- 4255 -
POLARIS - north star everyone knows? Polaris is the closest, brightest Cepheid
Variable. 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.
--------------------------
4255 - POLARIS
- north star everyone knows?
-
- 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 Cepheid Variable, a star classification that pulses regularly.
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- 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 star
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. 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. Many astronomers began 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.
-
- The radial
velocity (RV) measurements go back to 1888. This RV 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).
-
- The sample
included more than 3,600 RV measurements, including the nearly 1,200
spectroscopic observations carried out by the Lick Observatory over more than
60 years.
-
- This allowed
astronomers to trace the evolution of the pulsation properties of Polaris,
which showed how often pulses occur and their amplitude. 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 which 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,
astronomers were able to derive an improved spectroscopic orbit for the binary
system, something astronomers have been trying to resolve for generations. This
could also lead to more accurate estimates of the dynamical masses of each
stellar companion, which were also subject to uncertainty.
-
-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 the oscillations.
It is therefore important to keep an eye on it.
-
- Teach the kids how
to find the North Star, the Big Dipper, and impress them with new knowledge of
this orbiting binary star.
-
-
December 5, 2023
POLARIS - north star everyone knows? 4255
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Wednesday, December 6, 2023 ---------------------------------
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