THE USM SOUTHWORTH PLANETARIUM

207-780-4249     www.usm.maine.edu/planet

70 Falmouth Street     Portland, Maine  04103

43.6667° N                   70.2667° W 
Altitude:   10 feet below sea level

Founded January 1970

Julian date:  2458660.5

          "If you're in a spaceship traveling at the speed of light, and you turn on the headlights, does anything happen?"

                               -Stephen Wright

THE DAILY ASTRONOMER

Wednesday, June 26, 2019

Apple Core Nebula

We call it the "Butterfly analogy."   Perhaps you've heard us discuss it before and, if so, our apologies for the repetition.      Envision a butterfly scientist whose aim is to understand the human being.   The butterfly has the profound disadvantage of a fleeting life span, far more brief than the human life cycle.   So, one might wonder how our butterfly scientist could hope to fathom a specimen that is virtually immortal in comparison.    Our little lepidoptera can't linger about observing the human mature from infancy to the convalescent home.     However, the butterfly can observe humans in various development stages:  he can observe the baby, the codger, the thirtysomething woman; the middle-aged man; the teenager and young adult.     Were this butterfly a clever sort, he could piece together the evidence to form a human life cycle sequence.  The human is born a codger, shrinks down to an infant, expands into adulthood, alters its gender; and then blossoms into a teenager.  (Being clever doesn't mean one is always correct.)

The astronomer is like the butterfly.   We humans are ephemeral twerps relative to the cosmos.   The Universe is billions of years old and will likely exist for trillions of more years, if not longer.  Even a century is spark-quick by comparison.  So, how can astronomers know so much about the Universe?*   Well, first of all,  Earth has been blessed with more than one astronomer and so those alive today have benefited greatly from the work of preceding generations.   Also, we can observe stars and galaxies in various stages.     We can observe star birth in regions such as the Orion Nebula and we can watch a star's death throes in objects such as the Apple Core Nebula, also known as the Dumbbell Nebula.

The Apple Core or Dumbbell Nebula:  a planetary nebula "within" the constellation Vulpecula the Fox.     

Discovered by 18th century French astronomer Charles Messier, the Dumbbell is a planetary nebula, one formed when a solar-mass star ends its life cycle.   The star expels a rapidly expanding gaseous shell and leaves a highly dense white dwarf in its center.      English astronomer William Herschel coined the strange term "planetary nebula" for these objects as they resemble disc-like planets in the telescope.   We know that they have no association with planets, but the term remains.

Located 1,360 light years away in the constellation Vulpecula,* the Apple Core Nebula is quite luminous, exhibiting an array of different colors.   The copious ultraviolet radiation excites the nearby gas atoms, causing them to re-emit the absorbed photons as visible light.   When observed telescopically, the nebula's color are rather muted.  Only when captured through a long-time exposure image will the nebula's vibrant hues and filamentary structures become visible.

The nebula spans a 2.5 light year diameter and expands at 31 kilometers per second (69,230 miles per hour).    Observations of this expansion velocity, which isn't constant, have enabled astronomers to estimate its age between 9,400 - 13,000 years, or the time the star pushed its gases into outer space.     Seeing planetary nebulae such as the Apple Core or the Ring Nebula in Lyra the Harp gives us a view of the future Sun. Sol will eventually ends its life as a planetary nebula and white dwarf core.  Fortunately, this transformation won't occur for billions of years.  That we can see it happen to stars now enables us to understand the Sun's fate in the remote future.


*Let's have a moment to reflect on this misleading sentence.   One would think that the constellation Vulpecula (the fox) occupies a specific region, like a cube, and that the Apple Core Nebula sits neatly inside it.    Actually, the stars comprising the Vulpecula constellation are not physically connected, but just happen to be positioned along the same sight-line.    The Apple Core Nebula is also in the same direction, making it appear to be "inside" the Vulpecula constellation.