim Lochner

The Question

Is it at all possible for a companion star to survive a supernova explosion? Do any of the binary systems that we know of, where one of the bodies is a neutron star or black hole, have the original companion star present, especially where the compact object is near enough to draw surface gas off the visible star? Or have all these systems simply come into existence after the explosion and the nebula have dissapeared?

The Answer

Absolutely. All X-ray binaries have either neutron star or black hole components. By definition these stars must have experiences a supernova stage. Either their companions survived the supernova event or were captured tidally some time later. Certainly these companion stars did not form close to the compact object after the supernova. Energy from the compact source would prevent any cold gas from contracting in the vicinity.

with regards,
Martin Still & Jay Cummings
for "Ask an Astrophysicist"

The Question

What is the smallest nova ever found?

The Answer

I don't know the answer to your question, but there are several types of things called 'nova'.

Classical novae tend to be, roughly, about the same size, although there is some variation. Typical absolute magnitudes are around +4.0. Recurrent novae, which are seen to go off multiple times with intervals of 10 to 50 years, are somewhat dimmer. These two types of novae are thought to be due to hydrogen buildup on the surfaces of white dwarfs, which accumulate the material until it suddenly ignites and burns.

Dwarf novae are a different phenomenon, and are typically hundreds of times fainter. These are due to instabilities in accretion disks. Dwarf nova can be a variety of sizes, probably down to the point where they can be classified either as 'nova' or 'brightness fluctuation'.

See

http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html

for more details.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

The Question

Could you tell me the basics of accretion discs?

The Answer

Accretion disks arise when material (usually gas) is being transferred from one celestial object to another. "accretion" means collecting of additional material. Two major places where astronomers see accretion disks are in binary star systems (two stars orbiting each other) and active galactic nuclei.

I will discuss an accretion disk in a binary star system, but the basic ideas are the same for all cases. If one star in a binary system is a compact object such as a very dense white dwarf star and the other star is a normal star like the Sun, the white dwarf can pull gas off the normal star and accrete it onto itself. Since the stars are revolving around each other and since angular momentum must be conserved, this gas cannot fall directly onto the white dwarf, but instead spirals in to the white dwarf much like water spirals down a bathtub drain. Thus material flowing from the normal star to the white dwarf piles up in a dense spinning accretion disk orbiting the white dwarf. The gas in the disk becomes very hot due to friction and being tugged on by the white dwarf and eventually loses angular momentum and falls onto the white dwarf. Since this hot gas is being accelerated it radiates energy, usually in x rays which astronomers detect and use to identify and study accretion disks.

ou can find some more basic information at:
http://imagine.gsfc.nasa.gov/features/satellites/archive/asca_agndisk.html
and more at
http://antwrp.gsfc.nasa.gov/apod/ap991219.html
(part of Astronomy Picture of the Day).

Cheers,

Hans Krimm for "Ask an Astrophysicist"

The Question

I have a couple of questions for you on binary stars. I would just like to know how the stars got their name "Binary," which scientist discovered them, and why people find them so interesting.

The Answer

'Binary' just means having two parts, and you will find it used in many places (whenever there are two of something) and not just in connection with stars. The English astronomer William herschel coined the term binary star, after an investigation in the 1790s of stars that appear close together in the sky showed that many were indeed pairs of stars traveling together. Binary stars are very important in astronomy, because a lot of things which are hard to discover when stars are on their own can be easily measured when two are together. When two things are close together, the effect each other in many ways, and we can learn a lot from those effects. For example, two stars close together exert a gravitational pull on each other changing the way they move. By measuring their movements very carefully we can often figure out how much material is in each star - how heavy it is.

Paul Butterworth
for the Ask an Astrophysicist team

The Question

How can binary stars be detected even when a telescope like hst can't even see them together?

The Answer

There are stars called spectroscopic binaries that are detected when a spectrum is seen to contain two sets of lines which move slightly relative to each other, in step with the two stars movement about their common center of gravity. If we are roughly in the plane of the binary orbit, we will see first one set of lines slightly blue shifted and the other slightly red shifted (as one star moves towards us and the other away) and then the opposite effect (after another 180 degrees of mutual revolution). If the two stars are similar the two sets of lines will be similar, but if the stars are quite different the lines will be too.

Paul Butterworth
for the Ask an Astrophysicist team

The Question

What percentage of the stars are binary systems?

The Answer

Somebody once said that "3 out of every 2 stars are in a binary". Seriously, the fraction is very high, but it's difficult to be precise, because it's difficult to prove that a certain star is definitely single. Of the stars nearest to the Sun, about half are known to be in multiple systems.

Koji Mukai
for Ask an Astrophysicist

The Question

Binary stars are popular in science fiction. Star Wars' Tatooine, for example.

