Stellar Ghosts

Star Bubbles

  • Peering down into a vast cosmic maelstrom of gas and dust, the central star in this image of NGC 6853, also known as the Dumbbell Nebula, produced this gigantic bubble of material by shedding its outer layers. Located about 1,200 light-years from Earth, gas within the Dumbbell glows in visible light when it is struck by invisible ultraviolet radiation still being emitted by the central star. Several billions of years in the future, our Sun will undergo a similar transformation.
    Image credit: R. Jay GaBany
  • By: R. Jay GaBany

The Not-so-Ordinary Life of a Typical Star

Scattered like diamond chips across the Cosmos, stars look deceptively serene to earthbound observers. However, nothing could be farther from true because each one is a creature of unimaginable violence. Constant and unchanging, the nighttime stellar canopy appears essentially the same to us as it did to the ancient Greeks. But, this perception is misleading because, like people, even stars follow a cycle of birth and death.

A star, like our Sun, begins its life as a vast cloud of gas and dust drifting among the apparently empty spaces between the stars. These clouds of sparse material are truly immense and often span hundreds or thousands of light years in all directions. Even though they contain fewer atoms than the best vacuum on Earth, the total amount of material in an interstellar cloud, also known as its mass, is truly astronomical. Most of the stuff in one of these clouds is hydrogen, the simplest and most abundant element in the Universe, but they also contain other, more complex, elements, too.

For example, the cloud of dust and gas that formed our Sun must have included material from a previous generation star that blew itself apart. We suspect this because our Sun, and all of its planets including Earth, contain heavy elements that could only have been forged in the heart of a larger, long gone star that went supernova. Stars, like factories, create complex elements from hydrogen and release radiation as a by-product of their creation.
  • The life cycle of a typical star like our Sun.
  • Illustration credit: Wikipedia

For billions of years interstellar clouds wander more or less aimlessly between their starry relatives until they receive a gravitational nudge, possibly from a nearby exploding star or the close passing of another galaxy. Significantly, once the cloud is pushed, events take on a life of their own and the material begins to contract inward under its own massive weight. As the dust particles and gas molecules move toward the cloud's center, they inevitably begin bumping into each other. Like rubbing your hands together, this generates heat.
  • Stellar womb- am artist's impression of the central region inside an interstellar cloud where gravity is forging new stars.
  • Illustration credit: M. Kornmesser/ESO

Over time, these molecular collisions became more frequent. Eventually, the core of the cloud becomes so squeezed by gravity that pressures exceeded billions of atmospheres and temperatures reach over 50 million degrees. At this temperature, hydrogen nuclei collide with such speed, a thermonuclear reaction, known as fusion, is triggered. Fusion is the same process released when a hydrogen bomb is exploded. However, the amount of energy produced by a star is billions of times greater than any man-made nuclear weapon.

When fusion commences, hydrogen is converted into the next heaviest element (helium) and a tremendous amount of energy is released in the form of photons. As the photons rush from the core of the cloud, they push back on the inward pressure of gravity, which is still trying to compress the cloud, until an equilibrium is achieved and the cloud's collapse ceases. Eventually the photons escape into space as visible light and other forms of invisible radiation and the star is born. Thus, a star is a balance between the unrelenting inward pull of gravity and the ongoing thermonuclear explosion pushing back from its core.

Phase One- Nothing lasts forever

Although, most of a star's volume is comprised of hydrogen, hydrogen can only be fused into helium at its core and all stars eventually deplete their central store of hydrogen. Thus, the balance that creates a star is only temporary.

For example, although our Sun is supremely important to our existence, it is only an average star, about halfway through its 10 billion year life-cycle. Stars with less mass require less thermonuclear energy at their cores to counter balance the force of gravity, therefore they are cooler and shine for a longer period of time. Conversely, stars with greater mass than the Sun are hotter because they require more outward force to prevent gravity from squeezing them. Thus, larger stars are brighter and shine for a shorter period of time.

Our local star has sufficient mass in its core to fuse 700 million tons of hydrogen into 695 million tons of helium each second for 10 billion years. In the process, 5 billion tons of matter is also converted into energy per second as described by Einstein's famous equation E = mc2. At this rate, the Sun has already converted about 100 Earth-masses of matter into energy. But eventually, all the hydrogen in its core will fused into helium. When this happens, nuclear reactions will cease, the outward force of energy that has supported the star's weight will drop precipitously and the core will begin to compress under the force of gravity that has been patiently waiting all these years.

Phase Two- In the Footsteps of Giants

As the core collapses, the pressure and temperature within it will increase until the helium begins to fuse into the next heaviest elements, carbon and oxygen- two of the most important elements required for life as we know it. A huge amount of radiation will be released when this occurs - more than the amount released by the fusion of hydrogen. This further increases the pressure pushing back on gravity. The heat of the interior becomes so intense, a layer of hydrogen surrounding the core begins to fuse into helium. This also increases the amount of outward pressure and, combined, both these forces cause the star to grow enormous.

