Sakurai's Object: Stellar evolution in real time

Apr 02, 2014
The figure shows an oil painting done by Stephen Mack that represents what the present expanding shell of gas and dust around the star may look like. Mack is a member of the Tohono O’odham Nation, the Native American tribe on whose land the Kitt Peak National Observatory, which is managed by NOAO, is located. Credit: The National Optical Astronomy Observatory

( —Stellar lifetimes are measured in billions of years, so changes in their appearance rarely take place on a human timescale. Thus an opportunity to observe a star passing from one stage of life to another on a timescale of months to years is very exciting, as there are only a very few examples known. One such star is Sakurai's Object (V4334 Sgr). First reported by a Japanese amateur astronomer in 1996 as a "nova-like object," Sakurai's Object had been only a few years before the faint central star of a planetary nebula. In the 1990's Sakurai's Object brightened by a factor of 10,000. This brightening has been attributed to a final helium shell flash. In this process the burned out core of the star at the center of the planetary nebula re-ignites.

The final helium shell flash is violent, ejecting a cloud of dust and gas that forms a thick cocoon around the star blocking all visible light. By 2000 the was so thick that Sakurai's Object was not visible even with the Hubble Space Telescope (HST). Scientists at the National Optical Astronomy Observatory (NOAO) have been observing the sky in the area of Sakurai's Object waiting for infrared radiation to break through the dust cloud. Infrared radiation penetrates dust much more efficiently than optical light. A detection of the infrared light would mean that the dust cloud is breaking apart, ultimately permitting light from the star to escape.

Using the Altair adaptive optics (AO) system with the Gemini North telescope on Mauna Kea in Hawai'i to compensate for distortions to starlight caused by the Earth's atmosphere, two NOAO astronomers were able to observe the shell of escaping material around the star. According to Dr. Richard Joyce, who was in charge of the imaging program, "Using AO at Gemini gave us an unprecedented view into the heart of this object and showed us a number of faint where Sakurai's Object should be." The team compared the Gemini images to views by the Hubble Space Telescope, taken before Sakurai's Object had faded from view, to obtain a precise location for the object. The Gemini AO images have a resolution of 0.04 arc second (this is equivalent to asking someone to tell if you are holding up one finger or two—from a distance of 200 miles) which clearly resolved many of the stars that ordinarily would be blurred together from ground-based telescopic views. "The initial Gemini images in 2010 showed a faint fuzzy spot near the Sakurai location. It's amazing that we could see this level of detail," says Joyce. "By 2013 Sakurai's Object was obvious at this location with two ejected clouds thanks to these remarkable observations."

Dr. Kenneth Hinkle, lead author, says, "Sakurai's object appears to be forming a bipolar nebula: in the past three years two lobes of gas have been observed moving outward from the central star. The bipolar nebula is roughly aligned to the planetary nebula. The is formed from gas lost more than 10,000 years ago by the red giant. The co-alignment suggests that there is either a companion star or planet in the system." The accompanying artist's conception represents what the present expanding shell of gas and dust around the star may look like. Because it is enshrouded in dust, Sakurai's object is much brighter in the infrared region of the spectrum than in visible light. In this illustration the star appears bright red since blue light from the star is absorbed by the dust.

As stars like the Sun reach the end of their lives they expand and cool to become luminous red giants. When their nuclear fuel is exhausted a resulting stellar core, a cooling ember, is called a white dwarf. However, in 10-15 percent of stars like the Sun enough hydrogen and helium remains to start nuclear burning again, rapidly re-igniting the faint white dwarf. This phase is called a final flash. While not uncommon, this pulse lasts for such a short time that seeing it is very rare: there are only three stars currently known to be undergoing final flash evolution. Estimates of the frequency of such a final flash object in our galaxy suggest that one occurs about once every ten years. The previous one observed by astronomers erupted in 1919.

Located in the constellation Sagittarius, in the direction of the center of our Milky Way galaxy, the distance to Sakurai's object can be measured from the expansion of the dust cloud. The current data show that it is about 6,800 to 12,000 light-years from Earth. As the cloud of debris expands it will be possible to refine our knowledge of the distance and other parameters of this interesting .

The team's results will be published in the Astrophysical Journal.

Explore further: Planet formation relied on sweeping up of small glassy beads, new model suggests

Related Stories

Image: A storm of stars in the Trifid nebula

Jan 30, 2014

( —A storm of stars is brewing in the Trifid nebula, as seen in this view from NASA's Wide-field Infrared Survey Explorer, or WISE. The stellar nursery, where baby stars are bursting into being, ...

A close look at the Toby Jug Nebula

Oct 09, 2013

ESO's Very Large Telescope has captured a remarkably detailed image of the Toby Jug Nebula, a cloud of gas and dust surrounding a red giant star. This view shows the characteristic arcing structure of the ...

An anarchic region of star formation

May 02, 2013

( —The Danish 1.54-meter telescope located at ESO's La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside ...

Hubble sees J 900 masquerading as a double star

Apr 01, 2013

( —The object in this image is Jonckheere 900 or J 900, a planetary nebula—glowing shells of ionized gas pushed out by a dying star. Discovered in the early 1900s by astronomer Robert Jonckheere, ...

Recommended for you

White dwarf may have shredded passing planet

Apr 17, 2015

The destruction of a planet may sound like the stuff of science fiction, but a team of astronomers has found evidence that this may have happened in an ancient cluster of stars at the edge of the Milky Way ...

Giant galaxies die from the inside out

Apr 16, 2015

A major astrophysical mystery has centred on how massive, quiescent elliptical galaxies, common in the modern Universe, quenched their once furious rates of star formation. Such colossal galaxies, often also ...

Protosuns teeming with prebiotic molecules

Apr 16, 2015

Complex organic molecules such as formamide, from which sugars, amino acids and even nucleic acids essential for life can be made, already appear in the regions where stars similar to our Sun are born. Astrophysicists ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.