READER QUESTIONS ANSWERED
QI
HOW MANY STARS DIE IN THE MILKY WAY EACH YEAR?
Martin J. Heuer St. Petersburg, Florida
A IBefore diving into the astronomy here, we first need to acknowledge that we are borrowing the word die, which really belongs to biology. While we can try to apply the concepts of life and death to astronomy, so that stars burning fuel to produce energy are considered alive and those that are not are considered dead, the analogy is not perfect and there are edge cases where it all falls in a heap.
That said, there are predominantly two ways stars die. Normal low- to intermediate-mass stars (like our Sun), after swelling through their red giant phases, throw off their outer layers and dwindle away as white dwarf stars. This pathway to stellar death is trod about once every two years in our galaxy. For stars more than eight times heavier than our Sun, death becomes something of a spectacle, heralded by the explosion of a core-collapse, or type II, supernova.
Core-collapse supernovae happen about once every 60 years. They occur much less frequently than white dwarf deaths because these massive stars are much rarer than their low- and intermediate-mass counterparts. This brings our grand total of stellar deaths to about 52 per century or one every 1.9 years.
However, we have not accounted for brown dwarfs, whose masses fall between those of stars and planets. Brown dwarfs can’t sustain long-term nuclear fusion of hydrogen, but they can, for a fleeting stage, shine by burning deuterium (an isotope of hydrogen) or in some cases lithium. This continues until this limited fuel runs out and brown dwarfs eventually become dark objects made of cold gas. Although, what they lack in mass they make up for in sheer numbers — our galaxy is teeming with these objects and accounting for their life cycle would swamp the numbers above, bumping us up to something around one to three deaths per year.
But were these brown dwarfs ever sufficiently “alive” enough to die? That’s for the philosophers to decide.
David Sweeney and Peter Tuthill Ph.D. Student and Professor of Astronomy, respectively, Sydney Institute for Astronomy at the University of Sydney,
Australia
QI
HOW CLOSELY PACKED ARE JUPITER’S TROJAN ASTEROIDS COMPARED TO THE DENSITY OF THE MAIN BELT?
A IDoug Kaupa Council Bluffs, Iowa
Jupiter’s Trojans are asteroids that share the gas giant’s orbit around the Sun, clustering at one of two Lagrange points in the Jupiter-Sun system (L4 or L5, 60° ahead of or behind Jupiter in its orbit, respectively).
It turns out that main-belt asteroids (MBAs) and Trojan asteroids occupy a relatively similar volume in space. The bulk of MBAs lie between 2.1 and 3.3 AU from the Sun, while Jupiter’s Trojans are 5 to 5.3 AU from our star. Most of these bodies (in both groups) have orbital inclinations less than 30°.
You can approximate the volume these asteroids occupy using rings. So, picture a torus with a circular crosssection, with the inner and outer diameter being the inner and outer extents of the orbits just given, so that the volume each occupies is 61 AU3 and 58 AU3, respectively, for MBAs and Trojans. (One AU, or astronomical unit, is the average Earth-Sun distance of 93 million miles [150 million kilometers]. So, 1 AU3 = 8 x 1023 miles3 [2.7 x 1024 km3].)
There are about 108 MBAs larger than about 330 feet (100 meters) in diameter, and about 107 Trojans of this same size. That makes the volume density of asteroids in the main belt about 10 times higher than that of Jupiter’s Trojan asteroids.
Still, MBAs are hard to come by in the vastness of space. A sphere with a radius of about 249,000 miles (400,000 km) contains only one MBA larger than 330 feet (100 m) in diameter.
Simone Marchi Staff Scientist, Southwest Research Institute, Boulder, Colorado
M a i n
L 4 T r o j a n s b e l t
n s L 5 T r o j a
QI
IF THE JAMES WEBB SPACE TELESCOPE CAN SEE GALAXIES BILLIONS OF LIGHT-YEARS AWAY, WHY CAN’T IT FIND THE PROPOSED PLANET X SOMEWHERE IN OUR SOLAR SYSTEM BEYOND PLUTO?
A ITerry Murray
Your excellent question demonstrates that at times celestial reality can defy terrestrial intuition.
One would think that astronomical objects within our solar system would be more readily observable than galaxies billions of light-years away. However, in astronomy, apparent brightness is more important than proximity. For instance, the Andromeda Galaxy (magnitude 3.4) appears approximately 12,000 times brighter in our sky than Pluto at its maximum brightness (magnitude 13.6) despite the former’s 2.5-millionlight-year distance. All the same, one can observe Pluto with a sufficiently powerful telescope because its location is precisely known at any given time.
Finding Planet X, assuming it exists, is far more complicated. In January 2016, Caltech astronomers Konstantin Batygin and Mike Brown published a paper in The Astronomical Journal in which they cited evidence for a giant planet that might be five to 10 times more massive than Earth, with an average distance between 400 and 800 AU from the Sun. Pluto’s average heliocentric distance is 39 AU. Even if Planet X is truly a giant and reflects a lot of light, it will appear quite faint because the intensity of light diminishes with the square of the distance. At the minimum 400-AU distance, the Sun will appear at least 100 times fainter than it does at 39 AU. Any reflected light from Planet X will appear even fainter after traversing the solar system a second time to reach Earth.
Considering its distance, the uncertainty in its location, and the breadth of its orbital path, this planet would be exceedingly difficult to detect in conventional sky searches. And astronomers will not only have to detect Planet X, but also distinguish it from background stars by virtue of the motion it exhibits relative to them. Recall that Clyde Tombaugh (1906–1997) detected Pluto after a year of meticulous searching with a blink comparator, a machine that compares two different images of the sky to look for differences. The search field for Planet X is broader and, owing to its greater distance and commensurately slower orbital motion, its changes in position relative to the background will be smaller and more difficult to detect, even with current technology.
Edward Herrick-Gleason Planetarium Director, Southworth Planetarium, University of Southern Maine, Portland, Maine