|Hubble Spots a Carbon Star|
The object imaged below by Hubble is located about 3000 light years away from
Earth and is called AFGL 3068. Itís been known as a bright
infrared source for some time, but images just showed it as a dot.
This amazing Hubble image using the Advanced Camera for Surveys reveals an
intricate, delicate and exceedingly faint spiral pattern. Itís so
faint no one has ever detected it before!
This fascinating object is a binary star, two stars that orbit each other, and one is whatís called a carbon star, similar to the Sun but much older. The Sun is still in the first stage of its life, fusing hydrogen into helium in its core, but older stars run out of available hydrogen and eventually they move on to fusing helium into carbon. At this point the star swells up and becomes a red giant, and red giants tend to blow a lot of their outer layers into space in an expanding spherical wind. The star surrounds itself with a cloud of this material, essentially enclosing it in a cocoon. In general the material isnít all that thick, but in some of these stars there is an over-abundance of carbon in the outer layers which gets carried along in these winds. In these cases the material is very dense and opaque and it can completely block the light from the star, so all we see is the warm glow from the cocoon as an infrared glow.
AFGL 3068 is a carbon star and it is most likely that it evolved just like this, but with a difference: itís a binary system, and as the two stars swing around each other, the wind from the carbon star doesnít expand in a sphere. Instead, we see a spiral pattern as the material expands. This is called the sprinkler-head effect, just as when a sprinkler spins, the jet of water appears to take a spiral shape. The expansion rate of the spiral material has been measured as about 15 km/sec (9 miles/sec). Given that distance, the time it takes between spirals turns out to be a little over 700 years. So if you were hovering in space outside this object and an arm swept past you, youíd have another 700 years before the next one blew your way again.
Stars like this are very rare, because although carbon stars themselves are rare, in order top produce the spiral pattern they have to be in a binary system, and they are more rare.
This object is very is faint - this picture was made using 33 minutes of time on one of Hubbleís most sensitive cameras, and you can still barely see it. Why is it so faint? Well, the stars inside canít light it up; their light is blocked by the material closer to the stars. So what is lighting it up? The astronomers who observed AFGL 3068 think the spiral is being lit by galactic starlight. The spiral is actually slightly brighter on the right side than the left, and the galactic plane - where stars are most populous - is in that direction, so that supports the theory of illumination by galactic starlight.
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