Universe Today

W Cephei is one of the more popular variable stars. That is, stars that change their brightness. The reason why W Cephei is a variable star is because it is actually a system of two stars that, most of the time, appear as one. Such a system is called a binary system.

To be more specific, W Cephei is an eclipsing binary system. The term 'eclipsing binary' comes from the eclipsing behavior of the two stars. Since their orbital plane lies in the line of sight of an observer here on Earth, when one star is nearer to us than the other, the former obstructs the latter from our view. This eclipsing behavior also gives rise to the varying brightness of W Cephei.

This particular binary system is popular because W Cephei A, the larger of the two stars, is among the largest in the Universe. In fact, it is second only to VY Canis Majoris among all stars in our galaxy, the Milky Way. If we consider all known stars in the Universe, W Cephei A comes third, after WOH G64.

VY Canis Majoris has an estimated size of up to 2100 Solar radii. WOH G64 has about 2000 Solar radii, while VV Cephei has about 1900. All three are hypergiant stars, i.e., stars with very large masses, high luminosity, and also high rate of mass loss.

VV Cephei B, on the other hand, is a blue star in the main sequence. Main sequence stars are called as such due to their positions in plots of stellar color against brightness. The main sequence is the continuous, distinctive band of stars in this kind of plot.

You can find W Cephei within the constellation Cepheus, which is approximately 2,400 light years from Earth. Through the naked eye, you can spot two other big stars accompanying W Cephei in Cepheus. There's Mu Cephei or Herschel's Garnet Star and HR 8164.

Both W Cephei A and B orbit about a common center of mass. On the average, they are separated by a distance of about 25 AU. However, due to their orbit's high eccentricity, their distance can vary between a minimum of 17 AU and a maximum of 34 AU.

We've got a Youtube video here in Universe Today that features W Cephei A. It's in the article "What is the Biggest Star In the Universe?" Want to learn more about the constellation Cepheus instead? Click on that link.

There's more about it at NASA. Here are a couple of sources there:
What is a Binary Star System?
Why are binary stars interesting?

Here are two episodes at Astronomy Cast that you might want to check out as well:
Where Do Baby Stars Come From?
Where Do Stars Go When they Die?

Filed under: Astronomy | Comments Off

Did you know that our Solar System is a rarity with its single star? Astronomers believe that most star systems out there actually contain 2 or more stars – imagine seeing a sky with 4 suns. These binary and multiple star systems are a great target for new astronomers, and the dynamics of multiple stars keep astrophysicists busy too. Let's take a look at what it would be like to live on Tatooine.

Click here to download the episode.

Or subscribe to: astronomycast.com/podcast.xml with your podcatching software.

Binary Stars show notes and transcript.

Filed under: Podcasts | Comments Off

If you're a Star Wars fan, you may have noticed the two suns as seen from Tatooine, Luke Skywalker's (and Anakin Skywalker's; a.k.a. Darth Vader) desert home planet. According to the story, Tatooine was mostly dry because heat from the two suns prevented water from staying in liquid form very long. Those two suns, called Tatoo I and Tatoo II, comprise what is known as a binary star system.

While Tatooine is a purely fictional planet, binary star systems are not. In fact, majority of the stars in the universe come in pairs. The two stars that make up a binary system orbit around a common center of mass.

Recent studies have also shown that the earliest stars may have actually existed in pairs. Matthew Turk and his colleagues from Standford University performed computer simulations that revealed how a primordial star forming region that existed only some 200 million years from the Big Bang contained two pre-stellar cores which had the right conditions to form a binary star system.

When stars are found to exist in a binary system, they make the jobs of scientists lighter. The scientists can use information gathered regarding their orbits to calculate for their masses. In turn, this information can be used to solve for other parameters such as the stars' radii and densities.

There are four ways by which a binary star system can be detected: visually, and are hence known as visual binaries; spectroscopically, hence are known as spectroscopic binaries; photometrically, these are known as eclipsing binaries; and astrometrically, hence are called astrometric binaries.

If you want to catch a binary star system on your first telescope, we suggest Albireo. You can find it in constellation Cygnus. From your telescope, you'll notice one of the stars in the pair to be colored yellow (it's the brighter one) and the other, colored blue.

Going back to Tatooine, can such a planet actually exist? That is, wouldn't the combined gravitational forces of the two stars rip any surrounding planet apart? Also, wouldn't the heat coming from the two stars be too much for such a planet to be habitable (even for a Jedi)?

According to Erik Mamajek of Pennsylvania State University, 16 Cygni B and 55 Cancri A, which are both members of binary star systems, are believed to have planets orbiting them. That should answer the first and second question. As for the 3rd query, Mamajek adds that these systems may have "safe zones" in which planets can find habitable.

NASA has an article entitled "Planets around binary stars" that provides a more detailed discussion on this matter. Other interesting questions regarding binary stars can be found in this link: Binary Star Systems.

Related articles can be found here in Universe Today:

Earliest Stars Came in Pairs, New Simulation Shows

Astronomers Catch Binary Star Explosion Inside a Nebula

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

Moons and the Drake Equation, Stars in the Void, and Rings Around Stars
Stellar Roche Limits, Seeing Black Holes, and Water on Mars

Filed under: Astronomy | Comments Off

VV Cephei is an eclipsing binary star system. A binary star system is pair of stars orbiting about their common center of mass. Some of these systems are described as eclipsing due to their nature of occasionally obstructing their partner from an observer on Earth. This can happen when these systems' orbital plane lies near the observer's line of sight.

Found in the constellation Cepheus, some 2,400 light years away from Earth, VV Cephei is made up of the hypergiant VV Cephei A and VV Cephei B, the blue main sequence star. The distance between the two stars is, on the average, 25 AU (astronomical units). Because of its high eccentricity, this distance can change between 17 and 34 AU.

Eccentricity is a measure of an orbit's deviation from a perfect circle, where a high eccentricity means a large deviation and a low eccentricity means a small deviation.

Among all recorded stars, VV Cephei A is known to be the second largest in the Milky Way and the 3rd overall. Its diameter is at least 1,600 times wider than the Sun's, while its luminosity is at least 280,000 times greater. To give you an idea how large that is, imagine a star so huge, that if we replaced our own sun with it, it would occupy a space that would extend way past Jupiter's orbit.

It's smaller partner, VV Cephei B, on the other hand, is only about 10 times larger than the Sun and about 100,000 times as luminous.

One complete orbit of this system lasts for about 7,430 days or 20.3 years, with the eclipse lasting about 1,300 days. A total eclipse of this system takes about 40% of that time.

Much of the information known about VV Cephei, including those written above are very rough estimates. For example, its distance is too far away from us to use parallax, the triangulation method used to measure large distances in astronomy. The estimated distance of 2,400 light years is based upon the speculation that this binary star system belongs to the Cepheus OB2 association of hot blue stars, whose distance from us is somewhere around that value.

Some observers believe that VV Cephei A is nearing its end, and will soon depart by way of a supernova. Presumably, this will eventually leave VV Cephei B by its lonesome.

We have some related articles here that may interest you:

• Cepheus
• What is a Binary Star?

There's more about it at NASA. Here are a couple of sources there:

• What is a Binary Star System?
• Why are binary stars interesting?

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Where Do Baby Stars Come From
• Where Do Stars Go When they Die?

Filed under: Astronomy | Comments Off