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.
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 10000. 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 dust cloud 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 stars 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 planetary nebula is
formed from gas lost more than 10000 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 6800 to 12000 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 object.
The team’s results will be published in The Astrophysical Journal.
The National Optical Astronomy Observatory (NOAO) is operated by
Association of Universities for Research in Astronomy Inc. (AURA) under a
cooperative agreement with the National Science Foundation.
Media Contacts:
-
Dr. Katy Garmany
Deputy Press Officer
National Optical Astronomy Observatory
950 N Cherry Ave, Tucson AZ 85719 USA
Email: kgarmany@noao.edu
Desk: +1 520-318-8526
-
Peter Michaud
Pubic Information and Outreach Manager
Gemini Observatory, Hilo, HI
Email: pmichaud@gemini.edu
Cell: (808) 936-6643
Desk: (808) 974-2510
Science Contacts:
- Dr. Ken Hinkle
National Optical Astronomy Observatory
950 N Cherry Ave, Tucson AZ 85719 USA
Email: hinkle@noao.edu
Source: Gemini Observatory
