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Title: Evolutionary status of Polaris
Author: Yu. A. Fadeyev

Hydrodynamic models of short--period Cepheids were computed to determine the pulsation period as a function of evolutionary time during the first and third crossings of the instability strip. The equations of radiation hydrodynamics and turbulent convection for radial stellar pulsations were solved with the initial conditions obtained from the evolutionary models of population I stars (X=0.7, Z=0.02) with masses from 5.2 to 6.5 Msol and the convective core overshooting parameter 0.1 <= aov <= 0.3. In Cepheids with period of 4 d the rate of pulsation period change during the first crossing of the instability strip is over fifty times larger than that during the third crossing. Polaris is shown to cross the instability strip for the first time and to be the fundamental mode pulsator. The best agreement between the predicted and observed rates of period change was obtained for the model with mass of 5.4 Msol and the overshooting parameter aov=0.25. The bolometric luminosity and radius are L = 1.26e3 Lsol and R = 37.5 Rsol, respectively. In the HR diagram Polaris is located at the red edge of the instability strip.

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Title: The Hipparcos parallax for Polaris
Authors: Floor van Leeuwen

This letter follows a recent claim that the Hipparcos parallax for Polaris could be too small by 2.5 mas. It examines in detail the Hipparcos epoch astrometric data for Polaris, as well as the viability of other observations that were put forward to support a larger parallax. The Hipparcos determination of the Polaris parallax is shown to be sufficiently robust to fully exclude a significantly larger parallax, and there is no observational support from other observations, such as a supposed presence of a cluster, either.

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Title: The Pulsation Mode of the Cepheid Polaris
Authors: David G. Turner, V. V. Kovtyukh, Igor Usenko, N. Gorlova

A previously-derived photometric parallax of 10.100.20 mas, d=992 pc, is confirmed for Polaris by a spectroscopic parallax derived using line ratios in high dispersion spectra for the Cepheid. The resulting estimates for the mean luminosity of <Mv>=-3.070.01 s.e., average effective temperature of <Teff>=60251 K s.e., and intrinsic colour of (<B>-<V>)o=0.560.01 s.e., which match values obtained previously from the photometric parallax for a space reddening of E(B-V)=0.020.01, are consistent with fundamental mode pulsation for Polaris and a first crossing of the instability strip, as also argued by its rapid rate of period increase. The systematically smaller Hipparcos parallax for Polaris appears discrepant by comparison.

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Cause of North Star's decreasing mass revealed

Researchers at the University of Bonn in Germany believe they have unlocked the answer to the North Star's decreasing mass.
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Title: The period change of the Cepheid Polaris suggests enhanced mass loss
Authors: Hilding R. Neilson, Scott G. Engle, Ed Guinan, Norbert Langer, Richard P. Wasatonic, David B. Williams
(Version v2)

Polaris is one of the most observed stars in the night sky, with recorded observations spanning more than 200 years. From these observations, one can study the real-time evolution of Polaris via the secular rate of change of the pulsation period. However, the measurements of the rate of period change do not agree with predictions from state-of-the-art stellar evolution models. We show that this may imply that Polaris is currently losing mass at a rate of \dot{M} ~ 10^{-6} solar masses yr^{-1} based on the difference between modelled and observed rates of period change, consistent with pulsation-enhanced Cepheid mass loss. A relation between the rate of period change and mass loss has important implications for understanding stellar evolution and pulsation, and provides insight into the current Cepheid mass discrepancy.

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The Polaris Cluster

A Cepheid star is one whose mass and age results in physical conditions that generate periodic oscillations in its photosphere. A Cepheid thus varies regularly in brightness, with a period proportional to its intrinsic luminosity. This extraordinarily useful property of Cepheid variables, discovered and calibrated at Harvard by Henrietta Leavitt in 1908, allows them to act as reliable cosmic distance calibrators. By comparing the intrinsic brightness as determined from a period (which is easily measured) with the measured brightness, a precise distance can in principle be obtained. Cepheids in distant galaxies that are receding from us provide the basis for the famous distance-velocity relationship of galaxies that underpins the expanding universe model (the "big bang" model).
The North Star, Polaris, is not only renowned as a reliable beacon for early navigators. It is also the closest Cepheid to earth (about 425 light-years away), and a subject of intense study. One issue is whether, like many stars, it is associated with a cluster of small companion stars that could have affected its evolution. In fact Polaris itself ("Polaris Aa", whose mass is 4.5 solar-masses) is known to orbit with a close companion, Polaris Ab (whose mass of 1.3 solar-masses). The pair orbit at a separation of about 15 astronomical units, about as far apart as Uranus is from the sun. Another nearby star, Polaris B, seems to be orbiting around the other two at a distance 100 times farther away. Two more stars nearby, Polaris C and D, might also be faint companions that some astronomers think are gravitationally bound to the others.

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polaris9.gif

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View of Polaris. It can be seen that it its the center axis of rotation.
Taken at 101 inn, Israel on 14-08-09 During Perseids meteor shower.
20 second exposure intervals

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Polaris, or the North Star, is perhaps the most famous star in the northern sky, even though it is only 49th in brightness among them all.

"We think of Polaris as marking the exact position of the North Polethe place among the stars toward which the Earth's axis of rotation pointsand that it would be exactly overhead to someone standing at Earth's North Pole. But that's not true. Polaris is actually a small distance away from the North Pole. Moreover, the separation between them is changing from one year to the next" - Richard Teske, University of Michigan professor emeritus of astronomy.

The star's current distance from the exact North Pole of the sky is the width of one-and-one-half full moons, according to Teske, who added that their separation is slowly diminishing as the position of the pole moves among the stars. The pole and Polaris will be closest in the year 2102 when the width of just one full moon will fit between them. After that, their separation will begin to steadily widen.

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Title: Welcome back, Polaris the Cepheid
Authors: H. Bruntt, A. J. Penny, D. Stello, N. R. Evans, J. A. Eaton

For about 100 years the amplitude of the 4-day pulsation in Polaris has decreased. We present new results showing a significant increase in the amplitude based on 4.5 years of continuous monitoring from the ground and with two satellite missions.

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A star destined for a slow death that sits high above the North Pole, is puzzling astronomers after recent observations have revealed that it's breathing strong again.
Researchers from Australia, Scotland and the United States have found that the 'north star' Polaris, which was fading over the past century, has reversed and is increasing in activity.

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