June will be a very special month in the sky. We have a good chance of seeing a bright recurring nova, a short-lived explosion on the surface of a white dwarf star. A nova happens when a white dwarf, the dead core of a small star, is very close to a red giant star. As these stars orbit each other, the gravity of the denser white dwarf can pull gas from the red giant. As this gas falls onto the surface of the white dwarf, it is compressed and heated. When the gas is hot enough, it explodes in a runaway thermonuclear explosion. The explosion will look like a new star that lasts for a few days. However, if the gas that a white dwarf pulls from a red giant does not explode, and enough gas is added, the white dwarf will completely explode as an even brighter supernova.

When the explosion that formed the nova has cooled down, the system can reestablish itself and gas can again start to be drawn onto the white dwarf. This can lead to another explosion, but it may not happen for many centuries. In those rare instances when it happens within a few decades, it is called a recurring nova. The star that is being watched has exploded about every 80 years. The last time was in 1946, and it is showing signs that it will explode again sometime this summer.

The star that will form this predicted nova has been cataloged as T Coronae Borealis in the constellation of Corona Borealis (the Northern Crown). It is not visible to the naked eye, but when it explodes, it should reach second magnitude, about as bright as Polaris, the North Star. To find the location of the nova, extend the arc of the Big Dipper’s handle to almost overhead where you will come to the bright star Arcturus in Bootes (the Herdsman). Next look far toward the northeast during the evening, and you will see an even brighter star. This is Vega in Lyra (the Harp). About 1/4 to 1/3 of the way from Arcturus to Vega is the only fairly bright star near this line. It is Gemma (also called Alphecca), the brightest star in Corona Borealis. Watch this area, and if you see a star just to the east of Gemma, the new star is the nova.

While you are out looking for the nova, there is another interesting star that you can find by using the Big Dipper. When you come to the end of the Big Dipper’s handle, instead of “arcing to Arcturus,” bend the handle straight down, like you are snapping off the end of a longer Dipper’s handle. You should find Cor Caroli, which is translated as “Charles’s heart.” It is the only star named for an historical person. It was named by Edmond Halley to honor the English King Charles II (or the tragic Charles 1). It is the only moderately bright star in the dim and small constellation Canes Venatici (the Hunting Dogs), which is also modern. It was created in 1690 to fill an area of the sky that had not been assigned to a constellation. Through a telescope Cor Caroli is revealed to be a colorful double star. The brighter star is blue while the dimmer one is yellow.

The only planets visible during June are Saturn that rises at about 3:00 a.m. as the month begins and 1:00 a.m. by the end of the month. It will be followed about an hour later by Mars. Early risers should spot them easily in the eastern or southeastern sky.

Observing Highlights

*June 2: Before dawn, the Moon will be to the upper right of Mars. It will be to the lower left of Mars on the 3rd.

*June 8: Star Party sponsored by Grout Museum & Black Hawk Astronomy Club, Prairie Grove Park, Waterloo, 9-10:30. The Moon will be below Pollux, the brighter twin of Gemini (the Twins). Castor will be to the right of Pollux.

*June 11: The Moon will be close to the upper right of Regulus, the brightest star in Leo (the Lion).

*June 15: The Moon will be close to the upper right of Spica, the only bright star in Virgo (the Maiden). The Moon will move in its orbit and be closer to the left of Spica on the 20th.

*June 19: The Moon will be close to the upper right of Antares, the brightest star in Scorpius (the Scorpion).

*June 20: Summer will arrive in the northern hemisphere with the summer solstice. This is the shortest night of the year.

*June 27: After they rise at about 1:00 a.m., the Moon will be close to the right of Saturn.