Planetarium Newsletter - December 2017

Cosmic Curiosities

“The universe has a history; this is probably the greatest discovery of our century.”
~ Hubert Reeves

Zombie Stars

exploding starA sort-of supernova—that’s a zombie star.

A supernova is when a star runs out of fuel and collapses. As the massive star implodes, it quickly rebounds off its dense dead core in a ferocious outward explosion. A zombie star explodes, but does not die. The 2014 discovery of the star iPTF14hls, 500 million light years away in Ursa Major, was thought at first to be a regular supernova; but the stellar debris didn’t fade over a 100 days like a normal supernova -- it took 600 days. Then, it got brighter and dimmer five times. More fluctuations were discovered all the way back to 1954 after looking at old photos of the star. Astronomers think it will soon cease action all together and definitively join the cosmic graveyard.

Scientists are still in the dark about what’s going on there. It’s been proposed that the core of star iPTF14hls is so hot that energy is converted into matter and antimatter. When these two react, it causes explosions that might blow out the outer layers, but keep the center of the star intact.

This discovery reminds me of another death-defying, zombie-like star—Eta Carinae.

purple starBefore 1837, “Eta Car” was an average, fairly dim star at fourth magnitude. But that year, it got brighter -- much brighter. As its brilliance grew, it cracked the top-ten list of bright stars; then, the top three. For a short time in March 1843, it was the second brightest star in the entire night sky, but then faded rather rapidly. By 1856, it was invisible to the naked eye. Eta Car went through a brief reprise in 1892, becoming barely and rarely observed at sixth magnitude—the human eye limit—and then went missing again. In 1940, the mysterious star became visible once again. Today, it shines at 4.5 magnitude (rather feebly). This is where Eta Car started, before its astral onslaught back in 1837.

What was the cause behind the rise and fall of Eta Carinae? It turns out Eta Car is two stars. Both are extremely massive. The big one is about 100 times more massive than the Sun. The “smaller” star is 30 times more massive. The great eruption 180 years ago resulted from the larger star being so energetic and unstable. Though massive, the gravity of the mammoth star couldn’t contain the intense nuclear reactions at its core. It ejected two giant bubbles of hot gas in concentrated polar outflows, now the called the Homunculus Nebula. Back in the 1940s, astronomers thought it a looked like a little human, aka “homunculus.” The expanding gas and dust wreckage has the same mass of 10 to 20 Suns. The original eruption made Eta Car dazzling bright. The nebula eventually shrouded the two stars, causing it to fade from sight. Today, the Homunculus Nebula has inflated and thinned, allowing the stars to become more visible.

The zombie stars of Eta Carinae are located in the southern hemisphere in the constellation of Carina, the keel of a ship. Their distance is about 7,500 light years. The larger star is doomed, though; within a million years, astronomers predict it will go supernova and create a black hole.


Moon Mania


The solar system has 184 moons, but that’s only if you count those orbiting the eight major planets. If you count dwarf planets and asteroids, you end up with another 150 or so. That makes 330 plus moons in our cosmic neighborhood! But of course, this number is not stagnant. On January 1, 2019 the New Horizons spacecraft will fly MU69, an icy Kuiper Belt object beyond Pluto. There may be one or two moons there -- we will soon find out.

Scientists are also closing in on the first confirmed exomoon, a moon going around an exoplanet, which is a planet orbiting a distant star. The number of exoplanets went from 0 to 3,558 in the last 25 years. As telescopes and detectors advance, exomoon tallies will grow accordingly.

Back in our solar system, Jupiter leads the way with 69 natural satellites. Saturn is a close second at 61. The other two gas worlds, Uranus and Neptune, have 27 and 14 moons respectively. Mars has two very small moons, Phobos and Deimos. They look more like asteroids or big, rocky potatoes. In case you’re wondering, former planet Pluto has five moons. Finally, we have our Moon in the sky. Notice the capital “M” -- that is the officially recognized spelling to distinguish our nightlight from all the other 300+ moons.

The word “moon” descended from the Proto-Indo-European language, last spoken around 2500 B.C.E. They called it “menses,” meaning “month” or “moon.” Our 12 months derive from the cycle of the Moon’s phases, which equals 29.53 days. There are typically 12 full Moons during the year, though around every three years we have 13 full Moons. Luna is the Latin word for Moon while the Greeks called it Selene or Artemis.

Being rather small and secondary, moons often get shortchanged in all the numerous cosmic chitchats. Below are some fun top-ten “did you knows” on moons – enjoy!

  1. Our own Moon was born from a huge impact with Earth 4.5 billion years ago.
  2. Mars’s Deimos is only seven miles across—smaller than the city of Milwaukee.
  3. Dactyl, the moon of the asteroid Ida, is only one mile across—smaller than downtown Milwaukee.
  4. Jupiter’s Ganymede at 3,280 miles in diameter is the largest moon, bigger than the USA.
  5. Jupiter’s Io has active volcanoes.
  6. Saturn’s Mimas has a huge crater that makes it look like the Death Star.
  7. Saturn’s Iapatus has one side six times brighter than the other.
  8. The moons of Uranus are named after characters in Shakespeare’s plays.
  9. Neptune’s Triton has ice volcanoes.
  10. Pluto’s Charon is the largest moon relative to its parent body.

Interstellar Asteroid


A rogue asteroid called Oumuamua, about four football fields long, has wandered into our solar system from deep space. Astronomers have never observed and examined an interloper like this, even though they are expected to be numerous with at least one visiting our neighborhood every year. Usually, galactic debris like this is too small and dark to detect.

Oumuamua, a Hawaiian name from the discoverers at the Pan-STARRS telescope in Hawaii, has already swooped past the Sun and is heading outward at nearly 17 miles per second. Observers will watch the interstellar asteroid until it finally fades from sight. They want to ascertain its origin and where it’s heading on its galaxy tour.

Oumuamua passed nearest the Sun on September 9 and passed within 15 million miles of the Earth on October 14. It was discovered by Pan-STARRS telescope on October 19. The rock was never visible to the naked eye.


Sky Sights

The evening sky has gone planet-less! Saturn and Mercury  are now lost in the Sun’s glare.

The morning sky just got dimmer with Venus disappearing from view. The brightest planet will be lost for three months, finally starting to become visible in the March evening sky.

december sky

A much brighter Jupiter and a dimmer red Mars move closer together this month. By early January, they will pass each other in the morning sky. Look for Libra’s “Zubee” stars -- Zubenelgenubi and Zubeneschamali — near Jupiter. The Moon orbits by on the mornings of December 13-16.

december sky december sky

Watch the Moon pass above the brilliant constellation Orion twice this month. The first joining is from December 1-3 and then we see the Moon pass by from December 29-31. On Saturday night, December 30, the red star Aldebaran will be so close to the Moon it might not be visible. Challenge yourself to spot it. If no luck, try some binoculars.


When gazing at the majestic stars of Orion, see if you can spot some color. It’s fairly easy to see the red-orange of Betelgeuse, but try and discern the blue of Rigel. Notice in the diagram above the huge size of Betelgeuse compared to Rigel and our tiny Sun. Stars all work the same by the process of nuclear fusion, but they are not the same in size, nor do they generate the same amount of intrinsic light.


Star Map

Download the December Star Map.  

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