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The North Star has spots!

The spoted surface of Polaris
Click for original image.

Astronomers using an array of six ground-based telescopes have obtained best new data of Polaris, the North Star, including the first rough image of its surface, and discovered sunspots on its surface.

You can read the paper here [pdf]. The image to the right, taken from figure 4 of the paper, shows the surface as seen by the telescopes over two nights in April 2021. Polaris is what astronomers call a Cepheid variable star, which changes brightness on a very precise schedule as its diameter grows and shrinks. In the case of Polaris, that variation is four days long. The star’s brightness itself varies only slightly, and over the decades has even at times appeared to cease its variations.

As the true brightness of Cepheids is very predictable based on their pulse rate, these stars are one of the main tools astronomers use to determine distances to other galaxies. Knowing more about them thus has great importance to cosmological research.

The orbital motion showed that Polaris has a mass five times larger than that of the Sun. The images of Polaris showed that it has a diameter 46 times the size of the Sun.

The biggest surprise was the appearance of Polaris in close-up images. The CHARA observations provided the first glimpse of what the surface of a Cepheid variable looks like. “The CHARA images revealed large bright and dark spots on the surface of Polaris that changed over time,” said Gail Schaefer, director of the CHARA Array. The presence of spots and the rotation of the star might be linked to a 120-day variation in measured velocity.

The researchers plan to take regular images again of Polaris to better track the changes to its surface.

Genesis cover

On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.

 

The print edition can be purchased at Amazon. from any other book seller, or direct from my ebook publisher, ebookit. The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.


The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.
 

"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News

4 comments

  • David Ross

    What I didn’t know: Polaris A is a binary. Semimajor 6.2 AU = 29 times the stellar radius, Ab eccentricity = 0.635. Period ~29.4 years. This certainly helps gauge the mass-ratio.

  • John

    Or maybe triple star system? from linked article, “It is the brightest member of a triple-star system and is a pulsating variable star. Polaris gets brighter and fainter periodically as the star’s diameter grows and shrinks over a four-day cycle.

    All this time it was a Cepheid variable and I had no idea :(

  • John wrote, “All this time it was a Cepheid variable and I had no idea.”

    One of the first magazine articles I ever got published, in March 1995 for Astronomy, was specifically on Polaris and its variable star nature. I too was astonished then to discover it was a Cepheid that varied in brightness.

  • “But I am constant as the Northern Star,
    Of whose true fixed and resting quality
    There is no fellow in the firmament.
    The skies are painted with unnumbered sparks;
    They are all fire, and every one doth shine;
    But there’s but one in all doth hold his place”

    ‘Julius Caesar’ Act III Scene I The Bard 1599

    While I agree with McCoy’s assessment of Gen. Chang’s delivery, it was still core.

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