Gravity eventually balances this expansion, but only after the star has swollen to the orbit of Venus or possibly even farther out. In the case of our Sun, the planet Mercury will be absorbed along with Venus and Earth may be engulfed or thrown from its current orbit in the process. Since the star has the same amount of material, its heat will be spread over a larger surface area and thus it will be cooler, take on a reddish hue and become a star that is called Red Giant.
  • Red Giant Sun - In about 5 billion years our Sun will swell and possibly consume the Earth. In this artist conception, the distant moon transits the swollen solar disk as seen from our then lifeless, barren planet.
  • Illustration credit: Don Dixon Cosmographia

Phase 3- The Long Good-bye

As was the case with hydrogen, the amount of helium in the core will eventually become exhausted. When this happens, gravity will cause the surface of the star to suddenly shrink as it simultaneously exerts enormous pressure on the carbon and oxygen core. However, in a star like our Sun, core temperatures will never become sufficient to trigger the fusion of carbon and oxygen into the next heavier elements. It will only succeed in triggering fusion of the helium shell surrounding the core.

When thermonuclear reactions within the shell of helium commences, the outward radiation pressure will exceed the inward pull of gravity and the surface of star will expand once again returning the star to its Red Giant size. However, this time, the force of gravity will be insufficient to stop the expansion. So, the outer portions of the star will separate at up to 50 km/sec (over 100,000 miles/hour) and float away from the central region containing the star's core.

The ejected gas will begin to visibly glow in harlequin colors as it is ionized by invisible ultraviolet radiation still being released from the star's hot core. As the star's outer surface material departs the core will be progressively exposed. When the core of a star is revealed in this manner, the star is called a white dwarf. These stars are fantastically dense compared to anything on Earth, weighing over a ton per teaspoonful. Over billions of years, the core will slowly cool, cease to release any radiation and become a black dwarf- the corpse of a Sun that once was. If the star had a family of planets, those that weren't devoured or ejected from their solar system during its Red Giant phase will freeze.
  • The Dumbbell nebula (NGC 6853)
    The Universe is filled with spectacular cosmic displays, but few match the wonder of a planetary nebula.
  • Photo credit: R. Jay GaBany

The First Planetary

About 4,000 years ago an elderly, bloated giant red star, located toward the northern constellation of Vulpecula, gave its last gasp and shed its outer skin exposing its still pulsating heart. It wasn't until the mid-1700s that Charles Messier, the famous French comet hunter, noticed its distant faint fuzzy circular glow and placed it in his catalog to prevent him from mistaking it as a comet during future night sky expeditions. A few years later, the musician turned astronomer named William Herschel gave these objects their name because of their resemblance to planets.

Telescopes back then lacked the color definition and clarity of even the most inexpensive instruments available today. So, both Messier and Herschel would most likely be astonished had they lived to enjoy our current view of this planetary nebula. Today, we know Messier's 27th catalog designation as the Dumbbell nebula. Located about 1,200 light years from Earth. At that distance, our sun would appear 100 times fainter than the nebula. We happen to view the Dumbbell along its equator. If our line of site were more to its poles, our impression of the Dumbbell might be that of a ring. The Dumbbell is approximately half a light year in diameter but even at this great distance, the nebula appears quite large- equivalent to about half the diameter of the full Moon.

Our galaxy contains over 200 billion stars but only about 3,000 are surrounded by a planetary nebula. Planetary nebulae are usually no larger than a light-year in diameter which is actually enormous if you consider a light-year is almost 6 trillion miles in length. Because a planetary's bubble of gas is constantly expanding, the Dumbbell will only remain visible for around 10,000 years like most other planetary nebulae. Compared to the life of a star, which stretches into billions of years, a planetary nebula only exists for the twinkling of an eye and that helps explain their rarity.

Planetary nebulae come is a variety of shapes, no two are exactly identical and this has been a source of ongoing study by astronomers. Only about 20% of the known planetary nebulae have a spherical symmetry. Most, like the Dumbbell nebula, are elliptical or bi-polar with elongated somewhat mirrored structures protruding on opposite sides. The reason why most planetary nebulae are not spherical is not well understood. Several theories have been considered so far. One of them suggests the strange shapes of planetary nebulae might be due to some centrifugal force resulting from the rotation of red giant stars. Another theory proposes the symmetry of the star's wind may be affected by a companion star. However, the most recent theories explaining the shapes of the nebulae attribute their appearance to magnetic fields from the parent star.

Perhaps the best explanation for the structures seen within and surrounding the Dumbbell come from a 2008 investigation. The researchers proposed the existence of a jet spawned by an accretion disk, fed by material from a small stellar companion, surrounding the nebula's parent star. Over time, precession, similar to the wobble seen in a spinning top, causes the jet to slew in different directions. As the jet moves, it creates conical shaped structures that give the surrounding nebula its dumbbell appearance.

It's also amazing what scientists can learn from studying the colors released by stars, emitted or reflected by nebula. For example, the Dumbbell's teal coloration is released by oxygen atoms while the red hues are emitted by hydrogen when both are struck by the ultraviolet radiation sill being released from the core of the planetary's parent star.

A New Beginning

The creation of even ordinary stars requires the release of incredible energy, millions of times more powerful than every warhead and bomb in the world's combined nuclear arsenals. Even after its birth, the fury of a star can only be tamed by the relentless force of gravity. From our perspective, safely sequestered by great distance, the twinkling stars of the nighttime sky seem timeless, conjure thoughts of romance and appear dazzling to our eyes. But, as we have discussed, nothing could be farther from the truth.

Yet, if it weren't for the life cycle of stars, our existence would be impossible. Like stellar factories, stars transmute the building blocks of everything that surrounds us and in their deaths release a precious cargo of stellar material which may eventually find its way into creatures such as we, ourselves. In this way, instead of an end, every planetary nebula signals a new beginning.

Also read:
- The Cosmic Cycle
- Stellar Cauldrons