Starwatch: Polaris, center of the celestial pinwheel
Ask an average person to name a star and chances are they’ll come up with the North Star. Without a doubt that’s one of the best known stars in the night sky, but “North Star” is just a nickname. Its actual name is Polaris. If you really want to know, Polaris also has a scientific name — Alpha Ursae Minoris, because it’s the brightest star, or alpha star of the constellation Ursa Minor, which is Latin for Little Bear. To complicate things even more, Ursa Minor has its own nickname, the Little Dipper, because that’s what it actually resembles.
Polaris is a much larger star than our sun. Our home star is less than a million miles in diameter, but Polaris is nearly 40 million miles in girth. It also kicks out more than 2,000 times the light that our sun does. It’s so innocent looking in our night sky because it’s over 400 light years away, with one light year equaling nearly six trillion miles. Like many stars in the heavens that appear to be one star, it’s actually a triple star system. Polaris has two other tiny companion stars that can’t be seen unless you have one heck of a telescope.
Polaris, the North Star, is a very important star in our Rochester sky because it’s shining directly above the Earth’s North Pole. As a result all of the other stars in our sky, including the sun, appear to revolve around Polaris once every 24 hours. It’s not the stars that are moving, but rather the Earth rotating every 24 hours below the dome of the sky. If you were standing on top of the world at the Earth’s North Pole, Polaris would be directly over your head, and all of the other stars would be obediently revolving around the overhead North Star. None of the stars would rise or set. We would see the same set of stars every single night and we’d be really cold for most of those nights!
Since we’re not living on the North Pole, Polaris is not overhead in Rochester but instead it’s fixed about halfway from the northern horizon to the overhead zenith. It’s easy to find with the help of the nearby Big Dipper. This time of year, the Big Dipper, the rear end and tail of the official constellation Ursa Major or the Big Bear, is hanging nearly upside down in the high northern sky. The two stars that mark the side of the dipper’s pot section opposite the handle, Merak and Dubhe, act as pointer stars to Polaris. Just draw a line from Merak to Dubhe and continue that line below Dubhe, and that line will run almost exactly into Polaris. No matter where the Big Dipper is in the northern sky, the stars Merak and Dubhe will always point at Polaris.
The North Star, by the way, marks the end tip of the tail of the Little Bear or the end of the handle of the Little Dipper.
Constellations close to Polaris like Ursa Major and Minor, as well as Cassiopeia the Queen and Cepheus the King, make a tight daily circle around the North Star, and because they’re so close to Polaris they never set below the horizon. They graze the northern horizon at their lowest point. These Polaris huggers are known as circumpolar stars. The rest of the stars in the celestial dome that aren’t so close to Polaris also circle around the North Star once every 24 hours, but the northern portion of their circuits take them below the northern horizon for a time. So, from our perspective, those stars appear to rise at some point in the east and set at some point in the west similar to our sun and moon.
Because the Earth also revolves around the sun every year, not only do all of the stars circle Polaris every single 24 hours, but they also shift to the west a little bit each night. That’s a good thing because that results in us seeing different constellations from season to season through the course of the year. We gradually “lose” constellations below the western horizon over the weeks and months, and gradually “gain” constellations above the eastern horizon. This whole cycle takes one year.
By the way, the star Polaris is not permanently fixed above our North Pole. This is because the Earth’s axis is very, very slowly wobbling over a 26,000-year period, so hundreds of years from now Polaris won’t be our North Star, but for now it’s the hub of the sky!
This Week’s Celestial Huggings
The moon has a date with a couple of bright planets next week in the early morning pre-twilight southern sky. On Monday and Tuesday morning, May 20th and 21st, the waning full moon will be in a close celestial conjunction with the planet Jupiter. Later on next week on Wednesday and Thursday, May 22nd and 23rd, the shrinking gibbous moon will have a celestial hugging with Saturn. Don’t miss the show this week!