- 4266 - POLARIS - North Star my first star to learn? When you look up in the night sky and find your way to the North Star, you are looking at Polaris. Not only is it the brightest star in the Ursa Minor constellation (the Little Dipper), but its position relative to the north celestial pole (less than 1° away) makes it useful for orienteering and navigation.
------------------- 4266 - POLARIS - North Star my first star to learn?
- The North Star was
my first star, not counting the
Sun.. Since then it has been a growing
hobby learning astronomy. Polaris is
the closest, brightest Cepheid Variable. But. very recently, something changed.
-
- Since the age of
modern astronomy, scientists have understood that this star is a binary system
consisting of an F-type yellow supergiant (Polaris Aa) and a smaller
main-sequence yellow dwarf (Polaris B). Further observations revealed that
Polaris Aa is a classic Cepheid Variable, a stellar class that pulses
regularly.
-
- For most of the
20th century, records indicate that the pulsation period has been increasing
while the pulsation amplitude has been declining. But recently, this changed as
the pulsation period started getting shorter while the amplitude of the
velocity variations stopped increasing.
-
- These behaviors
could be attributed to long-term changes related to the binary nature of the
system, where the two stars get closer to each other, and the secondary
perturbs the atmosphere of the primary star.
-
- Cepheid variables
are stars that pulsate radially, causing them to vary in diameter and
temperature. These pulsations are directly related to changes in their
brightness, which makes them a useful tool for measuring galactic and
extragalactic distances.
-
- The variable
nature of Polaris was confirmed in 1911 by Danish astronomer Ejnar
HertzsDaprung, for whom the Hertzsprung–Russell diagram is named. Observations
conducted throughout the 20th century have shown that Polaris has a consistent
pulse period of about four days, which has been steadily increasing every year.
-
- This recently began
to change, leading many astronomers to question what is driving Polaris’
pulsations. For more than 150 years
and up until about 2010, the period had been getting longer by about 4 or 5
seconds each year. Modern observations
have shown that this trend has now reversed, and the pulsation period is
getting shorter. This is an unexpected change, showing that there is still much
that we do not understand about Polaris and other stars like it.
-
- To learn more about
Polaris’ pulsation period, radial velocity (RV) measurements going back to 1888
were studied. This technique consists of
measuring spectra from a distant star and looking for redshift and blueshift,
which are indications that the star is moving back and forth (this technique
also yields accurate estimates of its velocity).
With more than 3,600 RV measurements, including the nearly
1,200 spectroscopic observations carried out by the Lick Observatory over more
than 60 years.
-
- In the early
1990’s the amplitude had become so small that it was thought that the
pulsations were about to stop. However, Polaris decided otherwise, and by the
late 1990’s the amplitude had started to increase again, which lasted until
about 2015.
-
- The most recent
observations indicate the amplitude is no longer increasing, and may begin to
come down again. RVs have shown that
this behavior may be related to the fact that Polaris is orbited by another
star, which comes close to it every 30 years and may be perturbing the
Cepheid’s outer layers, where the pulsations take place.
-
- The changes in
Polaris’ pulsation period may result from its companion disturbing it whenever
they make their closest pass to each other. Once this is factored in they were
able to derive an improved spectroscopic orbit for the binary system, something
astronomers have been trying to resolve for generations.
-
- We now know that
Polaris behaves in an irregular and unpredictable manner. If it is confirmed
that this has to do with the presence of its companion, this may shed light on
the behavior of other pulsating stars with similar properties and help us
understand the nature of these oscillations.
-
-
December `12, 2023
POLARIS -
North Star my first star to learn? 4266
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