Unusual star mystifies astronomers

Some stars are bright enough to see with the naked eye. Others are so dim, they're visible only in a telescope.

A few stars are both, sometimes visible to the naked eye, sometimes only in a scope.

Until about 1840, Eta Carinae (“EH-tuh cah-RIN-eye”) was a rather mundane naked-eye star.

Around 1840, however, Eta Carinae suddenly grew bright … so bright, only one star — Sirius — outshone it.

Then Eta dimmed … and disappeared. It could be seen only with a telescope.

In 1890, Eta once again grew brighter – though only in a telescope; it remained invisible to the naked eye. And then, once again, it grew dimmer.

Prior to these brightenings, Eta Carinae looked sharp – perfectly ordinary – in the telescope. After the outbursts, however, it became a fuzzball.

With the launch of the Hubble Space Telescope, we could finally see what had happened. In the early 1840s, the star exploded at its poles, north and south … which is why it grew bright. Giant gobs of gas and dust ballooned out, engulfing the star in a light-absorbing cocoon … which is why it dimmed.

In 1890, Eta Carinae blew up again … only this time, at its equator. The Hubble image shows a sheet of dust expanding out from between the two lobes.

A bridge must endure a perpetual tug-of-war. All the cars, trucks and people on the bridge, and the bridge itself, are being drawn downward by gravity. The only thing opposing gravity, keeping the bridge erect, is the strength of its steel.

A star, too, endures a great tug-of-war. With hundreds of thousands, even millions of times the mass of the Earth, stars exert a tremendous gravitational pull … were gravity unopposed, it would collapse the star, down to almost nothing.

Two things opposing gravity

First, when something gets hot, it expands. A star is hot, so it expands.

Second, light exerts pressure — it can push on things. Sun, stars, flashlights and light bulbs all push on whatever they illuminate.

The pressure of light is not so strong, of course, that you risk getting knocked over when you step outside.

But, if space navigators did not account for the pressure of sunlight, radiation pressure, Mars-bound spacecraft would veer off-course by tens of thousands of miles.

Before it escapes from the star, the pressure of starlight pushes the star's atoms outward.

Thermal pressure and radiation pressure together fight gravitational collapse, and keep a star inflated. Most stars keep these forces in balance. But some, form time to time, become unbalanced.

Should energy escape faster than it's being produced within the star, the star would cool off.

If energy is produced faster than it escapes, the star grows hotter … and hotter … and hotter … until something gives: the star explodes.

Stars evolve. Like a fire that first burns through fine fuels (kindling) before igniting the heavier fuels (branches and logs) a star burns different fuels — different atomic nuclei — as it ages.

Could a change of fuel have triggered Eta Carinae's outbursts?

According to Nathan Smith of UC Berkeley, nothing in our understanding of stellar evolution predicts the sorts of outbursts we've seen in Eta Carinae.

Eta today is again growing brighter; it can again be seen with the naked eye, though only in a dark sky, where light pollution doesn't obliterate the view.

Eta Carinae may be heading for another explosion … perhaps even an explosion that will destroy it completely: a supernova.

The Hubble Space Telescope was scheduled for refurbishment this month. Last week, however, it suffered a serious equipment failure, and the servicing mission has been postponed to next year. Which begs the question: Should Eta Carinae again explode, will the Hubble be in shape to see it, and relay what it sees to us?