Cosmic fireworks
Q I WHAT ARE THE DIFFERENCES BETWEEN SUPERNOVAE, KILONOVAE, AND HYPERNOVAE?
Wolfgang Golser Tucson, Arizona
A IIn Latin, nova means “new.” In astronomy, that refers to a temporary bright “star” in the night sky. But the causes of these brief but brilliant stars are varied.
Classical novae occur in a binary star system with a white dwarf and a star close enough together that the white dwarf pulls, or accretes, material from its companion. The material — mostly hydrogen — sits on the surface of the white dwarf until enough has been gathered to kick-start a nuclear fusion reaction, the same process that powers the Sun. As the hydrogen is converted into heavier elements, the temperature increases, which in turn increases the rate of hydrogen burning. At this point, the white dwarf experiences a runaway thermonuclear reaction, ejecting the unburnt hydrogen, which releases 10,000 to 100,000 times the energy our Sun emits in a year. Because the white dwarf remains intact after blowing away this excess, a stellar system can experience multiple classical novae.
Kilonovae occur when two compact objects, like binary neutron stars or a neutron star and a black hole, collide. These mergers, as their name suggest, are about 1,000 times brighter than a classical nova, but not as bright as a supernova, which is 10 to 100 times brighter than a kilonova.
There are two basic ways to get a supernova. The type of supernova most people think of is a dying star’s last hurrah, known as a type II or core-collapse supernova. At the end of a massive star’s life, it no longer has the energy to support itself against gravity and collapses, the core squeezing itself into as tight a ball as possible. The implosion reverberates outward, exploding the leftover material into space. The other type of supernova, a type Ia supernova, occurs when a white dwarf in a binary star system gobbles up too much material from its companion. Unlike with a classical novae, this white dwarf experiences a thermonuclear reaction in its core. Once it crosses a critical mass threshold, it collapses and violently expels its outer layer, tearing itself apart. In both cases, a new stellar remnant — either a neutron star or a black hole — is born.
A hypernova — sometimes called a collapsar — is a particularly energetic core-collapse supernova. Scientists think a hypernova occurs when stars more than 30 times the mass of the Sun quickly collapse into a black hole. The resulting explosion is 10 to 100 times more powerful than a supernova.
Caitlyn Buongiorno Associate Editor
Q I HOW IS IT THAT EARTH IS THE ONLY KNOWN PLANET WITH ACTIVE PLATE TECTONICS? Erik McKenna Stamford, Connecticut
A IEarth is special in that it has two things that other terrestrial planets don’t: an abundance of internal heat, from when our planet was molten rock, and liquid water. To understand why our planet is unique in this regard, let’s first look at Earth versus Mars.
Earth is relatively large for a rocky planet. Its sheer amount of mass has allowed it to hold onto its internal heat over billions of years. The heat causes Earth’s surface to deform and plays a key role in ensuring that
Earth’s outer surface layer, called the lithosphere, doesn’t become too cold and thus too rigid to move. But Mars is smaller than our planet. Because of this, the Red Planet has cooled at a much faster rate. Mars’ lithosphere has become very rigid — too rigid to be broken into plates.
Heating isn’t the only thing at play when it comes to plate tectonics. Venus is about the same size as Earth, so theoretically, one might think it’s also likely to have moving plates. But it doesn’t. While heating is enough to stave off a rigid lithosphere, it isn’t enough to move Earth’s plates. That’s where liquid water comes into the equation. On Earth, interior water lubricates the tectonic plates, allowing them to flow and slide past one another, but there is no water in Venus’ interior.
To be clear, tectonic deformation is currently occurring in the outer layers of Venus and Mars, and once took place on Mercury. However, because the outer layers of these planets are not broken up into plates, we consider these planets to be one-plate planets. So, Venus and Mars still experience tectonics, just not plate tectonics.
Earth may not be the only body in the solar system to experience plate tectonics, however. Jupiter’s icy moon Europa is covered in a shell of cold, brittle ice that is believed to float atop a warmer, fluid ice layer. Like Earth’s plates, when two plates of this cold ice hit each other, one of these plates is able to slide beneath the other into Europa’s interior. Scientists have also observed evidence of water upwelling to the surface of this moon, much like magma wells up from vents on Earth.
Lynnae Quick Ocean Worlds Planetary Scientist, Goddard Space Flight Center, Greenbelt, Maryland
Q I SOME 50 YEARS AGO, ALAN SHEPARD HIT SOME GOLF BALLS ON THE MOON. JUST HOW FAR COULD A TOUR PRO HIT A GOLF BALL ON THE MOON IF THEY WEREN’T ENCUMBERED BY A SPACESUIT? Jim Knoll Vancouver, Washington
A I Alan Shepard shanked his first shot into a crater, but estimated that his second reached a distance of about 600 feet (183 meters). Recent evidence from remastered photos taken during the mission, however, suggests that Shepard managed to only hit his second golf ball some 120 feet (36.5 m).
To be fair, Shepard wasn’t just restricted by his spacesuit. His makeshift golf club wasn’t exactly regulation — just a 6-iron head attached to a collapsible tool designed to scoop lunar rocks.
According to PGA Tour stats for 2021, the average tour pro off the tee imparts a ball speed of 170.4 mph (274.2 km/h) and launches the ball at 10.52°. So, in lunar gravity, an average tour pro’s tee shot would carry about 4,170 feet (1,271 m). (On Earth, air actually helps a golf ball fly farther: Clubs impart backspin to a ball, which helps it generate aerodynamic lift and keeps it aloft.)
But a pro could still do better on the Moon. On Earth, golfers use low launch angles to send the ball further, minimizing the effects of drag. The lack of air resistance on the Moon means you could use a true ballistic trajectory with the ideal launch angle of 45°.
So, if you were able to launch a ball at a 45° angle at a speed of 170.4 mph (274.2 km/h) on the Moon, the ball would travel about 2.21 miles (3.55 km). Bryson DeChambeau, with his 2021 average ball speed of 190.72 mph (306.93 km/h), could hit a lunar golf ball even farther than that: 2.76 miles (4.58 km).