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Founded January 1970
Julian date: 2458762.5
2019-2020:  XXV


THE DAILY ASTRONOMER
Monday, October 7, 2019
Tau Ceti is No Sibling

[image: 1-taucetisunli.jpg]
*Tau Ceti: *at a distance of 12 light years, Tau Ceti is the closest single
Sun-like star to the Sun.     A Sun-like star is chemically similar to the
Sun. As it is so close and so Sun-like, could it be one of Sol's long lost
siblings?

Excellent question!

That was our initial response to a query sent after we posted Friday's
quiz.  This time, we're delighted to write, the quiz wasn't sullied by a
silly error, but instead, elicited a brilliant question.     If we engage
the mind's inexplicable ability to recall the irretrievable past, we
remember that in Friday's quiz we mentioned that Tau Ceti was the closest
Sun-like star to the solar system.   At a distance of only twelve light
years, Tau Ceti is certainly just an asteroid's toss away from Sol.
 (Granted, the Voyager I craft would require tens of thousands of years to
traverse that distance, but here we're referring to stellar standards.)
By Sun-like, we mean that Tau Ceti is spectrally (and therefore chemically)
similar to our own parent star.   Even though Tau Ceti is about 78%
smaller, we can still consider it to be very much like the Sun. The answer
to that question prompted the following question:

*"You mentioned that astronomers have found two stars that were the Sun's
siblings.   They know of them because of their chemistries.  If Tau Ceti is
about the same chemically and so close, why isn't it one of the Sun's
sisters too?"*
*           A.S.,  Norridgewock, Maine*

Greetings.
We'll answer the question with a simple statement and then we'll devote the
remainder of the response by justifying the statement.

*Tau Ceti is not the Sun's siblings because it is almost a billion years
older than the Sun and therefore formed much earlier.*

As the Sun and its companions formed over a period of approximately 10
million years, Tau Ceti couldn't have possibly have arisen from the same
cloud.    So, how can astronomers possibly know the star's age?

To answer this question, we first much discuss "metallicity," the relative
abundance of "metals" within stars.  In this context, 'metals' refers to
any elements heavier than helium.  The interstellar medium (ISM) and the
gaseous clouds formed out of it consists of 70% hydrogen and 28% helium,
with trace amounts of metals.    The metallicity of a given region of space
changes with time due to supernova explosions (the explosions of highly
massive stars at the end of their life cycles) that expel heavy element
material into space.  The incorporation of these remnants into a specific
cloud or ISM region increases its metallicity.    As the Universe ages, the
ISM becomes more metal-rich as more supernova explosions occur.

The actual measurement of metallicity involves the ratio of Iron to
Hydrogen.  Spectral analysis of Tau Ceti's light reveals the ratio in Tau
Ceti to be about a third that of the Sun.  This difference suggests that
Tau Ceti is significantly older as the material from which it formed
contained fewer "metals."     Tau Ceti's estimated age is approximately six
billion years: too old for sisterhood.

Tau Ceti is spectrally similar.  Its spectral type is G, just like the
Sun's. *  However, it is not chemically similar enough to the Sun indicate
that it is the Sun's sibling.




*A "G" type star has an effective temperature between 5,200 - 6,000 K with
stellar masses comparable to that of the Sun.

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