Hubble captures ‘crazy fireworks’ resulting from violent and chaotic deaths of two stars in stunning detail
For stars nearing the end of their lives, one thing is clear: they refuse to go quietly in the night. And the Hubble Space Telescope has captured the “crazy fireworks” happening in two nearby planetary nebulas — a massive cloud of gas and dust thrown out by the explosion of dying stars — in unprecedented detail. In particular, the images reveal how both nebulas — NGC 6303, dubbed as the Butterfly Nebula because of its wing-like appearance, and NGC 7027 that resembles a jewel bug (an insect with a brilliantly colorful metallic shell) — are splitting themselves apart on extremely short timescales, allowing astronomers to see changes over the past couple of decades. Some of this rapid change may be indirect evidence of a star merging with its companion star, say scientists.
“Most stars live placid lives for hundreds of millions to billions of years. But near the end of their lives, they can turn into crazy whirligigs, puffing off shells and jets of hot gas. Astronomers have employed Hubble's full range of imaging capabilities to dissect such crazy fireworks happening in two nearby young planetary nebulas,” says the study published in Galaxies. It adds, “These two objects represent key sources for purposes of understanding planetary nebulas' shaping processes.” The research team used Hubble's Wide Field Camera 3 to record the nebulas in every wavelength, from near-ultraviolet to near-infrared light. They found unprecedented levels of complexity and rapid changes in jets and gas bubbles blasting off of the stars at the centers of both nebulas, causing them to reconsider what is happening at their cores. “When I looked in the Hubble archive and realized no one had observed these nebulas with Hubble's Wide Field Camera 3 across its full wavelength range, I was floored. These new multi-wavelength Hubble observations provide the most comprehensive view to date of both of these spectacular nebulas. As I was downloading the resulting images, I felt like a kid in a candy store,” says Joel Kastner from the Rochester Institute of Technology, New York, in a statement.
Both are among the dustiest planetary nebulas known and both also contain unusually large masses of gas because they are so newly formed. This makes them a very interesting pair to study in parallel, say researchers. Based on the bizarre shape of these nebulas, the research team suspects that at the hearts of both nebulas are — or were — two stars circling each other, “like a pair of figure skaters.” Each has a pinched, dusty waist and polar lobes or outflows, as well as other, more complex symmetrical patterns, the study describes. According to experts, one theory for the generation of such structures in planetary nebulas is that the mass-losing star is one of two stars in a binary system. The two stars orbit one another closely enough that they eventually interact, producing a gas disk around one or both stars. The disk is the source of outflowing material directed in opposite directions from the central star.
A second theory is that a smaller star of the pair may merge with its bloated, more rapidly evolving stellar companion, which can also create outflowing jets of material that may wobble over time. This creates a symmetric pattern, perhaps like the one that gives NGC 6302 its Butterfly nickname, say scientists. Such outflows are commonly seen in planetary nebulas. “The suspected companion stars in NGC 6302 and NGC 7027 haven't been directly detected because they are next to, or perhaps have already been swallowed by, larger red giant stars, a type of star that is hundreds to thousands of times brighter than the Sun. The hypothesis of merging stars seems the best and simplest explanation for the features seen in the most active and symmetric planetary nebulas. It's a powerful unifying concept, so far without rival,” says team member Bruce Balick from the University of Washington in Seattle.
The image reveals that the Butterfly Nebula has an S-shaped pattern made by gas blown out at high speed by a star. The ‘S’ only appears when captured by the Hubble camera filter that “records near-infrared emission from singly ionized iron atoms.” This iron emission likely traces the central star system’s most recent ejections of gas, which are moving at much faster speeds than the previously expelled mass. The star or stars at its center are responsible for the nebula's appearance. In their “death throes,” they have cast off layers of gas periodically over the past couple thousand years. The ‘wings’ of NGC 6302 are regions of gas heated to more than 36,000 degrees Fahrenheit that are tearing across space at more than 600,000 miles an hour. “This iron emission is a sensitive tracer of energetic collisions between slower winds and fast winds from the stars. It's commonly observed in supernova remnants and active galactic nuclei, and outflowing jets from newborn stars, but is very rarely seen in planetary nebulas,” explains Balick.
NGC 7027's central star was identified in a new wavelength of light — near-ultraviolet — for the first time by using Hubble's unique capabilities. The near-ultraviolet observations will help reveal how much dust obscures the star and how hot the star is. “The nebula NGC 7027 shows emission at an incredibly large number of different wavelengths, each of which highlights not only a specific chemical element in the nebula, but also the significant, ongoing changes in its structure,” says Kastner. The researchers say that NGC 7027 had been slowly puffing away its mass in quiet, spherically symmetric, or perhaps spiral patterns for centuries, until relatively recently. In some respects, the changes within this nebula are even more dramatic than those within the Butterfly, says Kastner. “This object, which resembles a colorful jewel bug, is a visibly diffuse region of gas and dust that may be the result of ejections by closely orbiting binary stars that were first slowly sloughing off material over thousands of years, and then entered a phase of more violent and highly directed mass ejections,” says the study.