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Monday, November 9, 2009 / Duncan Forgan

Young Stars: Eager Gourmands or Reluctant Gourmets?


Some stars undergo a burst of increased brightness: why?How do you make a star brighter? One simple way is to feed it more matter.  As the matter spirals into the star, it releases its gravitational energy in the form of radiation, making the star more luminous.  But where does this material come from?

fu-orionis

Stars are born when a cloud of gas begins to collapse under its own gravity.  If the cloud is spinning, it will maintain this spin throughout the collapse, forming a pancake instead of a sphere (this is called the conservation of angular momentum – the classic example is the spinning ice skater.  As she moves her arms inwards, she speeds up).  The star will then be born in the centre of the pancake, with a disc of orbiting material around it (which we call the protostellar disc).  It can then grow by feeding on (or accreting) from this disc, in a steady, self-regulated fashion.

Or does it? Some stars seem to jump the gun, and feed on the disc in bursts, becoming many times brighter in days, and taking years to cool down.  We call these FU Orionis outbursts (they’re named after the first star we noticed that behaved in this way, pictured above).  What’s more, FU Orionis outbursts (FUors for short) are almost certainly necessary.  Stars don’t have a lot of time to build up to the sizes we see today, so these quick bursts of growth are probably essential to building stars like our Sun.

OK then, let’s assume that most stars go through an FUor stage at some point in their lives.  How do you make them gorge?  One way is to make the disc do all the work.  If the disc is really big in comparison to the host star, then the disc can become gravitationally unstable.  If it gets really big, then this instability funnels matter into the inner regions of the disc, until it gets so mashed together that the gas begins to become ionised (i.e. the gas gets electrically charged).  If this happens, then magnetic fields from the star play a role, and an even fancier instability (called the magneto-rotational instability) kicks in, pouring the piled up matter onto the star, producing a burst.  The problem is that getting the mass to pile up is difficult, and requires really big discs.

Maybe we don’t need the disc to work hard: what if we use a second star to drive matter in?  If another star comes close, the force of its gravity could work on the disc, forcing it to pile up matter and produce a burst.  Not to blow my own trumpet, but I did some research into this, and we found that yes, you can force a star to have an outburst from a close approach.  Sadly, this type of encounter doesn’t happen very often, and we see a lot of outbursts (in fact, as I said, probably all stars undergo outbursts throughout their early lives).

So, are stars compulsive or reluctant eaters? I think stars are a lot like human beings: some eat to live, others live to eat.

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