The Longest Eclipse Ever Found

This past week there was a beautiful total eclipse of our Sun visible for a few minutes from Indonesia, so it’s a good time to be thinking about all kinds of eclipses. A team of astronomers from several universities recently reported the discovery of a star that is eclipsed for more than 3 years every 69 years. This is the record holder for eclipses anywhere in the universe so far!

What would eclipse a star for such a long period of time? It can’t be a moon, or a planet, or another star, since all of those are too small to cause an eclipse that lasts three and a half years. There must be something around the star that extends over a much larger area and gets in the way of its light as seen from Earth.

After a lot of careful investigation -- using both recent observations by professional and amateur astronomers, and images going back over the last century -- the team concluded that they were seeing two aging stars whirling around each other in 69 years. One of the stars has lost a good deal of its material as it aged, and this lost material now forms a huge disk or doughnut around the emaciated star. It is this large disk that gets in front of the other star and cuts out its light as seen from our viewing angle.

We have seen other systems where such disks cause long eclipses. The best known case is the star Epsilon Aurigae, whose eclipses last about 700 days and happen every 27 years. It too has been studied by large teams of astronomers to uncover its secrets. 

But our star (which has no name, only a long catalog number TYC-2505-672-1) is further away and its eclipses take much longer to repeat. The only way astronomers were able to figure out that it has a 69-year cycle is because they were able to consult a hundred-year repository of pictures of the sky that has been lovingly kept up at the Harvard College Observatory.  

Those old images, many of them made on glass plates back in the days of chemical photography, are a treasure for astronomers. They are now being “digitized” – made into digital images that can be scanned and tracked via computer.

So if astronomers want to know if something interesting (like our eclipse) might have happened to a star sometime in the last century, they can now look up on the Harvard images what that star has been doing all that time. As someone who has trouble throwing out old files or magazines, my heart leapt when I heard about how important the old records were to the discovery!

And how far does stripped star with its disk of dark material have to be from the other star to only have an eclipse every 69 years? Their separation must be more than 20 times the distance between the Earth and the Sun, roughly the distance between the Sun and Uranus.

(The image shows an artist's impression of the dark disk moving in front of a red giant star.)

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By the way, to see a nice animation of the dark eclipse spot from the Indonesia eclipse on Earth, go to: https://commons.wikimedia.org/wiki/Category:Solar_eclipse_of_2016_March_9#/media/File:An_EPIC_Eclipse.gif

A Record Smashing Black Hole Merry-Go-Round

Astronomers using a European telescope in space have found a star and black hole that whip around a common center every two and a half hours, setting a new record for the movement of such weird companions in space. Like a merciless cannibal, the black hole is also stripping the poor star of its outer layers of hot gas and pulling them into its open "mouth."

To understand this system, you must first know that while some stars are single, like our Sun, many stars live in intimate relationships with each other.  Two stars may orbit a common center and spend their lives whirling around in a rhythmic cosmic dance. If one of those stars should be especially massive, it may end its life as a black hole -- a collapsed star corpse so dense that nothing, not even light, can escape from it.

In our present case, the black hole is estimated to contain enough material to make more than three of our Suns.   The little star it shares its dance with, on the other hand, is only about 1/5th the mass of our Sun. As their mutual gravity pulls them around, the star travels at 1.2 million miles per hour while the black hole moves at 90,000 miles per hour.  The two are only 600,000 miles apart (a very small distance for astronomy.)  Compare that to the 93 million miles that separate the Earth from the Sun.

Check out an animated video of this crazy pair of cosmic objects at: http://spaceinvideos.esa.int/Videos/2013/03/MAXI_J1659_152

You can see material being stripped from the living star and swirling in toward the black hole.  It is this doomed material which glows with tell-tale x-rays and identified the system to the XMM-Newton x-ray telescope being operated by the European Space Agency.  We can only detect these x-rays BEFORE the material enters the black hole -- nothing can escape from it once it falls into the black hole itself.

For a quick YouTube video where I explain the basics of what a black hole is, you can see: http://www.youtube.com/watch?v=7DX_cc-IjpY