A star that bears watching

By midnight, Bootes hangs from the top of the sky like some immense starry mobile. What so attracts the eye in this cone-shaped constellation, however, is the brilliant orange star Arcturus, fourth-brightest star in all the sky. Its name is Greek for 'bear watcher.'

Proximity is one reason Arcturus is so bright. The star is only 34 light-years away, which, in terrestrial terms, is a little over 200 trillion miles. Daunting as this distance may sound, it is a stone's throw across an astronomical pond. Another reason Arcturus is bright is because of its size, which relates directly to how much surface area is available to radiate light. Though its temperature is 4,200 kelvins,* or about 1,600 kelvins cooler than our Sun, its diameter is 25 times that of the Sun, which makes it nearly 22 million miles across.

Such a star, if placed where our Sun is, would be a shimmering red sphere with an apparent diameter of 12.5° in our sky - the width of 25 full Moons placed edge to edge. Earth would not be a pleasant place to live: Death Valley, where temperatures typically soar over 120 degrees Fahrenheit (50 °C), would be an oasis compared to what Earth would be like near such a star.

* The Kelvin scale corresponds to the Celsius scale but assigns zero degrees to absolute zero, or -273 degrees Celsius. In the Kelvin scale, room temperature (72 degrees Fahrenheit) is about 300 kelvins. The Sun's surface temperature, 5,800 kelvins, is equivalent to about 10,000 degrees Fahrenheit.

One of Arcturus' unique properties is its motion through space. The measure of a star's apparent motion in the sky over a year's time (in other words, how much it moves with respect to the more distant, and 'fixed,' background stars) is called the annual proper motion. The annual proper motion of a star includes both its angular rate of motion (in arcseconds per year) and its direction of motion in the sky.

When astronomers measure Arcturus' annual proper motion, they note the star shifts its position on the sky by 2.3 arcseconds per year in the direction of the constellation Virgo. Although this may seem like a very slight amount - the full Moon covers 1/2°, or over 1,800 arcseconds of sky - it's a remarkable amount for a star. In 5,000 years, Arcturus will have moved 3° on the sky in the direction of the constellation Virgo, equivalent to six apparent lunar diameters. This makes Arcturus one of the fastest moving first-magnitude stars in the sky (only Alpha Centauri moves faster).

The stars may look frozen in time, but they are not. If we could reduce several million years of proper motion to a minute's time, we'd notice that all stars move. There is no such thing as a 'fixed' star. Like Van Gogh's Starry Night, the heavens would be streaked with arcs of stars racing across the sky in all directions and at different speeds. Arcturus, though, would be one of the fastest moving stars, dashing across the heavens to become, in about 20,000 years, a new member of the constellation Virgo.

Also this week:

• Observers in the Southern Hemisphere may see the three stars in the end of the handle of the Big Dipper just above their northern meridian this week around 9 o'clock.

• Vega in the constellation Lyra the Lyre rises just before sunset at northerly latitudes and can be seen in the northeast at dusk.

May 24-30

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