I'm wondering about the orbit of a planet around binary stars. Is it possible?

Would the greater combined mass of two stars be more likely to pull in surrounding material, hence making the formation of planets less likely in a binary system?

Would the combined heat and radiation from binary stars mean that habitable planets would have to have a VERY large orbital radius?

The Answer

There are stable orbits for planets in binary star systems. There are various stability criteria which say when an orbit is stable. One such criteria (and I don't know the actual numbers) says that if all orbits are circular and the stars are the same size, then the planet must orbit one of them at less than /some fraction/ of the inter-star distance, or must orbit both combined at more than /whatever/ times the inter-star distance. Figure-eight orbits are unstable, and can eject the planet from the system.

If you have two Sun-like stars at the center of the system, a planet would be the same temperature as Earth if it were at sqrt(2) = 1.4 astronomical units away, rather than Earth's 1 au. This distance is closer than Mars's orbit (1.6 AU). Most stars are dimmer than our Sun, so the orbit could be even smaller.

The high energy astronomers at nasa don't know much about this subject, so we asked an expert: Eric Mamajek of Pennsylvania State University:

The solar-like stars 16 Cygni B and 55 Cancri A have been found to have Jupiter-size extrasolar planets orbiting them. So we do have indirect proof, through doppler spectroscopy methods (Marcy & Butler, SFSU, Lick Observatory), that planets indeed form in binary systems.

The formation mechanisms for forming stars and planets are very different. Planets require accretion to form, specifically accretion in a protoplanetary disk around a young star. Stars can form from the collapse of a molecular cloud core on their own, however planets can only form in the disk around a star. (Pulsar planets are likely formed "posthumously" around pulsars, and are a different beast all together). The main problem with forming planets in multiple star systems is dynamic ejection... stars can simply toss planetesimals out of the system all together (or even accrete them). An example of this is the Kirkwood gaps in the asteroid belt where Jupiter doesn't allow asteroids to exist in certain orbits, and conversely it "shepherds" asteroids in to certain other orbits. A companion star would have a similar effect, except there would be a lot less "shepherding" orbits. The vast majority of binary stars have eccentric orbits. It is difficult for bodies to exist in a system with two very massive bodies in an eccentric orbit. They can only exist very close to each star, or very far from both stars.

An excellent example in the lines of the Tatooine example is the nearby solar-like stars Alpha Centauri A and B. They orbit each other at an average distance of 23 AU, however the eccentricities of each orbit bring them to as close to 11 AU and as far as 35 AU. Numerical simulations by Paul Weigert at University of Toronto have shown that each star has a "safe zone" about 3 AU in radius in which planets could safely survive for billions of years. Objects placed further out from each star than about 3 AU are dynamically ejected in a matter of millions of years or less. Alpha Cen A is about 1.5 times as luminous as our Sun, and Alpha Cen B is about .45 times as luminous as our Sun, and if you do the simple physics, one can see that a "habitable zone" exists around BOTH stars within the 3 AU dynamic "safe zone." Indeed, it could be possible that BOTH Alpha Cen A and B have planets conducive to life. Theoretical models age them anywhere from 3-8 Gyr... plenty of time for life to develop if the planets have the right conditions...

David Palmer and Maggie Masetti
for Ask an Astrophysicist

The Question

I'm curious about why there is such a large percentage of binary star systems. What are the best theories about the reason for this? Are there any websites you would recommend to research this?

The Answer

A major barrier to star formation is angular momentum. A gas cloud collapsing to form stars can convert its angular momentum into the rotation of the stars, both as individuals and as members of larger groups rotating about a common center of mass. The binary stars tend to gain energy (i.e. become closer together and shorten their periods) through collisions with single stars in dense stellar environments, thus making it more difficult for single stars to form.

I hope this helps,

Tim Kallman
for Ask an Astrophysicist

The Question

How are binary star systems formed?

The Answer

This is a very good question and, perhaps much to your surprise, this can be a hot topic for a debate among astronomers.

In essence, a binary-star system emerges out of a cloud of gaseous material collapsing and forming more than a single star at the same time in a small proximity. This type of a collapsing event does not necessarily form only two stars -- it can form more than two, but it all depends on their unique environment in which stars form.

Also it is most unlikely for a single star to capture another star in a typical stellar space. When two stars encounter, they tend to swing by each other and almost never captures one to another by their own gravitational field. We are not going to explain why it is so, but you will need more than two stars (in fact, many stars) to do just that. Some cases of binary-capture may have been seen in a place like globular clusters where a million of stars are found in a very tiny volume of space.

Hope this helps,

Koji & Bish
for "Ask an Astrophysicist"