Astro Bob: Swollen star eats planet; Full Flower Moon blooms
Astronomers have long suspected stars eat planets when they swell up into giants. Now we have proof. Also, see the full moon and a meteor shower this week.
This week the full moon is in bloom. Called the Flower Moon, it rises with all its petals open around sunset on Friday, May 5 in the constellation Libra the scales. Libra is one of the 12 constellations of the zodiac. Its four primary stars form a diamond shape that unfortunately will be nearly impossible to see because of overwhelming moonlight. Wait a couple nights for the big, shiny guy to move out of the picture. Starting May 7, Libra will be in the clear in a dark sky. Get this free star chart to help guide you to the constellation.
That same night the moon passes through Earth's outer shadow, called the penumbra, in eclipse . The event won't be visible in the Americas but skywatchers in eastern Europe, Asia, Africa and Australia will see it. They'll have to pay attention though. Since the moon passes through the outer shadow, where the globe of the Earth doesn't completely block the sun from the moon's perspective, observers will see a dusky shading instead of a dark "bite". The odd coloration will appear across the upper third of the lunar disk for about 25 minutes centered on maximum eclipse.
While watching a full moon rise (find your local moonrise time at timeanddate.com/moon ) is a moving experience, a bright moon can kill a meteor shower. I'm afraid that's exactly what will happen on the morning of May 6, the peak of the annual Eta Aquariid shower.
Without a moon we might see a dozen or more meteors per hour in the two hours before dawn, but we'll be lucky to spot a handful this year because of moonlight. Every May, Earth crosses the orbit of Halley's Comet, which is strewn with dust and small rocks shed by the comet each time it passes near the sun. As Earth meets the particles head-on they slam into the atmosphere at 148,000 miles per hour (66 km/sec) and streak as meteors.
Are Earth's days numbered?
You may have heard that there's a good chance the Earth will be swallowed by the sun some day. In roughly 5 billion years our friendly star will expand into a red giant 100 times or more its current size. Earth will sizzle like a steak over hot coals as the sun balloons outward threatening to engulf the planet. We won't go it alone. Mercury and Venus will perish before us. While frighteningly inevitable, this scenario lies in the distant future. By then we'll have undoubtedly found a happier planet to call home.
As sun-sized stars age, their cores heat up while their atmospheres expand and cool. They evolve into red giants. To see one with your own eyes, face east and look halfway up the sky as soon as it gets dark. The bright, orange-red star up there is named Arcturus. It's a red giant with a mass similar to the sun but some 26 times larger. About the time the sun is on the verge of gobbling down the Earth it will look nearly identical to Arcturus.
By studying a myriad of stars at various stages of their evolution, astronomers have been able to piece together an understanding of their life cycles and how they interact with their surrounding planets as they age. Stars engulfing their planets are estimated to occur just a few times a year across the Milky Way galaxy. But until now no one had seen it happen.
An oddball "nova"
Astronomer Kishalay De was photographing explosive star systems called novae with CalTech's Zwicky Transient Facility telescope a few years back. A nova occurs in a very close double star when one star siphons gas from its companion. The material builds up on the surface of the hungry sun until it's hot enough to ignite via nuclear fusion and explode. The star usually survives the blast, but for a time it's surrounded by an expanding shell of hot dust and gas.
In 2020 De photographed a bright, explosive event in the constellation Aquila the eagle that resembled a nova. But follow-up observations revealed that instead of hot gas surrounding the star, it was embedded in cool dust. De called the newly discovered source ZTF SLRN-2020.
Hoping to unravel its origin he turned to archival data collected by NASA's NEOWISE spacecraft. Its telescope — designed to sense infrared light — is ideal for recording glowing dust around stars. NEOWISE images showed that the star had begun to slowly brighten almost a year before ZTF spotted the visible-light flash.
Combining the data from multiple ground-based telescopes and the NEOWISE spacecraft De and his team formulated a likely scenario to explain the star's slow, steady rise in brightness that concluded with a razzle-dazzle outburst.
As the red giant expanded, its outer atmosphere began to brush up against a Jupiter-sized planet in close orbit. Drag from the atmosphere slowed the planet and shrank its orbit until it spiraled to its doom, plunging into the star like some titanic meteoroid. So much energy was released during the big gulp the star temporarily expanded and brightened by several hundred times! That's what created the bright outburst De originally saw with ZTF.
Archival NEOWISE images showed that starting a year earlier the planet had tried to fight back. Though its fate was sealed, the Jupiter-mass world used its gravitational sway to rip away some of the star's gases and fling them into space, where they cooled and condensed to form dust. The event is the first-ever observation of an expanding red giant lunching on one its own planets. You can read all the juicy details in a May 3 paper published in the journal Nature.
The outburst from the engulfment lasted about 100 days. Based on the system's behavior we know the ejected material consisted of about 33 Earth-masses of hydrogen and 0.33 Earth-masses of dust. The team estimates that the star is similar in mass to the sun.
If the thought of stars eating planets makes you uncomfortable, the story has a happy ending. All the dust ejected into space during the tangle will provide material that will ultimately be recycled into new planets and stars.