(ORDO NEWS) — The days of our Sun are numbered. In about 5 billion years, the Sun will turn into a red giant, shed its outer layers, and then settle down and become a white dwarf.
This is the inevitable fate of most sun-like stars, and the process is well understood. But as a recent study has shown, we still have a few things to learn about dying suns.
A recent study looks at a star known as V Hydrae or V Hya for short. It is a red giant star at a distance of about 1300 light-years from us with a mass approximately equal to the mass of the Sun. This is what the Sun might look like in a few billion years.
However, there are a few things that make her different from our home star.
First, she has a companion star. The companion is too faint for us to see, but based on the motion of V Hya, we know it is most likely a red dwarf star that orbits V Hya once every 8.5 years.
On the other hand, V Hya seems to be dying in an unusual way.
In its current state, V Hya is classified as a Mira variable star. Its brightness changes by about 1-2 magnitudes every 530 days, give or take.
This is typical of dying red giant stars. Because they fuse heavier elements in their cores in an attempt to survive, red giants often go through periods of oscillation caused by the core heating and cooling.
More unusual is the fact that it is also a carbon star. This means that the carbon melted in its core got into the star’s atmosphere. So when astronomers look at the V Hya spectrum, they see a strong presence of carbon.
In fact, the atmosphere of the star is very “sooty”.
Above: Shown here in the composite, these leaky rings and the diffuse arc structure of the sixth ring are moderately visible in the 12CO carbon isotope emission line and become well visible when viewing 13CO carbon isotopes.
These two characteristics combine to make V Hya an asymptomatic giant branch star, or AGB. About 90 percent of sun-like stars enter an AGB period towards the end of their lives.
Astronomers generally believe that the AGB epoch is a gradual process of stellar death, where the outer layers of a star are shed off over a period of approximately 100,000 years. After that, the remaining core collapses into a white dwarf. However, V Hya shows that this is either partially or sometimes incorrect.
If the AGB were to gradually shed its outer layers, then we would expect to see an ever-expanding nebula known as the [planetary nebula] (/post/cold-embers/) surrounding most white dwarfs. And we do see many planetary nebulae as remnants of sun-like stars.
But observations with the Atacama Large Millimeter/Submillimeter Array (ALMA) show that Hydra V does not create such a nebula.
Instead, she throws out thick carbon-rich rings. The star ejected six rings over a period of approximately 2,100 years.
The team also observed jets of gas ejected from the star perpendicular to the rings. This suggests that V Hya experiences an unusually active period every few hundred years, which is very different from the common AGB model.
This period of active bursts is likely short compared to the full epoch of the AGB, so astronomers were lucky to catch a dying star at this stage. It is not known whether most AGB stars experience such active periods, or whether V Hya is particularly unusual.
Solving this mystery will require more observations of other dying red giant stars.
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