Eta Carinae
Credit NASA/CXC/GSFC/K.Hamaguchi, et al.
The Eta Carinae star system does not lack for superlatives. Not only
does it contain one of the biggest and brightest stars in our galaxy, weighing at least 90 times the mass of the Sun, it is also extremely volatile and is expected to have at least one supernova explosion in the future.
As one of the first objects observed by NASA's Chandra X-ray Observatory after its launch some 15 years ago, this double star system continues to reveal new clues about its nature through the X-rays it generates.
Astronomers reported extremely volatile behavior from Eta Carinae in
the 19th century, when it became very bright for two decades, outshining
nearly every star in the entire sky. This event became known as the
"Great Eruption." Data from modern telescopes reveal that Eta Carinae
threw off about ten times the Sun's mass during that time. Surprisingly,
the star survived this tumultuous expulsion of material, adding
"extremely hardy" to its list of attributes.
Today, astronomers are trying to learn more about the two stars in
the Eta Carinae system and how they interact with each other. The
heavier of the two stars is quickly losing mass through wind streaming
away from its surface at over a million miles per hour. While not the
giant purge of the Great Eruption, this star is still losing mass at a
very high rate that will add up to the Sun's mass in about a millennium.
X-ray, Optical & Infrared Images of Eta Carinae
This multi-panel image shows Eta Carinae in the same field of view using
three different telescopes. From left to right, the images are from
the Chandra X-ray Observatory, the Hubble Space Telescope in optical
light, with the ground-based 2MASS survey in infrared. The X-ray image
reveals an outer horseshoe-shaped ring, a hot inner core, and a hot
central source. These structures glow in X-rays because they have been
heated multi-million degrees by shock waves. The optical image shows
two giant bubbles expanding away from the center of the system at over a
million miles per hour. The infrared data reveal that Eta Carinae is
one of the most luminous systems in the Milky Way. Eta Carinae is
shrouded in a rapidly expanding cloud of dust that absorbs radiation
from the central star and re-radiates in in the infrared. Credit: Optical: NASA/STScI, Near-Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF
Though smaller than its partner, the companion star in Eta Carinae is also massive, weighing in at about 30 times the mass of the Sun. It is losing matter at a rate that is about a hundred times lower than its partner, but still a prodigious weight loss compared to most other stars. The companion star beats the bigger star in wind speed, with its wind clocking in almost ten times faster.
When these two speedy and powerful winds collide, they form a bow shock
- similar to the sonic boom from a supersonic airplane - that then
heats the gas between the stars. The temperature of the gas reaches
about ten million degrees, producing X-rays that Chandra detects.
The Chandra image of Eta Carinae shows low energy X-rays in red,
medium energy X-rays in green, and high energy X-rays in blue. Most of
the emission comes from low and high energy X-rays. The blue point
source is generated by the colliding winds, and the diffuse blue
emission is produced when the material that was purged during the Great
Eruption reflects these X-rays. The low energy X-rays further out show
where the winds from the two stars, or perhaps material from the Great
Eruption, are striking surrounding material. This surrounding material
might consist of gas that was ejected before the Great Eruption.
An interesting feature of the Eta Carinae system is that the two
stars travel around each other along highly elliptical paths during
their five-and-a-half-year long orbit. Depending on where each star is
on its oval-shaped trajectory, the distance between the two stars
changes by a factor of twenty. These oval-shaped trajectories give
astronomers a chance to study what happens to the winds from these stars
when they collide at different distances from one another.
Throughout most of the system's orbit, the X-rays are stronger at the
apex, the region where the winds collide head-on. However, when the two
stars are at their closest during their orbit (a point that astronomers
call "periastron"), the X-ray emission dips unexpectedly.
To understand the cause of this dip, astronomers observed Eta Carinae
with Chandra at periastron in early 2009. The results provided the
first detailed picture of X-ray emission from the colliding winds in Eta
Carinae. The study suggests that part of the reason for the dip at
periastron is that X-rays from the apex are blocked by the dense wind
from the more massive star in Eta Carinae, or perhaps by the surface of
the star itself.
Another factor responsible for the X-ray dip is that the shock wave
appears to be disrupted near periastron, possibly because of faster
cooling of the gas due to increased density, and/or a decrease in the
strength of the companion star's wind because of extra ultraviolet
radiation from the massive star reaching it. Researchers are hoping that
Chandra observations of the latest periastron in August 2014 will help
them determine the true explanation.
These results were published in the April 1, 2014 issue of The
Astrophysical Journal and are available online. The first author of the paper
is Kenji Hamaguchi of Goddard Space Flight Center in Greenbelt, MD, and
his co-authors are Michael Corcoran of Goddard Space Flight Center
(GSFC); Christopher Russell of University of Delaware in Newark; A.
Pollock from the European Space Agency in Madrid, Spain; Theodore Gull,
Mairan Teodoro, and Thomas I. Madura from GSFC; Augusto Damineli from
Universidade de Sao Paulo in Sao Paulo, Brazil and Julian Pittard from
the University of Leeds in the UK.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington, DC. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for Eta Carinae:
Release Date: August 26, 2014
Scale: Image is about 2 arcmin across (about 4.6 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 10h 45m 04s | Dec -59° 41' 03"
Constellation: Carina
Observation Date: 8 pointings between Sep 1999 and Feb 2009
Observation Time: 30 hours (1 day 6 hours).
Obs. ID: 50, 1249, 4455, 9933-9937
Instrument: ACIS
References: Hamaguchi, K. et al, 2014, ApJ 784, 125; arXiv:1401.5870
Color Code: X-ray (Red, Green, Blue)
Distance Estimate: About 7,500 light years
Source: NASA’s Chandra X-ray Observatory
