The month of December always marks the beginning of winter. This year that will happen at 5:44 a.m. on the 21st. This is the longest night of the year and the lowest point in the sky that the sun will reach for the year.

All five of the brightest planets will be visible this month along with two meteor showers, another super moon and, most exciting of all, a new nova in the constellation of Sagittarius.

SKY GUIDE: This chart represents the sky as it appears over Maine during December. The stars are shown as they appear at 9:30 p.m. early in the month, at 8:30 p.m. at midmonth and at 7:30 p.m. at month's end. To use the map, hold it vertically and turn it so that the direction you are facing is at the bottom. Sky chart by George Ayers

Just discovered by a Japanese amateur astronomer a little over one month ago, this nova can be seen with binoculars at about 9th magnitude in Sagittarius between the handle and spout of the teapot. It is only about 4 degrees to the left and below brilliant Venus, so that’s a good guide. That leaves you only a narrow window of time to see and appreciate this new star, since Sagittarius now sets just a couple of hours after sunset. An even brighter nova was discovered in the same constellation in March 2015. Now there’s also a second, fainter nova just below this one that requires a good telescope to see.

A nova is created when a red giant star wanders too close to a white dwarf. Gas spirals in from the red giant onto the surface of the white dwarf. The gas is compacted by the powerful gravity of the white dwarf, fully 350,000 times stronger than on the surface of the earth, and then heated until hydrogen fusion occurs after a thin shell of this gas has wrapped itself completely around the white dwarf after hundreds or thousands of years. This blows away the superheated thin shell of hot gas with the force of millions of hydrogen bombs, shooting it into space at 6 million miles per hour, which is six times faster than the solar wind that is always streaming out from our own very stable sun. This brightens that very dim white dwarf by 100,000 times in a matter of a few hours. It then becomes visible to people on Earth if it is close enough or if they have good telescopes.

Both the white dwarf and red giant will survive this great event, and it will occur again in a few hundred to a few thousand years. The extreme case of such an explosion is called a Type 1A supernova, when the white dwarf and the red giant will get blown out of the sky at 1.4 solar masses. Since the mass of this event is known, along with their apparent and absolute magnitudes, they can be used as standard candles or cosmic yardsticks to measure distances to galaxies billions of light years away.

About 6 percent of all the stars in the sky are white dwarfs and over 90 percent of all the stars in the sky will turn into a white dwarf when they run out of fuel, including our own sun. A white dwarf by itself is an incredible event, and it only gets more amazing when it interacts with a red giant.

After a star the size of our sun runs out of fuel and collapses down to the size of the earth, it becomes 125,000 times denser than steel, so that each cupful of this exotic material would weigh more than a cement truck. However, it is not nearly as dense as a neutron star, where each baseball- sized chunk would weigh 40 times the combined weight of all 7.3 billion humans on earth. Neutron stars shrink to the size of a city and only occur after a supernova explosion, which is much more powerful and rarer than the common explosion of an average star, which is called a planetary nebula.

Every white dwarf is basically pure carbon and some oxygen, but much more dense and valuable than the best diamonds on Earth.

You could think of each white dwarf as a spherical, very solid out-breath, indeed the very last breath of an average star, becoming very stable again and lasting for another 10 billion years on top of the 10 billion years that this star was living, and fusing hydrogen and helium to produce prodigious amounts of energy.

Venus is getting a little closer, brighter and higher each night, now setting nearly four hours after sunset by the end of the month. Notice that it is catching up with Mars, which is now in Aquarius and continues to set around 9:30 each night. Mercury will show up below and to the right of Venus low in the evening sky for the first two weeks this month.

Watch the slender waxing crescent moon pass just above Mercury, then Venus, then Mars on the first four successive nights this month.

Jupiter rises earlier and earlier each morning. It will rise by 1 a.m. by the end of the year. Saturn then returns to our morning sky by Christmas, which is a nice celestial present, though not nearly as valuable as a white dwarf.

The best meteor shower of the year, the Geminids, will be washed out by the December super moon on the 13th. But the less famous Ursid meteor shower will peak on the 22nd, just three days before Christmas. Caused by Comet 8P Tuttle, the Ursids usually only produce about 10 to 15 meteors per hour, but they have been known to go up to 50 per hour.

DECEMBER HIGHLIGHTS

Dec. 3: The moon is 7 degrees above and to the left of Venus this evening in Capricornus.

Dec. 4: The moon shines 5 degrees to the right of Mars this evening.

Dec. 7: First-quarter moon is at 4:03 a.m. Gerard Kuiper was born on this day in 1905. The Kuiper belt is named for him and Pluto is the most famous of its denizens, along with Sedna.

Dec. 13: The Geminid meteor shower peaks. Full moon is at 7:05 p.m. This is also called the Cold, Long Night, or Moon before Yule. This will also be a super moon within one day of perigee, the third one in a row, but not nearly as close as the tremendous super moon last month that many people watched, enjoyed and photographed. The next one that close will not occur until November 2034.

Dec. 14: Tycho Brahe was born on this day in 1546. He was the best celestial observer of his time and worked closely with Kepler to figure out that all the planets orbit in ellipses and not perfect circles as was previously thought.

Dec. 17: The first powered flight took place on this day in 1903. In just 66 more years we would fly all the way to the moon.

Dec. 20: Last-quarter moon is at 8:56 p.m.

Dec. 21: The winter solstice is at 5:44 a.m. Summer starts in the southern hemisphere today.

Dec. 22: The Ursid meteor shower peaks.

Dec. 23: The moon, Jupiter, and Spica in Virgo form a line about 9 degrees long this morning.

Dec. 25: Isaac Newton was born on this day in 1642.

Dec. 27: Johannes Kepler was born on this day in 1571. He discovered his three laws of planetary motion with the help of the excellent and detailed observations of Tycho Brahe.

Dec. 28: Sir Arthur Eddington was born on this day in 1882. He took the famous photographs of the total solar eclipse in May 1919 that proved Einstein’s General Theory of Relativity.

Dec. 29: New moon will be at 1:53 a.m.

Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.

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